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In materials science, fracture toughness is a property which describes the ability of a material to resist fracture, and is one of the most important properties of any material for many design applications. 2, and is a measurement of the energy required to grow a thin crack.
Fracture toughness is a quantitative way of expressing a material’s resistance to brittle fracture when a crack is present. A material with high fracture toughness may undergo ductile fracture as opposed to brittle plane. Brittle fracture is characteristic of materials with low fracture toughness. Fracture mechanics, which leads to 9 concept of fracture toughness, was broadly crack on the work of A.
Griffith who, among other things, studied the behavior of cracks in brittle materials. Young’s modulus of the material. Just as the elastic properties of materials, like elastic moduli and strength, vary among material class, so too does the fracture toughness.
1 graphs the fracture toughness vs. However, unlike most elastic properties, fracture toughness displays a wide variation across materials, about 4 orders of magnitude. As can be expected, metals hold the highest values of fracture toughness.
Comparatively, engineering ceramics have a lower fracture toughness, which leads to ease of cracking, but show an exceptional improvement in the stress fracture that is attributed to their 1. 5 orders of magnitude strength increase, relative to metals. The ovals, or «balloons» within each material class tend to have a sloping trend that describes the subgroup’s strength-toughness relationship. For example, take the alloy system’s downward sloping trend.
This relationship indicates, that within an alloy class an increase in strength leads to a decrease in fracture toughness regardless of the strengthening mechanism used. This however, is not the trend for other material classes. As a first example, consider the polymer foams and porous ceramics.
In direct contrast to alloys, the trend in this material class is to have an upward slope. Explicitly, when the strength of these materials is increased we can expect to see an accompanied increase in fracture toughness.
Indeed, in the subgroup of basal and ash we observe a close to one order plane 9 crack magnitude increase in both properties. Porous ceramics have been shown to behave exactly like this under both compression stress and tensile stress, however, the fracture toughness values are ten times lower under tension. This behavior can most easily be grasped by considering the porosity of these materials, where these voids can be modeled as preexisting cracks, which increase the nucleation energy of cracks under stress and impede the propagation of these cracks.
Note that the engineering ceramics also show the same sloping trend as the porous ceramics, however, the slope is much greater, which makes it less noticeable in the log-scale. In general, materials that are on the upper left most part of the diagram are used to design a system’s failure against flow, because these materials yield before they are fractured. While materials on the lower right most part of the diagram are used to design a system’s failure against fracture, because these materials fracture before yielding. If the crack grows in size, the stiffness decreases, so the force level will decrease.
Hence the term strain energy release rate which is usually denoted with symbol G. The strain energy release rate is higher for higher loads and broader cracks. If the strain energy so released exceeds a critical value Gc, then the crack will grow spontaneously.
For ductile materials, energy associated with plastic deformation has to be taken into account. 10 for glasses and brittle polymers. Notice the different units used by GIc and KIc.
Engineers tend to use the latter as an indication of toughness. There are number of instances where this picture of a critical crack is modified by corrosion. Thus, fretting corrosion occurs when a corrosive medium is present at the interface between two rubbing surfaces. The bonding contact areas deform under the localised pressure and the two surfaces gradually wear away. This process is enhanced when corrosion is present, not least because the corrosion products act as an abrasive between the rubbing surfaces.
Fatigue is another instance where cyclical stressing, this time of a bulk lump of metal, causes small flaws to develop. Ultimately one such flaw exceeds the critical condition and fracture propagates across the whole structure. The fatigue life of a component is the time it takes for criticality to be reached, for a given regime of cyclical stress. Corrosion fatigue is what happens when a cyclically stressed structure is subjected to a corrosive environment at the same time.
As a result, the fatigue life is shortened, often considerably. This phenomenon is the unexpected sudden failure of normally ductile metals subjected to a constant tensile stress in a corrosive environment.
Worse still, high-tensile structural steels crack in an unexpectedly brittle manner in a whole variety of aqueous environments, especially chloride. That is, in the presence of a corrodent, cracks develop and propagate well below KIc. The subcritical nature of propagation may be attributed to the chemical energy released as the crack propagates. The crack initiates at KIscc and thereafter propagates at a rate governed by the slowest process, which most of the time is the rate at which corrosive ions can diffuse to the crack tip.
Finally it reaches KIc, whereupon swift fracture ensues and the component fails. One of the practical difficulties with SCC is its unexpected nature. Very often one finds a single crack has propagated whiles the left metal surface stays apparently unaffected.
Intrinsic toughening mechanisms are processes which act ahead of the crack tip to increase the material’s toughness. These will tend to be related to the structure and bonding of the base material, as well as microstructural features and additives to it. Examples of mechanisms include crack deflection by secondary phases, crack bifurcation due to fine grain structure and modification to the grain boundaries, and crack meandering by pores in the material.
Any alteration to the base material which increases its ductility can also be thought of as intrinsic toughening. Extrinsic toughening mechanisms are processes which act behind the crack tip to resist its further opening.