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J. Eng. Mater. Technol. 2003;125(2):89. doi:10.1115/1.1562954.
Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS

J. Eng. Mater. Technol. 2003;125(2):90-96. doi:10.1115/1.1543966.

Glass fiber reinforced polymer composites are widely used as structural materials. These two-component materials can be tailored to suit a large variety of applications. A better understanding of the properties of the fiber-matrix “interphase” can facilitate optimum design of the composite structure. The interphase is a microscopic region around the fiber and hence nano-scale investigation using nano-indentation techniques is appropriate to determine mechanical property variations within this region. In this study the atomic force microscope adapted with a commercial nanoindenter has been used to determine the variation of the elastic modulus across the interphase for different silane coated glass fiber reinforced polyester matrix composites. A comparative study of the elastic modulus variation in the various interphases is reported. The results are discussed in the light of the current limitations of the instrumentation and analysis.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):97-106. doi:10.1115/1.1543967.

The propagation of a crack initiating at the surface was analyzed to simulate the fatigue wear behavior of glassy polymer materials. A crack in a material half plane was assumed to propagate along a predefined path as a result of contact loading by a cylinder sliding on the polymer surface. The crack path consisted of a vertical straight-line segment and a declined straight line originating at a branch point on the vertical crack segment. The stress intensity factors KI and KII along the crack path were computed by using finite element methods, and their values utilized in the Paris law to determine crack propagation rates. Because this process simulates surface pitting, component fatigue life is assumed to be proportional to the time needed for the propagating declined crack to intersect a neighboring vertical crack, a condition known to lead to pitting. This fatigue life is estimated by integrating the Paris law. Numerical results show that the branch point where the declined crack path originates can effectively hinder crack propagation, and that the rate limiting step in fatigue is crack propagation along a small segment of the declined crack near the branch point. Some important factors that affect the reliability of numerically predicted fatigue life cycles are discussed. Experimental crack propagation paths and lifetimes are shown.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):107-115. doi:10.1115/1.1543972.

Experimental validation of the calibration coefficients for integral hole-drilling method obtained from an improved three-dimensional FEM model was achieved using bending test of a cantilever beam. The experimental setup is a simple yet accurate method to validate the calibration coefficients obtained by a three-dimensional FEM model. With this experiment, we also validate the adequacy of the criterion applied for thin or thick plates in a previous work. The relieved stresses calculated from the calibration coefficients of the three-dimensional FEM model were compared with those calculated from two-dimensional model calibration coefficients. The results show that the accuracy of relieved stress calculation has been greatly improved as the calibration coefficients based on a three-dimensional model are used for integral hole-drilling method. Significant error in the residual stress measurement and calculation could be arisen if calibration coefficients for integral hole-drilling method were not chosen correctly for corresponding thin plate or thick plate cases according the results of the bending test of cantilever beam. A transitional dimensionless thickness was proposed by examining the calculated relieved stresses obtained from the calibration coefficients for different plate thickness. The probability bounds of relieved stress corresponding to both cases were also calculated to further reveal the improvement of the calibration coefficients obtained from the three-dimensional model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):116-124. doi:10.1115/1.1543973.

This paper illustrates the application of a new multiphase material model for simulating distortion and residual stresses in carburized and quenched gear steels. Simulation is focused on thin, metallic strips that are heat treated to introduce a through-thickness carbon gradient. Because the material properties are strongly dependent on the carbon content, quenching causes significant transverse out-of-plane distortion. The material model accounts for a multiphase alloy structure where inelasticity in the individual phases is temperature and rate dependent. The model is fit to an extensive matrix of experimental data for low carbon steels (0.2–0.8 percent) whose transformation kinetics and mechanical response are similar to 4023 and 4620 alloys used in experiments. While residual stress data are limited, reasonable agreement with X-ray diffraction measurements was obtained. Comparisons of transverse deflections predicted numerically showed excellent agreement with those measured experimentally for all five thicknesses reported. Accurate transformation and lattice carburization strains are critical to correctly predict the sense and magnitude of these transverse distortions and in-plane residual stresses.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):125-132. doi:10.1115/1.1555648.

Strength tests were performed to reveal the failure mechanisms of spot weld in lap-shear and cross tension test samples. It is shown the while the lap-shear (cross tension) sample is subjected to shear (normal) load at the structural level the failure mechanism at the spot weld is tensile (shear) mode at the materials level. Based on the observed failure mechanism, stress distribution is assumed and related to the far field load for the lap-shear and cross tension test samples. It appears that the failure load of the cross tension sample is 74 percent of the lap-shear sample based on the classical von Mises failure theory. The theoretical model is further extended to the mixed normal/shear loading condition. Data from strength tests as well as finite element numerical method are used to validate the model. Finally, the utility of the model in accessing the failure strength of spot welds is discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):133-140. doi:10.1115/1.1555651.

This paper describes an inverse approach (IA) formulation for the analysis of tubes under free hydroforming conditions. The IA formulation is derived from that of Guo et al. established for flat sheet hydroforming analysis using constant strain triangular membrane elements. First, an incremental analysis of free hydroforming for a hot-dip galvanized (HG/Z140) DP600 tube is performed using the Marc finite element code. The deformed geometry obtained at the last converged increment is then used as the final configuration in the inverse analysis. This comparative study allows an assessment of the predictive capability of the inverse analysis. The results are compared with the experimental values determined by Asnafi and Skogsgärdh. After that, a procedure based on a forming limit diagram (FLD) is described as a means to adjust the process parameters such as the axial feed and internal pressure. Finally, the adjustment process is illustrated through a re-analysis of the same tube using the inverse approach.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):141-147. doi:10.1115/1.1555652.

The forming of sheet metal into a desired and functional shape is a process, which requires an understanding of materials, mechanics, and manufacturing principles. Furthermore, producing consistent sheet metal components is challenging due to the non-linear interactions of various material and process parameters. One of the major causes for the fabrication of inconsistent sheet metal parts is springback, the elastic strain recovery in the material after the tooling is removed. In this paper, springback of a steel channel forming process is controlled using an artificial neural network and a stepped binder force trajectory. Punch trajectory, which reflects variations in material properties, thickness and friction condition, was used as the key control parameter in the neural network. Consistent springback angles were obtained in experiments using this control scheme.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):148-152. doi:10.1115/1.1555653.

A directionally attached piezoelectric (DAP) actuator is one method to control the twisting vibration of a plate with high authority. Although insuring proper performance for DAP actuators has been discussed in terms of the control law used, the optimization of the mechanical details of the actuator itself has received little attention. There is an interaction between the actuator and the controlled structure because more actuator material on the structure adds actuation power but also stiffens the structure. The effects of DAP actuator geometric parameters and material properties are explored in a systematic way for the case of a cantilever beam and it is shown that significant improvements in performance are possible. The material property study indicates that an optimum point exists whereby the weight and thus cost can be lowered while improving structure response by using a composite actuator. The actuator thickness, width, orientation angle, and offset from the clamped end have significant effects on structure response. In order of importance, the geometric parameters are: actuator thickness, orientation angle, width and offset. A study of the modal distribution for the structure shows that if the input disturbance that is to be suppressed is modally well characterized, the structure can be efficiently controlled by using more than one independent actuation voltage.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):153-162. doi:10.1115/1.1555654.

A new experimental method utilizing the Interferometric Strain/Slope Rosette (ISSR) and incremental hole-drilling is applied to measure in-plane residual stresses which vary with depth. For each depth increment, the ISSR measures three relieved strain components and two relieved slopes at point near the hole edge. Adapting the classical Integral Method to the ISSR, and incorporating a Tikhonov regularization scheme, stabilized residual stress solutions are back-calculated from the measured deformations. This paper describes the measurement principle of the ISSR, the derivation and numerical calculation of the ISSR hole-drilling coefficients, and the methodology used to back-calculate and regularize the residual stress solution. A verification test on a thin-plate with a through-hole, and residual stress measurements performed on a shot-peened Titanium-alloy block, are presented and discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):163-169. doi:10.1115/1.1555655.

Currently, fabrication of composite materials is of great interest in industry. By combining materials of different properties, we can produce new composite materials with synergetic functionality that individual materials do not possess. In this study, Al2O3 nanosized particles were coated on Al2O3 fiber substrates using a dry mechanical coating technique employing high shear and compression forces. The materials thus synthesized had high surface area with good dispersion for enhanced reactivity and were strong to sustain rigorous operation. Operating parameters, including rotor speed, processing time and initial loading percentage were varied to study their effects on the coating condition. The experimental results showed that the product surface area increased as the nanoparticle loading increased. The dispersion of nanoparticles improved as the processing time increased. A higher rotor speed resulted in a shorter product length while the nanoparticle loading had no effect on the product length. The durability test, conducted in a fluidized bed, indicated no significant change of the coating layer after 7 days of continuous testing.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):170-175. doi:10.1115/1.1555656.

In view of the well-known disparity between predictions from temper rolling models and measured values from rolling mills, there is a need to obtain a better understanding of the effect of roll asperities on the yielding characteristics of the rolled strip. This paper models a roll with asperities as a rigid body with a regular serrated surface and the rolling process as an indentation, and uses the plane strain model and slip-line theory to determine the critical pressure that is required to yield the strip throughout the thickness beneath the tips of the asperities. The strip is also under lateral tension at both ends. The emphasis of the paper is the effect of the lateral tension and the thickness of the strip on the critical pressure. For the case when the indenting surface has sharp teeth, the critical pressure can be found in close form. For the case when the indenting surface has blunt teeth, a robust approximate scheme for estimation of the critical values that does not require extensive computation is given and this scheme can be used in an on-line control process. It is found that when the tooth angle is smaller than a critical angle, the sharper the tooth, the lower the average critical pressure needed to make the strip yield. When the tooth angle is larger than the critical angle, then the blunter the tooth, the lower the pressure that is needed. The effect of the asperities is to reduce the critical pressure and it is found that this effect is more pronounced for thin strips than for thick strips. The lateral tension reduces the critical pressure further. These findings give some implications for the rolling of metal sheets.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):176-182. doi:10.1115/1.1555658.

The theoretical model for the blister test method was used to analyze the interface fracture toughness of zirconia coating deposited on an SUS304 stainless steel substrate by a plasma-spraying method. The elastic parameters of the debonded coating were determined by testing the oil pressure q and maximum deflection w(0). SEM observation, compliance method and ultrasonic detection were used to determine the radius of the debonded coating. The three methods gave the same results for the debonded coating radius. Micro-observations showed that the interfacial crack propagates by the growth of voids or microcracks ahead of the main crack and coalescence with the main crack. The energy release rate G0 with phase angle ψ=0 for type A coating and type B coating was, respectively, 14.54∼25.88 J/m2 and 11.88∼16.21 J/m2. The corresponding interface fracture toughness for type A TBC coating and for type B TBC coating is, respectively, 0.77∼1.02 MPa⋅m1/2 and 0.52∼0.61 MPa⋅m1/2. The stable phase angle was approximately −31.5° and −30.2° for coating A and coating B, respectively.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):183-190. doi:10.1115/1.1493804.

This paper presents a closure model for predicting the growth behavior of short cracks in the presence of large-scale yielding and residual compressive stresses, representative of structures that have been shot-peened. The plasticity-induced crack closure model developed by Newman is first extended by using the cyclic crack-tip opening displacement as the correlating parameter for fatigue crack growth rates. This new approach also enables a better characterization of the effect of large-scale yielding on short crack growth. The effect of residual stress on crack closure is then analyzed by adding to the loading spectrum an equivalent stress, which varies with the applied load level and the crack size. It is shown that predictions of the extended closure model are within a factor of two of the experimental results of etched specimens tested under spectrum loading, highlighting the capability of the predictive model along with some important issues for future research in this area.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):191-199. doi:10.1115/1.1491576.

Constitutive modeling and related numerical issues in conjunction with the macroscopic analysis and simulation of cold compaction and subsequent solid phase sintering of hard-metal are discussed. A key ingredient is the concept of sintering stress, which is derived as a thermodynamically consistent dissipative stress, based on a simplified microstructural arrangement with spherical pores. In order to account for the temperature effect on the rate-dependent response during the sintering phase, the concept of viscoplastic admissibility is employed as part of the model framework. A generic class of pressure-sensitive models is considered, and the particular model chosen for calibration is based on quasistatic and dynamic yield surfaces that are elliptic in the meridian planes of the stress space. Implicit integration is used for the pertinent evolution equations. The paper is concluded by a numerical investigation of the compaction and sintering of a specimen, whereby the model is calibrated using experimental data from free and uniaxially loaded sintering experiments (within a joint research project).

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):200-207. doi:10.1115/1.1562952.

This paper describes the implementation and modification of a previously proposed unified viscoplastic constitutive model to simulate the behavior of a Yttria Stabilized Zirconia plasma sprayed thermal barrier coating. The model was recast for use in finite strain situations and modified to have a more physically acceptable non-associated flow rule. Temperature dependent material constants were found for a specific material using a novel approach based on Genetic Algorithms.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):208-214. doi:10.1115/1.1562953.

A new method on estimating fatigue properties from simple tensile data was proposed for aluminum alloys. The method is based on experimental strain-life curves. An optimization technique was used to get the best fatigue properties. The four-point correlation method, the universal slopes method, Mitchell’s method, the modified universal slopes method, the uniform material law by Bäumel and Seeger, the modified four-point correlation method by Ong and a new method proposed in this work were evaluated in a quantitative manner. Mitchell’s method, the uniform materials law and the new method give good life predictions for aluminum alloys. In particular, the new method provided the best results.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):215-221. doi:10.1115/1.1543970.

This paper deals with an experimental methodology of the large deformation of cylinders under constrained sides and end conditions. A specific arrangement of two geometrically identical cylinders compressed laterally is studied under different quasi-static strain rates. Several tests are performed using two different structural situations. In the first case, the two cylinders are made from superplastic tin-lead alloy, while in the second case, one cylinder is made from superplastic and the other from steel. Different cylindrical geometries are investigated having the same cross sectional area with different ratios of inner to outer diameter (di/do). The load-deflection curves are recorded and then the energy absorbed per unit volume is determined. The experiments show obviously the remarkable sensitivity of the utilized superplastic to the strain rate in the range of 10−5/s–10−3/s. A two-dimensional finite element simulation is also conducted describing the collapse behavior of these cylindrical geometries in both structural cases under different strain rates. A confrontation with experimental observation shows that the predictions describe fairly well the experiments.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):222-226. doi:10.1115/1.1543968.

An incremental and thermal-electro-mechanical coupled finite element model has been presented in this study for predicting residual stress distribution in a spot welded steel joint. Approximate temperature dependent material properties, including physical and mechanical properties, have been considered. The spot nugget shape and the residual stress distribution were obtained by simulation. The results obtained have been compared with experimental measurements, and good agreement is observed. The highest tensile residual stress occurs at the center of the nugget and the residual stress decreases towards the edge of the nugget.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(2):227-233. doi:10.1115/1.1543969.

Residual life analysis of power plant components like boiler tubes, superheater outlet headers, reheater headers, steam pipes, etc., is important for life extension and avoidance of catastrophic failure. In this context, fracture toughness is very important. The fracture characteristics after prolonged exposure to high temperatures and pressures are likely to be different from that of the virgin material. 2.25Cr-1Mo reheater header pipe exposed at 813 K for 120,000 h was studied by instrumented impact tests (IIT) to evaluate dynamic fracture toughness and Charpy transition properties. The methods presented in this paper for estimating dynamic fracture toughness from IIT of Charpy specimens give reliably conservative results without the need for precracking. For estimating fracture appearance transition temperature (FATT) from IIT load-time traces, the equation for percent shear fracture, PSF3, gives the best 1:1 correlation with measured values from fracture surfaces. The lower bound equation for variation of fracture toughness with temperature derived in the present study is higher than that obtained from the FATT master curve (FATT-MC) approach. Comparison of Charpy indices like FATT and upper-shelf energy for the service exposed steel to results for the virgin material reported in the literature and the compositional J-Factor estimates for temper-embrittlement susceptibility indicate that the present steel, even after 120,000 h exposure to high temperature service, has probably undergone only very little or nil degradation in toughness properties.

Commentary by Dr. Valentin Fuster

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