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Abstract

The Flexural After Impact (FAI) behaviour of epoxy and vinyl-ester based carbon fiber reinforced composite laminates was investigated at elevated temperatures. Carbon/Epoxy (CE) and Carbon/Vinyl-ester (CV) laminates with cross-ply configuration (0/90/90/0)3S were manufactured via a compression moulding technique and subjected to Low-Velocity Impact (LVI) s at ∼1.5 and ∼2.5 m/s under temperatures 30, 60 and 90°C. The flexural behaviour of composite laminates was investigated via three-point bending tests. The non-impacted and impacted CE and CV samples' failure profile during the flexural tests was examined using the real-time Acoustic Emission (AE) monitoring technique via peak frequency analysis of AE events. Flexural after impact strength of CE samples at both the velocities were higher than that of the CV ones. For CE and CV samples, the flexural after impact strength increases at 60°C, and decreases when approaching 90°C. At 90°C, flexural strength degradation was considerably higher in the CE ones because the Co-efficient of Thermal Expansion (CTE) of the epoxy matrix occurs at a much higher rate than the vinyl-ester, which generates higher residual stress at the carbon fiber-epoxy interfaces. Acoustic emission (AE) monitoring allowed to capture the interface among the fiber and matrix due to the exposure temperature and impact velocity.

Keywords

Composite laminates Carbon Fiber Reinforced Polymer carbon epoxy vinyl-ester low-velocity impact temperature acoustic emission

Introduction

Due to their high specific strength/stiffness, chemical inertness, electrical conductivity, fatigue resistance, and ease of fabrication, carbon fiber-reinforced composite laminates have gradually replaced traditional metallic structures and have proven ideal in aerospace and automobile and marine sectors.¹ However, CFRP composite laminates have a high vulnerability to failure owing to transient low-velocity impact loads (LVI).^2,3 CFRPs can absorb the impact energy only via physical damage and not through plastic deformation.⁴ When laminated composites are impacted, they will generate barely visible impact damage (BVID), such as fiber/matrix delamination, fiber failure, etc.^5–7 The mechanical properties of the composites are largely affected by this internal damage; hence, it becomes vital to examine the influence of low-velocity impact on laminated composites.^8–10

Although several studies have demonstrated that CFRP composite laminates are competent materials for high-performance structural members, still, numerous challenges remain unexplained in the growing application of CFRP laminates.¹¹ One of them is to completely realize the mechanical behavior of CFRP laminates at high temperatures.¹² In general, composite laminates are exposed to different temperatures during service life.⁵ Temperature interactions with LVI loads induces complex manifestations of damage mechanisms within the composite materials.^13,14 The mechanical strength of CFRP laminates are weaken at elevated temperature.¹⁵ In Fiber Reinforced Polymer (FRP) laminate the matrix is reliant on the operating temperature and on the fabrication cycle, that influences the Glass Transition Temperature (Tg).¹⁶ Due to the temperature variations, thermal stresses can be generated at fiber-matrix interface. Hence, FRP composite laminates are influenced by operating temperature.¹⁵ The composites will degrade at exposure temperatures closer to the Tg of the matrix material.¹⁷ Matrix degradation affects the matrix system’s mechanical properties and the bonding between fibers and matrix leading to the rapid drop of the overall composite’s mechanical properties.¹⁵ Thus, the laminate’s durability exposed to elevated temperatures necessities to be studied more to tune them suitable for external uses.¹⁸

At present, FRP composite laminates for structural engineering applications largely use vinyl ester and epoxy as matrices.^19–23 Most of the previous literature on the degradation of composite materials owing to temperature exposure was concentrated on carbon fibers in an epoxy matrix.^24,25 Survana et al.²⁶ examined the influence of temperature on LVI response of carbon/epoxy laminates with an energy of 4.3 J at a range 30–90°C. The residual flexural properties raised with the exposure temperature owing to the lower impact damage. Im et al.²⁷ studied the influence of exposure temperature on the damage profile of epoxy and polyether ether ketone-based CFRPs subjected to LVI. They reported a direct correlation among the impact velocity and damage area at various temperatures.

Vinyl ester is one of the best matrix systems for carbon fibers with superior mechanical properties.²⁸ Compared to epoxy, vinyl ester possesses low viscosity and exhibits a lower hydrophilic feature.²⁹ Carbon/epoxy and carbon/vinyl ester composite laminates are often preferred owing to their superior mechanical properties, ease of processing and durability considerations.²⁰ Few comparative investigations on the durability behaviors of epoxy and vinyl-ester based composite laminates have been performed recently.^30,31 However, several aspects such as the fundamental understanding of temperature exposure and degradation mechanisms remain to be investigated. Detection and analysis of acoustic emission signals can help to infer information on the origin and importance of discontinuity in a material. Due to the versatility of Acoustic Emission Testing (AET), a variety of industrial applications, such as structural integrity assessment, flaw detection, leak testing or welding quality control, have also been widely used as a research tool.

The present investigation aims to evaluate the LVI impact behavior of carbon/epoxy (CE) and carbon/vinyl ester (CV) composites at elevated temperatures. The CE and CV samples exposed to temperatures 30, 60, and 90°C were impacted at 1.5 and 2.5 m/s and then subjected to flexural tests with AE monitoring. This investigation will help to understand the flexural after impact performance of epoxy and vinyl ester based CFRPs at elevated temperatures.

Experimental procedure

Materials and fabrication

Unidirectional carbon fibers of thickness 0.30 mm were used as the reinforcement. Epoxy LY556 resin with a hardener aradur HY951 (weight ratio 10:1) and vinyl ester resin VBR 2508 with a promoter VBR 1201, accelerator VBR 1206 and catalyst VBR 1204 (weight ratio 10:0.15:0.15:0.15) were employed as the matrix systems. Table 1 and Table 2 summarize the properties of different materials considered in this study. The carbon fiber mats were combined with the resin systems in a weight proportion of 1:1.

Table 1.

Properties of carbon fiber reinforcement.

Properties	Details	Tolerance
Area density (kg/m²)	0.240	±5%
Warp ends	8	±5%
Temperature resistance: (°C)
Continuous load	260	±5%
Short time resistance	600	±5%

Table 2.

Properties of epoxy and vinyl-ester resin systems.

Properties	Epoxy	Vinyl ester
Appearance	Clear, pale yellow liquid	Pale yellowish to pale brownish
Density at 25°C	1.15–1.20 × 10³ (kg/m³)	1.04–1.06 × 10³ (kg/m³)
Viscosity at 25°C (Pa s)	10–12	3–4
Storage Temperature (°C)	2–40	3–30

The CFRP laminates were fabricated in a compression moulding set-up of maximum load capacity 30 kN. In this moulding process, 12 layered CE and CV laminates of cross-ply orientation (0/90/90/0)_3S were manufactured by normal hand layup process in a mould of size 300×300 mm and then cured in the compression moulding set-up. The compression pressure of approximately 4.5 MPa was applied for 24 h at ambient temperature (30°C) by the pneumatic system. The laminates used in this study were post cured at 100°C temperature for 3 h and 24 h at room temperature and again post-cured at 90°C for 4 h. The schematics of the fabrication process is shown in Figure 1. ASTM D 790-03 standard CFRP samples of dimension 150×25×3.8 mm was trimmed from the fabricated laminates by employing a water jet machining set-up.

Figure 1.

Schematic depicting the fabrication procedure of carbon/epoxy (CE) and carbon/vinyl ester (CV) composite laminates using compression moulding.

Dynamic mechanical analysis

The dynamic mechanical properties of CE and CV samples were evaluated using a Metravib’s dynamic mechanical analyzer (DMA 25 01dB-Metravib) as depicted in Supplememtary Figure 1s. The dual cantilever method of loading was employed under a test temperature between 30 and 150°C with a heating rate of 5°C/min at a constant frequency of 0.1 Hz. Rectangular samples of dimension 80 mm × 10 mm × 3.8 mm were employed for the DMA analysis.

Low-velocity impact test

CEAST Fractovis 6875 impact tower was employed to perform the LVI tests at elevated temperatures (as per standard ASTM D790-03), as schematically depicted in Figure 2(a). A hemispherical faced impactor of mass (m) 1.926 kg and diameter 12.7 mm was employed for impacting the samples. A clamping load of 1 kN was applied to hold the samples (Supplementary Figure 2s). To prevent repeated impact on the tested samples, a pneumatically operated brake system was employed to hold the impactor after the first event.

Figure 2.

Schematic depicting (a) low-velocity impact test set-up and (b) Three-point flexural test set-up coupled with AE monitoring.

Piezoelectric transducers were employed to acquire the time histories of velocity and load. CEAST DAS 64k data acquisition set-up was employed to process the test signals acquired by the piezoelectric sensors. The LVI tests were performed at impact velocities (v) of 1.5 and 2.5 m/s under 30, 60 and 90°C. Supplementary Table 1S summarizes the impact energies⁴⁷ corresponding to the chosen impact velocities in this investigation. An electronic thermostatic controller was employed to set the temperature in the testing chamber. A thermocouple was used to ensure that the samples were tested at the desired temperature conditions. For each testing condition, at least four samples were tested.

Quasi-static three-point bending test with AE monitoring

To evaluate the flexural strength of the non-impacted and impacted samples, a quasi-static three-point-bending test was performed following standard ASTM D 790⁴⁵ using a Tinius Olsen H100KU universal testing machine (UTM). The flexural load was applied at a feed rate of 0.25 mm/min. Flexural tests on CE and CV samples were monitored (schematically shown in Figure 2(b)) up to the ultimate failure employing a PCI-8 AE set-up supplied by PAC, USA.

Results and discussions

Dynamic mechanical analysis

Figure 3 depicts the typical plots of normalized loss, storage modulus, and tanδ versus temperature for CE and CV samples tested in DMA. These tests allowed evaluating the glass transition temperature (Tg) for CE and CV composite laminates.³³ DMA test results for CE and CV composite laminates depicts an average Tg of about 105°C and 122°C, respectively. Above this temperature level, the laminates turn out to be unsuitable for structural applications. Thus, the maximum exposure temperature of CE and CV samples were chosen as 90°C, close to their Tg.

Figure 3.

Normalized loss, storage modulus, and tan δ versus temperature for (a) CE and (b) CV samples.

Low-velocity impact test results

Figure 4 illustrates the contact force versus deformation plots of the CE and CV samples. The peak contact force for each category of the samples is plotted against the corresponding impact temperature in Figure 5. The maximum contact force of the CE and CV samples is summarized in Supplementary Table 3S.

Figure 4.

Contact force versus deformation plots for (a) CE at 1.5 m/s, (b) CE at 2.5 m/s, (c) CV at 1.5 m/s, and (d) CV at 2.5 m/s.

Figure 5.

Peak contact force versus temperature plot for CE and CV samples impacted at 1.5 and 2.5 m/s.

The experimental force-deformation curves can be categorized into two groups, i.e. complete-rebound (e.g. CE and CV at 1.5 m/s in Figure 4(a) and (c)) and incomplete rebound (e.g. CE and CV at 2.5 m/s in Figure 4(b) and (d)). In general, at 1.5 and 2.5 m/s, the contact force raised almost linearly with displacement up to the incipient point for all the samples with various matrix systems. Few fluctuations arose before the contact force touched the peak value, induced by the onset and progression of some permanent damage.³³ Compared to the 1.5 m/s, the experimental curves at 2.5 m/s showed different behaviour after the incipient point, signifying that the impact velocity influenced the energy absorbing mechanism of the composite laminates.³³ The amplitude and duration of the oscillations were higher at 2.5 m/s.

From the hysteresis cycles of CE and CV samples at 2.5 m/s, it can be noticed that the descendant phase (after the peak contact force) of the experimental curve exhibited a very different characteristics compared to the respective samples at 1.5 m/s (Figure 4(b) and(d)). The experimental curves of some CE and CV samples at 2.5 m/s showed a sudden drop and/or higher residual deformation, signifying the occurrence of some major damage in the laminates.³³ Few oscillations further happened before the contact force-deformation curve attained the rebounding phase. In particular, the curves encountered a short plateau region with oscillating amplitude, as a result of progressive fracture of different layers of the laminates.⁷ In most of the samples tested at 1.5 m/s, during the unloading phase, the displacement returned perfectly near the axis origin (indicates recovery of some elastic energy). The response of CE and CV samples at 2.5 m/s exhibited an incomplete rebound trait. The occurrence of some permanent damages in the samples might have led to incomplete rebound behaviour.

From Figure 4, for the CE and CV samples at 1.5 and 2.5 m/s, a clear hierarchy pertaining to the linear stiffness with respect to exposure temperature, i.e. 60°C > 30°C > 90°C can be noticed. Besides, the trend was quite similar in terms of peak force for CV and CE samples at 1.5 and 2.5 m/s. For both CE and CV samples, at 1.5 and 2.5 m/s, the peak contact force increases at 60°C and then decreases when approaching 90°C, which is close to the Tg. As the velocity rises, the peak force for both CE and CV samples increases, signifying higher load-bearing capability of the composites. However, the increase in the peak contact force of CE and CV samples were different and these values further changed with the exposure temperature. The peak contact force of CE samples at both the velocities were higher than that of the CV samples.

To qualitatively evaluate the damage extent due to the impact load, it is common to examine the absorbed energy. The striker’s kinetic energy before it touches the sample is considered as the impact energy, while the absorbed energy is the amount of impact energy dissipated by the sample via various failure modes. During impact loading, some amount of impact energy gets dissipated to the surrounding depending upon the stiffness of the sample, remaining impact energy will be absorbed by the sample permanently leading to impact failure modes like laminate splitting, matrix crushing, and debonding.⁴⁶ Figure 6 illustrates the energy-time histories of the CE and CV samples at various impact velocities and temperatures. The absorbed energy of CE and CV samples impacted at 1.5 and 2.5 m/s are plotted against the corresponding temperature in Figure 7 and summarized in Supplementary Table 4S.

Figure 6.

Energy versus time plots for (a) CE at 1.5 m/s, (b) CE at 2.5 m/s, (c) CV at 1.5 m/s, and (d) CV at 2.5 m/s.

Figure 7.

Absorbed energy versus temperature plot for CE and CV samples at 1.5 and 2.5 m/s.

For both the CE and CV samples, at 1.5 and 2.5 m/s, the absorbed energy gets drops at 60°C. This behaviour might be caused by the result of superior molecular cross-linking in the polymer chain. The matrices such as epoxy and vinyl ester resins, when exposed to elevated temperatures, doesn’t melt or reflow, instead undergo chemical cross-linking or a phase change. At elevated temperature, in the polymer chain, the movement of the molecules rises significantly.

To check the extent of cure in Carbon epoxy and Carbon vinyl ester laminates, Differential scanning calorimetry (DSC) studies were carried out for neat epoxy resin and neat vinyl ester resin. From the DSC thermogram depicted in Figure 8, it is observed that a cure exotherm is obtained for neat epoxy and vinyl ester between 70–140°C and 60–100°C respectively. The thermogram also shows the absence of cure exotherm for carbon epoxy and carbon vinyl ester system, confirming the complete cure. Since there is no cure exotherm for carbon epoxy and carbon vinyl Ester composites in DSC thermogram, it confirms that the crosslinking reaction is complete.

Figure 8.

DSC Thermograms for the samples.

These events enhance the bond stuck between the fiber-matrix interfaces. This can explain why the CE and CV samples experienced a superior impact response at 60°C. When nearing to the glass transition temperature, the absorbed energy increases. CE and CV samples exposed to 90°C at 2.5 m/s, being more compliant, absorbed more impact energy through higher overall deformation (Figure 4). Alike interpretations can be made as regards the contact duration (Supplementary Figure 3s and Table 5S). The absorbed energy of both the CE and CV samples’ rises as the velocity rises. However, the absorbed energy of CV samples at 1.5 and 2.5 m/s were higher than that of the corresponding CE ones.

To better assess the role of temperature on the LVI performance, the percentage change in absorbed energy of CE and CV samples impacted at different velocities under 60 and 90°C are compared to the respective 30°C ones. Supplementary Table 6S depicts the percentage change in absorbed energy of CE and CV samples. The degradation of impact performance was considerably high in the CE ones at 90°C. As the exposure temperature rises, both the fiber and matrix systems try to expand and the rate of expansion depends on the CTE of the ingredients.³⁴

The CTE for the epoxy (∼45 × 10⁻⁶ m/(mK) is 38 times higher than that of the carbon fibers (∼1.18 × 10⁻⁶ m/(mK)). In contrast, the CTE of the vinyl ester (∼16 × 10⁻⁶ m/(mK)) is 13.55 times higher than that of the carbon fiber. Hence, expansion of epoxy matrix occurs at a much higher rate than the vinyl ester, which generates higher residual stress at the carbon fiber-epoxy interfaces. This fiber-matrix interface zones become a possible region for debonding and thus generates micro-cracks. As a result, compared to the CV samples, the impact performance of the CE composite laminates was substantially dropped at 90°C.

Post-impact residual flexural behaviour

The flexural strength of CE and CV samples impacted at 1.5 and 2.5 m/s are plotted against the corresponding temperature in Figure 9. Supplementary Table 7S summarizes the flexural test results of CV and CE samples. As expected, the non-impacted samples exhibited higher flexural strength than the impacted ones. The trends of the post-impact flexural strength and impact performance for CE and CV samples at various impact velocities remain the same for all the exposure temperatures considered. The post-impact flexural strength of both the CE and CV samples impacted at 1.5 and 2.5 m/s increases at 60°C and then decreases when approaching 90°C.

Figure 9.

Ultimate force versus temperature for CE and CV samples at 1.5 and 2.5 m/s.

To better assess the role of impact damage and exposure temperature on the flexural strength, the percentage change in flexural strength of CE and CV samples at 1.5 and 2.5 m/s under 60 and 90°C are compared to the respective 30°C ones. Supplementary Table 8S illustrates the percentage change in flexural strength of CE and CV samples. After being impacted at 1.5 and 2.5 m/s under 90°C, the CV samples loses almost 15.25 and 6.81% of its flexural strength while the CE samples loses 16.21 and 9.60% (poor damage tolerance at elevated temperature), respectively. This behaviour was owing to the critical energy absorption mechanism of CE samples at elevated temperatures, which absorbed higher energy due to the higher expansion of the epoxy matrix.⁴³ At 90°C, the CV composite exhibited a better response with a gradual degradation trend.

Acoustic emission monitoring

Figure 10 depicts the typical peak frequency distribution during the quasi-static flexural test on the CE and CV samples. Three clusters of peak frequency failure signals, namely 75 to 155, 158 to 235, and 240–400 kHz’ were acquired during the flexural tests of CE and CV samples. Most AE signals possess peak frequency in the range 75–155 kHz, which are typically recognized as matrix cracking,^37–39 whereas few AE signals in the range 205–kHz and 158–200 kHz can be attributed to fiber breakages and interfacial failures, respectively (Figure 10).^37,39,40 For the non-impacted case and the samples impacted at 30°C, higher AE signals pertaining to the fiber failure were recorded, confirming the localized-brittle fractures.^37,41,44

Figure 10.

Typical AE peak frequency versus time plot for (a) CE and (b) CV composite samples.

The percentage of AE events corresponding to different failure modes at various impact velocities and exposure temperatures are summarized in Table 3 and Table 4. The increased exposure temperature induced a rise in the matrix cracking and debonding failure modes (peak frequencies 75 to 155 and 158–235 kHz). An increase in the velocity (1.5–2.5 m/s) stimulated this transferal considerably. These events signify that, by the softening of the matrix at elevated temperature, the post-impact flexural load-induced more minor damage to the fiber system. In the CV samples, this transferal in the peak frequency scattering due to the exposure temperature and impact velocity was noticed as low as regards CE ones. These results can explain why CV samples exhibited a better response with higher retention of post-impact flexural strength with an increase in the temperature than the CE ones.

Table 3.

Percentage of AE events pertaining to various damage modes for the non-impacted and impacted CE samples at various impact velocities and exposure temperatures.

Temperature	AE events percentage
	Matrix cracking		De-lamination		Fiber breakage
	1.5 (m/s)	2.5 (m/s)	1.5 (m/s)	2.5 (m/s)	1.5 (m/s)	2.5 (m/s)
Non-impacted	90.91		4.39		4.7
Ambient (30°C)	91.12	91.82	4.66	4.88	4.22	3.3
(60°C)	92.72	93.16	4.86	4.96	2.42	1.88
(90°C)	93.71	93.93	5.58	5.8	0.71	0.27

Table 4.

Percentage of AE events pertaining to various damage modes for the non-impacted and impacted CV samples at various impact velocities and exposure temperatures.

Temperature (⁰C)	AE events percentage
	Matrix cracking		De-lamination		Fiber breakage
	1.5 (m/s)	2.5 (m/s)	1.5 (m/s)	2.5 (m/s)	1.5 (m/s	2.5 (m/s)
Non-impacted	91.33		6.06		2.61
Ambient (30)	91.53	91.65	6.5	6.67	1.97	1.68
60	91.6	91.92	6.56	6.68	1.84	1.4
90	92.8	92.83	6.7	6.74	0.5	0.43

The morphological investigation was carried out utilizing a Field Emission Scanning Electron Microscope (FESEM) model Quatro S produced by Thermofisher Scientific, United States. Before mechanical tests, this study gives information on the microstructural characterization of the CE and CV samples (Figure 11(a) and (b)). The fracture patterns of samples subjected to varied velocities and temperatures are depicted in the FESEM micrographs taken after the testing (Figure 12(a)–(d))

Figure 11.

FESEM micrographs of samples before mechanical testing a) CE before test b) CV before test.

Figure 12.

FESEM micrographs of fractured samples. (a) Fractured CE samples subjected to various temperatures at 1.5 m/s (b) Fractured CV samples subjected to various temperatures at 1.5 m/s (c) Fractured CE samples subjected to various temperatures at 2.5 m/s (d) Fractured CV samples subjected to various temperatures at 2.5 m/s.

Conclusions

In this paper, the influence of exposure temperature on the FAI response of carbon/epoxy (CE) and carbon/vinyl-ester (CV) composite laminates was experimentally investigated. CE and CV composite laminates with cross-ply configuration (0/90/90/0)_3S were manufactured via a compression moulding technique and were impacted at ∼1.5 and ∼2.5 m/s under temperatures 30, 60 and 90°C. The bending behaviour of the composite laminates was investigated using three-point bending tests. The following important conclusions were hereby drawn from the experimental results:

The impact performance of the CE samples at both 1.5 and 2.5 m/s were higher than that of the CV ones. For the CE and CV samples at both the velocities, a clear hierarchy pertaining to the impact performance with respect to exposure temperature, i.e. 60°C > 30°C > 90°C was noticed. The superior impact behavior at 60°C was due to the superior molecular cross-linking in the polymer chain and enhanced fiber-matrix interface bonding. When nearing to the glass transition temperature, the impact performance reduces. CE and CV samples exposed to 90°C and particularly at 2.5 m/s, being more compliant, dissipated more energy through higher overall deformation. The degradation in impact properties was considerably high (lower strength retention) in the CE ones at 90°C. This was because the CTE of the epoxy occurs at a much higher rate than the vinyl ester, which generates higher residual stress at the carbon fiber-epoxy interfaces. Similar observations can be drawn in terms of the post-impact flexural strength. After being impacted at 1.5 and 2.5 m/s under 90°C, the CV samples lose almost -9.29 and -5.16% of its flexural strength while the CE samples lose -17.44 and -15.21% (poor damage tolerance at elevated temperature), respectively.

Supplemental material

Supplemental matevarial - Flexural after impact behaviour of carbon/epoxy and carbon/vinyl-ester composite laminates at elevated temperatures

Supplementary material for Flexural after impact behaviour of carbon/epoxy and carbon/vinyl-ester composite laminates at elevated temperatures by R Bhoominathan, Arumugam V, Ashok Thompson, J J Andrew, and Hom Dhakal in Polymers and Polymer Composite

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) received no financial support for the research,authorship,and/or publication of this article.

ORCID iD

R Bhoominathan

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