Y. Marchal

Thickness dependence of cracking resistance in thin aluminium plates

The influence of thickness on the fracture toughness of aluminium 6082T0 thin plates of 1-6 mm thicknesses was investigated experimentally and numerically from tensile testing of cracked DENT specimens. The critical J-integral, J(c), critical CTOD, delta(CTODc), and essential work of fracture, w(e), are found to increase with thickness and to constitute equivalent measures of fracture toughness at small thickness. For larger thickness, J(c) and delta(CTODc) increase non-linearly with thickness and reach a maximum for 5-6 mm thickness Whereas iv, keeps increasing linearly with thickness. This difference is related to a more progressive development of the necking zone in front of the crack tip when thickness increases: at large thickness, cracking initiates well before the neck has developed to its stationary value during propagation, w(e) is more directly related to the steady-state crack growth resistance. A linear regression on the fracture toughness/thickness curve allows further separation of the two contributions of the essential work of fracture: the necking work and the fracture work spent for damaging. The maximum of the stress triaxiality ratio is shown to constitute a pertinent parameter for characterising how constraint affects cracking initiation in the present context where out-of-plane constraint dominates in-plane constraint. It allows justifying the shape of the J(c)/thickness relationship and results in the proposal of a 3D J(c)/thickness/triaxiality fracture locus. As fracture profiles are macroscopically flat with microscopic dimples and with only very small shear lips along the edges, a local criterion based on the growth and coalescence of voids has been used in order to predict fracture initiation

Essential work of fracture compared to fracture mechanics—towards a thickness independent plane stress toughness

The essential work of fracture (EWF) and the J-integral methods were applied in a study of the effect of the thickness on the cracking resistance of thin plates. The paper discusses two themes: (1) the relationships between the two methods or concepts is elucidated, and (2) a new, thickness independent plane stress toughness parameter is proposed. For that purpose, cracked aluminium 60820 thin plates of 1-6 mm thickness were tested in tension until final separation. The EWF, w(e), and the J-integral at cracking initiation, J(i), increase identically with thickness except at larger thickness for which the increase of J(i) levels off. J(i) reaches a maximum for 5-6 mm thickness whereas w(e) keeps increasing linearly with thickness. This difference is related to the more progressive development of the necking zone in front of the crack tip when thickness increases: at large thickness, cracking initiates well before the neck has developed to its stationary value during propagation. A linear regression on the fracture toughness/thickness curve allows partitioning the two contributions of the work of fracture: the plastic work per unit area for crack tip necking and a plane stress work per unit area for material separation. The pertinence of this new measure of the pure plane stress cracking resistance is critically discussed based on a micromechanical model for ductile fracture. The micromechanical void growth model incorporates void shape effects, which is essential in the low stress triaxiality regime

Statistical procedure for improving the precision of the measurement of the essential work of fracture of thin sheets

The precision of the measurement of the essential work of fracture requires an accurate determination of the critical ligament length below which the ligament is in a mixed mode stress state and the failure mechanisms considerably change. A statistical procedure is proposed which allows to determine easily and accurately this critical ligament length. After rejection of the specimens having too small ligament lengths, we is obtained by extrapolation of the remaining data for zero ligament length. The number of data points from which we is calculated is also shown to strongly influence the precision of the measurement. The procedure is applied to the measurement of the toughness of sheets of Al and Zn alloys and of a low density polyethylene.

Experimental and numerical investigation of fracture in double-edge notched steel plates

Double-edge notched (DENT) steel plates were pulled until complete fracture and several experimental observations were made (using profilometry and scanning electron microscopy). The essential work of fracture (EWF) model was found to be well verified. Numerical simulations—up to the maximum load only—of some experiments were performed using the finite element method (FEM), and incorporating geometric and material non-linearities (large deformation elasto-plasticity). Some experimental measurements were compared with the corresponding numerical computations and excellent agreement was found.

Influence of test parameters on the measurement of the essential work of fracture of zinc sheets

The Essential Work of Fracture (EWF) concept is used to characterize the fracture of thin plates of a zinc alloy. The consistency and applicability of the EWF approach are discussed. The successive stages of the fracture process were studied: the evolution of the shape of the crack tip was observed with a scanning electron microscope, the shape of the plastic zone and the evolution of necking in the ligament ahead of the crack tip were measured using a laser profilometer and the onset of cracking in the ligament was detected by means of a TV camera coupled with acoustic emission recording. The influence of both test parameters and material parameters on the Essential Work of Fracture (We) and on the CTOD (δc) are elucidated. The effects of specimen geometry, deformation rate, texture and grain size are especially investigated. The main advantages and drawbacks of the EWF method are highlighted.