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### Research Papers

J. Eng. Mater. Technol. 2009;132(1):011001-011001-7. doi:10.1115/1.3184027.

The role of temperature and load ratio $(R)$ on the crack propagation rate $(da/dN)$ of Alloys 276 and 617 under cyclic loading was investigated. The results indicate that the rate of cracking was gradually enhanced with increasing temperature when the $R$ value was kept constant. However, the temperature effect was more pronounced at $100–150°C$. Both alloys exhibited maximum $da/dN$ values at a load range of 4.5 kN that corresponds to an $R$ value of 0.1. The number of cycles to failure for Alloy 276 was relatively higher compared with that of Alloy 617, indicating its slower crack-growth rate. Fractographic evaluation of the broken specimen surface revealed combined fatigue and ductile failures.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011002-011002-10. doi:10.1115/1.3184029.

Motivated by a micromechanical determinist-probabilistic model coupled with damage recently developed by the authors, a new generalization is proposed to describe the nonlinear elasto-inelastic cyclic strain-stress behavior of polycrystals notably under biaxial cyclic loading paths. In this context, this generalization considers a compressible and linear anisotropic granular elastic strain behavior coupled with damage. The model is expressed in the framework of the time dependent plasticity for a small strain assumption. It is assumed that a damage variable initiates at the mesoscopic (granular) level where the plastic strain localization phenomenon takes place. The associated thermodynamic force of the damage variable is determined using the concept of total granular energy (elastic and inelastic). The transition of the elastic strain from the single to the polycrystal is modified due to its explicit coupling with damage. Comparisons between predicted and experimental results are conducted describing the low-cycle fatigue behavior of the aluminum alloy 2024 under different complex cyclic loading paths. It is demonstrated that the model has a reasonable ability in describing the cyclic behavior of this alloy. Qualitatively, the model is tested under different cyclic loading paths with stress-controlled condition describing especially the ratcheting behavior of the alloy. In fact, the effects of the applied mean stress on the predicted overall elasto-inelastic behavior and on the fatigue life are carefully studied. It shows the dependence of the fatigue life on the mean stress value.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011003-011003-7. doi:10.1115/1.3184030.

When measuring residual stresses using the hole-drilling strain-gauge method, plasticity effects arise if the residual stress level exceeds about 60% of the material yield strength. In this case the classical methods, which are based on the linear elastic material behavior, do not work properly and residual stresses are overestimated. This paper presents a numerical study of the influence of plasticity on residual stress measurement by using the hole-drilling strain-gauge method in those cases in which stress does not vary with depth. The study investigates the effects of the most important loading, measuring, geometry, and material variables. An iterative method, which can be applied to obviate these errors, is then presented. The method was implemented in ANSYS using the APDL macrolanguage (ANSYS Parametric Design Language Guide, Documentation for ansys 11.0) to automatically execute the procedure steps. A finite element model of the hole, which allows for plasticity, is requested. Employing the readings of a standard three elements strain-gauge rosette, the method makes it possible to extend the measurement limit in comparison to that of the ASTM E837 standard (ASTM E837-08, “Standard Test Method for Determining Residual Stresses by the Hole-Drilling Strain-Gauge Method”).

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011004-011004-10. doi:10.1115/1.3184031.

Detailed analysis of a residual stress profile due to laser microjoining of two dissimilar biocompatible materials, polyimide (PI) and titanium (Ti), is vital for the long-term application of bio-implants. In this work, a comprehensive three-dimensional (3D) transient model for sequentially coupled thermal/mechanical analysis of transmission laser (laser beam with wavelength of 1100 nm and diameter of 0.2 mm) microjoining of two dissimilar materials has been developed by using the finite element code ABAQUS , along with a moving Gaussian laser heat source. First the model has been used to optimize the laser parameters like laser traveling speed and power to obtain good bonding (burnout temperature of $PI>maximum$ temperature of PI achieved during $heating>melting$ temperature of PI) and a good combination has been found to be 100 mm/min and 3.14 W for a joint-length of 6.5 mm as supported by the experiment. The developed computational model has been observed to generate a bonding zone that is similar in width (0.33 mm) to the bond width of the Ti/PI joint observed experimentally by an optical microscope. The maximum temperatures measured at three locations by thermocouples have also been found to be similar to those observed computationally. After these verifications, the residual stress profile of the laser microjoint (100 mm/min and 3.14 W) has been calculated using the developed model with the system cooling down to room temperature. The residual stress profiles on the PI surface have shown low value near the centerline of the laser travel, increased to higher values at about $165 μm$ from the centerline symmetrically at both sides, and to the contrary, have shown higher values near the centerline on the Ti surface. Maximum residual stresses on both the Ti and PI surfaces are obtained at the end of laser travel, and are in the orders of the yield stresses of the respective materials. It has been explained that the patterned accumulation of residual stresses is due to the thermal expansion and contraction mismatches between the dissimilar materials at the opposite sides of the bond along with the melting and softening of PI during the joining process.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011005-011005-7. doi:10.1115/1.3184032.

Light weight high strength composites can be obtained by reinforcing resin with fillers such as hollow or solid particles and fibers. Composites were fabricated using microballoons (hollow particles) called syntactic foams. These foams can be used in various low density applications such as buoyancy aid materials for deep sea exploration and aerospace vehicles. These foams are usually utilized as light weight core materials for sandwich structures. The present study explores the procedure to fabricate functionally gradient syntactic foams (FGSFs) and further analyze their mechanical properties. The FGSFs produced are gradient structures consisting of four layers with four different types of microballoons, namely, S22, S32, S38, and K46, each having different wall thickness. The volume fraction of all microballoons is maintained constant at 60% to maintain light weight structures. Several FGSF specimens having similar density are fabricated with different layer arrangements. The different layers are integrated before major solidification takes place. Quasistatic compression testing is then performed on the cured FGSF samples using MTS-810 servohydraulic machine. Compressive strength and energy absorption values for each arrangement are compared. The stress plateau in integrated FGSF composites extends from 10% to 60% strain compared with plain syntactic foams. The integrated FGSF shows increment in yield strength and energy absorption compared with adhesively bonded FGSF. It is found that the compressive strength and energy absorption of integrated FGSF composites can be varied based on arrangement of the layers.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011006-011006-8. doi:10.1115/1.3184033.

Two types of defects normally occur in ultrasonically consolidated parts: (i) Defects that occur between mating foils in successive layers (“type 1” defects) and (ii) defects that occur within a layer between two foils laid side-by-side (“type 2” defects). While some success has been achieved in minimizing type 1 defects, type 2 defects, however, have been given very little attention. Both types of defects are undesirable and should be minimized if ultrasonically consolidated parts are to be used in structural applications. This work describes an investigation of how to minimize type 2 defects in ultrasonically consolidated parts. According to our hypothesis, a foil being deposited must overlap the adjacent deposited foil by an optimum amount to ensure a defect-free joint between the two foils. Transverse tensile specimens were fabricated with various amounts of foil overlap (by changing the foil width setting) to test this hypothesis. Metallographic and fractographic studies showed a clear correlation between foil overlap, defect incidence, and tensile strength. It was found that a foil width setting of 23.81 mm helps minimize type 2 defects in ultrasonically consolidated Al 3003 parts using standard foils of 23.88 mm (equivalent to 0.94 in.) nominal width.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011007-011007-5. doi:10.1115/1.3184035.

A novel dual-axis ESPI hole-drilling residual stress measurement method is presented. The method enables the evaluation of all the in-plane normal stress components with similar response to measurement errors, significantly lower than with single-axis measurements. A numerical method is described that takes advantage of, and compactly handles, the additional optical data that are available from the second measurement axis. Experimental tests were conducted on a calibrated specimen to demonstrate the proposed method, and the results supported theoretical expectations.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011008-011008-12. doi:10.1115/1.3184036.

Based on a well established nonincremental interaction law for fully anisotropic elastic-inelastic behavior of polycrystals, tangent formulation-based and simplified interaction laws of softened nature are derived to describe the nonlinear elastic-inelastic behavior of fcc polycrystals. Using the Eshelby’s tensor, the developed approach considers that the inclusion (grain) form is ellipsoidal. It has been clearly demonstrated by Abdul-Latif (2002, “Elastic-Inelastic Self-Consistent Model for Polycrystals,” ASME J. Appl. Mech., 69, pp. 309–316) for spherical inclusion that the tangent formulation-based model requires more calculation time, and is incapable to describe correctly the multiaxial elastic-inelastic behavior of polycrystals in comparison with the simplified model. Hence, the simplified nonincremental interaction is studied considering the grain shape effect. A parametric study is conducted showing principally the influence of the some important parameters (the grain shape $(α)$ and the new viscous parameter $γ$) and the effect of their interaction on the hardening evolution of polycrystals. Quantitatively, it is recognized that the model describes suitably the grain shape effect together with the new viscous parameter $γ$ on the strain-stress behavior of aluminum and Waspaloy under tensile test.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011009-011009-7. doi:10.1115/1.3184037.

In recent years functionally-graded composites have been proposed to develop strong surfaces that can withstand high contact and frictional forces. The present work presents a new graded composite that can be used for the development of surfaces with excellent strength properties. The composite is inspired by the human teeth, which nature builds as a hard and tough functionally-graded composite. The outer surface of teeth is of enamel, composed of prismatic hydroxyapatite crystallites, whereas the inner part of teeth is of dentine, composed collagen fibrils and hydroxyapatite. Enamel is hard, brittle, and wear resistant, while dentine is softer and flexible. The dentine-enamel junction is formed as a region at which enamel mixes with dentine in a continuous way. The nanomechanical properties of the transition zone have been recently revealed. Of particular interest in this investigation is the variation in the elastic modulus from the pure enamel to the pure dentine material, which leads to biomimetic graded composites that exhibit high surface strength. This work presents analytical solutions for the stress and displacement fields on an actual composite substrate, which is loaded by a line load. The elastic modulus of the substrate follows approximately the theoretical distribution.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011010-011010-9. doi:10.1115/1.3184038.

Functionally graded materials (FGMs) are composite materials that exhibit a microstructure that varies locally in order to achieve a specific type of local material properties distribution. In recent years, FGMs appear to be more interesting in engineering application since they present an enhanced performance against deformation, fracture, and fatigue. The purpose of the present work is to present evidence of the excellent strength properties of a new graded composite that is inspired by the human teeth. The outer surface of the teeth exhibits high surface strength while it is brittle and wear resistant, whereas the inner part is softer and flexible. The specific variation in Young’s modulus along the thickness of the presented composite is of particular interest in our case. The present work presents a finite element analysis and an experimental verification of an actual composite with elastic modulus that follows approximately the theoretical distribution observed in the teeth.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011011-011011-7. doi:10.1115/1.3184082.

The cold axisymmetric drawing of metals leads to effective strains that increase from the centerline to the surface of the material cross section. This strain heterogeneity depends on the die semi-angle and reduction in area related through a “$Δ$” parameter. The average strain in the product is evaluated through a redundant deformation coefficient, “$ϕ$,” which has a minimum value of unity and rises as $Δ$ is increased. Anomalous experimental results for this relationship ($ϕ$ values below unity and insensitive to variations in $Δ$) have been reported for the AISI 420 stainless steel. Strain path affects the work hardening of metals during sheet forming, where some materials harden more and others less than under pure tension, for the same strain path. The present paper analyses the possibility that a similar dependence of the work hardening on the strain path, during the axisymmetric drawing of AISI 420 stainless steel causes the anomalous $ϕ$ versus $Δ$ relationship. The strain path followed along various material streamlines in axisymmetric drawing involves the superposition of a radially varying reversed shear strain on a basic radial compression/longitudinal tension pattern. A new method was developed for the determination of the effective stress versus effective strain curves of the material along three material streamlines, located close to the material surface, along its centerline and following a midcourse between these two flow lines. A relationship between the local microhardness of the material and its flow stress was established and visioplasticity was employed for the determination of local strains in the deformation region. Data were obtained for six situations resulting from the combinations of two reductions of area (8% and 20%) and three die semi-angles (3 deg, 8 deg, and 15 deg). The various strain paths followed in axisymmetric drawing of AISI 420 stainless steel led to effective stress versus effective strain curves tending to be often lower than that obtained in pure tension. The degree of lowering seems to depend on the reduction in area and die semi-angle. The effect of strain path on the work hardening during axisymmetric drawing causes the anomalous experimental results for the $ϕ$ versus $Δ$ relationship of the AISI 420 stainless steel. The present paper seems to be the first report in literature covering such effects under cold bulk forming conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011012-011012-9. doi:10.1115/1.3184083.

The mechanism of the residual stress relaxation during the fatigue life of shot peened high-strength aluminum alloys was investigated. Experiments were conducted on specimens subjected to three different shot peening treatments and tested under reverse bending fatigue. x-ray diffraction (XRD) measurements were carried out to determine the initial and stabilized residual stress fields. The residual stress field created by the surface treatments has been introduced into a finite element (FE) model by means of a fictitious temperature distribution. The elastic-plastic response of the superficial layers affected by the shot peening treatments has been derived through reverse strain axial testing combined with microhardness tests and implemented in the FE model. The proposed numerical/experimental approach is able to satisfactorily predict the residual stress field evolution. Notably, relaxation has been correctly simulated in the low-cycle fatigue regime and imputed to plastic flow in compression when the superposition of compressive residual and bending stresses exceeds the local cyclic yield strength of the material. Conversely, the residual stress field remains stable at load levels corresponding to the $5×106 cycles$ fatigue endurance.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011013-011013-8. doi:10.1115/1.3184084.

This paper treats approximate solutions for a self-folding problem of carbon nanotubes. It has been observed in the molecular dynamics calculations (Buehler, Kong, Gao, and Huang, 2006, “Self-Folding and Unfolding of Carbon Nanotubes  ,” ASME J. Eng. Mater. Technol., 128, pp. 3–10) that a carbon nanotube with a large aspect ratio can self-fold due to the van der Waals force between the parts of the same carbon nanotube. The main issue in the self-folding problem is to determine the minimum threshold length of the carbon nanotube at which it becomes possible for the carbon nanotube to self-fold due to the van der Waals force. To the best of the author’s knowledge, no exact solution for this problem has been obtained. In this paper, an approximate mathematical model based on the force method is constructed for the self-folding problem of carbon nanotubes, and is solved exactly as an elastica problem using elliptic functions. Additionally, three other mathematical models are constructed based on the energy method. As a particular example, the lower and upper estimates for the critical threshold (minimum) length are determined based on both methods for the (5,5) armchair carbon nanotube.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011014-011014-6. doi:10.1115/1.4000217.

Many rubber products are reinforced with glass fibers to give dimensional stability, high modulus, and good fatigue life. To understand failure of these products, it is essential to understand the failure and strength degradation mechanisms of the reinforcing glass multifilament bundles. An empirical model has been developed to predict fast fracture and time-dependent failure of these bundles using the global load sharing approximation. The model is based on the statistical strength distribution of glass fibers, fracture mechanics of glass, and nonlinear stress distribution between individual fibers owing to sliding resistance of matrix. The model was also used to predict the residual strength of the bundle as a function of load and time.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011015-011015-7. doi:10.1115/1.4000218.

The process that combines selective laser sintering (SLS) and cold isostatic pressing (CIP) can manufacture nearly fully dense metal components with complex geometries. However, the SLS component will shrink to a significant extent during the CIP process. The modified cam-clay model was used to describe the constitutive equation of porous stainless-steel SLS components. The densification process of SLS porous components was simulated using the finite element method. The visual distributions of shape change, displacement, density, and stress were obtained, which showed that SLS components shrank with the same shape as the original design over the CIP process. In addition, a relative uniform density distribution appeared in cold isostatic pressed components. The experiment measurements displayed that the shrinkage rate of SLS components during the CIP process in height direction was a little higher than that on the forming plane due to the layer-by-layer manufacturing process of SLS. On the forming plane, the dimensional errors between simulations and experiments for the cuboid and the gear components are lower than 1% and 3%, respectively. However, the errors in height direction of the two components increase to the range of 6% and 9%.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011016-011016-7. doi:10.1115/1.4000220.

It has long been suspected that physical chain entanglements in the amorphous phase affect the strain hardening behavior of polyethylene. The precise number of chain entanglements in solid polyethylene cannot be measured using any current techniques. Since entanglements in the melt state are known to be preserved in the polymer upon solidification, determination of the molecular weight between entanglements $(Me)$ is used as a measure of chain entanglements for polyethylene. A decrease in molecular weight between entanglements means an increase in the number of entanglements in the polymer. As the $Me$ value decreases, increasing tensile strain hardening of polyethylene is observed. In addition to experimental work, parallel micromechanical modeling was carried out to study the entanglement effect in uniaxial tensile deformation. The model was able to shed more light over the earlier empirical speculations. By combining experimental observations and modeling results, the presence of physical chain entanglements in the amorphous phase was demonstrated to be the controlling factor in strain hardening behavior of polyethylene.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011017-011017-9. doi:10.1115/1.2931155.

The paper is concerned with occurrence of processing defects and resulting mechanical properties associated with material processing by metal injection molding (MIM). MIM process is a multistep one that consists first in the injection of metallic powders mixed with a thermoplastic binder, followed by a debinding stage that permits to evacuate the polymeric binder, and then followed by a sintering stage by solid state diffusion that normally leads to a nearly dense component. The main defects arising during MIM processing are associated with powder segregation during injection molding, and uncompleted or heterogeneous mechanical properties resulting from solid state diffusion. The paper first describes a biphasic fluid flow approach that can accurately predict powder volume fraction after injection molding and consequently the associated segregation defects. This analysis is followed and continued by a proper sintering model based on an elastic-viscous analogy that predicts the resulting local densities after sintering and also associated defects. So, from the two subsequent models, it becomes possible to get the final powder densities after processing and to localize the possible resulting defects. This analysis is completed by an analysis using a porous material model to get the final resultant mechanical properties after processing.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;132(1):011018-011018-5. doi:10.1115/1.4000302.

In this paper, two subspaces of the state space of constrained equilibrium states for solids are proposed and addressed. One subspace, constrained affinity space, is conjugate-force space with fixed temperature and internal variable. It is revealed in this paper that the remarkable properties of the kinetic rate laws of scalar internal variables, established by Rice (1971, “Inelastic Constitutive Relations for Solids: An Internal Variable Theory and Its Application to Metal Plasticity,” J. Mech. Phys. Solids, 19, pp. 433–455) and elaborated by Yang (2005, “Normality Structures With Homogeneous Kinetic Rate Laws,” ASME J. Appl. Mech., 72, pp. 322–329; 2007, “Normality Structures With Thermodynamic Equilibrium Points,” ASME J. Appl. Mech., 74, pp. 965–971), are all located in constrained affinity space. Furthermore, the flow potential function monotonically increases along any ray from the origin in constrained affinity space. Another subspace, constrained configuration space, is the state space with fixed external variables. It is shown that the specific free and complementary energies monotonically decrease and increase, respectively, along the path of motion of the thermodynamic system of the material sample in constrained configuration space. For conservative conjugate forces, Hamilton’s action principle is established in constrained configuration space, and the action is the entropy production of the thermodynamic system in a time interval. The thermodynamic processes in constrained configuration space are just creep or relaxation processes of materials. The Hamilton principle can be considered as a fundamental principle of rheology.

Commentary by Dr. Valentin Fuster