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.
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STI offers failure analysis services in the areas of
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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.
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:
- Crack Initiation
- Crack Propagation (power law growth)
- Unstable, rapid growth
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.
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.
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.
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.
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.
