Aspects of neutron residual stress analysis
Robert C. Wimpory
2134/25162
https://repository.lboro.ac.uk/articles/thesis/Aspects_of_neutron_residual_stress_analysis/9411053
This thesis is concerned with the physical principles, methodology and applications of
neutron diffraction in the measurement of residual stress. Work on three main areas is
presented. 1) Carbon steels 2) Data and Peak Broadening analysis and 3) Single lap
glue shear joints. The Carbon steels section shows the drastic effect of the content of
carbon on the measured stress. This is an aspect which has been somewhat neglected
in the past. The carbon is in the form of cementite, which is a hard compound and
causes the carbon steel to act like a composite material, the ferrite acting as a soft
matrix and the cementite as a reinforcement. The consequence of this is that the two
components develop high microstresses with plastic deformation. This is clearly
illustrated in the work of [Bon 97] where values of approx. 460 MPa in the residual
stress in the ferrite are balanced by negative residual stresses of 2300 MPa in
cementite yielding an overall macro residual stress of zero. In this work it has been
shown that even knowledge of the cementite and ferrite residual stresses and fractions
may not be sufficient to accurately calculate the macro stress since the ferrite
unloading curve is non linear. The use of a single valued constant modulus to convert
from strain to stress is hence not valid.
Peak shape analysis enables dislocation density and cell size estimates to be made.
The thesis examines several methods of data weighting and deconvolution in order to
asses the best means of extracting this information from standard residual stress data.
Care should be taken for the peaks with very low backgrounds when finding the
Gaussian and Lorentzian components. A weighting that avoids the strong bias of zero
and I counts in the detector channels should be used e.g. W = I / ( 10 + Y). Lorentzian
and Gaussian components can be successfully extracted from asymmetrical peaks (of
peaks that broaden symmetrically), using deconvolution method 1, although the data
should be of good quality. Reproducibility has been shown in the Gaussian,
Lorentzian and FWHM for different instruments at different institutes. This is
extremely important for the use of these values for peak broadening analysis and for
estimation of the plastic deformation within a sample.
The neutron diffraction technique has been used to investigate the longitudinal
stresses in the adherend produced as a result of cure and due to the application of a
tensile load in a single lap shear joint. The results throw doubt on widely used finite
element predictions.
2017-05-26 11:48:39
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