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Biaxial fatigue behavior of commercially pure titanium Ti-50A (Grade 2) and low-alloy titanium Ti-Code 12 (Grade 12) heat exchanger materialsTobias, Benjamin C. 06 May 1985 (has links)
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
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Characterising the stress-life response of mechanical and laser formed titanium componentsFidder, Herman January 2012 (has links)
This dissertation involves the experimental investigation of commercially pure titanium (CP Ti) which was subjected to laser forming and mechanical forming processes. Commercially pure titanium grade 2 was formed to a radius of curvature of approximately 120 mm using three forming procedures, i.e. i) laser forming; ii) mechanical forming (stretched forming) and iii) a combined forming process (laser-mechanical forming). Fatigue testing revealed, for all the forming processes, that samples produced by laser forming performed the best at high load settings. However, mechanically formed specimens performed the best at low load settings, whereas the laser-mechanical process resulted in midway performance between laser and mechanical processing. Considering microstructure vs fatigue; impact vs fatigue; and residual stress vs fatigue; at high load settings it is evident that the microstructure is the dominant contributor to crack initiation and growth. Crack morphology of fatigue samples revealed that secondary cracks (parallel to main crack front) followed the grain boundaries of the Widmanstätten microstructure, whereas irregular secondary cracks grew parallel and through the twinning planes and along the grain boundaries of the equiaxed microstructure. Laser forming resulted in microstructural changes from equiaxed grains to a Widmanstätten structure due to fast cooling rates. Excessive twinning is developed within the equiaxed microstructure after the mechanical forming procedure. This is due to cold working / strain hardening. The combined process shows a combination of equiaxed grains and Widmanstätten microstructure. Residual stress relieved for all forming processes revealed an increase in the magnitude of the residual stress compared to the parent plate and that the maximum values were obtained at the inner radius of curvature (i.e. 118.4 mm). Laser forming revealed the highest values in residual stress whereas the other two processes i.e. mechanical and laser-mechanical forming exhibited an increase midway between the parent plate and laser forming. The second most influential factor with regards to fatigue was the magnitude of the residual stress, especially at medium to low load settings. When considering theoretical models to predict fatigue life it was found that the Goodman model showed the closest relation to the actual fatigue data when considering the entire theoretical curve. Vickers microhardness profiling was applied to the thickness of the samples for the parent plate and all forming processes. No significant hardening occurred due to the forming processes and differences in hardness were considered negligible. Charpy impact testing revealed that the laser formed specimens exhibited the most brittle behaviour when compared to the parent plate results. Mechanical formed specimens showed a slight increase in brittleness compared to parent plate whereas the combined process yielded results midway between the laser and mechanically formed specimens. Mathematical equations are formulated and presented for predicting the fatigue life of CP Ti grade 2 for the parent plate and the three forming processes. This study proved that the laser forming process can be successfully used as a production stage in the forming of CP Ti grade 2.
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Laser based in-situ formation of ceramic coatings on titanium.Ochonogor, Onyeka Franklin January 2013 (has links)
M. Tech. Metallurgical Engineering / Titanium and its alloys exhibit poor tribological characteristics. The poor resistance to sliding wear of Ti6Al4V alloy makes it susceptible to severe wear at the surface during sliding contact. This could cause galling and seizing during sliding contact. Ti6Al4V alloy also have poor corrosion resistance under critical conditions. Some problems with Ti6Al4V MMCs produced by laser cladding technique in most cases is poor bonding as a result of wetting properties between the ceramic and metal powders for reinforcement. Occurrence of porosity is another factor which can reduce the mechanical properties of MMCs. Occurrence of agglomerates is also a concern due to poor mixing of reinforcement powders. This project is aimed at investigating the effect of laser cladding of titanium alloy substrate with zirconium (Zr), titanium carbide (TiC), titanium (Ti) reinforcement additions. The effect of combination of these powders using various fractions and variable cladding parameters on the substrate will be investigated.
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