Z-pin Diameter Research Articles

Improving Mode II delamination resistance of curved CFRP laminates by a Pre-Hole Z-pinning (PHZ) process

This paper presents an experimental investigation of the Mode II delamination resistance of curved CFRP laminates reinforced with Z-pins. A Pre-Hole Z-pinning (PHZ) process is developed to reduce the in-plane damage of the Z-pinned laminates. The microstructural observation of the Z-pinned laminate specimens indicates that the PHZ process can effectively decrease the defects including the Z-pin offset angle and the area of eyelet zone. The influences of the volume fraction and diameter of Z-pin on the fracture toughness and the delamination crack growth rate of the specimens under End Notch Flexure (ENF) loading are then determined experimentally. The test results show that Z-pin increases the interlayer stiffness of the laminate. The delamination crack growth rate is reduced with the increase of Z-pin diameter and volume fraction, and a reduction up to 40% is achieved compared with the specimens without pins. Furthermore, the Mode II fracture toughness is significantly improved with the increase of Z-pin volume fraction. When Z-pin volume fraction increases by 1%, the achieved fracture toughness is about 200% higher compared to the unpinned laminates.

Tuning interlaminar fracture toughness of fine z-pin reinforced polymer composite

This paper aims to improve the interlaminar fracture toughness of carbon fiber reinforced polymer (CFRP) composites by implanting fine z-pins with the minimum damage on in-plane fibers. Z-pins with diameters as small as 0.1 mm and 0.2 mm were prepared by using carbon fiber tows with different mechanical properties. The effect of the mechanical property of carbon fiber pin on the interlaminar fracture toughness of composite laminate was investigated to reveal the enhancement mechanism. The results show that fine z-pins significantly improve the interlaminar fracture toughness and simultaneously are favorable for maintaining high retentions of in-plane mechanical properties of composite laminate. Compared with control sample, the propagation GIC values of CFRPs implanted with 0.2 mm and 0.1 mm CCF800 z-pins increase by 276 % and 541 % respectively at a low pin volume fraction of 0.16 vol%. Both z-pin pull-out and z-pin fracture failure behaviors can be observed for these fine z-pin reinforced composites. The higher tensile properties of z-pins and better transverse shear resistance tend to result in the failure of z-pin pull-out. With the decrease of z-pin diameter, the probability of z-pin fracture failure becomes greater, and correspondingly stronger pinning effect and larger failure load are achieved.

Tensile properties of Z-pinned composite laminates with low damage insertion technique

In this paper, the effects of Z-pin diameter and Z-pin length on the tensile properties of carbon fiber reinforced laminates were studied. A new Z-pin insertion technique which causes less microdamage damage than the original method was developed. Experimental testing reveals that the in-plane tensile strength of Z-pin specimens is 96.98%-105.32% of the control group. The best performance of in-plane tensile properties is TF18 group which pinned by full-depth Φ0.18mm diameter. The tensile strength increases by 5.32%, and the Young’s modulus increases by 4.71% relative to the control group. Compared with the group of large diameter Z-pin, the in-plane performance of the ultra-fine diameter group is better due to the less in-plane damage. In the half-depth group, delamination occurred between the middle Z-pin sublayer and the upper and lower layers, thus reducing the in-plane tensile property. The Poisson’s ratio increases in both full depth and half depth after Z-pinned and it is only related to insertion depth.

A numerical study of the influence of Z-pin parameters on the elastic properties of laminates

A finite element model with periodic boundary conditions was developed to investigate the influence of different Z-pin parameters including diameter, spacing, and insertion angle of Z-pin on the elastic properties of composite laminates. Benchmark tests were carried out to verify the FE model and a series of parametric analyses were subsequently performed. In general, all the elastic moduli, excluding the through-thickness modulus ( Ez), decreased while Ez increased nonlinearly with increasing Z-pin diameter and decreasing spacing. The reduction of Ey (transverse modulus) was approximately 40% of that of Ex (longitudinal modulus), while the reduction of Gxy is similar to that of Ex. Besides, Gxz and Gyz were reduced by approximately half of the reduction of Gxy. Although the impact of insertion angle was obvious on Ez, it was negligible on the other five moduli.

Interlaminar Shear Properties of Z-Pinned Carbon Fiber Reinforced Aluminum Matrix Composites by Short-Beam Shear Test.

This paper presents the effect of through-thickness reinforcement by steel z-pins on the interlaminar shear properties and strengthening mechanisms of carbon fiber reinforced aluminum matrix composites (Cf/Al) with a short beam shear test method. Microstructural analysis reveals that z-pins cause minor microstructural damage including to fiber waviness and aluminum-rich regions, and interface reaction causes a strong interface between the stainless steel pin and the aluminum matrix. Z-pinned Cf/Al composites show reduced apparent interlaminar shear strength due to a change in the failure mode compared to unpinned specimens. The changed failure mode could result from decreased flexural strength due to microstructural damage as well as increased actual interlaminar shear strength. Fracture work is improved significantly with a z-pin diameter. The strong interface allows the deformation resistance of the steel pin to contribute to the crack bridging forces, which greatly enhances the interlaminar shear properties.

Strengthening and toughening of 2D C/SiC z-pinned joint via fiber bridging mechanism

Shear properties of 2D C/SiC z-pinned joint were aimed to improve through z-pin microstructural features optimization. Results demonstrated that joint shear properties could be effectively improved through z-pin density, diameter and ply orientation adjustment. As 2D C/SiC z-pin density increased by approximately 0.1g/cm3, joint strength increased linearly approximately 18%. As z-pin diameter increased from 3.8mm to 4.0mm, joint strength increased approximately 10.1% and 6.9% from 4.0mm to 4.3mm. When z-pin ply orientation changed from 0/90 to +45/−45, corresponding joint strength increased approximately 10.9%. Shear rupture of 2D C/SiC z-pin was the only failure mode of corresponding joint, whereas the failure was competing result of shear and bending damages in the z-pin. Corresponding strengthening and toughening mechanisms were related to SiC matrix cracking and carbon fiber bridging.

Comparative study of the mode I and mode II delamination fatigue properties of z-pinned aircraft composites

This paper compares improvements to the mode I and mode II delamination fatigue resistance of an aerospace composite material achieved by z-pin reinforcement. Mode I (cyclic crack opening) and mode II (cyclic crack sliding) interlaminar fatigue tests were performed on a carbon fibre reinforced epoxy composite reinforced in the through-thickness direction with different volume contents and diameters of z-pins. Paris curves obtained from displacement-controlled fatigue tests reveal that z-pins are more effective at resisting the initiation and growth of delamination cracks under mode I than mode II conditions. Both the mode I and II fatigue resistance increase with the z-pin content due to the formation of a large-scale extrinsic crack bridging toughening zone, although fatigue strengthening is greater for mode I. Improvements to the mode I and mode II delamination fatigue resistance are also dependent on the z-pin diameter. Similarities and differences in the mode I and mode II fatigue properties are related to the fatigue strengthening mechanisms induced by z-pins for the two load conditions.

Quasi-static mode II fracture tests and simulations of Z-pinned woven composites

Experimental results from quasi-static mode II ENF (end notch flexure) fracture tests for Z-pinned woven composites are used to investigate the influence of Z-pin density and Z-pin diameter on Mode II fracture toughness. The experimental results show that the 2% Z-pin composite and the 3% Z-pin composite have larger fracture toughness than the 1% Z-pin composite and a composite without Z-pins. Crack propagation changes from unstable propagation to stable propagation (crack arrest) as the Z-pin density increases. The additional fracture toughness provided by Z-pins prevents the primary crack to further propagate, enabling secondary and tertiary cracks to consume additional energy. While the primary crack is held from propagating by the frictional forces provided by Z-pins, the matrix in the composite is driven into the plastic state and enables the composite to absorb more energy. For numerical simulations, a discrete cohesive zone model (DCZM) is used to perform simulations. The simulations in conjunction with experimental results provide critical fracture toughness values.

Delamination Fatigue Properties of Z-Pinned Composites

This paper presents an experimental investigation into the mode I interlaminar fatigue resistance of carbon fibre-epoxy laminate reinforced in the through-thickness direction with z-pins. The effects of the volume content, diameter and length of z-pins on the interlaminar toughness, fatigue resistance and crack bridging toughening mechanisms are determined. The delamination growth rate also slowed when the volume content or length of the z-pins was increased or the z-pin diameter was reduced.

Improving the mode I delamination fatigue resistance of composites using z-pins

Abstract This paper presents an experimental investigation into the mode I interlaminar fracture toughness and fatigue resistance of carbon fibre–epoxy laminate reinforced in the through-thickness direction with z-pins. The effects of the volume content, diameter and length of z-pins on the interlaminar toughness, fatigue resistance, and crack bridging toughening mechanisms are determined. The delamination crack growth rate under cyclic interlaminar loading slowed rapidly with increasing z-pin content up to a limiting concentration (∼2% by volume), above which no further improvement to the fatigue resistance was achieved due to a transition in the fatigue cracking process. The delamination growth rate also slowed when the z-pin diameter was reduced or the z-pin length was increased. The dependence of the fracture toughness and fatigue resistance on the volume content, diameter and length of the z-pins is related to the crack bridging toughening processes induced by the z-pins.

Modeling and predicting the compression strength limiting mechanisms in Z-pinned textile composites

Motivated by experimental results on Z-pinned plain weave glass fiber textile composites that show kink banding of fiber tows to be a strength limiting mechanism of failure in compression, computational results are presented for the effects of Z-pin diameter and Z-pin density on compression strength. Distortion to the textile fiber tows introduced by the insertion of Z-pins is found to be the dominant cause for initiating kink bands while the type of bond between the Z-pin and the surrounding matrix is found to influence the post-kinking response. When the Z-pin diameter remains unchanged, the composite strength decreases as the Z-pin density increases, while, when the Z-pin density is fixed, the composite strength decreases as the Z-pin diameter decreases in agreement with experimental observations.

Influence of Compression and Shear on the Strength of Composite Laminates with Z-Pinned Reinforcement

The influence of compression and shear loads on the strength of composite laminates with z-pins is evaluated parametrically using a 2D Finite Element Code (FLASH) based on Cosserat couple stress theory. Meshes were generated for three unique combinations of z-pin diameter and density. A laminated plate theory analysis was performed on several layups to determine the bi-axial stresses in the zero degree plies. These stresses, in turn, were used to determine the magnitude of the relative load steps prescribed in the FLASH analyses. Results indicated that increasing pin density was more detrimental to in-plane compression strength than increasing pin diameter. Compression strengths of lamina without z-pins agreed well with a closed form expression derived by Budiansky and Fleck. FLASH results for lamina with z-pins were consistent with the closed form results, and FLASH results without z-pins, if the initial fiber waviness due to z-pin insertion was added to the fiber waviness in the material to yield a total misalignment. Addition of 10% shear to the compression loading significantly reduced the lamina strength compared to pure compression loading. Addition of 50% shear to the compression indicated shear yielding rather than kink band formation as the likely failure mode. Two different stiffener reinforced skin configurations with z-pins, one quasi-isotropic and one orthotropic, were also analyzed. Six unique loading cases ranging from pure compression to compression plus 50% shear were analyzed assuming material fiber waviness misalignment angles of 0, 1, and 2 degrees. Compression strength decreased with increased shear loading for both configurations, with the quasi-isotropic configuration yielding lower strengths than the orthotropic configuration.