Actual Stress State Research Articles

Design Philosophy of Pressure Vessels for Service Above 10 ksi (70 MPa)

This paper discusses design criteria for pressure vessels intended for service above 10 ksi (70 MPa). Since the ASME Boiler and Pressure Vessel Code cannot, in a practical sense, be applied to the construction of such vessels, suitable supplementary and alternative criteria are proposed to serve as a design basis for such vessels. It is hoped that they can also serve as the basis for development of Design Rules for High Pressure Vessels by the ASME. Topics considered include: Use of yield strength criteria; Use of alternative fatigue criteria; Use of burst and plastic flow criteria; Alternative hydrostatic test pressure criteria; Criteria for prestressing; Material quality standards. These show that suitable criteria for high pressure vessel design require a higher level of precision than now exists in the Code. Higher levels of material quality and testing are needed to insure required minimum levels of properties without the need to compensate (by elements of the factors of safety) for excessive variation. More detailed and comprehensive analysis, to make the mathematical models more truly reflect the actual stress conditions than are usually used in Section VIII of the Code, are found to be necessary.

Fatigue Damage Analysis and Life Prediction of Laminated Metal Plates

A linear cumulative damage model, using the actual state of stress ahead of a crack in the center layer of a 3layer, steel-aluminum-steel sandwich material, is used to predict the fatigue life of the plate. The state of stress ahead of the crack tip is calculated from a finite-element method, and is monitored as the crack grows. The model also is applied to a splice joint consisting of aluminum base plates and either aluminum or steel splice joints. I. Introduction ¥JONDED sandwich laminates are being used widely in -•^various industries. Both primary and secondary structural components in aircraft and space vehicles will, in the near future, be made of such composites. They also have been successfully used in pipes, chemical tanks, ship hulls, and in other structural applications in which a high strength-toweight ratio is a desirable feature. Such materials provide a number of advantages. They can be designed for maximum load carrying efficiency by placing the high strength, high stiffness component in preferential locations. Bonding does not require rivet holes, which are stress raisers and may cause premature failure either under static or fatigue loading. In fact, it has been shown that the fatigue strength of a stiffened panel in an aircraft structure is considerably improved when the stiffeners are bonded to the panel. In the compression design of a stiffened panel, rivets provide discontinuous attachment between the metal sheets and the stabilizing stringers. As a consequence, interrivet buckling may occur causing lower strength for riveted structures. The bonding of damping materials to metal sheets, to form a sandwich structure, currently is being considered as an effective way to control noise-induced fatigue (sonic fatigue) of air frames. It is well-known that there are imperfections existing in composite materials. These imperfections may be caused by metallurgical defects, or may be produced during the lamination and fabrication process of the structural component. In analyzing the fatigue strength of the material, the localized imperfection may be idealized as a crack in the material. A method of predicting the fatigue life of a cracked, layered composite laminate by using the existing S-N curves of

Evaluation of the load-carrying capacity of anchors in rock

1. The proposed method of analysis of the load-carrying capacity of an anchor on the basis of the strength of the plug-rock interface makes it possible to take into account more adequately the actual stress conditions at this interface in comparison with the schemes based on an arbitrary uniform stress distribution. 2. Failure of the rock within the limits of the so-called inverted cone is a highly complex process, which necessitates special examination and analysis on the basis of solutions using numerical methods. In addition, because of the higher probability of loss of load-carrying capacity through failure of the plug-rock interface than of failure of the rock itself, the approximate analysis using the inverted cone scheme may be retained by applying the results of the tests on uplift resistance of slabs.

Mechanical model for a jointed rock mass

When a rock mass weakened by a system of parallel horizontal joints is loaded at the surface, some settlement, comprised of the displacement due to deformation of the weathered material and the result of drawing together of joint edges, may take place. For such a system, the simplest model consists of a composite specimen subjected to one-dimensional compression. The advantage of such a procedure lies in the transformation of the three-dimensional stress state, reflecting actual stress conditions in the reek, into a simple one-dimensional system. The physicaI phenomena which take place in the specimens may be mathematically represented, very conveniently, in the onedimensional case. Subsequently, resulu obtained by such a treatment could be extended to a three-dimensiona l system with the help of well known methods. Figure 1 presents the stress-strain relationships obtained by M. Pancini [1] on composite gypsum-coal specimens subjected to one-dimensional compression. The number of joints simulated during the tests was less than the number of blocks by one. It is obvious that the total deformation Ah may be determined from the equation

Friction under high normal pressures

Torsion tests ∗∗ ∗∗ Thesis — Estudio Experimental del Coeficiente de Rose a altas Presiones, by R. Lauterbach, Universidad Catolica de Chile,in partial satisfaction of the degree of Ingeniero Civil, 1966. of totally enclosed test pieces of electrolytic copper and commercially pure aluminum were made by a rotating hardened punch whose flat end rotated under load against the constrained surface of the softer metal. This method essentially prevented plastic deformation of the test piece, except under certain conditions of load at or below the test surface. Static coefficients of friction as function of average normal pressure were obtained from the load and angle of twist signals transmitted to the two legs of a Variplotter, thus giving a load-twist diagram for each test. It was found that the theory of sublayer flow agreed with the experimental results for wire-brushed surfaces of both electrolytic copper and commercially pure aluminum, but not for copper surfaces finished with grade 180 abrasive paper. This failure is believed to be due to the state of stress in the softer of the two sliders in the metal between two adjacent junctions near its surface. This combined state of stress is unknown if the closely spaced junctions are the result of surface preparation with 180 abrasive paper. In this case the assumed simple compression in determining the flow condition by the von Mises flow rule (equivalent to a critical effective stress) must be modified to include the actual combined state of stress. For lubricated surfaces the values of torque and coefficients of friction were below the theoretical values for sublayer flow at pressures more than twice the original yield strength. The results thus indicate that the condition for sublayer flow permits the prediction of a limiting friction force and coefficient of friction, providing the state of work hardening in the test surface can be determined.

PRACTICE EFFECTS ON THE TRAIL MAKING TEST.

After six administrations, the Trail Making Test shows practice effects which significantly reduce performance rime. These effects were noted in Part A following the first trial and in Part B following the third trial. Neither Part A nor Part B appeared sensitive to anticipated or actual stress conditions.