Damage Mechanics Method Research Articles

Continuum damage mechanics analyses of type IV creep failure in ferritic steel crossweld specimens

A major high temperature failure mechanism for weldments in ferritic steel steam pipework is circumferential creep cracking within the region of the heat affected zone, adjacent to the parent material, that experiences the lowest temperatures during the welding process. This is commonly known as type IV cracking. In recent years a number of experimental studies have investigated the occurrence of type IV failure in laboratory test pieces, however, there have been few attempts at theoretical modelling of type IV failure to assist in the formulation of design and assessment procedures. This report discusses the use of the creep continuum damage mechanics method for the analysis of the deformation and failure of weldments that are known to fail within the type IV region. The creep behaviour of each of the material regions of a weldment is described with a set of physically based constitutive equations, which incorporate a number of state variables. The finite element creep continuum damage mechanics method is used, with the physically based constitutive equations, to analyse the deformation and failure of the welded testpieces. The computations are shown to be in good agreement with the experimental results. The implications of the analyses are discussed with reference to the assessment of weldments that are susceptible to type IV failure.

Combination of fracture and damage mechanics for numerical failure analysis

A new approach for the analysis of crack propagation in brittle materials is proposed, which is based on a combination of fracture mechanics and continuum damage mechanics within the context of the finite element method. The approach combines the accuracy of singular crack-tip elements from fracture mechanics theories with the flexibility of crack representation by softening zones in damage mechanics formulations. A super element is constructed in which the typical elements are joined together. The crack propagation is decided on either of two fracture criteria; one criterion is based on the energy release rate or the J-integral, the other on the largest principal stress in the crack-tip region. Contrary to many damage mechanics methods, the combined fracture⧹damage approach is not sensitive to variations in the finite element division. Applications to situations of mixed-mode crack propagation in both two- and three-dimensional problems reveal that the calculated crack paths are independent of the element size and the element orientation and are accurate within one element from the theoretical (curvilinear) crack paths.

A Method of Damage Mechanics for the Prediction of Fatigue Lives

A Method of Damage Mechanics for the Prediction of Fatigue Lives

LIFE PREDICTION FOR ADVANCED FERRITIC STEELS SUBJECT TO THERMAL FATIGUE

Abstract— Thermal fatigue is a well recognised source of damage in headers and steam piping of thermoelectric power plant. This topic has been extensively examined in the past for low alloy ferritic steels typically used in such applications. Experimental evidence obtained in low cycle fatigue testing with tensile hold times on Modified 9Cr1Mo and E911 steels suggests that the Linear Damage Summation rule conventionally used in engineering codes for high temperature damage analysis may not be particulary appropriate for the advanced 9Cr steel family. For this reason two alternatives have been examined: (a) a strain based creep damage evaluation using the R5 ductility exhaustion approach and (b) a creep‐fatigue continuum damage mechanics method. The potential advantages and disadvantages of both are discussed. In addition, results from low cycle fatigue and thermomechanical fatigue tests on crossweld specimens machined from welded joints in the Mod.9Cr1Mo alloy are evaluated. Even if the usual cyclic life reduction factor of 2 with respect to base material behaviour appears adequate to account for the mean trend of cross‐weld results, the large variability observed risks making the use of such a factor non‐conservative for accurate life prediction.

The use of supercomputer modelling of high-temperature failure in pipe weldments to optimize weld and heat affected zone materials property selection

The creep deformation and damage evolution in a pipe weldment has been mod­elled by using the finite-element continuum damage mechanics (CDM) method. The finite-element CDM computer program Damage xx has been adapted to run with increased speed on a Cray XMP/416 supercomputer. Run times are suffi­ciently short (20 min) to permit many parametric studies to be carried out on vessel lifetimes for different weld and heat affected zone (HAZ) materials. Finite-element mesh sensitivity was studied first in order to select a mesh capable of correctly predicting experimentally observed results using the least possible com­puter time. A study was then made of the effect on the lifetime of a butt welded vessel of each of the commonly measured material parameters for the weld and HAZ materials. Forty different ferritic steel welded vessels were analysed for a constant internal pressure of 45.5 MPa at a temperature of 565 °C; each ves­sel having the same parent pipe material but different weld and HAZ materials. From these studies it has been concluded that good creep rupture ductility and strength of the weld material are necessary to achieve long vessel lifetimes. Vessel lifetimes are not strongly influenced by the HAZ material properties provided that the rupture strength is maintained above that of the parent pipe material, and that the ductility is greater than one half of that for the parent pipe material. Also it is found that the optimum design can be achieved with both the weld and the HAZ materials having properties of high primary creep strain, mean sec­ondary creep strain, mean rupture strain, and long rupture lifetime, relative to the base or parent pipe material. A lifetime improvement has been demonstrated of 30% over that obtained for the initial materials property data. A methodology for weldment design has been established which uses supercomputer-based CDM analysis techniques; it is quick to use, provides accurate results, and is a viable design tool.

Fatigue failure in PbSnAg alloy during plastic deformation: a 3D-SIMS imaging study: Scandurra, A., Licciardello, A., Torrisi, A., Puglisi, O. and La Mantia, A. J. Mat. Res. Sept. 1992 7 (9) 2395–2402

A finite element (FE) approach for tension–tension fatigue damage in textile reinforced composites on meso-scale level (fabric unit cell) is proposed. S–N curves of a unidirectional composite (UD) out of the fatigue tests in fibre, transverse-fibre, in-plane-shear and out-of-plane-shear directions are used as the input for the impregnated yarns. The algorithm of multi-axial fatigue is applied to matrix-dominated fatigue, while the fibre-dominated fatigue is separately considered. The continuum damage mechanics method is used to describe the properties’ deterioration. The stress redistribution introduced by stiffness degradation conjugates the damage evolution. Averaged mechanical properties of the unit cell in pre- and post-fatigue damage stages are calculated at predefined load cycle numbers, using a numerical homogenization technique. The numerical results are validated by experiments of two types of carbon/epoxy plain weave composites, which have the same fibre/resin system but different textile structures. The validation showed good agreement between modelling and experiment.

A fracture damage evolution law for a cylindrical specimen irradiated by laser beam

The fatigue process is studied by the method of damage mechanics. The equations for the evolution of fatigue damage of one and the same specimen with different irradiation parameters are given, all obtained by experiment. The results show that the study of fatigue damage of a specimen after irradiation by laser beam is of definite significance for the selection of the optimum irradiation parameters.

A fatigue damage evolution law for laser beam irradiated cylindrical specimens

The fatigue process was studied by the method of damage mechanics. Equations for the evolution of fatigue damage of the same specimen with different irradiation parameters are given which were all obtained by experiment. The results show that the study of fatigue damage of a specimen after irradiation by a laser beam is of definite significance for the selection of the optimum irradiation parameters.