Libin Zhao

Mode I delamination growth of multidirectional composite laminates under fatigue loading

Fatigue delamination growth of multidirectional carbon/bismaleimide composites was investigated to disclose the specific role of ply orientations. Both fatigue crack growth rate and threshold values of strain energy release rate in Mode I were studied by double cantilever beam test. For specimens with rising delamination resistance effect, the fatigue crack growth rate was found to be dependent on normalized interlaminar energy, GI/GIC, rather than GI. The experimental data also show that the fatigue threshold Gth is almost proportional to the corresponding GIC, that is Gth /GIC is constant and hardly affected by the midplane-adjacent fiber orientation. Furthermore, fractographs revealed that the rising delamination resistance is dominantly aroused by fiber bridging and intra-ply fracture.Fatigue delamination growth of multidirectional carbon/bismaleimide composites was investigated to disclose the specific role of ply orientations. Both fatigue crack growth rate and threshold values of strain energy release rate in Mode I were studied by double cantilever beam test. For specimens with rising delamination resistance effect, the fatigue crack growth rate was found to be dependent on normalized interlaminar energy, GI/GIC, rather than GI. The experimental data also show that the fatigue threshold Gth is almost proportional to the corresponding GIC, that is Gth /GIC is constant and hardly affected by the midplane-adjacent fiber orientation. Furthermore, fractographs revealed that the rising delamination resistance is dominantly aroused by fiber bridging and intra-ply fracture.

A micromechanics-based degradation model for composite progressive damage analysis:

A new material degradation model only with fundamental material properties required is proposed for composite progressive damage analysis based on micromechanics. For different failure modes, the effects of fiber and/or matrix damage on the composite material properties are explored, from which the material degradation factors for these failure modes are deduced. The material degradation model is then implemented for progressive damage analyses, using user subroutines in the commercial code ABAQUS®, accompanying with a modified Hashin type failure criterion and finite element models for six commonly used double-lap composite bolted joints with various layups, geometry dimensions, and fasteners. The numerical predictions of failure loads, failure patterns, and load–displacement curves are compared with results obtained from static tests and further ultrasonic C-scan detection. Good agreements between numerical failure predictions and experimental outcomes indicate the effectiveness and suitability of the proposed model for progressive damage analyses of composite bolted joints.

Strength prediction of composite π joint under bending load and study of geometric and material variations effects

This article concentrates on the design of composite π joint, which is an effective connector for integrated composite structures in aerospace structures. A brief review of research on out-of-plane joints is first presented to current practice towards the design of such π joints. Progressive damage models are established to trace failure analysis from damage onset to ultimate collapse of joints, which highlight the failure mechanism and internal mechanical behaviours. Four kinds of failure criteria are used firstly, and after comparing the numerical prediction results with the experimental results, one failure criterion is chosen for the simulation of this joint. Influences of structural and geometric parameters such as structural configuration, overlaminate material, ply angle, fillet radius and upstanding/horizontal leg length of overlaminate on flexural resistance ability are studied to provide ‘optimal’ design schemes. Moment–displacement graphs from the mechanical experiments are presented and physical failure phenomena of the joint under bending load are depicted. The good agreement between the predictions and experimental results gives evidence of the efficiency of numerical analysis.

Modified maximum stress failure criterion for composite π joints

Composites are being widely used because of their high strength and stiffness, low density and high formability for creating complex shapes. An all-composite π joint is a structural connector known for its ability to reduce the weight and assembly cost while retaining a good load-carrying capability. Based on the material characteristics of unidirectional fiber composites used in composite π joints, a modified maximum stress failure criterion, which is able to assess damage onset, propagation and final failure, is presented for unidirectional fiber composites. The stiffness and strength of four types of composite π joints under tensile and bending loads are simulated by progressive damage models, involving a finite element analysis, failure criteria and a material degradation model. Numerical results from the application of this criterion in nonlinear element analysis show good agreement with experimental outcomes.

Three-dimensional progressive damage models for cohesively bonded composite π joint

Integrated composite structures can significantly reduce the assembling cost and improve the performance of the aircraft. All-composite π joint, which is a connector of integrated composite structures, has good ability to reduce the weight and assembly cost while retaining good load-carrying capability. To disclose the mechanics behavior of the out-of-plane, complex all-composite joint, the progressive damage methods are investigated for its good ability to trace the damage onset, damage propagation, up to collapse of composite structures. A progressive damage model is established using a new modified maximum stress failure criterion and a material degradation model developed from Chang’s model. The material degradation model takes account of the matrix crack direction, which is predicted by the modified maximum stress-failure criterion. Meanwhile, five other progressive damage models are established and applied to static tensile analysis to study the mechanics behavior of the joint. To evaluate the prediction of the six progressive damage models in joint initial failure, damage propagation, joint strength and stiffness, the numerical results of the six progressive damage models are compared with the experiment results. The effects of failure criteria and material degradation model on the prediction results are discussed in detail. By the comparison, it can be concluded that the progressive damage model consisting of the modified maximum failure criteria and its corresponding material degradation model is considered as the best one for the analysis of the all-composite π joints, because its initial failure prediction, failure process and ultimate failure prediction agree well with the experiment result.

Influence of end distances on the failure of composite bolted joints

To explore the physical effect of end distances on mechanical behaviors of composite bolted joints, series of single-bolt composite joints designed with different end distances were tested. In conjunction with the experimental work, a numerical progressive damage method is introduced to trace the damage process from the onset and propagation up to ultimate failure of the joints. A group of material degradation factors is presented by a trial-and-error method to establish three-dimensional progressive damage models of the bolted joints. The progressive damage analyses show that the predicted load–displacement curves, failure loads, and failure patterns of bolted joints with different end distances are in good agreements with the related experimental outcomes. From the experimental and numerical results, it follows that the cleavage failure gradually switches to the bearing failure with the increasing ratio of E/D ranged from about 2 to 4. An economic and suitable ratio of E/D ≈ 3 is provided for the bolted joints made of X850 carbon/epoxy composites with balanced and symmetric layups [45/0/−45/0/90/0/45/0/−45/0]s. The understanding of the effect of E/D on the mechanical behaviors including the strength, stiffness, and failure patterns of single-bolt joints is strengthened.

Determination method of stress concentration relief factors for failure prediction of composite multi-bolt joints:

Stress concentration relief factor, which reflects the linear relationship between the stress concentration factors of composite joint and those of elastic isotropic joint, was proposed by Hart-Smith for the failure prediction of composite multi-bolt joints. It is deemed to be determined by experiments from either unloaded- or loaded-hole laminates and is suitable for investigations of bypass and bearing stress concentration factors. To examine this viewpoint, in this paper, the stress concentration relief factor is determined by tensile tests of open-, filled- and loaded-hole laminates failing in tensile mode, respectively. It can be found that the loaded-hole has obviously high-stress concentration level, while the filled-hole has slightly higher stress concentration than that of open-hole. Detailed 3D-FE analyses are performed to disclose the stress concentration mechanisms of three hole-laminates. Various stress concentration relief factors are combined to calculate bearing and bypass stress concentration factors, which provide the failure envelope for the strength prediction of composite joints. Moreover, both conventional and modified failure envelopes are discussed. Experimental results of two-, three- and four-bolt joints are provided to investigate the suitability of different failure envelopes. It follows that the stress concentration relief factor for bearing stress concentration factor needs to be determined by a loaded-hole laminate, while that for bypass stress concentration factor can be determined by each of open-, filled- and loaded-hole laminates.

Investigation on characteristic length testing methods for failure prediction of composite multi-bolt joints

Characteristic lengths (CLs) are key factors in the characteristic curve method (CCM), which is widely used in engineering to predict the failure of composite multi-bolt joints. They directly affect the accuracy of predicting the joint failure. To identify suitable CL testing methods, six schemes have been designed and tested, in which two tensile CL specimens (open- and filled-hole laminates) and three compressive CL specimens (semi-circular notch laminates, pin-loaded hole laminates and single-bolt joints) have been compiled. Additionally, 3D FE analyses on the stress concentration have been performed and a discussion of the unit failure index curves of different specimens has been carried out to explore their load-carrying mechanism and determine why they result in different CLs. Failure predictions of composite multi-bolt joints by using the CCM with different CL testing schemes have been compared and evaluated using related static tensile experimental results. Filled-hole laminates for the tensile CL and single-bolt joints for the compressive CL are recommended because they best approximate the actual structure and obtain accurate failure predictions for composite multi-bolt joints.

A Numerical Method for Simulating the Microscopic Damage Evolution in Composites Under Uniaxial Transverse Tension

In this paper, a new numerical method that combines a surface-based cohesive model and extended finite element method (XFEM) without predefining the crack paths is presented to simulate the microscopic damage evolution in composites under uniaxial transverse tension. The proposed method is verified to accurately capture the crack kinking into the matrix after fiber/matrix debonding. A statistical representative volume element (SRVE) under periodic boundary conditions is used to approximate the microstructure of the composites. The interface parameters of the cohesive models are investigated, in which the initial interface stiffness has a great effect on the predictions of the fiber/matrix debonding. The detailed debonding states of SRVE with strong and weak interfaces are compared based on the surface-based and element-based cohesive models. The mechanism of damage in composites under transverse tension is described as the appearance of the interface cracks and their induced matrix micro-cracking, both of which coalesce into transversal macro-cracks. Good agreement is found between the predictions of the model and the in situ experimental observations, demonstrating the efficiency of the presented model for simulating the microscopic damage evolution in composites.

Influence of π overlaminates on the mechanical behavior of all-composite adhesively bonded π joints

An all-composite out-of-plane adhesively bonded π joint is an effective structural connector for integrated aircraft structures. Its application can not only enable both the weight and assembly cost benefits, but also enhance the load-carrying capability. In composite π joints, the π shaped overlaminate is a significant kernel and thus attracts more attention of designers. With progressive damage analyses, the influences of π overlaminates with different configurations, lay-ups and geometric dimensions on the mechanical behaviors of composite π joints are obtained. The failure mechanisms and final failure loads of composite π joints with different π overlaminates are compared to provide more knowledge for the optimal design of such joints. Final failure loads stemmed from the static tensile experiments and physical failure phenomena of three types of π joints are presented. The good agreements between the numerical and experimental results not only validate the effectiveness of the numerical models, but also give evidence of the predicted tendency of strength improvement for composite π joints.