摘要
本文证明了在裂纹小于某一个跟材料性能有关的值时,K_(lc)=σ_fa^(1/2)·y 为常量这个断裂力学的基本假设不成立。传统的 K_(lc)试件尺寸要求的理论依据是不正确的。在此基础上,通过求出断裂强度的上限而导出了 K_(lc) 公式的有效范围和尺寸要求的表达式。从理论上澄清了以下三个问题:1.为什么 K_(lc)的测试须有尺寸要求?2.怎么要求不同材料的试样尺寸?3.不满足要求时 K_(lc)如何变化?
In this paper,it is discovered that the basic hypothesis of fracture mechanics,K_(1c)=σ_f(ay)^(1/2) being a constant,is untenable when the crack size is smaller than certain val-ue which related to the material behavior.Thus,the measuring results with small samplesare incredible because the size of specimen for K_(1c) measurement is usually in proportionto the size of notch.The analytic expression of the requirement for specimen size on SENBmethod is derived by determining the upper limit of fracture strength.As fas as proportionalspecimen is concerned,the size requirement is showed as below:B,α,W-α≥ξ~2/[(1-η)~4Y^2](K_(lc)/σ_b)~2It is revealed that the reason of size requirement is neither for meeting plane straincondition nor meeting linear elastic state,but simply for the applicable range of the calcula-ting formula.The conventional explanation about size requirement is wrong.Some infer-ences are drawn and three questions as follows are clarified in theory.1.Why should the specimen meet size requirement for measuring fracture toughness?2.What is correct size requrement of brittle materials for measurement of K_(lc)?3.What results will occur if the specimen doesn't meet the size requiremet?
