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RESEARCH PAPERS

J. Eng. Mater. Technol. 1990;112(2):125-130. doi:10.1115/1.2903298.

Stitching has been found to be able to improve the interlaminar strength of composite laminates. Its application as composite joining has been also explored. This study examined the tensile strength of some stitched composite beams made of woven glass fabric and epoxy matrix. The effects of stitching parameters on joining strength were evaluated by both experimental technique and finite element method. It was found that there were two basic ways to improve the strength of stitching joint. One was to use high-density stitching in nonoverlap joint and the other was to use chain stitching in overlap joint. However, the former could give smoother surface and more uniform thickness than the latter. In addition, it was concluded from this study that stitching joint was more suitable for woven fabrics than for unidirectional prepreg tapes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):131-143. doi:10.1115/1.2903299.

The feasibility of laser cladding as a surface modification process was experimentally investigated. Emphasis was placed on identification of the effects of independent critical process parameters such as laser power, process speed (interaction time), and feed rate of cladding powder mixture on the microstructure, compositional homogeneity, geometry (e.g., thickness and width), and mechanical properties of the developed coatings. Rapidly solidified coatings metallurgically bonded to AISI 1018 steel substrates were formed in situ by using a 10 kW continuous wave CO2 laser to melt a thin layer of the substrate as well as a powder mixture consisting of Fe, Cr, C, and W with a weight ratio of 10:5:1:1 delivered to the substrate by means of blown shielding gas and a pneumatic screw feed system. Various diagnostic methods, indentation hardness measurements, and scratch-resistance testing of the laser-cladded coating materials revealed a high degree of grain refinement, increased solid solubility and uniform distribution of alloying elements, high hardness, and appreciable resistance against plastic shear deformation when the important process parameters were optimized. Microstructure studies demonstrated that coatings with very fine or relatively coarse dendritic, feathery, and particulate type microstructures were obtained, depending on the processing conditions. This investigation verified that due to the inherent rapid solidification and high concentration of key elements in the surface, hard coating materials of novel microstructures and physical properties which can be tailored to the surface requirements of the application can be produced with minimum dilution and thermal distortion. Implications of the laser cladding process in tribological applications are also interpreted qualitatively in light of the obtained results.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):144-150. doi:10.1115/1.2903300.

A closed-form elasticity solution is developed to predict stresses and strains in spiral paper tubes loaded axisymmetrically. No assumptions are made on stress distributions through the tube wall. Thus, the solution is valid for thick-walled tubes. The validity of this solution is established by comparison with experimental results. Measured strains in tubes subjected to external pressure showed remarkable agreement with the elasticity solution. After experimental verification, the elasticity solution is used to examine stress distributions in paper tubes loaded in external pressure. In both paper and isotropic tubes, the hoop stress dominates the other three stresses. However, the hoop stress distribution in paper tubes was radically different from the isotropic case. In paper tubes: (1) hoop stress was concentrated at the outer wall, especially for thicker tubes and (2) maximum hoop stress remained constant as tube thickness was increased. These differences can be attributed to the extremely small modulus in the radial direction of a paper tube. The hoop stress distributions indicate that isotropic, thick-walled cylinder theory is inapplicable for modeling paper tubes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):151-156. doi:10.1115/1.2903301.

The fracture toughness of soda-lime glass was measured by applying thermal stresses to center-cracked plates. Mode I cracking was achieved by chilling the crack faces. The stress intensity factor was obtained by combining temperature measurements with a finite element solution. The average value of KIC = 0.77 MN/m3/2 based on three tests agrees well with values in the literature for a water-free environment. Mode II cracking was achieved by applying a temperature gradient normal to the crack. A value KIIC = 1.6 MN/m3/2 was obtained in two tests using a finite element computation based on the temperature distribution computed from the specimen’s thermal boundary conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):157-163. doi:10.1115/1.2903302.

The paper outlines the use of the micromechanics model proposed by Aboudi in predicting the creep response of unidirectional composites consisting of linearly viscoelastic matrices and elastic fibers. The closed-form expressions for the effective elastic moduli given in terms of the phase moduli and volume fractions provided by the micromechanics model facilitate a straightforward application of the viscoelastic Correspondence Principle. The inversion of the effective moduli in the Laplace transform domain to the time domain is subsequently accomplished using the Bellman method. The predictions of the model are compared with the creep response of T300/934 graphite/epoxy unidirectional coupons at two different temperatures. Very good correlation between theory and experiment is illustrated for the linearly viscoelastic response characterized by relatively small creep strains.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):164-174. doi:10.1115/1.2903303.

A new concept of a multi-layered ceramic coating consisting of a porcelain enamel bond interlayer with thermal expansion characteristics tailored to match those of a cast iron substrate and a plasma-sprayed wear-resistant surface layer of chromium oxide, chromium carbide, or partially stabilized zirconia was investigated. Flat substrates of grit-blasted and surface-treated cast iron were slurry sprayed with a finely ground glass frit mixed with isopropyl alcohol and water. Subsequently, the specimens were dried in an oven before firing at 1023 K to produce a smooth, dense, and strongly adherent enamel coating 75 to 100 μm thick. Wear-resistant coatings of oxides and carbides with varying particle sizes and powder densities were then thermally sprayed, by means of a plasma spray gun, at controlled power ratings and gas flow rates. The produced ceramic coatings were subsequently ground to a thickness of about 120 μm. Adherence testing by the tensile pull-off technique revealed high interfacial strength between the enamel coating and the substrate resulting from enhanced chemical interdiffusion and mechanical interlocking. Unlubricated sliding wear experiments using a ball-on-flat tribotester and tungsten carbide balls as sliders demonstrated low initial and moderately high steady-state friction coefficients. Optical and scanning electron microscopy and surface profilometry of the tested ceramic-coated specimens verified that surface damage and wear rate were negligibly small. The important role of the main process parameters on the interfacial adherence and uniformity of the enamel and ceramic layers and the potentiality of the developed processes in the deposition of relatively low-friction and wear-resistant multi-layered thick ceramic coatings are discussed in the context of the obtained results.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):175-182. doi:10.1115/1.2903304.

Conventional ultrasonic C-scanning sometimes produces distorted and degraded images due to a variety of reasons, including surface roughness, beam dispersion, extraneous noise and imperfect fidelity of the total acquisition system. Enhancement techniques, using computer data acquisition and processing, can be used to enhance and restore the image. Enhancement techniques described include contrast stretching and median filtering, histogram equalization, thresholding, dynamic thresholding, thresholding depending on boundary characteristics, one-dimensional segmentation and intensity scans with hidden line removal. These enhancement techniques were applied and illustrated for five different types of damage in graphite/epoxy composite materials: (1) Embedded film patch in quasi-isotropic laminate; (2) impact damage in quasi-isotropic laminate; (3) matrix cracking due to static loading of crossply laminate; (4) fatigue damage in crossply laminate; and (5) thermally induced cracks in a thick crossply laminate. There is no single technique that is optimum in all cases. A suitable combination of techniques must be selected for optimum image quality.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):183-187. doi:10.1115/1.2903305.

A stress analysis that describes the crystal growing process requires a material model that is valid over a wide temperature range and includes dislocation motion and multiplication. The stresses developed in the growing process could induce residual stresses, changes in dislocation density and buckling into the growing crystals. The dislocation density is introduced as an internal variable in the constitutive model. The stress-strain and dislocation density-strain characteristics of silicon crystals are discussed as a function of temperature, strain rate, and initial dislocation density.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):188-197. doi:10.1115/1.2903306.

The titanium alloy Ti-6Al-4V is known to exhibit creep behavior at temperatures as low as room temperature. Consequently, for cyclic loading with hold times it is possible that the rate dependent behavior of Ti-6Al-4V can have negative bearing upon the low cycle fatigue life. If this effect is shown to be present at room temperatures, then it will certainly be magnified and, therefore, very important at elevated temperatures. In order to account for the effects of strain rate dependent deformation in fatigue life prediction methodology, it was considered necessary to incorporate a viscoplastic constitutive equation into the fatigue life calculational algorithm. After critical evaluation of a score of recently proposed viscoplastic constitutive theories, the Chaboche theory, which employs a yield condition, was considered to offer the most promise for description of a wide range of inelastic material behavior characteristics. The six viscoplastic material parameters that are required for nonelevated temperature applications were determined from data of uniaxial tests, conducted elsewhere and made avialable to this study. The fatigue life testing of smooth round bar specimens included load cycles with load hold times. Fatigue life predictions were performed using the equivalent fully reversed symmetric cycle, and the Smith-Watson-Topper parameter, for load cycles having varying stress amplitudes and varying hold times. The predicted fatigue life results indicate that: (i) For a given stress level above the initial yield stress, shorter load hold time periods result in longer fatigue lives. (ii) The higher the stress level (above the initial yield stress) the more pronounced becomes the effect of the load hold time on the fatigue life prediction. (iii) The rate of loading also has an effect on fatigue life. Analysis indicates that the slower the rate of loading, the higher the rate dependent (primary creep) deformation, and consequently, the lower the resulting fatigue life.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):198-201. doi:10.1115/1.2903307.

The strength and toughness of whisker- and fiber-reinforced composites are controlled to a large extent by the nature of the bonding between the fibers/whiskers and the matrix. An important problem in the development of metal matrix and ceramic-ceramic matrix composites is the simultaneous measurement of whisker and matrix residual stress. In certain systems, chemical bonding should be nonexistent and frictional forces at the interfaces provide the necessary link between the fiber and the matrix. In metal matrix composites, stresses in the fibers can lead to thermal-cyclic fatigue damage. Strain measurements have been acquired nondestructively for alumina/silicon carbide and titanium/silicon carbide composites by means of neutron diffraction techniques at the Argonne National Laboratory Intense Pulsed Neutron Source. Analysis of the experimental results shows that the in situ measurement of stress in the laboratory by neutron diffraction techniques is practical. The accumulated strain data could be used to calculate residual stresses in the fibers and the matrix (including stresses at the interface) simultaneously, or to determine stresses resulting from external loading.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):202-209. doi:10.1115/1.2903308.

The mathematical structure of Adachi and Oka’s model, a typical overstress type elasto-viscoplastic constitutive model, for normally consolidated clays is discussed. Since it has been recognized that the overstress type models cannot describe the acceleration creep and creep rupture, the Adachi and Oka’s model was modified so that it can explain the creep rupture including acceleration creep of normally consolidated clays. In addition, based on the Adachi and Oka’s constitutive model and Biot’s consolidation theory, one-dimensional consolidation problems were analyzed numerically by the finite element method. Results show that the proposed method can describe the effect of sample thickness and aging on consolidation phenomena.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):210-217. doi:10.1115/1.2903309.

An analytic study of planar beams and arches subjected to significant thermal cycling from ambient temperatures up to 800°C is presented. The study employs a recently unified nonlinear hereditary type of viscoelastoplastic constitutive law to characterize the time- and temperature-dependent properties of Hastelloy X, a typical aerospace alloy. The results demonstrate a strong interaction between the backstress variable of the constitutive law and the time-dependent stress distribution produced by the deformation. This interaction tends to control, in a highly nonlinear manner, the creep ratchetting response of the beam or arch. Moreover, temperature gradients in the thickness direction tend to exert an important influence during thermal cycling.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):218-222. doi:10.1115/1.2903310.

In this work, the use of ultrasonic velocity analysis for the quantitative nondestructive evaluation of the mechanical properties of composite laminates is discussed. The composite laminate is modeled as a layer or set of layers of transversely isotropic material of known orientation. The method described utilizes velocity measurements for ultrasonic wave propagation at multiple angles of incidence. Since the equations are algebraically cumbersome, a numerical procedure employing a commercially available nonlinear equation solving subroutine is utilized. Applications to composite laminates are presented.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):223-226. doi:10.1115/1.2903311.

Quality assessment of advanced materials like high strength ceramics and fibre reinforced composites requires detection and characterization of early damage and defects like cracks and pores even with linear dimensions of only 10 μm. Structures of the same size must be distinguishable in components of precision mechanics and electronics. This paper discusses some applications of microfocal X-ray radiography and high resolution computed tomography in nondestructive evaluation of such components. The latter can be looked at as a nondestructive microscopic technique imaging cross sections of an object. Results of investigations using a tomographic unit with a resolution capability of the order of magnitude of 20 μm are presented.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):227-230. doi:10.1115/1.2903312.
Abstract
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):231-235. doi:10.1115/1.2903313.

Based on a finite element modeling of the wire drawing process, a parametric study was conducted to investigate the effects of die geometry and area reduction factor on the magnitude and distribution of residual stresses through the wire cross section at the die exit. Two major variables of die geometry were considered in the study: the die radius and the die inlet angle. Three different die inlet angles of 12.5, 16.0 and 22.0 degrees were used in the analyses while the die radius was fixed at 25.4 mm. The die inlet angle was then set at 16.0 degrees and the die radius was varied from 12.7 mm through 38.1 mm in 12.7 mm increments. For each of the above cases, the area reduction factor was considered for 16.0 and 20.0 percents. In addition, the effect of initial stresses in wire was also investigated. The calculated results were compared to the analytical results published in the literature and an excellent agreement was obtained. The parametric study indicated that the die inlet angle has significant effect on the residual stresses at the surface of drawn wire. Specifically, smaller die inlet angle causes less tensile stresses at the surface and more compressive stresses at the center. The larger die radius reduces the level of residual stresses, but this reduction is only marginal. No significant change in either magnitude or distribution on patterns of residual stresses due to the initial stresses were found.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):236-240. doi:10.1115/1.2903314.

In a multilayer elastic plate, weak debonding in the horizontal direction parallel to the plate boundaries is insensitive to ultrasonic compressional (P) waves impinging perpendicularly, and can therefore not be detected by conventional vertically oriented reflection schemes. One of the methods for generating along the flaw line shearing stresses, to which the flaw responds, is to orient the input beam obliquely. Unlike vertically impinging beams, multiply reflected oblique beams eventually are converted into guided modes. The horizontal ranges over which the field in the unflawed plate is beam-like or mode-like must be well understood for selection of a diagnostic NDE scheme with “good” features for detection and identification. The present analytical model study explores the conversion and P-SV coupling processes for an oblique Gaussian P beam input in an aluminum plate. Special attention is given to physical features in the computed data that are relevant for “good” NDE algorithm construction.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):241-246. doi:10.1115/1.2903315.

This paper contains an extension of the uniaxial state variable constitutive model of Ramaswamy et al. (1988) to the case of multiaxial loading. The correlation between uniaxial and multiaxial loading conditions is achieved through the assumptions of material isotropy and conservation of inelastic volume. The multiaxial extension is based only on the material parameters evaluated from uniaxial loading. The research is accompanied by a multiaxial experimental program to evaluate the response of Rene’ 80 at 871°C and 982° C. Experiments in the program include torsion, proportional axial and torsion, and nonproportional loading. It was shown experimentally that there is no extra hardening from the multiaxial loading than results from uniaxial loading. Further, it is shown that the multiaxial model is successful in predicting the experimental results using only the parameters determined from the uniaxial experiments.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):247-252. doi:10.1115/1.2903316.

Rocket motor grain consists of rigid particles embedded in a soft, rubbery matrix. The ingredients are chosen to maximize the burning properties, but when cracks develop during curing or storage, burn properties are altered. The present study was directed towards a study of the loss of the inverse square root singularity in the neighborhood where a crack intersects the free surface in simulated motor grain. Experiments were conducted on both cracked simulated pure binder and cracked simulated motor grain and algorithms were developed for computing fracture parameters from the data. Severe crack tip blunting prior to crack growth initiation was observed in the simulated motor grain which required a modification of the algorithm used for single phase materials. Efforts to determine the dominant eigenvalue after crack growth have met with limited success to date due to increasing data scatter.

Commentary by Dr. Valentin Fuster

BOOK REVIEWS

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1990;112(2):254. doi:10.1115/1.2903318.
FREE TO VIEW
Abstract
Topics: Friction , Wear
Commentary by Dr. Valentin Fuster

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