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Studies in fatigue can be summarized into two stages, fatigue crack initiation and crack propagation. Fatigue damage may increase the risk of failure under cyclic load. Energy dissipation, termed damping, occurs in engineering metals and is a function of the cyclic loading history. Damping behaviour of materials has been estimated using many different experimental techniques, and parameters i.e. the loss factor vs. strain amplitude, frequency range, etc. However, micro-structural changes in the form of fatigue damage are also contributors to damping in engineering materials. In order to measure energy dissipation, a damping monitoring method has been used. Under a constant cyclic load up to the point of fatigue crack initiation, the effects of fatigue on damping factor were studied for 6061-T6511 aluminium alloy. In the experiments, the stress levels were below yield point, 50% and 70% of ultimate strength. Experimental results showed that the damping factor changes with the number of fatigue cycles. Percentage increase in damping energy was calculated using experimental data.

Hot Isostatic Pressing (HIP) is used extensively for consolidation of powers and particulates and near net shape fabrication of powder metallurgy products. The last few years have seen significant refinement of the HIP model proposed by Ashby and co-workers. It has been acknowledged by the developers of the maps that the accuracy of the predictions is limited by the precision with which material property data, used in the HIP Maps program are known. Even for materials processed similarly, there are, typically, variations in the values of material properties due to the inherent inaccuracies in the methods used to determine these properties. Such variations significantly affect the accuracy of the HIP model predictions. The objective of the present investigation is to determine the effect of variations in the values of material properties on the HIP model predictions by performing a sensitivity analysis with respect to material property data. The values for material property data used in the HIP Maps program are typically determined in one of the following ways: (a) experimental measurements at consolidation conditions; (b) estimates based on data on other materials; (c) through back calculations using the experimentally determined HIP data; and (d) by extrapolations to get appropriate materialmore » property values for the consolidation conditions. Methods other than (a) could result in the determination of a material property value which is highly unrealistic. The present study discusses this with respect to a recent experimental study [10] on TiAl and resolves the discrepancy by using PLC material parameters determined experimentally for conditions close to the actual HIP process conditions.« less

Substantial improvements have been reported in high temperature strength and creep resistance, and room temperature fracture toughness of molybdenum disilicide (MoSi2) reinforced with ductile or brittle reinforcements. The influence of Hot Isostatic Pressing (HIP) process parameters on the mechanical properties of MoSi2 based alloys was studied. Monolithic MoSi2 powder and MoSi2 powder blended with either niobium powder or silicon carbide whisker reinforcements were consolidated by HIP at 1200 − 1400°C, 207 MPa, and 1 - 4 hrs. The HIP'ed compacts were characterized for compression strength and creep resistance at 1100-1300°C. Fracture toughness was measured on single edge notched rectangular specimens at room temperature. Mechanical properties were correlated with post-HIP microstructural features.

In addition to excellent biocompatibility and corrosion resistance, the unique flow behavior of NiTi pseudo-elastic alloys renders them ideally suited for several medical device applications utilizing minimally invasive technologies. For example, self-expanding stents are indicated chronic therapies for treatment of a variety of vascular diseases. Long-term device integrity is thus critical with the FDA recommending a minimum 10-year life. Historically, the majority of published fatigue data on NiTi alloys have been generated under strain-controlled conditions using wire or single-diamond cell samples designed to replicate the behavior of the repetitive unit within a stent. Resulting data are then presented on modified Goodman or Soderberg diagrams in order to define regions of acceptable device life. Consistent with their unusual monotonic flow characteristics these alloys are found to exhibit unique cyclic behavior in fatigue: published data clearly demonstrate that increasing mean strain often increases high-cycle fatigue life. This anomalous behavior is generally attributed to any of a number of possible microstructural or mechanical effects associated with the reversible stress-assisted austenite-to-martensite phase transformation. Although the majority of published work has focused on these macroscopic properties in order to characterize and adequately define device life, various efforts are currently underway to better understand the more fundamental metallurgical and mechanical aspects controlling fatigue life in this alloy. Such studies include crack initiation and propagation rates, determining the roles of absolute and relative inherent flow behavior of the transforming structures, mechanical instabilities resulting from the heterogeneous nature of the transformation, latent heat effects during cyclic transformation, as well as the effects of testing frequency, crystallographic texture, grain-shape anisotropy, and stress-state on fatigue life. A review of existing literature delineating these behaviors and current efforts is presented in light of known beneficial effects of transformation plasticity and toughening in enhancing fatigue and fracture properties of a variety of metallic and nonmetallic materials.