Compression properties of z-pinned composite laminates

The effect of z-pinning on the in-plane compression properties and failure mechanisms of polymer laminates is experimentally studied in this paper. The reduction to the compression modulus, strength and fatigue performance of carbon/epoxy laminates with increasing volume content and diameter of pins is determined. The elastic modulus decreases at a quasi-linear rate with increasing pin content and pin diameter. Softening is caused by fiber waviness around the pins and reduced fiber volume content due to volumetric swelling of the laminate from the pins. A simple model is presented for calculating the compression modulus of pinned laminates that considers the softening effects of fiber waviness and fiber dilution. The compression strength and fatigue life also decrease with increasing volume content and diameter of the pins. The strength and fatigue properties are reduced by fiber kinking caused by fiber waviness around the pins and the reduced fiber content caused by swelling. The deterioration to the compression properties is also dependent on the fiber lay-up pattern of the laminate, with the magnitude of the loss in properties increasing with the percentage of 0° (load bearing) fibers in the laminate. The paper gives suggestions for minimizing the loss to the compression properties to laminates due to pinning.     

Full-field analysis of shear test on 3D orthogonal woven C/C composites

In this work, the digital image correlation (DIC) technique was used as full-field measurement to analyze the shear properties of the 3D orthogonal woven C/C composites. Both the in-plane and the through-the-thickness specimens were tested and the macroscopic average strain was obtained. The composites were composed of lots of periodic units and the macroscopic average strain was dependent on these meso-structures. There were three regions within one unit, which showed different mesoscopic strain. The relationship between the shear test region and the macroscopic average strain was systematically studied. Finally, the accuracy of conventional strain-gauge rosette measurement was also discussed.     

Unit yarn-reduction technique and flexural properties of tapered composites based on four-step row and column braiding

Mechanisms of unit yarn-reduction braiding were investigated and preform microstructures were characterized by digital image photography and topological analysis. Flexural properties and failure mechanisms of the unit yarn-reduction composites, cut composites and uniform composites were compared. Results indicated that continuity of the braiding process must be ensured after yarn reduction and distribution of the reduction units should be uniform. A smoothly trapezoidal profile appeared near the unit yarn-reduction cross-section and braiding angles and yarn lengths in the surface or interior yarn-reduction control volumes all increased. Flexural properties of the unit yarn-reduction composites were significantly higher than those of the cut composites and slightly lower than the uniform composites. The damage process of the yarn-reduction composites can be divided into the initial, developing and serious damage stages with yarn breakage being the dominant failure mechanism, while the primary failure mechanisms of the cut composites were matrix microcracking and fiber pulling-out.     

Three-dimensional needle-punching for composites – A review

The current literature on three-dimensional (3D) needle-punched composites tends to address the aspects of preforms fabrication and composites characterization respectively. This paper aims to bring together these two aspects to provide readers with a comprehensive understanding of the subject of 3D needle-punched reinforcements for composites. Consequently, this paper contains a detailed outline of the current state of 3D needle-punched technology for manufacturing advanced composite preforms. Properties of 3D needle-punched composites and some of the predictive models available for determining these properties are also reviewed. To conclude, a number of current and potential applications of 3D needle-punched preforms for engineering composites are highlighted, and issues impeding the use of 3D needle-punched composites are also summarized.     

Split Hopkinson pressure bar testing of 3D multi-axial warp knitted carbon/epoxy composites

High-strain-rate compression experiments were performed on 3D MWK carbon/epoxy composites with different fiber architectures at room and elevated temperature using an SHPB apparatus. Macro-fracture and SEM micrographs were examined to understand the failure mechanism. The results show the dynamic properties increase with the strain rate and show a high-strain-rate sensitivity. Meanwhile, composites with [0°/0°/0°/0°] have higher properties. Moreover, composites show temperature sensitivity and the properties decrease significantly, especially for composites with [0°/90°/+45°/−45°]. The results also indicate composites take on more serious damage and failure with the strain rate. The failure of composites with [0°/0°/0°/0°] behaves in multiple delaminating, overall expansion and 0° fibers tearing. While that of composites with [0°/90°/+45°/−45°] is mainly interlaminar delaminating, local fibers tearing and fracture on different fiber layers. In addition, with increasing the temperature, the composite shows less fracture and becomes more plastic. The damage of matrix yielding, interface debonding and twisting of fibers increase significantly.     

Effect of fabric compaction and yarn waviness on 3D woven composite compressive properties

3D-woven fabrics incorporate through-thickness reinforcement and can exhibit remarkable inter-laminar properties that aid damage suppression and delay crack propagation. However, distortions in the internal architecture such as yarn waviness can reduce in-plane properties, especially in compression. The degree of yarn waviness present in a 3D woven fabric can be affected by a range of factors including weave parameters and manufacturing-induced distortions such as fabric compaction. This paper presents a thorough analysis of the effect of fabric compaction and yarn waviness on the mechanical properties and failure mechanisms of an angel-interlock fabric in compression. Tests were conducted on coupons moulded to different volume fractions and data compared to previous measurements of local yarn angle. Major findings show the importance of yarn straightness on compressive strength and how this can be affected by optimising moulding thickness. Failure initiation was also found to be heavily influenced by weave style and yarn interlacing.     

Diametral compression of pultruded composite rods

Diametral compression tests were performed on pultruded composite rods comprised of unidirectional glass or carbon fibers in a common matrix. During compression tests, acoustic emission (AE) activity was recorded and images were acquired from the sample for analysis by digital image correlation (DIC). In both composite systems, localized tensile strain developed in the transverse plane under the load platens prior to failure, producing non-linearity in the load–displacement curve and AE signals. In situ SEM diametral compression tests revealed the development of matrix microcracking and debonding in regions of localized strain, perpendicular to the tensile strain direction (parallel to the load axis). Comparison of linear finite element simulations and experimental results showed a deviation from linear elastic behavior in the load displacement curve. The apparent transverse modulus, in plane shear modulus, and transverse tensile strength of the GF rod was greater than that of the CF rod, and fracture surfaces indicated greater fiber/matrix adhesion in the GF system compared to the CF system. A mixed mode fracture surface showed that two failure modes were active – matrix tensile failure and matrix compression failure by shear near the loading edge.     

Flexural and compression response of woven E-glass/polyester–CNF nanophased composites

A significant improvement in fiber reinforced polymeric composite (FRPC) materials can be obtained by incorporating a very small amount of nanofiller in the matrix material. In this work, an ultrasonic liquid processor was used to infuse carbon nanofiber (CNF) into the polyester matrix which was then mixed with catalyst using a mechanical agitator. Both conventional and CNF-filled glass-fiber reinforced polyester composites (GRPC) were fabricated using the vacuum assisted resin transfer molding (VARTM) process. Excellent dispersion of CNFs into the polyester resin was observed from the scanning electron microscopy (SEM) micrographs. Flexural and quasi-static tests were performed for investigating the mechanical responses. Fracture surface was examined using optical microscopy (OM) and SEM. Flexure tests performed on the conventional GRPC, 0.1–0.4 wt.% CNF-filled GRPC showed up to 49% and 31% increase in the flexural strength and modulus, respectively, compared to the conventional one with increasing loading of CNFs up to 0.2 wt.%. Similar trend was seen in quasi-static compression properties. SEM evaluation revealed relatively less damage in the tested fracture surfaces of the nanophased composites in terms of matrix failure, fiber breakage, matrix–fiber debonding, and delamination, compared to the conventional one. This might be the result of better interfacial interaction between matrix and fibers, due to the presence of CNFs.     

Experimental analysis of the through-thickness compression properties of z-pinned sandwich composites

This paper presents an experimental investigation into the flat-wise compression properties, strengthening mechanisms and failure modes of sandwich composite materials reinforced with orthogonal z-pins. The compression modulus of the sandwich composite increases rapidly with the volume content of z-pins due to their high longitudinal stiffness, however acoustic emission monitoring and X-ray computed tomography reveal that some z-pins are damaged during elastic loading. The compression stress to induce core crushing is increased greatly by z-pinning (up to nearly 700%), although a large percentage of the z-pins fail close to the elastic stress limit by longitudinal splitting and/or kinking. The total absorbed compressive strain energy of the sandwich composite is also improved greatly by z-pinning (more than 600%) due to the z-pins resisting core crushing, even though they are severely damaged. The results and observations presented in this paper have implications on the mechanical modelling of sandwich materials reinforced with brittle z-pins.     

Damage detection of glass fiber reinforced composites using embedded PVA–carbon nanotube (CNT) fibers

Polyvinyl alcohol–carbon nanotube (PVA–CNT) fibers were embedded in glass fiber reinforced plastic composites and used as strain sensors for damage monitoring of the composite. Sensing of the structural integrity of the composite was made by the in situ measurement of the electrical resistance of the embedded PVA–CNT fiber during the mechanical tests. The multi-functional materials were tested in tensile progressive damage accumulation (PDA) tests. These tests aimed to seek the electrical response of untreated and pre-stretched PVA–CNT fibers with known level of progressively induced damage to the composite. The advantages and disadvantages of each PVA–CNT fiber used as a sensor are analyzed; the electrical resistance readings of the PVA–CNT fibers were correlated with known parameters that express the induced damage of the composite.