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IN THIS ISSUE


RESEARCH PAPERS

J. Eng. Mater. Technol. 1994;116(4):439-445. doi:10.1115/1.2904310.

Novel polycarbonate (PC) foams with bubbles on the order of 10 μm and cell nucleation densities between 1 and 10 billion cells per cubic centimeter of foam have been produced using carbon dioxide as the blowing agent. The size and number of bubbles can be controlled to produce a wide range of foam densities. This paper presents the results of an experimental study of the tensile behavior of these unique microcellular foams. It was found that the tensile strength of microcellular PC foams is proportional to the foam density. The strength is less than that predicted by the rule of mixtures, suggesting that the microcellular structure is inefficient in carrying the tensile load. The saturation of PC by CO2 was found to reduce the tensile strength of the virgin material by approximately 20 percent. This showed that the sorption of a very high concentration of gas molecules by the polymer must be considered when characterizing and modelling the microcellular foam mechanical properties. The relative tensile modulus of microcellular foam was found to increase as the square of the foam’s relative density over the range of densities explored.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):446-450. doi:10.1115/1.2904311.

Large amplitude oscillatory shear (LAOS) experiments on three grades of LLDPE including one commercial film blowing resin showed an interesting transition in the shear stress response to LAOS. The shear stress response is initially a nonsinusoidal standing wave which then undergoes a transition to quasi-periodicity. Many line-broadened odd and even harmonics were found in the shear stress amplitude spectra of the quasi-periodic responses. The transition time depends on shear strain amplitude and frequency, but it did not correlate with the weight average molecular weight of the LLDPE’s.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):451-456. doi:10.1115/1.2904312.

The effect of a cycle of CO2 saturation and desorption on the yield strength, Young’s modulus, and fatigue life of polycarbonate was investigated. The fatigue life of the saturation-cycled polycarbonate exceeded that of unprocessed polycarbonate by up to a factor of thirty. The fatigue life of these rough-surfaced saturation-cycled specimens was approximately equal to or greater than that of polished smooth bar specimens. The increase in fatigue life is a function of the CO2 saturation pressure, reaching a maximum at the same saturation pressure at which there is no further reduction in static yield strength. The increased fatigue life is hypothesized to result from one or more of following mechanisms: (1) reduction of shear strength promoting slow-growing shear fatigue cracks: (2) “healing” or blunting of pre-existing flaws or microcracks by swelling strain or by stress relief strain resulting from plasticization by CO2 during saturation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):457-464. doi:10.1115/1.2904313.

The toughness of the low alloy ferritic steel material of structural components operating at elevated temperatures can degrade during service due to embrittling phenomena such as carbide coarsening and temper embrittlement. The extent of degradation and the current level of toughness are critical inputs to component structural integrity assessments and to operation and maintenance planning. Conventional test methods for measuring toughness require the removal of large material samples from the in-service component, which is generally impractical. However, the recent development of relatively nondestructive, miniature sample removal systems and the small punch test technique (which utilizes nonstandard, miniature specimens) now provides a convenient, practical means of evaluating the material of an in-service component for toughness and related mechanical properties. This paper describes the small punch test technique with selected examples of its application to various grades of low alloy ferritic steel.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):465-470. doi:10.1115/1.2904314.

In this paper a finite element analysis of steady-state dynamic crack growth under mode I plane strain small scale yielding conditions has been performed in a power law hardening rate dependent plastic material, characterized by the Perzyna over stress model. A modified version of the rate tangent modulus method has been used to update the stress. The main objective of the work is to obtain a quantitative relationship between dynamic fracture toughness ratio (K/Kss ) and crack speed. A plastic strain criteria proposed by McClintock (1968) has been applied to obtain this relationship. It is found that dynamic stress intensity factor increases with velocity for all values of β̂ (a normalized viscosity parameter). At a low value of β̂, which corresponds to high rate sensitivity, the fracture toughness ratio (K/Kss ) increases with hardening. On the other hand, at a higher β̂, the ratio increases initially and falls subsequently, with increasing hardening.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):471-478. doi:10.1115/1.2904315.

Manufacturing processes used to deposit hard coatings often produce cracks which reside within the coating, at the interface, or in the substrate. These cracks originate from material defects and thermal expansion mismatch. When subjected to stress (from solid body contact) the cracks can act as flaws which initiate and/or propagate subsequent fracture contained entirely in the coating, substrate, interface, or combinations thereof. The finite element method has been used in conjunction with a numerical interface fracture mechanics model to investigate the structural response of coated brittle materials subjected to normal and shear loads (tribo-contact). Residual stresses from depositing TiC onto a WC-TiC-TaC-Co substrate were superimposed with loads that simulate a single point scratch test. Experimental observations of metallographic cross sections, taken through scratched TiC films, were used for verification and guidance in modeling. This study has examined how flaw orientation affects crack propagation through the coating, interface, and substrate. The importance of interface fracture toughness and anisotropy in coating mechanical properties are discussed in light of wear particle formation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):479-482. doi:10.1115/1.2904316.

Temperature and strain rate dependences of low-cycle fatigue life can be represented by a modified Larson-Miller parameter. The parameter P is written by P=T(logN25 −Alog ε̇ + B), where T is temperature, N25 is fatigue life, ε̇ is strain rate, and A and B are constants. In the analysis, each data of several kinds of engineering materials from ferritic steels to austenitic stainless steels are used. These are the authors original data published in the documents of NRIM Fatigue Data Sheets. The result of 304 stainless steel has been compared with statistical analysis result by Diercks adopted in a design code. The fatigue life curves represented by the proposed parameter analysis fitted well test data in high-cycle region as well as ones in low-cycle region.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):483-487. doi:10.1115/1.2904317.

It is very difficult to monitor fatigue crack initiation at all parts of machine equipment day after day. In this study, a method of continuous monitoring for fatigue crack initiation is examined which uses the strain waveform of running machine. A notched section of the equipment is taken as the inspection point because many fracture accidents originate in fatigue cracks initiated from such a notch. Fatigue crack initiation can easily be detected by observation of the waveform of a strain function composed of the strains in the vicinity of the notch, because it changes shape on crack initiation. This change of the waveform is brought about by the change of the compliance of the material due to the crack closure behavior. It is found that crack initiation detection is improved by spectral analysis of the waveform.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):488-494. doi:10.1115/1.2904318.

This paper studies tension/torsion multiaxial low-cycle fatigue lives and creep-fatigue damage evaluation for Alloy 738LC superalloy. Tension/torsion creep-fatigue tests were carried out using hollow cylinder specimens and multiaxial creep-fatigue lives were obtained. The Mises’ equivalent strain correlated the multiaxial low cycle fatigue lives within a factor of two scatter band. An a.c. potential method is developed to detect the creep-fatigue damage associated with crack nucleation and extension. A.c. potentials at high frequencies accurately detect the creep-fatigue damage from the early stage of life while those at low frequencies detect that in the final stage of life. A.c. potentials at high frequencies detect the crack density, defined as the total crack length per unit area, and maximum crack length more sensitively than those at low frequencies.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):495-504. doi:10.1115/1.2904319.

A micromechanical principle is developed to determine the strain-rate sensitivity, relaxation behavior, and complex moduli of a linear viscoelastic composite comprised of randomly oriented spheroidal inclusions. First, by taking both the matrix and inclusions as Maxwell or Voigt solids, it is found possible to construct a Maxwell or a Voigt composite when the Poisson ratios of both phases remain constant and the ratios of their shear modulus to shear viscosity (or their bulk counterparts) are equal; such a specialized composite can never be attained if either phase is purely elastic. In order to shed some light for the obtained theoretical structure, explicit results are derived next with the Maxwell matrix reinforced with spherical particles and randomly oriented disks. General calculations are performed for the glass/ED-6 system, the matrix being represented by a four-parameter model. It is found that, under the strain rates of 10−7 /hr and 10−6 /hr, randomly oriented disks and needles at 20 percent of concentration both give rise to a very stiff, almost linear, stress-strain behavior, whereas inclusions with an aspect ratio lying between 0.1 and 10 all lead to a softer nonlinear response. The relaxation behavior of the composite reinforced with spherical particles is found to be more pronounced than those reinforced with other inclusion shapes, with disks giving rise to the least stress relaxation. The real and imaginary parts of the overall complex moduli are also established, and found that, as the frequency increases, the real part of the complex bulk and shear moduli would approach their elastic counterparts, whereas for the imaginary part, the increase shows two maxima, and then drops to zero as the frequency continues to increase. Finally, the complex bulk modulus is examined in light of the Gibiansky and Milton bounds, and it is found that, for all inclusion shapes considered, this modulus always lies on or within the bounds.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):505-511. doi:10.1115/1.2904320.

A thermomechanical analysis of advanced composites in a wide temperature range is presented. This analysis is based on the micromechanics method of cells. An incremental formulation of the micromechanics model is developed to facilitate the use of various inelastic constitutive theories. These theories incorporate time-dependent and temperature-dependent features for modeling different types of metal matrices. The constitutive models include the Bodner-Partom unified theory of viscoplasticity, the incremental plasticity model, and a power-law creep model. The effect of the cooling rate, taking into account temperature-dependent matrix properties, on residual thermal stresses is subsequently investigated for a SiC/Ti composite using the different models for the matrix phases. Predictions generated using the micromechanics method are compared with available results of finite-element analysis.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):512-516. doi:10.1115/1.2904321.

A mechanical model which can predict mode I delamination failure of laminated composites has been developed. A beam on a nonlinear spring foundation was used to model experimental results obtained from DCB type fracture specimens. The entire thickness of the beam specimen was used as a spring length, and a nonuniform strain distribution throughout the spring length was utilized, based on the 2-D asymptotic solution of the stress field near a crack tip. The failure condition of the spring foundation is based on an energy criterion. Mode I fracture tests were performed to verify the current model using two types of laminated composite DCB specimens. The current model reproduced the experimental results of pulling force versus crack opening displacement curves very closely for a wide range of resin layer thickness of the specimens. The current model has a potential capability of being extended to solve 2-D crack problems, where the evolution of an arbitrary shape of 2-D crack geometry will be predicted as a part of the solution of the current model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):517-523. doi:10.1115/1.2904322.

A micromechanics model is presented to predict thermoelastic properties of composites reinforced with plain weave fabrics. A representative volume element is chosen for analysis and the fiber architecture is described by a few simple functions. Equations are developed to calculate various phase fractions from geometric parameters that can be measured on a cross section. Effective elastic moduli and effective thermal expansion coefficients are determined under the assumption of uniform strain inside the representative volume element. The resulting model is similar to the classical laminated theory, and hence is easier to use than other models available in the literature. An experimental correlation is provided for a number of Nicalon SiC/CVI SiC and Graphite/CVI SiC composite laminates.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):524-532. doi:10.1115/1.2904323.

This paper is concerned with the development of a generalized approach for mesh refinement in a short fiber reinforced composite. Mesh refinement procedures are based on the calculation of the error in energy norm for global convergence and the traction differential approach at the fiber/matrix interface for local convergence. The mesh refinement strategy is based on the use of elongated elements at the fiber/matrix interface, yielding significantly different mesh patterns than obtained by conventional mesh refinement approaches. This difference may have a critical bearing on the subsequent thermo-mechanical properties predicted by finite element analysis (FEA). It is found that the use of elongated (i.e., high aspect ratio) elements for mesh refinement results in a much more rapid computational convergence rate than obtained by conventional meshes. Converged local solutions are obtained with significantly less degrees-of-freedom (DOF) than by conventional mesh refinement methods.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):533-538. doi:10.1115/1.2904324.

An analytical model is developed to compare the effects of voids and debonds on the interfacial shear stresses between the adherends and the adhesive in simple lap joints. Since the adhesive material above the debond may undergo some extension (either due to applied load or thermal expansion or both), a modified shear lag model, where the adhesive can take on extensional as well as shear deformation, is used in the analysis. The adherends take on only axial loads and act as membranes. Two coupled nondimensional differential equations are derived, and in general, five parameters govern the stress distribution in the overlap region. As expected, the major differences between the debond and the void occur for the stresses near the edge of the defect itself. Whether the defect is a debond or a void, is hardly discernible by the stresses at the overlap ends for central defect sizes up to the order of 70 percent of the overlap region. If the defect occurs precisely at or very close to either end of the overlap, however, differences of the order of 20 percent in the peak stresses can be obtained.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):539-544. doi:10.1115/1.2904325.

An experimental investigation was conducted using a CO2 laser to cut fibrous insulation blankets for the Space Shuttle and high speed aircraft. This investigation was unique in identifying important factors for controlling the cut depth, thus, allowing the laser to be used for making partial cuts in a composite quilt. The investigation determined an algorithm for essential physical parameters used in automating the laser machining of these materials.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):545-549. doi:10.1115/1.2904326.

During holographic inspection of glass fiber reinforcesd plastic (GRP) laminated plates, perturbation in the fringe patterns not only reveals the presence of an unbond but also enables estimation of its depth. The accuracy of estimation depends, among other factors, on the accuracy in determining the fringe orders within the boundary of perturbation. This study shows that phase shifting using a single reference beam enables determination of fractional fringe orders at any point on the test surface. When applied to the assessment of the detected unbond, this simple technique gives a more accurate estimation of the unbond depth than conventional holography.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):550-555. doi:10.1115/1.2904327.

Introducing a thin cut from the surface of a part containing residual stresses produces a change in strain on the surface. When the strains are measured as a function of the depth of the cut, residual stresses near the surface can be estimated using the compliance method. In previous work, the unknown residual stress field was represented by a series of continuous polynomials. The present paper shows that for stress states with steep gradients, superior predictions are obtained by using “overlapping piecewise functions” to represent the stresses. The stability of the method under the influence of random errors and a zero shift is demonstrated by numerical simulation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):556-560. doi:10.1115/1.2904328.

Residual stresses due to surface treatment are measured using the compliance method. The method makes use of the strains measured on the surface while a cut is extended progressively along a plane of interest. The experimental results for a shot peened specimen show good agreement with those obtained by the X-ray method. This experiment demonstrates that the compliance method is accurate and capable of measuring residual stresses which vary rapidly over a depth of less than 50 μm. Good general agreement with results by the X-ray method is also obtained for a laser treated specimen. Some advantages and disadvantages of the present method relative to hole-drilling, layer removal and X-ray methods are discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):561-566. doi:10.1115/1.2904329.

The semidestructive incremental hole-drilling method commonly used to evaluate residual stresses is exceedingly sensitive to experimental errors, with sensitivity increasing as hole depth increases. To determine stress variations through the engine thickness, it is necessary to use accurate drilling methods, as well as suitable mathematical models and procedures to minimize the errors associated with residual stress measurement. This work examines the effects of measurement errors on the evaluation of residual stresses with the integral method. An enhanced procedure for managing the experimental data is proposed that allows evaluation of the residual stresses with thickness variations.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):567-573. doi:10.1115/1.2904330.

The Waste Isolation Pilot Plant (WIPP) is a repository vault, mined deep into a salt strata. It eventually closes in on itself, encapsulating its contents. At room temperature salt may be regarded as a linear, isotropic, viscoelastic material. In this study, using triaxial compression test results on salt, we determine the relaxation functions and set up the boundary value problem for the encapsulation mechanism of a salt vault. Closure of the repository as a function of time is determined using a three-dimensional finite element model. The Tresca failure criterion is used to predict the stability of the repository. Finally, the study is validated by comparing our results to in-situ measured data.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(4):574-575. doi:10.1115/1.2904331.

The experimental drawing of SAE 1008 steel bars was performed for different reductions in area (r) and die semi-angles (α). The redundant deformation factor (φ) was evaluated for each case using two techniques: microhardness profiles and stress-strain curves superposition. The experimental values of φ obtained by the two techniques were not in agreement.

Commentary by Dr. Valentin Fuster

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