Fracture toughness
Plane strain conditions give the lowest fracture toughness value which is a material property.
[citation needed] Intrinsic toughening mechanisms are processes which act ahead of the crack tip to increase the material's toughness.
Examples of mechanisms include: Any alteration to the base material which increases its ductility can also be thought of as intrinsic toughening.
The conditions for fracture are the most favorable at the boundary between this plastic and elastic zone, and thus cracks often initiate by the cleavage of a grain at that location.
Cleavage is likely to initiate at the elastic-plastic zone boundary, and then link back to the main crack tip.
[8] Inclusions in a material such as a second phase particles can act similar to brittle grains that can affect crack propagation.
Since this process does not include a crystallographic phase transition, the unit cell volume remains constant.
Examples include Fracture toughness tests are performed to quantify the resistance of a material to failure by cracking.
There are several types of test used to measure fracture toughness of materials, which generally utilise a notched specimen in one of various configurations.
For the same characteristic dimensions, compact configuration takes a lesser amount of material compared to three-point flexural test.
Due to this importance ASTM has devised a standardized way of reporting the crack orientation with respect to forging axis.
Generally speaking, the lower bound of the toughness of a material is obtained in the orientation where the crack grows in the direction of forging axis.
When a test fails to meet the thickness and other plain-strain requirements, the fracture toughness value produced is given the designation Kc.
For example, when a relatively thin plate with high toughness is being tested, it might not be possible to produce a thicker specimen with plane-strain conditions at the crack tip.
ASTM E561 outlines a procedure for determining toughness vs crack growth curves in materials.
[18] This standard does not have a constraint over the minimum thickness of the material and hence can be used for thin sheets however the requirements for LEFM must be fulfilled for the test to be valid.
The criteria for LEFM essentially states that in-plane dimension has to be large compared to the plastic zone.
There are cases where even plane strain fracture ensues in rising R-curve due to "microvoid coalescence" being the mode of failure.
For the case of negligible plasticity, the load vs displacement curve is obtained from the test and on each point the compliance is found.
The compliance is reciprocal of the slope of the curve that will be followed if the specimen is unloaded at a certain point, which can be given as the ratio of displacement to load for LEFM.
J-toughness value signifies the resistance of the material in terms of amount of stress energy required for a crack to grow.
Now the single-valued JIC is determined as the toughness near the onset of the ductile crack extension (effect of strain hardening is not important).
The test thus performed yields several load vs crack mouth opening displacement (CMOD) curves, which are used to calculate J as following:-
The tear test can also be used for very ductile aluminium alloys (e.g. 1100, 3003), where linear elastic fracture mechanics do not apply.
A number of organizations publish standards related to fracture toughness measurements, namely ASTM, BSI, ISO, JSME.
The orientation of the grain boundary facets and residual stress cause the crack to advance in a complex, tortuous manner that is difficult to analyze.
[22] A mechanics of materials model, introduced by Katherine Faber and Anthony G. Evans, has been developed to predict the increase in fracture toughness in ceramics due to crack deflection around second-phase particles that are prone to microcracking in a matrix.
The actual crack tortuosity is obtained through imaging techniques, allowing the deflection and bowing angles to be directly input into the model.
The resulting increase in fracture toughness is then compared to that of a flat crack through the plain matrix.
The magnitude of the toughening is determined by the mismatch strain caused by thermal contraction incompatibility and the microfracture resistance of the particle/matrix interface.
