Variability of Strain-Controlled Fatigue Properties of USS Dual Phase 80 Steel

Because sheet-steel users are requesting strain-controlled fatigue properties of sheet steels for automotive use, a program is being conducted to characterize single grades of steel and to examine the variability of their fatigue properties. As part of that program, monotonic tension tests and low-cycle strain-controlled fatigue tests were performed on smooth longitudinal and transverse specimens from sheets of USS Dual Phase 80 steel. An 0.096-inch (2.4 mm) thick sheet from one heat and 0.131 inch (3.3 mm) and 0.188-inch (4.8 mm) thick sheets from a second heat were evaluated. The sheets were tested in the “as-shipped” (unstrained) and prestrained conditions. Prestraining was accomplished with a single uniaxial tension load sufficient to cause strains as high as 8 percent prior to fatigue testing.

The results of this study showed that the unstrained material was cyclically stable; that is, the cyclic yield strength was approximately the same as the monotonic yield strength. However the prestrained material cyclically softened so that the cyclic yield strength was considerably less than the monotonic prestrained value and approached or was lower than the cyclic yield strength of the unstrained material.

In this study the strain-controlled fatigue properties of Dual Phase 80 steel did not vary more among sheets from different coils and subjected to different amounts of prior uniaxial tensile prestrains than within a single sheet. Therefore this study gives little evidence that sampling from more than a single sheet is necessary to estimate the fatigue properties of this grade of steel. This finding is consistent with previous results for high-strength low-alloy sheet steels.