Combined Mechanical Loading Research Articles

Modelling of combined high-temperature creep and cyclic plasticity in components using continuum damage mechanics

A computer-based finite-element viscoplastic damage solver has been developed for the analysis of structural components subject to combined cyclic thermal and mechanical loading. The solver, which is based on continuum damage mechanics, is able to predict the combined evolution of creep and cyclic plasticity damage by solution of the combined boundary-initial value problem. The computational difficulties which arise due to the different timescales, associated with material behaviour at the different temperatures within a thermal field, have been overcome by the use of a normalization technique. The high computer demands associated with the detailed numerical solution of combined thermo-mechanical problems, which have prevented their use in design, have been overcome by the development of a novel ‘cycle jumping’ method which avoids repetitive calculations over those cycles in which the damage fields change insignificantly. Between the ‘jumps’ in the solution technique, the damage and the strain rate fields are coupled and hence allow stress redistribution. The finite-element solver has been used to successfully predict the high temperature behaviour of a slag tap component subjected to cyclic thermal loading generated by infrared heaters and water cooling ducts. The initiation of damage and micro-cracking has been found to occur early in the lifetime at approximately 3000 cycles adjacent to the cooling duct; and the propagation of failure zones has been found to stabilize at 60 000 cycles after which no further damage evolution occurs. A further development of the technique, which requires even less computer resource, is the ‘cycle leaping’ method which neglects the effect of stress redistribution over large numbers of cycles, by leaping from the first few cycles to the final state. With this method good predictions have been made of the fields of damaged and micro-cracked material in the final state of the slag tap component. The technique has potential for use at the early or conceptual stages of design.

Crack growth induced by thermal-mechanical loading

Advanced aerospace structures are often subjected to combined thermal and mechanical loads. The fracture-mechanics behavior of these structures may be altered by the thermal state existing around the crack. Hence, design of critical structural elements requires the knowledge of stress-intensity factors under both thermal and mechanical loads. This paper describes the development of an experimental technique to verity the thermal-stress-intensity factor generated by a temperature gradient around the crack. Thin plate specimens of a model material (AISI-SAE 1095 steel) were used for the heart transfer and thermal-mechanical fracture tests. Rapid thermal loading was achieved using high-intensity focussed infrared spot heaters. These heaters were also used to generate controlled temperature rates for heat-transfer vertification tests. The experimental results indicate that thermal loads can generate stress-intensity factors large enough to induce crack growth. The proposed thermal-stress-intensity factors appear to have the same effect as the conventional mechanical-stress-intensity factors with respect to fracture.

Fracture mechanics for piezoelectric ceramics

We Study cracks either in piezoelectrics, or on interfaces between piezoelectrics and other materials such as metal electrodes or polymer matrices. The projected applications include ferroelectric actuators operating statically or cyclically, over the major portion of the samples, in the linear regime of the constitutive curve, but the elevated field around defects causes the materials to undergo hysteresis locally. The fracture mechanics viewpoint is adopted—that is, except for a region localized at the crack tip, the materials are taken to be linearly piezoelectric. The problem thus breaks into two subproblems: (i) determining the macroscopic field regarding the crack tip as a physically structureless point, and (ii) considering the hysteresis and other irreversible processes near the crack tip at a relevant microscopic level. The first Subproblem, which prompts a phenomenological fracture theory, receives a thorough investigation in this paper. Griffith's energy accounting is extended to include energy change due to both deformation and polarization. Four modes of square root singularities are identified at the tip of a crack in a homogeneous piezoelectric. A new type of singularity is discovered around interface crack tips. Specifically, the singularities in general form two pairs: r1/2±iεand r1/2±iε, where ε. and k are real numbers depending on the constitutive constants. Also solved is a class of boundary value problems involving many cracks on the interface between half-spaces. Fracture mechanics are established for ferroelectric ceramics under smallscale hysteresis conditions, which facilitates the experimental study of fracture resistance and fatigue crack growth under combined mechanical and electrical loading. Both poled and unpoled fcrroelectrie ceramics are discussed.

Finite element validation studies of the revised PD6493/CEGB R6. Part 1—Failure assessment methodologies applied to welded flat plates and pipe/plate joints

Two-parameter failure assessment diagrams (FADs) are now widely used for the assessment of the significance of defects at different levels of complexity. Systematic investigations have been carried out for the validations and verifications of the FADs and the plasticity correction factors for taking into account the residual stress effects. FADs, for a wide range of welded structures under the effect of mechanical loading and combined mechanical loading and residual stresses, have been established using fracture mechanics data generated using finite element techniques. The safety margins and non-conservatism of the FADs and the plasticity correction factor have been quantitatively examined and modifications have been recommended where necessary and possible for these geometries.

Finite element validation studies of the revised PD6493/CEGB R6: Part 2—Failure assessment methodologies applied to welded tubular joints

Two-parameter failure assessment diagrams (FADs) are now widely used for the assessment of the significance of defects at different levels of complexity. Systematic investigations have been carried out for the validations and verifications of the FADs and the plasticity correction factors for taking into account the residual stress effects. FADs for a wide range of welded structures under the effect of mechanical loading and combined mechanical loading and residual stresses have been established using fracture mechanics data generated using finite element techniques. The safety margins and non-conservatism of the FADs and the plasticity correction factor have been quantitatively examined and modifications have been recommended where necessary and possible for these geometries.

Thermal downshock fatigue testing in AISI 316 stainless steel plate

Observations of mode I fatigue crack growth are presented for 316 stainless steel under combined thermal and mechanical loads, applied to a flat plate with coolmg along one or two edges. Experimental propagation rates obtained at different tempesatures are compared with isothermal crack growth data by usmg the range of strsm mtenslty factor. Steady mechanical end loads accelerate cracks, but arrest lS still observed under repeated downshocks, thereby preventing complete fracture of the plates

Some notes on J-integral under combined thermal and mechanical loading

Some notes on J-integral under combined thermal and mechanical loading

Kinetics of the deformation of specimens in combined thermal and mechanical loading. Communication 2. Results of calculation

Kinetics of the deformation of specimens in combined thermal and mechanical loading. Communication 2. Results of calculation

Tensile behaviour of hardboard under combined mechanical and moisture loading

Tests on tempered hardboard in tension were made with the purpose to study the effect of combined mechanical and moisture loading (mechano-sorption). The tests were performed both in a conventional way with constant load during the test period, and in quasirelaxation with the strain of each specimen prescribed during the test. In both cases the moisture content was varied. The tests show that the mechano-sorptive effects in the studied hardboard are small under constant load. In the relaxation tests the effects was found to be more significant. A constitutive model was suggested and quantified on the basis of test data from the constant load experiments. It was then checked independently against the other tests. The model was found to give good agreement with creep type experiments. When the model was checked against the relaxation type experiments, however, it showed poor agreement.

The numerical assessment of elastic-plastic sheets under variable mechanical and thermal loads using a simplified two-surface yield condition

A finite element method for the shakedown-assessment of sheets under combined thermal and mechanical loads is presented. The generalized standard material model is adopted allowing for a certain class of kinematical hardening materials; it is realized by a simple two-surface yield condition. Sample calculations illustrate the method.

Kinetics of the deformation of specimens in combined thermal and mechanical loading. Communication 1. Strain-kinetic criterion in combined thermal and mechanical loading

Kinetics of the deformation of specimens in combined thermal and mechanical loading. Communication 1. Strain-kinetic criterion in combined thermal and mechanical loading

Post-yield fracture mechanics analysis of the combined thermal and mechanical loading of a centre-cracked plate

The elastic-plastic stress analysis of a centre-cracked plate subjected to two types of loading is considered using the non-linear BERSAFE finite element system. The loading includes both uniaxial tension and a quadratic thermal gradient across the plate, both of which independently would tend to open the crack. In the present work the combined effect of these two load forms is investigated by applying one after the other using the restart facility. In the case of the thermal load followed by the tensile load, several restarts of the tensile part were made at successively lower thermal loads. Also, both combinations of load application were applied to a plate initially uncracked, then the crack was opened from the loaded state by using a nodal load release technique. In all cases, path independent contour integrals were calculated to assess the fracture conditions at the crack tip under the above styles of loading, particularly with a view to supplying results which can be compared to proposed CEGB fracture procedures.

Thermal and Structural Analysis of Ring Components by the Finite Element Method

in order to determine the effects of a shift in the electron vertical orbital plane, and the resulting displacement of the wiggler photon beam striking the inner surface of the vacuum chamber. Since then, due to some changes in the physical parameters of the vacuum chamber, a new design had been developed. The new geometry is quite different from the one described in [I], due primarily to an increase in the chamber vertical aperture which was opened to 23 mm. from an initial opening of 6 mm. Subsequently, a new study was conducted for this new chamber design that included a structural analysis in order to determine the stresses and deflections of the vacuum chamber due to the combined thermal and mechanical loading. It should be noted that various geometrical models were ana.lyzed before the final geometry was decided. This report will only describe in detail the results of the thermal and structural analysis of the final geometry using the finite element code, ANSYS[S].

Strength of magnesia refractories in combined thermal cyclic and mechanical loading of them

As the result of laboratory tests of periclase-chromite refractories produced by different methods (MKhS, MKhV, and MKhVP) under the combined action of thermocyclic and mechanical loads possibilities were revealed of the most effective use of them in relation to the service conditions of plasma and electric arc furnace linings. The results of production tests of MKhS, MKhV, and MKhVP refractories at various furnace lining operating temperatures agree with the laboratory data and make it possible to draw the following conclusions: MKhS parts, as the most heat resistant, may be used successfully in heating the furnace lining working surface to 1700–1750°C, which corresponds to a temperature on the boundary of spalling of the refractory of 1400–1500°C. An increase in the lining operating temperature to 1800–1900°C (1550–1650°C on the boundary of spalling) requires the use of refractories with a combination of such properties as heat resistance and high-temperature strength. These requirements are met to the greatest degree by MKhV parts. In the future for large tonnage plasma and electric arc furnaces with high mechanical loads MKhVP high-strength parts may be recommended for the lining. However, for successful use of these parts it is necessary to aim to increase their heat resistance.

Honeycomb sandwich structure for future space transportation systemswith integral cryogenic tankage

This paper presents the status of the structural development of an integral cryogenic-tankage/hot-fuselage concept for future Space Transportation Systems (STS). The concept consists of a honeycomb sandwich structure which serves the combined functions of containment of cryogenic fuel, support of vehicle loads, and thermal protection from an entry heating environment. The inner face sheet is exposed to a cryogenic (LH2) temperature of -423°F during boost; and the outer face sheet, which is slotted to reduce thermal stress, is exposed to a maximum temperature of HOOT during a high-altitude, gliding entry. A fabrication process for a Ren£ 41 honeycomb sandwich panel with a core solidity less than 1% has been developed, which is consistent with desirable heat treatment processes for high strength. Preliminary structural allowables and thermal properties for use in structural system studies have been determined; two 1 x6 ft panels have been tested with combined thermal and mechanical loads; and the effects of slots used to reduce stresses in the outer face sheet on the lower surface of the vehicle have been evaluated in the cryogenic environment associated with containment of LH2 fuel. Based on the work presented in previous system studies and the hardware development described herein, the Rene' 41 honeycomb sandwich appears to be a viable structural concept for an integral cryogenictank/hot-fuselage structure; however, additional in-depth studies, hardware development, and testing are required to verify the concept fully.

Effect of combined mechanical load and irradiation on hydrogen stabilization in LiF:OH

Abstractγ‐irradiated LiF:OH crystals are investigated by the ESR and IR‐spectroscopic methods. The possibility of stabilization of atomic hydrogen in a cation vacancy is shown. It is established that applied mechanical load during the irradiation process does not affect the hydroxide decay, but has essential influence on the redistribution of the decay products (conditions are created for stabilization of hydrogen atoms in two different positions).

Applications of Energy Release Rate Techniques to Part-Through Cracks in Experimental Pressure Vessels

In nonlinear applications of computational fracture mechanics, energy release rate techniques are used increasingly for computing stress intensity parameters of crack configurations. Recently, deLorenzi used the virtual-crack-extension method to derive an analytical expression for the energy release rate that is better suited for three-dimensional calculations than the well-known J-integral. Certain studies of fracture phenomena, such as pressurized-thermal-shock of cracked structures, require that crack tip parameters be determined for combined thermal and mechanical loads. A method is proposed here that modifies the isothermal formulation of deLorenzi to account for thermal strains in cracked bodies. This combined thermo-mechanical formulation of the energy release rate is valid for general fracture, including nonplanar fracture, and applies to thermo-elastic as well as deformation plasticity material models. Two applications of the technique are described here. In the first, semi-elliptical surface cracks in an experimental test vessel are analyzed under elastic-plastic conditions using the finite element method. The second application is a thick-walled test vessel subjected to combined pressure and thermal shock loading.

Evaluation of the strength of crown refractories under combined heat-cycling and mechanical loads

On the basis of tests of specimens with different configurations and sizes of working region, a shape of specimen has been developed which makes it possible to concentrate thermal and mechanical stresses simultaneously in the working region. In the laboratory tests, the specimens of the new form reflect most completely the exploitational properties of refractories. The results of the determination of the properties by the proposed method have been checked under industrial conditions when the wear-resistant refractories for lining the crown of plasma furnaces can be selected; the effectiveness of the new method was confirmed.

Resizing procedure for structures under combined mechanical and thermal loading

The fully-stressed design (FSD) appears to be the most widely used approach for sizing of flight structures under strength and minimum-gage constraints. Almost all of the experience with FSD has been with structures primarily under mechanical loading as opposed to thermal loading. In this method the structural sizes are iterated with the step size, depending on the ratio of the total stress to the allowable stress. In this paper, the thermal fully-stressed design (TFSD) procedure developed for problems involving substantial thermal stress is extended to biaxial stress members using a Von Mises failure criterion. The TFSD resizing procedure for uniaxial stress is restated and the new procedure for biaxial stress members is developed. Results are presented for an application of the two procedures to size a simplified wing structure.

Thermal shock in a nuclear containment wall due to loss-of-coolant accident

The safety analysis of reinforced concrete containments for nuclear reactors requires evaluation of thermal effects due to the loss-of-coolant accident. It is assumed that the inner surface of the containment is suddenly heated by the coolant getting out of the primary circuit. The wall is assumed to behave as a beam-column. Hoop forces and moments created by shell action are ignored. The plane wall section of the containment, normal to the reinforcing rods, is studied for evaluation of the stress increment due to the thermal shock. It is assumed that the section remains plane during the mechanical and thermal loading. The elastic-plastic model of material is chosen both for concrete and reinforcing steel. The section is considered cracked whereever the concrete is subjected to tensile stress. Thermal and mechanical material data are included in the program. The input-data for the computer program consist of the temperature of coolant inside the containment, the coefficient of heat transfer from the coolant to the wall, the axial force and the bending moment imposed by loading conditions before the thermal shock and the conditions of restraint for the considered wall section during the thermal shock. The computer program, based on the finite element method, consists of two sets of subroutines. The first set calculates the temperature increment after the prescribed time step. The second set calculates the elastic and plastic strain increments resulting from the increment of combined mechanical and thermal loading. The wall section of an actual containment is used, as an illustrative example, for the determination of thermal effect under various loading conditions. Results are presented in a diagram of axial force versus bending moment. A point on the diagram represents the load combination to which the wall section might be subjected. The thermal effect for various thermal loads, in form of the equivalent bending moment, it also plotted in the diagram.