Effect of various factors and hygrothermal ageing environment on the low velocity impact response of fibre reinforced polymer composites-a comprehensive review

Effect of various


PUBLIC INTEREST STATEMENT
Composite materials are extensively employed in aerospace, automotive, and construction industries due to their lightweight nature and outstanding mechanical properties. However, their performance can be significantly influenced by the combined effects of low-velocity impact and hygrothermal ageing, which involve exposure to moisture and varying temperatures over time. This review study aims to explore the effects of various factors including the hygrothermal environment on the low velocity impact response of composites, investigating the damage mechanisms, degradation behavior, and subsequent consequences. This article provides a thorough knowledge of the response of composites under low-velocity impact during hygrothermal conditions by employing a combination of experimental analysis, various material sequences, and accelerated ageing techniques. The results of this study will help to improve design methods, manufacturing techniques, and maintenance procedures for composite structures, assuring their durability and performance throughout time in demanding situations. exposure to such environments causes the material property degradation, which affects the overall LVI performance and durability of the structures. Hence, it is crucial to critically review the influence of various parameters and hygrothermal ageing on the LVI response of FRP composites. This paper reviews the primary factors affecting LVI behavior, following, a comprehensive review on the hygrothermal effect on LVI behavior of FRP composites.

Introduction
In recent decades, FRP composites have been increasingly popular in aircraft, automobile, and marine applications owing to the benefits such as light weight, high strength to weight ratio, and better mechanical properties (Agarwal & Broutman, 1990;Chawla, 2012;Imamura et al., 2011;Mallick, 2007;Po & Tak, 2012;Tanaka et al., 2002). FRP composites are frequently employed for structural applications due to their versatility in producing the required material properties and ability to absorb higher energy than metallic components. Hybridization of fibres in polymer matrix composites (PMCs) have greatly increased the potential for the production of cutting-edge composites that require mechanical properties unique to the field of application by offering enhanced material properties and improved impact resistance (Sovit Agarwal et al., 2023Kaustav Ghosh, 2018Po & Tak, 2012;Qiao et al., 2008;Stephen et al., 2022;Yahaya et al., 2014). The major drawback of composite is, they are vulnerable to damage from out-of-plane impacts, as those prompted by an accidental tool fall due to careless handling while manufacturing, hail impacts, bird strike, and impact brought on by flying debris which are the most common causes of impact damages (Abrate, 1998;Abrate & Abrate, 2009;Yan et al., 2018;Zubair & Pai, 2019). The aircraft industry has always been concerned about the possible risks from LVI on aircraft structures such as engine nacelles, leading edges, nose sections, lower parts of fuselage, and wings. Many researchers have reported that LVIs cause internal damages viz. matrix cracking, fibre splitting, and delamination which lead to significant reduction in the local strength of the laminate (Zai et al., 2019;Pai et al., 2022;Sadighi & Alderliesten, 2022a;Safri et al., 2014;Taraghi et al., 2014). Impact loading can be classified into four categories: low velocity (less than 10 m/s), intermediate velocity (between 10 and 50 m/s), high/ballistic velocity (between 50 and 1000 m/s), and hyper velocity (between 2 and 5 km/s) (Safri et al., 2014). The most commonly used test methods for LVI are the Charpy, Izod, and Drop weight impact test (Chow et al., 2007;Dogra et al., 2019;Naik et al., 2001;Pai et al., 2021;Salman et al., 2015). The Charpy and Izod impact tests can be employed to evaluate the toughness of composite materials. They are comparatively simple to perform and interpret the results, however they have several drawbacks, such as the fact that the commonly used specimens are thick as well as short, and thus are unable to accurately represent common engineering components (Bader & Ellis, 1974;Pai et al., 2021;Salman et al., 2015;Yahaya et al., 2016). The drop/falling weight impact test is a more suitable method to perform LVI tests because it allows testing to be done at low velocity as well as the simulation of a wide variety of real-world impact events while extracting detailed performance data (Hosur et al., 2005;Mathivanan & Jerald, 2010;Umer et al., 2017;van Hoof, 1999). Studies have proved that it is an effective method for investigating impact resistance in FRP composites for a variety of applications (Bandaru & Ahmad, 2016;Santiago et al., 2017;Shaoquan et al., 2019;Stephen et al., 2022).
The effect of hygrothermal ageing of composites is significant since many structural components are frequently subjected to circumstances where both moisture and temperature vary in a specific way. Hygrothermal ageing is the process in which temperature and moisture interact to reduce the mechanical rigidity and strength of the composite materials (Mokhtar et al., 2011;Yousaf et al., 2022;Zanni-Deffarges & Shanahan, 1995). The moisture diffusion primarily makes the polymer matrix more pliable or flexible and the process is called plasticization. Based on the type of moisture attack, several changes in characteristics have been noted, including a decrease in the composite glass transition temperature(T g ), the molecular mass of the polymer matrix, the operational temperature, mechanical strength, rigidity, and toughness of the components (Ahmad et al., 2021;Pai et al., 2021Pai et al., , 2022Shetty et al., 2020;Vieille et al., 2012). Long-term hygrothermal exposure of polymer composite materials generate physical and chemical changes in the matrix through a variety of mechanisms, including plasticization, hydrolysis, and swelling. The two major mechanisms by which hygrothermal conditions damage composite materials are plasticization and microstructure degradation. The literatures include reports of various types of damage modes such as matrix fracturing, delamination, and softening of the fibre-matrix component caused by hygrothermal conditioning (Niu et al., 2021;Vaddadi et al., 2003;Zai et al., 2018;Zhong & Joshi, 2015b). The fibres that are typically used as reinforcement, with the exception of boron fibres, do not clearly exhibit a clear tendency for absorbing moisture and are more resistant to temperature changes than polymers. Therefore, matrix dominated properties are said to be highly susceptible to temperature and moisture attack. However, both matrix and fibre dominated characteristics undergo deterioration according to the literature (Imieli & Guillaumat, 2004;Ray, 2006;Voth, 2018;Zhong & Joshi, 2015a). A crucial area of research focuses on how moisture/humidity and temperature affect composite materials response to various forms of loading. Most modern aircraft and automobile structures and body parts are made of composite materials. These components must function in a variety of environments and their capacity to withstand the load may change depending on the type of environment. Therefore, it is crucial for researchers to develop a relation between different service environments and how they affect composites, particularly in dynamic loading situations like impact (Hosur et al., 2007;Naresh et al., 2018;Sarasini et al., 2017;Yahaya et al., 2014).

LVI behavior
LVI test provides information about the impact behavior of the laminate through force vs time, deflection vs time, energy vs time curves. Figure 1(a) shows the force vs time and energy vs time plots for FRP composite subjected to LVI test. During the impact, as the impactor contacts the specimen, it produces regular undulations and contact force increases linearly with time until it reaches a threshold level. The first abrupt load drop (Hertzian failure) occurs at this threshold load indicating the onset of damage in the form of micro cracks in the matrix and minor delamination in the vicinity of impact point (Evci & Gülgeç, 2012;Lesser & Filippov, 1994;Moure et al., 2018;Zubair & Pai, 2019). The laminate demonstrates the ability to carry additional load after the initial load drops at the threshold level with load increasing non-linearly up to maximum force F max . While matrix damage is characterized by smaller load drops, high amplitude load oscillations in the curve indicate catastrophic failure involving fibre breakage and rapid increase in delamination at peak load. The force vs time plots also provide important information about dynamic properties such as contact stiffness which is represented by the slope of force vs time curves. Apart from that, "OA" on the x-axis shows the maximum time for which the composite is in contact with the impactor and consists of the time required for elastic and plastic deformation. Studies have reported that by comparing the contact time, the extent of damage can be evaluated. Moreover, increased contact duration corresponds to higher damages induced in the laminate (Güneş & Sinan, 2020).
The slope of the force vs deflection denotes the bending stiffness of composites, and the energy vs deflection plot displays the amount of energy absorbed by the composite during the impact event (Figure 1(b)) . Additionally, it displays the three different drop weight test cases: free fall-perforation, free fall-stop and free fall-rebound (Pai et al., 2021;Richardson & Wisheart, 1996).
The energy needed for damage initiation is indicated by point B on the force vs deflection plot as depicted in Figure 2, and energy absorption during damage propagation is indicated by line BA (matrix crack, plasticization, and fibre failure). Perforation of the impactor begins at point A due to the fibre failure, thus the energy remains constant on crossing this point. The perforation energy, abbreviated E perf , is made up of the energy required for initiation (E ini ) and propagation of damage (E prop ) .

Resin Toughness
The impact load bearing capacity of laminate is dependent on the type of reinforcement and matrix employed. The right portion of matrix and filler materials can increase various damageresistant abilities of the composite (Asmare et al., 2023). In addition, the composite's ability to withstand impacts is greatly influenced by the resin toughness. According to numerous studies, thermoplastic (TP) composites have much higher toughness and energy absorption capacity than thermoset (TS) composites, which improves performance under impact loading (Santiago et al., 2017;Striewe et al., 2018). This is due to the chemical structure of TPs which can be seen as collections of randomly packed and entangled linear chains and can undergo chain slippage under impact loading. Hence, they exhibit higher plastic deformation and have good damage tolerance (Cervenka, 1999;Coskun et al., 2022;Kayaaslan et al., 2022). A study has reported that TP resin based composite displayed lesser delamination compared TS resins (Kayaaslan et al., 2023). According to (Chen et al., 2006). study of the deformation response of TP and TS composites, TP composites demonstrated significantly higher contact force and duration. Additionally, during the LVI event, the dent depth of TP composites increased, improving the visibility while aiding visual observation approach. In the study on the effect of cooling rate on TP composites, Gao and Kim  observed that the ductility and Mode-II fracture toughness of the matrix are affected by the cooling rate. Carbon/PEEK (Polyether ether ketone) composites with fast cooling have greater impact resistance than those with slow cooling rates. Figure 3 shows the damage the LVI caused to the carbon/PEEK and carbon-/epoxy-based FRCs. Less damage occurs in TP resinbased composites due to their increased toughness and failure strain. Therefore, carbon/PEEK material is considered to be highly damage tolerant . Recently, there has also been interest in nanoparticle modification of polymer matrix to enhance the physical and mechanical properties of fibre-reinforced composites. In a study by (Demir et al., 2023). the modification of the matrix by multi-walled carbon nanotubes (MWCNTs) limited the propagation of impact damage, as demonstrated by C-scan analysis.

Fibre architecture
The reinforcing fibres in a uni-directional (UD) laminate are all aligned in the same direction i.e., loading direction. These composites exhibit high specific stiffness and high specific strength since most of the stresses are carried by these fibres. However, they are highly susceptible to impact damages because of their lower transverse tensile strength. Results from a study by (Kayaaslan et al., 2023). contrary to UD composite, woven fabric (WF) reinforced composites displayed greater bending stiffness, contact stiffness, and energy absorption capacity as they have fibres aligned in longitudinal as well as transverse direction allowing stress to be carried in both axes providing balanced in plane properties. One of the potential causes of WF composites is improved impact resistance properties due to substantially higher transverse tensile strength of WF composites than the UD composites (Naik et al., 2000). Thus, it can be stated that fibre densities in the transverse  and longitudinal directions are highly effective on the impact resistance for UD and WF reinforced composites. The majority of fibre architectures used in composite laminates are bi-directional (BD) plain-woven fabrics and fibres in the thickness direction are absent in typical BD composite materials as shown in Figure 4.
As a result, the matrix system controls transverse characteristics to a great extent. This is especially important under transverse impact loads because interply de-cohesion might develop even when the top and bottom layers appear to be intact. Reinforcing 3D fabrics into BD composites has proven to be a successful method of improving the transverse properties of BD composites ( Figure 5). Binder yarns are woven through the thickness of the many layers of orthogonal weft and warp threads that make up 3D woven preforms. Because of through-thickness direction fibres can withstand loads, 3D woven composites have better inter-laminar fracture toughness as well as higher impact energy absorption capacity than BD composites (Je et al., 2019). Figure 5 illustrates how 3D composites outperformed other materials in terms of penetration resistance and total energy dissipation (Agrawal et al., 2014). Due to strain & fracture of the z-tows mechanisms of damage, 3D laminates absorbed more energy and endured impacts longer than BD laminated systems. Pan et al., (2023) investigated the effect of heat treatment on the damage behavior and various impact mechanical responses of glass fibre/polypropylene/epoxy composites. Author reported that stress concentration due to heated z-binder yarns acts as an antidelamination mechanism, and further, 3D woven composite materials were more valuable to heat treatment under LVI. It is discovered that three-dimensional orthogonal weaves are severely stretched on the z-axis. These tows frequently have fractures, and surface weft tows are frequently pulled through the z-tows unbroken crimps. Z-crimps are new, a major source of energy loss, fail under tension, and also the surface weave steps across them under friction (Sadighi & Alderliesten, 2022b).

Hybridization
Mechanical properties are characterised through the hybridization process which combines reinforcements having high stiffness with ductile fibres that exhibit excellent impact resistance (Kaware & Kotambkar, 2022;Pai et al., 2021). The two primary methods of hybridization are the combination of various fibre types and the lamination of metal and composite layers, sometimes known as fibre metal laminate (FML).

Combination of different fibres.
The highest penetration resistance is provided by Kevlar and hybridizing it with carbon/glass textiles improves rigidity and reduces cost (Kaware & Kotambkar, 2022). It was found that hybridising the glass and graphite layers was successful in reducing damage accumulation (Sadighi & Alderliesten, 2022b). Low-velocity impact studies by (Demir et al., 2023). revealed that carbon/glass hybrid composites can achieve up to 70% higher impact peak forces than carbon fibres alone. The distribution of reinforcements also affects how well a hybrid composite performs, as seen in Figure 6 (a) interply hybridization, where layers of various fibres are stacked to form the laminate, and (b) intraply hybridization in which a ply/layer is made of different fibres. The inter-ply hybrid materials showed better impact performance than the intra-ply composites. LVI behavior of inter-and intra-ply hybrid basalt-aramid/epoxy composites was studied by Wang et al. (2008). The study revealed that compared to intra-ply composites, the inter-ply composite seemed to have the least peak contact force, the highest specific energy absorption, and higher ductility. Figure 6(c), is made up of layers of metal/composite material and layers of matrix composite material stacked in a specific order. FMLs bring together the more ductile characteristics of metals that possess desired fatigue and fracture properties of composites. Impact experiments on FML show that metal layers can limit the progress of delamination and the penetration of impactors (Je et al., 2019). Glass fibres in FML did not undergo failure at LVI until the outer aluminum layer cracked. Thus, the aluminium plate acted as a layer of sacrifice. The behavior of the laminate is additionally enhanced by the strain hardening action of the metal layer Sadighi & Alderliesten, 2022b).

Stacking Sequence
Delamination between plies with different orientations is a well-known occurrence in composites. Increasing interlaminar fracture toughness, that is inspired by stacking order and interfacial properties, can improve impact resistance & damage tolerance. It causes the load to change from tensile to compressive, which can delay or prevent the interlaminar fracture . Numerous researchers concluded that changing the order of the ply stacking had an impact on the maximum contact pressure, the delamination zone, the residual impact strength, and the damage area. Maier & Mandoc (2023). studied the LVI response of symmetric glass/carbon fibre woven reinforced composite materials focusing on different stacking sequences-one with alternating carbon and glass fibre layers and the other having four consecutive layers of glass fibre followed by consecutive layers of carbon fibre. Results demonstrated that layering two different materials in succession is a superior strategy to get a complementing effect for the two materials' disparate mechanical properties. Damage behavior observed proved that interlacing the fibres one after another allows the damage to spread more quickly through the specimen's elasticity and hence absorbs more of the impact. Another study reported that composites with ±45° surface layers had better impact resistance compared to those with 0° surface plies and improved residual impact strength. It was attributed that the composite's increased ability to absorb energy elastically is due to its higher flexibility (Leomand, 1975). Higher delamination occurred as the angle between adjacent layers grew (Pai et al., 2021). Previous studies demonstrate that hybrid interply composites with surface plies made of aramid fibres are more effective than inner aramid plies at resisting damage from delamination while subjected to impact loads (Dorey et al., 1978). During the impact phase, the outer layer of aramid absorbs most of the impact energy and spreads over a wider area from the impacted region and thus controls the delamination due to damage. Larger overall damage area indicates that more amount of energy is utilized in damage propagation than bouncing back of the impactor (Pai et al., 2022Vasudevan et al., 2018Vasudevan et al., , 2020Yahaya et al., 2015). By using ductile aramid plies as surface layers, it is possible to prevent impact damage to the load-carrying basalt plies at the core (0°/90°) (Pai et al., 2021(Pai et al., , 2022Sarasini et al., 2013). Jefferson et al. (2019). investigated the the LVI and quasi-static residual tensile behavior of GFRP (Glass Fibre Reinforced Polymer). Two stacking sequences with multiphase fibre systems-sandwich and embedded were investigated for four energies, namely 2, 4, 6, and 8 J. The sandwich-like layup showed enhanced impact and damage tolerance properties of up to 4 J, whereas the intercalated sample had good residual tensile capabilities as well as a consistent pattern of destruction for higher energy levels (Kaware & Kotambkar, 2022). (S. Behnia et al., 2016) conducted impact study to investigate effect of notch angle and stacking order on the hybrid composite including aramid, basalt, glass, & carbon to various stacking configurations. Results indicated that laminate where aramid fibre is utilized as the outermost layer showed higher energy absorption capacity than other configurations.

Effect of Hygrothermal environment on LVI
Hygrothermal ageing refers to a process in which the combined action of temperature and moisture causes reduction mechanical rigidity and strength of composite materials. The water diffuses between the chains of the macromolecules, separating them, and causes the polymer structure to expand, weakening the intermolecular bonding forces. As a result, polymer behavior becomes more ductile, and its stiffness reduces. This mechanical response is referred as "plasticization" (Vieille et al., 2012). Li et al. (2000) investigated the LVI response of UD and cross ply GFRP composite subjected to cyclic moistures for conditioning temperature from 50°C to 100°C. For comparison, three different conditioning temperatures were used viz. 50°C, 75°C, and 100°C, respectively. A wet cycle followed by dry cycle was referred to as one moisture cycle. The LVI behavior of GFRP laminate was significantly influenced by the first moisture cycle. The effect was not as obvious in successive moisture cycles as it was in the first moisture cycle. As the conditioning temperature increases the maximum impact load decreases and deflection increases. This indicates that cycling moisture level's damaging effect is enhanced by the elevated conditioning temperatures. Cross ply laminates exhibited greater LVI resistance than unidirectional laminates subjected to varying moisture content and high conditioning temperatures. Imielińska & Guillaumat (2004) studied the LVI response of hybrid aramid-glass/epoxy and interlayer aramid-glass/epoxy laminates after water immersion ageing. Accelerated ageing tests were performed on the samples by immersion in distilled water at 70°C for 8 weeks (about 2 months). The aramid-glass fibre configuration had no significant effect on moisture absorption and impact test characteristics. Moisture did not show a significant effect on the delamination threshold load or impact energy absorption. Author concluded that compared to wet hybrid samples, wet samples of interlayer composite showed higher resistance to impact. Kim et al. (2005) studied the damage mechanisms and compressive residual strength variation of glass/phenolic laminate by LVI loading under accelerated ageing environment. Accelerated ageing was conducted which consisted of xenon arc lamp irradiation at 60℃ and 60% humidity for 250 cycles (500 h.), 500 cycles (1000 h.), and 750 cycles (1500 h.). The results demonstrate that failure initiation energy and resultant compressive strength decrease as ageing time increased. Surface degradation due to ultraviolet light results in reduction of failure initiation energy as ageing cycles increase. The damage processes observed in the damage area were matrix cracking, delamination, fibre breakage, and finally penetration. Hosur et al. (2007) investigated the carbon/epoxy laminate's LVI response at increasing energies under cold-dry and cold moist conditions for a period of 3 and 6 months and compared them with the room temperature samples. It was observed that, as the cold dry samples were conditioned for longer duration their load bearing capacity decreased except at low energy level (15J) where the peak load was higher than room temperature specimens. The damage area was also observed to increase with conditioning period. After being treated to cold-moist conditioning, samples developed plasticity, which made them more ductile and able to withstand greater peak loads. At given impact energy, the damage area of 3-month samples did not show significant changes, however, 6-month samples showed larger area of damage. Aoki et al. (2008) studied the combined effects of thermal environment and water absorption on the impact characteristics of CFRP (Carbon fibre reinforced polymer) by keeping them immersed in distilled water at 71℃. The author reported that damages propagated catastrophically in dry specimens than that of wet specimens because the load oscillation of dry specimens tends to be greater than the wet specimens. After being infused with water, the water served as a plasticizer, separating the polymer chains, and softening the CFRP laminates. Berketis et al. (2008) investigated the hygrothermal effect of up to 30 months of prolonged submersion in water on the behavior of impact damage in glass/polyester composites. The author defined a point on the force time plot curve Figure 7(a,b) as critical impact force, point A which indicates the onset of delamination by a sudden load drop to point B, which indicates reduction in stiffness. However, such a characteristic point was not detected when the immersion time was beyond 12 months. The critical impact force decreases with increasing exposure time. Despite the impact strength, dry samples were stiffer than wet specimens, enabling them to retrieve from effects more rapidly and preserve a greater force after destruction has been initiated. Figure 8(a,b) the force-displacement curves indicated the softening behavior of laminate due to moisture absorption as there was an increase in maximum displacement with ageing. Boukhoulda et al. (2011) studied the impact behavior of polyester matrix E-glass fibres laminate under hygrothermal environment (50°C, 95% relative humidity) for an ageing period of 289 days. It was observed that the percentage drop in all impact forces becomes more significant as the weight of the absorbed water increases. These findings demonstrate that the material behaves  more elastically under hygrothermal conditions, absorbing higher elastic energy and lowering the contact force. It was also observed that when the material ages, the delamination zone reduces, and the material exhibits greater flexibility in the hygrothermal environment. Mokhtar et al. (2011) evaluated the effect of hygrothermal conditioning on impact response of carbon/epoxy composite by using three different stacking sequence, one having classic layup commonly used in aerospace industry and other two having quasi-isotropic and quasi homogeneous properties. Ageing was carried out in a climate chamber for 3 months, at a temperature of 70℃ and 85% relative humidity. The impact response test results showed that the composite is more brittle during the early stages of ageing than it is when the specimen is aged for a long time. As ageing time increases, it is seen that energy absorption increases, possibly indicating the matrix degradation of the composite. C-scan damage analysis revealed that the quasi-homogenous layup showed largest damage area while the quasi-isotropic showed the smallest damage area.
Zhong & Joshi (2015a) conducted LVI tests on three groups of carbon fibre/epoxy laminates specimen, each subjected to three hygrothermal conditions i.e. Group 1 consisted of a dry sample, Group 2 underwent cyclic hygrothermal conditioning (12 h. in water at 60°C and 12 hrs. in the freezer at −30°C), and Group 3 underwent isothermal immersion in water at 60°C. Three representative samples, Specimen 2, 6, and 13, from Groups 1, 2, and 3, with moisture content of 0.070%, 1.364%, and 1.732%, respectively, were chosen for comparison of impact responses of the laminates with various humidity levels. Impact resistance of the specimen varies inversely with contact time. This implies that a specimen with a shorter contact time exhibits higher impact resistance (and comparatively larger contact force). Therefore, it was determined that as the amount of absorbed moisture increased, CFRP laminates became more impact resistant. In test samples with higher moisture contents, the laminate dissipated greater impact energy through elastic deformation. The impact energy available to start and spread damage was therefore reduced. This observation accounted for the laminates with hygrothermal conditioning having minimized impact damage.
Zhong & Joshi (2015b) studied the impact resistance of carbon fibre based epoxy laminates, considering two types of laminate lay ups, normal and staggered which were subjected to hygrothermal conditioning by carrying out water immersion at 80 ℃ for increasing durations to achieve varying moisture levels. From a statistical perspective, the differences between the two lay-ups in terms of moisture absorption rate and impact resistance were not significant. Moisture interacted with the resin, weakening Van der Waals force which in turn decreased the amount of hydrogen bonds. As a result, the epoxy resin's ductility increased, which ultimately explained why hygrothermal conditioned laminates possessed superior impact resistance. Atas & Dogan (2015) studied the influence of hygrothermal ageing on single and repeated LVI response of glass/epoxy composites. Thermal ageing conditions were done at 95°C and 75% humidity. Ageing durations were 100, 400, 700, 1000 and 1300 h. Author observed a decrease in impact resistance of the laminates. This was mainly due to thermo-oxidative ageing of polymeric composites which causes surface deterioration and microcracks normal to the surface, which is followed by chain scission, oxygen diffusion into the matrix, diffusion of degradation products out of the matrix, and oxygen reactivity with the matrix. For repeated impact case, the absorbed energy until perforation of unaged samples were more than three times that of aged samples which implied that there is decrease in fatigue life. Dogan & Atas (2016) studied the variation of the impact properties of E-glass/epoxy laminates under hygrothermal ageing having the stacking sequence [0/90] 8s . Hygrothermal ageing was conducted in a climatic test chamber at 95° and 70% humidity having test duration from 0 to 1200 h. As ageing time increases, load to failure and the related deflections decreased. For ageing of 1000 h, the failure elongation decreased by 39% i.e., as the ageing time increases, samples showed more brittle nature. The absorbed energy increases for samples that have been aged for 100 hrs., and it essentially stays constant until 800 hrs., then it starts to decline. Deflection and peak force followed a similar trend. Ahmad et al. (2016). carried out experimental research on the effect of hygrothermal environment on LVI response of unidirectional (UD) carbon/epoxy composite plates. To produce various levels of moisture content, specimens were immersed in distilled water at 80°C for different time periods. Impact tests were conducted on specimens with varying moisture content in terms of weight percentage 0.0 % (dry), 0.75 %, 1.0 %, 1.1 %, and 1.42 % (fully saturated) under impact energy of 23.6 J. The experimental findings demonstrated that the moisture had a detrimental effect on composite plate impact resistance. Higher moisture content composite plates exhibit lower impact peak force, delamination threshold load, and initiation energy values. This was due to matrix component which gets plasticized at the molecular level by moisture absorption, lowering the stiffness of the composite. The size of the damage or delamination increased with the change in moisture content percentage. Zhong et al. (2019). conducted extensive experimental work to examine the hygrothermal performance of CFRP and GFRP composites. To speed up hygrothermal ageing, specimens were immersed completely in a general bath of water that was set to 80°C. The impact test results revealed that the impact characteristics of CFRP composites were improved by absorbed moisture during hygrothermal ageing. The peak impact force of CFRP laminate raised by 20.60% after absorbing 3.149 weight % of moisture. In contrast to CFRP composites, GFRP composites' impact behavior deteriorated noticeably with hygrothermal ageing. Peak impact force decreased by 33.44% for GFRP laminates. From the SEM images conditioned samples showed higher damage area than the unconditioned samples. van (1999). investigated the low-velocity impact behavior of carbon/epoxy composites with unidirectional, cross-directional, and quasi-isotropic layups under hygrothermal condition. Three specimens from each layup type were made to achieve dry, intermediate, and saturated moisture conditions. The samples were submerged for the requisite period in distilled water (80°C). Unidirectional laminates were shown to have a higher water absorptivity than cross-ply and quasiisotropic laminates. The drop tower test findings revealed that intermediate and saturated moisture concentrations yielded reduced impulses because moisture and temperature reduce the impact force as illustrated in Figure 9. Hygrothermal effects cause gradual increase in strain and a decrease in impulse. Regarding the stacking sequence, it was discovered that under all proposed conditions, the impulses in quasi-isotropic laminates were significantly higher than those in other stacking sequences. In every condition examined, quasi-isotropic laminates showed the lowest amount of strain.  Hawa et al. (2016) investigated the burst strength and impact behavior of hygrothermal aged E-glass fibre/epoxy composite pipes. To conduct the ageing process specimens were placed in tap water for 500, 1000 and 1500 h. and maintained at 80°C. Upon impact loading, the peak force was largest for the unaged specimen, and it gradually decreased as ageing duration increased. This was due to the water that diffused into the composite pipe's matrix, fibres, or interface region; leading to degradation and reduced stiffness. However, plasticization caused a slight increase in absorbed energy in aged pipes compared to unaged pipes. Shaoquan et al. (2019) studied the C/BMI (Carbon/Bismaleimide) composite's low-velocity impact behavior during supersonic hygrothermal flight cycles. The C/BMI composite underwent thermal cycling for each cycle, which included: (1) a high temperature interval of 180°C representing thermal ageing at supersonic speeds; (2) a low temperature phase of 60°C to resemble the subzero temperatures during subsonic flight speeds; and (3) a humid climate with 90% RH and temperature levels ranging at regular intervals around 0 and 100°C to reflect moisture absorption and thermal cycling of aircraft at varying altitudes. A secondary load reduction was visible on the force-time plot of hygrothermally cycled C/BMI matrix when the impact energy was above 20 J. Stiffness of the structure has a direct effect on contact duration t c. The BMI composite's oxidation and degradation brought on by hygrothermal cycles, along with the interfacial debonding in the fibre and matrix, are likely to have contributed to the matrix's softening, which in turn reduced stiffness. C/BMI laminate subjected to 300 cycles displayed lower F max , shortened t c , and greater absorption of energy compared to the unaged laminate, indicating a reduced tolerance to LVI when impact energy was between 30 and 40 J as shown in Figure 10.
Dogan & Arman (2019) studied the combined effect of temperature and humidity on the impact behavior of E-glass/epoxy composites. The conditioning was carried out at (C1) − 8 hrs. at 70°C and 70% humidity without UV radiation, followed by alternate phases of UV radiation at 40°C and 70% humidity, pure hygrothermal conditioning (C2) 90°C, (C3) 80°C, and (C4) 70°C at 70% relative humidity for all three conditions. Impact response curves showed that samples subjected to C2 have the lowest peak force in all the Figures 11(a-d) , while the unaged samples exhibit the highest contact force at all impact energies. This indicates that as temperature increases the load bearing capacity decreases. Unaged samples and samples exposed to conditions C1, C3, and C4, rebound at impact energies of 40 J, however sample exposed to C2 experienced penetration. All curves of aged samples were of the open type at impact energy of 80 J (implying perforation). The hygrothermally aged specimens at 90℃ had the lowest perforation threshold and impact bending stiffness while unaged samples showed highest values. The samples exposed to 90°C, 80°C and 70°C had reduced perforation thresholds of 40%, 34% and 6%, respectively. Chen et al. (2006) investigated the hygrothermal as well as thermal ageing effects on the LVI properties of C/BMI composite. The dent depth vs impact energy showed an inflection point for all of the sample groups. Up to the inflection point there was a slow increase in the dent depth with impact energy. On crossing this point, hygrothermally aged specimens showed lowest increase in depth followed by non-aged and thermally aged at a constant impact energy. The damage area increased with impact energy. At constant impact energy, the damage area grew in the following order: thermally aged> hydrothermally aged> and non-aged. Zhou et al. (2021) investigated the effect of hygrothermal conditioning on damage behavior of carbon/epoxy laminate after low-velocity impacts. Specimens were hygrothermally aged by immersion in water bath at 71℃ for 30, 60 and 90 days. Impact energies chosen were 25, 35, 45, 55, and 65 J. Author concluded that the equilibrium moisture absorption rate of the carbon fibre/epoxy resin composite was just 0.88%, which is a low moisture absorption rate. It was also observed that the increase in dent depth became significant after inflection point (35J). The impact damage area was unaffected by the ageing environment at low impact energy but increased as the impact energy increased and peaked at moisture saturation and the matrix's plasticization and hydrolysis degrees being near the limit. Paturel & Dhakal (2020) investigated the effect of temperature and moisture on LVI of flax/glass reinforced vinyl ester hybrid composites. The specimens were conditioned by submerging in ambient water and at an elevated temperature (70℃) in an environmental chamber. Results indicated that when flax composites were exposed to hygro environment, the fibres swell and cause microcracks in the resin. The hydrophilic properties of flax fibres allow them to absorb moisture more readily than glass fibres. With a load bearing capacity that was 25% higher than that of flax fibre laminates and an energy absorption capacity that was about 9% higher than that of glass fibre laminates, the hybridization of glass fibre onto flax fibre composite led to improved impact damage characteristics. Rubio-González et al. (2020) performed drop weight impact tests on sea water (SW) and distilled water (DW) aged glass fibre/polymer laminates to investigate its LVI performance. Epoxy and vinylester were the two types of resins utilized during the fabrication process for a comparative study. Both the resins showed higher absorption for DW than SW due to the presence of elements such as Na+ and Cl ions in SW. However, it was discovered that the composite laminates made with epoxy resin had a substantially larger moisture absorption capacity than those made with vinyl ester resin. This finding may be connected to the vinyl ester resin's residual cure reaction, which tends to prevent water molecules from absorbing into the polymeric network. It can be seen from the impact response curves in Figures 12 and 13 that the composite laminates made with epoxy resin as opposed to vinyl ester resin is significantly more affected by both SW and DW conditions. Peak force plateaus in glass fibre/epoxy composites occur for longer duration of time which can be useful in applications that require withstanding small loads for longer period during impact. Vinyl ester-based laminates do not absorb as much water content as epoxy-based laminates, so the maximum displacement for wet specimen made of epoxy resin increased in comparison to specimens prepared with vinyl ester resin, indicating a more ductile response ( Figure 13). However, authors concluded that the SW and DW aged epoxy resin-based laminates were better for design purposes than vinyl ester-based laminates due to their reduced damage area in the event of an impact. When Multi Walled Carbon Nano Tubes (MWCNTs) were added to laminates, there was tendency for the stiffness to increase. This results in further damage processes at CNT-rich areas during impact loading, which resulted in an increase of absorbed energy. Niu et al. (2021) examined the hygrothermal ageing mechanism of carbon fibre/epoxy resin laminates under LVIs. The specimens were conditioned in distilled water at 100℃ for 4, 8, and 16 hrs and tests were carried out. In the first stage 0-4 h, an increase in "soft interface" area occurs which causes a considerable reduction in flexural strength and shear strength as well as an improvement in impact resistance; second stage 4-8 h, the "soft interface" area keeps growing as the resin matrix goes through secondary curing, which results in the greatest impact resistance and a rebound of mechanical properties; After 8 hrs., the performance starts to deteriorate as a result of the resin swelling and plasticizing. Ahmad et al. (2021) carried out falling weight impact tests on dry and hygroscopically conditioned carbon fibre/epoxy quasi-isotropic [0°/45°/90°/45°] s composite plates. Conditioning was carried out with hot water heated to 80°C to achieve various moisture content (MC) levels: dry plate 0%, 0.75%, 1.0%, 1.1%, and fully saturated plate 1.36%. Wet composite plates often sustain less damage than dry composite plates as they are more elastically deformed. Initiation energies absorbed energy ratios, and residual energy all decreased as the proportion of MC rises. Therefore, the author concluded that presence of MC enhances the chain segmental mobility of the polymer molecules, which ultimately results in the improvement of the impact-resistance of wet quasiisotropic composite plates compared to the dry plate. This increases the epoxy materials ductility and elastic limit. Yousaf et al. (2022) conducted LVI experiments on carbon fibre/epoxy matrix laminated composites that had been dry and hygrothermally conditioned with variable impact energies of 30, 40, and 50 J. In an environment conditioning chamber, hygrothermal ageing was carried out at two different relative humidities (RH) [25°C and RH:85%] & [25°C and RH:100%]. As a result, under the same impact energy, deeper dents were found in hygrothermal conditions than in dry ones hence moisture softens the epoxy and increases viscosity in the composite. The indentation depth increased with impact energy as well as with increase in RH from 85% to 100%. Placet (2023) studied the impact and bending properties of flax/polypropylene (PP) composite and its stainless-steel hybrid laminates subjected to hydrothermal (water immersion at 70°C) and thermal ageing (heating to 120°C) for 177 and 252 days, respectively. The plasticization of the flax fibres by water, a reversible phenomenon along with irreversible mechanisms, such as the hydrolysis of flax fibres and the deterioration of the adhesive bond between steel and composite, flax fibres and PP was primarily responsible for the general decrease in stiffness of impacted and bending specimens. Thermal ageing for longer durations caused a loss in impact and bending properties mainly due to embrittlement of PP brought on by thermal oxidation and the resulting fibre/matrix debonding in the affected area. However, authors concluded that despite the degradation mechanisms observed, bending stiffness and strength of hybrid composite resulting from combination of plant fibre-based composite with fibre metal laminates still makes them appropriate for semi-structural applications.
Most studies have revealed that hygrothermal conditions have a degrading effect on polymer composites along with stiffness reduction. The combined effect of moisture and temperature induces changes in chemical properties which in turn aids in reduced mechanical performance. However, the moisture absorption behavior of these composites is very much dependent on the nature of polymer resin as well as the type of reinforcement used. Vieille et al. (2012) investigated the response of TS and TP composites subjected to extreme environmental conditions (temperature and humidity). Authors discovered that composites based on TP have greater humidity and temperature resistance. It was also discovered that TP composite had improved strength and a greater ability to regain its mechanical characteristics after hygrothermal ageing as strain levels and overstress were reduced. Studies also reported that the properties of the fibre matrix interface deteriorate as the temperature increases to the curing temperature in TS and TP composites. Under transverse loading, fibre pull out occurs due to poor fibre/matrix interface properties and the energy dissipation caused by this phenomenon improves LVI performance. Matrix contracts at low temperatures which strengthens the fibre/matrix interface and TS-based composite becomes brittle. It yields a greater contact force, less energy absorption, and a shorter contact time in comparison with exposure to ambient conditions. As a result, there will be more damage and the composite's impact resistance will be lower. Whereas the enhanced impact resistance of TP composite materials at low temperatures is a result of the increased matrix ductility .
Literature shows that a lot of researchers have focused on LVI studies on TS polymer resins under hydrothermal conditions, especially epoxy-based FRPs. Epoxy resins are common matrix material in CFRPs. It is seen that thermoset polymers readily absorb water because of the free space between the chains of molecules. The polymer is physically weakened by such absorption, and it could also be chemically attacked. Diffusion and chemical kinetics regulate the kinetics of these processes. They have an impact on cross-link strength, which in turn has an effect on mechanical characteristics, including impact properties (Hosur et al., 2007). (Zhang, 2023). provided a comprehensive review of ageing mechanism observed in polymers and their composites under various influencing factors using widely known molecular simulation methods. The authors mentioned moisture and temperature (hygrothermal ageing) to be an important environmental factor among others and summarized the main ageing mechanism under its influence as shown in Figure 14. It demonstrates primarily how water molecules participate in reactions or establish hydrogen bonds with polymers to influence the stability of polymers. In addition, the electronegative sites on these polymer chains attract the water molecules, enabling them to make their way deeper into the composite. When ageing is done in a saline environment the ions (Na + , Cl − ) enter the resin matrix along with water molecules causing the resin matrix to expand and degrade its mechanical properties.
Most studies have revealed that the aged specimens show lower peak loads, larger deflections, and longer contact durations than the dry specimens. Longer contact duration indicates a stiffness reduction. The dry specimens are stiffer than aged specimens and therefore witness holding higher peak loads. This enables us to draw the conclusion that the rise in absorbed moisture weight is mostly responsible for the decrease in impact forces. A hygrothermal environment makes the material structure more elastic, allowing it to absorb more elastic energy and decrease the force of impact.
Studies have revealed that the absorption of moisture is significantly influenced by the rise in temperature in FRP composites. Bibo et al. (1995). discovered that temperature has the ability to alter nature and extent of impact-induced damages. As the conditioning temperature increases the maximum contact load decreases and deflection increases. This indicates that cyclic moisture level's damaging effect is enhanced by the elevated conditioning temperatures (Li et al., 2000). Elevated temperature raises water uptake with a greater diffusion rate when contrasted to room temperature. This is possibly caused by a variation in the material's water diffusivity brought on by heat-induced thermal expansion. In fact, a interfacial bond failure in between fibre and matrix happens as the interface degrades due to heat, enabling the molecules to flow freely and creates a great path for water to travel rapidly. This encourages additional fibre swelling, which causes cracks and fibre- Figure 14. Aging mechanisms in composites due to moisture absorption (Zhang, 2023).
matrix interface degradation to occur more severely. Furthermore, at high temperatures, the time required for moisture saturation is significantly reduced (Paturel & Dhakal, 2020).
Commonly used synthetic fibres like carbon and glass fibres are hydrophobic in nature. They provide higher moisture resistance than natural fibres which are hydrophilic in nature. When a composite made of natural fibres is exposed to moisture, the fibres swell and cause microcracks in the resin. Since flax fibre has a high cellulose content of roughly 70%, water enters the interface through micro fractures at a faster rate, causing the fibres to swell more. Furthermore, the structures of flax fibres are rich in hydroxyl groups (−OH). These groups create many hydrogen bonds between the cellulose and polymer macromolecules. Large quantity of -OH groups cause low moisture resistance, which leads to reduced interfacial attachment at the fibre-matrix interface. As previously mentioned, in a study by Paturel & Dhakal (2020) glass fibres present on the outer layers of the samples prevented water penetration when flax fibres were hybridized with glass fibre. Water still permeates the sample sides and matrix voids before it reaches the fibres, which absorb the water through capillary diffusion more slowly than unhybridized flax vinyl ester samples. Therefore, hybridizing natural fibres with synthetic fibres leads to lower moisture gain, enhanced ductility and hence improved impact performance compared to flax fibre alone. The degrading effect of hygrothermal ageing can also be reduced by changing the stacking sequence of laminates. Quasi-isotropic laminates showed the lowest moisture gain compared to unidirectional and cross ply laminates. Lower strain and higher impulses at all moisture levels indicate improved impact resistance and higher load bearing capacity (Niu et al., 2021).

Conclusion
This paper has attempted to review the effect of primary factors specifically resin toughness, fibre architecture, hybridization and stacking sequence on the LVI performance of FRPs. A critical review of LVI behavior under hygrothermal environment has also been presented. Investigations have suggested that the effective selection of these primary factors can lead to enhanced impact resistance of the composite.
• When compared to thermoset resins, thermoplastic resin offers higher performance, superior impact resistance, and damage tolerance.
• Hybridization with ductile fibres such as Kevlar proved to significantly enhance the impact resistance as well as damage tolerance of the composite system. Inter-ply hybrid laminates are witnessed to perform better due to their ability to absorb higher energy under impact.
• 3D woven composites have better inter-laminar fracture toughness as well as higher energy absorption capacity than 2D composites due to the presence of Z-yarns.
• Interlaminar bond strength is influenced by the stacking sequence and fibre orientation. Higher delamination occurs as the angle between adjacent layers increased. Sandwich type laminate layup with surface aramid plies were effective in reducing the degree of delamination.
• Hygrothermal effect is primarily influenced by the moisture content absorbed by the composite, temperature, the ageing duration, type of polymer resin used and interfacial defects.
• It is important to note that while the combination of moisture and temperature is associated with a decrease in mechanical performance of FRP composites, increased matrix ductility can be considered as a curative benefit of moisture absorption. This is due to internal stresses that may be relaxed as a result of moisture diffusion during hygrothermal ageing. However prolonged exposure to moisture and temperature, which is often the case in real life applications, leads to material degradation which in turn has detrimental effects on mechanical properties. The water diffuses between the chains of the macromolecules, separating them, and causes the polymer structure to expand, weakening the intermolecular bonding forces. As a result, polymer behavior becomes more ductile, and its stiffness reduces. This mechanical response is referred to as "plasticization". Moisture ingress significantly deteriorates the bond strength due to mismatch in volume expansion coefficient between fibre matrix interfaces. Elevated temperatures cause thermal oxidation resulting in breakage of bonds. This leads to matrix swelling, cracks and delamination as the major damage modes. In general, it can be said that temperature and moisture have a negative effect on the impact properties of FRPs.
• Although the degrading effects of moisture and temperature are not completely avoidable, the degree of damage can be reduced to increase the service life of the structures. Since moisture is a major factor for degradation, results from the studies have shown that hybridizing synthetic fibres with natural fibres, change in stacking sequence such as a quasi-isotropic laminate, use of TP polymer-based matrix compared to TS, addition of nano particles such as MWCNTs in the matrix can reduce moisture absorption and hence potential damage can be lowered.
In the past two decades there have been extensive hygrothermal studies focused on thermoset polymer resin, epoxy resin in particular has been commonly used with carbon and glass fibres.
However, very few studies have investigated the hygrothermal impact performance of TP polymer resin. Future studies can be directed towards high-performance thermoplastics like PEEK (Polyether ether ketone) to investigate their benefit over popularly used thermosets under hygrothermal LVI application. Although most studies have identified the accelerating effect of elevated temperature to achieve moisture absorption saturation at a faster rate and implemented the same during hygrothermal conditioning. This method still does not successfully depict the real time hygrothermal temperatures and long-term submerged environments experienced in aerospace and marine applications. More attempts need to be made to correlate hygrothermally aged moisture uptake and properties against long-term water treatments for composites. Researchers need to extend their investigations in composites to wider environmental degradation factors such as UV radiation, oxygen, chemical exposure and fatigue apart from moisture and temperature. Future research should concentrate on generating predictive models that enable engineers to more accurately correlate accelerated ageing and natural ageing and capture the complex interactions between hygrothermal ageing and LV responses. If composite laminates are to be used in wide structural applications, more research must be done in the impacts on complicated geometry, as these effects are less well documented.