A B S T R A C T Defects in structural components are often associated with welds that may contain significant levels of residual stress. Whilst the primary load acting on the component may induce low constraint conditions at the crack tip, the presence of residual stresses, e.g. due to welding, can modify this constraint level and consequently influence the observed fracture toughness behaviour. This paper presents the results of a combined experimental and numerical programme aimed at exploring this issue. Cleavage fracture toughness data for high and low constraint specimens are presented with and without residual stresses. The results indicate that under certain conditions, the constraint-induced increase in cleavage fracture toughness may be eroded by the presence of a residual stress in the vicinity of the crack. The results are quantified with respect to two-parameter fracture mechanics in which the T and Q parameters are appropriately defined. Preliminary guidance is provided for the assessment of defects when residual stresses may influence crack-tip constraint.
A B S T R A C T Experimental data on tensile and compact geometry (CT) specimens of austenitic Type 316L(N) steel were obtained under sustained load conditions at room temperature. Timedependent crack growth, in some cases leading to failure, occurred in many of the CT specimens, dependent on the load level. However, rupture of the uniaxial specimens occurred only at stresses very close to the material's ultimate strength. The data validate the approach to assessing sustained loading effects in the R6 defect assessment procedure.In particular, sustained load effects in austenitic steel may be neglected for values of the R6 L r parameter less than unity. Uniaxial sustained load tests were also performed at 100 • C and 200 • C. The measured strain rates decreased with increasing temperature, becoming negligible at 200 • C. This is consistent with the advice in R6 that sustained load effects in austenitic steel can be neglected at temperatures between 200 • C and the high-temperature creep range.Keywords cold creep; fracture; sustained loading; 316 steel. N O M E N C L A T U R Ea = crack length, a o + a a o = initial crack length B = gross thickness B e = effective thickness, B n (2 − B n /B) B n = nett thickness E = Young's modulus E = E/(1 − ν 2 ) f (L r ) = failure assessment curve f 1 , f 2 = particular expressions for f J = crack driving force J 0 = value of J on load-up J mat ( a) = tearing resistance K = stress intensity factor K mat ( a) = fracture toughness K r = R6 parameter, K/K mat K r = modified R6 parameter, λK r L r = R6 parameter, P/P L L max r = R6 plastic collapse limit, σ f /σ y N = strain hardening exponent P = load P L = limit load t = time from start of sustained loading W = width Correspondence: P. J. Budden.
Currently, structural integrity safety cases are based on avoidance of fracture initiation, providing a pessimistic estimate of structure life. However, crack behaviour at heterogeneous junctions is an area where understanding is particularly limited. Improved data regarding the behaviour of defects at such junctions after initiation may lead to improved assessment of real structures containing welded joints. Furthermore, justification of stable tearing at a material interface would result in increased demonstrable safety and higher plant availability. An experimental study has been carried out to investigate the ductile fracture behaviour of a crack approaching material interfaces. The aim has been to examine and refine current guidelines in the R6 defect assessment procedure. The experiments have been supported by 3D finite element modelling of the specimens and by R6 calculations. A further assessment of the standard (25mm) CT specimen results indicated that the R6 guidance on usage of the toughness in the crack tip medium may not always be conservative. However, the assessment points are conservative when the lowest yield strength or the weighted average yield strength values are used in the R6 fracture assessment.
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