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A new longitudinal failure criterion for unidirectional fibre reinforced (UD) composites is proposed, incorporating the contribution of local fibre misalignment to fibre failure. The failure function was established based on the quadratic polynomial expansion of stress components in the local coordinate system parallel to the misaligned fibres, with coefficients rationally determined based on the basic strength parameters of UD composites. The local stress components were used to determine the failure status associated with fibre compression, while the tensile load along fibres was assumed to have negligible influence on the development of the fibre misalignment. The predicted failure envelopes agreed well with the experimental data for different stress combinations, particularly in the regimes where the fibre failure was influenced by transverse stresses. This validates the predictive ability of the proposed criteria and highlights the importance of the contribution of matrix-failure-related stresses to the fibre-dominated failure behaviour of fibre reinforced composites. [ABSTRACT FROM AUTHOR]

The capacities of Z‐pinning in improving the bending performance of composite T‐joints were examined experimentally and numerically. It was found that Z‐pinning was not effective in enhancing the maximum bending load capacities of the T‐joints. Delamination at the interfaces of flange/filler, flange/flange, flange/skin, in the arc‐flange, as well as the fracture of the upper corner of the filler was observed. Among those failure patterns, the notorious delamination formed in the arc‐flange assumed the main responsibility of the disability of Z‐pinning in enhancing the maximum bending load capacity. Despite this, Z‐pinning was found to be highly effective in improving the residual plateau load‐carrying capacities of the T‐joints and the maximum bending displacement could also be dramatically increased, making the T‐joint maintain considerable bending load capacity under large deformation and preventing the T‐joint from catastrophe failure once the maximum bending load was attained. [ABSTRACT FROM AUTHOR]

A new periodic unit cell model was proposed to quantify the longitudinal tensile strength of laminates with different Z‐pin parameters. A modified three‐dimensional Hashin criterion was used to predict the failure of the laminate and Z‐pin, while maximum stress criterion was utilized to analyze the damage in the resin‐rich zone. Benchmark tests were carried out to verify the model, and finite element (FE) results agreed well with the experimental data. Hence, a parametric study was carried out to examine the influence of Z‐pin parameters on the longitudinal tensile strength and damage mode of unidirectional laminates. FE results show that the changing trend of the tensile strength against Z‐pin parameter is obviously more complicated than that of the modulus and is related to the change of damage mode or microstructure. [ABSTRACT FROM AUTHOR]

Experiments and simulations were carried out to study the tensile performance of unreinforced and Z‐pinned composite T‐joints with low Z‐pin density (Z‐pin spacing larger than 5 mm). It was found that the initial load drop of the Z‐pinned specimen was obviously smoother than that of the unpinned specimen, and the improvement to the ultimate strength of the T‐joint was over 45% by Z‐pinning. Besides, the tensile performance of the Z‐pinned T‐joint was significantly related to the distance of the Z‐pin to the symmetry plane of the T‐joint and Z‐pin diameter, while was independent of the column spacing. Regarding the numerical study, the proposed macro T‐joint model was found to be capable of successfully predicting the tensile performance of Z‐pinned and unreinforced T‐joints in terms of both load‐displacement response and damage mode. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation into the effects of missing fasteners on the mechanical characteristics of double-lap, multi-row composite bolted joints. A highly efficient explicit finite element model, which was validated effective and accurate by experiments, was developed and employed to conduct the virtual tests. Single-column and multi-column joints with various positions of missing fastener were considered. It is shown that the removal of fasteners can reduce the joint stiffness significantly, especially in joints with fewer columns or missing fasteners in the outside rows. The removal of fasteners can also cause considerable reductions in both the initial significant failure loads and ultimate loads of multi-column joints, while in single-column joints only the initial significant failure loads are influenced. Considering the load distribution, it is suggested that bolts in the same column as or in the adjacent column to the missing fastener experience a notable growth in load. Meanwhile, if a bolt bears more loads in the pristine joint, the larger changes in stiffness, ultimate strength, and load distribution may be obtained when it is lost. [ABSTRACT FROM AUTHOR]

• The different values of the coefficient F 12 under different stress states were determined by using the basic strength values. • The improved Tsai–Wu criterion can distinguish the failure modes of composite materials. • The improved Tsai–Wu criterion has better prediction ability than the original, especially under the triaxial stress state. Different failure modes are not explicitly accounted for in the Tsai–Wu failure criterion for composite materials, and the factor F 12 in the expression cannot be determined by the basic strength values of the materials. In view of these two shortcomings of the Tsai–Wu criterion, an improved criterion was proposed based on the reasonable assumption that composites exhibit infinite strength under pure hydrostatic pressure. Some coefficients in the quadratic tensor expression of the Tsai–Wu failure criterion are redetermined using the basic strength value of the material, including the coefficient F 12 , under four different stress states (fiber tension and compression, matrix tension and compression). The reconstructed Tsai–Wu failure criterion can distinguish the failure modes based on different coefficient values under different stress states. In the progressive damage analysis of composite materials, the stiffness can be reduced in different manners based on each failure mode. Experimental verification for different kinds of unidirectional composites under various stress states was conducted, demonstrating that the improved Tsai–Wu failure criterion has a better prediction ability and accuracy than the original criterion. [ABSTRACT FROM AUTHOR]

To examine the tensile properties of T-joints reinforced by traditional lock stitching with stitch spacing not smaller than 5 mm, tensile experiments are conducted and the reinforcing effect of stitching and Z-pinning is compared. Results show that both the location of the innermost column of stitching and stitch spacing have a remarkable influence on the tensile property of stitched T-joints. The maximum average peak load achieved is 44.2% by stitching. Although the diameter of the stitch is 24% thinner than that of a thick Z-pin, the maximum load capacity of stitched T-joints is 14.2% greater than that of Z-pinned T-joints. [Display omitted] • Both the location of the innermost column and column spacing of stitching have a remarkable influence on T-joints. • The maximum average peak load achieved was 44.2%. • Stitching is superior to Z-pinning in terms of improving initial load drop, peak load, and stabilizing crack propagation. [ABSTRACT FROM AUTHOR]