Preventing Fatigue Failure with Ultrasonic Impact Treatment

The Devastating Effects of Fatigue Failure

Fatigue failure occurs when material is exposed to repetitive or fluctuating stresses that exceed its tensile strength threshold.  The effects are cumulative in their destructive potency, beginning as undetectable microscopic cracks that grow as they are continuously subjected to cyclic loading. 

Left untreated, seemingly insignificant fissures can grow to produce disastrous consequences, evidenced in such unfortunate cases as the Alexander L. Kielland oil platform disaster in 1980 and the midair disintegration of China Airlines Flight 611 in 2002.  And while not all outcomes of fatigue failure are as catastrophic, the breakdown of components greatly impact the operational capabilities of companies worldwide on a daily basis.  Ultrasonic impact technology (UIT) from Applied Ultrasonics has received mass recognition for its ability to successfully combat the contributing factors of fatigue failure throughout its stages of development.

The 3 Stages of Fatigue Failure

Fatigue failure develops in three stages: Initiation, Propagation, and Final Fracture: 

• Initiation – Minute, often undetectable cracks occur under repetitive loading and unloading of tensile stresses beyond the material’s capacity

• Propagation - After the original crack is formed and undergoes continuous loading, it develops into an exceptionally acute stress concentration that drives the crack deeper into the metal with each repetition of the stress.

• Final Fracture – Finally, as the fatigue crack proliferates wider and deeper, the material is weakened to the point of failure where final fracture occurs

UIT’s ability to quickly and effectively improve the surface quality, relieve tensile stresses within the material, and forge beneficial compressive stresses near the surface, allows it to successfully fight fatigue during initiation and/or propagation before it escalates to the devastating point of fracture.

Factors that Lead to Fatigue Failure

Numerous factors, man-made and natural, contribute to fatigue failure.  By utilizing ultrasonic energy, Applied Ultrasonics’ patented impact treatment is able to combat many of the following aspects that contribute to shortened fatigue life:

• Surface Quality - Smooth surfaces are considerably stronger than those plagued by imperfections.  Surface roughness creates microscopic stress concentrations that reduce fatigue strength. By introducing compressive stresses into the material, ultrasonic impact treatment (UIT) is able to improve surface quality and increase fatigue life.
• Residual Stresses: Manufacturing processes such as welding, cutting or casting that involve heat or deformation, produce high levels of tensile residual stress.  These tensile stresses greatly decrease fatigue strength and fatigue life.  Supplementing processes such as welding or machining with UIT helps to extend fatigue life and diminishes the harmful effects of various manufacturing process.
• Grain Size: For most metals, smaller grains yield longer fatigue lives.  UIT adjusts not only the grain size, but the intergranular composition of metals to their greatest capacity.
• Environment: Harsh environmental conditions lead to erosion, corrosion, or embrittlement.  UIT successfully removes corrosion and restores surface quality to counter the effects of aggressive environments to prevent corrosion fatigue and increase fatigue life substantially.  UIT also leads to greater erosion resistance without increasing brittleness.
• Material Type: Fatigue and behavior during cyclic loading varies across different materials.  UIT’s range of treatment extends to a variety of metals, including: aluminum, bronze alloy, iron, steel, titanium, and less common materials such as inconel and beryllium.
• Geometry: Notches and disparity in cross section throughout a component lead to stress concentrations where fatigue cracks initiate.  Again, UIT’s ability to greatly reduce detrimental stress concentrations at the point of initiation, limits the onset of cracks and the damaging effects of geometric factors.
• Size and Distribution of Internal Defects: Casting defects significantly reduce fatigue strength.  As UIT strengthens the metal on an intergranular level, the internal damage produced by casting-created defects, such as gas porosity, non-metallic inclusions, and shrinkage voids can be alleviated.
• Temperature: Extreme heat or cold can have a damaging effect on fatigue strength and fatigue life. UIT is effective in enhancing a component's resistance to thermal fatigue, and it is proven effective in extremely cold operating environments that frequently result in reduced fatigue performance.

By overcoming the numerous above factors before that contribute to fracturing, ultrasonic impact treatment from Applied Ultrasonics prevents the exorbitant costs and downtime of repairs, as well as the possibly devastating results in the aftermath of fatigue failure.