疲労き裂の開閉口挙動 : S形除荷曲線法による実験的検討

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A usual elastic compliance method for determining the crack opening point is based on an unloading curve whose uppermost part looks like a straight line. By this method there is the possibility of obtaining a wrong conclusion,since the crack closure behavior is always accompanied with plastic deformation. In this paper the tension-compression fatigue tests were carried out (stress ratio were taken as 0 and -1),and the opening and closure point is successfully measured by the S shaped unloading curve. The crack closure point measured from the S shaped unloading curve dose not coincide with the opening point. The crack growth rate can be expressed unifyingly by the effective stress intensity range, ΔK_<eff>^;<op>=K_<max>-K_<op>, or ΔK_<eff>^;<cl>=K_<max>-K_<cl>, independently of the stress ratio and the loading history. The inclination of the line of log(dl/dN) us, log ΔK_<eff>^;<cl> plot is approximately 2. This suggests that the proposed method for determining the crack closure point is rational. It is shown that the values of effective stress range ratio u_<op> (=ΔK_<eff>^;<op>/K_<max>) and (=ΔK_<eff>^;<cl>/K_<max>) plotted against K<max> respectively get together on one narrow band (when R is constant), if the cracks were propagated into a steady state. The effective stress range u_<op> and u_<cl> increase linearly with K_<max> in the range of K_<max>=7.5MPa√<m> to 12.5MPa√<m>. Especially,the u_<cl> based on K_<cl> is nearly in direct proportion to K_<max>. This is closely related to the background of the fact that the Paris crack growth low dl/dN=CΔK^4 holds good in the present materiel.
社団法人日本材料学会の論文
1986-07-15

疲労き裂の開閉口挙動 : S形除荷曲線法による実験的検討

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