page_title: Fatigue (material)
section_title: Life improvement
hierarchical_section_title: Fatigue (material) / Dealing with fatigue / Life improvement
caption_attribution_description: English: Example of a HiFIT-treated assembly.
context_page_description: In materials science, fatigue is the weakening of a material caused by cyclic loading that results in progressive and localized structural damage and the growth of cracks. Once a fatigue crack has initiated, each loading cycle will grow the crack a small amount, typically producing striations on some parts of the fracture surface. The crack will continue to grow until it reaches a critical size, which occurs when the stress intensity factor of the crack exceeds the fracture toughness of the material, producing rapid propagation and typically complete fracture of the structure.
  Fatigue has traditionally been associated with the failure of metal components which led to the term metal fatigue. In the nineteenth century, the sudden failing of metal railway axles was thought to be caused by the metal crystallising because of the brittle appearance of the fracture surface, but this has since been disproved. Most materials seem to experience some sort of fatigue-related failure such as composites, plastics and ceramics.
context_section_description: Change material. Changes in the materials used in parts can also improve fatigue life. For example, parts can be made from better fatigue rated metals. Complete replacement and redesign of parts can also reduce if not eliminate fatigue problems. Thus helicopter rotor blades and propellers in metal are being replaced by composite equivalents. They are not only lighter, but also much more resistant to fatigue. They are more expensive, but the extra cost is amply repaid by their greater integrity, since loss of a rotor blade usually leads to total loss of the aircraft. A similar argument has been made for replacement of metal fuselages, wings and tails of aircraft.
  Induce residual stresses Peening a surface can reduce such tensile stresses and create compressive residual stress, which prevents crack initiation.  Forms of peening include: shot peening, using high-speed projectiles, high-frequency impact treatment (also called high-frequency mechanical impact) using a mechanical hammer, and laser peening which uses high-energy laser pulses. Low plasticity burnishing can also be used to induce compresses stress in fillets and cold work mandrels can be used for holes. Increases in fatigue life and strength are proportionally related to the depth of the compressive residual stresses imparted.  Shot peening imparts compressive residual stresses approximately 0.005 inches (0.1 mm) deep, while laser peening can go 0.040 to 0.100 inches (1 to 2.5 mm) deep, or deeper.
  Deep cryogenic treatment. The use of Deep Cryogenic treatment has been shown to increase resistance to fatigue failure. Springs used in industry, auto racing and firearms have been shown to last up to six times longer when treated. Heat checking, which is a form of thermal cyclic fatigue has been greatly delayed.
  Re-profiling. Changing the shape of a stress concentration such as a hole or cutout may be used to extend the life of a component. Shape optimisation using numerical optimisation algorithms have been used to lower the stress concentration in wings and increase their life.
caption: Example of a HFMI treated steel highway bridge to avoid fatigue along the weld transition.
exif: {"Image ImageWidth": "3264", "Image ImageLength": "2448", "Image Make": "SAMSUNG", "Image Model": "GT-I9100", "Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "I9100XWKI4", "Image DateTime": "2012:01:12 13:40:47", "Image YCbCrPositioning": "Centered", "Image ExifOffset": "224", "Thumbnail ImageWidth": "320", "Thumbnail ImageLength": "240", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail Orientation": "Horizontal (normal)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "840", "Thumbnail JPEGInterchangeFormatLength": "43623", "EXIF ExposureTime": "1/100", "EXIF FNumber": "53/20", "EXIF ExposureProgram": "Aperture Priority", "EXIF ISOSpeedRatings": "40", "EXIF ExifVersion": "0220", "EXIF DateTimeOriginal": "2012:01:12 13:40:47", "EXIF DateTimeDigitized": "2012:01:12 13:40:47", "EXIF ShutterSpeedValue": "166/25", "EXIF ApertureValue": "281/100", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "281/100", "EXIF MeteringMode": "CenterWeightedAverage", "EXIF Flash": "Flash fired, compulsory flash mode", "EXIF FocalLength": "397/100", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "0", "EXIF ExifImageWidth": "3264", "EXIF ExifImageLength": "2448", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF SceneCaptureType": "Standard", "EXIF ImageUniqueID": "OCED11"}