| Normal operating stresses acting on a material are magnified by the high stress concentration effect at the edge of a hole, making the material susceptible to crack initiation and propagation at the hole surface, resulting in a reduction of the fatigue life. One way to increase the fatigue life of components with holes is to impart beneficial compressive residual stresses around the hole, thereby negating the effect of stress concentration during the normal operating condition. Cold-expansion technology is a pre-assembly technique by which a hole is expanded beyond the material's elastic limit, pulling an oversized mandrel through it, creating a beneficial compressive residual stress zone around the hole. The elastic-plastic boundary around a cold-expanded hole is an important measure of the amount of induced cold-working, since other parameters, such as radial expansion and pressure at the hole surface have their limitations.; An analytical closed-form solution of a cold-expansion process is extremely difficult to obtain due to the three-dimensional, elastic-plastic, and frictional-contact nature of this problem. Existing analytical and numerical solutions are based on many simplifying assumptions, failing to correlate satisfactorily with experimental results. The present industry practice has evolved mainly on the basis of trial and error methods, and is heavily dependent on the operators' experience. The basic objectives of this research were to establish realistic finite element models of the cold-expansion process validated by experimental results, to perform a parametric study on the effect of different process variables on the magnitude and the distribution of residual stresses, and to develop analytical equations for the elastic-plastic boundary radius and the residual stress distribution around a hole based on far-field strain measurements.; In this research, analytical equations, utilizing far-field strain as an input parameter, were developed, assuming an axisymmetric, plane stress problem. In finite element simulation, combined material and geometric non-linear analyses were performed, considering the motion of the mandrel and the frictional effect at all contact surfaces. In experimental work, far-field strains were measured during the cold-expansion, while the radial residual strain gradient and surface profile measurements were conducted after the completion of cold-expansions. The experimental/analytical and finite element results on the magnitude of the far-field strain, the elastic-plastic boundary radius, the radial residual strain gradient, and the thickness change showed good agreement. A parametric study was performed to investigate the effect of process variables, such as degree of cold-expansion, ratio of plate thickness to hole radius, and magnitude of friction coefficient on the residual stress generation around a cold-expanded hole, utilizing the experimentally validated finite element model. |
