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RESEARCH PAPERS

J. Eng. Mater. Technol. 1991;113(2):187-191. doi:10.1115/1.2903390.

Many metal forming operations, such as rolling and tube drawing, are known to induce orthotropic anisotropy. The change of axes of orthotropy with subsequent deformation has been studied in this paper. The change in the orthotropy directions is of great importance for understanding and interpreting the subsequent yield behavior of metals. Based on Hill’s hypothesis that the orthotropy axes coincides with the principal directions of stretch, the change in orthotropy directions has been studied theoretically and experimentally. Since the grain shape and its direction of elongation is a good indicator of the principal stretches and its directions, it has been used as an experimental means of determining, not only the directions of principal stretches in an as received material, but also to determine approximately the deformation it has undergone so far from a reference state. A fully annealed isotropic state is chosen as the reference state. The directions of the axes of anisotropy, induced as a result of finite deformation applied to this reference state, are characterized in terms of the principal directions of the Cauchy’s deformation tensor. An experimental scheme has been developed to determine the varying directions of orthotropy for comparison with the theoretical model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(2):192-198. doi:10.1115/1.2903391.

This paper presents a set of experimental results concerning the evolution of yield loci for anisotropic materials in the course of finite shear deformation history. Thin walled tubes of SAE 1020 steel are tested under triaxial loading conditions – tension, torsion and internal pressure, at room temperature. The specimens were prepared from cold-drawn tubes which had a prominent fibrous grain structure. They showed considerable amount of in-plane anisotropy and orthotropic yield properties with respect to the longitudinal and circumferential axes. The as-received specimens were further pre-strained by imposing simple shear which causes the change in orientations of these elongated polycrystalline grains. Our previous experimental results [1] have verified that the principal directions of the total stretch of the current grain shape match with the directions of orthotropy, and in this work, the subsequent yield loci for the pre-strained material are determined with respect to the new reference frame fixed along these directions of orthotropic symmetry. By utilizing the unique triaxial loading scheme of our servo-hydraulic testing system it has been possible to create the principal directions of a biaxial state of stress in a specified direction. The changes in the size and shape of the yield loci in the stress space are critically analyzed in this paper.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(2):199-204. doi:10.1115/1.2903392.

The use of thin cuts for residual stress measurement is referred to as the crack compliance method. A computational model is presented for the determination of normal and shear residual stresses near the surface by introducing shallow cuts. The optimum regions for strain measurement are obtained. This method is shown to be considerably more sensitive than the conventional hole drilling method and is capable of measuring residual stresses which vary with depth below the surface.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(2):205-209. doi:10.1115/1.2903393.

This first part of our study is concerned with the theoretical and variational formulations of the problem of elastic cyclic loading at very high frequency (or acoustic fatigue). The problem is treated by using the theory of longitudinal thermoelastic wave motion in a finite medium with and without running crack. Two methods are used to formulate the evolution problem: the first one deals with the use of classical time integration schema, and the second uses the Fourier transformation to solve the evolution problem in the frequency space. Comparison of our results with some closed form solutions of some classical problem is presented. In a second paper this method is used to calculate thermo-mechanical fields in specimens used in ultrasonic fatigue tests (endurance tests and crack growth tests).

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(2):210-214. doi:10.1115/1.2903394.

This work deals with the experimental study of the so-called “ultrasonic fatigue” or fatigue at very high frequency (some kHz). After describing experimental facilities such as specimen shape, testing machine and measured parameters, we use the numerical method developed in Part I of this study to calculate thermo-mechanical fields in cracked and uncracked specimens. Prediction of S-N curves and thresholds for some different materials are also presented and discussed using mechanical fields obtained numerically.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(2):215-221. doi:10.1115/1.2903395.

The high cooling rates normally encountered in the application of high intensity welding processes such as laser beam welding is often detrimental to the weldment, especially for high hardenability steels. To minimize this effect, the split-beam laser welding concept is proposed and analyzed. The analysis shows that even when the intensity of the single heat source is the same as the intensity of each of the dual heat sources, the resulting cooling rate at any specific temperature is lower for the dual source process than the single source process. For example, for mild steel, the cooling rate at a point 25 mm behind the heat source (where the temperature is 1364°C) was determined to be 382°C/s for the single source system, while that for a point 40 mm behind the major source (where the temperature is 1377°C) was determined to be 206°C/s for the dual heat source system. When the heat inputs for the dual system are reduced such that the total heat input is equal to that of the single source system, the resulting temperature rise is lower at all points of the weldment for the dual system. That also means a smaller weld pool size and heat affected zone. To maintain the same weld pool size and penetration as for the single heat source system then requires an increased total heat input for the dual heat source system, with the additional input depending on the spacing between the two heat sources.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(2):222-227. doi:10.1115/1.2903396.

In this paper, a direct approach based on the mixed mode Dugdale model, the accumulated plastic displacement criterion for crack propagation and the cyclic J-integral concept is used to develop equations to predict the mixed mode fatigue crack growth. A fourth-power stress intensity factor crack growth equation and a second-power cyclic J-integral equation are developed under conditions of mixed mode loading and small-scale yielding. By comparing the present analysis with the experimental results, the proposed fatigue crack growth rate equations lead to results that appear to yield reasonable magnitudes for various loading conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(2):228-235. doi:10.1115/1.2903397.

In this paper, a simple theoretical analysis of an old problem is presented. The analysis is more complete than earlier versions, but retains the mathematical simplicity of the earlier versions. The major thrust is to separate the material response into two phases. The first phase is dominated by strain rate effects and has a variable plastic wave speed. The second phase is dominated by strain hardening effects and has a constant plastic wave speed. Estimates for dynamic yield stress, strain, strainrate, and plastic wave speed during both phases are given. Comparisons with several experiments on OFHC copper are included.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(2):236-243. doi:10.1115/1.2903398.

The formability and fracture behavior of a high strength, aluminum alloy made to (French) specification AFNOR 7020 is reported. The formability study included the evaluation of the room temperature forming limit diagram (FLD) of the material in the T4 temper and an analysis of the strain distribution profiles obtained in punch stretching that involved different stress states. The fracture surfaces were examined by scanning electron microscopy and a correlation between the forming and fracture behavior could be obtained.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(2):244-253. doi:10.1115/1.2903399.

This paper describes the biaxial low cycle fatigue of unaged and aged 1Cr-1Mo-1/4V rotor steels at 823 K in air. Combined push-pull and reversed torsion tests were carried out on these steels and the biaxial fatigue data were obtained. Aging significantly reduced the hardness of the steel but had no effect on the crack direction in biaxial low cycle fatigue. Aging also had no effect on the data correlation using the biaxial strain parameters, but had a significant effect on the data correlation using the biaxial stress parameters. The change in the effective stress parameter in correlating the biaxial fatigue data due to aging is discussed in connection with the material softening.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(2):254-262. doi:10.1115/1.2903400.

The hardening behavior of materials in nonproportional cyclic process is related to the internal changes of materials, such as dislocation cell for wary slip material and ladder or vein substructures for planar slip material. The multiplicatively separated form of hardening function f, in terms of nonhardening region proposed by Ohno [1], and the measure of nonproportionality A proposed by Banallal and Marquis in 1987 [2], is then explained on this physical foundation. The new contributions of this hardening function are: (a) two parameters (f2 and f3 ) dependent on A are used to differentiate between the influence of latent hardening realized by a sudden change of loading direction, and hereditary hardening associated with nonproportional loading, (b) a general differential form fi (i = 1,2,3) is proposed, and memorial parameters a1 and a3 are introduced to describe different deformation history effects for wary and planar slip materials, (c) different hardening mechanisms through fi are embedded into thermomechanically constitutive relation. The stress responses of 304 and 316 stainless steels subjected to biaxial nonproportional loadings at room temperature are analyzed and compared with the experimental results obtained by Chaboche [3], Tanaka [4, 5] and Ohno [1].

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(2):263-270. doi:10.1115/1.2903401.

Assessment of the remaining life of steam turbine rotors in the presence of bore defects requires a knowledge of the fracture toughness (KIC ) of the rotors. Current procedures for estimating the KIC involve two steps; as a first step, the fracture appearance transition temperature (FATT) at the critical location is determined; the FATT value is then used to estimate the KIC , based on published correlations between the excess temperature (T-FATT) and KIC . Some problems arise in implementing both of these steps. To determine the FATT of the material, large pieces of material have to be removed, machined into charpy specimens and tested; this procedure is often time consuming and expensive and sometimes not feasible. The excess temperatures versus the KIC correlation that is used to derive the KIC values from the FATT data is based on a variety of low alloy steels and is therefore characterized by a large scatter band, thus leading to considerable uncertainty in the estimated KIC . In this work, FATT and KIC data reported for a number of retired CrMoV rotors were gathered and analyzed and correlations specific to CrMoV rotors were developed. Based on these correlations, a method for estimating KIC with greater accuracy, based on a knowledge of the steel chemistry alone, is proposed. The method offers the advantage that very small samples removed from noncritical locations in the rotor would be sufficient to get the desired data.

Commentary by Dr. Valentin Fuster

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