LOCAL VARIATIONS IN YIELD STRENGTH – A POSSIBLE MEANS FOR ENHANCING THE FRACTURE RESISTANCE OF STRUCTURAL COMPONENTS O. Kolednik Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, A - 8700 Leoben, Austria ABSTRACT In materials with local variations in yield stress, the effective near-tip crack driving force can become different from the nominally applied far-field value. The near-tip crack driving force is enhanced, if the yield stress decreases in the crack growth direction, and vice versa. This effect, termed as the yield stress gradient effect, can be used for the design of fracture resistant structural components by applying materials where the yield stress (and/or the elastic modulus) is intentionally varied. A model has been developed that allows us to derive analytical expressions to quantify the effect. The analytical expressions can be used to optimize monotonically or cyclically loaded components with smooth gradients in yield stress as well as interface and interlayer transitions. It is demonstrated, for example, that both a soft or a hard interlayer may act as a crack arrestor. Similarly, the fracture resistance of components with hard surface layers can be improved by appropriate interlayers between the surface layer and the base material. INTRODUCTION It is known long since, e.g., from papers by Erdogan and co-workers [1, 2, 3], that in materials with local variations in the elastic modulus the local near-tip crack driving force can become different from the nominally applied far-field value,. Recently, Kolednik [4, 5] has been demonstrated analytically that local variations in the yield stress can have a similar effect: in materials with a gradient in yield stress in the crack growth direction an additional crack-driving force term appears, called the yield stress gradient term. The yield stress gradient term is positive, i.e., it enhances the effective crack driving force when the yield stress decreases in crack growth direction. On the contrary, an increase of the yield stress in the crack growth direction induces a negative yield stress gradient term which diminishes the effective crack driving force. Experimentally, it was first shown by Suresh et al. [6] that a gradient in yield strength alone can influence the effective driving force of fatigue cracks. They conducted fatigue crack growth experiments on explosion clad bimaterial specimens consisting of austenitic and ferritic steel. For a crack approaching the interface from the softer ferritic steel (soft-hard transition), the crack growth rate dropped precipitously and the crack stopped before the interface. However, for a hard-soft transition, the crack advanced unimpeded through the interface; the crack growth rate remained roughly constant, except for some distance before the interface where a slight increase of the crack growth rate was noted. Subsequent experiments by Suresh et al. [7] and, recently, by Pippan and Flechsig [8, 9] further confirmed these findings. Finite element studies by Sugimura et al. [10] and Kim et al. [11] revealed that for a stationary, monotonically loaded crack the near-tip J-integral, Jtip, begins to deviate from the applied far-field value, Jappl, as soon as the plastic zone around the crack tip touches the interface: for a hard-soft transition, Jtip becomes higher than Jappl; for a softhard transition, Jtip < Jappl. Jtip can be interpreted as the intensity of the crack-tip field and approximated as
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