The state of the art in computational mechanics of dynamic fracture is reviewed. The concept of dynamic fracture mechanics is essentially based on asymptotic behaviors of a dynamically propagating crack and of a stationary crack subjected to dynamic loading. First, the asymptotic solutions of near-tip field of an elastodynamically propagating crack under steady-state conditions and under transient conditions are summarized. Next, the features of the dynamic J integral pertaining to nonlinear dynamic fracture mechanics as well as to elastodynamic fracture mechanics are described. The dynamic J integral may play important roles in both theoretical and computational aspects of dynamic fracture mechanics. Special attention is focused on various computational models of dynamic crack propagation, which are used in the finite difference method, finite-element method and boundary-element method. Various techniques to overcome inherent difficulties in the discretization of fast-moving surface problems associated with crack-tip singularities are thoroughly reviewed and summarized.

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In the beginning of paper, a solution technique for two-dimensional piezothermo-elastic problems in orthotropic solids of crystal class mm2 is proposed. The solution technique is formulated in terms of two piezothermoelastic potential functions, three piezoelastic potential functions and a single piezoelectric potential function. Next, by using the solution technique, a two-dimensional piezothermoelastic problem in an orthotropic thin plate of crystal class mm2 which is subjected to heating on one surface and electric surface charge on both surfaces is analyzed. The numerical calculations are carried out for cadmium selenide exhibiting class mm2 symmetry. The elastic displacements and stresses are compared with those obtained for a similar thermo-elastic problem without the piezoeffect. The effects of the electric surface charge on the elastic displacements, stresses, electric potential, electric field intensities and electric displacements are also investigated.

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When an external force acts on piezoelastic materials, elastic deformation and electric potential difference are produced. As an application of their intrinsic electro-mechanical coupling behavior to intelligent structures which function as a sensor, the present paper discusses an inverse problem of two-dimensional piezothermoelasticity in an orthotropic thin plate of crystal class mm2. An unknown thermal loading is inferred when the electric potential difference between the surfaces of the plate is given and when the mechanical and electric boundary conditions are prescribed. The solution technique proposed by the present authors is applied to analyze this problem. Numerical calculations are carried out for a cadmium selenide solid exhibiting class mm2 symmetry. The obtained temperature distribution of the heating medium is illustrated graphically.

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One of the difficult problems in the study of the constitutive equation for cyclic plasticity is the prediction of ratchetting behavior that is induced by the superposition of a cyclic secondary load to a constant primary load in a biaxial case, or by the mean stress in a uniaxial case. This paper shows the constitutive equation in which the memorization of the back stress is considered for ratchetting behavior, especially for biaxial ratchetting behavior. To verify the applicability of the constitutive equation to ratchetting behavior, a biaxial ratchetting test was carried out using SUS 304 stainless steel at room temperature. As a result, it was found that the simulations based on the constitutive equation have good agreement with the tests.

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The boundary element method (BEM) does not require a domain integral in steady-state thermoelastic problems without heat generation. However, with heat generation, the domain integral is necessary. This paper shows that the three-dimensional problem of steady-state thermoelasticity with nonuniform heat generation can be easily solved without a domain integral by means of the boundary element method. This method can also be applied to steady-state thermal stress problems under generally complicated heat generation. However, in this case, the domain must be divided into small areas, where distributions of heat generation satisfy the Laplace equation.

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In this paper, numerical solution of hypersingular integral equations in curved-crack problems is presented. The stress fields induced by two kinds of standard sets of force doublets are used as fundamental solutions. Then, the problem is formulated as a system of integral equations with the singularity of the form γ^-2. In the numerical calculation, two kinds of unknown functions are approximated by the products of the fundamental density functions and power series. The calculation shows that the present method gives rapidly converging numerical results for curved cracks under various geometrical conditions. In addition, a method of evaluation of the stress intensity factors for arbitrary-shaped curved cracks is proposed using the approximate replacement with a simple straight crack.

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This paper concerns a new boundary element analysis approach for the object divided into subregions. In this approach, the system of equations for each subregion, which is derived from the boundary integral equation, is transformed to an equation similar to the stiffness equation of the finite element method (FEM). The global equation is made by the superposition of these matrix equations. The present approach is derived theoretically and is compared with the existing ones in order to indicate its features. Then, it will be applied to analysis of two-dimensional elastic and potential problems.

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A numerical method to analyze two-dimensional stress waves in a transverse isotropic cylinder is presented, when an axisymmetric impulsive load is applied to the end surface of the cylinder. For the numerical analysis of two-dimensional stress wave propagation, the finite difference method based on integration along the bicharacteristics is employed. From the numerical results obtained by this method, it has been confirmed that the proposed method can be applied to analyze two-dimensional stress wave propagation in a transverse isotropic medium. The effect of the transverse isotropy on the propagation of stress waves in stainless steel-aluminum composites is also demonstrated.

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A new nondestructive inspection technique using infrared thermography was proposed, in which the thermal image of a heated test sample was used to identify flaws and defects. Joule effect heating by electric current was employed to heat up the sample instantaneously. Both numerical and experimental studies were made on the resolution and the applicability in the detection of the through-thickness and surface cracks embedded in steel plate samples. The results showed that a singular concentration in the surface temperature field was observed at the crack tips of the sample, and the cracks were found to be sensitively detected from the singular temperature field in the early stage of the heat conduction process. This technique was also applied to the inspection of the delamination defect in CFRP samples. The subsurface defect was found to be identified from a localized low-temperature region appearing on the sample surface. It was found that the proposed technique is applicable to damage inspection in CFRP as well as flow inspection in metallic materials.

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The development of an expert system for ultrasonic testing (UT) can contribute to the improvement and standardization of inspection levels if the technique and experience of the UT engineer can be incorporated into a knowledge data base. It is considered that such an expert system will be one way to solve the problem of the predicted shortage of UT engineers in the near future. The present system was originally developed as a system having a knowledge-based system and an inference engine which are combined with a blackboard model and frame model. In the developed expert system, which is combined with a man-machine interface, it may be advantageous to use a production rule and forward inference. The inference was performed in two steps. The causes of weld flaws were firstly inferred from the welding condition, and then the final result was determined from the UT data. When the present expert system was applied to diagnosis of the welding flaw of a welded structure, the results obtained by the present system agreed with those obtained by the UT engineer up to about 90% of the time. It was therefore confirmed that the expert system is applicable as a method of inference for weld flaws using ultrasonic testing data.

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We have developed an experimental and computational hybrid system which can be applied to the detection of flaws in structural members. The system consists of an infrared thermal video system by which the temperature distribution of the body surface can be measured, and an engineering work station which carries out image processing of the thermograms. We have applied the developed system to the detection of flaws embedded in honeycomb sandwich constructions. Various types of flaws lying between the honeycomb core and the surface sheet have been examined, and the effects of the thickness of the surface sheet and the combination of materials have been studied. We have found that the method is useful to detect flaws in the honeycomb sandwich structures.

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In order to investigate the relationship between surface roughening and deformation of respective grains, three-dimensional observation of the roughened surface of polycrystalline iron during uniaxial compressive deformation was carried out. The pattern of grain boundaries and the contour map of the surface roughening were superimposed on a personal computer monitor. The three-dimensional surface roughness data were also high-pass-filtered to obtain detailed figures of the surface. The fractal dimension was introduced to characterize the change in three-dimensional surface roughening during plastic deformation. The box-counting method was adopted to measure the fractal dimension. It was found that three-dimensional surface roughening showed fractal characteristics. The value of the fractal dimension was almost constant with increasing applied strain.

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The macroscopic elastic-plastic constitutive relation of inhomogeneous material is affected by the microscopic distribution of stress and strain in respective heterogeneous regions of the material. Several parameters are reviewed and a new parameter is proposed to describe the deformation behaviour of inhomogeneous material, and their relationship is discussed analytically as well as numerically. A plane model of in. homogeneous material composed of rectangular inhomogeneous regions was used for the numerical simulation, and the changes in the parameters during elastic-plastic deformation are calculated and compared. Uniaxial loading under plane strain condition is assumed. A modified plastic accommodations parameter is proposed, which is shown to be uniquely related to the constraint ratio reported previously. It is shown that the. initial aspect ratio has a marked influence on the deformation behaviour. The process to estimate the constitutive relation of inhomogeneous material is also discussed on the basis of the constraint model.

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Fracture of quasi-brittle materials such as concrete and rocks is known to result from the formation and development of microcracks. Microcracks reduce the macroscopic value of Young's modulus and therefore the corresponding wave velocity, so that shock wave formation is expected. The phenomenon was studied for granite as an example of quasi-brittle materials. By using of a constitutive law based on experiment, the nonlinear wave propagation was analyzed numerically for an infinite plate. The plate is subjected to an impact compressive stress at one of its surfaces. The other surface of the plate is assumed to be stress free. It was shown that the shape of the incident wave has a significant effect on the forming shock wave.

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Following creep-fatigue tests with Modified 9Cr-1Mo Steel in a high-vacuum environment, creep-fatigue surfaces were observed by SEM. In the cases where the creep-fatigue life is time- or rate-independent, the primary fracture mode is the transgranular type, but some differences were observed in the number of dimples being dependent on the strain wave form. When time- or rate-dependent life reduction takes place, the primary fracture mode is found to be of the intergranular type. This intergranular fracture mode is mainly observed on the inner side of the specimen, and the fracture mode becomes transgranular on the outer side. The area being fractured in the intergranular manner is correlated to the time-dependent damage variable calculated by the creep-fatigue life evaluation model based on the overstress.

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Fracture behaviors of brittle polycrystalline solids such as ceramic materials are deeply related to microcracking. Continuum damage mechanics is considered a powerful theoretical framework to deal with brittle microcracking solids. However, it is fairly difficult to obtain analytically, as well as experimentally, evolution equations for microcracking and reduced elastic compliances of microcracked solids. In the present study, a mesoscopic simulation method at grain scale using a discontinuum mechanics model is employed to obtain such information. The validity and limitations of the isotropic theory of continuum damage mechanics are studied here.

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The mesoscopic simulation method proposed in the first report of the present study has been applied to the analysis of a brittle polycrystalline solid under proportional loading of biaxial stresses. The validity and limitations of the anisotropic theory of continuum damage mechanics using a second-order tensor as damage variables have been studied through comparisons between simulated results and theoretical predictions. It has been shown that the existing anisotropic theory is inapplicable to the compression-dominant stress state because of an associated type of damage evolution equation, although it is effective in the tensile stress field.

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This paper deals with a method for predicting the three-dimensional distribution of internal defects from the two-dimensional observation of intercepted defects on an inspection plane, by inverse analysis. First, the fundamental relationships between the three-dimensional distribution and the two-dimensional observation of internal defects were analytically derived on the basis of a statistical model where multiple penny-shaped cracks in an infinite body were intercepted by an inspection plane. The validity of the method was confirmed by a numerical simulation. It was, moreover, applied to small creep-fatigue cracks distributed in a smooth specimen of Type 304 stainless steel, and the volumetric density and the distribution of crack size were successfully evaluated.

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The validity of the caustic method on fracture problems of partially stabilized zirconia at room and elevated temperatures up to 1200°C is confirmed, and we investigated the dynamic fracture behavior of this material from pictures taken using the apparatus of caustics together with an ultrahigh-speed camera, Imacon 790. Consequently, it is revealed that dynamic fracture toughness K<Id> abruptly decreases as the testing temperature increases over about 500°C, and the ratio of K<Id> to static fracture toughness is increased from 2.0 at room temperature to 5.0 at 800°C. Moreover, it is ascertained that the crack-propagation fracture toughness is increased with the crack velocity at any testing temperature up to 1200°C, and this tendency is decreased with the rise of temperature in this range. Finally, an example of the visualized acoustic emission (AE) wave is shown. It is revealed that the velocity of this AE wave decreases slightly as the temperature rises, and it coincides with the Rayleigh wave speed of this material.

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Using the shadow optical method of caustics, experimental evaluation of the stress intensity factor K_I was carried out for poly (methyl methacrylate) (PMMA) during the course of fatigue cracking at frequencies of 1 and 0.1 Hz. A critical comparative study was made of K_I values obtained experimentally by the caustic method (CM) and those deduced theoretically from linear fracture mechanics (LFM) at various rates of fatigue crack propagation (FCP). The relationship between △K_I and the FCP rate yielded two clear segmented curves in evaluations by both LFM and CM, results which are supported by fracture surface examination. When K_Imax was related to the FCP, good agreement was obtained for the data from both methods, while noticeable differences were recognized in the relation △K_I or K_Imax vs. FCP. The effect of the test frequency is also shown in the relations.

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The degradation of a plain woven carbon fiber/epoxy composite under tension-tension fatigue loading was studied in this work. An almost flat S-N diagram was obtained for this composite. As high as 85% fatigue strength of the static strength at 10⁶ cycles was recorded. A new specimen configuration was developed and used. That is, the specimen has two narrow sections. After the specimen fails at one section under static or cyclic loading, the residual strength of the other section will be statically measured. The residual strength for specimens failing under static and cyclic loadings is higher than the static strength for the virgin specimen. A typical S-shape stiffness reduction against logarithmic cycle number is found for high-cycle-fatigue specimens. However, sudden fatigue death of the specimens occurs with no indications in both S-S curves and AE events. Parameters of the parabolic function which fits the convex S-S curve are more sensitive to the stiffness reduction than the observed stiffness. Microscopic examination of the fracture surface was conducted. From those observations, a fatigue damage process model is proposed.

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We investigate the free vibration mode for Nickel-base single crystal and directionally solidified (DS) superalloy cantilever plates using three-dimensional finite-element analyses. The elastic anisotropy had a significant effect on the eigenfrequency and eigenmode of plates. The rotation of the crystal axis around the [001] axis had a smaller effect at low frequencies but a significant effect at high frequencies. The deviation of the crystal axis in the plate had a significant effect on the eigenfrequency at low frequencies as well as at high frequencies. The bulk elastic modulus of DS plates derived by means of the Reuss average was effective in estimating the eigenfrequencies of DS plates. Eigenfrequencies of DS plates could also be predicted by averaging the eigenfrequencies of single crystals which comprise DS plates.

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Acoustic emission (AE) monitoring was carried out during indentation tests of ceramic materials to reveal different types of emission and to correlate them with microscopic observation in order to identify plastic deformation or cracking phenomena. The results show good correlation between the AE characteristics and deformation mode of ceramics below the diamond indenter. The AE generated by surface cracks are of higher amplitude, longer duration and higher frequency, compared with the phenomenon during plastic deformation. Consequently, deformation processes of ceramics are distinguished by AE signal analysis. Moreover, this monitoring method is effective in examining the threshold load for cracking.

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Elastic wave propagation through an area discontinuity of two dissimilar, bonded, semi-infinite circular rods is investigated analytically. In particular, the variations of the coefficients of stress reflection and transmission are determined in terms of the nondimensional cross sectional area and mechanical impedance parameters. The coefficients of energy flux reflection and transmission are also included. Then, the case is generalized to include a rigid mass attached at the discontinuity

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This work is mainly an investigation of strip curvature caused by the work roll speed or work roll radius mismatch in the asymmetrical rolling process for an aluminum strip. At the same time, we analyze the variations in the temperature field and strain field, and use the method of speed mismatch of upper and lower work rolls to calibrate the deformation curvature caused by coolant condition mismatch in the hot rolling process. Finally, we will use the strain density theory to predict the possible initial fracture area in the strip. For completeness, all the boundary conditions of heat transfer which may be encountered in a realistic hot rolling process are taken into account, such as heat convection of the surrounding air and the cooling water, boiling phenomenon of cooling water and radiation heat loss. Based on the large deformationlarge strain theory, and by means of the updated Lagrangian formulation (ULF) and incremental theory, a coupled thermo-elastic-plastic analysis model for the hot rolling process is constructed. The flow stress of the material in this model is taken as a function of strain, strain rate and temperature. Finally, the numerical analysis method developed in this study is employed to analyze the changes in the aluminum strip's temperature and other changes during rolling. In addition, the average rolling force obtained from the simulation was compared with that from experiments at China Steel Co., and the model in this study was verified to be reasonable.

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