J. Eng. Mater. Technol. 1999;121(3):257-263. doi:10.1115/1.2812373.

An experimental technique for determining the dynamic indentation hardness of materials is described. Unlike the traditional static hardness measurements, the dynamic hardness measurements can capture the inherent rate dependent material response that is germane to high strain rate processes such as high speed machining and impact. The dynamic Vickers hardness (DHV) of several commonly used engineering materials is found to be greater than the static Vickers hardness (HV). The relationship between the hardness and yield stress under static conditions, i.e., HV =3σy , is also found to be valid under dynamic conditions. It is suggested that this simpler technique can be used to assess the rate sensitive nature of engineering materials at moderate strain rates in the range of around 2000/s.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):264-271. doi:10.1115/1.2812374.

A flexible statistical modeling framework for the analysis of creep rupture data is proposed, which offers an improvement on traditional methods of deriving creep rupture strength values and confidence limits. The paper reviews a family of models that can be used to represent the trend relationship between failure times about the trend line, and examines the reliability of extrapolations. Areas of statistical research which would lead to model improvement are discussed, such as variance heterogeneity, left censoring and allowance for the cluster (cast) structure of the data.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):272-277. doi:10.1115/1.2812375.

Paper is a highly anisotropic and nonlinear material. Solid mechanics research on paper structures has been hampered by limited data on the nonlinear material properties of paperboards. In particular, it has been difficult to measure shear and z-directional properties. In this paper, nonlinear material response of paper laminates under simple loading conditions (uniaxial compression in three directions and in plane shear) are presented. Also, data is presented for various Poisson’s ratios. It was found that paper behaves differently along the thickness direction than along the in-plane directions. Poisson’s ratios are usually nonlinear with large deformation. In-plane Poisson’s ratios decrease, while out-of-plane Poisson’s ratios increase with the applied load.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):278-285. doi:10.1115/1.2812376.

The low-cycle fatigue damage initiation in Waspaloy under complex cyclic loading (out-of-phase) is studied from experimental and theoretical viewpoints. Special emphasis is put on the transgranular damage development and results are compared to those reproduced in the literature. A physico-phenomenological model based on slip theory is used to predict the damage initiation lives as well as the directional aspect of the damage distribution. In this model, the micro-damage is supposed to initiate and then evolve on the activated crystallographic slip systems. The theoretical results are compared to both the experimental ones concerning the same material (Waspaloy) as well as other experimental results extracted from the literature.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):286-293. doi:10.1115/1.2812377.

Multiaxial fatigue under variable amplitude loading is investigated using Kandil et al.’s parameter, rainflow cycle counting on the shear strain history, and the Miner-Palmgren damage rule. Fatigue data are obtained on tubular specimens of S45C steel under proportional and nonproportional tension-torsion loading. The approaches using the maximum shear strain range (Δγmax ) plane and the maximum damage (Dmax ) plane as the critical plane are investigated. The damage is computed for each reversal or for each cycle. The results show that both Δγmax and Dmax approaches yield acceptable fatigue lives irrespective of the damage computation method. Damage computation for each reversal tends to shift fatigue life toward the nonconservative side for some nonproportional loading. It is concluded that the overall procedure used in this study is viable for multiaxial life prediction under variable amplitude loading for the test material.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):294-304. doi:10.1115/1.2812378.

We measure the impact forces and deflections resulting from drop tests of a mass with a flat impact surface onto flat pads of various elastomeric materials, and show that the forces can be predicted quantitatively with no adjustable parameters by using a theory whose only inputs are the linear viscoelastic characteristics of the materials, measured in small-amplitude oscillatory deformations. The theory, which models the elastomer as a nonlinear neo-Hookean material, is accurate for several elastomeric solids including polyurethanes, polynorbomene, and poly-vinyl-chlorides (PVCs), over a wide range of impact velocities, masses, temperatures and pad thicknesses. Some steps are taken to extend the model to surfaces which are not flat. The application in mind is the rational design of elastomeric components in impact-tolerant portable electronic equipment.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):305-312. doi:10.1115/1.2812379.

This paper describes a damage Parameter for predicting fatigue life under biaxial-tensile loadings. Several studies have focussed in the past on the situations where the in-plane biaxial stress ratio is negative; however, little attention has been paid for the cases involving both principal stresses in tension. A new testing method is used to carry out biaxial fatigue tests, at room and 427°C, on Type 304 stainless steel for different positive values of the stress ratio. In the experimental procedure, a disk-shaped specimen was used in connection with a spatial-arms mechanism which converts the uniaxial force generated by a conventional testing machine to radial forces extending the disk specimen. A modified virtual strain energy parameter is then suggested to normalize fatigue data obtained under a wide range of stress states. The proposed parameter accounts for the mean stress and the mean strain effects in an explicit form. In addition, the COD equivalent stress and strain concepts are adopted to account for the stress state biaxiality. The predictions of the proposed parameter are compared with the obtained experimental data and the correlation between the applied stress states and the experimental fatigue lives is discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):313-320. doi:10.1115/1.2812380.

The stress distribution ahead of a notch tip is the prerequisite to calculating the driving force for cracks emanating from notches. This article first examines whether two commonly used engineering methods, which are often employed to determine the response at a notch tip, can be applied to evaluate the elastic-plastic response ahead of a notch tip. It is found that both methods would significantly underestimate the stress-strain distribution ahead of a notch tip. Based deformation theories of plasticity, and analytical method is then developed, taking into account of the effects of stress redistribution induced by notch plasticity and the in-plane and through-thickness constraints near the notch root. Predictions are shown to be in close correlation with finite element results.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):321-329. doi:10.1115/1.2812381.

The fatigue crack growth rate (FCGR) characteristics of Laser Shock Peened (LSP) titanium 6Al-4V were examined and compared to those of unprocessed material. The FCGR resistance of LSP processed material tested at low stress ratios (R) is shown to be significantly greater than for unprocessed, baseline material. The increased damage tolerance can be attributed to the large residual compressive stresses generated by the LSP process. Differences in the growth rate behavior due to LSP can be accounted for by using the closure corrected effective stress intensity range, ΔKeff , which takes into account only the portion of loading above the crack opening load. The rationale of using ΔKeff is also demonstrated through fractographic investigations.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):330-335. doi:10.1115/1.2812382.

The model describing the behavior of resistance brazing process developed here is a one-dimensional, explicit, finite difference model. The finite difference method uses a central difference scheme for the spatial approximation and a forward difference scheme for the transient iteration. The complex geometry of the contact is reduced to simple element via conformal mapping. The model incorporates joule heating, interfacial resistance, forced convective cooling of the electrodes. Radiation and ambient convection are neglected. The model simulates resistance brazing of powder metallurgy contacts to copper terminal quite well. The anticipated transient response is reproduced. The model, in conjunction with an experimental reference, could be used to troubleshoot or develop similar processes. As a result, it is concluded that the method is successful.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):336-340. doi:10.1115/1.2812383.

A traversing water jet was used to impact the surface of 1100 series aluminum specimens in an effort to generate compressive residual stresses on the surface. Stresses induced by the water peening operation were measured using X-ray diffraction, and compressive stress increases as large as 60 percent of the monotonic yield strength resulted. Surface roughness and hardness were also measured. Finite element modeling of a stationary water jet impinging on an elastic-plastic half-space was performed to characterize the water peening process. Surface residual stresses were found to be a result of sub-surface plastic deformations.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):341-345. doi:10.1115/1.2812384.

The bulge forming performance of Pb-Sn superplastic sheet materials is evaluated under different forming pressure-time profiles. In addition, an “optimum” pressure-time profile based on variable strain rates is proposed. Results of forming time, achieved amount of deformation, and thinning behavior are reported for the different forming pressure profiles. Not only the forming time was significantly reduced, when forming with the optimum pressure profile, but the integrity of the formed part was also maintained. Furthermore, the results show that forming profiles based on uniaxial models cannot successfully predict the actual forming behavior when the loading path is biaxial. In fact, the one-dimensional strain rate hardening relation, along with von Mises criterion, predict failure at a much higher strain (time) than the actual case.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):346-351. doi:10.1115/1.2812385.

The fatigue resistance of components is greatly influenced by their surface integrity. In particular, residual stresses and surface roughness are held to be the primary factors influencing fatigue resistance. This paper presents an experimental study of the influence of turning parameters on surface roughness and residual stresses. Two steels were machined by turning, the results of variations in four process parameters were measured and analyzed. These results show that in the typical range of industrial machining conditions both surface finish and residual stresses are influenced mainly by the feed rate and the nose radius; the cutting velocity and the primary rake angle play a minor, negligible role. Moreover, two empirical models were identified that can be used to predict residual stresses and roughness as a function of the two major turning parameters. These models can be used to optimize the turning conditions of components when their functionality requires the control of residual stresses and surface roughness.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):352-359. doi:10.1115/1.2812386.

The coalescence conditions of plastic zones are calculated for multiple cracks in a riveted stiffened sheet using a strip yield model. The multiple cracks and their plastic zones are treated as a fictitious crack, and algebraic equations are formulated on compatibility of displacements, no stress singularity at the fictitious crack tips, and zero displacement at the coalesced points of plastic zones. These equations are iteratively solved, and critical values of remote stress, fastener forces, plastic zone sizes, and crack tip opening displacements are calculated. Some numerical results are presented for two cracks in a sheet with and without stiffeners.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):360-366. doi:10.1115/1.2812387.

A micromechanics based failure initiation predictive capability for analyzing notched composite laminates loaded remotely in multiaxial compression is reported. The model relies on the results from a previous experimental study that investigated compression failure mechanisms in special “uniply” composite laminates. The finite element method (FEM) was used in the solution process. The experimental results showed that the dominant mode of failure initiation was kink banding near the hole edge. The kink band was confined in extent to a distance within one half of the hole radius. The fibers within the kink band were rotated both in plane and out of the plane of the laminate. The position of the kink band with respect to the center of the notch depended on the remote biaxial load ration. In the FEM, the region in which kink banding takes place is contained within a finite size rectangular area, and is meshed as an alternatingly stacked region of fiber and matrix layers. The values of boundary loads on this rectangular area which correspond to kink banding is related to the remotely applied loads via an available closed form analysis for orthotropic laminates. Good agreement is found between experiment and analysis for a wide range of notch sizes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):367-373. doi:10.1115/1.2812388.

The cryogenic interlaminar shear behavior of G-10CR glass-cloth/epoxy laminates has been discussed through theoretical and experimental characterizations. The use of the double-notch shear test for measuring the interlaminar shear strength of glass-cloth/epoxy laminates at low temperatures is evaluated first. The interlaminar shear tests were carried out with double-notch shear specimens at room temperature, 77 K and 4 K to evaluate the interlaminar shear strength (ILSS) of G-10CR glass-cloth/epoxy laminates. The double-notch shear specimen was loaded on its ends in compression with a supporting jig to prevent buckling. These tests were conducted in accordance with ASTM D3846-79. The effects of temperature, specimen thickness, and notch separation on the apparent ILSS are shown graphically. Fracture surfaces were examined by scanning electron microscopy (SEM) and optical microscopy to verify the failure mechanisms. A three-dimensional finite element analysis was also performed to investigate the effect of specimen thickness and notch separation on the shear stress distribution in the expected fracture plane. Effective elastic moduli were determined under the assumption of uniform strain inside the representative volume element. The numerical findings are then correlated with the representative volume element. The numerical findings are then correlated with the experimental results. The validity of this test technique has been established.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):374-380. doi:10.1115/1.2812389.

A model of a taper-taper adhesive-bonded joint under cylindrical bending has been derived using first-order laminated plate theory. Shear correction factors were used to account for transverse shear deformation. A FORTRAN program was written to integrate the resulting system of twelve simultaneous, linear, first-order, differential equations with variable coefficients. The Linear Shooting Method was used to solve the model. A finite element model was developed using the COSMOS/M commercial finite element package to verify the analytical model for a cross-ply laminate. The analytical model results agreed well with the finite element models and predicted peak adhesive stresses within about 2% of the finite element model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):381-385. doi:10.1115/1.2812390.

The purpose of this study is to investigate the delamination growth behavior of a glass fabric reinforced laminated composite under Mode I fatigue loading and to examine the applicability of a new fatigue crack growth rate model to this material. In this study, double contilever beam specimens were subjected to tension-tension cyclic loads with three different load ratios and the delamination growth rate was measured using the compliance method. The delamination growth rate was related to the strain energy release rate during fatigue cycling by a power law equation that takes into account not only the effect of the strain energy release rate range, but also the effect of delamination growth at various stages of loading using a weight average strain energy release rate. It was observed that this new model can represent the delamination growth rate of the fabric reinforced laminated composite at three different load ratios in a single unifying curve.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):386-392. doi:10.1115/1.2812391.

In this paper, the microstructure of “Saffil”-Al2 O3 short fiber reinforced Al-Mg5.5 metal matrix composite material is simulated by computer. In the simulation it is taken into account of that the lengths, diameters, orientations, and locations of short fibers, etc. For the 3-D randomly distributed short fibers in composite materials, the typical distributions of short fiber microstructures on different planes are obtained for different short fiber volume fractions. The microstructural effects of average fiber length, diameter and their standard deviations on the overall strength of metal matrix composite materials are analyzed. From the short fiber microstructural distribution in metal matrix composite materials, the short fiber diameter coefficient ξd and short fiber length coefficient ξ1 are obtained for different standard deviations σd and σl , respectively. The short fiber orientation coefficient ξa is obtained, also. The results of these coefficients may be useful to the manufacture and use of short fiber reinforced composite materials. Considering these coefficients ξa ξd and ξl , the improved formula is given for the direct calculation of overall strength of short fibers reinforced composite materials. The improved formula may reflect the microstructural characteristics of short fibers reinforced composite materials.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1999;121(3):393-398. doi:10.1115/1.2812392.

Effect of thickness on fatigue crack growth (FCG) behavior has been investigated at varying stress ratio in a quaternary Al-Li alloy. The thickness has been observed to influence FCG behavior in two opposite ways, depending on stress ratio. The crack closure concept has been invoked to rationalize the observed behavior. The roughness-induced crack closure is the main governing mechanism in the alloy under investigation. Factors such as fracture surface asperity size and severity of plane strain condition have been argued to affect the extent of crack closure, and consequently, fatigue crack growth behavior.

Commentary by Dr. Valentin Fuster


J. Eng. Mater. Technol. 1999;121(3):399-401. doi:10.1115/1.2812393.

Two parameters that are linearly related for a crack-free cantilever but cease to be so when a crack is present, are derived. A simple procedure for detecting the crack and assessing its location and depth is subsequently described.

Commentary by Dr. Valentin Fuster

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