Effect of Residual Stress on Interlaminar Fracture Toughness of CFRP Laminates

In the present study, an effect of residual stress in CFRP laminates for the interlaminar fracture toughness was investigated in detail. The boundary element method based on the Laplace transform were employed to calculate the residual stress and interlaminar fracture toughness of the CFRP laminates. The relaxation moduli of the CFRP laminates were estimated by unidirectional compression creep test experimentally. Using CFRP specimens of orthogonal stacking sequence [0°/90°/0°], double cantilever beam (DCB) test for opening mode crack and end notch flexure (ENF) test for shear mode crack were introduced to obtain the actual fracture toughness of CFRP laminates, where the effect of the residual stress in the cooling sequence of the making process was taken into account. From the experimental results and numerical demonstrations by BEM, it was shown that there is a possibility of underestimation of the interlaminar fracture toughness if it is calculated by elastostatic analysis neglecting the residual stress. On the other hand, there is a possibility of overestimation of the toughness when thermoelastic analysis is applied without thermo-viscoelastic properties.     

Thermal Stress Analysis for Octagonal Quasicrystals

The complex variable method for solving the two-dimensional thermal stress problem of octagonal quasicrystals is stated. The closed-form solutions for octagonal quasicrystals containing an elliptical hole subjected to a remote uniform heat flow are obtained. When the hole degenerates into a crack, the explicit solutions for the stress intensity factors and energy release rate are presented.     

J-integral and local damage fracture analyses for a pump casing containing large weld repairs

Three-dimensional J-integral and two-dimensional Local Approach finite element studies are described for postulated crack-like defects in a large repair weld to the casing of a light water reactor circulation pump. The repair weld residual stress field is simulated and plant operating pressure and thermal transient loads are applied. Crack tip constraint effects are quantified through detailed analysis of the cracked structure and compact tension fracture toughness specimens. Fracture initiation crack sizes are shown to be larger than conceivable fabrication defects that are detectable using modern ultrasonic inspection techniques. The Local Approach study demonstrates the benefits of quantifying crack tip constraint conditions, compared with conventional J-estimation schemes and cracked body J-integral analysis. The method of introducing the crack into the finite element model is shown to have a large effect on calculated crack tip fracture parameters; a slowly developing crack in the residual stress field being more benign.     

EDGE CRACK PROBLEMS IN HOMOGENOUS AND FUNCTIONALLY GRADED MATERIAL THERMAL BARRIER COATINGS UNDER UNIFORM THERMAL LOADING

In this study the axisymmetric crack problem for thermal barrier coatings under a uniform temperature change is considered. Modes I and II stress intensity factors and the strain energy release rate are calculated for various sizes and locations of the crack. The main variables in the problem are the material inhomogeneity parameter of the functionally graded material coating, the size and the location of the crack, and the relative dimensions of the specimen. The effect of the temperature dependence of the material properties on the stress intensity factors and the strain energy release rate is also investigated. The finite element method is used to solve the problem. The material property grading is accounted for by developing special inhomogeneous elements and the stress intensity factors are calculated by using enriched crack tip elements.     

MODELING OF THERMAL CRACKING IN ELASTIC AND ELASTOPLASTIC TWO-PHASE SOLIDS

A review is given about fracture mechanical investigations concerning the thermal crack initiation and propagation in one of the segments or in the material interface of two-arid three-dimensional self-stressed two-phase compounds. The resulting boundary value problems of the stationary thermoelasticity and thermoplasticity for the cracked two- and three-dimensional bimaterial structures considered are solved using the finite element method. Furthermore, by applying an appropriate crack growth criterion based on the numerical calculation of the total energy release rate G of a quasistatic mixed-mode crack extension the further development of thermal crack paths starting at the intersection line of the material interface with the external stressfree surface of the two- and three-dimensional elastic bimaterials could be predicted. In the case of the disklike two-phase compounds, the theoretically predicted crack paths show a very good agreement with results gained by associated cooling experiments. Several specimen geometries consisting of different material combinations and subjected to uniform and nonuniform temperature distributions have been studied using the relevant methods of fracture mechanics. Thereby thermal cracks propagating in one segment of an elastic bimaterial only obey the condition G_II = 0, whereas for interface cracks a mixed-mode propagation is always existent where the G_II values play an important role. Moreover, by applying the proposed crack growth criterion the possible crack kinking direction ? of an interface crack tip out of the interface could be predicted by taking into consideration the finite thickness of an interlayer (interphase). In addition, an analysis of the stress and strain fields in the vicinity of thermal interface cracks in the discontinuity area of two- and three-dimensional elastoplastic two-phase compounds has been performed by using the FE-method. Thereby a heat source Q was assumed in one of the two materials in the neighborhood of an interface crack tip. The corresponding stress states in the bimaterial structuresand especiallyin the vicinity of the interface crack tip have been calculated by applying the incremental I₂-plasticity and using a bilinear hardening material law and based on a sequentially coupled solution of the heat transfer and the thermal stress boundary value problems. Finally, the failure assessment has been performed on the basis of the local J-integral which, for three-dimensional interface cracks, was recently generalized by two of the authors.     

Ductile crack growth of semi-elliptical surface flaws in pressure vessels

Experiments on ductile crack growth of some axial surface flaws in a pressure vessel have revealed the well-known canoe shape, i.e. a larger crack extension has occurred in the axial direction than in the wall thickness direction. Two tests have been analyzed by finite element calculations to obtain the variation of the J-integral along the crack front, and the stress and strain state in the vicinity of the crack. The local crack resistance depended on the local stress state. To predict ductile crack extension correctly, J_R-curves have to account for the varying triaxiality of the stress state along the crack front.     

Thermal Fracture Analysis Of Nonhomogeneous Plate with Interfaces Under Uniform Heat Flow

The mixed-mode thermomechanical fracture problem in a nonhomogeneous material plate with two interfaces is studied in this research. Uniform heat flow conditions are considered. The interaction energy integral method for the thermal fracture problem is developed to calculate the thermal stress intensity factors (TSIFs) in nonhomogeneous materials. This method is proved to be domain independent for nonhomogeneous materials even when the integral domain is cut by one interface or many interfaces. Combining the interaction energy integral method with the extended Finite Element Method (XFEM), the temperature fields, the displacement fields, the thermal stress fields, and the TSIFs are calculated. In this article, both the edge crack and the internal crack are considered. Some examples are presented to study the influence of the material properties on the TSIFs. It can be found that the mismatch of the elastic modulus and thermal expansion coefficient can affect the TSIFs dramatically; however, the thermal conductivity interface will not arouse a kinking behavior of the TSIFs. It can be concluded that the existence of an interface (especially for elastic modulus and thermal expansion coefficient) affects the TSIFs greatly.     

Fracture analysis of hydrogen storage composite cylinders with liner crack accounting for autofrettage effect

An understanding of fracture behavior is crucial to the safe installation and operation of high-pressure composite cylinders for hydrogen storage. This work has developed a comprehensive finite element model to investigate axial surface flaws in cylinder liners using the fracture mechanics and a global–local finite element technique. Since the autofrettage process has a strong influence on cylinder fracture behavior, it is also considered in this analysis. The simulation process is broken down into three steps in order to precisely extract fracture parameters and incorporate the autofrettage effect. In the first step, the global model performs the autofrettage simulation to study the residual stress with consideration of both material hardening and the Bauschinger effect. In the second step, the global model uses residual stress to compute displacement for the local model. Finally, in the third step, the local model extracts the values of stress intensity factor and J-integral. Comparison is conducted on the fracture parameters with various autofrettage levels and crack shapes. The vicinity of the crack front is also studied by the size and shape of the plastic zone, and the validity of stress intensity factor and J-integral dominances is examined.     

TRANSIENT THERMOELASTIC FRACTURE OF INTERFACE CRACKS

Abstract

The transient behavior of an interface crack at the center and edge of two finite dissimilar materials free to bend and subjected to a transient thermal load was studied. It was first assumed that the crack was insulated. The effect of allowing heat to conduct through the crack upon closing was also investigated. The effects of the mechanical and thermal material property ratios as well as the thickness ratio on the crack deformations and the transient strain energy release rate were calculated.     

Crack propagation of planar and corrugated solid oxide fuel cells during cooling process

The corrugated solid oxide fuel cell (SOFC) can effectively improve energy density and transformation efficiency compared with conventional planar SOFC, but its stability and durability have not been systematically analyzed. The residual stress of SOFC may lead to crack initiation and propagation during cooling process, so stress distributions of planar and corrugated SOFCs are simulated to analyze the location of crack initiation. The materials of electrolyte, anode, and cathode in this paper are yttria‐stabilization zirconia (YSZ), Ni‐YSZ, and strontium‐doped lanthanum manganite (LSM), respectively. The result shows that the edge of cell is more prone to cracking. Therefore, precracks including edge crack and middle crack are introduced into anode‐electrolyte interfaces to investigate crack propagation of two types of SOFCs during cooling process. For corrugated SOFC, the cracks propagate more slowly, and the cell is less prone to interfacial delamination compared with planar SOFC. In addition, the interface energy release rates are obtained to further analyze crack propagation of two types of SOFCs, and the corrugated SOFC has lower energy release rate. The research in this paper provides guidance for stability analysis and lays a foundation for future mechanical analysis of corrugated SOFC.     

IMPORTANCE OF INERTIA TERM IN DYNAMIC CRACK PROBLEMS CONSIDERING LORD–SHULMAN THEORY OF THERMOELASTICITY

A numerical technique is presented for the accurate calculation of stress intensity factors as a function of time for generalized coupled thermoelastic problems. In this task, the effect of the inertia term is investigated, considering different theories of thermoelasticity, and its importance is shown.

A boundary element method using the Laplace transform in time domain is developed for the analysis of fracture mechanics; dynamic coupled thermoelasticity problems with relaxation time are considered in the two-dimensional finite domain. The Laplace transform method is applied to the time domain and the resulting equations in the transformed field are discretized using the boundary element method. Actual physical quantities in the time domain are obtained using the numerical inversion of the Laplace transform method.

The singular behavior of the temperature and displacement fields in the vicinity of the crack tip is modeled by quarter-point elements. The thermal dynamic stress intensity factor for mode I is evaluated using the J-integral method. The accuracy of the method is investigated through comparison of the results with the data available in literature.

The J integral, which represents the dynamic energy release rate for propagating cracks, contains a boundary integral and a domain integral. The boundary integral contains strain energy, tractions, and strains whereas the domain integral contains inertia and strains. The J-integral method allows these two terms to be calculated separately. In this way, the importance of each term may be investigated by considering different theories of dynamic thermoelasticity.     

Crack Propagation and Initiation Lives for Surface Pitting Due to Rolling/Sliding Contact

The stress intensity factors of a multiply kinked crack in a half-space are analyzed under rolling/sliding contact with frictional heat. On the basis of these analytical results, rolling contact propagation fatigue life is predicted numerically for the case of high carbon-chromium bearing steel. The initial crack is assumed to be inclined at the angle of maximum energy release rate, and the initial crack length can be determined by the threshold value criterion of the energy release rate. Applying the maximum energy release rate criterion to each kinked angle, the crack growth path can be described, and employing a mixed mode fatigue crack growth law, the associated propagation fatigue life can be predicted. Making use of the dislocation dipole accumulation model to the assumed initial crack, the crack initiation life also can be estimated. Combining the initiation life and the propagation life, the total rolling contact fatigue life for surface pitting can be predicted. The thermomechanical effects on these lives and induced surface pitting are considered.     

The effect of a nearby circular thermal inclusion on the irregular shape of a coated inhomogeneity with internal uniform hydrostatic thermal stresses

We show that under a uniform temperature change, a uniform hydrostatic residual thermal stress state can still be maintained inside a coated inhomogeneity of irregular shape despite its interaction with a nearby circular thermal inclusion. Our analysis indicates that the internal uniform hydrostatic thermal stress field inside the inhomogeneity, the constant mean stress within the coating and the constant hoop stress along the inhomogeneity-coating interface on the coating side are unaffected by the existence of the nearby circular thermal inclusion. In contrast, the irregular shape of the coated inhomogeneity is attributed solely to the presence of the circular thermal inclusion.     

Thermal Stress Analysis of Sandwich Structures

ABSTRACT

Significant thermal and interlaminar transverse stresses often occur in sandwich structures subjected to thermal loading due to the mismatch in thermal expansion of different materials in structures. A kind of 32-node, 3-layered shell element with relative degrees-of-freedom accompanied by a post-processing method is presented based on 3D FEM to accurately describe thermo-structures behavior and easily connected with brick elements. The present method is verified by a theoretical result. The thermal stress fields in a sandwich cylinder with a cutout with and without reinforcement are calculated by the method successfully. The results show that reinforcement may cause significant transverse thermal stresses, although it is beneficial to reducing in-plane stress.     

Effect of interface morphology on the residual stress distribution in solid oxide fuel cell

Solid oxide fuel cell directly and efficiently converts chemical energy to electrical energy. However, the necessity for high operating temperatures can result in mechanical failure. Fuel cell is a multilayer system and its stress distribution is greatly affected by the interface morphology. In this work, cosine interfaces with different amplitudes are used to approximate the fluctuation of actual interface. The effects of interface morphology on stress state, energy release rate of crack and creep behavior have been investigated. The results show that if the interface is planar, the residual normal stress component is zero on the interface, while the nonplanarity of interface can cause the normal stress S_n and shear stress S_t on the interface. When the amplitude is relatively small, the max values of S_n and S_t on the interfaces vary linearly with increasing amplitudes in both anode and cathode. Above a certain value, nonlinearity of the interface becomes important. Max tensile S_n always occurs at the peak of convex interface, but the position of max compressive S_n varies. Max shear stress is prone to occur at 1/4 of the wavelength at small amplitude and moves towards 1/2 of the wavelength when the amplitude increases. Fracture mechanics analysis shows that the surface crack possibly penetrates into the anode function layer and then is constrained by the stiff electrolyte. On the other hand, the horizontal crack likely penetrates into the electrolyte layer when the interface is not planar. Creep analysis shows that 11 800 hours of continuous operation at high temperature cannot remove stress undulation introduced by nor-planar interface but can make max value of Sn and St decrease around 30%.     

THERMAL STRESS ANALYSIS OF A RECTANGULAR PLATE AND ITS THERMAL STRESS INTENSITY FACTOR FOR COMPRESSIVE STRESS FIELD

Abstract

In this study, a transient thermal stress problem of a rectangular plate due to a nonuniform heat supply is treated theoretically and, thereafter, fracture behaviors of the plate with a crack are examined for compressive stress states. Assuming that a crack located on an arbitrary position, with an arbitrary direction, is sufficiently small and is closed because of the compressive stress field, a temperature field, in a transient state, is analyzed by taking into account the effect of relative heat transfer on both surfaces of the plate. Thereafter, the corresponding thermal stress analysis is developed on the basis of the two-dimensional plane stress problem using Airy's stress function method, and the stress intensity factor is analyzed for the biaxial stress state. As an analytical model, we consider mechanical boundary conditions of prescribed displacement and estimate the stress intensity factor of a crack tip using parameters of the crack configuration such as the location, direction, length, and coefficient of friction. These numerical results are shown in graphical form.     

The net-section stress associated with the extension of a part-through full-circumference crack in a pipe

Earlier workers have used a simple net-section stress approach, based on collapse-type analyses, to predict the size and shape of a part-through circumferential crack that will cause failure of a pipe fabricated from a ductile material: stainless steel. The equations of equilibrium are applied, assuming that the cracked section behaves like a plastic hinge, with a region of uniform tensile stress, σ*, acting above the neutral axis and a region of uniform compressive stress, — σ*, acting below the neutral axis; σ* is the average of the yield and ultimate stresses. Both experimental and fracture mechanics calculations have hither to shown that crack extension occurs at net-section stresses that are approximately the same as that used in the collapse analysis, when the crack is of the through-wall type, thus providing partial vindication of the net-section stress approach. The present paper extends this work and a fracture mechanics analysis shows that the net-section stress associated with the extension of a part-through full-circumference crack can be appreciably higher than that for a through-wall crack. The paper therefore provides further support for the usefulness of the simple net-section stress approach for predicting the failure of a stainless steel pipe containing a circumferential crack, since its predictions should be conservative in this situation.     

An explicit integral expression for the stress intensity factor of a semi-elliptic surface crack subjected to thermal transient loading

An explicit integral expression for the stress intensity factor of a semi-elliptic surface crack in a plate subjected to thermal transient loading was developed. The stress intensity factor of a semi-elliptic surface crack in a plate, which is exposed to a step change of fluid temperature, was calculated on the basis of the weight function method. The change of the stress intensity factor for a semi-elliptic surface crack subjected to an arbitrary change of the boundary fluid temperature was obtained by Duhamel integration for the product of the step function result and the time varying fluid temperature. The result obtained by the present method has shown good agreement with those obtained by the influence function method. As a practical application, a parametric analysis was performed for the crack behavior during the emergency cool down of reactor coolant in the reactor pressure vessel. Also, the present expression can be effectively applied to the simulation of fatigue crack growth of a semi-elliptic surface crack subjected to various thermal transient loading.     

Life improvement in high temperature components for advanced power engineering

Advanced power engineering rely on technological solutions allowing the design, construction and operation of power plants according to the state of the art, extending the limits of available materials for higher cycle efficiencies, for improved reliability and availability of the systems, and assuring in the meanwhile longer service lives.

Elevated temperature components, high pressure and high energy sections might be considered relevant with respect to time behaviour of the plant, since load-bearing structural materials are mainly subjected to metallurgical ageing, creep processes due to temperature exposure, thermomechanical and low cycle fatigue induced by load histories, as well as crack initiation and growth. Life improvement objective has to be fully considered and assumes a twofold application: i) at the design and construction stage, with the introduction of new materials and/or adopting better reference data and validated extrapolation procedures; ii) for existing plants, managing proper life extension methodologies integrated with plant rejuvenation, refurbishment or repowering decisions.

In the following, attention will be devoted to still open questions which are susceptible to contribute to the goal of safe and reliable, high efficiency thermal power plants, as residual life evaluation in components under multiaxial stress state, on-line plant monitoring, non destructive techniques and expert systems for residual life assessment and in situ repair/treatment techniques.     

Investigation Methods for Thermal Shock Crack Problems of Functionally Graded Materials–Part II: Combined Analytical-Numerical Method

Fractures phenomena can be often found in functionally graded materials (FGMs) subjected to thermal shock loadings. This paper aims to develop a set of analytical-numerical methods for analyzing the mixed-mode thermal shock crack problems of a functionally graded plate (FGP). First, a domain-independent interaction energy integral method is developed for obtaining the mixed-mode transient thermal stress intensity factors (TSIFs). A perturbation method is adopted to obtain the transient temperature field. Then an analytical-numerical method combining the interaction energy integral method, a perturbation method, and the finite element method is developed to solve the present crack problem. Particularly, the influences of the materials parameters, crack length, and crack angle on the TSIFs and the crack growth angle are investigated. The results show that the present analytical-numerical method can be used to solve the thermal shock crack problem with high efficiency. The present work will be significant for the fracture mechanics analysis and design of FGM structures.