Discontinuous Reinforcement Research Articles

A theoretical and numerical stability analysis for composite micro-structures by using homogenization theory

A theoretical and numerical non-linear analysis of the homogenized response of composite solids with periodic micro-structure subjected to macroscopically uniform strain is here developed, by including the effects of instabilities occurring at both macroscopic and microscopic levels. The theory, formulated for incrementally linear materials, provides an original closed-form representation of homogenized material response which puts in evidence the competing effects of local constitutive response and of microstructural heterogeneity. Macroscopic constitutive stability measures are introduced corresponding to the positivity of the homogenized moduli tensors relative to a class of conjugate stress–strain pairs, and their relations with microscopic stability are investigated by using the proposed theoretical framework.These results are then illustrated and verified by analyzing numerically the in-plane stability problem of hyperelastic models of composites with continuous and discontinuous reinforcements and defected cellular solids. To this end a coupled finite element approach is implemented for a micro-structure driven along prescribed monotonic macro-strain paths. Numerical applications provide the hierarchy of critical loads for both macroscopic and microscopic stability measures and show how the proposed macroscopic constitutive stability region (determined by simple unit cell calculations) can be included within the exact microscopic stability region (obtained by including microstructural details and thus more computational expensive) in accordance with the theoretical developments given before. This gives the possibility of obtaining a conservative estimation of the microscopic critical load, essential to determine the validity of homogenization procedure and to predict instability-induced material failure, and thus makes the paper significant from the engineering’s point of view.

Influence of SiC particles distribution on mechanical properties and fracture of DRA alloys

In the last 20 years a new class of metal matrix composite material (DRA – Discontinuously Reinforced Aluminum) with aluminum alloy matrix and SiC particles as secondary phase has been developed. The most important step during composite production is the homogenization process of metal and ceramic powder particles. Quantitative analysis of a SiC particles distribution in the aluminum alloy matrix (CW67) was used to determine the optimum homogenization parameters of different powders. It was found out that the level of mixture homogeneity largely depends on the amount of mixing dish filling, homogenization time and characteristics of reinforcing particles. By introducing the concept of homogeneity index, it was shown that the lowest values of the mentioned parameter correspond to the best uniformity of SiC particles in the CW67 matrix. Composite with the lowest value of homogeneity index was the one with 5 vol.% of SiC, homogenized during 60 min and the amount of mixing dish filling of 20 vol.%. This composite displayed the best values of mechanical properties and fracture resistance.

Assessment of Mixed Uniform Boundary Conditions for Predicting the Mechanical Behavior of Elastic and Inelastic Discontinuously Reinforced Composites

The combination of heterogeneous volume elements and numerical analysis schemes such as the Finite Element method provides a powerful and well proven tool for studying the mechanical behavior of composite materials. Periodicity boundary conditions (PBC), homogeneous displacement boundary conditions (KUBC) and homogeneous traction boundary conditions (SUBC) have been widely used in such studies. Recently Pahr and Zysset (2008) proposed a special set of mixed uniform boundary conditions for evaluating the macroscopic elasticity tensor of human trabecular bone. These boundary conditions are not restricted to periodic phase geometries, but were found to give the same predictions as PBC for the effective elastic properties of periodic open cell microstructures of orthotropic symmetry. Accordingly, they have been referred to as “periodicity compatible MUBC” (PMUBC). The present study uses periodic volume elements that contain randomly positioned spherical particles or randomly oriented short fibers at moderate volume fractions for assessing the applicability of PMUBC to modeling composite materials via volume elements that deviate from orthotropic symmetry. Macroscopic elasticity tensors are evaluated with PBC, PMUBC and KUBC for elastic contrasts in the range 2 ≤ sr ≤ 30. For one configuration the isotropic contributions to the macroscopic elastic tensors obtained with PBC and PMUBC are extracted and compared. In addition, macroscopic elasticplastic responses for different hardening behaviors are studied with PBC and PMUBC. Only small differences between the predictions obtained with PBC and PMUBC are found, validating the PMUBC for studying volume elements the overall behavior of which shows minor contributions of lower than orthotropic symmetry. keyword: Mixed Uniform Boundary Conditions, Periodicity Boundary Conditions, Finite Element Simulation, Composites 1 Institute of Lightweight Design and Structural Biomechanics, Vienna University of Technology, A-1040 Vienna, Austria

Interface Effects on the Quasi-Static and Impact Toughness of Discontinuously Reinforced Aluminum Laminates

Trilayer laminates consisting of two layers of aluminum alloy 7093 surrounding one layer of 7093/SiC/15p were produced via two roll-bonding techniques as well as by adhesive bonding. The effects of systematic changes in interface characteristics (i.e., weak bond via roll bonding with a thin ductile interlayer material, stronger bond via roll bonding without a thin ductile interlayer, and strongest bond via adhesive bonding with a semibrittle material) on the subsequent laminate toughness was studied. The fracture resistance of the laminates and the constituent materials was examined via instrumented Charpy notched impact testing in the crack-arrester orientation as well as by fracture-toughness testing of bend bars tested in the crack-divider and the crack-arrester orientations. The notched impact resistance of the trilayer crack-arrester laminates was found to be greater than both monolithic 7093/SiC/15p and 7093 samples of similar global thickness. The laminated structure promoted crack arrest, deflection, and large-scale deformation of the unreinforced layers, producing R-curve behavior. The tendency for interface delamination was predicted and confirmed based on recent mechanics-based analyses. The trilayer laminate structures tested in the crack-divider orientation exhibited a greater R-curve than either of the 7093/SiC/15p or 7093 samples tested at similar global thickness. Both types of roll-bonded laminates (i.e., stronger interface and weak interface containing a thin metal interlayer) exhibited a greater enhancement in Charpy impact toughness and mode I fracture toughness than did the adhesively bonded (i.e., semibrittle interface) laminates. These relative improvements in toughness were rationalized by estimating the contributions to energy absorption by the delamination and crack bridging in these systems and by the effects of the interface type on these processes. These results are generally relevant to the performance of these materials under impact and under certain blast loading and penetration situations.

Effects of fiber aspect ratio evaluated by elastic analysis in discontinuous composites

An elastic stress analysis to investigate the effects of fiber aspect ratio in short fiber reinforced discontinuous composite materials has been done for different fiber volume fractions. In order to examine the elastic internal behavior, an evaluation of the load bearing capacity of discontinuous reinforcements is needed in advance. Accordingly, analytical derivation of composite mechanics has been carried out to predict fiber stresses and fiber/matrix interfacial shear stresses in discontinuous composites. The model is based on the theoretical development of conventional shear lag theory developed by Cox. However, the major shortcoming of the Cox model is due to the calculation without normal stress transfer from the end of fibers. In order to overcome the shortcoming, both of the normal and shear stress transfer mechanisms between the fiber and the matrix are accounted for with the stress concentration effects as well as material and geometrical properties. Results of predicted stresses concerning the various fiber aspect ratios are described by using the present model that is the closed form solution and compared with the Cox model and Taya model. It is found that the effect of fiber aspect ratio is significant to composite strengthening through load transfer from the matrix to the fiber, whereas the effect of fiber volume fraction is not so sensitive, relatively. It is also found that the present model has the capability to correctly predict the values of fiber stresses and fiber/matrix interfacial shear stresses.

Behavior of Slab-Column Connections of Existing Flat-Plate Structures

Experimental results of five large-scale isolated slab-column connections subjected to three types of loading histories are presented. The subassemblies represented typical flat-plate construction designed prior to the 1980s that had low slab reinforcement ratios and discontinuous bottom reinforcement at the column. The specimens failed in punching after extensive slab flexural yielding under different loading conditions. The damage induced by simulated seismic loading did not reduce the connection punching capacity under gravity loading. The post-earthquake connection stiffness was significantly reduced. The flexural reinforcement ratio had a significant effect on connection strength and stiffness.

Spherical dome formation by transformation of superplasticity of titanium alloys and titanium matrix composites

Titanium alloys and titanium matrix composites are useful materials in aerospace applications due to their high strength and stiffness, good corrosion resistance and low density. The gas pressure bulging of metal sheets has become an important forming method. As the bulging process progresses, significant thinning in the sheet material becomes obvious. This paper presents a simple analytical procedure for obtaining the dome height with respect to the forming time useful to the process designer for the selection of initial blank thickness as well as non-uniform thinning in the dome after forming. By thermally cycling through their transformation temperature range, coarse-grained, polymorphic materials can be deformed superplastically, owing to the emergence of transformation mismatch plasticity (or transformation superplasticity) as a deformation mechanism. This mechanism was examined under biaxial stress conditions during thermal cycling of titanium alloys with and without discontinuous reinforcements. For the transformation superplasticity, the strain-rate sensitivity index is considered as unity. The radius of curvature, thickness and height of the dome with respect to the forming time are obtained. The analytical results were found to be reasonably in good agreement with the test results.

Development of New Low Cost Discontinuously Reinforced Titanium Composites for Commercial Applications

A new cost effective process to produce discontinuously reinforced (TiB) TMCs has been developed. The article presents general features of the composites, microstructural characteristics and mechanical properties. The production and characterization of two potential commercial applications are also discussed.

Investigation of the thermoelastic response of long-fibre reinforced thermoplasticsby comparison with different non-contactstrain measurement techniques

Abstract Processing of long-fibre reinforced thermoplastics with a discontinuous reinforcement of long glass fibres, especially by direct-processing, leads to inhomogeneous and anisotropic properties. Because of this, locally resolved characterisation of the material properties is necessary. Non-contact testing methods are required to derive these material properties. Locally resolving hysteresis measurement, full-field strain measurement techniques and thermoelastic stress analysis (TSA) are applied to an incremental step test and to a fatigue test. It is shown that there is good correspondence between the local TSA-signals and the locally measured values of the major strain. This correlation is valid not only for lower linear elastic load levels, but also up to nonlinear-viscoelastic levels. A model is presented to describe the correlation between the TSA-signal and the measured major strain.

Corrosion Behavior of a Discontinuously Reinforced Composite in Salt Water Environment

In this study, the corrosion behavior of Al 6092/17.5 SiC(p) alloy is investigated. The alloy offers a good resistance to corrosion in 3.5 wt% NaCl as shown by weight loss and electrochemical studies conducted in a smoothly stirred condition up to 70 C. The alloy also offered a strong resistance to corrosion in a salt spray chamber. The corrosion behavior of the alloy, however, declined with an increase in velocity, temperature, and addition of polystyrene particles in the electrolyte. The declining corrosion resistance is attributed to the cracking and breakdown of the protective boehmite layer at elevated temperatures, higher velocities, and impact of polystyrene particles. Microanalytical studies reveal the intermetallic precipitates to be the preferred sites for initiation of pits. The alloy offers good service prospects in a marine environment at controlled flow velocities and temperatures.

Recycling Mechanism of Discontinuously Reinforced Al Matrix Composites

In the present work, the effects of reinforcement type, reinforcement surface treatment, alloying additions, remelting time and remelting number upon the chemical and property degradations of discontinuously reinforced Al matrix composites, are investigated by means of Differential Scanning calorimeter (DSC), Transmission Electron Microscopy (TEM) and thermodynamic calculations. The results show that the SiCp/Pure Al and SiCp/wrought Al alloy composites could not be recycled via remelting, nevertheless SiCp/foundry Al alloy and Al2O3p/Al composites could be recycled via remelting. The remelt-recycling mechanism of discontinuously reinforced Al matrix composites is also discussed in detail in the present work.

Thermal cycle response of a lead-free solder reinforced with adaptive shape memory alloy

Shape memory alloy (SMA) discontinuous reinforcements in a solder matrix have the potential to reduce inelastic strains and improve thermo-mechanical fatigue (TMF) life of solder joints. In this work, concept viability was tested on a simpler configuration consisting of a single NiTi wire embedded in a cylindrical Sn–3.5Ag solder matrix. The composite samples were thermally cycled at a constant shear stress under double shear loading perpendicular to the fiber axis, and the local shear displacement was monitored. The local displacement plots indicate significant reduction in creep rate of the SMA reinforced composite versus a control Cu-wire reinforced composite, suggesting that SMA particulate reinforcements may be a feasible option. In order to understand and model the composite response, tests were also conducted on the bare NiTi wire. A preliminary model based on Eshelby's transformation strains provided reasonable agreement with the displacement response of the composites.

Investigation on the influence of heat-treatment parameters on the hardness of AA6061-SiC p composite

The development of metal matrix composites with discontinuous reinforcement represents a well-established method for improving the strength and stiffness of a material. The above class of composites are age-hardenable and can be strengthened through heat treatment. Experiments were conducted based on the design of experiments (DOE) technique to determine the effect of heat-treatment parameters on the hardness of AA6061-SiC p composite. Subsequently, the results were used for the formulation of a mathematical model. The adequacy of the developed model was checked by calculating the correlation coefficient. It is observed that the second-order interaction of the heat-treatment parameters has the maximum influence on hardness.

Discontinuous fiber-reinforced composites above critical length.

Micromechanical physics of critical fiber length, describing a minimum filament distance for resin impregnation and stress transfer, has not yet been applied in dental science. As a test of the hypothesis that 9-micron-diameter, 3-mm-long quartz fibers would increase mechanical strength over particulate-filled composites, photocure-resin-pre-impregnated discontinuous reinforcement was incorporated at 35 wt% into 3M Corporation Z100, Kerr Corporation HerculiteXRV, and an experimental photocure paste with increased radiopaque particulate. Fully articulated four-point bend testing per ASTM C 1161-94 for advanced ceramics and Izod impact testing according to a modified unnotched ASTM D 256-00 specification were then performed. All photocure-fiber-reinforced composites demonstrated significant improvements over particulate-filled compounds (p < 0.001) for flexural strength, modulus, work of fracture, strain at maximum load, and Izod toughness, with one exception for the moduli of Z100 and the experimental reinforced paste. The results indicate that inclusion of pre-impregnated fibers above the critical aspect ratio yields major advancements regarding the mechanical properties tested.

Bounds of the Thermal Conductivity in Discontinuously Reinforced Metal-Matrix Composites

A simple model based on the network simulation method is proposed to estimate numerically the thermal conductivity of particulate reinforced metal-matrix composites. The estimation is carried out running the model in the standard Pspice code, the computing time being negligible. The 3-D solid is discretized in 1000 cubic volume elements which represent an acceptable approximation of the shape of the particles. For each reinforcement percentage and each combination of matrix and reinforcement more than 200 tests were carried out, so that the results may be considered close to the exact values. The limit values are scarcely influenced by the effect of the 3-D geometry and basically depend on the amount of the reinforcement. Applications to aluminum and titanium matrix composites reinforced with different types of particles are presented covering a wide range of reinforcement percentages.

Modeling and Optimization of Machining Process in Discontinuously Reinforced Aluminium Matrix Composites

This article presents development of an Artificial Neural Networks (ANN) based model for the prediction of surface roughness during machining of composite material using Back Propagation algorithm. Statistically designed experiments based on Taguchi method were carried out on machining of Al/SiCp composite material. The experimentation helped generate a knowledge base for the ANN system and understand the relative importance of process, tool and work material dependent parameters on the roughness of surface generated during machining. The ANN model trained using the experimental data was found to predict the surface roughness with fair accuracy. An optimization approach was also proposed to obtain optimal cutting conditions that yield the desired surface roughness while maximizing the metal removal rate.

Continuum Models for the Thermomechanical Behavior of Discontinuously Reinforced Materials

Continuum micromechanical models have become important tools for understanding the thermomechanical behavior of composite materials. This work presents the most important continuum-level approaches for modeling the thermomechanical behavior of discontinuously reinforced composites. Analytical and numerical models are covered, special emphasis being put on multi-inclusion unit cell methods. The fields of application of the different models are discussed and selected applications are demonstrated.

Numerical approach for estimating the elastic modulus in MMCs as a function of sintering temperature

The main problem of metal matrix composite (MMC) when submitted to high temperature is the degradation of the reinforcement. Normally, this effect is studied through degradation at the interface of the reinforcement. In this case, the high temperatures produce the formation of intermetallic compounds. Generally, particle or whisker type reinforcements produce an improvement in the mechanical properties. In either case, to do a prediction of this influence through a mathematical model is very difficult and the single utilisation of the rule-of-mixtures is unsuitable for evaluating the effect that a discontinuous reinforcement would produce in these materials, because the different properties involved are very sensitive to the changes that occur in the microstructure during the production of the composite. This paper presents a numerical model to evaluate the Young’s modulus of the MMCs as a function of the sintering temperature. This model is similar to that presented in the rule of mixtures. However, it considers the porosity of the sintered material itself and the diffusion phenomena produced during the sintering of these materials. The suggested model utilises experimental data of the sample sintered in a vacuum at 1120, 1160, 1200, 1230 and 1250 °C for periods of 30 min. The Young’s modulus was obtained from the tensile curves of stress–strain. The utilised material is constituted of powder particles with size 99.8%<150 μm for 316L and 99%<100 μm for the intermetallic γ-TiAl. This intermetallic is added to the matrix 316L in the proportions of 3, 6 and 9% in volume. Powders were dry mixed in a rotating laboratory mill at 80 rpm for 30 min. Afterwards, they were uniaxially pressed at 700 MPa.

Closely Spaced Footings on Geogrid-Reinforced Sand

The results discussed in this paper are based on a total of 74 tests performed on closely spaced strip and square footings on geogrid-reinforced sand. The study was carried out to evaluate the effect of spacing between the footings, size of reinforcement, and continuous and discontinuous reinforcement layers on bearing capacity and tilt of closely spaced footings. The interference effects on bearing capacity and settlement of closely spaced square footings on reinforced sand were almost insignificant in comparison to those on isolated footings on reinforced sand; whereas a significant improvement in the tilt of adjacent square footings has been observed by providing continuous reinforcement layers in the foundation soil under the closely spaced footings. A considerable improvement in bearing capacity, settlement, and tilt of adjacent strip footings has been observed by providing continuous reinforcement layers in the foundation soil under the closely spaced strip footings.

Particle strengthened alloys for high temperature applications: strengthening mechanisms and fundamentals of design

Particle strengthening of metallic alloys, subjected to high temperatures, is reviewed in this article. Distinction is made between nanometer-size dispersoids and discontinuous reinforcements with meso- or macroscopic dimensions. Dislocation climb and detachment are considered in conjunction with the first particle type, demonstrating that creep behaviour is dominated by the thermally activated detachment process. A number of implications resulting from that finding, such as the existence of an optimum dispersoid size, are discussed in this context. Reinforcement strengthening is reviewed, discussing load transfer strengthening, generation of geometrically necessary dislocations, relaxation of load transfer by diffusion, dislocation climb and fiber failure as key mechanisms. Finally, dual scale particle strengthening is addressed. The basic idea is to purposefully combine dispersoids and reinforcements, i.e., particles of two different size scales. The interesting point here is that a synergistic interaction between both particle types, leading to superior elevated temperature strength, is predicted by theoretical analysis.