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### Research Papers

J. Eng. Mater. Technol. 2010;132(2):021001-021001-7. doi:10.1115/1.4000230.

Nanoindentation is widely used to characterize the mechanical and interfacial properties of thin film systems. However, the effects of substrate compliance on the indentation response of compliant substrate systems are not well understood. This paper investigates the effects of the large compliance mismatch between the film and the substrate and of the film thickness for model systems using nanoindentation tests, finite element simulations, and an analytical model based on a classical plate-bending solution. The results showed that for displacements less than the film thickness and for ratio of the substrate to film modulus less than 100. The indentation force-displacement response exhibits a linear relationship that can be predicted accurately by the linear plate-bending model. The effective stiffness depends linearly on the film thickness and also on the substrate and film moduli. For larger displacements, the indentation response exhibits the scaling relationship of the nonlinear plate-bending model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021002-021002-8. doi:10.1115/1.4000667.

This paper is concerned with a novel elasto-plastic reformulation of the Theory of Critical Distances (TCD) specifically devised to estimate lifetime of notched metallic materials (ferrous and nonferrous) failing in the low/medium-cycle fatigue regime. We used the classic Manson–Coffin and Smith–Topper–Watson approaches, but applied in conjunction with the TCD. We assumed that the material’s critical distance is a constant whose value does not depend on either the sharpness of the notch or on the number of cycles to failure. The accuracy and reliability of the proposed approach was checked by using a number of experimental results generated by testing cylindrical specimens made of En3B, which is a commercial low-carbon steel, and Al6082, which is a conventional aluminum alloy, containing different geometrical features and tested at applied load ratios of $R=−1$ and $R=0$. The resulting predictions of fatigue life were highly accurate, giving estimates falling within an error factor (in lifetime) of about 2. This result is undoubtedly encouraging, especially in light of the fact that the pieces of experimental information needed to calibrate our method can easily be generated by using standard testing equipment, and the necessary stress/strain fields acting on the fatigue process zone can be determined by directly postprocessing elasto-plastic finite element results.

Topics: Fatigue , Stress , Cycles , Failure , Testing
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021003-021003-8. doi:10.1115/1.4000668.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021004-021004-6. doi:10.1115/1.4000669.

In this paper, some interesting, experimentally determined actualities referring to the 50CrMo4 steel are presented. That way, the mechanical properties of the material are derived from uniaxial tensile tests at lowered and elevated temperatures. Engineering stress versus strain diagrams for both mentioned temperatures, curves representing the effect of temperature on specimen elongation, and short-time creep curves are given. Notch impact energy test was also carried out. Taking into consideration the service life of the final product of the mentioned steel widely used in engine and machine technology, all of the mentioned data may be relevant during design and manufacturing procedure.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021005-021005-8. doi:10.1115/1.4000670.

By far, carbon and glass fibers are the most popular fiber reinforcements for composites. Traditional carbon composites are relatively expensive since the manufacturing process requires significant heat and pressure, while the carbon fibers themselves are inherently expensive to produce. In addition, they are often flammable and their use is restricted when fire is a critical design parameter. Glass fabrics are approximately one order of magnitude less expensive than similar carbon fabrics. However, they lack the stiffness and the durability needed for many high performance applications. By combining these two types of fibers, hybrid composites can be fabricated that are strong, yet relatively inexpensive to produce. The primary objective of this study was to experimentally investigate the effects of bonding high strength carbon fibers to E-glass composite cores using a high temperature, inorganic matrix known as geopolymer. Carbon fibers were bonded to E-glass cores (i) on only the tension face, (ii) on both the tension and compression faces, or (iii) dispersed throughout the core in alternating layers to obtain a strong, yet economical, hybrid composite laminate. For each response measured (flexural capacity, stiffness, and ductility), at least one hybrid configuration displayed mechanical properties comparable to all carbon composite laminates. The results indicate that hybrid composite plates manufactured using 3k unidirectional carbon tape exhibit increases in flexural capacity of approximately 700% over those manufactured using E-glass fibers alone. In general, as the relative amount of carbon fibers increased, the likelihood of precipitating a compression failure also increased. For 92% of the specimens tested, the threshold for obtaining a compression failure was utilizing 30% carbon fibers. The results presented herein can dictate future studies to optimize hybrid performance and to achieve economical configurations for a given set of design requirements.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021006-021006-10. doi:10.1115/1.4000222.

There is considerable worldwide interest in magnesium (Mg) sheet as a replacement for heavier steel and aluminum alloys in vehicle closure components. As Mg gains acceptance in the automotive industry, there will be an increasing demand for accurate material properties for finite element simulations of Mg structures. In this paper, we investigate the extent to which average grain size and postformed tensile properties vary across a Mg AZ31B hood inner component formed at $485°C$ for 20 min under a constant gas pressure. Tensile specimens were extracted from six regions of the hood inner, which underwent varying degrees of thinning. A state-of-the-art digital image correlation (DIC) algorithm and custom image acquisition software provided true stress-true strain data for each specimen. Tensile data acquired during room temperature testing was compared with that from baseline (undeformed) Mg AZ31B in a fully recrystallized condition (O-temper). Due to its importance in finite element simulations, particular emphasis was placed on the variation of postformed yield strength with specimen thickness and average grain size. Finally, we compute local strain fields during fracture in a tensile specimen with DIC grids positioned in the failure region.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021007-021007-11. doi:10.1115/1.4000223.

The effects of static Mode I (opening mode) loading on orthotropic-orthotropic bimaterial interface cracks have been investigated using the experimental technique of transmission photoelasticity. For successful implementation of this experimental technique transparent and birefringent glass fiber reinforced polyester composite materials were developed, enabling the direct observation and recording of photoelastic fringes in the vicinity of the interface crack in the orthotropic bimaterial. It is our belief that this is the first of such experimental investigation. Opening and shearing mode stress-intensity factors and the strain energy release rates have been calculated for various combinations of the bimaterial halves. Results have been verified by mathematically regenerating the observed isochromatic fringe patterns. This study has made it feasible to investigate interfacial fracture in orthotropic-orthotropic bimaterials and orthotropic-isotropic bimaterials using photoelasticity.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021008-021008-13. doi:10.1115/1.4000224.

This paper discusses cyclic deformation and fatigue behaviors of stainless steel 304L and aluminum 7075-T6. Effects of loading sequence, mean strain or stress, and prestraining were investigated. The behavior of aluminum is shown not to be affected by preloading, whereas the behavior of stainless steel is greatly influenced by prior loading. Mean stress relaxation in strain control and ratcheting in load control and their influence on fatigue life are discussed. Some unusual mean strain test results are presented for SS304L, where in spite of mean stress relaxation fatigue lives were significantly longer than fully-reversed tests. Prestraining indicated no effect on either deformation or fatigue behavior of aluminum, while it induced considerable hardening in SS304L and led to different results on fatigue life, depending on the test control mode. Possible mechanisms for secondary hardening observed in some tests, characterized by a continuous increase in the stress response and leading to runout fatigue life, are also discussed. The Smith–Watson–Topper parameter was shown to correlate most of the experimental data for both materials under different loading conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021009-021009-6. doi:10.1115/1.4000225.

A simple phenomenological constitutive model has been proposed to describe dynamic deformation behavior of various metals in wide strain rate, strain, and temperature regimes. The formulation of the model is, $σ=[A+B{1−exp(−Cε)}][D ln(ε̇/ε̇0)+exp(E⋅ε̇/ε̇0)][1−(T−Tref)/(Tm−Tref)]m$, where $σ$ is the flow stress, $ε$ is the strain, $ε̇$ is the strain rate, $ε̇0$ is the reference strain rate, $T$ is the temperature, $Tref$ is the reference temperature, $Tm$ is the melting temperature, and $A$, $B$, $C$, $D$, $E$, and $m$ are the material parameters. The proposed model successfully describes not only the linear rise of flow stress with logarithmic strain rate for many metals, but also the upturn of the flow stress at strain rate over about $104 s−1$ for the case of copper. It can also describe the exponential increase in the flow stress with logarithmic strain rate for the case of tantalum, and is capable of predicting thermal softening of various metals at high as well as low temperature. The current model can be used for the practical simulation of many high-strain-rate events with improved precision and as a more rigorous comparison standard in the development of a physical model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021010-021010-8. doi:10.1115/1.4000227.

Understanding and quantifying the effects of overloads/overstrains on the cyclic damage accumulation at a microscale discontinuity is essential for the development of a multistage fatigue model under variable amplitude loading. Micromechanical simulations are conducted on a 7075-T651 Al alloy to quantify the cyclic microplasticity in the matrix adjacent to intact or cracked, life-limiting intermetallic particles. An initial overstrain followed by constant amplitude cyclic straining is simulated considering minimum to maximum strain ratios of 0 and $−1$. The nonlocal equivalent plastic strain at the cracked intermetallic particles reveals overload effects manifested in two forms: (1) the cyclic plastic shear strain range is greater in the cycles following an initial tensile overstrain than without the overstrain and (2) the initial overstrain causes the nonlocal cumulative equivalent plastic strain to double in subsequent tensile-going half cycles and triple in subsequent compressive-going half cycles, as compared with cases without an initial tensile overstrain. The cyclic plastic zone at the microdiscontinuity corresponds to that of the maximum strain during the initial overstrain and the nonlocal cyclic plastic shear strain range in the matrix near the intact or cracked inclusion is substantially increased for the same remote strain amplitude relative to the case without initial overstrain. These results differ completely from the effects of initial tensile overload on the response at a macroscopic notch root or at the tip of a long fatigue crack in which the driving forces for crack formation or growth, respectively, are reduced. The micromechanical simulation results support the incorporation of enhanced cyclic microplasticity and driving force to form fatigue cracks at cracked inclusions following an initial tensile overstrain in a fatigue incubation model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021011-021011-11. doi:10.1115/1.4000229.

A computational scheme for estimating the effective elastic properties of a particle reinforced matrix is investigated. The randomly distributed same-sized spherical particles are assumed to result in a composite material that is macroscopically isotropic. The scheme results in a computational efficient method to establish the correct bulk and shear moduli by representing the three-dimensional (3D) structure in a two-dimensional configuration. To this end, the statistically equivalent area fraction is defined in this work, which depends on two parameters: the particle volume fraction and the number of particles in the 3D volume element. We suggest that using the statistically equivalent area fraction, introduced and defined in this work, is an efficient way to obtain the effective elastic properties of an isotropic media containing randomly dispersed same-size spherical particles.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021012-021012-5. doi:10.1115/1.4000231.

The properties of nanocomposite materials depend on the dispersion of the nanoparticles/nanofibers within the matrix. The addition of surfactants and varied processing techniques are used to increase the dispersion of the nanoparticles in the final composite. A method for the quantitative prediction of the interactions between nanoparticles in solution would aid in the design of processing schedules. In this study, molecular dynamics simulations are used to compute for the potential of mean force as a function of the distance and orientation between a pair of single-walled carbon nanotubes (CNTs) in water. An adaptive biasing force method is used to speed up the calculations. Simulation results show that CNT orientation and the addition of surfactant can significantly affect CNT interactions and inturn dispersion.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021013-021013-7. doi:10.1115/1.4000822.

The increasing demand of reliable creep design for very long lives (exceeding 100.000 h), as those for high stress-low temperatures and high temperature-low stress regimes, requires a model formulation capable to account for the nonlinearity in the stress dependence of the logarithm of the creep rate as a result of the combination of both diffusional and dislocation type creeps. In this paper, a creep model, where the effect of mechanism change has been accounted for through an explicit dependence of the creep exponent $n$ on stress, has been proposed. The model has been also extended, incorporating damage processes and characteristics of tertiary creep stage, adopting a time independent damage formulation proposed by the authors. An application example of the proposed approach to high purity aluminum is given.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021014-021014-7. doi:10.1115/1.4000283.

A fivefold increase in adhesion energy is observed for poly(acrylic acid) (PAA) modified Cu/TaN interfaces in which the thin copper films are deposited by the hydrogen assisted reduction of bis(2,2,7-trimethyloctane-3,5-dionato) copper in supercritical carbon dioxide. The PAA adhesion layer is sacrificial at the reaction conditions used, and X-ray photoelectron spectroscopy has shown that the Cu/TaN interface is free of contamination following deposition. The resulting average interfacial adhesion energy is just above $5 J/m2$, which meets adhesion requirements for integration in Cu interconnects. The adhesion measurements are performed with a custom built four-point bend fracture mechanics testing system. Comparison of the copper film thickness to the measured adhesion energy indicated that there is no effect on the adhesion energy as the film thickness changes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021015-021015-7. doi:10.1115/1.4000671.

Mechanical properties and microstructure of heat-treated samples of A356 and AA2024 aluminum alloys, which were rheoforged by varying the change in pressure and temperature were investigated, preventing defects such as porosity, liquid segregation, and insufficient filling occurring during rheoforging process. The rheology material was fabricated by an electromagnetic stirring process by controlling stirring current so that shearing force and temperature of the molten metal were controlled during electromagnetic stirring. As a result, by crushing dendrite and rosette type microstructures, fine and globularized rheology material was obtained and the feasibility of the rheoforging process was found to be positive. In the case of the direct rheoforging process, excessive applied forging-pressure caused material spattering, which in turn caused eutectic segregation. This segregation brought about a shrink hole and thus led to a deterioration of mechanical strength. According to varied applied forging pressures, agglomeration phenomena of primary particles of wrought aluminum alloy remarkably increased as compared with an as-cast aluminum alloy.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(2):021016-021016-8. doi:10.1115/1.4000823.

Based on the critical plane approach, the drawbacks of the Wang–Brown (WB) model are analyzed. It is discovered that the normal strain excursion in the WB model cannot account for the additional cyclic hardening well. In order to solve this problem, a new damage parameter for multiaxial fatigue is proposed. In the meantime, the procedure for multiaxial fatigue life assessment incorporating critical plane damage model is presented as well. In the new damage parameter, both strain and stress components are considered, and the effect of the additional cyclic hardening on the fatigue life during nonproportional loading is taken into account as well. In addition, the proposed model is modified when the mean stress is existence. It is convenient for engineering application because of no material constants in this parameter. The capability of fatigue life assessment for the proposed fatigue damage model is checked against the experimental data found in literature for tubular specimens of 1045HR steel, hot-rolled 45 steel, S460N steel, GH4169 alloy at elevated temperature, and the notched shaft of SAE 1045 steel, which is under cyclic bending and torsion loading. It is demonstrated that the proposed criterion gives satisfactory results for all the five checked materials.

Commentary by Dr. Valentin Fuster

### Technical Briefs

J. Eng. Mater. Technol. 2010;132(2):024501-024501-10. doi:10.1115/1.4000221.

Dual phase steel tubes can be produced by a variety of methods. In the present study, comparisons between dual phase steel tubes produced directly from a dual phase steel sheet and tubes produced using a new method are made. The conventional method is to process dual phase sheet steel through a tube making operation with electrical resistance welding. In the new processing method, a ferrite/pearlite sheet steel is formed into a tube, which is then normalized, induction heated to an intercritical temperature, quenched, and tempered, producing a dual phase microstructure. Tubes produced directly from a dual phase steel sheet have variations in microstructure and mechanical properties between the weld region and nonweld region material; whereas, tubes that have been produced from ferrite/pearlite steel sheet and treated to create a dual phase microstructure following the tube forming operation show little or no variation between the weld and nonweld regions. Dual phase tubes produced by the new method appear to have three main advantages over tubes produced in the traditional manner: (1) microstructural uniformity between the weld and nonweld material, (2) mechanical property uniformity between the weld and nonweld material, and (3) compressive rather than tensile residual stress components on the outer surface of the tubes.

Commentary by Dr. Valentin Fuster