Verification of a Cohesive Zone Model for Ductile Fracture

Abstract

In the present paper, ductile crack growth in an aluminium alloy is numerically simulated using a cohesive zone model under both plane stress and plane strain conditions for two different fracture types, shear and normal modes. The cohesive law for ductile fracture consists of two parts—a specific material’s separation traction and energy. Both are assumed to be constant during ductile fracture (stable crack growth). In order to verify the assumed cohesive law to be suitable for ductile fracture processes, experimental records are used as control curves for the numerical simulations. For a constant separation traction, determined experimentally from tension test data, the corresponding cohesive energy was determined by finite element calculations. It is confirmed that the cohesive zone model can be used to characterize a single ductile fracture mode and is roughly independent of stable crack extention. Both the cohesive traction and the cohesive fracture energy should be material specific parameters. The extension of the cohesive zone is restricted to a very small region near the crack tip and is in the order of the physical fracture process. Based on the present observations, the cohesive zone model is a promising criterion to characterize ductile fracture.

Fracture and damage mechanics modelling of thin-walled structures – An overview

“…These are, however, encouraging as Li and Siegmund [77] have shown for the cohesive zone approach and Kim et al [139] for a modified Gurson approach.…”

“…Li and Siegmund, in [77] showed that the CTOA, instead of being constant, reduces rapidly when the crack tips start to interact and falls even to zero during the link up. That means that the parameter can be used only as long as no interaction between the crack tips takes place which is, however, sufficient for many practical applications.…”

“…Although its applicability has been demonstrated in some cases [77][78] care should always be exercised. Scheider et al, in [71] give an example where the modeling of a stiffened structure by shell elements would cause unrealistic results due to undue deformation of the finite elements in the stiffener region.…”

Crack propagation analyses with CTOA and cohesive model: Comparison and experimental validation

“…Cohesive models [8,9] allow for separation of interfaces between continuum elements, if some critical value of a separation is reached locally, whereas the material outside deforms according to elasto-plastic constitutive equations without any damage. The so-called process zone is a material volume, and material separation and fracture are controlled by a cohesive law, which has the general form σ = f(δ), where δ = [u] is the corresponding value of the displacement jump between adjacent continuum elements.…”

“…Various functions exist for the cohesive law [8][9][10], which have in common, that they contain two characteristic parameters (per fracture mode), a cohesive strength, σ 0 , and a critical separation, δ c . The cohesive law proposed by Scheider [12], …”