Compression after impact & vibrational analysis of aramid-basalt/epoxy interply composites under hygrothermal conditions

Abstract With the application of composites in various industries viz. aerospace, automobile, defence and marine, it becomes essential to carry out the Compression After Impact (CAI) and vibration tests to study their behavior in different environmental conditions. This study investigates the significance of CAI and damping characteristics of the aramid-basalt/epoxy interply composites under different hygrothermal conditions. In this examination, laminates were exposed to three different ageing conditions, namely, ambient (ageing in distilled water at 25°C), sub-zero (ageing in distilled water at −10°C), and humid (ageing in an environmental chamber maintained at 40°C and 60% relative humidity) for a duration of 180 days. Moisture saturated specimens were subjected to low velocity impacts (LVI) of 10 J and 15 J energy levels using drop weight impact method. CAI test was carried out on post impact specimens to analyze the residual compressive strengths. Furthermore, impact hammer and impedance tube tests were also conducted to compute the damping properties and sound transmission loss (dB) of the specimens. The results were compared with the pristine samples to analyze the effect of hygrothermal conditions on the CAI and vibrational properties. The results indicated that the moisture has a detrimental effect on the compressive residual strength, natural frequency, and sound transmission loss of the specimens. The Scanning Electron Microscopy (SEM) of fractured CAI specimens displayed the occurrence of various types of damages such as fiber fractures, delamination, matrix fractures, etc as the primary reason for failure of the specimens.


Introduction
With the advancement in research and technology, industries today are transforming toward increased use of composites due to their high resistance to impact, high strength to weight ratio, and longer life cycle as compared to traditional metals or alloys (Mallick, 2007)- (Chawla, 2012).As the industries are moving toward sustainability, the conventional materials are being replaced by sustainable fiber reinforced composites due to their biodegradable and environmentally friendly nature (Fiore et al., 2016)- (Nunna et al., 2012).Basalt is one such material derived from volcanic deposits that are being widely used for reinforcement in composites (Dhand et al., 2015).Basalt material is obtained from the extrusive igneous rock and is continuously extruded from a hot temperature melt of selected basalt stones (Cziga & Szabo, 2003)- (Zubair & Pai, 2019).The main drawback of basalt fiber is their brittle nature and have poor impact strength compared to glass fiber (Fiore et al., 2015)- (Bulut, 2017).Thus, by hybridizing the basalt fibers with more ductile fibers such as aramid, the mechanical properties are enhanced, and the impact load bearing capacity can be improved.As a result, hybrid material is well-suited for applications requiring high impact resistance, such as aerospace, automotive, marine or sports equipment (Behnia et al., 2016)- (Sabu et al., 2012).
Composite structural components in various applications are often subjected to periodical heat and moisture environments.Long-term exposure of these components to such conditions will have significant effects on their performance, durability, and structural integrity (Pai et al., 2021;Prolongo et al., 2012;Dogra et al., 2019).Additionally, composite structures are potentially vulnerable to damage threats from outside factors, particularly low velocity impact phenomena such as bird strikes, hail impacts, falling of tools due to careless handling, and impact of flying debris (Richardson & Wisheart, 1996;Shyr & Pan, 2003;Zubair & Pai, 2019).These impacts have the potential to cause serious microscale damage to composite materials, significantly reducing their strength and stiffness without leaving any obvious physical indicators of damage on the impacted surface.The combined effects of moisture, temperature and low velocity impacts under hygrothermal circumstances can have a substantial impact on the behavior of composite materials.The absorbed moisture can alter the mechanical properties of the impacted object, potentially changing energy absorption and damage propagation (Zanni-Deffarges & Shanahan, 1995)- (Fernandes et al., 2023).
Components subjected to the low velocity impact test may experience internal delamination, which is hazardous because the damage to the impact area is invisible.Therefore, the components that have been damaged by low velocity impact and non-destructive testing will undergo another test named compression after impact (CAI) (Kravchenko et al., 2021;Li et al., 2020).This is done to avoid undetectable damage by visual inspections, which will cause reduction in strength, and to assess the effect of the reinforcement by evaluating the performance of the composites.The compression test evaluates the ability of the material to endure compressive stresses and determines the level of damage that might have any effect on its overall performance by applying compressive loads to the damaged region.Moreover, ageing conditions can significantly affect the damping and sound absorption properties of the composites (Agarwal et al., 2023).Comprehending how vibrational characteristics are affected under different ageing situations helps in assessing the structural integrity and predicting the fatigue life of composites, and hence, potential performance deterioration can be minimized.Therefore, understanding material behavior such as impact characteristics, CAI and vibrational properties under hygrothermal conditions is critical for accurately estimating the effects of low velocity impacts and building resilient structures and systems that can tolerate these environmental influences.
Several researchers have worked in the past on the CAI and damping characterisation of composites subjected to different hygrothermal ageing conditions.Sarasini et al. (2013aSarasini et al. ( , 2013b)).investigated the LVI behavior of basalt-glass/epoxy hybrid composites with various layup sequences.The results revealed that basalt and hybrid laminates had better impact energy absorbing capacity and more damage tolerance compared to glass laminates.Jefferson et al (Andrew & Ramesh, 2015).studied the LVI and CAI characteristics of glass-basalt successively stacked hybrid composites.Results indicated that glass laminate had maximum resistance to damage caused by impact and the hybrid configuration had poorer CAI test results compared to plain glass and basalt composites.Haibao et al (Zhang et al., 2013).investigated the CAI behavior of hybrid unidirectional and woven carbon composites.The CAI test findings revealed that the hybrid composites with carbon woven plies on the laminate's surface showed measurable improvement in residual strengths than unidirectional plies.Hongliang et al (Tuo et al., 2019).investigated the damage and failure mechanism of CFRP laminates under the influence of LVI and CAI tests.Results of CAI indicated that damage always originates at the impact location and spreads instantly within the laminate to the outside edges.The fiber damage area was discovered to be small, and the matrix damage area was adequately large to encompass the fiber damage area.The greatest and most asymmetrical damage area was caused by delamination.Pavan et al (Pavan et al., 2019).experimented the vibrational characteristics of glass/epoxy composites under the influence of sea water medium.Results showed that natural frequency of aged specimens in ambient conditions decreased by 5.7%, while the natural frequency of aged specimens in sub-zero temperatures reduced very minutely i.e., less than 1% as matched to pristine specimens.Ismail et al (Ismail et al., 2019) carried out experiments on kenaf-glass/epoxy composites to understand the LVI and post LVI behavior.Results concluded that hybrid composites are capable of withstanding up to 40 J impact energy against maximum value of load.The energy absorption increased with the rise in impacted energy.CAI test inferred that materials that undergo less damage display higher compressive strength.Yan li et al (Li et al., 2020) compared the residual compressive strength and damage patterns of flax fiber reinforced composites (FFRP) and compared with glass fiber (GFRP) composites.Authors reported that localized cracking was seen in the FFRP composite, whereas the GFRP composite had general delamination.In line with this, the CAI strength of the GFRP composites was greater than FFRP composites.
Previous examinations have shown that for impact applications, interply composites with aramid surface plies have greater ability of controlling the delamination damage (Dorey et al., 1978;Vasudevan et al., 2018).The surface aramid plies absorb most of the impact energy and spreads over larger area from the impacted zone.Thus, aramid layers protect the core basalt layers from the impact damage (Pai et al., 2021(Pai et al., , 2022)).Hence, this study is to understand the CAI and vibrational behavior of interply aramid-basalt/epoxy hybrid composite.Numerous studies on CAI and damping characteristics of composites have been carried out in the literature.However, researchers have predominantly focused on synthetic fibers and with the interest now shifting toward natural fibers in the past decade.Moreover, among all the studies carried out majority of them have been conducted on the pristine specimen without exposing the specimen to different ageing conditions (Alizadeh & Guedes Soares, 2021)- (Jeya, 2019).Only limited studies are available on natural bio-degradable fiber composites which creates an opportunity to research in this area where the composite laminates are aged in different environmental conditions.Therefore, this work will help in understanding the feasibility of the aramid-basalt/epoxy laminates and its applications in various industries where thin-walled composites are widely being used.

Materials and composite fabrication
The parent interply laminates were fabricated with epoxy resin (CT/E 556) and woven bidirectional aramid fabric (480 g/m 2 ), basalt fabric (400 g/m 2 ) supplied by M/S Composite Tomorrow, Gujarat, India.To speed up the curing process, the epoxy resin was combined with hardener (CT/H 951) in a weight ratio of 10:1.By hand layup technique, the epoxy resin was reinforced with three layers of basalt fiber mats sandwiched between surface aramid mats to manufacture panels with dimensions of 350 mm × 350 mm and an average thickness of 2.8 mm as shown in Figure 1.A compression molding machine (loading capacity of 30 kN) was used to consolidate the handlaid laminates for roughly 24 hours at ambient temperature and pressure of 5 MPa.A thick spacer plate of 2.8 mm was placed between the molds to maintain a consistent thickness throughout the panel.In the aramid-basalt/epoxy laminates, the nominal volume fractions of fiber, matrix, and porosity were 58.9, 42.5, and 1.58%, respectively, and reported in (Pai et al., 2021(Pai et al., , 2022)).The constructed laminates were cut into ASTM D3763 (ASTM D3763, 2014) standard panels for the low velocity impact test.After the impact, specimens were cut using a water jet machining method as per the ASTM D7137 (ASTM D7137/D7137M-17, 2017) with the dimensions of 150 mm × 100 mm for the compression after impact test.

Ageing condition and LVI testing
Moisture absorption test was performed to observe the moisture uptake characteristics as per the standard ASTM D5229 (ASTM D 5229, 2020) for a period of 180 days.The following ageing conditions were chosen to study the influence of moisture on the CAI and vibrational characteristics of the composites.The detailed procedure of specimen preparation and measurement of moisture diffusion at different time periods and the calculation of diffusion coefficients are reported in (Pai et al., 2022).
The moisture saturated specimens were subjected to LVI tests by employing a drop-weight impact testing machine as per ASTM D3763 (ASTM D3763, 2014) with a dimension of 150 mm × 150 mm and a corner radius of 2 mm.The aged specimens were tested for two different energy levels of 10 J and 15 J.

Compression after impact testing
Post LVI test, specimens were cut as per ASTM D7137 (ASTM D7137/D7137M-17, 2017) for the CAI testing using the Omax built Proto Max Abrasive water jet machine as shown in Figure 2(a).As per the ASTM standard, dimensions of the specimens were 150 mm x100 mm x 2.8 mm (Figure 2(b)).
The CAI test were carried out using Indian make FIE universal testing machine with 50 kN loading capacity at a rate of 1.25 mm/min in the ambient conditions as depicted in Figure 3.The CAI specimens were compressed till the point of failure which is associated with a sudden drop in  the load as displayed by the force versus deformation plot.The graph obtained after the tests were studied thoroughly to draw conclusions and compare the compressive strength across specimens.
The CAI test were conducted with a standard setup on the pristine and aged specimens, and the force versus displacement graphs were obtained for the complete duration of loading.The residual compressive strength σ c from the peak compressive load F max of the specimen after CAI test is calculated using equation 1.
Where b is the width (100 mm) and d is the thickness of the specimen (2.8 mm).

Impact hammer test
In the impact hammer test, three specimens of each ageing conditions were tested to determine the damping properties and natural frequency of the specimens.The specimens were measured to be 250 mm × 25 mm as per ASTM E756-05 (E756-05, 2005).The specimen was set like a cantilever beam with one end fixed and other end free, a free boundary condition was created, and the fixed end was impacted by the impact hammer as depicted in Figure 4. Using an accelerometer, the displacement of the fixed specimen was detected, and with the software LabVIEW and the NI 9234 data acquisition interface, the data acquisition was carried out.The fixed end when impacted, generated numerous peaks on the acceleration amplitude versus frequency graph of which the first peak denoted the first natural frequency of the specimen while the second peak denoted the second natural frequency.The impact hammer test helped in identifying the natural frequency of the composite laminate for its suitable use in the safe frequency range to avoid superimposition of the amplitude at resonant frequency.
The damping ratio which is an essential property that helps in keeping a limitation on the vibrations of the specimen was obtained using the Half power bandwidth method.In this method, the X max value, which is the peak impact magnitude divided by √2, and across Xmax p 2 , a parallel line drawn to the x-axis intersects the frequency versus impact magnitude graph at two points and the corresponding values on the x-axis at these two points gives us ω 1 and ω 2 .The peak natural frequency is ω n on the x-axis as depicted in Figure 5.Using equation 2, the damping ratio ξ is calculated.The stiffness coefficient K is directly dependent on mass m of the specimen and the natural frequency ω n and can be calculated using equation 3.
The mean of the test results were taken to evaluate the vibrational characteristics of the specimens under the influence three different ageing conditions.

Impedance tube test
The impedance tube test method (BSWA tech) was used to determine the transmission loss of the pristine and aged aramid-basalt/epoxy specimens as shown in Figure 6.The specimens were of two diameters of 99.5 mm and 29.5 mm, respectively, as per ISO 10,534-2:1998(ISO-10534, 1998) as shown in Figure 7.The testing frequency was in the range of 63 Hz to 6300 Hz.Four microphones of different sensitivities were placed in the large tube and small tube, and the values were measured for 10 minutes; with the help of transfer function method, the transmission loss was measured.At the end, the results of large tube, large tube with wide spacing and small tube were combined to obtain a graph of transmission loss versus frequency.The impedance tube helps in understanding the acoustic characteristics of the specimen using a sound source.A noise is generated from the loudspeaker and the composite specimen acts as a barrier.During this process, certain percentage of sound is absorbed by the specimen while the remaining is transmitted further.The four microphones detect this sound transmission over varied frequency and the transmission loss coefficient is obtained.

Moisture absorption behavior
The moisture absorbed by the specimens in different ageing conditions were measured at regular intervals, and the graph has been plotted and compared with the theoretical values using Fick's model and reported in (Pai et al., 2022).The specimens at ambient condition absorbed maximum amount of moisture, i.e., 5.44% at saturation, specimens aged in sub-zero condition absorbed 3.12% of moisture and the least moisture absorption was recorded for the humid specimens i.e., 1.80% at equilibrium.

CAI test
The impacted specimens of 10 J and 15 J energy levels were tested for their residual compressive strength using a CAI test setup.The specimens were fixed in this setup in such a manner that the application of load is perpendicular to the 100 mm × 2.8 mm face of the specimen, and the load axis is in line with the centre of the laminate, which helped in preventing early buckling of the laminate.CAI test results are as shown in Table 1.
After the CAI tests were conducted, the compressive load vs displacement graph was obtained for both 10 J and 15 J impacted specimens as depicted in Figure 8.
It was observed that the 10 J pristine specimen had a peak load value of 10.54 kN at a displacement of 2.37 mm while in the aged specimens the peak loads and displacement values decreased with increase in moisture absorption.The degradation of compressive strength was 10.62%, 22.5%, 36.15% for 10 J and 12.47%, 19.60%, 31.87% for 15 J sub-zero, humid, and ambient aged specimens, respectively.The ambient aged specimens displayed least resistance to compressive loading followed by humid and sub-zero specimens.A steeper slope was observed in the load versus displacement plot for all 10 J specimens, which indicate the higher stiffness of the specimens.However, for 15 J specimens, slope decreased due to the higher amount of damage occurred during the LVI event, which reduced the stiffness and resulted in breaking of fibers, increased delamination, causing the overall strength of the specimens to decrease.Likewise, pristine specimens showed higher amount of stiffness and load bearing capacity in both 10 J and 15 J category compared to aged specimens.The pristine specimens have more intact and homogeneous microstructure due to which specimens have fewer or no microstructural defects and presence of strong interfacial bonding between the fiber-matrix result in increased stiffness and maximum load bearing capacity.However, the aged laminates experienced higher amount of damage during the LVI process and, primarily the moisture absorption resulted in swelling of the matrix material by undergoing the dimensional changes which degrades the matrix strength, ultimately resulting in early failure of the specimens and reduced compressive strength.The ageing under different condition leads to swelling, softening of epoxy resin and absorbed water causes fiber-matrix interfacial degradation resulting in localised stress concentrations and reduced load transfer efficiency between the matrix and fiber of the laminate (Padmaraj et al., 2021;Zhong & Joshi, 2015).The ambient aged 10 J and 15 J specimen displayed maximum value of displacement compared to other aged specimens which can be attributed to improved ductility of the fibers and moreover, absorbed moisture act as plasticiser, weakening the intermolecular bonding due to higher amount of moisture absorption (Ahmad et al., 2016;Soles & Yee, 2000).The compressive residual strength for the sub-zero aged specimen was found to be higher when compared to humid and ambient aged specimen in both 10 J and 15 J specimens.The 10 J subzero aged specimens showed higher residual strength than 15 J sub-zero aged specimens.This is because higher impact energy typically causes more material damage, resulting in decreased mechanical characteristics and, as a result, a lower CAI value.The higher impact energy also  generates more internal stresses, delamination and other forms of damage that compromise the structural integrity of the material (Pai et al., 2023).The absorbed moisture at sub-zero condition freezes and undergoes volumetric expansion.This frozen moisture act as a binder and enhances the interfacial strength between the fiber and the matrix.This result in higher compressive load bearing capacity of the laminate.Hence, sub-zero specimen showed higher compressive strength compared to other ageing conditions.
The fractured CAI specimens of pristine and aged specimens are shown in Figure 9.The specimens primarily failed due to delamination buckling and the cracks near the LVI impacted site.
The cracks generated on the laminate have caused matrix failure and the fracture started at the indented site at the centre and extended to the laminate's edges.A white region is visible on the laminate where butterfly type cracking has occurred which denotes delamination.In pristine and ambient aged specimens, the cracks that occurred were passing through the LVI impacted site, but in case of humid and sub-zero aged specimens, it was seen that the cracking occurred few centimetres above the LVI impact site primarily due to early buckling which led to failing of the specimen and propagation of cracks across the laminates.
Figure 10 shows the cross-sectional view of the fractured pristine and ambient aged CAI specimen taken for the SEM analysis.Pristine specimens have strong adhesion between fiber-matrix interface which result in effective load transfer from matrix to fiber.As a result, more fiber dominated damages such as fiber fractures, micro buckling of fibers was visualised in pristine specimens (Figure 10.a).However, moisture absorption in aged specimens results in swelling of the matrix which introduces the internal stresses within the composite and premature degradation of the polymer matrix occurs.Hence, higher amount of matrix dominated failures such as delamination, matrix stress fracture was visualised in aged specimens as witnessed in Figure 10(b).Furthermore, aged specimens have undergone interlaminar stress fracture due to reduced stress transfer between the layers of the laminate resulting in early failure of the specimens.Figure 11 shows the residual strength retention of aged 10 J and 15 J specimens compared to pristine specimens.The sub-zero aged specimens showed higher amount of residual strength retention, which is 89.38 % and 87.53% for 10 J and 15 J, respectively.The least amount of residual strength was observed in ambient aged specimens, which were 63.85 % and 68.13 % for 10 J and 15 J, respectively.The moderate amount of residual strength was witnessed in humid aged specimens, which were 77.50 % and 80.40 % for 10 J and 15 J, respectively.During the ageing process, the composites undergo chemical reaction such as polymer matrix hydrolysis where absorbed water molecules breaks the down the chemical bond of the polymer system, resulting in loss of structural integrity and reduced strength retention of the laminates (Faguaga et al., 2012).

Scanning electron microscopy (SEM)
The fractured CAI specimens were analyzed using the SEM to identify the type of damage occurred and effect of moisture on the properties of the laminate.The SEM specimens shown in Figure 12 indicate the fracture modes such as delamination, fiber breaking, matrix failure, splitting of laminate, and cracks development.The SEM images imply that the aramid-basalt/epoxy laminate have undergone significant damage due to the different ageing conditions.The moisture absorption has led to poor adhesion between the matrix and fibers, and matrix cracking has primarily caused failure of aged aramid-basalt/epoxy laminate.The pristine specimens exhibited delamination, fiber cracking and multiple micro cracks, resulting in failure of the laminate.In ambient aged specimens, due to higher moisture absorption, increased development of cracks were witnessed in the matrix phase, which resulted in degradation of mechanical properties and further reduction in  compressive residual strength of the specimen, which can be validated from the CAI results.However, the absorbed moisture in sub-zero aged specimens were in frozen state, and the frozen moisture enhanced the load transfer mechanism resulting in fiber dominated failure.Delamination, matrix degradation and fiber fractures were the primary damage modes observed in sub-zero specimens.

Impact hammer test
Understanding the effect of moisture absorption on damping properties and natural frequencies is critical for precisely forecasting the dynamic performance, durability, and reliability of the composite structures in real-world applications.Damping assists in the dissipation of mechanical energy, lowering the danger of fatigue failure, enhancing structural integrity, and reducing the possibility of damage.Moisture can have a substantial impact on damping behavior, and understanding how it affects the material enables better composite material design and selection for certain applications.
The vibrational characteristics of the composite aged under three different conditions is shown in the Table 2.The test examined three specimens from each pristine and aged conditions, and average results were calculated to investigate the effect of moisture on vibrational characteristics.
From Table 2, it is deduced that the natural frequency decreases with increased water absorption and the order of natural frequency is pristine > sub-zero > humid > ambient aged specimen.The pristine specimen had an average first natural frequency of 22.21 Hz while the sub-zero aged specimen had an average first natural frequency of 19.45 Hz.The natural frequency for humid and ambient specimens were 17.76 Hz and 16.11 Hz respectively.
The acceleration amplitude vs natural frequency plots have been depicted in Figure 13.The first natural frequency is denoted by the smaller peak and the second natural frequency is denoted by bigger peak in the below graphs.
The percentage natural frequency retention was plotted for various ageing conditions as shown in Figure 14.The natural frequency retention was highest in case of sub-zero specimens, which was 87.58%, followed by humid and ambient specimens, which were 79.70% and 72.54%, respectively, compared to pristine specimens.The Meirovich's beam model helps in understanding the relation between storage modulus E and natural frequency.The natural frequency (f n ) is denoted by equation 4 (Pavan et al., 2019).
where, L, E, ρ, I, and A represent the beam length, storage modulus, material density, moment of inertia, and cross-sectional area of the specimen, respectively.In the frequency equation, the natural frequency is directly proportional to storage modulus of the material and with decrease in  storage modulus, the natural frequency decreases and the damping ratio increases.Moreover, natural frequency of the system is inversely proportional to the mass of the composite specimen.
During the diffusion process, mass of the composite materials rises due to moisture absorption.Moisture molecules increase the composite's total mass, which has an impact on the system's inertia characteristics.Since more force is needed to produce the same degree of vibration with the same mass, an increase in mass often results in a drop in the natural frequency and the same can be validated by multiple studies (Zai et al., 2009)- (Bulut & Kanmaz, 2019).
Particularly in polymer matrix composites, moisture has the potential to change the internal structure and characteristics of composite materials, including mass distribution and stiffness, which in turn alters the natural frequency.The reduction in the stiffness of the composite material due to moisture absorption can be explained with the help of various mechanisms.The polymer matrices are hydrophilic in nature, this increases the moisture diffusion and causes the polymer resin to plasticize and lose rigidity.The total stiffness of the composite system is impacted by this loss in stiffness, which lowers the natural frequency.Moreover, laminates experience dimensional changes due to moisture absorption, such as swelling or expansion.Internal stresses are developed within the composite because of these dimensional changes, and these internal stresses have an impact on the material's mechanical characteristics, including stiffness.This decline in the composite's effective stiffness causes the natural frequency of the composite to drop (Chandra et al., 1999).
Natural fibers like basalt are known for higher moisture absorption and with hydrophilic nature of epoxy resin, the absorbed moisture results in chemical and mechanical degradation in the matrix of polymer.From Table 2, it is also witnessed that the damping ratio is maximum for the ambient and humid aged specimens due to increased moisture absorption which resulted in swelling and matrix plasticisation (Poveda et al., 2013).Sub-zero aged specimens displayed lowest value of damping ratio among the aged specimens due to the diffused moisture was in frozen condition which improved the fiber-matrix interfacial bonding and hence stiffness increased.Basalt fibers when compared to aramid fibers have lower damping properties and the hybridisation of the two fibers results in enhanced damping properties (Bulut et al., 2016).The absorption of moisture in aramid-basalt/epoxy laminate has resulted in decrease in storage modulus and increase in the damping ratio.

Impedance tube test
A material's or structure's capacity to attenuate or obstruct the transmission of sound waves from one side to the other is referred to as sound transmission loss.It is frequently expressed in decibels (dB) and reflects the decrease in sound intensity caused by the material.Materials having the largest sound transmission loss are typically regarded as being more effective for acoustic applications where sound insulation or reduction is desired.Figure 15 shows the transmission loss vs frequency plot as obtained from the impedance tube test.
The pristine specimens showed steady increment in transmission loss values for lower frequencies with steep increment between the frequency ranging from 400 Hz to 630 Hz with transmission loss values ranging from 11.75 dB to 26.80 dB.A steep decrement was observed for higher frequency until 800 Hz, after which the pristine specimen showed increase in the transmission loss with an optimum value of 36.01 at 6300 Hz frequency.The sub-zero aged specimen showed initial decrement in transmission loss in the frequency range of 80 Hz to 400 Hz, and at higher frequency, the transmission loss curve showed a steady increment with the optimum value of 35.34 dB at 6300 Hz.The ambient and humid aged specimens also showed negative slope in the lower frequency with humid aged specimen showing poor transmission loss values until frequency of 4000 Hz.The optimum values for ambient and humid aged specimens were found to be 30.14dB and 34.12 dB, respectively, at 6300 Hz as shown in Table 3.
Basalt fiber is a natural fiber which is viscoelastic in nature having high porosity.When sound waves pass through theses pores, attenuation of sound waves occurs.Hence, basalt fibers can be widely used in aircrafts and big ships as insulation material with great sound absorption characteristics.The hybridisation of basalt with aramid does help in reduction to moisture exposure of the basalt fiber and is desirable when compared to pure basalt fiber.However, moisture absorbed by the specimen result in filling up of the voids and pores present in the specimens which causes the reduction in porosity of the specimen.The sound waves transmitted from the source passes through these pores and with presence of moisture the sound absorption efficiency is decreased.In ambient and humid aged specimen due to higher levels of moisture absorption, they exhibit poor sound absorption characteristics; also, there is an increase in density of the specimen with moisture absorption in aged specimens when compared to pristine specimen, which affects acoustic performance.In case of sub-zero specimen aged at −10°C, the moisture present in the voids is crystalized, resulting in filling of the voids in the laminate and affecting the sound absorption.Thermal contraction in composite materials happens at below-freezing temperatures, increasing the density of the material.Higher density significantly improves the sound transmission loss by enhancing the mass and stiffness, thus reducing the sound absorption.
The highest transmission loss was seen in pristine specimens, and hence, it has the best sound absorption characteristics while ambient specimens with maximum moisture absorption had the least transmission loss.Therefore, the hygrothermal ageing of the aramid-basalt/epoxy laminate results in the decrease of sound absorption capabilities due to moisture absorption.

Conclusion
The present work is focused on the influence of various ageing conditions on the CAI and vibrational behavior of aramid-basalt/epoxy composite through experimental investigation.The following results may be drawn based on the results obtained: • Pristine specimens had the maximum compressive residual strength, and the residual strength decreases with increase in moisture absorption by the specimens.Similar trend was seen across both the energy levels of 10 J and 15 J.The residual strength was in the order Pristine > Sub-zero > Humid > Ambient.
• The SEM images indicated that failure of the laminate is due to continuous compressive loading, resulting in micro buckling, delamination, and fiber fracture in the pristine specimen.In case of aged specimens, matrix degradation, weak interfacial bonding between matrix and fiber, multiple matrix cracking, and enhanced fiber failure were observed.
• The impact hammer test results indicated the trend of decrease in natural frequency with increase in moisture absorption.The moisture diffusion in laminates resulted in plasticization and swelling of the matrix and led to reduction in natural frequency.The order of first natural frequency was Pristine > Sub-zero > Humid > Ambient.
• The damping ratio was found to be the highest for ambient aged specimen while lowest for the pristine specimens.It was also found that moisture absorption affects the stiffness coefficient of the specimen but improves damping properties.
• The transmission loss graph implies that the pristine specimen has the best sound absorption characteristics, and with increase in moisture absorption, the density of the specimens is increased and the porosity is degraded, which resulted in poor sound absorption capabilities.
• The outcomes of the research work confirmed that moisture had detrimental impacts on the composites' mechanical characteristics.However, before utilizing the full potential of hybrid laminates for structural applications, it is required to validate additional design elements on a case-bycase basis.

Figure 3 .
Figure 3. (a) CAI test set up (b) enlarged view of the specimen.

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Figure 5. Half power bandwidth method.

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Figure 7. Impedance tube test specimen of diameter (a) 99.5 mm and (b) 29.5 mm.

Figure 8 .
Figure 8. Compressive load vs displacement curves from CAI test (a) 10 J and (b) 15 J energy level.

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Figure 10.Cross section of fractured (a) pristine (b) subzero (c) humid and (d) ambient aged specimen considered for SEM analysis.

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Figure 11.Residual strength retention of aged specimens.

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Figure 12.SEM images of fractured (a) pristine, (b) sub-zero aged (c) humid aged and (d) ambient aged CAI specimens.

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Figure 13.Acceleration amplitude vs frequency plots of (a) pristine (b)Sub-zero (c) humid and (d) ambient aged specimens.

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Figure 15.Transmission loss vs frequency graph of pristine and aged specimens.