Effect of the Strain Rate on Fracture Surface and Structure Figure10 shows the SEM micrographs of the fracture surfaces of the 5052-H112 and 5083-H112 alloys at various strain rates. Both Al-Mg series alloys provide the flat surfaces at static load conditions. However, the fracture occurred by a combination of two different dimple sizes at higher strain rates, one of which is approximately 10μm and the other is several micrometer in the case of 5083-H112 alloy having higher magnesium content. The fracture surface of low magnesium 5083-H112 alloy is mainly dominated by dimples of approximately 20μm in diameter. Although the fracture surfaces mainly exhibited the shear type dimple pattern under low strain rates, ordinary equiaxed dimple fracture surfaces were observed under high strain rates. The specimen was largely necked at high strain rate. In both materials, the dimple size and depth increased with the strain rate. Figure 10: SEM micrographs of fracture surfaces at various strain rates. CONCLUSIONS The effect of strain rate on mechanical properties and microstructural changes in 5052-H112 and 5083-H112 aluminum alloys were examined over a wide range of strain rate. The following conclusions could be drawn. 1. The strain rate dependencies of two kinds of Al-Mg series alloys were very sensitive at high strain rate. As the strain rate increased, 0.2% proof stress and ultimate tensile strength increased. Under the same strain rate conditions, the 5083-H112 alloy showed more clear increase in strength than the 5052-H112 alloy. 2. The strain hardening exponent can be seen to increase with increasing strain rate. Strain hardening exponent showed similar degree of strain rate dependency with the 0.2% proof stress and ultimate tensile strength. However, the increasing rate of the strain hardening exponent was constant up to the strain rate of approximately 102s-1 for both materials. 5052-H112 5083-H112 =1000s-1 =3000s-1 =0.0004s-1 50μm
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