On some basic problems of fatigue research in engineering

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.

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

The progress in the research on the fatigue crack propagation (fcp) in metals at low temperatures is briefly reviewed in this chapter. According to whether the ductile—brittle transition of the fcp mechanism (DBTF) occurs at low temperature or not, the metals can be divided into two kinds: 1) the metals without DBTF, 2) the metals with DBTF at a certain temperature which is denoted as T dbt . For the first kind of metals, the ductile striation mechanism operates until 4°K, and the fcp rate decreases with decreasing temperature. For the second kind of metals, the fcp rate decreases with decreasing temperature in the same pattern as that in the first kind of metals when T>T dbt . When T dbt , however, the fcp mechanism changes from the ductile mechanism into the brittle one such that the fcp rates in the intermediate region (d a / d N = 10 −8 ∼ 10 −6 m / cycle) increase sharply with decreasing temperature. The DBTF also induces the brittle fracture of the cracked elements. It has been shown that the fcp formula developed by Zheng and Hirt can be used to fit the fcp test results at low temperatures and to explain well the variation patterns of fcp rates with decreasing temperature. More interesting is that the fcp rates at low temperatures can be predicted from the tensile properties of metals by using the above—mentioned fcp formula.

Fatigue crack propagation in metals at low temperatures

Study of the effect of defects on the fatigue behaviour and the fracture toughness for low carbon steel (API 5L grade B) gas transmission pipelines

In this paper, the fatigue crack initiation (FCI) from notches and fatigue crack propagation (FCP) in different zones of welds in butt joints of 30CrMnSiNi2A steel were investigated. General formulas for the FCI life and the FCP rate of the welds are examined, and an approach to predicting the FCI life is offered. The research indicates that the postweld isothermal quenching treatment improves both the microstructure and fatigue resistance of the welds. Little difference is found in the FCI resistance of the base metal, weld metal and weld interface, although the microstructural effect on the FCP rate is observed. In the near-threshold region, the FCP rates of the weld interface are higher than those of the base metal but lower than those of the weld metal. The discontinuities on the notch surface have a detrimental influence on the FCI resistance of the welds investigated. The results further indicate that, in order to predict the fatigue life of the welded structures accurately, the test results of FCI life and FCP rate should be properly selected according to the FCI location and FCP path in the structures.

Tienxie Cui

Papers

Life prediction of butt welds containing welding defect