The types of fracture surface characteristics, know as fractography, are classified into four modes of fracture: microvoid coalescence or dimple rupture, transgranular cleavage, fatigue, and decohesive rupture.

• Microvoid Coalescence or Dimple Fracture usually occurs under single load or tearing. This is shown by depressions in the microstructure called dimples, which occur from microvoid emergence in places of high local plastic deformation. Under increased strain micrvoids grow, coalescence, until rupture occurs, thus dimple rupture. Dimple size and shape depends on the type of loading and extent of microvoid emergence. When a material is put under uniaxial tensile loading, equiaxed dimples appear which have complete rims. Under a tear loading the dimples are elongated, the rims of the dimples are not complete and the dimples are in the same direction as the loading. Shear loading has the same features as tear loading except the dimples are in opposite directions. The following sentences provide other common terminology associated with dimple fracture. Oval dimples occur when a large void intersects a smaller subsurface void, the dimples form an oval shape and exhibit complete rims. A serpentine glide is an interwoven pattern of glide plane decohesion steps. Ripples are partially smoothed out areas of serpentine glide. A stretched area is a flat, featureless area resulting from further straining of a ripple pattern. Intergranular dimple rupture occurs along grain boundaries due to nucleation and coalescence of voids at grain boundaries.

• Transgranular Cleavage occurs at well defined planes in the crystal which are shown in body centered cubic and hexagonal close packed materials. Cleavage results from high stress along three axes with a high rate of deformation and at low temperatures. Characteristics of cleavage are cleavage steps, feather markings, herringbone structure, tongues and microtwins, Wallner lines and quasi-cleavage. A cleavage step is a step on a cleavage facet joining two parallel cleavage fractures. Feather markings are very fine, fan like markings on a cleavage fracture. Tongues and microtwins tongues are fine slivers of metal on cleavage facets which form from cleavage across microtwins which, in turn, are formed by plastic deform at the tip of the main crack. Wallner lines a distinct "V" shaped by pattern intersection of two groups of parallel cleavage steps, primarily found in brittle materials. Quasi cleavage is a fracture mode resembling cleavage because of its planar facets but where the fracture facets are not specific well-defined planes.

• Fatigue failure is classified into two types, high and low cycle. Striations show the propagation of the crack front. High cycle fatigue (HCF) failure often shows close, well-defined striations. Low cycle fatigue (LCF) failure commonly shows sparse and less detailed striations than failure in HCF. Common terms associated with fatigue failure are: beach marks or clam shells, ratchet marks, secondary cracks and tire tracks. Beach marks or clam shells are crescent shaped macroscopic marks on fatigue fractures representing positions of the crack propagation. Ratchet marks are macroscopic step marks which are formed when cracks emanating from multiple origins meet. Secondary cracks are cracks seen at the fatigue striations (prevalent in some alloys). Tire tracks microscopic features observed in high-stress LCF fractures caused by repeated impacting and relative motion of the mating surfaces.

• Decohesive Rupture occurs along weak material surfaces. Surfaces such as grain-boundary precipitation, low-strength phases, defect structures, stress-corrosion cracking and hydrogen embrittlement. In layman's terms, surfaces such as: in-between the individual grains; an area of the material which is not completely crystallized (see crystallized) or due to conditions, is undergoing a phase change (see phase change); vacancies and interstials in ceramic materials; rupture due rusting and rupture due to oxidation. Decohesive rupture frequently occurs with a combination of other modes of fracture. Stress-corrosion cracking is identified as intergranular and transgranular cracks which form in stressed materials due to interaction between the material and the corrosive environment. Hydrogen embrittlement of the material caused by rapid diffusion of hydrogen into grain boundaries and other areas of some materials. Hydrogen embrittlement promotes fracture failure under sustained load. Rubbing or attrition marks surface features resembling striations which result from mutual rubbing of the mating surfaces after fracture. Mud cracking fine fissures found in some corrosion products in fractured surfaces. Glide plane decohesion separation of material by glide along a slip plane.
  
  
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  Submitted by Ron Halahan

  Virginia Tech Materials Science and Engineering

  http://www.eng.vt.edu/eng/materials/classes/MSE2094_NoteBook/97ClassProj/exper/halahan/www/halahan.html

  Last updated: 4/25/97