Increasingly high-temperature equipment is expected to operate in harsher environments and to survive well beyond the initial required design life. This can lead to low equipment life expectancy and low overall production. To this end, researchers have devoted efforts towards the development of effective service and life extension methods. With extensive changes in modern design, the adoption of damage tolerant design has recently proved to be of great importance. For instance, it can be used in diagnostics and prevention of failure mechanisms through defect assessment.
Presently, various methods for predicting creep crack growth behavior, a common failure at high temperatures, have been developed. In particular, C* and C(t) concepts have been applied for steady-state creep crack growth and time-dependent growth respectively. Under multiaxial stress states, plane strain concepts apply and current code methods uses a multiplication of thirty times the plane stress crack growth rates. To improve the accuracy of the results and to decrease current conservatisms, it would be highly desirable to obtain biter predictions for plane strain C* growth rates.
To this note, Dr. Warwick Payten from the Australian Nuclear Science and Technology Organization presented a study where he revaluated the upper bound plane strain multiaxial C* crack growth rates for different materials. Here, a different approximation method based on the integral formulation of the multi-axial ductility factor was utilized. Specifically, the multiaxial ductility factor was approximated from the integration of the dimensionless deviatoric and hydrostatic strain energy density of the Hutchinson Rice Rosengren (HRR) singular fields. His research work is currently published in the journal, Engineering Fracture Mechanics.
Based on the equivalency approximation between the plastic and elastic strain energy, the author determined the crack tip triaxial constraint using the damage mechanics method. The newly determined multiaxial factor was observed to be independent of power-law behavior. As such, a multiaxial factor value close to one was recorded for the plane stress Hutchinson Rice Rosengren fields. Interestingly, the new factors were effectively used to estimate the upper bound crack rate under plane strain. To justify the integrity of the new approximations of the upper bound crack growth rates, creep crack growth data of nine different materials were employed. From the results, nearly all the materials showed the possibility of decreasing the current upper bound plane strain crack growth rates from thirty times to three times the plane stress.
Carbon Manganese and stainless 316 material were among the investigated materials using this new approach. It was worth noting that the author observed that for the two materials, the experimental data was in line with the predicted finite elements results for the NSW-MOD model. The results were consistent with the experimental data at low C* values. However, Nikbin Smith Webster- strain energy model was relatively insensitive to creep stress exponent especially with crack rate ratios between plane stress and plane strain. This was largely evident when the model was extended to a range of different materials including low alloy ferritic, Inconel alloys and ferritic martensitic among others.
In a statement to the Advances in Engineering, Dr. Warwick Payten highlighted the importance of investigating the creep crack growth in enhancing the performance of different materials. For example, his research results suggested that the current upper bounds could be lowered from the current thirty times to three times the plane stress creep crack growth rate for most materials.
Payten, W. (2019). A reassessment of the multiaxial ductility C* creep crack growth equation based on the strain energy integral of the HRR singular field terms. Engineering Fracture Mechanics, 217, 106530.