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超薄(小于2 mm)球墨铸铁件强度的方法，寻找提高薄壁铸件可靠性的途径。讨论了拉伸性能、韧性、疲劳性能和无损评价1)拉伸试验：提出了一种宽4.5 mm、厚2 mm、长度12 mm的小试件，其抗拉强度和延伸率与直径14 mm的标准试件相当。2)韧性试验：采用10×10×0.5(Mm)试件的小冲头试验(SP试验)。由测得的载荷挠度曲线确定裂纹萌生挠度δ^*，并估算等效断裂应变ε_&lt；Qf&gt；，由此确定断裂韧性J&lt；IC&gt；。3)疲劳试验：认为应采用应力比R=-1.0的薄板试件进行疲劳试验。研制了试验专用夹具。将观察到疲劳裂纹从铸件表面萌生，以及裂纹的初始和稳定扩展过程。4)无损检测：采用电涡流检测技术对试件的硬度和显微组织进行检测。提高测量精度和不同于厚样品的新标准可以使估计成为可能。

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.