Striation (fatigue)

Last updated
Scanning electron microscope image of fatigue striations produced from constant amplitude loading. The crack is growing from left to right. Striation sk02.jpg
Scanning electron microscope image of fatigue striations produced from constant amplitude loading. The crack is growing from left to right.

Striations are marks produced on the fracture surface that show the incremental growth of a fatigue crack. A striation marks the position of the crack tip at the time it was made. The term striation generally refers to ductile striations which are rounded bands on the fracture surface separated by depressions or fissures and can have the same appearance on both sides of the mating surfaces of the fatigue crack. Although some research has suggested that many loading cycles are required to form a single striation, it is now generally thought that each striation is the result of a single loading cycle. [1]

Contents

The presence of striations is used in failure analysis as an indication that a fatigue crack has been growing. Striations are generally not seen when a crack is small even though it is growing by fatigue, but will begin to appear as the crack becomes larger. Not all periodic marks on the fracture surface are striations. The size of a striation for a particular material is typically related to the magnitude of the loading characterised by stress intensity factor range, the mean stress and the environment. The width of a striation is indicative of the overall crack growth rate but can be locally faster or slower on the fracture surface.

Striation features

The study of the fracture surface is known as fractography. Images of the crack can be used to reveal features and understand the mechanisms of crack growth. While striations are fairly straight, they tend to curve at the ends allowing the direction of crack growth to be determined from an image. Striations generally form at different levels in metals and are separated by a tear band between them. Tear bands are approximately parallel to the direction of crack growth and produce what is known as a river pattern, so called, because it looks like the diverging pattern seen with river flows. The source of the river pattern converges to a single point that is typically the origin of the fatigue failure. [2]

Striations can appear on both sides of the mating fracture surface. There is some dispute as to whether striations produced on both sides of the fracture surface match peak-to-peak or peak-to-valley. The shape of striations may also be different on each side of the fracture surface. [3] Striations do not occur uniformly over all of the fracture surface and many areas of a fatigue crack may be devoid of striations. Striations are most often observed in metals but also occur in plastics such as Poly(methyl_methacrylate). [4]

Small striations can be seen with the aid of a scanning electron microscope. [5] Once the size of a striation is over 500 nm (resolving wavelength of light), they can be seen with an optical microscope. The first image of striations was taken by Zapffe and Worden in 1951 using an optical microscope. [1]

The width of a striation indicates the local rate of crack growth and is typical of the overall rate of growth over the fracture surface. The rate of growth can be predicted with a crack growth equation such as the Paris-Erdogan equation. Defects such as inclusions and grain boundaries may locally slow down the rate of growth.

Variable amplitude loads produce striations of different widths and the study of these striation patterns has been used to understand fatigue. [6] [7] Although various cycle counting methods can be used to extract the equivalent constant amplitude cycles from a variable amplitude sequence, the striation pattern differs from the cycles extracted using the rainflow counting method.

The height of a striation has been related to the stress ratio of the applied loading cycle, where and is thus a function of the minimum and maximum stress intensity of the applied loading cycle. [8]

The striation profile depends on the degree of loading and unloading in each cycle. The unloading part of the cycle causing plastic deformation on the surface of the striation. Crack extension only occurs from the rising part of the load cycle. [9]

Striation-like features

Other periodic marks on the fracture surface can be mistaken for striations.

Marker bands

Variable amplitude loading causes cracks to change the plane of growth and this effect can be used to create marker bands on the fracture surface. When a number of constant amplitude cycles are applied they may produce a plateau of growth on the fracture surface. Marker bands (also known as progression marks or beach marks) may be produced and readily identified on the fracture surface even though the magnitude of the loads may too small to produce individual striations. [10]

In addition, marker bands may also be produced by large loads (also known as overloads) producing a region of fast fracture on the crack surface. Fast fracture can produce a region of rapid extension before blunting of the crack tip stops the growth and further growth occurs during fatigue. Fast fracture occurs through a process of microvoid coalescence where failures initiate around inter-metallic particles. The F111 aircraft was subjected to periodic proof testing to ensure any cracks present were smaller than a certain critical size. These loads left marks on the fracture surface that could be identified, allowing the rate of intermediate growth occurring in service to be measured. [11]

Marks also occur from a change in the environment where oil or corrosive environments can deposit or from excessive heat exposure and colour the fracture surface up to the current position of the crack tip. [10]

Marker bands may be used to measure the instantaneous rate of growth of the applied loading cycles. By applying a repeated sequence separated by loads that produce a distinctive pattern the growth from each segment of loading can be measured using a microscope in a technique called quantitative fractography, the rate of growth for loading segments of constant amplitude or variable amplitude loading can be directly measured from the fracture surface. [12]

Tyre tracks

Tyre tracks are the marks on the fracture surface produced by something making an impression onto the surface from the repeated opening and closing of the crack faces. This can be produced by either a particle that becomes trapped between the crack faces or the faces themselves shifting and directly contacting the opposite surface. [13]

Coarse striations

Coarse striations are a general rumpling of the fracture surface and do not correspond to a single loading cycle and are therefore not considered to be true striations. They are produced instead of regular striations when there is insufficient atmospheric moisture to form hydrogen on the surface of the crack tip in aluminium alloys, thereby preventing the slip planes activation. The wrinkles in the surface cross over and so do not represent the position of the crack tip.

Striation formation in aluminium

Environmental influence

Striations are often produced in high strength aluminium alloys. In these alloys, the presence of water vapour is necessary to produce ductile striations, although too much water vapour will produce brittle striations also known as cleavage striations. Brittle striations are flatter and larger than ductile striations produced with the same load. There is sufficient water vapour present in the atmosphere to generate ductile striations. Cracks growing internally are isolated from the atmosphere and grow in a vacuum. [14] When water vapour deposits onto the freshly exposed aluminium fracture surface, it dissociates into hydroxides and atomic hydrogen. Hydrogen interacts with the crack tip affecting the appearance and size of the striations. The growth rate increases typically by an order of magnitude, with the presence of water vapour. [15] The mechanism is thought to be hydrogen embrittlement as a result of hydrogen being absorbed into the plastic zone at the crack tip. [16]

When an internal crack breaks through to the surface, the rate of crack growth and the fracture surface appearance will change due to the presence of water vapour. Coarse striations occur when a fatigue crack grows in a vacuum such as when growing from an internal flaw. [15]

Cracking plane

In aluminium (a face-centred cubic material), cracks grow close to low index planes such as the {100} and the {110} planes (see Miller Index). [3] Both of these planes bisect a pair of slip planes. Crack growth involving a single slip plane is term Stage I growth and crack growth involving two slip planes is termed Stage II growth. [17] Striations are typically only observed in Stage II growth.

Brittle striations are typically formed on {100} planes. [17]

Models of striation formation

There have been many models developed to explain the process of how a striation is formed and their resultant shape. Some of the significant models are:

Related Research Articles

<span class="mw-page-title-main">Fracture</span> Split of materials or structures under stress

Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displacement develops perpendicular to the surface, it is called a normal tensile crack or simply a crack; if a displacement develops tangentially, it is called a shear crack, slip band or dislocation.

<span class="mw-page-title-main">Fatigue (material)</span> Initiation and propagation of cracks in a material due to cyclic loading

In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Once a fatigue crack has initiated, it grows a small amount with each loading cycle, 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.

<span class="mw-page-title-main">Fracture mechanics</span> Field of mechanics that studies the propagation of cracks in materials

Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics to characterize the material's resistance to fracture.

<span class="mw-page-title-main">Hydrogen embrittlement</span> Reduction in ductility of a metal exposed to hydrogen

Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbed, hydrogen lowers the stress required for cracks in the metal to initiate and propagate, resulting in embrittlement. Hydrogen embrittlement occurs most notably in steels, as well as in iron, nickel, titanium, cobalt, and their alloys. Copper, aluminium, and stainless steels are less susceptible to hydrogen embrittlement.

<span class="mw-page-title-main">Stress corrosion cracking</span> Growth of cracks in a corrosive environment

Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments. The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metal. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for SCC to go undetected prior to failure. SCC often progresses rapidly, and is more common among alloys than pure metals. The specific environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating and unexpected failure.

<span class="mw-page-title-main">Intergranular fracture</span>

Intergranular fracture, intergranular cracking or intergranular embrittlement occurs when a crack propagates along the grain boundaries of a material, usually when these grain boundaries are weakened. The more commonly seen transgranular fracture, occurs when the crack grows through the material grains. As an analogy, in a wall of bricks, intergranular fracture would correspond to a fracture that takes place in the mortar that keeps the bricks together.

<span class="mw-page-title-main">Fracture toughness</span> Stress intensity factor at which a cracks propagation increases drastically

In materials science, fracture toughness is the critical stress intensity factor of a sharp crack where propagation of the crack suddenly becomes rapid and unlimited. A component's thickness affects the constraint conditions at the tip of a crack with thin components having plane stress conditions and thick components having plane strain conditions. Plane strain conditions give the lowest fracture toughness value which is a material property. The critical value of stress intensity factor in mode I loading measured under plane strain conditions is known as the plane strain fracture toughness, denoted . When a test fails to meet the thickness and other test requirements that are in place to ensure plane strain conditions, the fracture toughness value produced is given the designation . Fracture toughness is a quantitative way of expressing a material's resistance to crack propagation and standard values for a given material are generally available.

Hydrogen damage is the generic name given to a large number of metal degradation processes due to interaction with hydrogen atoms. Note that molecular gaseous hydrogen does not have the same effect as atoms or ions released into solid solution in the metal.

<span class="mw-page-title-main">Embrittlement</span> Loss of ductility of a material, making it brittle

Embrittlement is a significant decrease of ductility of a material, which makes the material brittle. Embrittlement is used to describe any phenomena where the environment compromises a stressed material's mechanical performance, such as temperature or environmental composition. This is oftentimes undesirable as brittle fracture occurs quicker and can much more easily propagate than ductile fracture, leading to complete failure of the equipment. Various materials have different mechanisms of embrittlement, therefore it can manifest in a variety of ways, from slow crack growth to a reduction of tensile ductility and toughness.

<span class="mw-page-title-main">Paris' law</span>

Paris' law is a crack growth equation that gives the rate of growth of a fatigue crack. The stress intensity factor characterises the load around a crack tip and the rate of crack growth is experimentally shown to be a function of the range of stress intensity seen in a loading cycle. The Paris equation is

<span class="mw-page-title-main">Fracture (geology)</span> Geologic discontinuity feature, often a joint or fault

A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. A fracture will sometimes form a deep fissure or crevice in the rock. Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane. Fractures can provide permeability for fluid movement, such as water or hydrocarbons. Highly fractured rocks can make good aquifers or hydrocarbon reservoirs, since they may possess both significant permeability and fracture porosity.

<span class="mw-page-title-main">Fractography</span> Study of the fracture surfaces of materials

Fractography is the study of the fracture surfaces of materials. Fractographic methods are routinely used to determine the cause of failure in engineering structures, especially in product failure and the practice of forensic engineering or failure analysis. In material science research, fractography is used to develop and evaluate theoretical models of crack growth behavior.

<span class="mw-page-title-main">Forensic materials engineering</span>

Forensic materials engineering, a branch of forensic engineering, focuses on the material evidence from crime or accident scenes, seeking defects in those materials which might explain why an accident occurred, or the source of a specific material to identify a criminal. Many analytical methods used for material identification may be used in investigations, the exact set being determined by the nature of the material in question, be it metal, glass, ceramic, polymer or composite. An important aspect is the analysis of trace evidence such as skid marks on exposed surfaces, where contact between dissimilar materials leaves material traces of one left on the other. Provided the traces can be analysed successfully, then an accident or crime can often be reconstructed. Another aim will be to determine the cause of a broken component using the technique of fractography.

Dynamic strain aging (DSA) for materials science is an instability in plastic flow of materials, associated with interaction between moving dislocations and diffusing solutes. Although sometimes dynamic strain aging is used interchangeably with the Portevin–Le Chatelier effect, dynamic strain aging refers specifically to the microscopic mechanism that induces the Portevin–Le Chatelier effect. This strengthening mechanism is related to solid-solution strengthening and has been observed in a variety of fcc and bcc substitutional and interstitial alloys, metalloids like silicon, and ordered intermetallics within specific ranges of temperature and strain rate.

Polymer fracture is the study of the fracture surface of an already failed material to determine the method of crack formation and extension in polymers both fiber reinforced and otherwise. Failure in polymer components can occur at relatively low stress levels, far below the tensile strength because of four major reasons: long term stress or creep rupture, cyclic stresses or fatigue, the presence of structural flaws and stress-cracking agents. Formations of submicroscopic cracks in polymers under load have been studied by x ray scattering techniques and the main regularities of crack formation under different loading conditions have been analyzed. The low strength of polymers compared to theoretically predicted values are mainly due to the many microscopic imperfections found in the material. These defects namely dislocations, crystalline boundaries, amorphous interlayers and block structure can all lead to the non-uniform distribution of mechanical stress.

Crack closure is a phenomenon in fatigue loading, where the opposing faces of a crack remain in contact even with an external load acting on the material. As the load is increased, a critical value will be reached at which time the crack becomes open. Crack closure occurs from the presence of material propping open the crack faces and can arise from many sources including plastic deformation or phase transformation during crack propagation, corrosion of crack surfaces, presence of fluids in the crack, or roughness at cracked surfaces.

<span class="mw-page-title-main">Crack tip opening displacement</span>

Crack tip opening displacement (CTOD) or is the distance between the opposite faces of a crack tip at the 90° intercept position. The position behind the crack tip at which the distance is measured is arbitrary but commonly used is the point where two 45° lines, starting at the crack tip, intersect the crack faces. The parameter is used in fracture mechanics to characterize the loading on a crack and can be related to other crack tip loading parameters such as the stress intensity factor and the elastic-plastic J-integral.

Metal-induced embrittlement (MIE) is the embrittlement caused by diffusion of metal, either solid or liquid, into the base material. Metal induced embrittlement occurs when metals are in contact with low-melting point metals while under tensile stress. The embrittler can be either solid (SMIE) or liquid. Under sufficient tensile stress, MIE failure occurs instantaneously at temperatures just above melting point. For temperatures below the melting temperature of the embrittler, solid-state diffusion is the main transport mechanism. This occurs in the following ways:

<span class="mw-page-title-main">Crack growth equation</span>

A crack growth equation is used for calculating the size of a fatigue crack growing from cyclic loads. The growth of fatigue cracks can result in catastrophic failure, particularly in the case of aircraft. A crack growth equation can be used to ensure safety, both in the design phase and during operation, by predicting the size of cracks. In critical structure, loads can be recorded and used to predict the size of cracks to ensure maintenance or retirement occurs prior to any of the cracks failing.

Fastran is a computer program for calculating the rate of fatigue crack growth by combining crack growth equations and a simulation of the plasticity at the crack tip.

References

  1. 1 2 McEvily, A. J.; Matsunaga, H. (2010). "On fatigue striations". Transaction B: Mechanical Engineering. 17 (1).
  2. ↑ Hull, Derek (1999). Fractography: observing, measuring and interpreting fracture structure topography. Cambridge University Press.
  3. 1 2 Nix, K. J.; Flower, H. M. (1982). "The micromechanisms of fatigue crack growth in a commercial Al-Zn-Mg-Cu alloy". Acta Metallurgica. 30 (8): 1549–1559. doi:10.1016/0001-6160(82)90175-4.
  4. ↑ Johnson, T. A. (1972). "Fatigue Fracture of Polymethylmethacrylate". Journal of Applied Physics. 43 (3): 1311–1313. Bibcode:1972JAP....43.1311J. doi:10.1063/1.1661271.
  5. ↑ Brooks, Charlie R.; McGill, B. L. (1994). "The Application of Scanning Electron Microscopy to Fractography". Materials Characterization. 33 (3): 195–243. doi:10.1016/1044-5803(94)90045-0.
  6. 1 2 McMillan, J. C.; Pelloux, R. M. N. (1967). "Fatigue Crack Propagation under Program and Random Loads". Fatigue Crack Propagation. ASTM STP 415. ASTM International. pp. 505–535.
  7. ↑ Schijve, J. (1999). "The significance of fractography for investigations of fatigue crack growth under variable-amplitude loading". Fatigue and Fracture of Engineering Materials and Structures. 22 (2): 87–99. doi:10.1046/j.1460-2695.1999.00147.x.
  8. ↑ Uchida, Y.; Shomojop, M.; Higo, Y. (1999). "Relationship between fatigue striation height and stress ratio". Journal of Materials Science. 34 (10): 2411–2419. doi:10.1023/A:1004510615621. S2CID   134254877.
  9. ↑ McMillan, J. C.; Pelloux, R. M. (1970). "Fatigue crack propagation under programmed loads and crack tip opening displacements". Engineering Fracture Mechanics. 2: 81–84. doi:10.1016/0013-7944(70)90031-7.
  10. 1 2 Lynch, S. P. (2007). "Progression markings, striations, and crack-arrest markings on fracture surfaces". Materials Science and Engineering A. 468–470: 74–80. doi:10.1016/j.msea.2006.09.083.
  11. ↑ Barter, S. A.; Molent, L.; Wanhill, R. J. H. (2009). "Marker loads for quantitative fractography of fatigue cracks in aerospace alloys". 25th ICAF Symposium – Rotterdam, 27-29 May 2009.
  12. ↑ McDonald, M.; Boykett, R.; Jones, M. (2012). "Quantitative fractography markers for determining fatigue crack growth rates in aluminium and titanium aircraft structures". 28th International Congress of the Aeronautical Sciences.
  13. ↑ "Characteristics of a fatigue failure in metals" . Retrieved 29 June 2019.
  14. ↑ Schijve, J. (1978). "Internal fatigue cracks are growing in vacuum". Engineering Fracture Mechanics. 10 (2): 359–370. doi:10.1016/0013-7944(78)90017-6.
  15. 1 2 Ruiz, J.; Elices, M. (1996). "Effect of water vapour pressure and frequency on fatigue behaviour in 7017-T651 aluminium alloy plate". Acta Materialia. 45 (1): 291–293.
  16. ↑ Robertson, Ian M.; Sofronis, P.; Nagao, A.; Martin, M. L.; Wang, S.; Gross, D. W.; Nygren, K. E. (2015). "Hydrogen Embrittlement Understood". Metallurgical and Materials Transactions A. 46A (6): 2323–2341. Bibcode:2015MMTA...46.2323R. doi: 10.1007/s11661-015-2836-1 .
  17. 1 2 Suresh, S. (2004). Fatigue of materials (Second ed.). Cambridge University Press.
  18. ↑ Laird, Campbell (1996). "Fatigue". In Cahn, R. W.; Haasent, P. (eds.). Physical Metallurgy (Fourth ed.). Elsevier Science BV.
  19. ↑ Neumann, P. (1974). "Coarse Slip model of fatigue". Acta Metallurgica. 17 (9): 1219–1225. doi:10.1016/0001-6160(69)90099-6.
  20. ↑ Zhang, J. Z. (2000). "A shear band decohesion model for small fatigue crack growth in an ultra-fine grain aluminium alloy". Engineering Fracture Mechanics. 65 (6): 665–681. doi:10.1016/S0013-7944(99)00148-4.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.