Crack Compliance Method Applied to FSW Stringer Skin Lap Joint to Determine
Residual Stress Profile
Michael Bach, Carleton University, Canada.
Conventional riveted stringers increase structural rigidity of aluminum skin panels but
also increase cost and weight to these aerospace structures. Friction stir welding (FSW)
invented by TWI was applied to these stringer skin lap joints as a solution to this problem.
The 7075 aluminum stringer was FSW to a 2024 aluminum skin panel. Distortion was apparent
in the panel after welding. A post-welding technique of hammer peening reduced the distortion
in the lap joints. The crack compliance method, a combination of experimental slitting, FEM
modeling, and analytical solutions, was applied to the lap joint to detail the residual
stress profile in the stringer lap joints before and after the hammer peening. Four test
pieces were cut from original plate for testing. Uniaxial strain gages on the test pieces
recorded measured strains. Removal of elements in FEM model simulated slitting method
used to calculate compliance functions. Strains and compliance functions were combined
with series expansion solution based on Legendre polynomials to calculate the residual
stress profile. The residual stress profile in the transverse, longitudinal, and
through-thickness direction were determined. Fatigue testing and fractograpy was used
to study the effects of residual stress on fatigue life of FSW plates before and after
post-welding processes.
Measurement of Bulk Residual Stress Distributions in Thick-section Components
using the Contour Method
Adrian T. DeWald, Hill Engineering, McClellan, CA.
Prediction of the fatigue and fracture performance of large, monolithic components depends
on knowledge of the bulk residual stresses that they contain. The contour method is a new
way to measure bulk residual stress fields that provides data useful for forecasting fatigue
and fracture performance, and it can be applied to thick-section parts. Relying on simple
assumptions and straightforward experimental procedures, the method provides the two-dimensional
spatial distribution of residual stress normal to a plane of interest within the component.
When the method is applied at sections with high failure risk, the measured residual stress
field may be used directly with standard methods for predicting fatigue crack initiation and
growth. The presentation provides a summary of the experimental details of the contour method
and examples of its application with specific emphasis on aerospace forgings and thick-section
welds.
Effects of Laser Peening on Fatigue Life in an Arrestment Hook Shank Application
for Naval Aircraft
M. J. Leap, Naval Air Warfare Center, Maryland,
J. Rankin, L. Hackel, T. Heidenberger and S. Marley, Metal Improvement Corp., Livermore, CA,
J. Harrison, Metal Improvement Corp., Wellington, KS, and
J. Nemeth, Concurrent Technologies, Johnstown, PA.
Laser peening is evaluated relative to and in combination with other means of improving fatigue
resistance in a particularly severe arrestment hook shank application for a carrier-based Naval
aircraft. A large-scale test specimen was designed and manufactured from Hy-Tuf steel to
geometrically simulate conditions in the arrestment hook shank, and fatigue tests were conducted
on peened specimens under conditions of spectrum loading that simulate aircraft arrestments.
Laser peening substantially increases the resistance to crack initiation relative to conventional
shot peening due to its ability to impart deep levels of plastic deformation and thus deep residual
stress. Samples were laser peened and residual stress measured via three techniques: x-ray
diffraction, slitting (crack compliance) and hole drilling. Similarities and differences in the
stress results for otherwise identical coupons were observed from the three techniques and will
be discussed. Fatigue samples were laser peened/shot peened or shot peened only and then fatigue
tested in an axial pull mode on a 100 kip rig with a fatigue spectrum replicating that observed in
carrier landings. Samples that included laser peening showed extended lifetime before crack
initiation of 250%.
Complementary Residual Stress Analysis of a Ball Bearing by Diffraction of Synchrotron
Radiation and Neutron
S. Hirai, T. Fujiwara and M. Uesaka, University of Tokyo, Japan.
Ball bearings are among the most important components of rotating equipment in power plants.
Investigations on ball bearing condition have been conducted by measuring vibrational acceleration
and acoustic emission, counting wear particles and by doing finite element method (FEM) stress
analysis. However, these methods give only indirect information of bearing fatigue or behavior
occurring near the surface of the ring and balls. In this study, stress-strain analysis on rolling
elements of bearings with different deterioration levels was done using neutron diffraction, lab
X-ray and synchrotron radiation to estimate its 3D destruction mechanism from the surface to the
deepest part of the interior. The measured samples, the rolling elements of high carbon-chromium
bearing steel, initially had compressive residual stress, approximately 800 MPa, on its surface
by heating with high-frequency wave, and lost the stress due to deterioration by cracking on the
surface. The following experiments were done to assess the complementarity of measurement by
diffraction of three types of radiation: a thermal neutron source JRR-3, a spallation neutron
source J-PARC, and a synchrotron radiation facility SPring-8. Each method showed the abilities
to estimate residual stress and strain distribution of the rolling elements.
Evaluation and Characterization of Stress and Texture in Lightweight Materials for
Vehicle Applications
Camden R. Hubbard and Thomas R. Watkins, Oak Ridge National Laboratory, TN.
A selection of vehicle light weight materials related stress and texture studies is
addressed each year at the DOE-Vehicle Technology sponsored High Temperature Materials
Laboratory User Facility (HTML), Residual Stress User Center. Recent projects include
studies of stresses about spot welds, in situ studies of phase transformations and
intergranular stresses in advanced high strength steels under applied loads, role of
microstructure on the strength of AZ60 Mg alloys, validation of computational tools that
predict casting properties and performance, and mapping of peening stresses through thickness
for glass and steel shot peening performed at constant Almen number. Facilities include
laboratory and synchrotron X-rays instruments and a reactor based neutron strain mapping
instrument for through thickness measurements.
Residual Stress Measurement in Dental Prostheses by Hole-Drilling
Amélie K. Mainjot and Alain J. Vanheusden, University Hospital of Liège, Liège, Belgium,
Gary S. Schajer, University of British Columbia, Canada, and
Michaël J. Sadoun, University Paris Descartes, Paris, France.
Mismatch in thermal expansion properties between veneering ceramic and metallic or
high-strength ceramic cores can induce residual stresses and initiate cracks in bilayered
dental crowns and bridges when combined with functional stresses. Knowledge of the stress
distribution within the veneering ceramic is a key factor for understanding and predicting
fracture failures, which are well-known problems with Yttria-tetragonal-zirconia-polycrystal
based dental prostheses. The objectives of this study are to develop a method for measuring
the stress profile in veneering ceramics and to compare ceramic-fused-to-metal compounds to
veneered Yttria-tetragonal-zirconia-polycrystal ceramic. The hole-drilling method was adapted
for use with bilayered disk samples characteristics of dental crowns. Because of the high
sensitivity needed in comparison with industrial applications, a special drilling procedure
and a high sensitivity electrical measurement chain were developed (10nV). The hole drilling
method was shown be a practical tool for measuring residual stresses in veneering ceramics
used for dental prostheses.
Through-Thickness Variations of the Residual Stresses in a Thick Weld
Plate Using Neutron Diffraction
Wanchuck Woo, Vyacheslav Em, and Baek-Seok Seong, Korea Atomic Energy Research Institute,
Gyu-Baek An, Technical Research Laboratories, South Korea, and
Pavel Mikula, Nuclear Physics Institute and Research Center, Czech Republic.
We determined residual stresses through the thickness of the 50-mm thick steel weld plate
without cutting using neutron diffraction. For decreasing the beam attenuation at depth, the
wavelength of 2.39 Å was selected, which is located at the minimum total cross-section but
avoided the 211 Bragg edge. The gauge volume was less than 2x2x20 mm³ and ‘stress-free’
lattice spacing (d0) was also considered. Significant amounts of the tensile longitudinal stresses
(94% of yield strength) were observed along the heat-affected zone.