An experimental study on quasi-static indentation,low-velocity impact and damage behaviors of laminated composites at high temperatures

Abstract

Nowadays, fiber reinforced laminated composites are widely used in many applications due to their high strength/weight ratio. However, these materials are very sensitive to transverse loading. The low-velocity impact test has been widely used by researchers to simulate the transverse loading. However, the low-velocity impact tests are highly toilsome, and this test requires expensive hardware and software systems. To reduce the experimental costs of the low-velocity impact test, it will be more attractive, much simpler, cheaper and more widely available to achieve impact behavior using quasi-static tests. Thus, to compare both tests, in this work the absorbed energy and force-deflection curves obtained by low-velocity impact and quasi-static indentation loading in two different fiber reinforced epoxy composites have been investigated. The Carbon-Kevlar hybrid fabrics and S2 glass fabrics were used as reinforcements. For low-velocity impact tests, a range of energies was used between 20 and 80 J. For quasi-static indentation test, the crosshead speeds were increased gradually from 1 mm/min to 60 mm/min. In addition, tests at 23°C, 40°C, 60°C and 80°C were made to examine the effect of temperature on these tests. As a result of the quasi-static tests performed, the amount of energy required to perforate the samples at a certain test speed is at the same level as the low-velocity impact test. Thus, the required energy amount for the perforation of the materials can be found by performing a quasi-static test at an appropriate speed, rather than the low-velocity impact test.

Introduction

Nowadays, fiber reinforced laminated composites are very popular in many applications such as aerospace, automotive, defense and wind power industries due to their high strength to weight ratio and improved mechanical properties as compared to metals. ¹ However, fiber reinforced laminated composite materials are very sensitive to transverse impact load which decreases the material properties and reduces the load-carrying capacity of the composite structure. It has become a serious threat to the composite structures because it produces large internal damages such as delamination, matrix cracking, and fiber/yarn breakage. ²

The impact of an object on a composite material is a complex and dynamic event, global deflections are usually large, and membrane effects or shear deflections are usually significant. ³ Therefore, the low-velocity impact responses of multistitched woven E-glass, weft-knitted spacer glass fabrics and laminated composite materials have attracted the attention of many researchers.^4-9 However, studies on temperature effect for low-velocity impact behaviors of laminated composites were limited.^10,11 The authors have published a series of studies to investigate various aspects of low-velocity impact response on the laminated composites. Aktaş et al. ¹² investigated the effect of low-velocity impact on the compression-after impact (CAI) strength of E-glass reinforced epoxy composites. The CAI strength decreases from 152 MPa for the undamaged specimen to 90 MPa for specimen impacted at 70 J. Barouni and Dhakal ¹³ presented the results of full investigation of impact damage mechanisms on flax/vinyl ester laminated composite and flax-glass/vinyl ester hybrid composite laminates caused by low-velocity impact using a drop-weight impact machine. Düring et al. ¹⁴ investigated the composites consisting of carbon and glass fiber reinforced plastics in combination with steel and elastomer layers subjected to experimental drop-weight impact tests and compression-after impact tests. Ismail et al. ¹⁵ focused on analyzing the effects of hybridizing kenaf and glass fiber to develop hybrid composites with varying weight ratios on the low-velocity impact response and the post-impact properties of the obtained composites. Tuo et al. ¹⁶ investigated the damage and failure mechanism of carbon fiber reinforced epoxy thin composite laminates under low-velocity impact and compression-after-impact (CAI) loading conditions. Four levels of impact energy were included in the test matrix. Delamination induced by the low-velocity impact was captured using ultrasonic C-scan, and a three-dimensional (3D) digital image correlation (DIC) system was employed to measure full-field displacement during the CAI tests. Caprino et al. ¹⁷ analyzed the effect of impact velocity, mass, and energy on glass/aluminum composite laminates. Arikan and Sayman ¹⁸ investigated the effect of resin type on the impact response of E-glass fiber reinforced composites manufactured with two types of resin, polypropylene(thermoplastic) and epoxy (thermoset) subjected to the low-velocity single and repeated impacts.

The low-velocity impact test is a direct solution to investigate the effects of transverse impact load on composite materials, but also has its restrictions such as expensive impact equipment to record the dynamic response is required, filtering of the data due to oscillations in the signals may be problematic, and analysis of the results is complex. Also, since full-scale impact testing is usually prohibitively expensive, the problems of scaling up a laboratory-scale component or coupon behavior to that of the in-service structure must be addressed. ³ To reduce the experimental costs of the low-velocity impact test, it will be more attractive, much simpler, cheaper and more widely available to achieve impact behavior using quasi-static tests. However, since several mechanisms control the impact response of composite material, it should be verified under which conditions the quasi-static test will give a good representation of the effective results. James and Sun ¹⁹ have carried out quasi-static indentation tests on uniaxial and 45° off-axial Kevlar K706 coupons with and without an elastic foundation to investigate failure modes and mechanisms. Failure occurs in the 45° off-axial case because of yarn sliding at the corners. Goodarz et al. ²⁰ investigated the effect of structural parameters of weft-knitted spacer fabrics on the impact behavior of fabric reinforced composites, experimentally and analytically. It is concluded that the impact behavior of composites depends on the structural parameters of weft-knitted spacer fabrics and the geometrical parameters of fabric also affect on influence the impact behavior of textile composites. Weirdie and Lagace ²¹ reported that the impact tests fall within the so-called large-mass, low-velocity regime, where previous findings for composite plates indicate that quasi-static tests represent the impact response accurately; i.e. impact and quasi-static tests can be considered equivalent. Aymerich et al. ²² presented that for graphite/PEEK laminates the use of static tests would not give the same responses as impact tests and that it would be hazardous to try to simulate dynamic events with equivalent static tests. However, this approach has not been validated for the types of E-glass/polyester laminates ubiquitous in the marine industry.

Comparison of the quasi-static and low-velocity impact behaviors of advanced composites are very limited in literature. The objective of this study is to compare quasi-static crosshead speed and impact energy behaviors of S2-glass fabric/epoxy and Carbon-Kevlar hybrid fabric/epoxy composites under the quasi-static and low-velocity impact loadings at the different temperatures. HEXION MGS L160 epoxy and H260S hardener system were used as the matrix material in the manufacturing of advanced composites. With the aid of contact force-deflection, absorbed energy-impact energy and absorbed energy/crosshead speed diagrams that have been obtained for different impact energy levels and different crosshead speeds, comparisons have been done between the two different types of tests and materials.

Materials and method

Manufacture of composites

The thermoset composite materials (TS) used in this work were produced by the vacuum assisted resin infusion method, Figure 1. The areal densities were 190 g/m² and 210 g/m² for the woven S2-glass fabrics and woven Carbon-Kevlar hybrid fabric, respectively. An epoxy system consisted of MGS L160 and hardener H260S was used as matrix material in manufacturing. The curing process was performed on a specially designed heating table, at 80°C for 8 h, followed by cooling at room temperature. S2-glass fabric reinforced composite was manufactured with 18 layers and Carbon-Kevlar hybrid fabric reinforced composite was produced with 12 layers. The average thicknesses of composites were measured as 3.03 mm and 3.27 mm, respectively. As a result of the materials produced, it has been observed that the thickness of the materials is not directly proportional to the number of layers. These two thicknesses are the values found closest to each other. The fiber volume fraction of composites is approximately 50%.

Figure 1.

Composite manufacturing system.

Low-velocity impact tests

The low-velocity impact tests were carried out by CEAST-Fractov is Plus drop-weight impact testing machine. A load-cell capacity of 22.4 kN was used in the tests. The striking mass including crosshead, impactor and its nose was approximately 4.926 kg. A hemispherical nose with a diameter of 12.7 mm was used in low-velocity impact tests. S2-glass fabric/epoxy composite specimens with dimensions of 100 mm × 100 mm × 3.03 mm and Carbon-Kevlar hybrid fabric/epoxy composite specimens with dimensions of 100 mm × 100 mm × 3.27 mm were fixed by a pneumatic fixture with a 76.2 mm hole diameter. The impact energy was gradually increased from 20 J to 80 J to determine the perforation impact energy. In addition, all tests were performed at the room temperature (23°C), 40°C, 60°C and 80°C.

Quasi-static tests

The quasi-static punch shear apparatus with a 76.2 mm hole diameter has been manufactured and equipped on the Universal Shimadzu tensile test machine with 100 kN load capacity to perform the quasi-static test, Figure 2. Various crosshead speeds, 1mm/min, 10 mm/min, 20 mm/min, 40 mm/min and 60 mm/min were chosen for quasi-static tests. S2-glass fabric/epoxy composite specimens with dimensions of 100 mm × 100 mm × 3.03 mm and Carbon-Kevlar hybrid fabric/epoxy composite specimens with dimensions of 100 mm × 100 mm × 3.27 mm were used for quasi-static test. Experiments were performed at the room temperature (23°C), 40°C, 60°C and 80°C.

Figure 2.

Universal shimadzu tensile test machine with punch shear apparatus.

Results and discussion

In a low-velocity impact event, the variation of the contact force versus deflection of the sample (displacement of the impactor nose or top surface of the sample) gives significant information about the impact response of the materials. ²³ In Figure 3, the contact force-deflection curves obtained from low-velocity impact tests at temperatures of 23°C, 40°C, 60°C and 80°C for both samples have been given. It is observed from Figure 3 that a higher contact force occurs in S2-glassfabric (S2) reinforced composites than Carbon-Kevlar hybrid fabric (HY) reinforced samples in all conditions and impact energies, while the maximum deflection presents the opposite trend. In addition, as seen from the figure, three different types of curves are obtained. The first one is “closed curve” which imply the rebounding case, as the curve for 20 J in Figure 3(a) illustrates this case. In this case, after the impactor has been in contact with the material for a while, it cannot penetrate the material and then it returns to its initial position. A closed curve consists of an ascending section of loading and a descending section combining loading and unloading. The second one is called the penetration case. Curves obtained for 30 J in Figure 3(a) and 50 J in Figure 3(b) are examples of this case. The third one is the perforation case, which happened at 80 J impact energy level for all specimens. For the penetration and perforation case force starts at zero, reach a maximum and drops to zero with an ever-growing displacement.

Figure 3.

Contact force-deflection diagrams at different temperatures for HY and S2 composites under impact loadings at. (a and b) 23 C, (c and d) 40 C, (e and f) 60 C and (g and h) 80 C.

For the Carbon-Kevlar hybrid fabric /epoxy sample, the rebounding case has occurred only at 20 J impact energy for all temperatures. However, when the impact energy increased to 30 J, the penetration case has been seen. The rest of the applied impact energies led to the perforation of the sample. Moreover, in S2-glass fabric reinforced composite, the rebounding case has been observed at 20 J and 30 J for all temperatures. For all temperatures except 80°C, a penetration case occurred at 50 J impact energy, while at 80°C perforation took place. Finally, at 80 J impact energy, perforation case was observed for all temperature values. Furthermore, the maximum contact force increased with the increasing of temperature in Carbon-Kevlar hybrid fabric reinforced composites. On the other hand, it is nearly constant in S-glass fiber reinforced composites.

The slope of the ascending section of curve is called impact-bending stiffness. As shown in figures, the slope of Carbon-Kevlar hybrid fabric reinforced composites curves has increased with increasing temperature, which implies that the impact-bending stiffness of composites increased. This shows that Carbon-Kevlar hybrid fabric/epoxy composite materials have become more rigid with increasing temperature. However, on the slope of S2-glass fabric reinforced composites almost no change was observed. Because of the more rigidity, the failure modes of Carbon-Kevlar hybrid fabric/epoxy composites are almost penetration threshold and perforation for 30 J and 50 J impact energies at all temperatures while failure modes are rebounding and almost penetration threshold in S2 glass/epoxy, respectively. However, failure mode is perforation in both composites for 50 J and 80°C. In addition, in S2 glass fabric/epoxy composite, failure mode changes from rebounding to perforation by increase of temperature from 60°C to 80°C. These situations show that S2 glass fabric/epoxy composite absorbs more energy than Carbon-Kevler hybrid fabric/epoxy composite. Therefore, S2-glass fabric/epoxy can be preferred in energy absorption applications.

Figure 4 presents some specimens tested with the hemispherical no seat room temperature under different impact energies. As it can be seen form Figure 4, the major mode of failure occurred as delamination, penetration and perforation. At 20 J energy, both specimens underwent rebounding but the behaviors were not the same when the impact energy increased progressively. The increase of impact energy causes fiber breakage. Matrix cracking and delamination occur and failure mode is rebounding at 20 J impact energy for both types of composites. With the increase of impact energy to 30 J, delamination area increases, and rebounding case still occurs for S2-glass fabric/epoxy. For 50 J impact energy, perforation and penetration exist for S2-glass fabric/epoxy (S2) and Carbon-Kevlar hybrid fabric/epoxy (HY) composites, respectively. Perforation occurs at 80 J for both composites. Temperature does not significantly affect the damages of both composites, so, damage photos of samples made at different temperatures are not given.

Figure 4.

Photographs of impact damaged samples of HY and S2 composites for various impact energies at 23°C temperature.

A quasi-static indentation test method for modeling low-velocity impact events of composites would prove to be very beneficial to the researchers since applying static tests are much easier than low-velocity impact test. In Figure 5, the force-deflection curves of quasi-static indentation tests at 23°C, 40°C, 60°C and 80°C for Carbon-Kevlar hybrid fabric/epoxy and S2-glass fabric/epoxy at different speeds are given. From the graphs of quasi-static tests (Figure 5(a) to (h), the peak force for Carbon-Kevlar hybrid fabric reinforced composites is higher than S2-glass fabric reinforced composites. Similarly, the deflection at maximum contact force is higher for Carbon-Kevlar hybrid fabric reinforced composites. However, the maximum deflection is higher for S2-glass fabric reinforced composites.

Figure 5.

Contact force-deflection diagrams at different temperatures for HY and S2 composite under quasi-static loadings at (a and b) 23 C, (c and d) 40 C, (e and f) 60 C and (g and h) 80 C.

The maximum contact force of Carbon-Kevlar hybrid fabric reinforced composites has increased with increasing temperature. For instance, although the maximum contact force is around 5000 N at 23°C, this value is 5500 N for 1 mm/min when the temperature rises to 80°C. Similarly, the maximum deflection value also increased with temperature. In addition, while the descending part of the curves occurs smoothly at 23°C, sharp decrease has been observed with the increasing temperature because of the formation of the plug after maximum contact force. Contrary to the Carbon-Kevlar hybrid fabric reinforced composites, there is no significant change on the maximum contact force of the S2-glass fabric reinforced composites with increasing temperature. The average maximum contact force at 23°C was nearly 4050 N and it is 4030 N at 80°C. In addition, there was no change in the deflection in the maximum contact force with temperature. The contact force increases generally as the test speed increases.

After the fiber fracture without a sudden drop in load and the formation of the plug, the friction region observed has been considerable so increasing the displacement.

The area under force-deflection curves yields the absorbed energy. In Figure 6, the variation of the absorbed energy versus impact energy and crosshead speed is given which was calculated from a software of the low-velocity impact machine and the static test machine. For Carbon-Kevlar hybrid fabric reinforced composites, the absorbed energy is nearly 11 J at 20 J impact energy. However, the composite absorbed all energy for 30 J impact energy and this value does not change significantly from 50 J to 80 J at the temperature of 23°C. It means that the perforation threshold of Carbon-Kevlar hybrid fabric reinforced composites is 30 J for this type of penetrator. On the other hand, the absorbed energy increased to nearly 40 J for 50 J impact energy at 40°C, 60°C and 80°C. Figure 6(c) represents the result graph of absorbed energy by hybrid fiber reinforced specimens under quasi-static test with hemispherical tip indenter. From the rate of 1 mm/min to 60 mm/min, generally an increase of the absorbed energy is noticed at 23°C and 40°C temperatures but the graphs at 60°C and 80°C temperatures present almost the same behavior and nearly constant value over all the velocities. The absorbed energy has increased from 34 J to 57 J at 40°C when the crosshead speed increased from 1 mm/min to 60 mm/min. However, the absorbed energy shows a constant trend around 30–40 J energy band wide at all speeds at 60°C and 80°C.

Figure 6.

(a and b) Absorbed energy vs impact energy for HY and S2 composites under impact loadings. (c and d) Absorbed energy vs impact energy for HY and S2 composites under quasi-static loadings, at various temperatures.

Nevertheless, in S2-glass fabric composites, no significant change occurred in the variation of absorbed energy with temperature for low-velocity impact tests, as seen in Figure 6(b). The perforation threshold of S2-glass fabric reinforced composites is 50 J at all temperatures. The absorbed energies are 10 J, 20 J, 50 J and 55 J for impact energies of 20 J, 30 J, 50 J and 80 J, respectively. A significant decrease in the absorbed energy has occurred with the increase in temperature for the quasi-static test. For example, the absorbed energy is 42 J at 23°C, while it is 29 J at 80°C for 1 mm/min crosshead speed. In addition, as the speed increased, this difference in the absorbed energy increased. When the crosshead speed increased from 1 mm/min to 60 mm/ min, the absorbed energy has increased from 42 J to 62 J and 33 J to 69 J, respectively, at 23°C and 40° C. However, significant change is not observed on the absorbed energy at 60°C and 80°C with increasing of crosshead speed.

Conclusion

The quasi-static indentation and the low-velocity impact tests are compared experimentally for the composites with two different reinforcements. Behaviors of Carbon-Kevlar hybrid fabric/epoxy and S2-glass fabric/epoxy composites under the low-velocity and quasi-static tests at different impact energy levels and crosshead speeds were investigated experimentally. In addition, the tests were made at various temperatures. Conclusions obtained from this study could be summarized as follows:

For Carbon-Kevlar hybrid fabric reinforced composites, the maximum contact force was always higher nearly 15% in the quasi-static indentation tests, while the maximum deflection was observed very close to each other for both tests. In addition, when temperature increased, the maximum contact force has raised in both tests.

For S2-glass fabric reinforced composites, it was observed that the maximum contact force was higher in the low-velocity impact tests than quasi-static tests. There was no significant change in the contact force with increasing of temperature.

With the S2-glass fabric/epoxy specimen in general the contact force increased as the test speed increased. After the fiber rupture without an abrupt drop of load, the friction zone observed has been considerable thereby increasing the displacement.

The effects of crosshead speed and temperature on the damage were insignificant while impact energy had a significant effect on impact damage for both composites.

The damage area in S2-glass fabric/epoxy was bigger than that in Carbon-Kevlar hybrid fabric/epoxy.

The absorbed energy that caused perforation was significantly affected in quasi-static tests with increasing of crosshead speed, especially in high temperature.