Niti Shape Memory Alloy Fatigue Fracture Properties

With experimental results, finite element method (FEM) was used to analyze the fatigue and fracture behavior of superelastic NiTi shape memory alloys (SMAs). Ferdinando Auricchio's phenomenological model based on extended plasticity theories was used in the FEM analysis. The main conclusions are as follows:1. A set of uniaxial pull-pull experiments of NiTi wires was performed under different temperatures. The input parameters of Auricchio's model are obtained and calibrated. The modelling results are in good agreement with the experimental results.2. The martensitic transformation fields at the notch of specimen without pre-cracks and that at the crack tips of specimens with different crack length were studied. And the transformation field at room temperature under different load levels and the temperature influence on the transformation field were also analyzed. The presence of pre-cracks increases the volume fraction of the martensite and the transformation zone evidently, but the transformation zone is centralized around the crack tip. The specimen with larger crack length has larger volume fraction of the martensite and larger transformation zone. The loads deducing the transformation increase with the increasing temperature for the specimen without pre-cracks, while those for the specimens with pre-cracks almost do not change. The influence of the temperature on the transformation is not as obvious as the pre-cracks. The martensitic transformation zones of both specimens decrease with the increasing temperature. The martensitic transformation is dependent not only on the load level but also on the loading path.3. FEM analysis was used to quantitatively study the stress distribution at the cross sections of the NiTi wires and the influence of the TiC inclusions on the fracture behaviour under bending load. A primary explanation was given to the bending-rotation fatigue (BRF) experimental results. The position of the maximum stress at the cross sections is dependent on the position of the inclusions, the load level and the loading path. The maximum stress is not always at thesurface of the specimen without inclusion. The maximum stress increases with the increasing distance from the inclusion to the neutral axis. When the inclusion is at the specimen's surface, the maximum stress is greatly increased and may induce the fatigue crack to form.4. A pre-twist was added to analyze the mechanical behaviour of NiTi wire under bending load. A small angle of twist can change greatly the stress distribution at the cross section of the specimen in the BRF experiment. Under the same bending load, stress distribution is different from that under pure bending.5. In order to study the mechanical behaviour of NiTi alloys under multiaxial loading conditions, the extension-torsion cyclic loading experiments were performed on a thin-wall tube. The transformation stresses under pure torsion experiments decrease with the increasing loading cycles. Keeping the torsion angle constant, the normal stress increases with the increasing tensile displacement, while the shear stress decreases greatly. Then unloading the tensile displacement, the shear stress cannot completely come back to its original level. The equivalent stress-strain curve under the biaxial loading state is not smooth.6. Fracture toughness experiments were performed on a domestic NiTi superelastic material and the KIC was obtained.