A Damage Mechanics Method for Predicting Strength in Composite Joints

Composite materials have been extensively used in the aerospace industry and also in various engineering structural designs due to the several advantages they offer. Nonetheless their applicability is still limited by the complexity of their analysis and their high cost. A particular interest for industry is the prediction of damage and ultimate failure of mechanically fastened joints. On aircraft, mechanically fastened joints are commonly used to attach composite components to main frames. The bolts within the multi-bolted connections are commonly subjected to two types of load, a bearing load in the bolt-hole contact area and a load that bypasses the bolt (bypass load). Currently, there are very few numerical models of bearing versus bypass loading and those that do exist do not perform a damage progression analysis within the composite. Furthermore, they do not consider factors such as friction, bolthole clearance and clamping forces. In an attempt to overcome this issue, a three-dimensional damage progression study in composite pinned and bolted joints subjected to bearing-bypass loads has been performed. In 'addition, a parametric study of well-known stress-based failure criteria and degradation models was carried out in order to assess their applicability for the case of bearing versus bypass loads. From this work, a new failure criteria and degradation method considering energy release is proposed. Furthermore, bearing versus bypass experimental testing of a single-lap pinned and bolted composite plate considering different joint parameters (clearance, clamping forces, and laminate lay-up) was performed. The damage model was verified against the experimental data obtained. The model and the experimental results showed good correlation. The numerical model was also validated with experimental data from literature. Some of the main contributions of the work presented herein are that it provides a more robust method, to predict joint behaviour under bearing versus bypass loading conditions, than previous approaches. Additionally, it provides guidance on the applicability of stress-based failure criteria to predict composite ultimate strength. The problem of obtaining accurate results using a damage mechanics approach is highlighted. Furthermore, it shows the effect that the different joint parameters have on the ultimate strength of a composite bolted joint subjected to bearing versus bypass loading.EThOS - Electronic Theses Online ServiceGBUnited Kingdo