Strain concentration in beams under cyclic plastic straining

Abstract

Cyclic plastic-straining tests with controlled deflections have been conducted on beams subject to uniform bending, three-point bending, and a cosine distribution of bending moment; the second and third beams represent cases of strain concentration. Three different materials were used, namely mild steel, stainless steel, and aluminium.

The strain-concentration tests show stainless steel and aluminium to be more resistant to deflection cycling than mild steel. A similar difference is not found in the uniform bending tests to anything like the same extent. Stainless steel shows a more pronounced strain-hardening characteristic in the cyclic stress-strain curve than does mild steel and it is concluded that this produces a more favourable strain distribution along the length of the beam, so that the maximum strain is less and the endurance is correspondingly greater.

For materials which show settled cyclic stress-strain relations, reasonable predictions may be made of life in deflection cycling of beams under strain-concentration conditions if the strains are calculated from the cyciic relations and the corresponding endurance is obtained from uniform bending tests. If, for design purposes, the strains determining the life are calculated from monotonic stress-strain relations, the design will be safe, provided the material does not soften with cyclic straining.