Material failures in heat exchangers are often
closely tied to events associated with the conditions
of service and operating parameters. These events can
generally be attributed to adverse load application and
higher than optimum operating temperatures that could
lead to changes in the microstructure of the materials
and fatigue failure of the component. However, fatigue
failure in heat exchangers is usually associated with
the presence of a biaxial stress condition. Two nonparallel
forces create a two-dimensional stress field
at the free surface of the structural element where
the process and mechanism of fatigue failure normally
initiate.
An experimental investigation was conduct6d to
evaluate the biaxial fatigue behavior of commercially
pure titanium Ti-50A (Grade 2) and low-alloy titanium
Ti-Code 12 (Grade 12) heat exchanger materials. The
biaxial state of stress was composed of an axial stress
and a superimposed torsional stress, applied in a thin-wall
tubular specimen machined from titanium tubing.
Torsional stress was applied independently using a torsion
machine and a torque fixer assembly devised as part of
this study. After applying the desired torsion, the
torsionally stressed specimen was mounted on a closed-loop
electrohydraulic machine for the application of
axial cyclic loading. A minimum of four tests were
conducted for each of three alternating stress levels at
both high and low torsional stresses. The biaxial
fatigue test under load control condition was done under
fully reversed cycles equivalent to a biaxiality ratio
of -1. These test parameters were determined from an
analytical formulation based on Mohr's circle.
The results are presented in terms of the various
measured or calculated quantities versus number of cycles
to fracture. Biaxial fatigue curves were drawn through the
experimental points corresponding to Weibull's mean life
criterion. The four data points exhibit scatter that
appears to be related to the applied stress amplitude. It
was also found that a correlation exists between the magnitude
of applied cyclic biaxial stress and fatigue life to
failure. In addition, the results have been discussed
taking existing failure criteria into account. / Graduation date: 1985
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/32368 |
| Date | 06 May 1985 |
| Creators | Tobias, Benjamin C. |
| Contributors | Saletore, Murli |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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