Failure Analysis of Mechanical Components


Failure analysis requires a broad and comprehensive understanding of the many different failure modes that exist for any given system. Frequently two or more modes contribute to a component failure. STI Technologies has extensive, first hand experience in failure analysis for industrial equipment, turbomachinery and rotating components in a wide range of applications.

  • Steam Turbine Engines
  • Gas turbine engines
  • Plastic Structures
  • Machine Tools and Fixtures
  • Compressors
  • Office Equipment
  • Pumps
  • Turbine Blades
  • Rotating Shafts
  • Heating Fans
  • Exhaust Hoods
  • Rotors
  • Turbines
  • Impellers
  • Support Structures
  • STI offers failure analysis services in the areas of




    Creep

    Experience has shown that equipment subjected to sustained loading at elevated temperatures can show a gradual deformation or creep, even at stresses below the proportional limit. 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.

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    Fatigue

    Fatigue is generally understood as 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. STI has extensive experience with analyzing applications with variable loads and defining the associated application criteria.

    Fatigue is normally separated into three stages:

    1. Crack Initiation
    2. Crack Propagation (power law growth)
    3. Unstable, rapid growth

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    Structural Resonance

    The dynamics of equipment can be a major factor in design life. Consideration of natural frequencies and modes of excitation can and often does 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 of the forcing functions analyzed to avoid interaction. STI Technologies has the capability to do 3D FEA models to define vibration modes and frequencies of housings, rotating shafts and complex support structures.

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    Crack Initiation

    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. While each cycle represents an infinitesimally small amount of energy, when this process is repeated over and over again, the total energy can be quite significant. STI has done extensive analysis of the crack initiation phase of fatigue failure.

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    Crack Propagation

    The material properties, the stress range, and the crack size govern the crack propagation rate for a part. A key factor that can effect 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 effect growth rate include the temperature and structural resonance. The technology of fracture mechanics, crack infiltration, and growth is well understood at STI Technologies. Databases and experience for a broad range of problems have been used to develop several computer codes. These codes have been successfully used for both 2D and 3D applications.

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    Spalling and Pitting (Gears and Bearings)

    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 end result is easily observed, the underlying cause can be a challenge. STI has found that understanding complex dynamic interaction, such as resonance and environmental factors are often important to solving these types of problems.

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    Fretting and Wear

    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.

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