Recent advances in friction-stir welding : Process, weldment structure and properties

Nitinol, a near equiatomic intermetallic of nickel and titanium, is the most widely known and used shape memory alloy. Owing to its capacity to undergo a thermal or stress induced martensitic phase transformation, Nitinol displays recoverable strains that are more than an order of magnitude greater than in traditional alloys, specifically as high as 10%. Since its discovery in the 1960s, Nitinol has been used for its shape memory properties for couplings and actuators, although its contemporary use has been in for medical devices. For these applications, the stress induced transformation (‘superelasticity’) has been used extensively for self-expanding implantable devices such as endovascular stents and vena cava filters, and for tools such as endodontic files. Most of these applications involve cyclically varying biomechanical stresses or strains that drive the need to fully understand the fatigue and fracture resistance of this alloy. Here we review the existing knowledge base on the fatigue of Nitinol, both in terms of their stress or strain life (total life) and damage tolerant (crack propagation) behaviour, together with their fracture toughness properties. We further discuss the application of such data to the fatigue design and life prediction methodologies for Nitinol implant devices used in the medical industry.

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Mechanical fatigue and fracture of Nitinol

Grain size effects on the fatigue response of nanocrystalline metals

Many manufacturing processes can induce residual stresses in components. These residual stresses influence the mean stress during cyclic loading and so can influence the fatigue life. However, the initial residual stresses induced during manufacturing may not remain stable during the fatigue life. This paper provides a broad and extensive literature survey addressing the stability of surface and near-surface residual stress fields during fatigue, including redistribution and relaxation due to static mechanical load, repeated cyclic loads, thermal exposure and crack extension. The implications of the initial and evolving residual stress state for fatigue behaviour and life prediction are addressed, with special attention to fatigue crack growth. This survey is not a critical analysis; no detailed attempt is made to evaluate the relative merits of the different explanations and models proposed, to propose new explanations or models or to provide quantitative conclusions. Primary attention is given to the residual stresses resulting from four major classes of manufacturing operations: shot peening and related surface treatments, cold expansion of holes, welding and machining.

A literature survey on the stability and significance of residual stresses during fatigue

Many of the degradation mechanisms relevant to power plant components can be exacerbated by stresses that reside within the material. Good design or structural integrity assessments require therefore, an accounting of residual stresses, which often are introduced during welding. To do this it is necessary to characterise the stresses, but this may not be possible in thick components using non-destructive methods. These difficulties, and a paucity of relevant engineering data, have led to an increasing emphasis on the development and validation of suitable modelling tools. Advances are prominent in the estimation of welding residual stresses in austenitic stainless steels. The progress has been less convincing in the case of ferritic alloys, largely due to the complexities associated with the solid state phase transformations that occur in multipass welding. We review here the metallurgical issues that arise in ferritic steel welds, relate these to the difficulties in calculating residual stresses,...

Welding Residual Stresses in Ferritic Power Plant Steels

Superior fatigue crack growth properties in newly developed weld metal

Fatigue crack propagation rates and threshold stress intensity factors for welded joints of ht80 steel at several stress ratios

Unique fatigue threshold and growth properties of welded joints in a tensile residual stress field

Fatigue strength evaluation of welded joints containing high tensile residual stresses

The potential improvement in fatigue performance of lap welded joints in 2 mm thick 540MPa and 780MPa class steels from the introduction of compressive residual stress by welding with low transformation temperature (LTT) welding wire was investigated. Comparative fatigue tests in bending at a stress ratio of R=0 on specimens made with conventional and LTT welds confirmed that the fatigue performance of such thin specimens was improved, particularly in the long-life regime. Furthermore, the benefit was greatest in the high-strength steel, the improvement in fatigue strength at 107 cycles being 1.4 times for 540MPa steel and 1.6 times for 780MPa steel.

Akihiko Ohta

Papers

Superior fatigue crack growth properties in newly developed weld metal

Fatigue crack propagation rates and threshold stress intensity factors for welded joints of ht80 steel at several stress ratios

Unique fatigue threshold and growth properties of welded joints in a tensile residual stress field

Fatigue strength evaluation of welded joints containing high tensile residual stresses

Fatigue Strength Improvement of Lap Welded Joints by Low Transformation Temperature Welding Wire — Superior Improvement with Strength of Steel