When designing engine parts of motor vehicles, it is important
to evaluate internal residual stresses that cause crack growth and
influence the strength of parts. Internal stresses can be measured
nondestructively by the neutron diffraction method. However, it is
difficult to apply this method to aluminum alloy castings because
they consist of coarse crystal grains. As for cylinder heads, the
grain size ranges up to approximately 400 μm and there are few
grains contributing to intensity of diffraction in each gauge
volume. In the case of X-ray diffraction, "the oscillation
method" has been employed for materials with coarse
grains.
In this study, the applicability of the oscillation method to
aluminum alloy castings was investigated with the aim of
establishing a method of measuring internal stresses and strains. A
related objective was to determine the accuracy of stresses. For
the purpose of the study, specimens made of an aluminum alloy
casting and measuring 20x20x140 mm in size were prepared and
subjected to a bending load. A neutron diffraction experiment with
oscillations was conducted, and the strain values were compared
with the results measured with strain gauges. The experimental
results show that an oscillation range of ±15 degrees provides
enough intensity of diffraction to read peak positions. Under this
measurement condition, the differences between the strain values
obtained by neutron diffraction and with strain gauges were less
than 250x10-6 and the accuracy of the stresses was within 20-25
MPa. We also applied this method to an actual cylinder head. As a
result, sufficient intensity of diffraction was observed with
oscillations of ±15 degrees. It was found that there were internal
tensile residual stresses. We verified the efficacy of the neutron
diffraction method with oscillations for aluminum alloy
castings.
