Cohesive zone model
The cohesive zone model (CZM) is one of the most modern evolutions in the area of fracture mechanics in which fracture formation is regarded as a gradual phenomenon in which the separation of the surfaces involving in the crack takes place across an extended crack tip, or cohesive zone, and is resisted by cohesive tractions. The origin of this model can be traced back to 1959, when Barenblatt came up with the concept of CZM attributed to brittle fracture.[1] Cohesive zone models: a critical review of traction-separation relationships across fracture surfaces. This was followed by Dugdale in 1960, who introduced the concept that cohesive stress is equal to the Yield stress (
) of the material considered.[2] CZM can be considered as an alternative method to model separation.
The major advantages of the CZM over the conventional methods in fracture mechanics like those including LEFM (Linear Elastic Fracture Mechanics), CTOD (Crack Tip open Displacement) are:
- It is able to adequately predict the behaviour of uncracked structures, including those with blunt notches.
- Size of non-linear zone need not be negligible in comparison with other dimensions of the cracked geometry in CZM, while in other conventional methods, it is not so.
- Even for brittle materials, the presence of an initial crack is needed for LEFM to be applicable.
Another important advantage of CZM falls in the conceptual framework for interfaces.
The Cohesive Zone Model does not represent any physical material, but describes the cohesive forces which occur when material elements are being pulled apart.
As the surfaces (known as cohesive surfaces) separate, traction first increases until a maximum is reached, and then subsequently reduces to zero which results in complete separation. The variation in traction in relation to displacement is plotted on a curve and is called the traction-displacement curve. The area under this curve is equal to the energy needed for separation. CZM maintains continuity conditions mathematically; despite physical separation. It eliminates singularity of stress and limits it to the cohesive strength of the material.
The traction-displacement curve gives the constitutive behavior of the fracture. For each material system, guidelines are to be formed and modelling is done individually. This is how the CZM works. The amount of fracture energy dissipated in the work region depends on the shape of the model considered. Also, the ratio between maximum stress and the yield stress affects the length of fracture process zone. Smaller the ratio, longer is the process zone. The CZM allows the energy to flow into the fracture process zone, where a part of it is spent in the forward region and rest in the wake region.
Thus, CZM provides an effective methodology to study and simulate fracture in solids.