Kenji Nakai

Uniaxial Compressive Response and Constitutive Modeling of Selected Polymers Over a Wide Range of Strain Rates

The present work deals with constitutive modeling of the compressive stress–strain response of selected polymers at strain rates from 10−3 to nearly 103xa0s−1. Six different commercially available extruded polymers—ABS, HDPE, PC, POM, PP and PVC—are tested at room temperature. Cylindrical specimens with a slenderness ratio (=length/diameter) of 0.5 are used in high strain-rate tests, and those with the slenderness ratios of 1.0 and 2.0 are used in low and intermediate strain-rate tests. High strain-rate compressive stress–strain loops up to a strain of nearly 0.08 are obtained on a standard split Hopkinson pressure bar. Low and intermediate strain-rate compressive ones are measured on an Instron testing machine. By fitting experimental loading stress–strain data to a modified Ramberg–Osgood equation, material parameters are uniquely determined using a linear least-squares procedure. Experimental results indicate that all polymers tested exhibit intrinsic dynamic viscoelastic–plastic characteristics and a higher elastic after-effect following complete unloading. It is shown that the modified Ramberg–Osgood constitutive model is appropriate for describing the monotonic loading compressive stress–strain relations of the three semi-crystalline polymers over a wide range of strain rates. The advantages and limitations of the constitutive model are also discussed.

High Strain-Rate Compressive Properties and Constitutive Modeling of Bulk Structural Adhesives

The present paper is concerned with constitutive modeling of two different bulk structural adhesives at strain rates from 10−3 to 103/s using a modified Ramberg-Osgood equation. Commercially available two-part methacrylate structural adhesive and epoxy structural adhesive were tested. The high strain-rate compressive stress-strain loops for the two bulk structural adhesives were determined with a conventional split Hopkinson pressure bar using a tapered striker bar. Static and intermediate rate compressive stress-strain curves were measured in an Instron® testing machine. The five parameters for the modified Ramberg-Osgood equation were determined by fitting to the experimental stress-strain data using a least-squares fit. The compressive stress-strain curves at three different strain rates predicted from the modified Ramberg-Osgood model were compared with the experimental results. It is shown that the compressive stress-strain curves up to the maximum stress can successfully be predicted by the modified Ramberg-Osgood equation. The limitations of the modified Ramberg-Osgood model were discussed.

High Strain-Rate Compressive Characteristics of a Unidirectional Carbon/Epoxy Laminated Composite: Effect of Loading Directions

Composite material exhibits superior mechanical properties such as high specific stiffness and strength over metallic materials and, hence, are increasing used in various industries. In many practical situations, the composite structures are often subjected to impact loading. Therefore, it is required to characterize the mechanical properties of composite materials at high rates of strain.

Determination of High Strain-Rate Compressive Stress-Strain Loops of Selected Polymers

Compressive stress-strain loops of selected polymers at strain rates up to nearly 800/s are determined in a strain range of nearly 8% on the standard split Hopkinson pressure bar. Four different commercially available extruded polymers are tested at room temperature. The compressive stress-strain loops at low and intermediate strain rates are measured on an Instron testing machine. The effects of strain rate on the Youngs modulus, flow stress and dissipation energy are discussed. It is shown that the area included within the stress-strain loop increases with increasing strain rate as well as a given strain, that is, all four extruded polymers tested exhibit intrinsic strain-rate dependent viscoelastic behavior and a high elastic aftereffect following complete unloading.

Impact Compressive Failure of a Unidirectional Carbon/Epoxy Composite: Effect of Loading Directions

The impact compressive failure behaviour of a unidirectional T700/2521 carbon/epoxy composite in three principal material directions is investigated in the conventional split Hopkinson pressure bar. Two different types of specimens with square cross sections are machined from the composite in the plane of the laminate. The uniaxial compressive stress-strain curves up to failure at quasi-static and intermediate strain rates are measured on an Instron testing machine. It is demonstrated that the ultimate compressive strength (or maximum stress) increases slightly, while the ultimate compressive strain (or failure strain) decreases marginally with strain rate in the range of 10-3 to 103/s in all three directions. Dominant failure mechanisms are found to significantly vary with strain rate and loading directions along three principal material axes.

Tensile properties of 6061-T6 friction stir welds and constitutive modelling in transverse and longitudinal orientations

Abstract Tensile stress–strain properties of Al alloy 6061-T6 (AA6061-T6) and its butt welds produced by the friction stir welding (FSW) process were characterized in two different loading orientations. AA6061-T6 FS welds were made under three sets of welding conditions. Micro-hardness tests were performed to investigate microstructural evolution during the FSW process. Flat tensile specimens were machined normal and parallel to the weld line. Transvers and longitudinal tensile tests were run on the base material (AA6061-T6) and its FS welds in an Instron testing machine. The strength and ductility (or fracture strain) of the FS welds observed in the transverse orientation were substantially less than those in the longitudinal orientation. Constitutive modelling of uniaxial tensile stress–strain behaviour in both orientations was presented using a rate-independent Ludwik equation. In addition, microstructures of the base material and its FS welds were examined with optical and transmission electron microscopy to discuss the decrease in the flow stress level and the increase in the strain hardening rate of the FS welds.

High Strain-Rate Compressive Behavior and Constitutive Modeling of Selected Polymers using Modified Ramberg-Osgood Equation

The present paper is concerned with constitutive modeling of the compressive stress-strain behavior of selected polymers at strain rates from 10-3 to 103/s using a modified Ramberg-Osgood equation. High strain-rate compressive stress-strain curves up to strains of nearly 0.08 for four different commercially available extruded polymers were determined on the standard split Hopkinson pressure bar (SHPB). The low and intermediate strain-rate compressive stress-strain relations were measured in an Instron testing machine. Six parameters in the modified Ramberg-Osgood equation were determined by fitting to the experimental stress-strain data using a least-squares fit. It was shown that the monotonic compressive stress-strain behavior over a wide range of strain rates can successfully be described by the modified Ramberg-Osgood constitutive model. The limitations of the model were discussed.

Dynamic Tensile Characteristics of Nuclear-Grade Graphite IG-11

The effect of strain rate up to nearly = 102/s on the tensile stress-strain properties of isotropic fine-grained nuclear-grade graphite IG-11 was investigated. Cylindrical tensile specimens machined out of graphite bars were used in both static and dynamic tests. The dynamic tensile stress-strain curves up to fracture were determined using the split Hopkinson bar (SHB). The low and intermediate strain-rate tensile stress-strain relations up to fracture were measured on an Instron 5500R testing machine. It was demonstrated that the ultimate tensile strength increases slightly, while the fracture strain and absorbed energy up to fracture decrease dramatically with increasing strain rate. Macro and microscopic examinations revealed a slight difference in the fracture surfaces between the static and dynamic tension specimens.

High Strain-Rate Compressive Stress-Strain Loops for Structural Adhesive

The high strain-rate compressive stress-strain loops for bulk specimens of an epoxy structural adhesive are determined on the standard split Hopkinson pressure bar. The compressive stress-strain data including unloading curves are obtained over a wide range of strain rates from 10-3 to 103/s. The effects of strain rate on the initial (secant) modulus, flow stress, dissipation energy and hysteresis loss ratio are discussed. The experimental results show that the bulk structural adhesive exhibits dynamic viscoelastic behavior like polymers.

High Strain-Rate Compressive Response of Friction Stir Welded AA7075-T651 Joints

High strain-rate compressive responses of AA7075-T651 and its welds as produced by the friction stir welding (or FSW) process are investigated using the conventional split Hopkinson pressure bar. Cylindrical specimens machined along the thickness direction of the base material (AA7075-T651) and the friction stir (FS) welds are used in the static and impact compression tests. The micro-hardness tests are conducted across the centerline of a FS welded AA707-T651 joint in order to examine the microstructural change. It is shown that FSW reduces the compressive flow stress of the FS weld (weld nugget) to below that of the base material, and both the base material and the FS weld exhibit almost no strain rate effects up to nearly € ε˙ =103/s.