Application of fracture mechanics concepts in ultrasonic fatigue

Ultrasonic fatigue experiments are not carried out in load control but in displacement control. In addition, the thickness of the specimen center in ultrasonic fatigue must be thinner to increase the stress concentration. This means that it is not possible to apply the usual stress intensity factor calculations. It is a key point to study crack growth rate in ultrasonic fatigue. In this paper, a finite element method (FEM) based on the theory of linear elasticity is presented for the plane-strain problem. Eight-node isoparametric quadratic elements were used and two small singular elements were installed at the crack tip to compute the singular field. After taking account of the temperature variation, the specimen thickness and the dynamical effect, the stress intensity factor was computed. Finally, the process was standardized. The program was verified through a calculation for a standard example, the error being small for a/w < 0.5. Several experimental results are given for fatigue crack growth of nickel-base alloys at room and elevated temperature.