J. Eng. Mater. Technol. 2007;129(3):349-355. doi:10.1115/1.2744392.

The material behaviors of two types of bearing steels at hot working conditions are investigated. Stress-strain curves at various temperatures (900–1300°C) and strain rates (1–50/s) are obtained by compression tests with a computer controlled servo-hydraulic Gleeble 3800 testing machine. Elongation and reduction of the area are also obtained by tensile tests with the Gleeble 1500 testing machine. Flow stresses are calculated from the experiments and are used to predict the temperature distribution and the metal flow of a workpiece during a multistage hot forging process of a bearing race. A rigid-thermoviscoplastic finite element method is applied. The experimental and numerical results are summarized to reveal the reasons for internal crack formation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2006;129(3):356-366. doi:10.1115/1.2744393.

Much of the fatigue damage in aircraft structures can be linked to the stress concentration arising at the rivet/skin interface in fuselage lap-joints. Fatigue damage can degrade the strength of the structure and reduce structural integrity. The stress distribution around the rivet holes, which depends on several loading conditions, is therefore of prime importance. Critical manufacturing process variations must be taken into account to observe the effect on local stresses at the hole. This paper presents three-dimensional (3D) nonlinear finite element analyses to investigate the stress state at rivet holes in fuselage lap joints. Initially, a 3D single rivet model of the riveting process was developed to characterize the unsymmetric residual stress distribution resulting from rivet installation. Then a global three-rivet model of the fuselage lap-joint, which takes into account the residual stresses from rivet installation and fuselage pressurization, was analyzed and compared to observations available from teardown inspection. The models were then implemented to observe the effects of rivet interference, sealant, and drill shavings on the stress state. A multiaxial fatigue criterion was implemented to predict cycles to crack nucleation for the modeled parameters. The effect of underdriven rivets and sealant were observed to be the most critical on the stress state of the fuselage splice. Excellent comparison with the damage characterization of the fuselage lap-joint provides validation to the finite element model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2006;129(3):367-379. doi:10.1115/1.2744395.

A methodology for incorporating a description of material structure into a finite element formulation is presented. This work describes an experiment/simulation - based methodology for characterizing attributes of material structure, and then incorporating those attributes into a modeling framework. The modeling framework was used to study the development of deformation induced surface roughening in thin sheets machined from AA 7050 thick plate. Predicting this roughening phenomenon necessitates the quantification and representation of material structure and processes that exist over several size scales. Electron backscatter diffraction experiments were used for material structure characterization, which included crystallographic texture, distributions in grain sizes, and a distribution in intragrain misorientation. These distributions in structure were incorporated in digital microstructures which represented virtual specimens composed of finite element-discretized crystals. A continuum slip-polycrystal plasticity model was coupled with the digital microstructures to study the differences in roughening seen in specimens deformed along the rolling direction and transverse direction of the plate material. The success of these simulations build additional insight into how to incorporate material structure into deformation simulations, and build representative virtual specimens that can be used to study the complicated processes that underlie deformation mechanics in polycrystalline materials.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(3):380-389. doi:10.1115/1.2744396.

An established dislocation density related, one-internal variable model was used, with some modifications, as a basis for modeling the mechanical response of aluminum alloy AA6111. In addition to conventional rolling, equal channel angular pressing (ECAP) was used to produce a wide range of grain sizes, down to the submicrometer scale. The samples were heat treated before and after both processes to optimize tensile ductility. Implementation of the model to uniaxial tensile response of the conventionally rolled and the ECAP processed materials confirmed its good predictive capability. The model was further used to formulate simple relations between true uniform strain and the constitutive parameters that allow reliable prediction of the uniform elongation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(3):390-396. doi:10.1115/1.2744397.

Titanium/graphite hybrid composites (TiGr) are a potentially enabling technology which satisfies the low structural weight fraction and long operational lifetime required for the high-speed civil transport. TiGr composites are made of thermoplastic polymer matrix composite plies with titanium foils as the outer plies. The two materials are assembled by bonding the polymer matrix composite plies and titanium foils to form a hybrid composite laminate. Both experimental and analytical work has been performed to characterize major hole quality parameters and cutting mechanisms encountered in drilling of TiGr composites. The effects of consolidation processing, such as induction heating press and autoclave processes, on drilling characteristics of TiGr composites were examined. The hole quality parameters and hole exit damage was investigated and discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(3):397-406. doi:10.1115/1.2744399.

In this paper, an optimum design is carried out with finite element analysis to determine process parameters which reduce the amount of springback and improve shape accuracy of a deep drawn product with the channel shape. Without springback simulation usually performed with an implicit solving scheme, the study uses the amount of stress deviation through the sheet thickness direction in the deep drawn product as an indicator of springback. The simulation incorporates the explicit elasto-plastic finite element method for calculation of the final shape and the stress deviation of the final product. The optimization method adopts the response surface methodology in order to seek the optimum condition of process parameters such as the blank holding force and the draw-bead force. The present optimization scheme is applied to the design of the variable blank holding force in the U-draw bending process and the application is further extended to the design of draw-bead force in a front side member formed with advanced high-strength steel (AHSS) sheets made of DP600. Results demonstrate that the optimum design of process parameters decreases the stress deviation throughout the thickness of the sheet and reduces the amount of springback of the channel shaped part. The present analysis provides a guideline in the tool design stage for controlling the evolution of springback based on the finite element simulation of complicated parts.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2006;129(3):407-413. doi:10.1115/1.2744400.

Despite the high demand for industrial applications of magnesium, the forming technology for wrought magnesium alloys is not fully developed due to the limited ductility and high sensitivity to the processing parameters. The processing window for magnesium alloys could be significantly widened if the lower-bound ductility (LBD) for a range of stresses, temperature, and strain rates was known. LBD is the critical strain at the moment of fracture as a function of stress state and temperature. Measurements of LBD are normally performed by testing in a hyperbaric chamber, which is highly specialized, complex, and rare equipment. In this paper an alternative approach to determine LBD is demonstrated using wrought magnesium alloy AZ31 as an example. A series of compression tests of bulge specimens combined with finite element simulation of the tests were performed. The LBD diagram was then deduced by backward calculation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2006;129(3):414-421. doi:10.1115/1.2744406.

This paper aims to evaluate the stress-strain characteristics of tubular materials considering their anisotropic effects by hydraulic bulge tests and a proposed analytical model. In this analytical model, Hill’s orthogonal anisotropic theory was adopted for deriving the effective stresses and effective strains under a biaxial stress state. Annealed AA6011 aluminum tubes and SUS409 stainless-steel tubes were used for the bulge test. The tube thickness at the pole, bulge height, and the internal forming pressure were measured simultaneously during the bulge test. The effective stress-effective strain relations could be determined by those measured values and this proposed analytical model. The flow stress curves of the tubular materials obtained by this approach were compared with those obtained by the tensile test with consideration of the anisotropic effect. The finite element method was also adopted to conduct the simulations of hydraulic bulge forming with the flow stress curves obtained by the bulge tests and tensile tests. The analytical forming pressures versus bulge heights were compared with the experimental results to validate the approach proposed in this paper.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2006;129(3):422-430. doi:10.1115/1.2744407.

The potential of cast magnesium alloys for being used as structural materials in lightweight applications is assessed. The ability of the alloys for mechanical performance is evaluated and compared against the ability of widely used structural aircraft cast aluminum alloys. The specific quality index QDS, devised for evaluating both cast and wrought aluminum alloys, will be exploited to evaluate the ability of a number of cast magnesium alloys for mechanical performance. The exploited quality index QDS involves the material’s yield strength Rp to account for strength, the strain energy density W to account for both tensile ductility and toughness, and the material’s density ρ. The effects of differences in chemical composition and heat treatment conditions on the mechanical performance of cast magnesium alloys have been assessed. The use of the quality index QDS has been proved to appreciably facilitate the evaluation of the mechanical performance of cast magnesium alloys and also the comparison between alloys of different base materials. The results quantify the gap to be closed such as to involve cast magnesium alloys in aircraft structural applications.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2006;129(3):431-439. doi:10.1115/1.2744408.

An equivalent orthotropic representation (EOR) of the nonlinear elastic behavior of multiwalled carbon nanotubes (MWCNTs) was developed based on a nested shell structural representation of MWCNTs. The EOR model was used together with the finite element method to simulate the large deformation of MWCNTs under bending, axial compression and radial compression. Results were compared with those of the nested shell model for four-, eight-, nine-, 14-, and 19-walled carbon nanotubes. The EOR model provides a dramatic improvement in computational efficiency and successfully quantitatively replicates the overall deformation behavior including the initial linear elastic behavior, the onset of local buckling, and the post-buckling compliance. The proposed EOR model together with the finite element method offers a computationally efficient method for simulating large and complex systems of MWCNTs.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(3):440-445. doi:10.1115/1.2744416.

An effective procedure is presented that allows stable hole-drilling residual stress calculations using strain data from measurements taken at many small increments of hole depth. This use of many strain measurements is desirable because it improves the data content of the calculation, and the statistical reliability of the residual stress results. The use of Tikhonov regularization to reduce the noise sensitivity that is characteristic of a fine-increment calculation is described. This mathematical procedure is combined with the Morozov criterion to identify the optimal amount of regularization that balances the competing tendencies of noise reduction and stress solution distortion. A simple method is described to estimate the standard error in the strain measurements so that the optimal regularization can be chosen automatically. The possible use of a priori information about the trend of the expected solution is also discussed as a further means of improving the stress solution. The application of the described method is demonstrated with some experimental measurements, and realistic results are obtained.

Topics: Stress , Errors , Drilling
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(3):446-452. doi:10.1115/1.2744417.

Understanding effects of welding on strength and formability is critical to support wider application of advanced high strength steels in automotive components. In this study, High Strength Low Alloy (HSLA) and DP980 (Dual Phase, 980MPa) sheet steels were welded with a 4kW diode laser. Mechanical properties of welds and parent metals were assessed by tensile and limiting dome height tests, and related to microhardness distribution across the welds. The formability of HSLA welds was insensitive to the welding process and comparable to that of parent metal. For the DP steel, weld formability was much lower than that of corresponding parent metal, which appeared to be due to the formation of soft zones in the outer region of the Heat affected zone (HAZ) of the welds. It was found that increase of welding speed resulted in a slight increase of formability of the DP steel, associated with a reduction in the microhardness difference between base metal and HAZ soft zones.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2006;129(3):453-461. doi:10.1115/1.2400282.

A material composed of a mixture of distinct homogeneous media can be considered as a homogeneous one at a sufficiently large observation scale. In this work, the problem of the elastic mixture characterization is solved in the case of linear random mixtures, that is, materials for which the various components are isotropic, linear, and mixed together as an ensemble of particles having completely random shapes and positions. The proposed solution of this problem has been obtained in terms of the elastic properties of each constituent and of the stoichiometric coefficients. In other words, we have explicitly given the features of the micro-macro transition for a random mixture of elastic material. This result, in a large number of limiting cases, reduces to various analytical expressions that appear in earlier literature. Moreover, some comparisons with the similar problem concerning the electric characterization of random mixtures have been drawn. The specific analysis of porous random materials has been performed and largely discussed. Such an analysis leads to the evaluation of the percolation threshold, to the determination of the convergence properties of Poisson’s ratio, and to good agreements with experimental data.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(3):462-467. doi:10.1115/1.2744418.

Nano-Y2O3 particulates containing ductile magnesium nanocomposites were synthesized using blend-press-sinter powder metallurgy technique followed by hot extrusion. Microstructural characterization of the nanocomposite samples showed fairly uniform reinforcement distribution, good reinforcement-matrix interfacial integrity, significant grain refinement of magnesium matrix with increasing presence of reinforcement, and the presence of minimal porosity. Mechanical properties characterization revealed that the presence of nano-Y2O3 reinforcement leads to marginal increases in hardness, 0.2% yield strength and ultimate tensile strength, but a significant increase in ductility and work of fracture of magnesium. The fracture mode was changed from brittle for pure Mg to mix ductile and intergranular in the case of nanocomposites.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(3):468-482. doi:10.1115/1.2744419.

The response of metal matrix composites is affected by factors such as inclusion distribution and shape, inclusion/matrix interfacial bond, residual stresses, and fabrication-altered in situ matrix properties. These effects are studied using a finite-volume micromechanics model whose extensive modeling capabilities are sufficient to account for these diverse factors. A consistent micromechanics-aided methodology is developed for extracting the unknown in situ matrix plastic parameters using a minimum amount of experimental data. Subsequent correlation of the micromechanics-based predictions with carefully generated data on off-axis response of unidirectional boron/aluminum composite specimens under tensile and compressive axial loading validates the model’s predictive capability and quantifies the importance of each factor.

Commentary by Dr. Valentin Fuster


J. Eng. Mater. Technol. 2007;129(3):483-487. doi:10.1115/1.2744435.

To investigate the constitutive relation of a plant tissue regarded as a deformable continuum, stress and strain must be determined experimentally for the same configurations. Such experiments are hindered by the inherent theoretical complexity of continuum mechanics, and by the technical difficulties of effecting external stress loads or body forces on the tissue without invasion, especially on a small scale. An understanding of appropriate mechanical problems and their solutions can help the experimentalist overcome these difficulties to a certain extent. Based on recent work on fiber-reinforced material, we formulate a constitutive theory for the root of different angiosperm species and suggest a set of loading conditions to determine the parameter values in a specific tissue sample. The loading conditions are formulated with a view toward experimental realization in vivo or with minimal invasion. For each loading condition, we formulate the corresponding mechanical problem and show how to obtain the values of the elastic parameters from known solutions. This framework can be used to analyze the interplay between mechanical and metabolic behavior in plants and to study the elastodynamics of plant tissues.

Commentary by Dr. Valentin Fuster


J. Eng. Mater. Technol. 2007;129(3):488-495. doi:10.1115/1.2744436.

Cyclic stress-strain measurements have to be performed in order to determine the cyclic plasticity parameters of material models describing the Bauschinger effect. For thin wires, the performance of tensile tests is often not possible due to necking of the specimen on exceeding the yield stress, whereas compression tests are uncritical. This paper presents an approach to determine the cyclic plasticity parameters by performance of compression tests for wires before and after drawing. Here, a simple analogous model is used instead of finite-element (FE) simulations. This approach has been applied for two different integration time steps in order to evaluate their influence on the fit and the accuracy of the integration. It is shown that good accuracy can be obtained for the cyclic plasticity parameters. For FE simulations using larger integration time steps, large deviations have been noted. However, there the analogous model could also be adopted in order to find appropriate model parameters. In general, it is the intention of this paper to show that searching an analogous model can be a very time- and cost-saving task.

Commentary by Dr. Valentin Fuster

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