J. Eng. Mater. Technol. 1991;113(3):271-279. doi:10.1115/1.2903405.

High temperature crack growth behavior is investigated over a wide range of R-ratios, frequencies, and temperatures in Alloy 800H. It is found that high R-ratio, low frequency, or high temperature can enhance creep damage and thus induce an intergranular crack growth mode. At low frequencies, the nonlinear fracture mechanics parameter, C*, is found to correlate time-dependent fatigue crack growth rate well if the applied mean stress is used in calculating C*. On the other hand, the Paris crack growth law using K eff is proven to be an adequate expression to use when fatigue (time-independent) damage dominates. These conclusions correlate well with damage mechanisms observed from sample fracture surfaces.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(3):280-284. doi:10.1115/1.2903406.

A new method is proposed for analyzing the stress intensity factors of multiple cracks in a sheet reinforced with riveted stiffeners. Using the basic solution of a single crack and taking unknown density of surface tractions and fastener forces, Fredholm integral equations and compatibility equations of displacements among the sheet, fasteners, and stiffeners are formulated. After solving the unknown density, the stress intensity factors of multiple cracks in the sheet are determined. Some numerical examples are analyzed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(3):285-291. doi:10.1115/1.2903407.

This paper presents one general theory of large elastic deformations of a rubber sphere in simple compression, as the removal of restrictions of the constant Young modulus and small deformation in the prevailing Hertzian theory in contact of elastic bodies. It derives a set of five equations associated with approach, radii of contact surface without and with lateral extension of free surface, the lateral extensive displacement on the contact surface and the position of the contact surface in a very large range of applied forces, on the basis of the Hertz theory (half-space elastic body model) with an extensive term, in consideration of the rubber-elastic nonlinear elasticity, the lateral extension and the symmetry of the deformed shape of the rubber sphere. In Part 2 it is shown that results calculated by the set of the equations fit experimental data for a rubber sphere.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(3):292-295. doi:10.1115/1.2903408.

This paper presents experimental results of simple compression of a soft rubber sphere in a very large range of forces attaining at 5000 N, presenting calculational results by a set of five equations presented in Part 1. The calculational values of approach, the radius of contact surface, and lateral expansion agree well with the experimental data in the large range of deformations. It is thus verified experimentally that the set of the equations (12), (13), (31), (40), and (43) in Part 1 is approximately valid in large deformations for rubber sphere. Program using a personal computer in calculating five quantities from the five nonlinear equations associated with the five quantities is noted.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(3):296-302. doi:10.1115/1.2903409.

An analysis of the flow patterns and associated solidification phenomena during the die casting process is presented. In the filling stage of the die cavity, the flow patterns are predicted using the volume of fluid method. These predictions are compared with experiments using water for a variety of geometries including an obstacle within the die cavity and multiple gates. In the solidification analysis, an enthalpy technique on a fixed grid is used to model the phase change with convection and diffusion occurring after the cavity is filled with liquid aluminum. A feature of this model is the prediction of the shape of the liquid-solid interface. Emphasis is placed on the effect of velocities from the residual flow field on this interface.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(3):303-306. doi:10.1115/1.2903410.

Experimental investigations have been carried out to study the creep crack growth in types 316, 308 Cb, and 304 L stainless steel in the temperature range of 873–1073 K under plane stress condition. Testings have been carried out with both the base metal and the welded composite joints, because such joints are commonly used in nuclear power industries. Among the various parameters tried to correlate the creep crack growth, the energy rate line integral has been found to give the best description of the crack growth rate. The steady-state energy rate line integral has been found to correlate well with the rupture time. Based on this observation, life estimations are presented for thin components containing various initial defect sizes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(3):307-313. doi:10.1115/1.2903411.

In this paper, single and multiple overloading tests under cyclic controlled stress were carried out on specimens of Worm-like Graphite Cast Iron (WGCI), a brittle material. The results showed that the overloading had a strong retardation effect on this material when the overload ratio r (ratio between the overload range and the baseline load range) is larger than 1.2. For example, the number of cycles of retardation due to overloading, N’, may be as high as ten times the cycles, N, required for the same crack length under baseline loading. In addition, for the testing conditions examined, there is no significant difference in the retardation effect under single or multiple overloading for this material. An analytical model based on the concept of crack closure and the effective stress intensity range was developed for evaluating the effect of the retardation. The development led to a correlation between the number of applied cycles and the crack size after overloading and permits to calculate the retardation cycles N’. Finally, the mechanisms attributed to the retardation effect for WGCI are discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(3):314-323. doi:10.1115/1.2903412.

In this part a rate-dependent elastic-plastic constitutive model is presented which is an extension of our earlier rate-indpendent model. The effect of prior creep on the subsequent inelastic deformation is also included. The model can be used to predict inelastic processes with variable strain (stress) rates. It is shown, through comparison with the experimental results, that most of the rate-effect features of the material response can be simulated by the model. Despite the wide range of application, the model is relatively simple and incorporated a few material constants which could be easily determined from standard tests.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(3):324-328. doi:10.1115/1.2903413.

In this part, a constitutive model for creep deformation is presented, which includes the effect of prior plastic straining. The model is capable of predicting the influence of plastic or creep prestrain and step loading on the subsequent creep deformation as well as cross effect between prestrain and creep strain directions in a multiaxial stress state. The predictions of the model are compared with the experimental data and the agreement is found to be very good. This model, in conjunction with that presented in Part I, would enable one to predict material response to complex loading sequences in the creep-plasticity range.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(3):329-335. doi:10.1115/1.2903414.

A long-term laboratory study has confirmed that temper embrittlement (TE) in 2-1/4 Cr-1 Mo plate, forgings, and weldmetals levels off over time. Fifty-five heats of 2-1/4 Cr-1 Mo steel from several vendors were aged for up to 75,000 hours at 427 to 482°C (800 to 900°F), and extent of embrittlement was determined from shifts in 54 J (40 ft-lb) transition temperature for each heat. Embrittlement levelled off within 35,000 hours at 427°C (800°F) and 454°C (850°F), and within 10,000 hours at 482°C (900°F). Plate material embrittled least, while submerged arc and shielded metal arc weldments embrittled most. Although steel chemistry has previously been used to predict TE susceptibility, our analysis of these 55 heats, and of 23 heats of 2-1/4 Cr-1 Mo in a 1982 API study, shows that a step-cool treatment simulating long-term exposure is a more reliable way to screen susceptible materials.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(3):336-343. doi:10.1115/1.2903415.

A three-dimensional transient thermomechanical analysis has been performed for the Gas Metal Arc Welding process using the finite element method. Because the heat generated due to elasto-visco-plastic straining in welding is negligible in comparison to the arc heat input, the thermomechanical analysis is uncoupled into two parts. The first part performs a three-dimensional transient heat transfer analysis and computes entire thermal history of the weldment. The second part then uses results of the first part and performs a three-dimensional transient thermo-elastoplastic analysis to compute transient and residual distortions, strains and stresses in the weld. The thermomechanical model incorporates all the thermophysical and mechanical properties of the material as functions of temperature. Boundary conditions used in the numerical simulation are quite general and are matched with the experiment carried out to measure transient strains in the mild steel (0.22 percent carbon steel) weld. Good qualitative agreement was achieved between calculated and measured transient strains.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(3):344-349. doi:10.1115/1.2903416.

This paper provides an appraisal of a recent analysis for estimating the plastic component of the deformation J integral for a stationary crack. Such an estimation is a prerequisite when describing ductile crack extension in nuclear reactor pressure vessel materials. The assumptions in the analysis are emphasized, and the approximations arising as a consequence of introducing these assumptions are demonstrated.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(3):350-353. doi:10.1115/1.2903417.

A preceding paper reported the results of biaxial strain measurements at the roots of notched aluminum specimen subjected to monotonic tension loading. The specimens had different amounts of constraints at the notch root, and it was shown that a modified version of the Neuber relation gave some improvement in its predictive capability. Several of those specimens were also subjected to fully reversed cyclic loading until microcracks formed at the notch roots, and the results of those biaxial strain measurements are reported here. The modified Neuber relation in the cyclic form was used to predict the strains at the notch roots. Reasonably close agreement between the predicted and the measured load-strain loops was found for all three levels of constraint.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1991;113(3):354-362. doi:10.1115/1.2903418.

The general characteristics of surfaces machined with abrasive-waterjets (AWJs) are discussed in terms of surface texture and surface integrity. Data are presented from several studies involving different machining operations, including cutting, milling, turning, and drilling, and various advanced materials. The effects of AWJ parameters on surface texture effects, such as waviness, kerf taper, and burr height, in thin sheet metals typically used in the aerospace industry are presented and discussed. Other surface texture parameters, including surface finish and lay, are also addressed for selected machining applications such as milling and turning. Abrasive particle size is found to be the dominant parameter affecting the surface finish. The surface integrity effects are defined as particle deposition, delamination, gouging, microstructural changes, cracking, chipping, work hardening, and heataffected zones. Particle deposition occurs in the machining of ductile materials, but deposited particles can easily be removed by cleaning. Minor work hardening may result but can be totally eliminated if the dominant material removal mechanism is the cutting wear mode, which prevails under certain jet and traverse conditions. Thus, although the machining of advanced materials with AWJs may result in minor but controllable surface texture effects, it does not generally affect the integrity of the material.

Commentary by Dr. Valentin Fuster

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