Identification of damage mechanism and validation of a fracture model based on mesoscale approach in spalling of titanium alloy

AbstractThe subject of this paper is identification of the physical mechanisms of spalling at low impact velocities for Ti–6Al–4V alloy and determination of the macroscopic stress of spalling via meso-macro approach. Spalling is a specific mode of fracture which depends on the loading history. The aspects of the initial microstructure and its evolution during plastic deformation are very important. In order to identify the spalling physical mechanisms in titanium alloy, numerous pictures by the optical microscopy of the spall surfaces created by plate impact technique have been taken. The scenario of failure observed is in complete agreement with known physical micro-mechanisms: namely nucleation, propagation and coalescence by adiabatic shearing of micro-voids. The most interesting point in spall fracture of Ti–6Al–4V alloy is the nucleation of micro-voids. A significant amount of small micro-voids in the region of the expected spall plane has been observed. It appears that microstructural effects are important due to dual α–β phase microstructure, called Widmanstätten structure. The orientation of microstructure has a direct influence on nucleation mechanism by means of distribution of nucleation sites and decohesion between the softer particles (α-phase lamellae) and the harder lattice (β-phase). According to these observations, a fracture model has been developed. This model is based on the numerous post-mortem microscopic observations of spall specimens. The goal is to determine the macroscopic stress of spalling in function of loading time and damage level via a meso-macro approach

Spalling uniaxial strength of Al2O3 at high strain rates

In this article research into the uniaxial tensile strength of Al2O3 monolithic ceramic is presented. The experimental procedure of the spalling of long bars is investigated from different approaches. This method is used to obtain the tensile strength at high strain rates under uniaxial conditions. Different methodologies proposed by several authors are used to obtain the tensile strength. The hypotheses needed for the experimental set-up are also checked, and the requirements of the set-up and the variables are also studied by means of numerical simulations. The research shows that the shape of the projectile is crucial to achieve successfully tests results. An experimental campaign has been carried out including high speed video and a digital image correlation system to obtain the tensile strength of alumina. Finally, a comparison of the test results provided by three different methods proposed by different authors is presented. The tensile strength obtained from the three such methods on the same specimens provides contrasting results. Mean values vary from one method to another but the trends are similar for two of the methods. The third method gives less scatter, though the mean values obtained are lower and do not follow the same trend as the other methods for the different specimens

Failure dynamics: New area of research

This paper is a short review of the present situation and recent advances in dynamic fracture and failure. More precisely, two main research areas are discussed: Dynamic Fracture Mechanics and Dynamic Failure Mechanics. After brief characterization of the Dynamic Fracture Mechanics, a well developed research area, a more detailed discussion of the Dynamic Failure Mechanics is offered. Although in the past some research efforts can be spotted in the literature describing failure of materials during impact or explosions there is a lack of systematic characterization of this area. The main subject in this contribution is a discussion how dynamic plasticity coupled with temperature effects intervenes in failure by adiabatic heating. Some examples of failure by adiabatic shear band formation, including some new failure criteria, are discussed

Plastic shearing at high and very high strain rates

Nous analysons la déformation de cisaillement pour différentes vitesses de chargement dans le domaine des grandes et très grandes vitesses de déformation. Ainsi, nous présentons brièvement les différentes techniques expérimentales utilisées dans ce domaine. Une nouvelle technique expérimentale basée sur l'impact direct utilisée en double cisaillement ainsi que la géométrie modifiée de l'éprouvette sont discutées. Pour étudier le cisaillement simple, nous avons développé un modèle unidimensionnel où la loi de comportement tient compte de la sensibilité à la vitesse de déformation et à la température. Pour différentes vitesses de déformation, nous avons discuté et déterminer les conditions d'instabilités dans le cas du cisaillement adiabatique. Finalement dans le cas de la propagation unidimensionnelle des ondes plastiques, nous avons analysé la déformation engendrée par les instabilités thermoplastiques. Nous présentons une analyse précise sur la vitesse critique d'impact dans le cas du cisaillement, ainsi, nous avons effectué une comparaison entre les résultats théoriques et expérimentaux.Shear straining at different velocities, including high and very high strain rates, is discussed. Some experimental techniques which allows for experiments at high and very high strain rates are briefly reviewed. A new experimental technique based on direct impact on Modified Double Shear a specimen (MDS) is also reviewed. A one-dimensional model for simple shear is applied together with a constitutive relation which takes into account rate and temperature sensitivity a generalised condition for instability of adiabatic shearing at different rates has been derived and discussed. Finally, an analysis of deformation trapping in shear due to thermoplastic instability in one-dimensional plastic wave propagation is analysed. The critical impact velocity in shear is analysed in detail and values are compared with preliminary experiments

INITIATION OF FRACTURE AT DIFFERENT LOADING RATES, AN ATTEMPT OF MODELLING BASED ON DYNAMIC PLASTICITY

La sensibilité de la vitesse de chargement pour l'initiation de la rupture est analysée par rapport à quelques aciers, la solution HRR combinée avec le critère RKR est aussi envisagée, /8-11/. La sensibilité de la vitesse de déformation plastique est introduite en parallèle avec l'analyse, /3/. Le minimum de la ténacité observée en une région particulière des vitesses de chargement est analysé avec ce modèle.The loading rate sensitivity of fracture initiation is discussed for some steels within the framework of HRR solution and RKR. criterion /8-11/. Strain rate effects of plastic flow are also introduced into the analysis /3/. The minimum of fracture toughness at a particular region of loading rates is analysed by this model

Dynamic instabilities and failures in impact tension, compression and shear

Several aspects of Dynamic Fracture Mechanics and Dynamic Failure Mechanics are discussed in this paper. In the first part the local fracture criteria are briefly discussed, but the main part is focused on the failure criteria to be applied in fast or impact loading. A phenomenon of delayed fracture is discussed when a stationary crack is loaded by an impulse in tension or shear. Some local failure criteria are evoked and applied to cases of dynamic failures in tension, compression and shear. Conditions for stability in different modes of dynamic loading leading to failures are defined. The Critical Impact Velocities (CIV) in tension and shear, as the last stage of dynamic failure, are discussed as well. Another case of dynamic failure is the phenomenon of spalling. A local failure criterion based on delayed accumulation of defects and generalized to a wide range of temperatures is introduced

CONSTITUTIVE MODELING IN DYNAMIC PLASTICITY BASED ON PHYSICAL STATE VARIABLES - A REVIEW

En rappel des derniers développements des modèles de la plasticité dynamique appliqués aux métaux et alliages. Une approche cohérente pour fournir une loi constitutive doit se baser sur l'évolution des variables physiques qui caractérisent la microstructure. Le glissement et l'évolution de la microstructure sont déduits tous deux à partir de l'analyse de l'activation thermique et de la vitesse de déformation. Concrètement, il est possible d'établir quantitativement les effets de l'histoire de la vitesse de déformation et de la température sur des structures du type C.F.C., C.C. ou H.C. On montre également l'existence d'une très forte production de dislocations aux grandes vitesses de déformation (- 104 s- 1), ce qui conduit un accroissement notable du seuil mécanique athermique (contrainte d'écoulement en l'absence d'activation thermique).A review is presented on recent developments in constitutive modeling in dynamic plasticity of metals and alloys. A consistent approach to constitutive modeling is based on evolution of physical state variables which characterize microstructure. The thermal activation strain rate analysis is employed for both, the kinetics of glide and the kinetics of structural evolution. As a result quantitative description of strain - rate history and temperature - history effects in FCC, BCC and HCP structures can be accomplished. It is also demonstrated that at high strain rates (- 104 s-1) an excessive dislocation generation occurs which leads to a substantial increase of the mechanical threshold (the flow stress in absence of thermal activation)

Behavior of Ti-6Al-4V alloy at high strain rates, shear testing up to 6 x 10E4 1/s and failure criterion

Plastic behavior in shear of Ti-alloy : Ti-6Al-4V is the theme of this paper. A fast hydraulic machine, and the direct impact technique with impact velocities from 2 m/s to 120 m/s were applied to study this alloy. The following range of strain rate was applied : l0E-3 to 5* 10E4 1/s using the MDS specimen geometry, [2]. The role of the stress concentrator is discussed in triggering of failure by Adiabatic Shear Bands (ASB). A local failure criterion is proposed which is based on the critical strain of the final localization. The criterion has been applied in a numerical code, [10], permitting to calculate dynamic failure by adiabatic shearing. Finally, a brief discussion is includes on the Critical Impact Velocity in shear, [1,2,7,10]

Impact tension of sheet metals - Effect of initial specimen length

It is well known that a specimen for impact testing of materials must be optimized concerning its dimensions. The main reason is to reduce strain gradients due to the effects of elastic-plastic wave propagation. On the other hand, when Split Hopkinson Bar (SHB) is applied for tension test, the net displacement of the specimen ends is very limited, usually from 2.0 to 3.0 mm. Thus, to reach maximum strain 0.5 the specimen length must be reduced to dimensions from 4.0 mm to 6.0 mm. Consequently small diameter, or lateral dimensions in case of flat specimen, must be applied to assure one-dimensional deformation. Such small lengths substantially perturb the real material behavior to be determined. So the main motivation of this study was to perform a systematic analysis, numerical and analytical, to find differences in behavior of short and long specimens loaded in impact tension. The FE code Abaqus/Explicit has been used to simulate several specimen lengths from 10 mm to 40 mm, and several velocities from 10 m/s to 100 m/s