LC4CS铝合金的疲劳裂纹扩展速率表达式

General Expression for Fatigue Crack Growth Rates in LC4CS Aluminum Alloy

采用实验方法测定了 LC4CS 铝合金板材在不同应力比下的近门槛区疲劳裂纹扩展速率。所得的实验结果可很好地用公式 dα/dN=(ΔK-ΔK_(th))~2拟合。对实验结果的分析表明,对于 LC4CS 铝合金,公式中 B 的值可由材料的杨氏模量估算;根据实验结果按照上述公式外推求得精确的ΔK_(th)值是可行的。依据实验和分析结果提出了 LC4CS 板材疲劳裂纹扩展速率的一般表达式。该式的有效性为文献中的实验结果所证实。本文的研究结果进一步表明,如果已知材料的ΔK_(th)值和中区(dα/dN=10^(-8)～10^(-6)m/cycle)疲劳裂纹扩展机制,则可根据材料的拉伸性能预测其疲劳裂纹扩展速率。

Although several expressions have been proposed to predict fatigue crack growth (FCG) rates in both near-threshold and intermediate regions[11],most of them are inconvenient in application because these expressions contain some empirical constants.In present study,the FCG rates in near-threshold region and the FCG thresholds ΔK_(th) were experimentally determined on LC4CS alu- minum alloy sheet,which is similiar to 7075-T6 in both composition and mi- crostructure,under the stress ratio range of 0.1 to 0.7.The test results and analysis show that a general expression for FCG rates of LC4CS sheets under the stress ratio R≤0.7 can be obtained by using the formula developed due to Zheng and Hirt[3]to fit the test data and that the FCG threshold ΔK_(th) is the governing parameter for FCG rates,especially in the near-threshold region. If the value of ΔK_(th) is known,the FCG rates in both the near-threshold and intermediate regions can be predicted from tensile properties of metals. The regression analysis of FCG test data,by using eq.(1),the formula mentioned above,and the procedure developed in Ref.[3],shows that eq.(1) gives a good fit to FCG test results under the stress ratios of R≤0.7 for LC4CS sheets: da/dN=B(ΔK-ΔK_(th))~2 (1) where the values of ΔK_(th),the theoretical FCG threshold,obtaind by the above metioned regression analysis can be expressed as: ΔK_(th)=2.7(1-R)^(0.74),for R≤0.7 (2) It is observed in LC4CS alloy that the fracture mode operating in the intermediate region is predominately formation of striation.Consequently,the value of B,the FCG coefficient,can be predicted by the formula:B=15.9/E^2 [7] and is equal to 3.40×10^(-9) MPa^(-2).Substituing eq.(2) and the value of B into eq.(1)gives the general expression for FCG rates of LC4CS sheets in range of stress ratios R≤0.7 as follows: da/dN=3.40×10^(-9)(ΔK-ΔK_(th))~2 (3) The validity of eq.(3) is objectively checked by using the test data given in Ref.[1].As shown in Fig.5,eq.(3) can accurately predict the FCG rates of LC4CS sheets. The FCG experimentally measured is defined as the value of ΔK at da/dN =3.0×10^(-10)m/cycle and denoted as ΔK_(thE).The values of ΔK_(thE) are in good agreement with those of ΔK at da/dN=3.0×10^(-10)m/cycle calculated from eq. (3),which is denoted as ΔK_(thC).Therefore,it may be thought that the ΔK_(th) can be obtained by the extrapolation of FCG test data according to eq.(1). It should be pointed out,however,that the predicted value of B is slightly higher than those determined by tests.The reason behind it can be attributed to the resonant fatigue test machine used in the present study[6-8]. Obviously,eq.(3)expresses the existence of ΔK_(th) and the stress ratio effect on FCG rates and is,thus,of pratical importance in predicting the FCG life of components under variable loading.