Thermal Fatigue

Thermal fatigue (TF) is related to structures subjected to cyclic thermal loadings that are likely to lead to initiation of cracks, crack propagation and failure. Thermal fatigue is the gradual deterioration and eventual cracking of a material by alternate heating and cooling during which free thermal expansion is partially or completely constrained.  If a mechanical loading is also involved along with thermal loading, it is termed as thermo-mechanical fatigue (TMF).  Thermal strains induced and related to the dilation of materials lead to some mechanical strains due to clamped boundary conditions or due to a structural effect from thermal gradients across the material, for example, hot zones surrounded by colder zones, or vice versa.  The constraints can be external, such as bolting or welding, or they can be internal, such as different thermal expansion due to different materials connected.  Cracks may develop after many cycles of heating and cooling.

When a break disc is subjected to the friction of the break pad, the surface of the disc is suddenly heated over a relatively thin layer which leads to a surface compressive stress due to the constraint of the colder material.  When the stress becomes significant at high temperature the material can plastify.  When it cools down, residual tensile stresses can develop on the disc surface.  It can lead to the development of a cyclic plasticity after each heating-cooling cycle, and thus low cycle fatigue damage.

rotor_disc

Thermal fatigue cracks on break disc

Many industrial structures are subjected to rapid temperature variations during their use:  gas and steam turbines, boilers, automobile engines, forming tools, nuclear reactors, etc.  Thermal fatigue is a major concern for these structures.

Thermal fatigue is common in castings in which part of the casting experiences a fluctuating high temperature whilst other parts of the casting remain a lower temperature. The phenomenon is seen in aluminum alloy cylinder heads and pistons for internal combustion engines, particularly diesel engines and air-cooled internal combustion engines. It is also common in the casting industry with the crazing and sometimes catastrophic failure of high-pressure-die-casting dies made from steels, and gravity dies and ingot molds made from gray cast iron.

The valve bridge between the exhaust valves in a four-valve diesel engine is an excellent example of the problem.  In brief, the majority of the casting remains fairly cool, its temperature controlled by water cooling.  However, the small section of casting that forms a bridge, separating the exhaust valves, can become extremely hot, reaching a temperature in excess of 300°C.  The bridge therefore attempts to expand by αΔT where α is the coefficient of thermal expansion and ΔT is the increase in temperature.  For a value of α about 20 × 10-6 K–1 for an alluminum alloy, an expansion of 300 × 20 × 10–6 = 0.6% can be predicted.  This is a large value when it is considered that the strain to cause yielding is only about 0.1%.  Furthermore, since the casting as a whole is cool, strong, and rigid, the bridge region is prevented from expanding.  It therefore suffers a plastic compression of about 0.6%.  If it remains at this temperature for sufficient time, stress relief will occur, so that the stress will fall from above the yield point to somewhere near zero.

However, when the engine is switched off, the valve bridge cools to the temperature of the rest of the casting, and so now suffers the same problem in reverse, undergoing a tensile test, plastically extending by up to 0.6%.

The starting and stopping of the engine therefore causes the imposition of an extreme high strain and consequent stress on the exhaust valve bridge.  For those materials, such as a poorly cast alluminum alloy, which has perhaps only 0.5% elongation to failure available, it is not surprising that failure can occur in the first cycle.  What perhaps is more surprising is that any metallic materials survive this punishing treatment at all.  It is clear that modern cylinder heads can undergo thousands of such cycles into the plastic range without failure.

High temperature fatigue is a vast and complex problem in which many materials are adopted nowadays, the vast majority falling into the following three families:

1) 9% – 12% Cr , plus Mo, steels in power generation industry;

2) austenitic stainless steels, in power generation industry;

3) Ni-based superalloys, in gas turbines in jet engine industry.

With increased temperature, there is a tendency for more homogeneous band slip, which delays the onset of fatigue crack initiation.  However, in most cases increasing the temperature reduces the lifespan of the material.  This decreased fatigue life is directly related to oxidation.

Thermal fatigue life can be improved by reducing the temperature and temperature gradient or alleviate the geometric constraints.  For example, while larger wall thicknesss of a steam drum will reduce mechanical stress due to internal pressure, it will however increase thermal gradient and thermal stresses, hence reduce fatigue life.  In other words, thinner wall thickness is beneficial to the fatigue life of steam drums and other hot pressure vessels.  Using slots or grooves in the component may eliminate the constraints for thermal expansion.