Fatigue Design Limit Curves for Different Materials János Lukács Ph.D., associate professor Department of Mechanical Engineering, University of Miskolc, Hungary Introduction The fatigue life can be divided into several phases: crack nucleation, micro-crack growth, macro-crack growth and failure. For the engineering point of view the phase of macro-crack growth is the most important one, since crack size can be detected by nondestructive testing methods, the fatigue crack growth and the residual life-time can be predicted based on fracture mechanical parameters. In connection with the previously mentioned facts the objectives of my presentation are the followings:
Experiments and results Kinetic diagram of fatigue crack growth can be described using different characteristics. The most widely used fracture mechanical properties are the stress intensity factor range (D K), the crack tip opening displacement range (D CTOD), the J-integral range (D J) and their effective values. Reliability of a structural element having crack or crack like defects is determined by the geometrical features of the structural element and the flaws, the loading conditions as well as the material resistance to fatigue crack propagation. There are different documents and codes containing fatigue crack propagation limit or design curves and rules for the prediction of crack growth. The background of the limit curves and rules consists of two parts: statistical analysis of numerous experiments and fatigue crack propagation law. Experiments were carried out with a large number of specimens, the tested material grades represent wide variety of industrial applications. CT and TPB specimens were tested for base materials and welded joints under mode I loading condition. The specimens were cut from the sheets parallel and perpendicular to the rolling direction, so the directions of fatigue crack propagation were the same. Axes of welded joints were parallel and perpendicular to the rolling direction, too. The location of the specimens and the notches represent the different possibilities of cracks in the base materials and welded joints. CTS specimens were used for tests under mixed mode I+II loading condition. The specimens were cut parallel to the rolling direction, so the cracks were propagated perpendicular to the rolling direction. Experiments were performed by D K-decreasing and constant load amplitude, methods, at room temperature, in air, following sinusoidal loading function. Stress ratio was constant, 0.1, and propagating crack was registered by optical and compliance methods. The determination of fatigue crack propagation limit curves consists of 6 step. 1st step: calculation of measuring results. The threshold stress intensity factor range (D Kth) and the two parameters of Paris-Erdogan law (n and C) were calculated according to ASTM code, the fatigue fracture toughness (D Kfc) was calculated from crack length measured by stereo microscope. 2nd step: sorting measuring results into groups and calculation of the statistical parameters of the samples. For the first aim Wilcoxon-probe was applied and the calculated statistical parameters are average, standard deviation and standard deviation coefficient. Standard deviation coefficients are generally less than 20%, which means reliable, reproducible testing, data processing and evaluating system. 3rd step: selection of the distribution function type. For this aim Shapiro-Wilk probe, Kolmogorov or Kolmogorov-Smirnov probe and c 2-probe were used. It was concluded, that Weibull distribution is the only function suitable for describing all the samples. 4th step: calculation of the parameters of the three parameter Weibull-distribution functions. 5th step: selection of characteristic values of each distribution function for limit curves. The new method is the following:
6th or last step: calculation of parameters of design curves. Different design curves were determined for base materials and their welded joints, and for mode I and mixed mode I+II loading conditions. The minimum value of the Paris-Erdogan exponent under mode I loading condition is less than 2. Conclusions For microalloyed steels and their welded joints both the threshold stress intensity factor range (D Kth) and the exponent of the Paris-Erdogan law (n) decrease with the increase of the strength of steel, while fatigue fracture toughness (D Kfc) increases. Both the exponent of the Paris-Erdogan law (n) and fatigue fracture toughness (D Kfc) for welded joints are higher then those of base material. The proposed method is suitable for determination of fatigue crack propagation design curves under mixed mode I+II loading condition. For this case stress intensity factor range (D K) should be replaced by effective stress intensity factor range (D Keff). Summary The proposed method for determining fatigue crack propagation design curves can be generally applied for different materials and their welded joints; under mode I and mixed mode I+II loading conditions. Limit curves determined with this method represent the compromise of rational risk and striving for safety. Based on these fatigue design limit curves integrity assessment calculations can be done for operating structures. Acknowledgements Author wish to aknowledge the assistance given by the National Scientific Foundation for supporting the research (OTKA T 022020). Copyright © 2000 |