Failure Analysis of Plant Fibre-Reinforced Composite in Civil Building Materials Using Non-Destructive Testing Methods: Current and Future Trend

ABSTRACT Natural plant fiber-reinforced composite (NFC) has become a preferred component in modern-day civil building construction materials because it offers, among others, an environment-friendly solution without compromising stringent engineering requirements. Such green-based composites have exhibited noteworthy level of competitiveness comparable to that of the existing commercially available nongreen materials. Furthermore, NFC can also be tailored to align with the desired functional attributes. However, lack of comprehensive guidelines and recommended applications of suitable methods to assess composite failure of such novel NFC have raised significant concerns. This paper provides a comprehensive review of the latest developments in nondestructive testing (NDT) that can be applied to investigate into NFC failures. The study further explores alternative nondestructive testing methods and technologies exhibiting potential use in plant fiber composites studies, hence paving the way to future investigation trends. Precise characterization of defects and identification of damages in NFCs present a major challenge, demanding application of advanced nondestructive testing (NDT) methodologies accompanied with expert interpretation. Findings in this review can be applied to identify and explore new areas of research to analyze failure modes and fractures in NFC by applying NDT or by integrating NDT with other advanced technologies including machine learning.


Introduction
Composite materials are generally applied in buildings, bridges, highways and other structures.Increasing application of composite materials in recent decades is attributable to their strength to weight ratio advantages, versatility and other desired properties (Hannah 2022).Emerging trend shows natural fiber composite, also known as "green composite," is slowly replacing existing commercially available materials.This can be attributed to growing global demand for new and more ecofriendly solutions for future sustainability.Figure 1 shows typical plant fiber types and conditions for green composite from plant fiber.
Extensive studies conducted to characterize and evaluate plant fibers in reinforced composites show promising potentials in view of their lightweight and low-density nature, high strength to weight ratio, good molding flexibility, and cost-effectiveness, among others (Najeeb et al. 2021).To date, a host of studies have shown enhanced material performance in plant fiber-reinforced polymer as well as concrete composites.As shown in Figure 2(a), relevant literatures were sourced from Web of Science Master Journal List using keywords which are composite and plant.Subsequently, the refine tab was applied to screen and many relevant articles related to various topics published over the last 10 years were selected.The outcome of the studies shows a positive trend toward incorporation of plant fiber in composite application as reflected by the number of publications as shown in Figure 2(a).Figure 2(b) shows some research areas utilize plant fiber, namely material science, engineering, chemistry, polymer science, science technology and construction building technology.Further, Figure 2(c) shows the top 5 countries leading the overall plant fiber composite category.This shows countries across the divide are keenly investing in research and development involving sustainable materials.This database indicates the viability of applying Natural Fibre Composite (NFC) as advanced engineering material in building constructions.
However, significant damages associated with NFC have also been reported.This is attributable to the anisotropic and inhomogeneous nature of NFC materials that are not as well established as metals.While in service, composites are exposed to static, fatigue and impact loads that can compromise performance.Exposure to such loads can lead to material failure and inability to serve its function.Destructive tests such as fatigue, bending, impact and compression can be carried out on natural fiber composite to simulate impact related to real-life service challenges (Siwowski and Rajchel 2019).
Nondestructive testing technique (NDT) is among the methods that can be applied to effectively detect and evaluate failure.Common nondestructive tools that can be applied to assess plant fiber composite include X-ray computed tomography (CT), ultrasonic test and visual inspection.Visual inspection is carried out as a first step to examine abnormalities on the surface of a structure or material.Upon visual detection of anomalies, additional inspection tools such as nondestructive testing (NDT) techniques are applied to further elucidate the extent of damage.X-ray CT offers threedimensional view of interior features within solid objects (Withers et al. 2021).For example, X-ray CT technology can be applied to characterize the pore size distribution and topological structure of foamed concrete (Guo et al. 2021).As of ultrasonic test, it involves transmitting high frequency waves through an object or material to ascertain its characteristics and detect flaws (Papa, Lopresto, and Langella 2021).On the other hand, scanning electron microscope (SEM) technique is used to further characterize the damage to give a clearer damage interpretation result from NDT.It can provide details including fiber-matrix adhesion, fiber distribution in polymer matrix, surface roughness/morphology, adhesive failure and surface fracture in microscale.Figure 3 shows examples of NDT application in plant fiber composite damage evaluation.
This paper presents comprehensive review on the latest technology, knowledge and understanding pertaining to the detection and analysis of failure modes and ascertaining characteristics of composite materials.Thus, the study presents two objectives.Firstly, to provide an overall review on current research and literature pertaining to plant fiber composite failures using nondestructive and destructive tests while highlighting the current composite reinforcement material trend.The study also aims to review the future trends of nondestructive technologies in order to provide greater insight into the next area of research to further explore potentials of plant fiber composite.The findings of this review can be extrapolated and applied in other industries utilizing plant fiber-reinforced composites in their materials or structures.

Failure analysis of composite materials
The non-homogeneous properties of natural fiber-reinforced composites contribute to the complexity of the material (Beaumont 2020).This presents a significant challenge in conducting failure analysis.In the field of civil engineering, plant fiber reinforcement is frequently applied in two primary categories of composites, namely polymer matrix composites and concrete matrix.In-depth analysis of failure with the application of NDT methods for the respective composite materials is provided in the following sections.

Polymer matrix composites (PMCs)
Fatigue behavior, impact loading and bending properties of composite structures are reviewed in this section.The section also provides insights into drilling-induced damage fracture analysis.

Fatigue
Goumghar et al., have illustrated fatigue behavior of non-hybrid and hybrid twill flax and glass fabricreinforced epoxy laminates.The finding indicates that substituting two interior glass layers with two twill flax layers results in a decrease in maximum stress loss by approximately 13% and an improvement in damping characteristics by approximately 60% at a frequency of 10 Hz, compared to the glassepoxy layer (Goumghar et al. 2022).Furthermore, visual inspection of fatigue failure reveals a significant degree of stress concentration and localized delamination along the horizontal stitching lines of the kenaf laminates.Similar observation was noted in the hybrid kenaf/glass composite where the appearance of white lines indicate distinct areas of elongation (Barouni et al. 2022).Further analysis of fracture imaging captured using SEM indicates that the interlayer defects in the fatiguetested composite are closely associated with manufacturing defects (Müller et al. 2022).Furthermore, detailed analysis of SEM imaging enables the detection of brittle fiber failure, fatigue striations and fiber imprints at the nanoscale level.Present findings demonstrate that SEM is capable of providing detailed failure mode of plant fiber composite at the microscale level despite seemingly flawless appearance at the macroscale level (Javanshour et al. 2022).

Impact
Impact loading analysis is important in structural application.To date, in most studies, plant fiber composites were subjected to low velocity impact (LVI) range occurring at a speed of less than 10 m/s (Ismail et al. 2019).Although the impact-induced damage may not be readily apparent, it must not be disregarded because it can potentially cause catastrophic failure in composite structures (Razali, Sultan, and Jawaid 2019).The global damage modes in LVI are also distinctly unique, as large deflections often occur.These deflections depend highly on the shear properties (both in-plane and interlaminar) of the material.Hence, it is crucial to characterize the impact resistance and assess damage under LVI conditions as a prerequisite for material selection in structural design (Syed Abdullah 2021).
Furthermore, Javanshour applied X-ray computed tomography (CT) as a tool to capture internal damage patterns, such as delamination lengths and cracks transcending multiple layers of flax-PMMA cross-ply composites that were subjected to impact (Javanshour et al. 2022).The damage is attributed to the occurrence of radial crack, which serves as a connecting link in the progressive delamination between the layers (Demirci and Şahin 2022).Amir et al. reported similar findings where post-impact delamination and fiber breakage were detected on oil palm EFB/kevlar hybrid composite (Amir et al. 2022) using X-Ray CT.Ultrasonic C-scan is a viable alternative to X-ray CT to measure the length of crack, track damage pattern and ascertain the area of damage.According to Mahesh et al., the sisal/ epoxy composite experienced greater crack propagation in the warp direction compared with the weft direction due to impact-induced damage, as observed using ultrasonic C-scan.A linear increase in damaged area was observed from the initial area of damage measuring 607 mm 2 at 20J to up to 50J.The highest degree of damage covered an area of 3733 mm 2 .The severity of the impact on the sisal/ epoxy composite at higher impact energy is reflected in the increase in crack length along both the scan axis and index axis, as well as the progression of damage (Mahesh et al. 2021).Moreover, a study on drop weight impacted hemp/banana composite and E-glass composite shows the former was severely damaged and broken with multiple long open cracks while the latter only displayed compression with closed crack.This observation is consistent with visual examination.Despite the severity of the damage exhibited by hemp/banana composite, it demonstrated a capacity to absorb 550J of impact energy, surpassing E-glass composite which can only absorb up to 410J of impact energy (Ravi et al. 2022).In addition, the failure mechanism of sandwiched composites differed from that of nonsandwiched composites.In a report, Ude, Ariffin and Azhari described occurrence of delamination and matrix cracks as two dominant damage modes rather than fiber fracture in reinforced woven natural silk fiber (RWNSF), RWNSF/Epoxy/Honeycomb and RWNSF/Epoxy/Foam sandwich composites, while fiber breakage, tear, penetrations, and perforations were dominant in RWNSF/Epoxy composite (Ude, Ariffin, and Azhari 2013).In addition, it was found that the size of damage on Palm/ kenaf fiber-reinforced MWCNT phenolic composites using dye penetration and Digital Detector Array (DDA) radiography were different by 3% to 15%, where the DDA radiography method showed greater detail of damage (Loganathan et al. 2022).In a study on composite laminate comprising pineapple leaf and flax, Kumar and Saha noted that the laminate experienced debonding at every layer when subjected to low velocity impact.This is attributed to the brittle fracture in reinforced fibers, as reflected in the SEM analysis (Kumar and Saha 2022).

Bending
Kumar and Saha carried out an experiment subjecting pineapple leaf-flax fiber composite to a compression test.Visual inspection showed composite failure due to delamination and buckling behavior following layerby-layer breakage of the natural fiber material, further contributing to the damage complexity of the composite.The upper half of the composite experienced failure as a result of compressive forces, while the lower half failed due to tensile forces (Kumar and Saha 2022).In addition, an analysis of post-bending image captured via SEM at low magnification reveals linear fracture in the resin, stopping before the fiber.This indicates that the composite's weakest region is the resin area, whereas fiber reinforcement prevents propagation of crack (Laraba et al. 2022).In addition to SEM, X-ray CT is applied to analyze the porosity of polymer composites reinforced with chitosan and flax fibers.It was found that the porous area is more sensitive to tensile properties than bending properties (Rath et al. 2023).As for fiber metal laminates (FML), detailed analysis of images captured using SEM showed Alfa/epoxy (core) metal (skin) composite exhibiting two failure modes: shear cracking caused by a brittle break and core/skin delamination representing adhesive-bond failure.Brittle failure may begin from between the Alfa/epoxy interface or porous area and propagate almost perpendicular to the thickness of the composite (Laraba et al. 2022).In addition, the flexural test revealed damage mode of the core banana fiber of the FML composite that experienced extended buckling compared to the core aramid fiber, thus affirming that plant fiber increases the strain capacity of FML composites (Pai et al. 2022).Table 1 presents the key highlights of the failure analysis of the plant-fibre polymer composite.

Others
Prior to the assembly process, it is generally necessary to perform drilling on composite structures such as beams to facilitate the installation of bolts and to establish slots for joints.Such drilling action can potentially cause significant damage in the composite material.Several studies focussing on investigating into the impact of drilling on woven flax, hemp and jute epoxy composites showed visible signs of delamination.To further understand the delamination damage mode, image processing tools such as X-ray CT and Phased Array Ultrasonic Testing (PAUT) were applied.The results demonstrated that the X-ray CT provides a more accurate assessment of the damage area compared to PAUT.The advantages of X-ray CT include ease of accessibility apart from serving as a tool to accurately characterize drilling-induced damage in terms of the shape and size of delamination.Additionally, this technique does not require the service of a skilled operator and is relatively easy to perform (Maleki et al. 2022).

Ceramic matrix composites (CMCs)
To date, there has been no incorporation of natural plant fibers as fillers in Ceramic Matrix Composites (CMC).However, recent studies focusing on adding ceramic fillers such as SiO 2 and B 4 C to natural fiber polymer composites resulted in notable improvements in tensile, flexural and impact strength.Extensive investigation into the fracture mechanism using SEM images revealed favorable resin transfer across the interlaminar structure of the fibers.SEM analysis of a particular composite containing 15% SiO 2 demonstrated a homogeneous distribution within the polyester matrix, resulting in significant enhancement in polymer-fiber bonding, hence consequently improving the mechanical properties of the composite.Nevertheless, it should be noted that exceeding a SiO 2 concentration of 15 wt% leads to the formation of agglomerates, which in turn weakens the bonding strength in composites, ultimately resulting in the deterioration of its mechanical characteristics (Hariprasad et al. 2022).

Concrete composite
Concrete is widely used in construction applications.Analysis of bending and compressive fracture of concrete composite structure is reviewed in this section.

Bending
In an experiment carried out by Tunje, Onchiri and Thuo, it was found that bending strength is further improved by adding sisal fiber in sugarcane bagasse ash (SCBA) concrete, as SEM analysis affirms the effectiveness of sisal fiber in bridging micro cracks (Tunje, Onchiri, and Thuo 2021).SEM analysis on composite subjected to bending test shows a 10 µm-60 µm wide gap between coconut fiber and the matrix as a result of debonding.Micro cracks were also observed on the cement paste near the gap (Khan and Ali 2018).Visual observation showed different damage modes with and without the addition of banana stem fiber in concrete.Concrete with void space experienced total rupture, whereas the fiber-reinforced concrete displayed partial rupture, suggesting fibers provide resistance to control stress.Analysis of SEM imaging at 100 μm magnification showed only minor debonding and microcracks near the banana stem fiber (Ali et al. 2022).

Compressive
When a concrete specimen is subjected to axial compressive force, the possible failure mode is multiple cracks.Upon being subjected to compression test, photograph observation showed the length of crack is reduced in flax fiber reinforced concrete compared to normal concrete.This is attributed to the bridging effect and high compressibility characteristic of fibers, hence resulting in improved compressibility, fracture energy and ductility of earth concrete (Kouta, Saliba, and Saiyouri 2020).Further, in a fire endurance test, kenaf fiber reinforced concrete (KFRC) specimens were exposed to heat ranging from 100°C to 800°C.Failure mode was recorded at a high temperature of 800°C, where the kenaf fiber begins to disintegrate.The existence of large voids in the specimen failed to improve the composite strength (Aluko et al. 2022).Wang, Mo, Zhang and Chouw investigated the dynamic splitting tensile behavior of plain concrete (PC) confined by flax-fiber-reinforced polymer (FFRP) and glass-fiberreinforced polymer (GFRP) with the impact velocity range of 0.1-6.5 m/s.The result showed that despite the damage, both the FFRP-PC and GFRP-PC remained clumped together, as cracked FRP layers confine the crushed concrete core.The deformation process progressed gradually until specimen failure when the outer FRP package is finally squeezed.Such ability of the crushed components to remain clumped together is beneficial in reducing casualty in the event of structural collapse.The study showed similar damage pattern observed in both the FFRP-PC and GFRP-PC specimens.This is in contrast to the PC specimen that crushes into fragments when damaged (Wang et al. 2022).In another compression study, inverted cone damage mode was observed in concrete confined by jute fiber epoxy, compared to concrete composite that displayed longitudinal cracks and flaking off bulk concrete rubble (Gao et al. 2022).Madhavi concluded that the compressive strength of both the jute fiber composite and polypropylene fiber composite in external concrete strengthening application is comparable (Madhavi et al. 2021).Table 2 shows summary of various plant fibers reinforced in different matrix composition and the types of analysis tools applied in studies.It is observed that in-depth analysis of damage is seriously lacking in most of the studies.

Research gap on plant-fiber composites: meteorological conditions
The findings show that previous studies on plant fiber composites focused primarily on nondestructive tests conducted under static loading conditions.Hence, there is a significant gap in research pertaining to the influence of meteorological conditions on the failure properties of plant fiber-reinforced composites despite their widespread use in various applications.For instance, a variety of conditions including dynamic loading from wind and vibrations, significant changes in temperature and humidity, radiation, effects of cable tension-induced creep, as well as mechanical flaws, can potentially have a negative impact on the performance of composite crossarms in transmission towers.Therefore, it is crucial to investigate how such environmental factors interact with the composite material and the potential impact on structural integrity.
In-depth research is necessary to further understand the effect of environmental conditions on properties and the failure mechanisms of plant fiber-reinforced composites, particularly in applications where dynamic loading and varying environmental conditions are prevalent (Zhu and Schoenoff 2018).These aforementioned factors can ultimately result in structural failure of the transmission tower (Amir et al. 2021).Studies have demonstrated that exposing composites to wet-thermal and chloride salt environments can result in fracture and a reduction of mechanical properties by approximately 10-25% (Tencom Ltd 2022).Furthermore, the absorption of water into the composite has been found to decrease strength, elastic modulus and hardness (Randhawa and Patel 2022).The findings suggest that increasing fiber content in fiber-reinforced polymer composite leads to higher water absorption (Sanjeevi et al. 2021).Furthermore, it has been observed that the tensile and flexural strength of both synthetic and natural fibers experience significant degradation when immersed in aqueous solutions, regardless of the pH value.The rate of degradation was found to be influenced by the solution temperature and specific polymer used in the composite (Yan and Chouw 2015).In certain scenarios, the structural design of composite transfer beams used in high-rise buildings must take into account the impact of ground vibrations (Nie et al. 2019).
Moreover, the quality of material can be affected by surrounding radiation.A study showed that C/SiC composites exposed to radiation levels of 2.8 dpa and 9.8 dpa at a temperature of 350°C resulted in axial shrinkage of carbon fibers (Yang et al. 2023).In another scenario involving a different type of radiation, specifically gamma radiation from a source, the results indicate that sulfur polymer concrete composites (SPC) irradiated with doses up to 1.5 MGy using a Co-60 gamma source exhibited a significant increase of up to 35% in compressive strength (Szajerski et al. 2020).It was also found that regardless of the thickness of plant-fiber hybrid composite, at 50 kGy gamma radiation, the compressive strength increases around 45% compared to non-irradiated composite (Muhammad Amir et al. 2018).In maritime applications, in US warships, for example, a sandwich panel comprising balsa core reinforced with T700 carbon fiber in vinyl ester is incorporated as a crucial component.It is essential to emphasize the effect of seawater on composite aging to ensure long-term sustainability (Composites World 2014).It is worth noting that the impact of such environmental factors on material failure in plant-fiber composites has not been extensively investigated using nondestructive testing methods.

Future adoption -other non-destructive test & machine learning for composite material analysis
A wide range of nondestructive tests can be applied not only to characterize failure modes of materials or structures but also to inspect and detect defects or abnormalities (Su et al. 2020).Among the widely used nondestructive tests include acoustic emission, infrared thermography, terahertz testing, shearography and neutron imaging (Wang et al. 2020).Each technique has its own set of strengths and limitations, and its suitability depends on the condition of the respective materials or structures.For example, a nondestructive test using C-scan ultrasound is applied for composite sizing.The test indicated a sound velocity reference value of 31,700 mm/s, which was used for thickness mapping of flax fiber-reinforced plastic composite material (Wang et al. 2020).Additionally, infrared thermography can be used in real time to identify crack during the tensile testing of plant fiber composite.Loganathan et al., found that peak in the temperaturetime graph spiked as the phenolic composites reinforced with palm fiber began to crack.This indicates that the composite material dissipated its mechanical energy in the form of heat energy on its surface (Loganathan et al. 2022).On the other hand, an advanced NDT method combining more than one NDT method has been developed.For example, Fathi, Nasir and Kazemirad have developed a novel hybrid method known as acoustic shearography that combines ultrasonic excitation with shearography optical imaging.In a carbon fiber composite material defect characterization study, comparison was drawn between X-ray radiography, X-ray CT and acoustic shearography imaging.The results showed that the novel acoustic shearography method is capable of providing sufficiently accurate imaging of defect within 2 seconds, compared to X-ray CT that takes a significantly longer scanning process time (Fathi, Nasir, and Kazemirad 2020).Another novel hybrid tool is the FLIR Lepton and thermoIMAGER TIM200 infrared camera that can be utilized to identify lightning damage on flax bio-composite.However, this technique has its limitations, as composites are also exposed to other sources of radiations including from surrounding objects.It, therefore, requires further imaging processing to improve blurry images and eliminate noise.
The author noted the need to harmonize variable parameters such as heating duration, composite distance to the IR camera and halogen lamp, for better thermal contrast and to reduce noise (Anwar et al. 2021).
The emergence of machine learning has taken nondestructive test analysis to new heights.Machine learning algorithms are now being integrated with nondestructive technology to interpret multiple signals or images, enabling comprehensive analysis, inspection and assessment of material structure integrity.Numerous studies have explored the feasibility of this modeling approach.In one such study, the author compared various machine learning algorithms for anomaly assessment, utilizing different feature analyses on ultrasonic signals recorded by sensor networks.Comparisons were drawn using various machine learning algorithms namely hidden Markov models (HMM), support vector machines (SVM), isolation forest (IF) and reconstruction autoencoders (AEC).The author concluded that a more accurate quantitative analysis on the severity of damage is achievable with the availability of calibration data during operation or by leveraging on the expertise of field professionals (Kraljevski et al. 2021).The author developed an autonomous concrete crack detection method utilizing a convolutional neural network, specifically GoogLeNet, combined with vision and infrared thermography images.This approach has been experimentally validated, and the results demonstrate automatic visualization of both macrocracks and microcracks while effectively minimizing false alarms (Jang, Kim, and An 2019).Furthermore, by harnessing the power of machine learning tools, the author discovered that the application of high-speed decision-making algorithms further enhances the efficiency of nondestructive test evaluation analysis.This advancement enables the processing of a greater number of affected parts on a daily basis, ultimately improving the working conditions of human operators (Niccolai et al. 2021;Shipway et al. 2021).In addition to characterizing damage, NDT is also applied to assess mechanical properties of materials.Integrating the group method of data handling (GMDH) neural network has further enhanced the evaluation of such properties using NDT.GMDH is a self-organized machine learning algorithm renowned for its ability to solve highly complex nonlinear problems, thereby bolstering the effectiveness of NDT-based analysis of material properties (Ebtehaj et al. 2015).The results demonstrated that the tool can be applied to successfully predict the modulus of elasticity (MOE) and modulus of rupture (MOR) of wood material with remarkable accuracy (Ebtehaj et al. 2015).Figure 4 showcases a comprehensive summary of next generation advancements in the field of nondestructive testing and analysis.

Conclusion
Composites are widely recognized as a superior alternative for civil building materials.However, the ever-expanding variants of reinforcement materials have contributed to greater complexity in composites, hence posing greater challenges in analyzing fractures and failures.In addressing this issue, application of relevant nondestructive test methods can greatly assist in detecting and characterizing damage and enhancing understanding of fractures in plant-fiber composite materials.It is critical to stay abreast of the latest advancements and trends in keeping pace with the ongoing green composite revolution.As technology continuously evolve, it is imperative to remain up to date with the latest developments including technological advancement in the NDT methods.This review article is divided into two main sections in meeting two primary objectives.Firstly, it aims to provide a comprehensive review of current research and literature on plant-fiber composite failures, focusing on nondestructive tests and the current trends in composite reinforcement materials and testing.Secondly, the article reviews future trends in nondestructive technologies.Application of existing and advanced nondestructive tools plays an important role in assessing and evaluating complex characteristics of novel composite materials.
Most of the existing NDT studies pertaining to fracture analysis of plant-fiber composites had primarily focussed on static conditions, hence neglecting the assessment of damage caused by volatile environmental factors such as moisture, radiation and temperature.The paper noted scarcity of studies pertaining to plant fiber reinforcement in ceramic matrix composites and carbon matrix composites over the last five years.This can be attributed to the fabrication process and high curing temperatures that are not suitable for plant fibers, as it can consequently reduce composite strength.Modification of the fabrication process and materials of plant fiber is therefore necessary to ensure the success of hybridization process.
Lastly, the combination of nondestructive testing tools and machine learning, which is one of the Artificial Intelligence (AI) branches, has been proven to be advantageous in terms of reducing analysis time and improving overall productivity.However, further research and development is crucial in order to establish a strong foundation before applying machine learning techniques in plant-fiber composite fracture analyses.

Figure 1 .
Figure 1.Typical plant fibers and its condition for composite material applications.

Figure 2 .
Figure 2. Publication output related to plant-fiber composite: (a) the publication output of the plant-fiber composites over the last 10 years (b) some of research areas working with plant-fiber composite and (c) top 5 countries contribute to plant-fiber composite by publication.

Figure 3 .
Figure 3. Non-destructive methods and schematic diagram for composite material set-up (a) Infrared thermography method (b) X-ray computed tomography method (c) Digital detector array radiography method (Najeeb et al. 2021).

Table 1 .
Plant-fiber polymer composite failure analysis summary.

Table 2 .
Research works on natural plant fiber reinforcement in concrete matrix composite.