Predicting fracture and fatigue crack growth properties using tensile properties

Abstract

The safe-life assessment of components requires information such as the plane stress ( K c ), plane strain ( K Ic ), part-through fracture toughness ( K Ie ), and the fatigue crack growth rate properties. A proposed parametric/theoretical approach, based on an extended Griffith theory is used to derive fracture toughness properties and generate fatigue crack growth rate data for a range of alloys. The simplicity of the concept is based on the use of basic, and in most cases available, uniaxial stress–strain material properties data to derive material fracture toughness values. However since the methodology is in part based on an empirical relationship a wide ranging validation with actual data is required. This paper uses steel, aluminum and titanium based alloys from a pedigree database to quantify material properties sensitivity to the predictions for K Ic and K c and the subsequent estimation of Δ K th threshold and the Paris constants, C and n values. A sensitivity analysis using experimental scatter bounds show the range of d a/d N predictions can be achieved. It is found K Ic /Δ K th ratios designated as α has a range of 5–25 irrespective of tensile ductility, ε f , and is insensitive to it. The value of Δ K th for all the alloys considered was found to be proportional to the final elongation, ε f , and an empirical relationship describing Δ K th as a function of ε f was established. Furthermore it is suggested that, with the knowledge of appropriate tensile properties and the estimated range of K Ic /Δ K th ratios for the different alloys applying this method could be an appropriate tool that can be used to conservatively predict fracture and fatigue in similar alloy categories. Thus helping to reduce costs and optimize the number of experimental tests needed for alloy characterizations.