Abstracts - M.B. Prime
M. B. Prime, "Measuring Residual Stress and the
Resulting Stress Intensity Factor in Compact Tension Specimens"
The measurement of residual stress through the remaining ligament of a
compact tension specimen was studied. In the crack compliance method, a
slot or notch is successively extended through the part, and the resulting
strain is measured at an appropriate location. By using a finite element
simulation of a specimen preloaded beyond yield, three techniques for
determining the original residual stress from the measured strains were
compared for accuracy and sensitivity to measurement errors. A common
beam-bending approximation was substantially inaccurate. The series
expansion method proved to be very versatile and accurate. The fracture
mechanics approach determined the stress intensity factor caused by the
residual stresses with a very simple calculation. This approach offers the
exciting possibility of determining the stress intensity factor prior to a
fatigue or fracture test by measuring strains during the specimen
preparation.
P. Rangaswamy, M. B. Prime, et al., "Comparison of
Residual Strains Measured by X-ray and Neutron Diffraction in a Titanium
Matrix Composite"
The objective of this research was to compare matrix thermal residual
strains measured in a continuous fiber reinforced SiC/Ti-6Al-4V titanium
matrix composite (TMC) using X-ray and neutron diffraction techniques with
finite element predictions. In addition to this, the hkl dependence on the
strains for several reflections (105,204,300,213,312) of the matrix were
explored at the surface (X-ray) and in the bulk (neutron). A variation on
the x-ray d vs. sin2y method was used in X-ray diffraction technique to
determine the longitudinal surface strain (e11) rather than the
conventionally determined strain difference (e11-e33), which gives
longitudinal stress. Finite element predictions were used to compare
surface and bulk strain measurements in the direction parallel to the
fibers. Continuum micro-mechanics based multi-ply finite element models
(FEM) simulating rectangular and hexagonal fiber distributions were used
for calculating average surface and bulk strains. Differences in the
surface measured strains ranged from + 1904 to + 2974 me and the bulk
measurements ranged from + 2269 to + 3022 me. These values contrast with
the FEM predictions of + 3200 me determined for both the surface and the
bulk. The 10% differences between the averaged bulk and averaged surface
strains could not be explained by the spatial variation in the finite
element predictions.
M.B. Prime and Ch. Hellwig, "Residual Stresses in a Bi-Material
Laser Clad Measured Using Compliance"
In this research we used the compliance method to measure residual stresses
in a laser-clad layer and the underlying substrate. Surface strains were
measured as a slot was incrementally introduced using wire electric
discharge machining (EDM). The elastic modulus of the layer, a copper
alloy, was about 85% greater than that of the base material, an aluminum
alloy. Because of this large difference in elastic constants, a new solid
mechanics solution was needed in order to apply the compliance method
accurately. The stress profile was also measured using x-ray diffraction
and electrochemical layer removal. Results from the two techniques were
compared. Additionally, the error caused by using an older single material
solution was shown to exceed 50% for stresses measured near the
interface.
M.B. Prime and I. Finnie, "Surface Strains due to Face Loading of a Slot
in a Layered Half-Space"
A solution is presented that will allow the compliance method for
residual stress measurement to be applied to layered materials. The
solution is for surface strains due to arbitrary normal stress loading on
the faces of a slot in a layered half-space. The materials are homogeneous
isotropic elastic with different elastic constants and the slot may
penetrate into the substrate. The solution is accomplished using the body
force method and the solution for a point force in two bonded half spaces.
The results indicate that, for residual stress measurement, the effects of
the substrate properties are significant for materials with elastic moduli
differing by 50 percent or more when the slot penetrates to at least
one-half of the layer thickness.
M.B. Prime and D.W. Shevitz, "Linear and Nonlinear
methods for Detecting Cracks in Beams"
This paper presents experimental results from the vibration of a
polycarbonate beam containing a crack that opens and closes during
vibration. Several techniques were employed to detect and locate the crack
making use of the nonlinearity. "Harmonic mode shapes" proved to be more
sensitive to damage than conventional mode shapes. Instantaneous frequency
and time-frequency methods also showed clear signatures for the crack. The
results indicate that nonlinearities may provide increased capabilities for
structural damage detection and location.
M.B. Prime, "Residual Stress Measurement in Layered Media"
(Ph.D. dissertation)
The significant contribution of residual stresses to failure, the current inability to predict them accurately, and the widespread use of layered materials all illustrate the need for an experimental technique to measure residual stresses in layered materials. This work develops the compliance method for the case of a layer on an infinite substrate, where both are isotropic elastic materials. In this method, a slot is introduced incrementally into a body containing residual stress. The resulting surface deformations are measured and used to calculate the stress variation with depth, both in the layer and into the substrate.
A brief review of existing residual stress measurement techniques, especially as applied to layers, is given. A review of the theoretical development of the compliance method is also given. It is necessary to calculate the compliance functions, or surface strains for arbitrary face loading of a slot, for a layered half-space. The body force method is used. Continuously embedded point forces are applied to the boundary of the prospective slot and free surface and adjusted to satisfy the boundary conditions. Complete details of the numerical implementation, not available elsewhere for the body force method, are given.
Results for compliance in a layered half-space are presented for various slot and layer geometries and combinations of elastic properties. Specific attention is given to the errors introduced by ignoring the different elastic properties of the substrate, an oft made assumption in previous measurements in clads or coatings. Finally, the compliance method is applied to the experimental measurement of residual stress in a beam composed of a 0.039 inch (1.0 mm) thick layer of tantalum brazed to a copper substrate.
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