The energy density exhaustion method for assessing the creep-fatigue lives of specimens and components

Abstract

This paper attempts to link the energies to failure involved in low-cycle fatigue at elevated temperature with those involved in forward creep. These energy levels are connected by a suitable sliding strain-rate scale. The principle is then applied to (i) dwell tests on 1CrMoV, 304 and 20Cr25Ni/Nb steels at 550, 650 and 750°C respectively and (ii) service failure of a thick-walled ferritic steam vessel at 570°C. Calculated creep-fatigue endurance using the linear damage summation rule is then plotted on Damage Diagrams. Sensitivity analyses are carried out as regards the linking of energy levels in fatigue (upper-shelf) with those in creep (lower-shelf). These involve knowledge of creep laws or stress relaxation relations and the methods of connecting energy levels (power law or those giving an S-shaped curve). Within this paper, sufficient information has been provided for investigators to undertake their own analyses on relevant laboratory tests or service assessments.

A New 3D Creep-Fatigue-Elasticity Damage Interaction Diagram Based on the Total Tensile Strain Energy Density Model

“…where a and b are model constants and can be fitted by experimental data. The ∆w c+ can be considered as creep component [24], and is expressed as…”

“…Though these methods can account for the loading waveform effect, the damage interaction diagram, which is very crucial in the design and safety assessment of high-temperature structures, cannot be established. Since experiments and theory [20,21] have revealed that a stable value of hysteresis energy is arrived after a short initial cycle, even the materials possess the cyclic hardening or softening behavior, many energy-based fatigue life prediction methods have been proposed, and good prediction accuracy is obtained [22][23][24][25][26][27][28]. For example, Wang et al [26,27] proposed a modified strain energy density exhaustion model for creep-fatigue life prediction in which the mean stress effect is considered.…”

An efficient fatigue and creep‐fatigue life prediction method by using the hysteresis energy density rate concept

“…S AEFB that represents the energy dissipated during stress relaxation is obtained by 41 : where σ o is the initial stress at the creep period, σ r is the final stress of the stress relaxation period, as shown in Figure 2A. E is the elastic modulus.…”

An Initial Assessment of the Design Margins of Different ASME Section III, Division 5 Design Rules

“…The key to the approach is a work-based damage model that captures the available tensile, creep, fatigue, and creep-fatigue data. This type of damage model has been applied to high-temperature components in the past [3,4], but this report is the first to use the method to predict the margin of the ASME rules for a particular component.…”