Residual stresses can exist in materials even when no external
loads are applied to the material. In fact, most fabrication processes
leave residual stresses in the material. Most residual stresses in
materials are not well understood, and can
be either detrimental or beneficial to the life of a component.
At MURR we observe stresses in materials by using neutrons to measure
the change in the spacings of atomic planes as a result of stress
with a specially designed instrument, the MU Residual Stress Instrument
(MURSI). Our research aims to improve materials by characterizing
residual stresses and their influence on material performance.
Neutron reflectometry is a very powerful tool for investigating
the near surface structure of many materials. Typical measurements
are carried out by scattering neutrons in the grazing incidence
geometry, beyond the critical angle for total external reflection,
as a function of wave vector transfer Q (2sinq
/ l), perpendicular to the reflecting
surface. This profile of reflectivity then can be related to the
neutron scattering length density (Nbc) at
depths of up to several thousand Å, with an effective length
resolution of a few Å. The samples must have a smooth, flat
surface, the individual layers must be smooth and have uniform thickness
throughout, and preferably several cm2 in area. The method is extensively
used for studies of polymer films and interfaces, and thin films
and multilayers of metals and superconductors, both magnetic and
non-magnetic.
MURR Center researchers also use other characterization tools that
provide information complementary to neutron scattering studies.
These include a SQUID magnetometer, a Deep Level Transient
Spectroscopy system and photoluminescence.
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University of Missouri-Columbia
