Comparative Study of Dynamic Impact Response and Microstructure of 304L Stainless Steel With and Without Prestrain
| Autor: | Chi-Feng Lin, 林奇鋒 |
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| Rok vydání: | 2002 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 90 This study compares the dynamic plastic deformation behaviour and microstructural evolution of 304L stainless steel with and without metal forming prestrain using the compressive split Hopkinson pressure bar technique under strain rates ranging from 8×102 to 4.8×103 s-1 at room temperature, with true strains varying from yield to 0.3. Results show that the flow stress of unprestrained and prestrained 304L stainless steel is sensitive to applied strain rate, but the prestrained material exhibits greater strength. Work hardening rate, strain rate sensitivity and activation volume depend strongly on prestrain, strain and strain rate. Under dynamic range, work hardening rate decreases with increasing strain and strain rate, the 0.15 prestrained specimen having the highest value compared to the other specimens. Increasing the strain rate of impact loading and prestrain increases strain rate sensitivity. However, the inverse tendency is observed for the activation volume. The effect of loading rate on mechanical response and impact substructure of unprestrained and prestrained 304L stainless steel are found directly related to dislocation density, twin density and the amount of transformed martensite. By using a physically-based constitutive equation with the experimentally determined specific material parameters, the flow behaviour of unprestrained and prestrained material can be described successfully for the range of test conditions. OM and SEM fracture feature observations reveal adiabatic shear band formation is the dominant fracture mechanism. Adiabatic shear band void and crack formation are along the direction of maximum shear stress and induce specimen fracture. Microstructural observations reveal the dislocation substructure morphologies of mechanical twins, micro-shear bands and martensite formation are strongly influenced by prestrain, strain and strain rate. Equiaxed dislocation cells are found in unprestrained materials and elongated cells in prestrained materials. Mechanical twins are only found in prestrained material. Micro-shear bands and martensite are more evident at large strain and strain rate, especially for the prestrained material. Quantitative measurement reveals that dislocation, twin density and the volume-fraction of martensite increase with prestrain, strain and applied strain rate, but twin density decreases at 0.5 prestrain. These microstructrual changes are functions of work hardening stress and together account for the observed strengthening effects. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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