Study On The Ultimate Strength Of Laminated Composites And The Minimization Of Delamination Stresses

By virtue of its excellent properties, such as the high specific strength and high specific modulus, anisotropy and designability, the Fiber Reinforced Plastics (FRP) has been widely used as structure materials in aircraft, space, marine and automobile, etc. Fiber metal hybrid laminate (FML) is a new class of composite material structure arisen in recent years. It consists of thin aluminum alloy sheets bonded together with fiber-reinforced epoxy prepreg. These laminates exhibit favourable characteristics, such as excellent impact properties, fire and corrosion behavior and fatigue properties. These advantages further facilitate the use of FML for primary structures in aerospace industry. In order to guarantee secure use of FRP or FML in aeronautical applications, their strength characteristic and evaluation is of great concern. For a loaded laminate, there are several failure modes among which delamination is a primary failure mode unique to composite laminates. For laminates prone to delaminating, the predicted ultimate failure strength will be overestimated if delamination is neglected. Thus, it is crucial to take the delamination into account when predicting the ultimate failure strength of laminated composites, and it is essential to minimize delamination tendency by optimization technology.Aiming at the problems mentioned above, the main contents and achievements in this paper are listed as follows:(1) A numerical method for predicting ultimate strength of composite laminates is proposed. The proposed method adopts FEM to conduct structural analysis and Tsai-Wu criterion is utilized to account in-plane failure of laminates. The progressive failure process is modeled through APDL incorporated in ANSYS and the ultimate failure strength of laminates is obtained. The failure characteristics of CF/EP quasi-isotropic laminates are discussed and the effect of loading direction and stacking sequence on the ultimate strength is investigated through experiments and numerical analyses.(2) The proposed method above considering only in-plane failure is improved by accounting delamination failure attributed to the interlaminar stresses at the free-edge additionally. The improved method is used to predict the onset strength of delamination and the ultimate strength of various lay-ups. The predicted results show excellent agreement with the experimental results available in literature. Thus, in comparison to the method considering only in-plane failure, the improved method can lead to a more comprehensive failure process. It is of great significance in further understanding the failure mechanism and exploiting laminated composites more safely and reliably.(3) Considering that delamination failure is mainly attributed to interlaminar normal stresses, an objective function is presented to minimize the interlaminar normal stresses at the free-edge. In practical engineering problems, optimization may deal with functions which are discontinuous or un-differentiable, highly nonlinear, or multiple local extreme. It makes the optimization problems difficult or even impossible to be solved by traditional methods which require at least the first derivative of the objective function with respect to the design variables. In view of this, the zero-order method (ZOM) incorporated in ANSYS which requires only the value of the objective function and the constraint functions is firstly adopted for optimization. ZOM can yield a satisfactory optimal solution. However, the solutions of ZOM exhibit sensitivity to the initial designs. In order to overcome this disadvantage, a technique of applying an evolution-based optimization algorithm PSO integrated with the general FE code ANSYS is developed for optimization. Examples dealing with the optimization of three different functions of interlaminar normal stresses are presented to demonstrate the feasibility and superiority of the proposed approach.(4) Aiming at the new class of composite structure FML, traditional CLT is modified and extended to nonlinear cases. Modified CLT and FEM are utilized to simulate the stress-strain behavior of FML under tensile loading. The simulated stress-strain curves agree well with the experimental results available in literature. Subsequently, ANSYS is utilized to perform the structural analysis of laminates under different loads, and the failure indices are obtained through failure criterions. The maximum and minimum failure index of FRP and FML have been found out respectively via altering the fiber orientations of prepreg layers. Based upon the optimization results, the strength behavior of FML and FRP are compared. The optimization results demonstrate that owing to the substituting of metal alloy sheet for prepreg layer, FML is of better capability to withstanding biaxial load and out of plane concentrating load (low-velocity impact load) than FRP.