Theoretical and practical implications of creep curve shape analyses for 2124 and 2419
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Theoretical and Practical Implications of Creep Curve Shape Analyses for 2124 and 2419 H. BURT and B. WILSHIRE Creep and creep fracture properties are presented for two commercial aluminum alloys, 2124 and 2419, tested at stresses giving creep lives up to about 1000 hours at 373 to 463 K. The results are analyzed using the methodology, which quantifies the shape of individual creep curves and the variations in curve shape with changing stress and temperature. The relationships are shown to allow reasonable prediction of 100,000-hour stress rupture values, while also explaining the complex creep behavior patterns widely observed with precipitation-hardened alloys.
I.
INTRODUCTION
SINCE the 1930s, aluminum alloys have dominated airframe construction for commercial airliners, cargo, and military transport, with the resulting design and production experience ensuring their continuation as the “material of choice” over future decades.[1] However, aluminum has a low melting point (Tm 933 K). Hence, creep and creep fracture become important design considerations for airframe components that must operate without failure for long periods under load at temperatures around 0.4Tm (373 K). For instance, in the European Commission CREEPAL Thematic Network, which began in 1998,[2] the creep life planned for next generation supersonic civil transport was 80,000 flight hours under stresses from 130 to 170 MPa at operational temperatures of 363 to 373 K (maximum 403 K). Because of the long service lives specified, experimental acquisition of creep design properties for different alloys represents a protracted and expensive task. Only modest reductions in the scale and costs of such long-term programs can then be achieved using standard parametric methods for data extrapolation,[3,4] because these empirical techniques allow estimation of creep lives to just 3 times the longest reliable test results available.[5,6] Moreover, confidence in the validity of the long-term predictions would be enhanced by the availability of extrapolation procedures based on a sound theoretical understanding of the processes controlling strain accumulation and damage evolution during creep. In the present program, the creep and creep fracture properties of 2419 and 2124 over stress ranges giving creep lives up to about 1000 hours at 373 to 463 K are considered in relation to information derived from studies involving optical and transmission electron microscopy. In addition, the results are analyzed using the methodology,[7,8] which seeks to quantify the variations in creep curve shape as the stress and temperature conditions are changed. In this way, an assessment is made of the accuracy with which 100,000-hour H. BURT, Senior Research Officer, and B. WILSHIRE, Professor and EngD/IGDS Director, are with the Materials Research Centre, School of Engineering, University of Wales Swansea, Swansea, SA2 8PP, United Kingdom. Contact e-mail: [email protected] Manuscript submitted November 20, 2003. METALLURGICAL AND MATER
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