Showing papers on "Stress–strain curve published in 2000"

Cement reinforced with short carbon fibers: a multifunctional material

Abstract: This is a review of cement-matrix composites containing short carbon fibers. These composites exhibit attractive tensile and flexural properties, low drying shrinkage, high specific heat, low thermal conductivity, high electrical conductivity, high corrosion resistance and weak thermoelectric behavior. Moreover, they facilitate the cathodic protection of steel reinforcement in concrete, and have the ability to sense their own strain, damage and temperature. Fiber surface treatment can improve numerous properties of the composites. Conventional carbon fibers of diameter 15 μm are more effective than 0.1 μm diameter carbon filaments as a reinforcement, but are much less effective for radio wave reflection (EMI shielding). Carbon fiber composites are superior to steel fiber composites for strain sensing, but are inferior to steel fiber composites in the thermoelectric behavior.

Time and temperature dependence of the scratch properties of poly(methylmethacrylate) surfaces

Abstract: Most existing models describing the scratch properties of materials take into account forces acting at the interface between the material and a grooving tip, but do not consider the stress and strain properties of the material far beneath or ahead of the tip. In the case of polymer scratches, there are no models at all which take into account the viscoelastic viscoplastic behaviour of the material. In standard indentation tests with a non moving tip, the elastic plastic boundary and the limits of the region subjected to hydrostatic pressure beneath the tip are known. These models were used to analyse the geometry of the grooves left on the surface of a viscoelastic viscoplastic body by a moving cone-shaped diamond tip having a radius of about 40 μm. A new apparatus was built to control the velocity of the tip over the range 1 to 104 μm/s, at several different temperatures from −10°C to 100°C. The material was a commercial grade of cast poly(methylmethacrylate) (PMMA). The normal and tangential loads and groove size were used to evaluate the dynamic hardness, which behaved like a stress and temperature activated process. Values of the activation energy and volume of the dynamic hardness and of the interfacial shear stress were in good agreement with those usually attributed to the mechanical properties of PMMA.

Macroscopic material properties from quasi-static, microscopic simulations of a two-dimensional shear-cell

Abstract: One of the essential questions in the area of granular matter is, how to obtain macroscopic tensorial quantities like stress and strain from ``microscopic'' quantities like the contact forces in a granular assembly. Different averaging strategies are introduced, tested, and used to obtain volume fractions, coordination numbers, and fabric properties. We derive anew the non-trivial relation for the stress tensor that allows a straightforward calculation of the mean stress from discrete element simulations and comment on the applicability. Furthermore, we derive the ``elastic'' (reversible) mean displacement gradient, based on a best-fit hypothesis. Finally, different combinations of the tensorial quantities are used to compute some material properties. The bulk modulus, i.e. the stiffness of the granulate, is a linear function of the trace of the fabric tensor which itself is proportional to the density and the coordination number. The fabric, the stress and the strain tensors are {\em not} co-linear so that a more refined analysis than a classical elasticity theory is required.

Continuum damage mechanics-based fatigue model of asphalt concrete

Abstract: A fatigue performance prediction model of asphalt concrete is developed from a uniaxial constitutive model based on the elastic-viscoelastic correspondence principle and continuum damage mechanics through mathematical simplifications. This fatigue model has a form similar to the phenomenological tensile strain-based fatigue model. Therefore, a comparison between the new model and the phenomenological model yields that the regression coefficients in the phenomenological model are functions of viscoelastic properties of the materials, loading conditions, and damage characteristics. The experimental study on two mixtures with compound loading histories demonstrates that the fatigue model maintains all of the strengths of the constitutive model such as its accuracy and abilities to account for the effects of rate of loading, stress/strain level dependency, rest between loading cycles, and mode-of-loading on fatigue life of asphalt concrete.

Micromechanics-based stress-strain behaviour of soils at small strains

Abstract: A model that describes the small-strain behaviour of soils is derived using micromechanics theory. The model allows examination of the effects of fabric anisotropy, stress conditions and contact characteristics on the small-strain modulus of soils. The closed-form solutions of the small-strain modulus are presented for the case of an isotropic fabric assembly under isotropic stress conditions. A fabric tensor is used to model the fabric anisotropy, and the small-strain modulus for the case of cross-anisotropic fabric assembly under general stress conditions is numerically calculated. The effect of the contact condition between soil particles is examined by incorporating three different contact laws (the linear elastic, Hertz–Mindlin and rough-surface contact models) into the model. It was found that the numerical results using the rough-surface contact model compare well with the published experimental data, and that the model provides microscopic insight into the small-strain behaviour of soils observed ...

An improved methodology for determining temperature dependent moduli of underfill encapsulants

Abstract: Finite element analyses (FEAs) have been widely used to preventively predict the reliability issues of flip-chip (FC) packages. The validity of the simulation results strongly depends on the inputs of the involved material properties. For FC packages Young's modulus-temperature relationship is a critical material property in predicting of the package reliability during -55/spl deg/C to 125/spl deg/C thermal cycling. Traditional tensile tests can obtain the modulus at selected temperatures, but are tedious, expensive, and unable to accurately predict the Young's modulus-temperature relationship within a wide temperature range. Thus, this paper is targeted to provide a simple but relatively accurate methodology to obtain the Young's modulus-temperature relationship. In this paper, three commercial silica filled underfill materials were studied. A simple specimen (based on ASTM D638M) preparation method was established using a Teflon mold. A dynamic-mechanical analyzer (DMA) was used to obtain the stress-strain relationship under controlled force mode, storage and loss modulus under multi-frequency mode, and stress relaxation under stress relaxation mode. A simple viscoelastic model was used and an empirical methodology for obtaining Young's modulus-temperature relationship was established.

Time and temperature dependence of the scratch properties of poly(methylmethacrylate) surfaces

Abstract: Most existing models describing the scratch properties of materials take into account forces acting at the interface between the material and a grooving tip, but do not consider the stress and strain properties of the material far beneath or ahead of the tip. In the case of polymer scratches, there are no models at all which take into account the viscoelastic viscoplastic behaviour of the material. In standard indentation tests with a non moving tip, the elastic plastic boundary and the limits of the region subjected to hydrostatic pressure beneath the tip are known. These models were used to analyse the geometry of the grooves left on the surface of a viscoelastic viscoplastic body by a moving cone-shaped diamond tip having a radius of about 40„m. A new apparatus was built to control the velocity of the tip over the range 1 to 10 4 „m/s, at several different temperatures from i10 ‐ Ct o 100 ‐ C. The material was a commercial grade of cast poly(methylmethacrylate) (PMMA). The normal and tangential loads and groove size were used to evaluate the dynamic hardness, which behaved like a stress and temperature activated process. Values of the activation energy and volume of the dynamic hardness and of the interfacial shear stress were in good agreement with those usually attributed to the mechanical properties of PMMA. C ∞ 2000 Kluwer Academic Publishers

Deformation and damping behaviors of foamed Al–Si–SiCp composite

Abstract: The tensile and compressive as well as damping behaviors of A356/20SiCp composite foams with different porosity and cell sizes, which were produced by a foaming route, have been studied. The tensile stress–strain curves of the composite foams show a quite low fracture elongation, and the compressive stress–strain curves demonstrate three distinct regions, i.e. the linear elastic region, the crushing collapse region and the densification region. The linear elastic region only appears at very low strain (<0.03). In the linear elastic region, the mechanical property (modulus) can be described by the model of Ashby. The compressive stress–strain curves in the crushing collapse region show marked serrations. The A356/SiCp composite foam exhibits characteristics of typical brittle foam. The relative density of the foam exhibits a significant influence on the damping capacity, while its influence on the dynamic modulus of the composite foam is very small.

A Method for Testing Tensile Strength in Unsaturated Soils

Abstract: This paper presents a new method for testing tensile strength in unsaturated soils. A rigid tensile mold has been designed and simple testing procedures developed. A series of tension tests has been carried out on densely compacted unsaturated soil specimens to evaluate the method. The material tested in this study was a highly plastic 50:50 (by dry weight) mixture of sand and Na-rich bentonite. Cylindrical specimens were the same size and shape as those used for triaxial tests.

Comparative study of large strain methods for assessing failure characteristics of selected food gels

Abstract: Vane rheometry was compared with uniaxial compression and torsion in evaluating the effects of strain rate on failure shear stress and deformation of soybean protein (tofu) and gellan gum gels. A Haake VT 550 viscotester was used for torsion and vane tests, and compression was performed with an Instron/MTS universal testing machine. Strain or angular deformation at failure was independent of strain rate in the three testing modes. In vane rheometry, failure shear stress increased with increasing low shear rates (< 0.100 s−1) and was rate independent at higher rates. This strain rate dependency was also evident in compression, varying with the material. For torsion, fracture stress appeared to be rate independent. Shear fracture stresses measured in torsion and compression were in good agreement at strain rates above 0.025 s−1 and 0.100 s−1 for tofu and gellan gels, respectively. Shear stresses from the vane method were lower than shear stresses of torsion and compression. Similar texture maps of the food gels studied were generated by plotting stress and strain or angular deformation values of the three testing methods. The findings validate the vane technique as an alternative to torsion and compression for rapid textural characterization of viscoelastic foods.

Stress–strain relation of entangled polymer networks

Abstract: The non-linear stress–strain relation for cross-linked polymer networks is studied using molecular dynamics simulations. Previously we demonstrated the importance of trapped entanglements in determining the elastic and relaxational properties of networks. Here, we present new results for the stress versus strain for both dry and swollen networks. Models which limit the fluctuations of the network strands like the tube model are shown to describe the stress for both elongation and compression. For swollen networks, the total modulus is found to decrease like ( V 0 / V ) 2/3 and goes to the phantom model result only for short strand networks.

Stiffness nonlinearity of three sands.

Abstract: This paper presents the results of 254 drained triaxial compression tests. The results are analyzed with the aim of obtaining a phenomenological description of secant Young's modulus variation according to different test conditions. Using empirical fitting equations, the influences of different factors such as axial strain, vertical consolidation stress, consolidation stress ratio, and stress history on the secant Young's modulus under axisymmetric (triaxial) loading conditions at small and intermediate axial strains are singled out and presented. The main purpose of this paper is to identify the factors affecting the nonlinear secant stiffness of sand and to quantify their relevance at different levels of strain.