Binjun Fei

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.

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.

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.

An average failure index method for the tensile strength prediction of composite adhesive-bonded π joints

An average failure index method based on accurate FEA was proposed for the tensile strength prediction of composite out-of-plane adhesive-bonded π joints. Based on the simple and independent maximum stress failure criterion, the failure index was introduced to characterize the degree of stress components close to their corresponding material strength. With a brief load transfer analysis, the weak fillers were prominent and further detailed discussion was performed. The maximum value among the average failure indices which were related with different stress components was filtrated to represent the failure strength of the critical surface, which is either the two curved upside surfaces or the bottom plane of the fillers for composite π joints. The tensile strength of three kinds of π joints with different material systems, configurations and lay-ups was predicted by the proposed method and corresponding experiments were conducted. Good agreements between the numerical and experimental results give evidence of the effectiveness of the proposed method. In contrast to the existed time-consuming strength prediction methods, the proposed method provides a capability of quickly assessing the failure of complex out-of-plane joints and is easy and convenient to be widely utilized in engineering.