eFunda: Factors Affect Fatigue Life
engineering fundamentals Factors Affect Fatigue Life
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As mentionsed in the introduction, many factors may affect the fatigue life of an engineering material, including
  • Material defects
  • Surface roughness and surface treatments
  • Imperfection in assembly or functionality requirements in design
  • Size
  • Loading types
  • Harsh environments
  • Damage in service
  • Poor maintenance and improper repair

Some of these factors are addressed in this page. Please click the links or scroll down for details.

Size Effect

Generally speaking, the large the component, the more initial imprefections within the component. Experiments done on carbon steel alloy found that the diameter of steel rod does affect the fatigue life when the rod is under axial (tension-compression) loading.

where Cs is called fatigue reduction factor.

For common steel alloys, the size-effect can be approximated by the following formula:

where f1 is the unknown fatigue limit for a critically stressed V1 and f0 is the known fatigue limit for a critically stressed V0. Critically stressed volume is defined as the volume near the surface of the specimen that is stressed to at least 95% of max.

Surface Roughness

For many common types of loading, such as bending and torsion, the maximum stress occurs at the surface. Furthermore, the surface is exposed to harsh environments such as corrosion, or unexpected loads such as scratch and impact. In addition, experiments show that even the failure of axial loading usually begins at the surface. With all the evidences pointing to the surface, it is necessary to understand the effect of surface roughness and the treatments improving surface properties.

Experiments done on SAE 3130 steel specimens under completely reversed stress at 655 MPa show that a uniform and smooth surface does have positive influence on the fatigue life.

Finish Surface Roughness
(micron)
Median Fatigue Life
(kilo-cycles)
Lathe-formed 2.67 24
Partial hand-polished 0.15 91
Hand-polished 0.13 137
Ground 0.18 217
Ground and polished 0.05 234
Superfinished 0.18 212

A more systematic approach to understand the "surface factor" is to plot it against the fatigue limit of the material. Each line represent a different surface treatment.

Surface Treatments

Since the fatigue life is closely related to the condition of surface, many researchers devote their effort to surface treatments. Generally speaking, treatments that produce compress residual stress at the surface have positive impact on the fatigue life and vise versa. Many widely used surface treatments may affect the fatigue in different ways. They are briefly discussed as follows:

  • Electroplating, especially chromium plating, while improves corrosion resistance and/or the looking of surface finish, generally decreases the fatigue limit of steel.
  • Grinding is a necessary process to improve surface finish, abrade hard materials, and tighten the tolerance. However, it often introduces surface tension and the heat generated in the grinding process might temper the previously quench hardened components.
  • Stamping is the process that punches through sheet metal to make parts. Stamping introduces discontinuities and irregularities to the material and thus high stress concentration.
  • Forging is the process that compresses heated metal such that the metal palsticaaly deformed into a desired form. Forging refines the grain structure and improves physical properties of the metal. Nevertheless, forging can cause decarburiztion (loss of surface carbon atoms) which is harmful to fatigue life.
  • Hot rolling can also cause decarburiztion (loss of surface carbon atoms), a damaging lose regarding the fatigue life.
  • Carburizing and nitriding produce higher strength and hardness at the surface and thus improves fatigue life.
  • Cold Rolling is the process that compresses and squeezes sheet metal between rollers. It improves the accurancy of thickness and surface finish. Cold rolling produces compress residual stress layer on the surface such that postpone the cracks propergate from the surface and extend the fatigue life. Note that cold rolling may reduce the overall strength of the material for it push the surface to plastic deformation which acompanies strain hardening.
  • Shot Peening is the process that blasts steel or glass beads on to the sruface to produce residual compress stress at the surface. It is an effective low-cost treatment but leave small dimples on the surface. Shot peening can be used to particialy offset the surface tension introduced by plating, decarburization, corrosion and grinding. Shot peening becomes less effective for high stress low-cycle fatigue components (>60% of yield stress) or high temperature applications (> 450°F for steel, 250°F for aluminum). Note that shot peening may have negative impacts on the corrosion resistance. Since the surface is pocked with small divits and dimples and not smooth any more, non-smooth surface is easier to trap moistures, has more surface area exposed to air, and thus is more vulnerable to corrosion.

In summary, cold rolling and shoot peening which introduce compress residual stress at the surface have positive impacts on the fatigue lives, especially favorable for low-stress long-cycle components. They are both widely used in commercial products.

Thermal Fatigue

Thermal fatigue can be catergorized into thermal loading, high temperature, and low temperature fatigue. The mismatch of thermal coefficients of expansion between two mating parts induces thermal stress when temperature changes. Another thermal loading case is the component exposes to heat or cold on one side or back and forth repeatly. When the changes of temperature are severe (thermal shock) and/or frequent (thermal loading), the thermal fatigue or thermal failure occues.

Low temparature often make the usually ductile material more brittle. The cracks in brittle materials propagate more rapidly than in ductile materials which worson the fatigue life.

High temperature (temparatures above half of melting point of the material) often reduce the stiffness of materials which implies that the bounding between crystals are weakened. The intercrystalline creep failure more likely occures than transcrystalline fatigue failure under high temperature environments.

Fretting and Wearing

Fretting is the surface damage resulting from the relative motion of two contact surfaces which should not move relatively. Wearing, on the other hand, is the the surface damage of two contact surfaces which are designed to have relative motion. Both surface damages may range from surface pitting, deterioration, oxide debris, cracks, to functional or structural failures. Note that chemical effects (corrosion) may play an important role, since the surface damages may remove the protecting layers.

To reduce the fatigue of fretting, one may eliminate the relative motion or reduce the coefficient of friction between mating parts. To reduce the fatigue of wearing, sufficient lubricaiton and higher precision in mating parts are beneficial.

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