Finite Element Analysis On Bending Fatigue And Damage Evolution Of Three-dimensional Four-step Braided Composite Materials

In three-dimensional four-step braided preform, the yarns are interwoven and integrated to form the spatial network, enabling a versatile designability of this structure. The structures with a great number of shapes, including the variable cross section can be produced using four-step braiding technique. The three-dimensional four-step braided composite has the advantages of near-net-shape manufacturing, good structural stability, high inter-laminar shear strength and impact damage tolerance, leading to the wide application in engineering industry. Fatigue life is a key concern in the engineering applications, especially in the load bearing structural parts; therefore, it is of great importance that more attention is paid to fatigue analysis. In this paper, based on the experimental results, the three-point bending fatigue of three-dimensional four-step braided composite was studied by means of a unit-cell model and a meso-scale model in finite element method with an aim of analyzing its mechanical response, structural effect and damage mechanism. The detailed studies are described below:1. The static three-point bending test was conducted to obtain the ultimate bending strength, bending modulus, load-deflection curves and damage morphologies. The failure mechanism and structural effect were investigated. The stress level of fatigue loading was determined according to the ultimate bending strength. It was found that the three-dimensional four-step braided composite had distinct bending rigidity, and the damage tends to occur at the middle region of specimen. The damage modes consist of debonding between fiber and resin, fiber pull-out and fiber breakage. No delamination was observed.2. The three-point bending fatigue test was carried out at different stress levels. The typical’three stages’progressive damage, fatigue response and damage mechanisms were found via load-deflection curves and the stiffness degradation curves. The fatigue life span is proved to be inversely proportional to the stress level. Under different stress levels, the load-deflection curves and stiffness degradation curves have the typical’three stages’features.3. Based on the braiding technique and the spatial pattern of braiding yarns, the three-dimensional four-step braided composite was divided into three kinds of unit cells (RUCs), i.e., interior RUCs, surface RUCs and corner RUCs. The RUCs volume proportion, yarn packing factor, fiber volume fraction and stiffness matrix of the three RUCs were obtained on the basis of yarn distributions and braiding angles. The surface and corner RUCs showed high bending resistance when subjected to loading.4. The finite element analysis was performed using three RUCs model under static bending loading. The results were compared with the inner RUCs method. The damage mechanism of the three-dimensional four-step braided composite under static bending loading was investigated. By comparison, the three RUCs model was verified to be more consistent with experimental results than the inner RUCs method. The bending performance under static bending loading can be accurately analyzed using three RUCs model and this model can be further applied in bending fatigue analyses.5. The stiffness and strength degradation model was established from the bending stiffness damage equation. A user-defined material (UMAT) subroutine which characterizes the stiffness matrix and fatigue damage evolution of the three-dimensional braided composite was developed. The bending fatigue modellings were carried out at three stress levels. The fatigue damage mechanism was discussed by observing the stress and strain concentration areas and damage propagation morphologies. Stress concentration was found at the middle of specimen and the restricted regions of bottom surface. The strain changed dramatically during cyclic loading, especially in the middle to late periods of loading. The stiffness degradation and deflection change conformed to’three stages’ features, which demonstrated the close relation between the failure mode and loading cycle.6. The small size and full size meso-scale models were set up by considering the resin impregnation and recalculation of yarn properties. The load bearing of yarns and resins, the debonding between the two and the damage morphologies were analyzed qualitatively. The structural effect on fatigue property was discussed. From the stress distribution, the stress concentration region of yarn was observed to be similar to that of resin. The damage appeared at the central loading area and the restricted areas were where the supporting rollers are. The interface crack was caused by the stress difference. The crack propagation position was affected by the yarn damage condition.7. The finite element analysis of full size meso-scale model was conducted on the80%stress level. The stress concentration and energy conversion were studied. The facture morphologies were compared with the experimental results to reveal the structural effect on the mechanical degradation of three-dimensional four-step braided composite. It was discovered that during fatigue loading, the stress concentration formed a unique triangle area and changed with the structure, braiding angle and thickness due to the spatial entanglement of yarns. The morphologies on the surface and bottom differed greatly because of the braiding angle and loading pattern.To sum up, this paper reports the three-point bending deformation and damage of4-step3-D braided composite. By establishing the unit-cell model, the fatigue life was effectively predicted. The structural effect on the mechanical degradation was investigated using meso-scale models. The results were compared with experimental to verify the effectiveness of models. It was proved that the methodology developed in this paper can be further applied in other structural composite, and it can provide the basis of improving the fatigue resistance and increasing the fatigue life of composites.