Thermo-Mechanical Fatigue Lifetime Assessment of Spheroidal Cast Iron at Different Thermal Constraint Levels

Abstract

In previous work on the thermo-mechanical fatigue (TMF) of compacted graphite iron (CGI), lifetimes measured under total constraint were confirmed analytically by numerical integration of Paris’ crack-growth law. In current work, the results for CGI are further validated for spheroidal cast iron (SGI), while TMF tests at different constraint levels were additionally performed. The Paris crack-growth law is found to require a different CParis parameter value per distinct constraint level, indicating that Paris’ law does not capture all physical backgrounds of TMF crack growth, such as the effect of constraint level. An adapted version of Paris’ law is developed, designated as the local strain model. The new model considers cyclic plastic strains at the crack tip to control crack growth and is found to predict TMF lifetimes of SGI very well for all constraint levels with a single set of parameters. This includes not only full constraint but also over and partial constraint conditions, as encountered in diesel engine service conditions. The local strain model considers the crack tip to experience a distinct sharpening and blunting stage during each TMF cycle, with separate contributions to crack-tip plasticity, originating from cyclic bulk stresses in the sharpening stage and cyclic plastic bulk strains in the blunting stage.

Metal Plasticity and Fatigue at High Temperature

“…As a part of this Special Issue, researchers were invited to submit their innovative research papers aimed at providing a state-of-the-art knowledge on the topic of metal plasticity, creep deformation and fatigue strength of metals operating at high temperatures, with emphasis on both experimental characterization and numerical modeling of material behavior. A total of eleven research papers were published [1][2][3][4][5][6][7][8][9][10][11].…”

“…Thermo-mechanical fatigue (TMF) is studied in [5][6][7]. The paper written by Szmytka et al [5] provides an overview of a TMF design protocol through the analysis of the specific case of a cylinder head, which summarizes the studies led by the authors over the last five years.…”

“…In [6], Wagner et al work on the identification of a criterion for crack initiation to provide the basis for a nondestructive quality control of TMF-loaded porous components with improved statistical safety. The paper by Ghodrat et al [7] presents an adapted version of Paris' fatigue crack-growth law where cyclic plastic strains at the crack tip are considered as the parameter that controls crack growth. Finally, it is found that the modified Paris' law is able to assess TMF lifetimes of spheroidal graphite iron (SGI) very well for all constraint levels with a single set of parameters.…”

“…Papers in this Special Issue addressed different types of material, from stainless steel studied by [1,2,4,10], to aluminum alloys investigated by [3,6,9], Ni-based supper alloy in [8], spheroidal graphite iron by [7], and copper alloy discussed in [11].…”

“…The developed constitutive methods have, in most cases, been compared to experimental data obtained with tensile tests [3], fatigue tests [3,4,8], TMF tests [6,7], creep tests [2], and stress relaxation, aging tests presented in [9].…”