# Residual stress measurement using X-ray diffraction

measurements.

Residual stresses can be defined as the stresses which remain in a material in the

absence of any external forces. There are many stress determination methods. Some of

those methods are destructive and some are nondestructive. X-ray residual stress

measurement is considered as a nondestructive method.

X-ray diffraction together with the other diffraction techniques of residual stress

measurement uses the distance between crystallographic planes as a strain gage. The

deformations cause changes in the spacing of the lattice planes from their stress free

value to a new value that corresponds to the magnitude of the residual stress. Because of

Poisson?s ratio effect, if a tensile stress is applied, the lattice spacing will increase for

planes perpendicular to the stress direction, and decrease for planes parallel to the stress

direction. This new spacing will be the same in any similarly oriented planes, with

respect to the applied stress. Therefore the method can only be applied to crystalline,

polycrystalline and semi-crystalline materials.

The diffraction angle, 2?, is measured experimentally and then the lattice spacing

is calculated from the diffraction angle, and the known x-ray wavelength using Bragg's

Law. Once the d-spacing values are known, they can be plotted versus 2 sin ?, ( ? is the

tilt angle). In this paper, stress measurement of the samples that exhibit a linear behavior

as in the case of a homogenous isotropic sample in a biaxial stress state is included. The

plot of d vs. 2 sin ? is a straight line which slope is proportional to stress. On the other

hand, the second set of samples showed oscillatory d vs. 2 sin ? behavior. The oscillatory

behavior indicates the presence of inhomogeneous stress distribution. In this case the xray

elastic constants must be used instead of E and ? values. These constants can be

obtained from the literature for a given material and reflection combination. It is also

possible to obtain these values experimentally. Calculation of the residual stresses for

these samples is beyond the scope of this paper and will not be discussed here.

Advisor:Yavuz, Mustafa; McIntyre, Peter; Karaman, Ibrahim

School:Texas A&M University

School Location:USA - Texas

Source Type:Master's Thesis

Keywords:x ray diffraction residual stress

ISBN:

Date of Publication:12/01/2004