Lu Baotong

Predicting fatigue crack growth rates and thresholds at low temperatures

In this paper an attempt was made to offer a new method for predicting fatigue crack growth (FCG) rates and thresholds of metals in low temperature structural applications. The experimental results available have indicated that a decrease in temperature can influence the FCG mechanism, as well as the FCG rates, and the fatigue ductile-brittle transition (FDBT) may occur in some metals. Accordingly, metals can be divided into two groups, i.e. alloys without an FDBT and those with an FDBT. In the former, the fracture mode of FCG is ductile transgranular for temperatures down to 4 K and the crack growth rates in the intermediate region (da/dN = 10−8−10−6 m cycle−1) depend mainly upon Youngs modulus. In the latter, the FCG behaviour above the FDBT temperature is similar to that of the alloys without an FDBT. When the temperature is below the FDBT temperature, the FCG rates (especially at higher ΔK levels) will be enhanced by the brittle transition of the FCG mechanism and both the strength and the ductility of the alloys will have significant effects on the FCG rates. The expression previously proposed by one of the present authors and a coworker for FCG rates not only explains the low temperature FCG behaviour of the metals mentioned above but also predicts the low temperature FCG rates in both the near-threshold region and the intermediate region. Finally, a new method to predict ΔKth at low temperatures is tentatively proposed based on the stratic fracture model.

Fatigue tests and life prediction of 16 Mn steel butt welds without crack-like defect

Fatigue test results of 16 Mn steel butt welds without crack-like defect under both constant and variable amplitude loads are reported and new procedures are used to predict fatigue crack initiation (FCI) life, fatigue crack propagation (FCP) life and total life of the butt welds. The results indicate that the FCI life and FCP life should be calculated separately and the total life is the sum of the FCI life and FCP life. For the butt welds investigated, stress cycles to initiate a crack of engineering size may occupy more than 70 percent of the total life of the butt welds and it is more suitable to express the total life as a power function of the equivalent stress amplitude {ie275-1}. In predicting the FCI life, the expression of FCI life obtained from the test results of notched specimens is used but the effects of microstructure, surface condition, macro- and micro-geometrical discontinuities at weld toe should be taken into account. In predicting the FCP life, the formula developed by Zheng and Hirt is used and the stress ratio is taken as 0.6 to account for the residual stresses effect on the FCP rate. Because overload produced by the maximum load in a load spectrum has no effect on the FCI life of 16 Mn steel and weldment of the steel, according to the procedures outlined in the paper, one can use the FCI life expression mentioned and the linear damage accumulation rule proposed by Miner to predict the FCI life of 16 Mn steel butt welds under variable amplitude loads. A good agreement is achieved between the predicted results and the test data.

On a fatigue formula under stress cycling

Abstract Based on recent research, a new formula is developed for fatigue life under stress cycling and substantiated using fatigue life test results given in the literature. The new formula reveals a correlation between fatigue life, stress range, stress ratio, tensile properties and theoretical endurance limit. The latter, expressed by the equivalent stress amplitude, is a material constant independent of the stress ratio. Using this concept to re-analyse the effect of stress ratio on the endurance limit, it is shown that the expression given in the present study is in good agreement with endurance limit test results while the empirical relationships of Goodman and Geber both overestimate the endurance limit when R > −1. The theoretical endurance limit can be obtained by regression analysis of fatigue life test data or predicted from the conventional endurance limit without any additional tests. An approximate expression is given for the fatigue strength coefficient as a procedure for predicting the fatigue life. Finally, the cause of scatter in fatigue test results is analysed.

Prediction of probability distribution of fatigue life of 15MnVN steel notched elements under variable-amplitude loading

Attempts are made to develop procedures for predicting the probability distribution of the fatigue life of notched elements of a high-strength low-alloy steel, 15MnVN steel, under variable-amplitude loading (VAL). First, expressions for the fatigue life of 15MnVN steel with given survivabilities are obtained by fitting the test data of the fatigue life with corresponding survivabilities. Then procedures are outlined for predicting the probability distribution of the fatigue life of notched elements of 15MnVN steel, which has a non-continuous strain hardening characteristic, under VAL, based on the assumption that Miners rule and the expression with given survivability could be directly used to predict the fatigue life with corresponding survivability under VAL. The predicted probability distribution of the fatigue life of 15MnVN steel notched elements under VAL follows a log-normal distribution and is checked by testing notched specimens under two types of block loading. The procedures mentioned above are of general applicability in predicting the probability distribution of the fatigue life of notched elements of metals with non-continuous strain-hardening characteristic. A criterion is also tentatively proposed for omitting small loads in the load spectrum in life prediction under VAL.