Assessment of ageing effect on the mechanical and damping characteristics of thin quasi-isotropic hybrid carbon-Kevlar/epoxy intraply composites

Abstract

Hybrid polymer laminates employed in advanced engineering fields are susceptible to atmospheric conditions, such as moisture and temperature throughout their service life, which may impact their mechanical and damping properties. This research focuses on the influence of three distinct environmental situations: ambient ageing at 25°C, sub-zero ageing at −10°C, and environmental chamber humid ageing at 40°C with 60% RH (relative humidity) on the mechanical and damping characteristics of quasi-isotropic carbon-Kevlar fibre reinforced intraply hybrid composites. Compression molding process was incorporated for the fabrication of the laminates and specimens were aged until the moisture saturation point was reached. The mechanical and vibrational properties of post-aged specimens, including tensile, three-point flexure, short-beam shear strength (SBS), and hammer impact tests, were investigated and compared with pristine specimens. The results showed that the experimental degree of moisture absorption closely followed the Fick’s first law, with a greater moisture diffusion rate in the early phase of ageing and moisture saturation in the laminates were ranging from 1.162% to 4.167%. The study also unveiled that diffusion of moisture has a negative impact on the mechanical and vibrational behaviour of the composites. Mechanical strength deterioration was discovered to be highest in the ambient aged specimens followed by humid and sub-zero aged specimens. To evaluate the damage morphology, post-tensile test coupons were analysed by employing a Scanning Electron Microscope (SEM). The predominant failure mechanisms found in aged composites were fibre fractures, interfacial degradation due to matrix deterioration, matrix cracks, and delamination.

Keywords:

• hybrid composites
• quasi-isotropic
• mechanical characterization
• hygrothermal ageing
• damage morphology

PUBLIC INTEREST STATEMENT

The quasi-isotropic laminates are commonly used in components subjected to complex loading conditions including aircraft wings, helicopter rotor blades due to their balanced properties and improved performance characteristics. In these applications, composites are more often subjected to different environmental conditions, such as moisture and temperature throughout their service life, which may impact their mechanical and damping properties. Therefore, this research focuses on the influence of three distinct environmental situations namely, ambient ageing at 25°C, sub-zero ageing at -10°C, and environmental chamber humid ageing at 40°C with 60% RH (relative humidity) on the mechanical and damping characteristics of quasi-isotropic intraply carbon-Kevlar/epoxy composites. The outcome of this research work may be useful in designing and predicting the durability of the aerospace, automotive, and marine structures which are frequently exposed to various ageing environments.

1. Introduction

In recent years, the desire for high-performance, lightweight materials has led to the use of fibre reinforced composite materials in place of traditional metal components (Agarwal & Broutman, 1990; Chawla, 2012; Mallick, 2007). The qualities of reinforcements are critical in determining the performance of the composites. Composite materials can attain the necessary dimensional stability and mechanical strength while being molded into complicated geometrical components by regulating the orientation and volume percentage of fibres (Demircan et al., 2015; Harris et al., 2002; Kaustav Ghosh, 2018; Sarasini & Santulli, 2013). Another method for achieving the intended or better performance is to combine several types of reinforcements. In comparison to typical composites (single reinforcement type), hybrid composites offer unique features and may suit a wide range of design requirements at a lower cost (Kretsis, 1987; Singh & Samanta, 2015; Yahaya et al., 2014). According to the literature, there are numerous scenarios in which a high modulus material is required, such as carbon fibre reinforcements, however it is generally linked with catastrophic brittle failure, which is undesirable. The combination of carbon fibres and other forms of fibre with adequate ductility is the answer to such a problem. The predominant material used in military ballistic protection is carbon-Kevlar hybrid composite, which has great impact resistance (Bilisik, 2021; Gustin et al., 2005; Jia et al., 2019; Taraghi et al., 2014; Vasudevan et al., 2020).

Several investigations have demonstrated that the intra-ply composite design produces excellent results compared to inter-ply hybridization (Pegoretti et al., 2004; X. Wang et al., 2008; Zubair & Pai, 2019). In the experimental study on inter/intra ply reinforced with glass and carbon hybrid composite designs carried out by Jacob and Vijaya (O’Donnell & Chalivendra, 2021) discovered that the intra-ply configuration with loading axis in the glass fibre direction exhibited the greatest ultimate strength and flexural properties. Jeyaraj and Marugan (Rajesh & Pitchaimani, 2017) investigated the mechanical characteristics of natural fibre conventional and braided yarn woven composites, discovering that intra-ply combination of conventional yarn jute and banana fibres outperformed conventional jute composites. Bandaru et al (Bandaru et al., 2020) investigated hybrid Kevlar/basalt both inter-ply and intra-ply composites for compression tests at higher strain rates and concluded that hybrid laminates, with intra-ply alternating Kevlar and basalt fibres possessed the highest maximum stress, stiffness, and ability to withstand higher strain rates than hybrid inter-ply composites. Ebrahimnezhad-Khaljiri et al (Ebrahimnezhad Khaljiri et al., 2020). investigated the effect of interlayer hybridization of oxidized polyacrylonitrile fibres (OPF) with glass, carbon, and Kevlar layers on the tensile and flexural behavior of epoxy composites. The results of the tensile and flexural test’s energy absorption showed that hybrid laminates with two plies of OPF and two plies of glass, carbon, and Kevlar layers had the maximum hybridization parameter. Arpatappeh et al (Arpatappeh et al., 2020). studied the Charpy impact characteristics of hybrid laminates reinforced with basalt, Kevlar and fibre metal laminates (FML). Authors revealed that specific energy absorption of aluminium FML was 2.5 times higher than steel FML and composites. Khazaie et al (Khazaie et al., 2018). investigated the repeated low-velocity impact of composite reinforced with basalt and Kevlar fibres. The authors revealed that basalt fibres reinforced composites exhibit higher resistance to damage in the repeated impact, in comparison to Kevlar-basalt reinforced hybrid composites. Abu et al (Shaid et al., 2020). discovered that the specimen with complete carbon layers in the center had a greater tensile strength due to stronger carbon–carbon interaction, whereas the specimen with carbon layers on the exterior had higher flexural and impact properties. Furthermore, symmetrical stacking configurations have higher flexural stiffness than any other asymmetric configurations composed of the same layers (Mechanics of Laminates, 2007).

The carbon-Kevlar/epoxy intraply composites are widely used in aviation, automobiles and wide range of structural applications. In these applications, composites are often subjected to different environmental conditions, which results in severe degradation of the mechanical properties (Behnia et al., 2016; Pai, Kini, et al., 2021; Pai, Pai, et al., 2021; Vasudevan et al., 2020; Ye et al., 2016). The most common damages observed in FRPs due to ageing degradation are micro matrix cracking, debonding of fibre-matrix interface and delamination resulting in ultimate failure of composite structures. Diffused moisture in FRPs is widely recognised to have impacts on the polymer matrix, reinforcements, and interfaces between the laminae. Since the matrix resin is organic compound in nature, prolonged exposure to such situations may cause physical as well as chemical changes in composites, resulting in decline in mechanical properties (Imieli & Guillaumat, 2004; Pai, Pai, et al., 2021; Vieille et al., 2012; Zanni-Deffarges & Shanahan, 1995). Plasticization effect and polymer matrix swelling are the two most common physical deterioration of polymer resins, and they are capable of causing the polymer to become more ductile, lowering its glass transition temperature. Moisture diffusion may lead to hydrolysis of the polymeric matrix, resulting in degradation of the interface between fibre and matrix. As a result, several research have been carried out to evaluate and comprehend the impacts of ageing environments on the mechanical features of laminates (Ahmad et al., 2021; Aoki et al., 2008; Costa et al., 2005; Pérez-Pacheco et al., 2013; Zhong et al., 2019). Interestingly, most of these studies are primarily centered on a single type of composite structure and loading situation. Moreover, only a handful of studies have investigated the effect of ageing conditions on carbon-Kevlar/epoxy intraply composites on testing for mechanical and damping properties to provide a comprehensive assessment for their long-term reliability (Effect, 2018; Pai et al., 2022; M. Wang et al., 2016).

The composite components in aerospace and automotive sectors require strength in all directions, due to the loads and stresses acting randomly or in multiple directions, indicating that a material with isotropic properties is preferable (Ahmad et al., 2021). However, due to the directional characteristics of the embedded fibres, true isotropic behavior cannot be attained in composites. Therefore, the study of quasi-isotropic laminates is necessary to imitate the characteristics of an isotropic material (Murat et al., 2009; Padmaraj et al., 2021). The quasi-isotropic intraply carbon-Kevlar/epoxy composite is investigated in this work in a configuration appropriate to applications needing mechanical characteristics as that of thin and isotropic materials. Based on the preliminary studies and survey of the literature, only few research studies focused on the intra-ply fabrics with quasi-isotropic laminate designs of carbon-Kevlar composites subjected to various hygrothermal ageing conditions, despite the fact that these composites offer significant potential for use in a wide range of industry sectors. Thus, the intention of this research study is to analyse the vibrational and mechanical characteristics of carbon-Kevlar intraply fabric embedded quasi-isotropic design composite under the influence of various aging circumstances by experimental examination. Static mechanical tests, and impact hammer vibration tests have been carried out in this study. Above mechanical tests offer material characteristics which are significant criteria to consider when selecting materials for different applications. In addition, void estimation test is performed to determine the laminates manufacturing quality, and all testing was performed in accordance with ASTM standards.

2. Experimental procedures

2.1. Materials selection

The material chosen for the investigation is a carbon-Kevlar intraply fabric purchased from Easy Composites Ltd, United Kingdom. The fabric is made of high strength TR30S 3K carbon and genuine Kevlar® yarn and sewed in a simple weave pattern. The fabric features an uneven weave with two carbon tows for every one Kevlar tow in the warp direction and conversely for the weft direction. Figure 1 depicts the weaving pattern of the fabric employed for the experiment.

Figure 1. (a) Intraply fabric and (b) Representation of intraply weaving design.

Araldite® LY 5052 and Aradur® 5052 CH epoxy resin-hardener combinations were purchased from Huntsman Advanced Materials in India. The epoxy resin used was of relatively moderate viscosity epoxy resin utilized in aerospace and industrial composites, as well as aircraft maintenance. The material characteristics are summarised in Tables 1 and 2.

Table 1. Details of the fabric used

Type of fibre	Fabric type	Style	Weight	Thickness	Yarn Count	Balance	Manufacturer
Carbon Kevlar Hybrid	Woven intraply Fabric	Plain weave	188 GSM	0.3 mm	3K	Unbalanced	Dupont

Table 2. Details of the matrix used

Epoxy Resin	Viscosity at 25° C	Density at 25° C	Hardener	Viscosity at 25° C	Density at 25° C
Araldite® LY 5052	1000–1500 mPa s	1.17 g/cm^3	Aradur® 5052 CH	40–60 mPas	0.94 g/cm^3

2.2. Laminate fabrication

A 300 mm × 300 mm composite laminate was manufactured by cutting the cloth as per the orientation [90º/+45º/-45º/0º]_s. The imbalance of the fabric was corrected by adding a 90º laminae for every 0º ply and a −45º laminae for every + 45º ply. Here, 90º represent the orientation angle of the fabric in the direction of loading (two Kevlar tows for every one carbon tow) and all other angles + 45º, −45º, and 0º are measured in the clockwise direction with respect to loading direction 90º. A schematic illustration of the stacking procedure and fibre orientation is shown in Figure 2.

Figure 2. Fibre orientation and stacking arrangement of the fabricated laminate (1 and 8 represent the top and bottom layers).

The traditional hand layup method was implemented for the fabrication on an open mold mild steel plate with dimensions 360 mm × 360 mm, followed by compression molding, as indicated in Figure 3(a,b). To begin, the mild steel plate surface is washed, scraped, and cleaned with turpentine to remove any remaining residue. This is necessary to avoid staining the fabricated laminate during curing. After cleaning the plates, a releasing agent is applied to both the top and bottom plates prior to the placement of the fibre yarns. The fibre layers are then cut into 350 mm × 350 mm dimensions with specific orientations. The binding matrix was created by combining the epoxy resin with the hardener in the manufacturer’s recommended ratio of 100:38 and a fibre to resin ratio of 60:40. The yarn layers are subsequently incorporated with the epoxy matrix and laid up in the previously specified quasi-isotropic sequence. Finally, the top mild steel plate is put onto the layup, and the entire setup is pressed under the pressing machine until a laminate thickness of 2.8 mm is achieved. Maintaining 2.8 mm thickness spacers on all four sides of the plate assured the laminate’s thickness. Curing was done in ambient conditions for around 24 h. The laminate was removed from the compression molding machine after curing and cut using a water jet cutting machine in accordance with ASTM standards.

Figure 3. (a) Compression molding machine (b) Cured laminate.

2.3. Void content

The weight density of carbon-Kevlar hybrid laminates has been determined using ASTM D792 (D792 − 20, 2013). As per the standard, 10 mm x 10 mm specimens were created. To establish the experimentally measured density of the laminates, the principle of Archimedes was implemented. The mass of every specimen was determined using an electronic scale for weighing which had a lowest count of 0.001 g. The total quantity of water expelled was employed to compute the volume of the samples. The ratio of mass to the total volume of the sample is then used to measure experimental density. A total of five samples have been selected for testing from various regions of the laminate, and their average density was computed. Equation (1)(1) ${\rho }_{\mathit{th}}=\frac{1}{\frac{w_f}{{\rho }_f}+\frac{w_m}{{\rho }_m}}$(1) has been employed to obtain the theoretical densities of the samples (Agarwal & Broutman, 1990).

(1) ${\rho }_{\mathit{th}}=\frac{1}{\frac{w_f}{{\rho }_f}+\frac{w_m}{{\rho }_m}}$(1)

The % void in the laminate was computed by employing equation (2)(2) $\mathit{Void}(\mathrm{\%})=\frac{{\mathit{\rho }}_{\mathit{th}}-{\rho }_{\mathit{ex}}}{{\rho }_{\mathit{ex}}}\times 100$(2) (Agarwal & Broutman, 1990).

(2) $\mathit{Void}(\mathrm{\%})=\frac{{\mathit{\rho }}_{\mathit{th}}-{\rho }_{\mathit{ex}}}{{\rho }_{\mathit{ex}}}\times 100$(2)

Where w_f, w_m, ρ_th and ρ_ex represent the weight fraction of fibre, weight fraction of matrix, theoretical density and experimental density, respectively.

2.4. Ageing methods and testing procedure

Moisture diffusion experiment was carried out to investigate the moisture absorption properties and to analyse the weight percentage of moisture accumulated by the specimens after a time period of 180 days. Before conditioning, the test samples were cut following ASTM specifications and dried in a drying oven at 50°C. To achieve uniform moisture diffusion throughout the laminate, the cut edges were covered with polymer epoxy prior to exposing to different ageing conditions. The specimen weights before conditioning (dry weight) were used as a reference and were measured with a digital weighing machine (of accuracy ±0.001 g). Five specimens for each ageing condition were used for the measurement. Specimen weights during ageing were monitored periodically and recorded till specimen reached moisture equilibrium. The ageing conditions listed below were employed to investigate the influence of absorbed moisture on the performance degardation of laminates.

1. Ambient ageing (immersion in water maintained at 25°C ambient temperature)

2. Sub-zero ageing (immersion in water at −10°C temperature in a deep freezer)

3. Humid ageing (ageing in an environmental chamber set at 40°C temperature and relative humidity of 60%)

ASTM D5229 (Materials, 2020) is employed to measure the moisture gain of the composites at regular intervals. Equation (3)(3) $\mathit{M}\left(\mathit{t}\right)\left(\mathrm{\%}\right)=\frac{m_t-m_0}{m_0}\times 100$(3) is used to estimate the % moisture gained by the samples at various time periods (Pai et al., 2022).

(3) $\mathit{M}\left(\mathit{t}\right)\left(\mathrm{\%}\right)=\frac{m_t-m_0}{m_0}\times 100$(3)

In equation no. (3), M(t) indicates the percentage gain of moisture at time t, m_o and m_t are the mass of the pristine specimen and mass during time t respectively.

The theoretical values of moisture absorption at various intervals were determined as per the Fick’s law using following Equation (4)(4) $\frac{M_t}{M_s}=1-\frac{8}{{\pi }^2}\sum _{\mathit{n}=0}^{\mathrm{\infty }}\frac{1}{{\left(2\mathit{n}+1\right)}^2}exp\left[-\frac{{\left(2\mathit{n}+1\right)}^2{\pi }^2{D}_z}{h^2}\right]$(4) (Moudood et al., 2019)

(4) $\frac{M_t}{M_s}=1-\frac{8}{{\pi }^2}\sum _{\mathit{n}=0}^{\mathrm{\infty }}\frac{1}{{\left(2\mathit{n}+1\right)}^2}exp\left[-\frac{{\left(2\mathit{n}+1\right)}^2{\pi }^2{D}_z}{h^2}\right]$(4)

Where M_t and M_S are the mass of moisture diffused at time t and at saturation, n is the summation index, h is the specimen thickness and D_z is the coefficient of diffusion in the composite.

The diffusion coefficients of the specimens were estimated using equation (5)(5) $D_Z=\mathit{\pi }{\left(\frac{h}{4M_S}\right)}^2\left(\frac{M_2-M_1}{\sqrt{t_2}-\sqrt{t_1}}\right)=\mathit{\pi }{\left(\frac{h}{4M_S}\right)}^2{k}^2$(5) (Shetty et al., 2020)

(5) $D_Z=\mathit{\pi }{\left(\frac{h}{4M_S}\right)}^2\left(\frac{M_2-M_1}{\sqrt{t_2}-\sqrt{t_1}}\right)=\mathit{\pi }{\left(\frac{h}{4M_S}\right)}^2{k}^2$(5)

In the above equation, D_Z is the diffusion coefficient, h is the depth of the specimen, M₁ and M₂ are the weight percent of moisture diffused at time periods t₁ and t₂, M_S is the saturation moisture diffusion and k indicates the slope of the initial part of the curve.

2.5. Mechanical characterisation

2.5.1. Tensile test

Intraply carbon-Kevlar/epoxy composites have been extensively utilised in a broad range of engineering applications. Therefore, understanding long-term reliability predictions such as the maximum strength under tensile load, modulus, and breaking strain are essential. Tensile tests were performed on an Indian-made BISS Universal Testing Machine (UTM) with 50 kN maximum load capacity, in accordance with the ASTM D3039 (ASTM D3039/D3039M–14, 2014) standard as depicted in Figure 4. The total span dimension was maintained constant at 150 mm with an overall length of 250 mm and width 25 mm, while the speed of the crosshead was kept constant at 2 mm/min. The force exerted as well as the elongation of the specimens were determined using the Data Acquisition System and the corresponding tensile properties were calculated.

Figure 4. (a) Tensile test setup (b) Test specimen.

2.5.2. Flexural and Short Beam Shear Strength (SBS) test

Flexural examinations are used to assess the specimen’s ability to resist the bending under the application of external loads using 3-point bending. Test was carried out in compliance with ASTM D7264 (ASTM D7264/D7264M–07, 2007) employing an Indian-made UNITEK-9450 UTM having maximum load capacity of 50 kN. Five specimens with dimension of 88.32 mm (overall length) × 73.6 mm (span length) × 13 mm (width) were experimented. The flexural coupons were held in place by a pair of supports and a centred load at 1 mm/min was imposed at the middle of the specimen as depicted in Figure 5. Displacement of the specimen at the loading point were measured and corresponding flexural properties were computed. After analysing five specimens from each ageing conditions, average outcomes were documented.

Figure 5. Flexural test setup.

Inadequate shear strength between the lamina is the primary cause for composite’s delamination failure in several fields of engineering. The SBS examination assesses the ability of a composite to tolerate the delamination damage. The experiment was performed using a German-made Instron UTM equipped with a 50 kN load cell in compliance with ASTM D2344 (ASTM D2344/D2344M–13, 2013). This test method is similar to 3-point bending with shorter specimen dimensions and it computes the shear strength between the lamina by generating solely shear forces between the layers and reducing the stresses due to bending. The specimen length was determined to be six times greater than the thickness, while the width was determined to be twofold the thickness, i.e.,, 13.8 × 4.6 × 2.3 mm. The entire test procedure is similar to the flexural test with a crosshead rate of 1 mm/min.

2.5.3. Damping characterisation (Impact hammer test)

The damping properties associated with different ageing conditions were evaluated experimentally using the ASTM E756–05 standard (ASTM E756–05, 2005). Specimens were cut as per the standard with dimensions of 250 mm length and 25 mm width for each ageing conditions. As illustrated in Figure 6, one of the ends of the test specimen was tightly attached to the bench whereas the opposite end was left free, resulting in a cantilever beam set up.

Figure 6. (a) Impact hammer test setup (b) Test specimen.

An accelerometer was attached to the open end of the specimen, and an impulse hammer was utilized to stimulate the specimen. The sensitivity of the accelerometer and impact hammer were 101.6 mV/g and 10 mV/lbf, respectively. The NI 9234 data collection device connected to LabVIEW 16 was employed to capture the frequency vs time, coherence, and phase parameters. Using equations (6)(6) $\mathit{\zeta }=\frac{\mathit{\delta }}{\sqrt{4{\pi }^2+{\delta }^2}}$(6) to (9(9) $E_s=\frac{16{\pi }^2{f}_n^2\mathit{m}{L}^3}{\mathit{b}{h}^3}$(9) ), the logarithmic decrement (δ), damping ratio (ζ), and storage modulus (E_s) for pristine and each aging condition were calculated from the gathered data. The natural frequency (f_n) was determined using the acceleration amplitude vs frequency plot. The logarithmic decay was determined using the acceleration amplitude vs time plot.

(6) $\mathit{\zeta }=\frac{\mathit{\delta }}{\sqrt{4{\pi }^2+{\delta }^2}}$(6)

(7) $f_n=\frac{1}{2\mathit{\pi }}\sqrt{\frac{k}{m}}$(7)

(8) $\mathit{k}=\frac{3E_s\mathit{I}}{L^3}$(8)

(9) $E_s=\frac{16{\pi }^2{f}_n^2\mathit{m}{L}^3}{\mathit{b}{h}^3}$(9)

In the above equations L, m, b, h and I denote the length, mass, width, thickness, and moment of inertia of the specimen, respectively.

The specimen coding and details of the tests carried out in this research are summarised in Table 3.

Table 3. Specimen coding and test details

Specimen Coding	Ageing condition	Tests carried out for each ageing condition	No. of samples tested for each ageing condition
Quasi-isotropic carbon-Kevlar/epoxy composite	Pristine	Tensile	5
	Ambient	Flexural	5
	Sub-zero	Short beam shear (SBS)	5
	Humid	Impact hammer	5

3. Results and discussion

3.1. Void content

A void is an unexpected additional phase that exists in almost all materials made of composites. Voids are vacant, empty spaces occupied by gas rather than solid substances. They are typically the consequence of production flaws and are regarded to be unwanted since they might impair the mechanical characteristics and hence the durability of the laminates. The theoretical and experimental densities were computed to be 1.104 ± 0.14 and 1.143 ± 0.21 respectively. The void content of the laminate was computed using equation (2)(2) $\mathit{Void}(\mathrm{\%})=\frac{{\mathit{\rho }}_{\mathit{th}}-{\rho }_{\mathit{ex}}}{{\rho }_{\mathit{ex}}}\times 100$(2) found to be 3.53%.

3.2. Moisture uptake characteristics

Figure 7 displays the experimental findings of the composite’s moisture uptake under three different ageing situations.

Figure 7. Moisture absorption behaviour of the laminate for different ageing conditions.

The graph shows how the sample’s amount of moisture absorption changes with immersion time. The theoretical values of absorption of water were estimated using Equation (4)(4) $\frac{M_t}{M_s}=1-\frac{8}{{\pi }^2}\sum _{\mathit{n}=0}^{\mathrm{\infty }}\frac{1}{{\left(2\mathit{n}+1\right)}^2}exp\left[-\frac{{\left(2\mathit{n}+1\right)}^2{\pi }^2{D}_z}{h^2}\right]$(4) , it is observed that theoretical moisture uptake closely follows the experimental values. In the beginning stage, weight gain was quickly increased, which eventually slowed down after long-term water immersion and the curves exhibited Fickian behaviours. Results primarily declared that the moisture diffusion values were not uniform in all the three aged specimens. The laminates that were aged in water at room temperature acquired the greatest quantity of moisture content (4.167%) at equilibrium. The laminates aged in sub-zero environment absorbed 2.762% of moisture at equilibrium, whereas the laminates that were aged at 40°C and 60% RH in a controlled environment collected the lowest amount of moisture, 1.162%. Clearly, the moisture weight gain of specimens rose significantly across all ageing environments in the initial stage till 125 days, following Fickian behaviorus, thereafter, increased quite slowly until it attained a saturation condition. After 150 days of ageing, the composite had nearly achieved saturation. As the ageing time was increased to 180 days, the amount of weight gain remained negligible as compared to 150 days. It improved from 4.132% to 4.167%, a 0.84% increment for ambient specimens. Furthermore, after 180 days of ageing, the specimens had nearly reached saturation. The humid aged specimens recorded lowest water absorption than ambient and sub-zero aged specimens. Table 4 shows the % moisture absorption and diffusion coefficients for three distinct ageing conditions.

Table 4. Moisture diffusion parameters

Ageing Condition	Saturation moisture uptake (%)	Ageing time	Diffusion Coefficient DZ (mm^2/s)
Ambient	4.167	180 days	8.944 × 10^−8
Sub zero	2.762	180 days	8.177 × 10^−8
Humid	1.162	180 days	7.897 × 10^−8

The difference in the moisture diffusion of specimens is clearly explained by the Fick’s first law of diffusion, which indicates the amount of diffusion of a substance through a specific area of the cross-section is in direct proportion with the concentration gradient. The diffusion coefficient (mm²/sec) is the proportionality constant. The first law is especially important in polymer composites reinforced with fibres, since moisture content varies with time. The major driving factor underlying moisture absorption is the concentration gradient, which persists until the gradient is equalized. The specimens which were immersed in water at ambient ageing conditions, is identical to a situation of 100% relative humidity (RH). Since the concentration gradient between the specimen and surrounding water was more, diffusion of water molecule occurred at a quicker rate, resulted in higher slope and the composites absorbed the highest moisture at saturation. However, in humid environment, composites were subjected to 60% RH in an controlled environment, causing a slower rate of moisture diffusion and a slighly lower saturation moisture content than ambient conditions. In sub-zero condition, specimens were completely submerged in water and the ageing temperature was much lower than ambient temperature which resulted in moderate amount of moisture absorption in laminates.

Moisture absorption in composites is a complicated process that is influenced by a variety of factors, including the kind of reinforcement and matrix used, the percentage of voids in the laminate, the method of fabrication, and the type of ageing scenario (Almeida et al., 2016). Furthermore, strong bonding between the polymer and the reinforcement hinders the rate of moisture diffusion. The strong bonding at the fibre and the polymer interface result in closer packing inside the laminate, which decreases the average free path between the water molecules (the distance that is travelled between the two subsequent impacts by the absorbed molecules of water) leading to reduced moisture absorption.

3.3. Tensile test

Table 5 displays the results of tensile test experiment attained from carbon-Kevlar/epoxy interply laminates exposed to three different ageing conditions.

Table 5. Tensile properties of the specimens

Type of Ageing	Ultimate strength (MPa)	Tensile modulus (GPa)	Tensile failure strain (mm/mm)
Pristine	270.03 ± 9.22	7.00 ± 0.57	0.050 ± 0.0025
Ambient	223.29 ± 6.93	5.86 ± 0.43	0.045 ± 0.0026
Sub-zero	247.93 ± 8.21	6.48 ± 0.58	0.048 ± 0.0025
Humid	231.56 ± 10.24	6.25 ± 0.62	0.047 ± 0.0026

It has been found that as the moisture absorption percentage of the sample increases, so does the deterioration of the mechanical characteristics. The level of deterioration is determined by the type of ageing experienced by the specimen. Mechanical characteristics deterioration was in the decreasing sequence of: ambient condition > humid condition > sub-zero condition > pristine samples. Figure 8 shows the stress-strain characteristics of pristine and aged specimens. The tensile characteristics of all aged specimens were evidently changed after hygrothermal ageing. Ultimate strengths, modulus and strain at fracture all decreased as ageing time increased. However, sub-zero aged composites displayed improved resistance on hygrothermal ageing tensile characteristics.

Figure 8. Stress vs strain graphs of pristine and aged specimens.

Figure 9 depicts the comparison of ultimate strengths and modulus of aged specimens with respect to pristine specimens.

Figure 9. Ultimate tensile strength and modulus of the specimens.

The highest tensile strength and modulus recorded were 270.03 MPa and 7.0 GPa, respectively, in pristine specimens. Exposing the specimens to distilled water for 180 days at ambient temperature led to a greater degree of tensile strength deterioration, with a tensile strength drop of 20.85% in comparison to pristine specimens. Specimens treated with sub-zero ageing had the lowest drop in tensile strength among the three ageing conditions, with a reduction of 10.85%. Figure 10 depicts the retention of tensile strength under three different ageing situations.

Figure 10. Strength retention after tensile testing.

When compared to pristine specimens, the lowest strength retention (79.15%) was recorded in specimens aged under ambient environment, and the highest strength retention (89.15%) was seen in sub-zero aged specimens. The considerable loss in tension strength was noticed in ambient temperature-aged samples owing to a greater degree of moisture absorption, which culminated in the swelling of matrix material. Carbon and kevlar fibres do not absorb water as they are synthetic in nature, water diffusion induced the deterioration of the fibre-matrix interface, resulting in an overall reduction in tension characteristics. Water absorption is an usual occurrence in all polymeric substances, and it has a significant impact on mechanical characteristics (Costa et al., 2005). The absorption of moisture by the composite material primarily degrades the polymer matrix, lowering its capacity to sustain external loads. This may ultimately result in reduction in tensile properties, causing the material more prone to failure. Moreover, voids occupied by the moisture inside the composite can disturb the transfer of load between the fibres and the polymer matrix, resulting in a reduction in the composite’s ultimate stiffness or modulus. This decrease in stiffness can have an impact on the composite’s capacity to withstand distortion under tensile forces. The absorption of moisture may trigger the matrix substance to expand, which can result in interfacial problems including delamination and debonding of the fibre–matrix interface. These flaws deteriorate the structure of the composite and lower its tensile strength dramatically. Even though, the moisture absorption was higher in sub-zero specimens than humid aged specimens, they were able to retain higher amount of tensile properties due to the frozen moisture present in the voids and fibre–matrix interfaces. These frozen moisture acts as a binder between the fibre and matrix and helps in transferring the loads from matrix to fibres, thereby increasing the load carrying capacity of the specimens. However, in ambient and humid aged specimens, increased moisture absorption decreases the interfacial bonding between reinforcing fibres and the surrounding matrix. The weakening of the bonding affects the efficiency of load transfer between the components, resulting in a decrease in tensile strength. Furthermore, absorbed moisture reacts with the hydroxyl groups of the polymer matrix and initiate the plasticization process. The plasticisation process makes the polymer more flexible by disrupting the polymer chains, enhancing mobility and reducing the glass transition temperature. These factors ultimately result in reduction in material’s tensile strength and stiffness. Additionally, incorrect manufacturing processes also result in weak interfacial adhesion across the reinforcement and the polymer, allows the moisture to effortlessly penetrate and reducing its mechanical durability.

Figure 11 shows a scanning electron microscope examination of ruptured tensile test samples to determine the root causes of damages. The SEM images of fractured pristine samples revealed the existence of significant amount of matrix cracks and fibre ruptures. The fibres were tightly held by the matrix, indicating the existence of excellent fibre-matrix interfacial bonding, which aids in successfully transmitting tensile load from matrix to fibre, and consequently pristine samples were able to take up higher loads than aged samples. The SEM images of aged samples demonstrate that, the matrix-dominated mechanical characteristics were greatly influenced by the ageing parameters and the degree of molecular diffusion. Tensile characteristics deteriorate due to the moisture diffusion through the matrix, which causes plasticization and breakdown of structure of the polymer. The process of ageing increased the number of matrix cracks at the micro level and deterioration of the matrix leading to weakening of fibre-matrix interfacial bonding. This resulted in fibre debonding and fibre fractures during the tensile examination. Multiple fractures, matrix degradation, delamination, and interfacial debonding at the fibre and matrix interface have been noticed in both ambient and humid samples. However, sub-zero aged samples revealed the presence of brittle type of matrix fractures with excessive fibre ruptures. Previous studies have shown that when the temperature decreases the composite materials becomes brittle in nature, due to the absorbed moisture is in its frozen form, and the specimens exhibit brittle fractures (Padmaraj et al., 2021; Ray, 2004).

Figure 11. SEM micrographs of fractured (a) Pristine (b) Ambient (c) Sub-zero and (d) Humid aged tensile test specimens.

3.4. Flexural and Short Beam Shear (SBS) test

The three-point flexural experiment showed that the degree of moisture absorption has a substantial impact on the bending strength and SBS strength of carbon-Kevlar/epoxy composites. In both cases test results demonstrated an identical pattern with sub-zero samples had the highest strengths, followed by humid and ambient samples. The outcomes of the three-point flexural experiment under three distinct ageing conditions are shown in Figure 12. Specimen failure in three-point flexural test occurs predominantly by shearing and bending between the different layers. Compressive stresses are developed in the layers above the neutral surface and tension in the layers below the neutral surface during specimen loading. The maximum stresses in laminate will be produced at the specimen’s outermost surfaces, the pristine specimens exhibited maximum bending strength and modulus which were 196.60 MPa and 10.80 GPa, respectively.

Figure 12. Flexural strength and modulus of pristine and aged specimens.

The decrease in flexural characteristics is caused due to degradation of fibre-matrix interfacial bonding due to polymer matrix swelling. In the case of aged specimens, flexural force easily deteriorated the bonding strength between the fibre and matrix. The existence of a more void content causes the laminate to pick up greater amounts of moisture, resulting in decreased interfacial bonding. Tensile and bending characteristics commonly known as reinforcement dominant properties, any moisture uptake by the composite weakens the interfacial bonding and reduces the effective load transfer, affecting the tensile and flexural characteristics (Ahmad et al., 2021; Zhong et al., 2019). Ambient, sub-zero, and humid aged specimens’ flexural strengths had been reduced by 20.84%, 10.84%, and 16.89%, respectively. The most significant decrease in characteristics was recorded in ambient aged samples, whereas the minimal decrease in characteristics was recorded in sub-zero aged samples. The absorption of moisture molecules, led to the deterioration of the fibre–matrix interface, is considered to be the primary cause of the decrease in bending strength. Fractured test specimens and the enlarged view of the fractured region is as shown in Figure 13. All the specimens were bent after the testing without complete rupture due to the presence more ductile Kevlar fibres. Tensile cracks were observed on the bottom surface of the specimens and crack intensity were increased with increase in the moisture contents.

Figure 13. (a) Post flexural test samples (b) Enlarged view of the fractured area.

The deterioration of flexural properties of laminates can be attributed to several factors. The primary reason being the softening of the matrix. The absorption of moisture by the polymer matrix could end up in material plasticization or softening. The molecules of water interfere with the polymer matrix’s intermolecular forces, lowering its overall strength as well as stiffness. This softening phenomenon decreases the resistance to external flexural forces, resulting in reduction in bending properties. Additionally, Moisture can also reduce the bonding between the two components. Moisture absorbed at the interface between the fibres and the matrix can weaken the adhesion of fibres to the matrix, resulting in fibre-matrix debonding or fibre expulsion during bending experiments. The lack of efficient transfer of load between the fibres and the matrix leads to reduced stress transfer and, as a result, reduces the composite’s flexural properties. Additionally, increased moisture absorption in ambient and humid specimens can cause hydrolysis, resulting in chemical deterioration of the polymer matrix. The reaction of moisture molecules with polymer chains causes chain scission, weakening of polymer bonds, and a drop in molecular weight. Chemical degradation weakens the matrix, making it more prone to fracture under bending loads and lowering flexural strength. However, sub-zero aged specimens showed higher flexural strengths due to the frozen moisture at the fibre–matrix interface which acts as a binder and assist the flexural load transfer from matrix to fibre and hence improved flexural load carrying capacity compared to other aged specimens.

Furthermore, absorption of moisture may trigger multiple kinds of damages within the structure of the composite. Matrix cracks are developed in the laminates due to the phenomena of osmosis at the interface of fibre and matrix, and hydrolysis (breaking of bonds made from hydrogen), which ultimately weakens the interfacial adhesive bond. Moisture-induced swelling and dimensional transformations (volumetric expansion) can result in internal strains, microcracks, or the formation of voids, which function as stress concentration areas. These flaws interfere with the microstructure of the composite, resulting in early failure at much lower applied loads and decreased flexural properties (Arun et al., 2010; Saha & Bal, 2018; VanLandingham et al., 1999).

SBS specimens used for testing have been loaded so as to eliminate tensile and compressive stresses, resulting in only shear between the layers. In response to the shearing of adjacent layers of the laminate in different directions, the sample generates transverse interlaminar cracks and delamination. Figure 14 depicts the SBS strength and SBS strength retention of pristine and aged specimens.

Figure 14. SBS strength and SBS strength retention of pristine and aged samples.

Pristine samples had the maximum shear resistance of 21.40 MPa. Sub-zero samples recorded the lowest drop in SBS strength at 5.84%, while humid specimens showed a reasonable reduction of 12.19%, followed by ambient specimens with the highest reduction of 18.78%. Since interlaminar characteristics are more sensitive to environmental ageing, the deterioration rate of shearing characteristics upon ageing is usually greater than the deterioration rate of other mechanical properties. values. In general, the matrix structure and interface of composite materials determine the interlaminar characteristics, which are more vulnerable to environmental ageing than fibres.

The primary reason for the degradation of SBS strength is the stresses developed in the laminate due to swelling of the polymer resin and its effect at the interface between the fibre and matrix. Samples failed predominantly because of buckling at micro level, fibre breaking, or cracking and delamination due to shear between the lamina as shown in the optical micrograph images (Figure 15).

Figure 15. Light microscope pictures of fractured (a) Pristine (b) Ambient (c) sub-zero and (d) humid aged SBS test specimens.

Shear strength decreased significantly in ambient aged samples owing to the increased amount of moisture diffusion, resulting in higher interfacial deterioration. The moisture absorbed primarily degrades the specimen as well as promotes the production of residual stresses due to hygroscopic conditions. In sub-zero samples, diffused water undergoes volumetric expansion during freezing, and frozen moisture improves the bonding between fibre and matrix, leading to enhanced interlaminar shear strength.

The other factor which causes deterioration of interfacial shear strength is believed to be the bonding water. A linear correlation was observed between shear strength and content of the bonding water regardless of surrounding temperatures. During the moisture absorption process, bonding water in the laminate breaks the original inter-chain Van der Waals forces and bonds of hydrogen, resulting in resin plasticization. This results in the reversible impact of resin plasticization and the irreversible action of interfacial debonding, leading to linear deterioration of shear strength with water concentration (Guo et al., 2021).

3.5. Impact hammer test

Absorption of moisture can affect the damping properties of laminates. Damping is a material’s capacity to disperse energy during oscillation. Whenever a composite accumulates moisture, the viscoelastic characteristics of the polymer matrix change, causing variations in damping performance. The altered damping properties may influence the speed at which the oscillations in the laminate decay, thereby influencing the natural frequency.

The vibrational properties of the pristine and aged specimens under various conditions is presented in the Table 6. Three specimens from each ageing condition were considered for the test and the average values were computed to understand the effect of moisture on the damping properties. From Table 6, the pristine specimen had the highest logarithmic decay followed by sub-zero, humid and ambient aged specimens. Sub-zero aged specimens recorded lowest percentage reduction in decay (9.09%) followed by humid aged specimens (29.54%) and maximum decrement was seen in ambient specimens (45.9%) compared to pristine specimens. Accordingly, the damping coefficient was maximum for pristine specimens and decreased as the moisture content increased in the specimens.

Table 6. Damping properties of pristine and aged specimens

Ageing condition	logarithmic decay (δ)	Damping coefficient (ζ)	Natural frequency(fn)	Storage modulus, Es (GPa)
Pristine	0.22	0.036	21.47	23.37
Ambient	0.119	0.019	16.51	13.83
Sub-zero	0.20	0.030	19.81	19.91
Humid	0.155	0.024	18.16	16.73

Damping characteristics of the specimens are as depicted in Figures 16–19.

Figure 16. Damping characteristics of pristine specimens (a) Acceleration amplitude vs frequency (b) Acceleration amplitude vs time.

Figure 17. Damping characteristics of ambient aged specimens (a) Acceleration amplitude vs frequency (b) Acceleration amplitude vs time.

Figure 18. Damping characteristics of sub-zero aged specimens (a) Acceleration amplitude vs frequency (b) Acceleration amplitude vs time.

Figure 19. Damping characteristics of humid aged specimens (a) Acceleration amplitude vs frequency (b) Acceleration amplitude vs time.

The Pristine specimens exhibited highest natural frequency and the ambient specimens showed least values of natural frequency. The percentage reduction in natural frequencies for the specimens aged in ambient, sub-zero and humid conditions were 23.07%, 7.68% and 15.38%, respectively, with respect to pristine conditions. The variations in the damping characteristics of the specimens can be explained with the help of Meirovich’s continuous beam model. As per this model, the frequency of free vibration of a cantilever beam is determined as per Equation (10)(10) $\mathit{f}=\frac{1}{2\mathit{\pi }}{\left(\frac{1.875}{L}\right)}^2\sqrt{\frac{\mathit{EI}}{\mathit{\rho A}}}$(10) (Pavan et al., 2019)

(10) $\mathit{f}=\frac{1}{2\mathit{\pi }}{\left(\frac{1.875}{L}\right)}^2\sqrt{\frac{\mathit{EI}}{\mathit{\rho A}}}$(10)

In the above equation, L, E, ρ, A, and I represent the overall length, material stiffness, material density, area of cross section and moment of inertia of the specimen respectively. From the frequency equation, it is evident that, the natural frequency decreases as the stiffness of the material reduces. The quantity of moisture gained by the sample influences the frequency of free vibration of the specimen. The higher moisture absorption in the specimen initiates the matrix to undergo plasticisation effect and subsequently reduces the specimen’s stiffness, which ultimately reduces the frequency of free vibration of the specimen. In sub-zero aged specimens, the absorbed moisture was in frozen condition which increased the stiffness and ultimately resulted in higher retention of natural frequencies compared to the pristine specimens. The ambient specimens collected maximum moisture content which resulted in swelling and softening of the matrix. As a result, the stiffness of the specimen reduced and led to least natural frequency retention of 76.93% compared to pristine specimens.

4. Conclusion

The present study focuses on the impact of various ageing environments on the static mechanical and damping properties of the quasi-isotropic carbon-Kevlar/epoxy laminates. The following inferences could be derived from the findings:

• The nature and amount of moisture absorption of carbon-Kevlar/epoxy composites depends on the type of ageing condition. The theoretical values of moisture absorption closely resemble the experimental values for the initial period of ageing and follows Fick’s diffusion law.

• The saturation moisture uptake was 4.167%, 2.762%, and 1.162% for specimens aged in ambient, sub-zero, and humid ageing conditions, respectively.

• The ambient aged specimens experienced maximum deterioration of tensile strengths (20.85%) followed by humid aged specimens (16.90%) and sub-zero specimens (10.85%) in comparison to pristine samples.

• Bending and short beam shear strength reduction was greatest in laminates aged under ambient environments, trailed by laminates aged under humid and sub-zero environments.

• The damping characteristics indicated that, in the aged category, sub-zero specimens retained maximum natural frequency compared to humid and ambient specimens. Reduction in damping ratio for ambient, humid and sub-zero specimens were found to be 47.22%, 33.33% and 16.6%, respectively, in comparison to pristine specimens.

• SEM examination of post-tensile specimens showed that the primary reason for strength reduction was due to plasticization and swelling of the matrix which led to degradation of the polymer matrix. Fibre fractures, matrix deterioration, matrix fractures, and delamination were some of the major failure modes observed in the aged specimens.

• The outcome of this research work may be useful in designing and predicting the durability of the aerospace, automotive and marine structures which are frequently exposed to various ageing environments.

Correction

This article has been corrected with minor changes. These changes do not impact the academic content of the article

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

Yogeesha Pai

Yogeesha Pai is a faculty in the Department of Aeronautical and Automobile engineering at Manipal Institute of Technology – Manipal Academy of Higher Education (MAHE) (Institute of Eminence)- Manipal, India. He has obtained his PhD degree in Polymer Composites from MIT Manipal and M.Tech degree in Aerospace Engineering from IIT Kharagpur, India. His research interests include: Composite materials and characterization, Aircraft structures, Smart materials, Vibrational analysis.

Dayananda Pai

Dayananda Pai K has been working as a Professor in the Department Aeronautical and Automobile Engineering at Manipal Institute of Technology, Manipal. He has obtained his PhD degree in Mechanical Engineering from NITK Surathkal, India. His research interests include: Composite materials, Manufacturing, Design and optimization.

G. T. Mahesha

GT Mahesha, has been working as Professor in the Department of Aeronautical and Automobile Engineering, Manipal Institute of Technology, Manipal. He has published more than 15 research articles in reputed journals and his areas of interest are automotive power trains, pollution control, alternative fuels, bio-based materials, and electric vehicles.