Toughening
The crack tip plasticity is important in toughening of metals and long-chain polymers.
Ceramics have limited crack tip plasticity and primarily rely on different toughening mechanisms.
For the case of a ductile material such as a metal, this toughness is typically proportional to the fracture stress and strain as well as the gauge length of the crack.
Therefore, some safety-critical structural part such as pressure vessels and pipelines to aluminum alloy air frames are manufactured in relatively low strength version.
In an AISI 4340 alloy, interstitial carbon exist in a bcc (body centered cubic) matrix and show an adverse effect on toughness.
In 18%Ni-maraging steel, the carbon content is lower and martensite is strengthened by substitutional Ni atoms.
Multiple vacuum arc melting (VAR) technique can be used to minimize the oxygen content and increase the toughness of the alloy.
[6] If the dephosphorization is improved during steelmaking, the steel will be toughened for a lower phosphorus content.
After appropriate processing of steel, crystalline grains and second phases that are oriented along rolling direction can improve toughness of materials by delamination which can relax triaxial stress and blunt the crack tip.
Though the irreversible work is decreased because of grain boundary energy, the fracture area is increased in intergranular crack propagation.
[8] Crack deflection mechanisms bring about increased toughness in ceramics exhibiting abnormal grain growth (AGG).
Additional microcracks will cause stress to concentrate in front of the main crack.
The distance between microcrack and fracture plane should be larger than grain size to have a toughening effect.
As demonstrated most prominently by Katherine Faber in 1981, the toughening induced by the incorporation of second-phase particles subject to microcracking becomes appreciable for a narrow size distribution of particles of appropriate size.
The stress field near the crack tip triggers the martensitic transformation at velocities hypothesized to approach that of sound in the material.
[13] From another point of view, the work associated to this phase transformation contributes to the improvement of toughness.
The phenomenon of abnormal grain growth, or AGG, can be harnessed to impart a crack bridging microstructure within a single phase ceramic material.
The presence of abnormally long grains serves to bridge crack-wakes and hinders their opening.
Abnormally large grains may also serve to toughen ceramics through crack deflection mechanisms.
In ceramic matrix composites (CMCs), the additions can toughen materials but not strengthen them.
In bulk metallic glass composites(BMGs), dendrites are added to hind the movement of shear band and the toughness is improved.
[17] If fibers have larger fracture strain than matrix, the composite is toughened by crack bridging.
These processes correspond to plastic deformation and pull-out work and contribute to toughening of composite.
When fiber is ductile, the work from plastic deformation mainly contributes to the improvement of toughens.
From the equation, it can be found that higher flow stress and longer debond length can improve the toughening.
However, longer debond length usually lead to a decrease of flow stress because of loss of constraint for plastic deformation.
The toughness in a composite with ductile phase toughening can also be shown using stress intensity factor,
by linear superposition of the matrix and crack bridging based on solutions by Tada.
In high-impact polystyrene (HIPS), the elastomeric dispersion is used to improve crack propagation resistance.
When main crack propagates, microcracks form around elastomeric dispersion above or below the fracture plane.
