A new method for assessing high‐temperature crack growth

Abstract

A B S T R A C T Experimental creep crack growth (CCG) test data are obtained by following standards that characterize CCG rates using the C * parameter. Such data are then used in hightemperature failure assessment procedures. An alternative approach to defect assessment at high-temperature failure is an extension of the R6 failure assessment diagram (FAD). At high temperature, creep toughness, K c mat , can be estimated from CCG tests and replaces low-temperature toughness in R6. This approach has the advantage that it is not necessary to establish a creep fracture regime, such as small-scale, primary or widespread creep. Also, a new strain-based FAD has been developed, potentially allowing variations of stress and temperature to be accommodated. In this paper, the results of a series of crack growth tests performed on ex-service 316H stainless steel at 550 • C are examined in the light of the limitations imposed by ASTM for CCG testing. The results are then explored in terms of toughness and presented in FADs.Keywords creep crack growth; failure assessment diagram; material toughness; strain based. I N T R O D U C T I O NCreep-ductile materials are used extensively in the petroleum and power generation industries to fabricate pressurized equipment, such as piping and pressure vessels. These materials are usually tough and ductile at lower temperatures and retain good strength at high temperatures. Many austenitic and ferritic steels exhibit creepductile behaviour, and typically, these materials have ductilities that exceed 20%. Extensive experimental and theoretical studies on creep crack growth (CCG) have been carried out generally on materials that show considerable creep ductility. Materials classified as creep-ductile have the ability to sustain significant amounts of deformation prior to crack growth and final failure.In contrast, creep-brittle materials are more resistant to creep deformation, and their creep ductility is relatively low. However, in the next generation of power plants there is the need to reduce CO 2 emission and limit the use of natural resources, and this has provided an additional incentive to increase efficiency. 1 The main enabling technology is the development of stronger high-temperature materials, capable of operating under high stresses at ever increasing temperatures. Many of these advanced materials applicable to such operations are creep-brittle. Other

The effects of long-range residual stress, elastic follow-up and applied load on creep crack incubation and material toughness

“…All specimens experienced significant plastic deformation prior to creep. As observed by Fookes and Smith, 10 this is a feature of the behaviour of Type 316H stainless steel when specimens are subjected to reference stresses in excess of the yield strength (172 MPa at 550°). Notably, during initial loading of specimens with elastic follow-up, the creation of plastic deformation in the specimens led to elastic follow-up and initial relaxation of the initial misfit and total forces on the specimens prior to creep.…”

“…In addition to the results shown in terms of reference stress in Figures 12 and 13, a fracture mechanics approach was adopted. A material toughness was calculated using procedures developed in earlier work 10 with the toughness K mat [Δ a ], corresponding to the increment of crack growth from the initial notch, given by…”

“…The test rigs were also designed so that initial misfit longrange residual stresses could be introduced in combination with applied stresses. These experiments differ from earlier approaches [8][9][10] where selfequilibrating residual stresses were introduced directly into fracture mechanics specimens.…”

“…A material toughness was calculated using procedures developed in earlier work 10 with the toughness K mat [Da], corresponding to the increment of crack growth from the initial notch, given by…”

“…Based on load-controlled tests, the times for existing cracks to grow a certain distance have been related to material toughness, K mat . 9,10 This is equivalent to fracture toughness for elastic-plastic fracture. For high-temperature fracture, K mat is a function of the material's elastic-plastic and creep properties.…”