Amin H. Almasri

Effect of Strain Rate on the Dynamic Hardness in Metals

phenomena in a slender rod to determine the dynamic Vickers indentation hardness in metals. The results were verified by obtaining the constitutive response of materials at similar strain rates and then correlating the yield stress with the corresponding hardness. The characteristics of the induced plastic zone under static and dynamic indentations were investigated by contouring microindentation hardness measurements of the indented regions. The results showed that the size of the plastic zone highly depends on yield stress under static and dynamic conditions. Anton and Subhash 3 performed static and dynamic Vickers indentations on six different brittle materials to study the rate effects in hardness and fracture toughness. The dynamic indentations were performed on the same aforementioned hardness tester that is based on elastic stress wave propagation phenomena. Under dynamic indentations, an increase in hardness was observed in all the brittle materials compared to their static hardness measurements. Lu et al. 4 developed a dynamic indentation technique to measure time-resolved depth and load responses during the process of indentation. A moire interferometry-based displacement measurement technique was utilized to measure the depth of indentation and a quartz load transducer was used to measure the load. They introduced a new methodology to deduce the dynamic rate sensitivity of materials using the measured data. Andrews et al. 5 investigated the impact of a sharp indenter at low impact velocities for elastoplastic materials. They developed a one-dimensional model based on the assumption that under dynamic conditions—as well as under static conditions—the variation of indentation load is a parabolic function of the depth Kick’s law. The motion of the indenter as it indents and rebounds from the target was investigated. For rate-independent materials agreement with the model was good provided the impact velocity did not exceed certain critical values. For rate-dependent materials, the relationship between load and depth in the impact problem is no longer parabolic and the model predictions cannot be applied to this case. It was suggested that the rate-dependent case can be solved by incorporating the relationship between the motion of the indenter and the dynamic flow properties of the material into the equation of motion for the indenter. Initially Voyiadjis and Buckner 6 and Voyiadjis et al. 7 studied the axisymmetric contact problem for the elasto-plastic behavior of materials subjected to spherical static indentation using the finite element method with mixed boundary conditions. Vasauskas 8 divided the complete

Formulation and Verification of a Concrete Model with Strong Coupling between Isotropic Damage and Elastoplasticity and Comparison to a Weak Coupling Model

AbstractIn this work, a new concrete model that strongly couples continuum-damage-mechanics to elastoplasticity is presented. The model incorporates a plasticity yield criterion written in terms of the nominal/damaged, rather than effective, stress space. Tensile and compressive behaviors are modeled through the damage-affected-multi-hardening nature of the plasticity yield criterion and the introduction of two (tensile and compressive) plasticity-affected isotropic damage initiation/growth criteria, where plasticity variables are added to the definition of the tensile and compressive growth functions of damage. A nonassociative plastic flow rule is used to control inelastic dilatancy. Specific expressions of the elastic/damage and plastic/damage components of the Helmholtz free energy function are presented. The constitutive equations of the model are consistently derived within a sound framework of irreversible thermodynamics. These free energy expressions are used to define three plasticity and damage ...

Experimental Study and Fabric Tensor Quantification of Microcrack Distributions in Composite Materials

In this study, the microcrack distributions of samples of a metal-matrix composite material (titanium aluminide (Ti-14Al-21Nb(α2)) reinforced with continuous silicon carbide (SiC) (SCS-6) fibers) are measured experimentally, and then quantified using the fabric tensor approach. The laminated composite material samples have two lay-up configurations: [0/90]s and [±45]s. These samples are tested under uniaxial tension up to different levels of loads, to understand how microcrack distributions develop with applied loads. The [±45]s samples are shown to have more microcracks and a wider range of orientations of microcracks than the [0/90]s samples. The microcracks can be divided into two categories: fiber microcracks and fiber-interface microcracks. Distributions of both types are shown to be similar in shape but different in orientation. In addition, microcrack distributions weighted by the microcrack lengths are presented. Fabric tensors of zero, second, fourth, sixth, eighth, and tenth order are used to approximate these microcrack distributions. Fabric tensors are seen to give a very good approximation when eighth or tenth-order fabric tensors are used for both fiber and fiber-interface microcrack distribution types. Finally, damage variable and damage evolution relations are derived in terms of the fabric tensor based on a thermodynamically consistent framework.

Stress concentration around a central hole as affected by material nonlinearity in fibrous composite laminated plates subject to in-plane loading

Abstract The distribution of stresses in laminated composite plates with a central circular hole and having various stacking sequences, different geometric dimensions and subjected to in-plane axial tensile loading was investigated. The ANSYS computer program was utilized using the finite element method to study the linear and nonlinear material effects. A new method was proposed for the purpose of incorporating the material nonlinearity model into the ANSYS computer program using the secant modulus material model. The aim of the authors is to analyze the effect of D/b and a/b ratios (where D is hole diameter, b is plate width, and a is plate length) on stresses induced in such plates. Analysis was carried out for angle-ply, four-layered symmetric laminated rectangular plates with various stacking sequences [±θ]s.

Numerical Evaluation of Steel Columns Stability under Various Cases of Thermal Loads

This paper presents the results of a simulation study of fire tests of restrained (fixed-hinged) hot rolled steel column sections subjected to axial compression in addition to constant and variable thermal loads. Finite element computer software is utilized to study steel columns stability under thermal loads using three approaches; non-linear buckling analysis, thermal structural-Load Transfer Method (LTM) analysis, and Direct Coupled Field analysis. The finite element results are in good agreement with experimental results found in literature. The Direct Coupled Field analysis is also used to study a non-uniform temperature distribution on one side of the steel column. The LTM shows the best capabilities in studying such problems compared to the other two approaches

Analytical solution for shear bands in cold-rolled 1018 steel

Abstract Cold-rolled 1018 (CR-1018) carbon steel has been well known for its susceptibility to adiabatic shear banding under dynamic loadings. Analysis of these localizations highly depends on the selection of the constitutive model. To deal with this issue, a constitutive model that takes temperature and strain rate effect into account is proposed. The model is motivated by two physical-based models: the Zerilli and Armstrong and the Voyiadjis and Abed models. This material model, however, incorporates a simple softening term that is capable of simulating the softening behavior of CR-1018 steel. Instability, localization, and evolution of adiabatic shear bands are discussed and presented graphically. In addition, the effect of hydrostatic pressure is illustrated.