Using Thermomechanical Conditioning Cycles to Improve Fracture Toughness of Low Carbon Steel
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INTRODUCTION
FRACTURE toughness is a measure of a material’s resistance to unstable crack growth. Improving a material’s fracture toughness has major implications for the manufacturing and continued operation of components containing defects or cracklike discontinuities. In this study, the fracture toughness, achieved after the application of thermomechanical conditioning (TMC) cycles, is termed as the apparent fracture toughness (Ka). By modifying the stress distribution around the crack tip region, a material’s resistance to unstable crack growth or its apparent fracture toughness (Ka) can be improved. A technique called TMC was developed[1–5] for introducing compressive residual stress at the critical locations. This technique involves heating the precracked fracture toughness specimen to a moderate temperature with subsequent application of a tensile load for 15 minutes. After unloading, the specimen is cooled to room temperature. A schematic diagram of the theoretical basis for generation of compressive residual stress ahead of the crack tip is described in Figure 1. Curve 1 shows the distribution of the axial elastic stress ahead of the crack tip at room temperature. However, in reality, the stresses in the region around the crack tip are high F.W. WU, Doctor, Research Fellow, and R.N. IBRAHIM and R.K. SINGH RAMAN, Associate Professors, are with the Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria 3168, Australia. Contact e-mail: raafat.ibrahim@ eng.monash.edu.au R.K. SINGH RAMAN, Associate Professor, is also with the Department of Chemical Engineering, Monash University, Clayton, Victoria 3168, Australia. R. DAS, Doctor, Research Scientist, is with CSIRO Mathematical and Information Sciences, Clayton, Victoria 3168, Australia. Manuscript submitted December 18, 2007. Article published online March 11, 2009 1118—VOLUME 40A, MAY 2009
enough to exceed the yield strength of the material. Thus, plastic deformation will occur and the stresses will be redistributed (curve 2). Upon unloading, a residual stress distribution will remain, as given by curve 3, which is obtained by subtracting curve 1 from curve 2. Curve 3 clearly suggests the establishment of a compressively stressed region around the crack tip. Let us now consider the case when the material is uniformly heated during tensile loading, so the yield stress is reduced, giving rise to a larger zone of yielded material around the crack tip (curve 4). After unloading, a higher compressive residual stress field will be created, as shown by curve 5, which is obtained by subtracting curve 1 from curve 4. It is important to note that the extended zone of compressive residual stress in curve 5 is formed as a result of a larger zone of yielded material (as shown in curve 4). In the first instance, it may appear an unsound practice to improve fracture strength or fatigue life by overloading the component followed by heating. However, it must be borne in mind that the conservative design of most engineering structures ensures that
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