Ductile Damage Mechanics Research Articles

The Analysis of High-Speed Shearing Mechanic and Cross-Section about Sheet Metal

In the shearing process, these were many problems in shearing, the regularity of metal deformation of shearing was analyzed combined with theory of ductile damage and fracture mechanism of high-speed, the rules of factors which could be affected the shearing was analyzed on the velocity and shearing stress state. These theories would improve the shearing section.

A novel damage variable to characterize evolution of microstructure with plastic deformation for ductile metal materials under tensile loading

Abstract Damage and fracture of ductile metal materials are greatly associated with evolution of their microstructure under loading, which meso-dimensionally corresponds to grain deformation as well as nucleation, growth, and coalescence of microvoids in later local deformation. The evolution behavior of microstructure is important to realize ductile damage and fracture mechanism of materials. In the present work, a novel damage variable of shape factor of grains was put forward to quantitatively describe the character of microstructure; its evolution with the plastic deformation was built-up by microanalytical and mechanical experiments for Armco iron and mild steel tensile bars. The evolution rule based on the damage variable of shape factor is possible to be extended into all of ductile metal materials.

Experimental analysis of strain path dependent ductile damage mechanics and forming limits

This paper contributes to the physical understanding of sheet metal micro-mechanics by addressing the influence of damage evolution on localization and eventually ductile fracture in different strain paths. For this purpose, two steels of different microstructure are deformed in different strain paths, along which forming and fracture limit curves are measured. Microstructural damage mechanisms are characterized and compared for different strain paths (i.e. uniaxial, plane strain and biaxial tension) and at different stages of deformation (i.e. before localization, after localization and at fracture). Interesting results are obtained revealing generic relationships between microstructure evolution (e.g. damage accumulation), localization (forming limit curve) and fracture (fracture limit curve). For single phase microstructures with limited damage sources, damage is initiated as a result of a developing plastic instability (localization), and therefore does not have a significant role on the forming limits. For microstructures with more damage mechanisms, however, the damage accumulates even before localization, and significantly affects both necking and fracture limits.

Revisiting single-point incremental forming and formability/failure diagrams by means of finite elements and experimentation

In a previously published work, the current authors presented an analytical framework, built upon the combined utilization of membrane analysis and ductile damage mechanics, that is capable of modelling the fundamentals of single-point incremental forming (SPIF) of metallic sheets. The analytical framework accounts for the influence of major process parameters and their mutual interaction to be studied both qualitatively and quantitatively. It enables the conclusion to be drawn that the probable mode of material failure in SPIF is consistent with stretching, rather than shearing being the governing mode of deformation. The study of the morphology of the cracks combined with the experimentally observed suppression of neck formation enabled the authors to conclude that traditional forming limit curves are inapplicable for describing failure. Instead, fracture forming limit curves should be employed to evaluate the overall formability of the process.The aim of this paper is twofold: (a) to compare the mechanics of deformation of SPIF, namely the distribution of stresses and strains derived from the analytical framework with numerical estimates provided by finite element modelling; and (b) to compare the forming limits determined by the analytical framework with experimental values. It is shown that agreement between analytical, finite element, and experimental results is good, implying that the previously proposed analytical framework can be utilized to explain the mechanics of deformation and the forming limits of SPIF.

Defect tolerance assessment of ARIANE 5 structures on the basis of damage mechanics material modelling

Damage mechanics based material models have been applied to establish fracture control procedures for the failure prediction of ARIANE 5 main structural components as the booster cases including welds, the main stage, and the upper stage cryogenic tank. The main goals of the damage mechanics based investigations were the accurate failure prediction, the clarification of dependency of fracture toughness on geometry, the calibration of analytical methods and the interpretation and optimisation of small scale fracture tests for material characterization and quality assurance. The results of these investigations allowed a failure prediction accuracy with analytical tools which is close to 3D numerical simulations. This could be demonstrated both with ductile (Gurson) and brittle (RKR) damage mechanics models.

Single‐point incremental forming and formability—failure diagrams

In recent work, the present authors constructed a closed‐form analytical model that is capable of dealing with the fundamentals of single‐point incremental forming (SPIF) and explaining the experimental and numerical results published in the literature over the past couple of years. The model is based on membrane analysis with in‐plane contact frictional forces but is limited to plane strain, rotationally symmetric conditions. The aim of the present paper is twofold: first, to extend the previous closed‐form analytical model into a theoretical framework that can easily be applied to the different modes of deformation that are commonly found in general single‐point incremental forming processes and, second, to investigate the formability limits of SPIF in terms of ductile damage mechanics and the question of whether necking does, or does not, precede fracture. Experimentation by the present authors, together with data retrieved from the literature, confirms that the proposed theoretical framework is capable of successfully addressing the influence of the major parameters of the SPIF process. It is demonstrated that neck formation is suppressed in SPIF, so that traditional forming limit diagrams are inapplicable to describe failure. Instead fracture forming limit diagrams should be employed.

The Ductile Damage and Fracture Mechanisms Analysis with Random Dispersion Multivoids

The cell model with twenty-five random dispersion voids was employed to analyze the damage and fracture mechanism of the nodular cast iron. The results show that the growth velocity of the voids has obvious difference with each other due to the random dispersion of voids. In the early stage of the deformation, the growth of the voids is mainly determined by the distance of the voids since the triaxiality stress parameter of the matrix around the voids is approximately equal. With the increase of the triaxiality stress parameter of the matrix materials around the void, the evolution velocity of the voids increases quickly. At the same time, this will influence the neighboring voids to grow quickly. The chain reaction of the rapidly increase of voids lead to the final material failure. The results can explain the fracture appearance of the smooth bar specimens under uniaxial tensile load really.

Modeling of Anisotropic Damage for Ductile Materials in Metal Forming Processes

The primary goal of this study is to model the anisotropic effect of ductile damage in metal forming processes. To represent the ductile metals, an anisotropic ductile plasticity/damage formulation is considered within the framework of continuum mechanics. The formulation is motivated from fracture mechanisms and physical observations in Al–Si–Mg aluminum alloys with second phases. The ductile damage mechanisms are represented by the classical ductile process of nucleation of voids at inclusions, followed by their growth and coalescence. Functions associated with each mechanism are related to different microstructural parameters. The damage, represented by a second rank tensor, is coupled to the Bammann–Chiesa–Johnson (BCJ) rate-dependent plasticity using the effective stress concept. The constitutive equations are integrated using a trapezoidal implicit scheme and implemented into an explicit finite element code. This implementation is used to predict damage during the forward axisymmetric extrusion of an aluminum bar. This example illustrates the applicability of the model to predict the initiation and the evolution of anisotropic damage in metal forming processes.

Impact failure of beams using damage mechanics: Part I—Analytical model

A simple theoretical method, which is based on ductile damage mechanics and which retains strain rate effects, is presented for predicting the failure of beams made from a perfectly plastic material and subjected to impact loads. For this class of materials, the strains can be estimated by defining a hinge length. The definition adopted here leads to reasonable predictions for the plastic strains and the strain rate, as shown by comparing the results with numerical calculations and experimental data. The equivalent strain and the strain rate can be used in the damage model to predict the failure of beams, as shown in a companion paper (Alves, Jones, Int J Impact Eng 2002;27(8):863–90).

Three-dimensional damage theory of crack growth in large centre-cracked panels using the element removal technique

The conventional use of continuum ductile damage mechanics in finite element analysis identifies the crack tip or crack front by some criterion which, on the basis of developing parameters, deems a certain region to be cracked. In the region deemed to be cracked there is thus only a reduction in stresses, which thereafter falls continually. This paper incorporates in the damage model for both two and three dimensions, a facility which enables elements to be ‘switched off’ ehind the predicted crack front so that the stresses there are zero. A three-dimensional finite element analysis using this element removal scheme is performed to predict the deformation of centre-cracked panels under uniaxial and equibiaxial loading and to study the applicability of this scheme.

MESH INDEPENDENT CELL MODELS FOR CONTINUUM DAMAGE THEORY

Abstract— The conventional use of continuum ductile damage mechanics in finite element analyses identifies the “cell” in which damage occurs with the finite elements in which the distribution of stress and strain is modelled. Since the cell size is a fixed, metallurgically‐defined, property of the material being analysed, this methodology forces a minimum size for the finite element mesh. Mesh refinement is thereby disallowed. This paper presents one way of avoiding the problem by developing a mesh‐independent cell model which, with a fixed cell size, allows the finite element mesh to be refined to any degree within the cells. Procedures which average some state variables within the cells are introduced to prevent the localisation of damage after a certain critical stage is reached. The method has been tested in numerical simulations of (a) the deformation of a notched tensile bar, (b) a 35 mm compact tension specimen and (c) the first of the AEA spinning cylinder tests. There is a reasonable agreement between the results of the computer simulations and those of the experiments.