A finite element analysis of dynamic fracture initiation by ductile failure mechanisms in a 4340 steel

Abstract

In some recent dropweight impact experiments [5] with pre-notched bend specimens of 4340 steel, it was observed that considerable crack tunneling occurred in the interior of the specimen prior to gross fracture initiation on the free surfaces. The final failure of the side ligaments happened because of shear lip formation. The tunneled region is characterized by a flat, fibrous fracture surface. In this paper, the experiments of [5] (corresponding to 5 m/s impact speed) are analyzed using a plane strain, dynamic finite element procedure. The Gurson constitutive model that accounts for the ductile failure mechanisms of micro-void nucleation, growth and coalescence is employed. The time at which incipient failure was observed near the notch tip in this computation, and the value of the dynamic J-integral, Jn, at this time, compare reasonably well with experiments. This investigation shows that J-controlled stress and deformation fields are established near the notch tip whenever Jd increases with time. Also, it is found that the evolution of micro-mechanical quantities near the notch root can be correlated with the time variation of J,~. The strain rate and the adiabatic temperature rise experienced at the notch root are examined. Finally, spatial variations of stresses and deformations are analyzed in detail.

Implicit stress integration procedure for small and large strains of the Gurson material model

“…The same approach was followed by other authors, e.g. Lee and Zhang [29], Jha and Narasimhan [9], Doege et al [7], Zhang [30; 31], Lee and Zhang [6] and Brunet and Sabourin [8].…”

“…An extension of the yield condition (1) to a material with the kinematic or mixed hardening response was proposed by Lee and Zhang [6], and to an orthotropic-type model by Doege et al [7], and Brunet and Sabourin [8]. The yield condition (1) was employed by Jha and Narasimhan [9] as the ow potential in a rate-sensitive viscoplastic model for dynamic fracture in steel.…”

Thermal effect of plastic dissipation at the crack tip on the stress intensity factor under cyclic loading

“…Analytical [6,7], experimental [8][9][10][11] and numerical [12,7] methods allowed estimation of the temperature variation field near the crack tip and these studies show that this increase in the temperature can change the fracture mechanisms under monotonic loading.…”

Numerical simulation of anvil interactions in the impact testing of notched bend specimens

“…Such a theoretical model would considerably relax the need for direct measurement of K~ on the specimen, which is particularly suitable for routine material testing, and provide insight into various physical phenomena involved in the impact process. Although the method of applying measured tup load and anvil reaction force histories as traction boundary conditions in a finite element model [15,[17][18] yields predictions close to experimental results, it doesn't provide insight into the physics of the system dynamics. Also, this method requires additional instrumentation.…”