Influence of Stress State on the Failure Behavior of Cracked Components Made of Steel

Abstract

One of the decisive factors influencing the safety of components is the capacity for plastic deformation of the material employed. This depends not only on the actual material properties, such as reduction of area or notch impact energy, but also on the stress conditions prevailing in the component. With sufficiently sharp transitions of geometrical form, or at cracks, such high multiaxial stress states can arise in components, that in spite of excellent plastic deformation capability of the malterial, practically deformationless fractures are inevitable. If one generates from the principal normal stresses (σ1, σ2, σ3) the multiaxiality quotient q, which represents a characteristic quantity for the degree of multiaxiality of the stress state, the effect of the stress states on the strength and deformation behavior of a component can be estimated. With the aid of the Sandel fracture theory, which includes the von Mises yield theory as a special case, the critical q value qc, which characterises the stress conditions leading to cleavage fracture if q < qc, can be calculated. The fracture mechanics evaluation of the sharply notched specimens of dimensions similar to components shows no dependence of the effective crack initiation value on the specimen size or stress state, since at the load free crack tip, plane stress conditions generally prevail. The further failure process after crack initiation in the form of stable crack extension is very strongly controlled by the stress state. This phase could also be estimated from consideration of the pattern of the q value in the remaining cross section. The investigations have shown that the multiaxiality quotient q, which characterizes the degree of multiaxiality of the stress state, represents a characteristic quantity with which, in combination with fracture mechanics methods, the failure behavior of components may be estimated, even with respect to stable crack extension.

Description of failure modes in welded components operating in the creep regime

“…The factor of multiaxiality [14] q is introduced as a quotient of von Mises stress and hydrostatic stress: (4) with and (5) The need for the implementation of the factor of multiaxiality q of the stress state and the coupling with damage evolution in the constitutive equations is demonstrated in the following. Results of numerical simulations of P91 hollow cylinders under internal pressure are depicted in Fig.…”

“…higher creep cavity density can be found close to the centre of the specimen, where the multiaxiality of the stress state has its maximum. The factor of multiaxiality of the stress state q can be described [14] as a quotient of von Mises stress σ vM and hydrostatic stress σ hyd , eq. (4).…”

Micromechanical modelling of weldments using GTN model

“…The original idea was a unique fracture resistance curve would suffice to characterise the material. However, testing of different types of specimens under different loading conditions revealed considerable different J R curves especially in the slope (Clausmeyer et al 1991). This raises the question of transferring fracture parameters from specimens to component level.…”

Unified characterization of in-plane and out-of-plane creep constraint based on crack-tip equivalent creep strain

“…The characterization of constraint has been widely investigated within the elastic-plastic fracture mechanics frame, and leading to the development of two-parameter or three-parameter fracture mechanics, such as two-parameter concepts K-T [2], J-Q [3,4], J-A 2 [5], J-T Z [6][7][8], J-h [9] and three-parameter concept J-T z -Q T [10]. Most of these parameters are only used to quantify the in-plane or out-of-plane constraint separately, but not the interaction between in-plane and out-of-plane constraint and the overall level of constraints.…”