Fatigue is probably the most common mode of failure. It is generally understood to be the gradual deterioration of a material when subjected to repeated loads. Key factors are the mean stress (average of maximum and minimum), and the variation (difference between the maximum and minimum) components. While most data have been developed for the fully reversing application, there are techniques for applying these data to alternate fatigue situations. Sometimes the stresses vary due to normal operation, and for other cases there is an excitation of a harmonic frequency which acts to amplify the applied load. SimuTech has extensive experience with analyzing applications with variable loads and defining the associated application criteria.
There are several means of evaluating fatigue:
- Fracture Mechanics
The dynamics of equipment can be a major factor in design life. Consideration of natural frequencies and modes of excitation can have a major impact on localized stress. Excitation of a part at or near its natural frequency can produce an amplification of stresses. A complete system will usually have many modes. These must be defined, and all the forcing functions analyzed to avoid interaction. SimuTech Group has the capability to do 3D FEA models to define vibration modes and frequencies of housings, rotating shafts, and complex structures. For existing equipment, SimuTech Group can offer testing services to measure vibration frequencies, modes, etc. FEA and testing services are complementary tools for precision analysis of complex structures.
Significant excursions of temperature can also create stress. The range of temperatures and the rate of change during heat-up and cool-down have a major impact on the associated stress. SimuTech Group has analyzed engine components where thermally induced stresses were the controlling factor for early fatigue failure.
Crack initiation is related to the stress-strain hysteresis loop. The area within this loop is the dissipated energy that promotes crack initiation and propagation. Each cycle represents an infinitesimally small amount of energy, but when this process is repeated, the total energy can be quite significant. SimuTech Group has done extensive analysis of the crack initiation phase of fatigue failure.
The material properties, the stress range, and the crack size govern the crack propagation rate for a part. A key factor that can affect the crack growth rate is the environmental chemistry at the crack tip. The combined action of corrosion and fatigue does substantially increase the crack growth rate. Other factors that can affect growth rate include the temperature and structural resonance. The technology of fracture mechanics, crack infiltration, and growth is well understood at SimuTech Group.
SimuTech Group has recently developed an advanced “state-of-art” fatigue life prediction computer code called LifeCycle. This software is based on proven NASA technology. SimuTech Group has used this program to predict the remaining life of turbine blades and for failure analysis under known operating conditions. This proprietary code has been developed to solve fatigue problems involving components subject to complex load sequencing.
Spalling and pitting are surface fatigue phenomena. The failure occurs when high contact stresses produce sub-surface tensile and shear (Hertzian) stresses that exceed the material fatigue limits. Gears and bearings are often subjected to large contact stresses. While the result is easily observed, the underlying cause can be a challenge. SimuTech Group has found that understanding complex dynamic interaction, such as resonance and environmental factors, is often important to solving these types of problems.
Fretting damage occurs on the mating surfaces of components subject to normal pressure and tangential oscillatory motion. The surface damage can take the form of wear, or for high normal forces, fatigue. The process can be accelerated in the presence of chemical attack.
For some applications, corrosion and fatigue can act in a synergistic fashion. Corrosion generates local damage in the form of pits. The growth of the pits is a complex electro-chemical reaction. The rate of corrosion is dependent on the metal type, corrodent concentration, temperature, and the availability of oxygen. When these pits grow at a location with high stress, the pit can be the nucleation point for the initiation of a fatigue crack. SimuTech Group has experience with analysis of this type of problem and is presently participating in leading-edge research on this technology.
Experience has shown that equipment subjected to sustained loading at elevated temperatures can show a gradual deformation or creep. This can occur at stresses below the proportional limit and well below yield. Both metallic and non-metallic materials exhibit these tendencies. In metals, creep is the deformation caused by slip along the crystallographic directions in individual crystals combined with some flow at the grain boundaries. Creep can also be a factor in thermal cycle applications.