Fracture prediction in metal sheets

Abstract

One of the most important failure modes of thin-walled structures is fracture. Fracture is predominantly tensile in nature and, in most part, is operated by the physical mechanisms of void nucleation, growth, and linkage. For ductile sheet, fracture is preceded by necking. Prediction of necking which limits sheet metal formability is well established and has been developed over the past several decades. However, an in-depth understanding of the mechanical process inside the neck leading to sheet metal fracture is lacking. This is true for both static and high intensity, short duration loads. Furthermore, there is an ever increasing need to raise the safety envelope of existing protective structures against localized extreme loading. The present thesis addresses four parts of the many outstanding issues in sheet metal fracture. In the first part, the new Bao-Wierzbicki (BW) fracture criterion formulated in terms of the accumulated equivalent plastic strain with the stress triaxiality as a weighting function is considered. Using the equations of plane stress von-Mises plasticity and the strain-to-stress mapping procedure, the BW fracture criterion is transformed to the spaces of the principal tensile strains and stresses in a sheet and compared with experimental results for various materials. An extensive comparative study of the most widely used fracture criteria is then conducted.

(cont.) The applicability and expected errors of those criteria are investigated. In the second part, calibration methods for the determination of the stress-strain curve after necking and critical damage parameters are discussed. Most importantly, a simple method of calibrating for fracture from a round or flat specimen tensile test is developed ans shown to be valid in a wide range of stress triaxiality. In the third part, experimental, numerical, and analytical studies on the deformation and fracture of thin plates subjected to localized static and impulsive loadings are conducted. A new method of constructing a Fracture Forming Limit Diagram (FFLD), which is understood as the locus of fracture strain in the principal strain space, is proposed and confirmed by the classical problem of punch indentation in thin plates. Moreover, it is demonstrated that the present fracture criterion captures the formation and propagation of cracks in thin plates. In the fourth part, extensive parametric studies on the transient responses and fracture of various core arrangements in sandwich structures under explosive loading are carried out. A new Blast Resistant Adaptive Sandwich (BRAS) structure is proposed, which substantially increases fracture resistance during static and dynamic loading events.

(cont.) In particular, the threshold impulse to initial fracture of the optimized BRAS is 1.8 times higher than the optimized conventional sandwich structure (USDH). Also the maximum reduction of the ruptured area with the optimized BRAS is 90%, as compared to the optimized USDH.