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RESEARCH PAPERS

J. Eng. Mater. Technol. 1988;110(4):297-304. doi:10.1115/1.3226053.

Residual stresses in a heat treated weld clad plate and test specimens obtained from the plate are determined using a combination of experimental residual stress analysis and a finite element computational model. The plate is 102 mm thick and made of A 533-B Class 2 steel with 308 stainless steel cladding. The plate is heated to 538 C and allowed to cool uniformly. Upon cooling, residual stresses are set up in the clad plate because of the difference between the coefficients of thermal expansion of the plate and the cladding. Residual stress in the clad plate is determined using both a previously verified experimental residual stress analysis technique and a computational model. Removing test specimens from the clad plate can relax the stresses in the cladding. Thus, residual stress distributions were also determined for two types of clad test specimens that were removed from the plate. These test specimens were designed to examine the effect of cladding thickness on residual stresses. Good agreement was found between the experimentally obtained residual stress values and the residual stresses calculated from the computational model. Because of the interest in tests conducted at elevated temperatures and the inherent difficulty in doing experimental residual stress analysis at elevated temperatures, the computational model was applied to examine the effect of elevated temperature on the residual stresses in the test specimens. Peak stresses in the heat treated clad plate were found to approach the yield stress of the cladding material. It was also found that removing a 32 mm clad specimen with cladding on one side reduced the residual stresses in the cladding. However, the residual stresses in the cladding were found to increase when one-half of the cladding thickness was machined away to form the second test specimen geometry. Residual stresses parallel and perpendicular to the weld direction were very similar in magnitude for all cases considered. The effect that heating the test specimens to 204 C has on residual stress distributions was to reduce the residual stress in the cladding and the plate.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):305-312. doi:10.1115/1.3226054.

Part distortion during sintering results from nonhomogeneous and nonisotropic shrinkage. Shrinkage is highly dependent on density and particle size distributions within a part, thus, “shrinkage factors” are not constant and may vary throughout the part. An energy based approach is presented which can be used to form accurate shrinkage laws for eventual use in a finite element solution of part shape change. This energy method can be easily modified to include the important effects of particle size variations, contact forces and irregular particle shape in the formation of accurate of shrinkage laws. Linear particle shrinkage and rotations due to contact stresses are examined using simple two particle geometries to test the validity of the approach. Energy methods give results which agree well with observation and the direct application of the diffusion equation, however, the advantage of this formulation over traditional diffusion based analysis will be fully realized in the eventual task of specifying time dependent shrinkage laws of realistic multi particle models.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):313-318. doi:10.1115/1.3226055.

The toughness of low alloy steel components can degrade in service due to temper embrittlement. This embrittlement is caused by segregation of impurity elements to the grain boundaries of the steel. Nondestructive techniques are needed to assess the extent of toughness degradation, since this information is vital in making run-retire decisions. EPRI has successfully completed evaluation of a chemical etching technique for determining the change in the fracture appearance transition temperature (Δ FATT) of rotor steels. Laboratory steels and core samples removed from actual rotors, with varying degrees of embrittlement, have been evaluated. Depths of grain boundary grooves, produced by chemical etching, correlate well with Δ FATT and with the grain boundary coverage of impurity elements as determined by Auger spectroscopy. To render the technique even more nondestructive, a 3-stage replica technique has been developed. Groove depths measured from replicas of etched samples also yield an excellent correlation with the Δ FATT of the samples.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):319-324. doi:10.1115/1.3226056.

The effect of laser welding on prevention of the fracture of cracked shafts of a high carbon steel are presented. Static tensile strength and rotary bending fatigue strength were obtained using shaft specimens of AISI W1 which were welded by a CO2 laser around a precrack. Since the welded region became extremely hard and brittle, both the strengths of laser-welded specimens were lower than those of non-welded specimens. However, the strengths were increased higher than those of non-welded specimens after tempering the specimens. It was found that when tempered at 600°C after laser welding, the rotary bending fatigue strength of specimens with a crack smaller than 12 mm rose similar to that of the base metal, and when tempered at 700° C, the static tensile strength of specimens with a crack smaller than 12mm rose similar to that of the base metal. Therefore, it was shown that the laser welding is very effective to prevent fracture of high carbon steels.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):325-331. doi:10.1115/1.3226057.

An experimental method is described whereby the dynamic fracture initiation toughness of ceramics and ceramic composites can be measured in pure tension or pure torsion at stress intensity factor rates of 105 to 106 MPam/s. In this procedure, circumferentially notched cylindrical rods are subjected to uniaxial cyclic compression at room temperature to introduce a self-arresting, concentric Mode I fatigue pre-crack, following the technique presented by Suresh et al. (1987) and Suresh and Tschegg (1987). Subsequently, dynamic fracture initiation is effected by stress wave loading with a sharp-fronted pulse which subjects the specimen to a dynamic load inducing either Mode I or Mode III fracture. Instrumentation appropriate to the loading mode provides a record of average stress at the fracture site as a function of time. The capability of this method to yield highly reproducible dynamic fracture initiation toughness values for ceramics is demonstrated with the aid of experiments conducted on a polycrystalline aluminum oxide. The dynamic fracture toughness values are compared with the results obtained for quasi-static Mode I and Mode III fracture in the ceramic material at stress intensity factor rates of 10−1 to 1 MPam/s. Guidelines for the dynamic fracture initiation testing of ceramics and ceramic composites are discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):332-337. doi:10.1115/1.3226058.

The variation in Young’s modulus and Poisson’s ratio of metals during uniaxial deformation is examined. Predictions are made on the basis of a nonlinear description of the constitutive relation between stress and strain involving both second- and third-order elastic constants. A comparison of measured and predicted variations shows good agreement, with the relative change in Young’s modulus being on the order of 10 times the elastic strain for most metals. The material nonlinearity is then examined as a possible cause for changes in the natural frequency of a beam subject to an axial residual stress. An upper bound for the frequency change involving the maximum stress, initial Young’s modulus and a nonlinearity parameter is derived.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):338-343. doi:10.1115/1.3226059.

The Incremental Strain, Average Stress, Power Series, and Integral methods are examined as procedures for determining non-uniform residual stress fields using strain relaxation data from the hole drilling method. Some theoretical shortcomings in the Incremental Strain and Average Stress methods are described. It is shown that these two traditional methods are in fact approximations of the Integral Method. Theoretical estimates of the errors involved are presented for various stress fields. Also, some simple transformations of stress and strain variables are introduced so as to decouple the stress/strain equations and simplify the numerical solution.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):344-349. doi:10.1115/1.3226060.

The Integral Method for calculating non-uniform residual stress fields using strain relaxation data from the incremental hole-drilling method is examined in detail. Finite element calculations are described which evaluate the calibration coefficients required for practical applications of the method. These calibration data are tabulated for a range of hole sizes and depths. It is found that the hole drilling method is not well adapted to measuring stresses remote from the surface, and a theoretical depth limit for stress measurements of 0.5 of the mean radius of the strain gauge rosette, rm , is identified. A practical depth limit is in the range 0.3–0.4 rm .

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):350-354. doi:10.1115/1.3226061.

This paper describes a method of estimating the fatigue life of adhesively bonded lap joints on the basis of the stress analysis in adhesive layer with finite element method. First, cyclic tensile fatigue tests were conducted for adhesively bonded lap joints with different lap length and adhesive layer thickness. The results were evaluated from the viewpoint of the maximum values of both tensile and shear stress obtained numerically, instead of the apparent stress. Then these standardized fatigue strength were compared with those of adhesively bonded butt joints of a thin wall tube under cyclic tensile and fully reversed torsional load conditions. The results indicate that fatigue strength of lap joints evaluated from the maximum tensile stress of the adhesive layer agrees well with the fatigue strength of adhesively bonded butt joints of thin wall tube under cyclic tensile load condition. It is confirmed that fatigue strength of lap joints can be estimated adequately based on the fatigue strength of the butt joint of thin wall tube and the numerical results for the stress state of adhesive layer.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):355-360. doi:10.1115/1.3226062.

A theoretical attempt to clarify the reason why the compacts of powder media have uniform density distributions as the density of the compacts becomes high, is made for the compaction of the copper powder medium of a simple type by punch impaction. Based on the one-dimensional equation of motion including the effect of die wall friction force, there are two main factors which influence the density distribution of the medium during the compaction process; one is the propagation of the shock wave passing through the medium, while the other is the friction force between the circumferential surface of the medium and the die wall. The equation reveals that the effect of the force increases little as the density becomes high as a result of the repetitive traveling of the shock wave between the punch and plug. The propagation or more definitely the repetitive traveling, on the other hand, increasingly unformalizes the density distribution during the process as the number of the traveling increases. Owing to the aforementioned effects of the two factors on the density distribution during the process, the high density compacts become uniform.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):361-363. doi:10.1115/1.3226063.

The article presents strain distributions of oxygen-free copper up to finite natural strains of 1.53. The drawing operations were performed at room temperature with a split specimen having a gridded cross section for revealing the deformation pattern. It was found that the final effective strain distribution was nearly constant except at the center of the bar, where it was lower.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):364-371. doi:10.1115/1.3226064.

To formulate the constitutive equations for cyclic plasticity, the subsequent yield surfaces should be examined carefully. In this paper, the subsequent yield surfaces have been examined from the experiment for the initially isotropic material of SUS304 subject to cyclic loading, using a 50µm/m offset strain criterion for yielding probed at the current center of the yield surface. The experiment shows a translation, distorsion, and rotation of the subsequent yield surfaces because of the deformation-induced-anisotropy due to proportional or nonproportional cyclic loading in tension-torsion space. These yield surfaces could be represented by the quadratic function of stresses with fourth rank anisotropic coefficient tensor components. These anisotropic coefficient tensor components are found to be represented by the strain amplitude of cyclic loading. As a result, the loading function obtained shows availability to derive the constitutive equations of cyclic plasticity.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):372-379. doi:10.1115/1.3226065.

A mathematical model was developed for calculating the temperature, stress-strain field, and their interaction in a metallic material undergoing phase transformation during quenching processes. The model considers the fully coupled thermomechanical field equations, with coupling through the dilatational work, the plastic work, and the heat release during phase transformation in the energy equation. Computations were performed for a hollow steel cylinder. Several test problems were evaluated to make certain that the computer model worked properly. Final results demonstrate qualitative agreement with experiments. They also reveal detailed descriptions of material response.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):380-388. doi:10.1115/1.3226066.

Two materials, an Inconel 718 and a 1045 steel, are used to verify the extension of a shear strain-based parameter developed to account for out-of-phase cyclic strain hardening to multiaxial mean-stresses. Shear strain amplitude on the maximum shear strain amplitude plane and the maximum stress normal to this plane are the nominal stress-strain parameters considered in this approach. Tension-torsion and axial-internal pressure loadings using tubular specimens are employed to investigate stress-strain states that exhibit mean-strains and/or mean-stresses. Deformation response relevant to the proposed fatigue damage algorithm such as mean-stress relaxation is discussed. Adequate fatigue life correlations are obtained by implementing the proposed analysis. It is also demonstrated that methodologies successful for correlating uniaxial mean-stress data often lead to erroneous multiaxial life predictions.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):389-394. doi:10.1115/1.3226067.

Quantitative relationships have been obtained between restraining force and shape deviations, such as springback and side wall curl, in flanged channels made of SKDQ and high strength streets. It is observed that shape deviation is greatly reduced if the applied restraining force is beyond the yield strength of the material. However, due to the restriction of die entrance and punch corners, this condition cannot be readily achieved in the conventional bead system as side wall fracture intervenes. To circumvent this constraint, a so-named “intermediate restraining” process has been developed to form high quality flanged channels in one single operation. The suggested process is displacement controlled and, in practice, should be applicable to any material once its physical properties are known.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):395-400. doi:10.1115/1.3226068.

The effects of material strain-rate dependence on necking and neck propagation in polymeric fibres are analyzed. Using a finite element scheme together with an elastic-viscoplastic constitutive model the entire load-deformation histories of round specimens are computed. Stress-strain distributions as well as the evolution of the specimen profiles are given at various stages of the axisymmetric stretching process. Comparisons with rate-independent response are also presented.

Commentary by Dr. Valentin Fuster

BOOK REVIEWS

J. Eng. Mater. Technol. 1988;110(4):401. doi:10.1115/1.3226069.
FREE TO VIEW
Abstract
Topics: Polymers
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(4):401-402. doi:10.1115/1.3226070.
FREE TO VIEW
Abstract
Topics: Reliability
Commentary by Dr. Valentin Fuster

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