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超薄（小于2mm）球墨铸铁的小试件强度评价方法，寻找提高薄壁铸件可靠性的途径。讨论了材料的拉伸性能、韧性、疲劳性能和无损评价。1)拉伸试验：提出了宽4.5 mm、厚2mm、规长12mm的小试件，其抗拉强度和伸长率与直径14mm的标准试件相当。伸长率应根据所测载荷-伸长率曲线确定。2)韧性试验：采用10×10×0.5 （mm）试件的小冲孔试验（SP试验）。裂纹起裂挠度δ^*由实测载荷挠度曲线确定，估算出等效断裂应变ε_<qf>，由此确定断裂韧性J_<Ic>。通过HRB硬度可以估计出J_< ic>。3)疲劳试验：认为应进行应力比R= -1.0的薄板试件疲劳试验。为此研制了专用夹具。观察到铸件表面的疲劳裂纹萌生和初次裂纹扩展和稳定裂纹扩展过程。提出了用普通熔体处理获得良好薄板试样的铸造方法。4)无损评价：采用涡流检测对硬度和显微组织进行评价。提高测量精度和采用不同于厚试件的新标准可以使估算成为可能。

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.