抄録
When evaluating the durability of engine parts, it is important to know the internal residual stresses that cause crack growth and influence fatigue strength. The only way to measure internal stresses nondestructively is by the neutron diffraction. However, it is difficult to apply this method to aluminum alloy castings with coarse crystal grains because there are few grains contributing to intensity of diffraction in gauge volumes. In this study, the applicability of "the oscillation method" to aluminum alloy castings was investigated with the aims of solving the problem and establishing a method of measuring internal stresses and strains of this material. Specimens made of an aluminum alloy casting were subjected to a bending load and a neutron diffraction experiment with oscillations was conducted. The experimental results show that an oscillation range of ±15 degrees provides enough intensity of diffraction to read peak positions. Under this condition, the differences between the strain values obtained by diffraction and with strain gauges were less than 250 X 10-6 and the accuracy of the stresses was within 20-25 MPa. We also applied this condition to actual cylinder heads. It was found that there were internal tensile residual stresses in regions where cracks grow easily. We verified the efficacy of the oscillation method for aluminum alloy castings.
