Abstract
In this paper, a strain-intensity-factor-based method is proposed to calculate the fatigue crack growth under the fully reversed loading condition. A theoretical analysis is conducted in detail to demonstrate that the strain intensity factor is likely to be a better driving parameter correlated with the fatigue crack growth rate than the stress intensity factor (SIF), especially for some metallic materials (such as 316 austenitic stainless steel) in the low cycle fatigue region with negative stress ratios R (typically R = −1). For fully reversed cyclic loading, the constitutive relation between stress and strain should follow the cyclic stress-strain curve rather than the monotonic one (it is a nonlinear function even within the elastic region). Based on that, a transformation algorithm between the SIF and the strain intensity factor is developed, and the fatigue crack growth rate testing data of 316 austenitic stainless steel and AZ31 magnesium alloy are employed to validate the proposed model. It is clearly observed that the scatter band width of crack growth rate vs. strain intensity factor is narrower than that vs. the SIF for different load ranges (which indicates that the strain intensity factor is a better parameter than the stress intensity factor under the fully reversed load condition). It is also shown that the crack growth rate is not uniquely determined by the SIF range even under the same R, but is also influenced by the maximum loading. Additionally, the fatigue life data (strain-life curve) of smooth cylindrical specimens are also used for further comparison, where a modified Paris equation and the equivalent initial flaw size (EIFS) are involved. The results of the proposed method have a better agreement with the experimental data compared to the stress intensity factor based method. Overall, the strain intensity factor method shows a fairly good ability in calculating the fatigue crack propagation, especially for the fully reversed cyclic loading condition.
Highlights
The fatigue life prediction is of great significance to the safety and reliability of structural components in many engineering projects
It is clearly observed that the scatter band width of crack growth rate vs. strain intensity factor is narrower than that vs. the stress intensity factor (SIF) for different load ranges
The paper is organized as follows: First, the strain intensity factor concept is introduced briefly; Second, the transformation relationship between the SIF and strain intensity factor is delivered, and a theoretical analysis is given to demonstrate the effectiveness of the proposed model; Third, the fatigue life prediction method based on strain intensity factor and modified equivalent initial flaw size (EIFS) is addressed; Fourth, the proposed model is validated by using the fatigue testing data in 316 austenitic stainless steel and AZ31 magnesium alloy; And some conclusions are given
Summary
The fatigue life prediction is of great significance to the safety and reliability of structural components in many engineering projects. Some researchers propose local plastic characteristics (such as the plastic CTOD range and the plastic flow intensity factor) to correlate with the crack growth rate [19,20] These driving parameters can describe the plastic deformation near the crack tip better than the SIF, especially under the “elastic-plastic” condition. The paper is organized as follows: First, the strain intensity factor concept is introduced briefly; Second, the transformation relationship between the SIF and strain intensity factor is delivered, and a theoretical analysis is given to demonstrate the effectiveness of the proposed model; Third, the fatigue life prediction method based on strain intensity factor and modified EIFS is addressed; Fourth, the proposed model is validated by using the fatigue testing data in 316 austenitic stainless steel and AZ31 magnesium alloy; And some conclusions are given
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