Experimental investigation of flexural strength and plane strain fracture toughness of carbon/silk fabric epoxy hybrid composites

2.1 Materials

In this work, the composite is processed using an epoxy resin matrix (LY556), a hardener (HY 5200), and carbon/silk satin fabric for reinforcement.

2.2 Fabrication method

The manual lay-up process is used to prepare the composite. Here, carbon and silk satin cloth is cut into the required size of 390 mm × 340 mm fabric plies. Five fabric plies of silk satin and carbon fabric are selected from each. Subsequently, the resin, hardener, and textiles were weighed. Using a stirrer, hardener and epoxy matrix were combined in a bowl. Air bubbles should be prevented as much as possible. Air bubbles trapped in a matrix, for instance, can indicate material failure in that specific instance. The manufacturing process involves initially coating the mold surface with wax (a release film), followed by the application of a polymer layer on the sheets. Subsequently, a carbon fabric layer is positioned on the mold and meticulously rolled. After this, the resin is applied by laying down a silk fabric layer, which is then also rolled. This rolling step is performed using a cylindrical steel rod. This sequence of laying and rolling carbon and silk fabric layers is repeated until a total of ten layers (comprising five carbon and five silk fabrics) are stacked. To enhance surface finish, a polymer coating is applied to the topmost layer, and then wax (a release film) is applied to the punch. The composite is then subjected to a 20 kg f force via a hydraulic press machine and is left to cure and harden for 7 h.

2.3 Sample preparation and testing

After the curing process, the specimens were cut according to ASTM guidelines. To evaluate the FS and Mode-I plane strain fracture toughness, the composite test specimens were prepared in alignment with ASTM standards, specifically ASTM D790 for FS and ASTM D5045 for Mode-I fracture toughness. The dimensions of length and width relative to the thickness of each specimen were determined based on these standards. For the Mode-I fracture toughness testing, as dictated by ASTM D5045, a notch was carefully introduced at one edge of the specimen using a blade with a thickness of 125 µm. The crack extension was monitored using the compliance method.

2.4 FS

Also referred to as modulus of rupture or bend strength, this characteristic represents the stress level a material can withstand just before it reaches the yield point. In this context, the specimen, which may have a rectangular or circular cross-section, is subjected to bending until it either yields or fractures through a technique known as 3-point bend flexure. Typically, specimens are flat bars, and both ends are subjected to uniaxial load during the test. Key indicators to observe during testing include the offset yield strength, the peak or ultimate tensile strength – marking the threshold just before permanent deformation begins – and the fracture point at which the specimen breaks apart. These flexural tests are conducted using a Universal Testing Machine, and the obtained results are analyzed to evaluate the FS of composite materials.

2.5 Mode-I plane strain fracture toughness

When conducting a fracture toughness examination, the single-edge notch bend (three-point bend or SENB) specimen is typically the preferred choice for the test sample configuration. The geometry of the three-point bend specimen is illustrated in Figure 1. To precisely gauge the Mode-I plane-strain fracture toughness, the thickness of the specimen must surpass a certain critical thickness (B). Experimental evidence suggests that plane-strain conditions are predominantly achieved under the following conditions:

where B represents the minimum thickness required to attain a state where crack tip plastic strain is minimized. K _Q denotes the conditional or trial value of K _Ic for the composite, and σ _y stands for the material’s yield stress.

Figure 1

Three-point bend (SENB) test specimen configuration.

When testing a specimen/material with an unknown fracture parameter value, the specimen’s thickness is either the full material section or sized according to estimated fracture toughness. If the resulting fracture toughness value from the test does not meet the specified requirement, the test procedure must be redone for a thicker specimen. Additionally, apart from thickness, the test specifications entail other criteria, such as shear lip size, to determine the K _IC value. The failure to meet thickness criteria and other test requirements for ensuring plane-strain conditions results in assigning the fracture toughness value as K _C. At times, it is impractical to meet thickness requirements, such as when testing a thin plate specimen with high toughness where achieving plain strain conditions at the specimen’s crack tip may not be feasible.

3.1 FS

In accordance with ASTM standards, the three-point bending flexural test was conducted. The specimens used were rectangular with dimensions as follows: length 78 mm, width 8 mm, and thickness 3 mm, with a span length of 48 mm. The FS test of the carbon fabric composite was performed using the three-point bend method, and the details and results are outlined in Table 1.

The FS obtained using the following equation was an average of 526.18 MPa for the carbon fabric composite. Figure 2 illustrates the FS results for the carbon fabric specimens, depicting the load versus deflection curve.

In accordance with ASTM standards, we conducted tests on four specimens to determine the FS. For clarity, the data related to load versus displacement are presented for one of the specimens in Figure 2. The average value of FS for carbon fabric composite is 526.180 MPa. The FS test for the hybrid composite (carbon and silk satin fabric) was also performed using the three-point bend method. Detailed specifications and results are given in Table 2.

Figure 2

Load vs displacement data of the carbon fabric-reinforced composite.

The hybrid composite (composed of reinforcement carbon fabric and silk satin fabric) exhibits an average FS of 383.17 MPa. For clarity, the data related to load vs displacement for one of the specimens are shown in Figure 3.

Figure 3

Load vs displacement data of the silk and carbon fabric-reinforced hybrid composite.

In accordance with ASTM standards, eight specimens (four carbon fabric composites and four hybrid composites) were tested for FS. For the sake of clarity in comparison, one of the specimens was selected for the analysis of FS. Based on the load vs displacement diagrams provided in Figures 2 and 3, it is evident that carbon fabric-reinforced composite (CFRC) exhibits significantly greater FS compared to the hybrid composite consisting of reinforcements of silk satin and fabrics. This disparity can be attributed to the fact that the CFRC sustains a maximum load of 502.9 N, surpassing the 412.4 N load experienced by the hybrid composite. Additionally, the CFRC demonstrates a displacement of 2.5 mm, whereas the hybrid composite fails within a narrower displacement range in comparison to the CFRC.

3.2 Mode-I plane strain fracture toughness

Fracture toughness testing of the carbon fabric composite material was carried out in accordance with the ASTM D5045 standard, focusing on plane strain conditions. This methodology has emerged as a fundamental approach for evaluating fracture toughness and has spurred further research endeavors aimed at refining alternative fracture toughness characterization techniques. The testing protocol involves subjecting a notched specimen to 3-point bend loading. Autographic recording of load versus displacement along the notch at the composite specimen edge enables the determination of the load corresponding to crack length expansion. Conditional K _Q is subsequently derived from this load by established equations based on elastic stress analysis. K _Q attains the designation of K _IC provided specific conditions outlined in subsequent sections are met.

Thickness of composite specimen:

Crack length:

K _Q is calculated using the following expressions:

where x = a/W and 0 < x < 1:

To ascertain the validity of K _Q using size criteria, Eq. (1) is employed. If the resultant value is smaller than the dimensions of the specimen, such as crack length (a), thickness (B), and ligament (W − a), then K _Q = K _Ic. Conversely, if the value exceeds these dimensions, the test is deemed invalid.

3.2.2 K _Ic of the carbon and silk fabric-reinforced PMC

Table 5 indicates that as the a/W ratio increases, the maximum load (P _max) decreases. At a/W = 0.508, the maximum load is 402 N, while at a/W = 0.493, it drops to a minimum of 290 N. This suggests that even with the notch covering approximately 40% of the material width, it still withstands over 400 N of load. Based on these experiments, the average Mode-I plane strain fracture toughness of the carbon and silk fabric-reinforced polymer composite is determined to be 9.58 (MPa m^1/2).

Table 5

Specimen specifications of the carbon and silk fabric-reinforced PMC

Specimen no.	W (mm)	B (mm)	a (mm)	a/W	f (a/W)	P Q (N)	P Max (N)	K Q (MPa.m1/2)	K Max (MPa m1/2)	P max/P Q
1	10.15	3.83	5.003	0.493	10.417	378	396	10.20*	10.69	1.04
2	9.89	3.74	4.925	0.498	10.582	305	311	8.67*	8.84	1.01
3	10.01	3.60	5.125	0.512	11.067	350	370	10.75*	11.36	1.05
4	9.97	3.69	5.014	0.503	10.752	370	402	10.79*	11.73	1.08
5	10.14	3.71	5.151	0.508	10.925	310	325	9.06*	9.50	1.04
6	9.77	3.66	4.855	0.497	10.549	275	290	8.01*	8.45	1.05

Average K _Q = 9.58 MPa m^1/2*.

a – crack length; B – specimen thickness; W – specimen width; P _max – maximum load; K _Q – apparent fracture toughness; P _Q – onset fracture conditional load; * – valid K _IC.

The Mode-I plane strain fracture toughness of the carbon fabric-reinforced composite, yielding a value of K _Ic = 11.23 MPa m^1/2. This finding indicates that the hybrid composite of carbon and silk fabric exhibits a fracture toughness approximately 17% lower than that of carbon fabric-reinforced PMC. The enhanced fracture toughness in the latter is attributed to the superior strength and modulus of carbon fibers within the composite. Moreover, in accordance with ASTM D 5045 and as discussed in preceding sections, analysis of Table 6 reveals that the computed P _Max/P _Q values for all specimens remain below 1.1. Consequently, the obtained results are deemed valid, with all K _Q values satisfying the validity criteria outlined in ASTM standards Eqs. (2) and (3), as demonstrated in Table 6. Thus, the findings regarding fracture resistance are considered reliable.

Table 6

Validity calculations

Specimen no.	Width, W (cm)	Thickness, B (cm)	Crack length a (cm)	2.5 (K Q/σ y)2
1	1.015	0.383	0.5003	0.00168
2	0.989	0.374	0.4925	0.00124
3	1.001	0.360	0.5125	0.00201
4	0.997	0.369	0.5014	0.00209
5	1.014	0.371	0.5151	0.00150
6	0.977	0.366	0.4855	0.00121

Figure 4 displays load versus displacement data for all specimens, with only data from four specimens presented for clarity. It is evident from Figure 4 that increasing the a/W value leads to a decrease in both the maximum fracture load and absorbed energy by the specimen. In other words, as the crack length (a) increases, the maximum load and area under the curve decrease. Consequently, at lower values of a/W, the composite exhibits a higher fracture energy release rate.

Figure 4

Load vs displacement details of the hybrid composite in fracture toughness testing.

4.1 Fractography of the carbon fabric-reinforced composite

The specimen failures resulting from flexural loading (3-point bending), specifically when specimen failure transpires at the central roller, were examined. Figure 5, an SEM image, illustrates the flexural failure of a carbon fiber-reinforced composite material fracture surface, revealing a distinct demarcation between the regions of tension and compression. The fractured surface displays two distinct zones: a smooth area indicative of compressive mode of fracture, delineated by a distinct boundary from a rough surface area representative of tensile mode of fracture. Notably, the boundary between these regions runs parallel to and often in proximity to the neutral axis or non-aligned of the composite laminate.

Because carbon fiber-reinforced composites exhibit lower compressive strength relative to tension, coupled with stress concentration from the intermediate roller, flexural failures predominantly occur on the compression surface. Analysis of the fractured surface reveals exposed fiber pull-out, which serves as a crack-bridging mechanism, enhancing fracture energy. Additionally, SEM images indicate strong bonding between the matrix and fibers, contributing to FSs and high inter-laminar shear in these composites. Subsequently, these types of flexural failures typically result from a combination of compression and tension forces.

4.2 Fractography of hybrid composite (carbon and silk satin fabric reinforced)

The fractured surface of the hybrid or dual reinforced composite, comprising silk satin and carbon fabric, was examined using SEM (refer to Figure 6(a)–(d)). Figure 6(a) clearly depicts a strong bond between the carbon fiber and epoxy matrix. Despite undergoing a two-stage heat treatment process in the hydraulic press, minimal disturbance to the alignment of carbon fibers on the fractured surface was observed. Additionally, an increase in the diameter of the carbon fiber compared to its original state was observed. Figure 6(b) also indicates some carbon fiber pull-out, contributing to enhanced fracture energy.