Youssef Hammi

Compressive behavior of a turtle's shell: experiment, modeling, and simulation.

The turtles shell acts as a protective armor for the animal. By analyzing a turtle shell via finite element analysis, one can obtain the strength and stiffness attributes to help design man-made armor. As such, finite element analysis was performed on a Terrapene carolina box turtle shell. Experimental data from compression tests were generated to provide insight into the scute through-thickness behavior of the turtle shell. Three regimes can be classified in terms of constitutive modeling: linear elastic, perfectly inelastic, and densification regions, where hardening occurs. For each regime, we developed a model that comprises elasticity and densification theory for porous materials and obtained all the material parameters by correlating the model with experimental data. The different constitutive responses arise as the deformation proceeded through three distinctive layers of the turtle shell carapace. Overall, the phenomenological stress-strain behavior is similar to that of metallic foams.

An Anisotropic Damage Model for Ductile Metals

An anisotropic ductile damage description is motivated from fracture mechanisms and physical observations in Al-Si-Mg aluminum alloys with second phases. Ductile damage is induced by the classical process of nucleation of voids at inclusions, followed by their growth and coalescence. These mechanisms are related to different microstructural and length scale parameters like the fracture toughness, the void size, the intervoid ligament distance, etc. The classical thermodynamic constraints of irreversible processes with material state variables are used to model the tensorial damage evolution coupled to the Bammann-Chiesa-Johnson (BCJ) rate-dependent plasticity. The damage-plasticity coupling is based on the effective stress concept, assuming the total energy equivalence, and written through a deviatoric damage effect tensor on the deviatoric part and through the trace of the second rank damage tensor on the hydrostatic part. The damage rate tensor is additively decomposed into a nucleation rate tensor, a void growth rate scalar, and a coalescence rate tensor. The induced damage anisotropy in mainly driven by the nucleation, which evolves as a function of the absolute value of the plastic strain rate tensor. Finally, some experimental data of cast A356 aluminum alloy are correlated with predictive void-crack evolution to illustrate the applicability of the anisotropic damage model.

Micromechanics Study of Fatigue Damage Incubation Following an Initial Overstrain

Understanding and quantifying the effects of overloads/overstrains on the cyclic damage accumulation at a microscale discontinuity is essential for the development of a multi-stage fatigue model under variable amplitude loading. Micromechanical simulations are conducted on a 7075-T651 Al alloy to quantify the cyclic microplasticity in the matrix adjacent to intact or cracked, life-limiting intermetallic particles. An initial overstrain followed by constant amplitude cyclic straining is simulated considering minimum to maximum strain ratios of 0 and —1. The nonlocal equivalent plastic strain at the cracked intermetallic particles reveals overload effects manifested in two forms: (1) the cyclic plastic shear strain range is greater in the cycles following an initial tensile overstrain than without the overstrain and (2) the initial overstrain causes the nonlocal cumulative equivalent plastic strain to double in subsequent tensile-going half cycles and triple in subsequent compressive-going half cycles, as compared with cases without an initial tensile overstrain. The cyclic plastic zone at the microdiscontinuity corresponds to that of the maximum strain during the initial overstrain and the nonlocal cyclic plastic shear strain range in the matrix near the intact or cracked inclusion is substantially increased for the same remote strain amplitude relative to the case without initial overstrain. These results differ completely from the effects of initial tensile overload on the response at a macroscopic notch root or at the tip of a long fatigue crack in which the driving forces for crack formation or growth, respectively, are reduced. The micromechanical simulation results support the incorporation of enhanced cyclic microplasticity and driving force to form fatigue cracks at cracked inclusions following an initial tensile overstrain in a fatigue incubation model.

Plasticity and Fracture Modeling/Experimental Study of a Porous Metal Under Various Strain Rates, Temperatures, and Stress States

A microstructure-based internal state variable (ISV) plasticity-damage model was used to model the mechanical behavior of a porous FC-0205 steel alloy that was procured via a powder metal (PM) process. Because the porosity was very high and the nearest neighbor distance (NND) for the pores was close, a new pore coalescence ISV equation was introduced that allows for enhanced pore growth from the concentrated pores. This coalescence equation effectively includes the local stress interaction within the interpore ligament distance between pores and is physically motivated with these highly porous powder metals. Monotonic tension, compression, and torsion tests were performed at various porosity levels and temperatures to obtain the set of plasticity and damage constants required for model calibration. Once the model calibration was achieved, then tension tests on two different notch radii Bridgman specimens were undertaken to study the damage-triaxiality dependence for model validation. Fracture surface analysis was performed using scanning electron microscopy (SEM) to quantify the pore sizes of the different specimens. The validated model was then used to predict the component performance of an automotive PM bearing cap. Although the microstructure-sensitive ISV model has been employed for this particular FC-0205 steel, the model is general enough to be applied to other metal alloys as well. [DOI: 10.1115/1.4025292]

Modelling and experimental study of fatigue of powder metal steel (FC-0205)

Abstract The microstructure sensitive multistage fatigue model captured the fatigue life of a powder metal FC-0205 steel alloy. Uniaxial strain controlled fatigue data and microstructure information from sets of high and low porosity specimens calibrated the model. Strain–life behaviour depicted that above the plastic strain limit of 0·002 mm mm−1 in the low cycle fatigue regime, where ubiquitous plasticity occurred, the different porosity levels gave distinct, visibly different results. However, specimens tested below the plastic limit in the high cycle fatigue regime, where failure was dominated by local cyclic microplasticity, showed unclear fatigue lives at different porosity levels. Fractography using scanning electron microscopy showed no clear presence of striations; however, asserted striations in powder metal specimens were similar to geometrical features observed on fracture surfaces of monotonically loaded specimens. The experimental and microstructure data calibrated a fatigue model that allowed for satisfactory prediction of the varying porosity specimen strain–life curves.

An Experimental Study of Mechanical Behavior of Resistance Spot Welded Aluminum 6061-T6 Joints

In this study, the mechanical behavior, joint characterization and process sensitivity of aluminum 6061-T6 alloy Resistance Spot Welded (RSW) joints are investigated. Tensile tests were conducted on single weld lap-shear coupons until failure in order to determine the optimum welding parameters. Welding currents, forces and times have been investigated to establish the correlation between failure loads and nugget sizes. The experimental and preliminary finite element analysis (FEA) results indicate that failure loads and nugget sizes are strongly dependent upon welding parameters and the material anisotropy.© 2010 ASME

Constitutive modeling of metal powder behavior during compaction

In this paper, a constitutive modeling of the metal powder compaction is presented as a first step of ongoing research of a multiscale and multistage mathematical based model concept for powder metallurgy component design and performance prediction using the finite element method. In recent years, techniques such as finite element analysis have received wide attention for their high applicability to powder metallurgy (PM) industry. These techniques provide a valuable tool in predicting density and stress distributions in the pressed compact prior to the actual tooling design and manufacturing process. However, the accuracy of FE prediction highly depends on the possibility to obtain appropriate experimental data to calibrate and validate the powder material model. Within the framework of continuum mechanics, the plasticity model depends on external and internal state variables such as temperature, stress, hardening, relative density and contact between metal powder particles. The material model considers different microstructural and mechanical aspects such as dislocation dynamics, friction, porosity/density evolution, pressure dependent yield surface, particle size/shape distribution, and rate effects related to press speed. To represent the powder densification, we describe a double-surface plasticity model, based on a combination of a convex yield surface consisting of a failure envelope, such as a Mohr-Coulomb yield surface and, a hardening cap model with the use of internal state variables that capture the structure of the structure-property relations. The model is aimed to be implemented in the finite element program. The Molecular Dynamics is also introduced in this multiscale methodology as numerical experiments to study the metal compaction and sintering processes at the nanoscale level. These numerical experimental data are aimed to determine and correlate the micrpstructure-plasticity relationships at the macroscale.

Computational Investigation of High Velocity Penetration of Copper Subjected to Impact From Nickel Projectiles

A computational simulation of penetration between nickel projectile and copper plate with high velocity in macro scale has been built. It is the first time a comprehensive investigation of penetration between these two materials. A threshold velocity between a nickel projectile and a copper plate is determined by mathematical equation in this paper. ABAQUS/Explicit is used to verify this threshold velocity by setting different velocities under the same condition and displaying visual animation of penetration results. John-cook model has been chosen to represent two material plastic behaviors. The shapes of penetrator and target are shown in order to expose penetration procedure and phenomena. Stress patterns and perforation features are adopted to make a good understanding of Ni-Cu penetration mechanism from computational study.Copyright

Application of a Microstructure-Based ISV Plasticity Damage Model to Study Penetration Mechanics of Metals and Validation through Penetration Study of Aluminum

A developed microstructure-based internal state variable (ISV) plasticity damage model is for the first time used for simulating penetration mechanics of aluminum to find out its penetration properties. The ISV damage model tries to explain the interplay between physics at different length scales that governs the failure and damage mechanisms of materials by linking the macroscopic failure and damage behavior of the materials with their micromechanical performance, such as void nucleation, growth, and coalescence. Within the continuum modeling framework, microstructural features of materials are represented using a set of ISVs, and rate equations are employed to depict damage history and evolution of the materials. For experimental calibration of this damage model, compression, tension, and torsion straining conditions are considered to distinguish damage evolutions under different stress states. To demonstrate the reliability of the presented ISV model, that model is applied for studying penetration mechanics of aluminum and the numerical results are validated by comparing with simulation results yielded from the Johnson-Cook model as well as analytical results calculated from an existing theoretical model.

Damage Progression and Fragmentation in Atomistic, Single Crystal Copper at High Strain Rates

We show a correlation between nanoscale damage and fragmentation length scale through atomistic simulations. We simulated homogeneously expanding perfect, single crystal copper at rates ranging from 1E+08 to 3E+10 s-1 and temperatures from 200 to 1000 K. Damage was quantified in terms of void number density, average void volume, and void volume fraction. We quantified fragmentation size in terms of a length scale parameter, the solid volume per void surface area. A-1⁄2 power law relationship between the fragment length scale and strain rate was observed following the predictions of Mott. The fragmentation length scale and the maximum void number density are strongly correlated for this damage mechanism. We can scale up the relationships between damage and fragmentation observed in the molecular dynamics simulations to motivate a continuum scale fragmentation model.