Damage mechanisms of bismaleimide matrix composites under transverse loading via quasi-static indentation

2.1 Materials

A bismaleimide resin (BMI-6421) was supplied by the National Key Laboratory of Advanced Composites at the Beijing Institute of Aeronautical Materials [29,30].

The commercial U3160 carbon cloth (T300 grade, 160 ± 7 g/m², Weihai Tuozhan FIBER Co., Ltd.) was selected as reinforcement materials.

The toughing polymer was an amorphous high-performance engineering thermoplastic polyethersulfone (PES) (VW-10200RFP, Solvay Specialty Polymers). The toughening polymer applied on ES^TM fabric has an areal density of 35 g/m².

2.2 Fabrication of laminates

The CFRP composites were fabricated by the RTM method with a quasi-isotropic stacking sequence [+45/0/−45/90]_4s. The thickness and fiber volume of the obtained CFRP composites were controlled at 5.3 mm and 55 ± 2 vol%, respectively.

The proposed temperature and time are programmed as in Figure 1. First, the ES^TM fabrics were placed in an RTM mold. Then, the BMI-6421 was filled in the RTM mold to impregnate the porous ES^TM fabrics at 115°C under a pressure of 0.2 MPa. The mold temperature was raised at a rate of 3°C/min to the three-step curing temperatures (150, 160, and 180°C), followed by a post-curing step (200°C, 8 h). Finally, the mold was cooled down to 60°C to take out the composite sample.

Figure 1

The manufacturing process of CFRPs.

2.3 Quasi-static indentation experiment

The CFRPs composites were cut into 150 × 100 mm² specimens for Quasi-Static indentation. All the quasi-static indentation tests were performed by an Instron 4400R testing machine (Instron, USA) at a constant indenter nose and support fixture displacement rate of 0.5 mm/min (ASTM D7136).

As reported [31], quasi-static indentation testing was used to measure the BMI matrix composite’s impact resistance at low velocity. And the quasi-static indentation testing was correlated successfully with the low-velocity impact of BMI matrix composites in terms of delamination parameters. Hence, it is convinced that the principles of damage development suggested here can also applied to the BMI matrix composite during low-velocity impact.

2.4 Microstructure characterization

According to the Quasi-static indentation, loading levels with significance were selected for later studies: (a) 5.5 kN, the load-increasing process before damages occurred, (b) 6.2 kN, a sudden drop of loading accompanied with sharp noise, (c) 8.2 kN, the propagation of delamination, (d) 13.3 kN, the initial breaks of fiber, (e) 14.4 kN, the maximum loading, and (f) 9.3 kN, a plateau region for penetration failure, respectively.

The ultrasonic C-scan system (SDI-5420 Delta X, Structural Diagnostics Inc., CA) was used to identify the areas of damage for all specimens. Specimen-sustained quasi-Static indentation experiments with different loading levels were collected and sectioned into slices in the 0-direction close to the contact point of the indentation. The sliced specimens were first mounted in epoxy resin and then grounded by sandpaper with increasing mesh numbers. Subsequently, the cross-section was polished with the diamond polishing agent for microstructure characterization.

A fluorescent penetrant dye (Rhodamine B, by Tianjin BASF Chemical Co., Ltd.) was added to the mounting resin to enhance the contrast between the cracks and the surrounding matrix. An optical microscope (Leica DM750 fluorescence microscope, Leica microsystems co., Ltd) [32] was adopted to characterize the cracking patterns under polarized light. In this study, the Rhodamine-B-stained epoxy and BMI presented different colors, i.e., red and blue-green, respectively, under ultraviolet light.

An SEM (Hitachi S-3000N) was used to characterize the morphology of fracture induced by indentation tests. Cross-sectional samples were cut and polished from the indentation-tested specimens. The fractured surface and cross-section were thoroughly etched with methylene chloride to increase the contrast.

3.1 Damage process investigation via indentation tests

Damage analysis was performed by a continuous indentation test, as shown in Figure 2, the data points (a–f, 5.5, 6.2, 8.2, 13.3, 14.4, and 9.3 kN, respectively), corresponding to (a) load-increasing process before damages occurred, (b) a sudden loading drop accompanied by a sharp noise, (c) the propagation of delamination, (d) the initial breaks of fiber, (e) the maximum loading, and (f) a plateau region corresponding to penetration failure, respectively. The indentation test was paused at each selected loading level, where specimens were removed for visual and ultrasonic C-scan examinations.

Figure 2

Typical load–displacement curves for different load levels.

As shown in Figure 3, the optical photographs of the front surface (contact with the indenter) and the back surface, as well as the ultrasonic C-scan images of the specimens under different loading levels, are presented. During the quasi-static indentation test, the indentation becomes more pronounced (in depth and size) with increasing loading. First, there is a significant loading decrease around 6.2 kN (point b) in Figure 2, where a small indentation on the top surface and short fiber splitting on the back represent the onset of delamination. As the loading increase to 8.12 kN (point c), a tear emerges on the top surface which is perpendicular to graphite fibers. In addition, there are more tears occur on the back surface, which ultimately leads to ply splitting with continuously increased loading. Furthermore, combined with the C-scan images under different loading, where the shape of the delamination area would turn from generally circular to diamond with increased loading. The evolution in damage area size monitored via ultrasonic C-scan further evidence the damage behavior observed from optical photography observations.

Figure 3

The corresponding optical photographs and ultrasonic C-scan images of specimens under different loadings (points a–f in Figure 2), respectively.

Figure 4a shows the optical micrograph of the cross-section for Point a (in Figure 2), where the black shading and blue-green areas stand for the fibers and matrix, respectively, while the red region close to edge of the image is attributed to the mounting epoxy resin. It is found that fiber stacks in the 90° direction formed a horizontally positioned elliptical-like shaped area. The micrograph exhibits no epoxy resin (red) within the BMI matrix (blue-green), indicating no damage until the corresponding loading level. A complete wet out of fabric architecture by resin infusion and impregnation occurs. In addition, the BMI matrix composite was fabricated by the RTM process during which resin was injected into ordered dry fabric layers in close contact with varying degrees in a mold. The neighboring layers were nesting or mechanically interlocked at different locations owing to the compact structure. Consequently, composite structure within the specimen may vary due to differences in the orientations of local fabrics, and the sizes of resin-rich zones differ not only between layers but also different regions within the same layer [33].

Figure 4

The corresponding optical micrographs for points a–f in Figure 2, respectively. Inset: cross-section for microscopic observation.

Figure 4b is the micrograph of the cross-section at point b (in Figure 2) corresponding to the onset of delamination. Here, the red area is clearly observed inside the BMI matrix, which is the representation of the cracks. The damage is identified as a small indentation, matrix cracks, and delamination damage. The failure mode is close to the pattern suggested by Freitas et al. [8], where a pine-tree-patterned damage with the bottom layer forms cracks due to the bending of the vertical matrix.

This conical shape of the damaged area in the direction of thickness increases from the impact surface to the backside with the formation of multiple parallel cracks. Transverse shear cracks, owing to shear failure, appear initially in the −45° layer in the top half of the sample and in the +45° layer in the bottom half and then propagated through the 90° layer from the top down. Coincident with this shear crack extension into the 90° layer, longitudinal cracks, that is delamination, grew mostly in the 45° layer and lastly arrested at the 0° layer interfaces. These transverse shear cracks inclined about ±45° from the vertical position, and connected with the longitudinal cracks (delamination). It is worth noting that many matrix cracks existed in the termination spacing of a crack with the indication of the large deformation of the matrix resin ahead of the crack tip. Additionally, an approximately undamaged zone with a cone shape is visible inside the conical shape under the indentation point as well. Previous works also have reported this phenomenon [34,35,36,37], which is caused by the low-stress field surrounding the area where the matrix does not reach the delamination required failure point. However, the matrix cracks would generate when the laminate suffered tensile-bending stress, especially below the undamaged zone.

As the loading increased to point c (in Figure 2), more enhanced shear cracks near the edge of the undamaged zone under the indentation point are observed, as shown in Figure 4c, resulting in a change in the shape of the undamaged conical zone. Meanwhile, the delamination almost solely propagates within the 0° ply mostly on the bottom half of the sample with a large increase in the extent. The largest delamination occurs in the lowest 0° ply. In addition, bending cracks were developed within a limit while no delamination in the central zone under the indentation point. Therefore, the delamination and shear cracks are the major failure mode at point c.

After continuously increasing the loading to point d (in Figure 2) where a slight decrease of the loading occurred, as shown in Figure 4d, there are remarkably increased damage areas containing shear cracks and delamination. The permanent indentation damage becomes more severe whereas the delamination on the 0° ply opens further. The undamaged zone under the indentation point still exists but with its size decreasing. Meanwhile, a new large delamination running through all the lowest 0° layer is formed in the bottom of the undamaged conical zone, which may be induced by the bending cracks. Due to tensile-bending stresses, a new fiber breakage was observed in the lowermost 45° ply of the specimen, which is consistent with the optical photography where a remarkable fiber splitting is observed. The delamination and shear cracks are suggested as dominant-failure modes of laminate.

The ply breakage occurred when the increase of loading to the maximum value (point e in Figure 2), where almost all 0° layers were under the indentation point at the bottom of the sample break (Figure 4e). Additionally, the serious shear cracks developed far from the central line of the indentation while tensile cracks developed close to it. And an almost undamaged zone below the indentation point still exists but is limited to the central zone in the top half of the specimen. In this zone, there are several small matrix cracks, with little signs of fiber breakage or delamination. As a result, delamination and fiber failure are the biggest failure modes.

After the maximum value of the loading, penetration failure occurred and the corresponding loading began to decrease slowly. Increasing the displacement of the indenter led to a larger and increased frequency of delamination along with an increase in depth of fiber failure beginning at the bottom half of the laminate. Figure 4f also shows that delamination cracks appear in a small localized cone under the indenter point, which is likely to be initialized from the concentration of stress as well as the contact indentation effect. Because of the complexity of the stress profile surrounding the contact indentation, the excessive matrix cracking would not follow the pattern and direction, which is different from the formation of the bending and shearing cracks. Ply splitting becomes more severe, which is in accordance with the result observed in optical photography.

3.2 Process of damage development

Figure 5 shows the fracture surface of the BMI matrix composite. The BMI matrix laminate presented more resin-rich zones in the intralaminar, inter-fiber-bundle regions, and the interlaminar regions, as indicated in Figure 4d. The nodular structure of BMI in the PES-rich matrix was observed at the fractured surface of the BMI matrix composite, where the BMI nodule exhibited a circular shape with a size of nearly 2 μm. Moreover, there was a large size difference in various nodular structures in the interlaminar regions, owing to the discontinuous distribution of the PES particle. Here, BMIs acted as the sites of high-stress concentration, which homogeneously form the crazes/micro-cracks around microscopic and submicroscopic. These crazes/micro-cracks were frequently difficult to coalesce into a true crack since they were stabilized by plastically deformed matrix resin.

Figure 5

SEM of fracture surface of BMI matrix composite.

Crazes/micro-cracks may initiate tears when stresses were well beyond the resin bulk shear yield strength. Then, the non-continuous cracks (intralaminar cracks) propagated along the surface of the individual bundles. These intralaminar cracks could release the residual stresses developed within individual fiber bundles owing to a combination of cure shrinkage and thermal expansion mismatch between the carbon fibers and the BMI resin, as well as the stress concentration due to the transverse load. The distinguishing feature of intralaminar cracks is that they propagate along the bundle and exist within a fiber bundle. There were three types of intralaminar cracks observed in the BMI matrix composite, as shown in Figure 6. Type I cracks, namely bending cracks, was perpendicular to the bundle axis owing to tensile/bending stress, and type II cracks, namely shear cracks, were inclined to the bundle axis due to shear stress, whereas type III cracks run parallel to the bundle axis, often being formed at the interface between two bundles in close contact. Type III cracks were also seldom observed lonely, existing rather accompanied by Type II cracks. According to Karbhari and Rydin [33], Type II cracks were the result of the relative motion of adjacent reinforcing layers under transverse load.

Figure 6

Schematic showing types of intralaminar cracks.

The growth of intralaminar cracks could be associated with the deformation of the fiber bundle itself. As mentioned, the fiber bundle is compacted together tightly in RTMable composite, which can be regarded as a composite with a very high fiber volume fraction (about 80% [3]) [33]. However, the nominal fiber volume fraction in the BMI matrix composite is 57%. Under stress, the loading can transfer through the plastic deformation of the corresponding small volume of the matrix in a bundle as well as the elastic deformation of the individual fibrils. The intralaminar cracks developed when exceeding the failure strain. For the BMI matrix composites, the bending cracks occur on the bottom half of a conical damage zone while shear cracks mainly occur on the inside edge of one.

Delamination or interlaminar cracking is shown in the schematic (Figure 7) and appears as a result of crack propagation between layers of reinforcing fabric. Depending on the resin-rich regions between the fiber bundles, there are four modes of onset of delamination identified in Figure 7. Type a delamination appears to be initiated from an intralaminar crack and propagates along the interface, namely micro-delamination. The onset of Type b delamination is similar to the formation of Type a delamination but propagates on both sides of intralaminar cracks. In Type c delamination, the intralaminar crack initiates the micro-delamination separately and eventually joins together. However, in Type d delamination, intralaminar cracks coalesce into a main crack in the bundle and then result in delamination. The degree of intra- or interlaminar crack is related to the stress concentration, which depends on the applied load and local fabric structure. The current study suggests that shear cracks are only responsible for the delamination at lower loading whereas bending cracks-resulted delamination happens in the lower half of the cross-section at higher loading. The shear cracks-resulted delamination could happen independently, with no relationship to the bending cracks-induced delamination, which is consistent with that of Kuboki et al. [35]. In addition, it should be clarified that delamination can appear to mainly grow along 90° ply at lower loading and then transfer to propagate downward through 0° ply with increasing loading. The characteristic of craze, intralaminar cracks, as well as delamination cracks are shown schematically in Figure 8. The fiber breakage occurs on the reverse side of the laminate with a further increase in the loading up to the maximum value, which led to the ultimate failure of the composite.

Figure 7

Schematic showing intralaminar crack-induced interlaminar cracking (blue line: Intralaminar crack; red line: Delamination).

Figure 8

Optical micrograph of the cross section with matrix damage and delamination.

It is worth noting that a conical undamaged zone in the thickness direction under the indentation point existed until the penetration failure occurred. The size of the undamaged zone decreased with the increasing loading. As expected, the BMI matrix composite shows phase separation and inversion microstructure, which can dissipate the impact energy effectively and then release the stress concentration, eventually leading to no delamination at low-stress field under the indenter.