Establishing Strength Evaluation Method of Very Thin Ductile Cast Iron and the Way of Improving Reliability of Thin Wall Castings.

本研究的目的是建立用小试件评定特薄（小于2 mm）球墨铸铁强度的方法，并寻求提高薄壁铸件可靠性的途径。1）拉伸试验：提出了一种宽度为4.5mm、厚度为2 mm、标距为12 mm的小试样，其抗拉强度和伸长率与直径为14 mm的标准试样相当。伸长率应根据测得的载荷-伸长率曲线确定。2）韧性试验：使用10×10×0.5（mm）试样进行小冲杆试验（SP试验）。由实测载荷-挠度曲线确定裂纹萌生挠度δ^*，估算等效断裂应变ε<qf>，由此确定断裂韧度J<Ic>。3<Ic>）疲劳试验：认为应采用薄板试件进行应力比R=-1.0的疲劳试验。开发了用于测试的专用夹具。观察了疲劳裂纹从铸件表面萌生的过程以及初始裂纹稳定扩展的过程。4）无损检测：采用涡流检测技术检测合金的硬度和显微组织。提高测量精度和新的标准不同的厚标本可能使估计。

The purpose of this research is to establish the way of strength evaluation of very thin (less than 2 mm) ductile cast iron by small test pieces and to find the way to improve reliability of thin wall castings. Tensile properties, toughness, fatigue properties and nondestructive evaluation were discussed.1) Tensile test: A small test piece 4.5 mm wide and 2 mm thick with 12 mm gauge length was proposed, with which, tensile strength and elongation equivalent to the ones from a 14 mm diameter standard test piece are obtained. Elongation should be determined from the load-elongation curve measured.2) Toughness test: Small punch test (SP test) by 10×10×0.5 (mm) test piece was applied. Crack initiation deflection δ^* is determined by the measured load deflection curve, and equivalent fracture strain ε_<qf> is estimated, from which, fracture toughness J_<Ic> is determined. J_<Ic> can be also estimated from HRB hardness.3) Fatigue test: It was concluded that fatigue tests with stress ratio, R= -1.0, by thin plate test pieces should be carried. Special jigs for the test were developed. Fatigue crack initiation from the cast surface and the process of primary and steady crack propagation will be observed. Casting method to get sound thin plate specimens with ordinary melt treatment was also developed.4) Nondestructive evaluation: Eddy current test was applied to estimate the hardness and the microstructure. Improving the measurement accuracy and new standards different from the thick specimens may enable the estimation.