Special Issue "Creep and Fracture of Engineering Materials and Structures"

Guest Editor
Prof. Dr. Bill Plumbrige
Materials Engineering, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
Website: http://materials.open.ac.uk/staff/Staff_wjp.htm
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Phone: +44 1908 652 630
Fax: +44 1908 653 858

Creep is time-dependent deformation under constant load or stress. It has been recognised as an engineering challenge for more than a century, and traditionally, the key design parameter has been ‘time to fracture’ as a function of applied stress and temperature. More recently, in applications, such as close-tolerance turbines and miniaturised equipment, ‘time to a critical strain’ has become a more appropriate failure criterion.

It is often regarded as a high temperature phenomenon, although this can be misleading since it is the homologous temperature (for metallic materials, this is the ratio of the current temperature to the melting temperature, expressed in degrees Kelvin) that is the salient factor in determining the significance of creep. Consequently, creep may still be a problem in solders for electronics at temperatures as low as −50 °C.

Creep may occur in all classes of materials (metals, ceramics, polymers and composites) although the phenomena involved are quite disparate. Even for a single material type, such as metals, one or more mechanisms may be involved, depending upon the operating conditions (stress, temperature, strain rate). For example, the dominant deformation process may be occurring at the grain boundaries or within the grains themselves. Further sub divisions are possible which explains the plethora of constitutive expressions for describing creep that have been developed over the years. With the advent of further miniaturisation and nanostructures, their number will undoubtedly mushroom, but the need to identify the dominant creep mechanism, both in the laboratory and in the field, will remain paramount, if reliable creep performance is to be achieved.

That this Special Issue contains a diverse range of papers should be regarded as strength. Potentially, there is much to gain from cross-fertilisation between different material-specific and application-specific approaches.

Submission

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• creep processes in materials
• failure criteria
• life prediction
• dominant mechanisms

Title: Transformation-Induced Creep and Creep Recovery of Shape Memory Alloy
Author: Kohei Takeda and Hisaaki Tobushi
Affiliation: Aichi Institute of Technology, 1247 Yachigusa, Yakusa-cho, Toyota, 470-0392 Japan; E-Mail: tobushi@aitech.ac.jp (H.T.)
Abstract: If the shape memory alloy is subjected to the subloop loading under the stress-controlled condition, creep and creep recovery can appear based on the martensitic transformation. In the design of shape memory alloy elements, these deformation properties are important since the deflection of shape memory alloy elements can change under constant stress.  he conditions for the progress of the martensitic  ransformation are discussed based on the kinetics of the martensitic transformation for the shape memory alloy. During loading under constant stress rate, temperature increases due to the stress-induced martensitic transformation. If stress is held constant during the martensitic transformation stage in the loading process, temperature decreases and the condition for the progress of the martensitic transformation is satisfied, resulting in the transformation-induced creep deformation. The details for these thermomechanical properties are investigated  xperimentally for TiNi shape memory alloy which is most widely used in practical applications. The volume fraction of the martensitic phase increases in proportion to an increase in creep strain.