Vicinity Of Crack Research Articles

Optimization of Digital Image Correlation for High-Resolution Strain Mapping of Ceramic Composites

Digital image correlation (DIC) is assessed as a tool for measuring strains with high spatial resolution in woven-fiber ceramic matrix composites. Using results of mechanical tests on aluminum alloy specimens in various geometric configurations, guidelines are provided for selecting DIC test parameters to maximize the extent of correlation and to minimize errors in displacements and strains. The latter error is shown to be exacerbated by the presence of strain gradients. In a case study, the resulting guidelines are applied to the measurement of strain fields in a SiC/SiC composite comprising 2-D woven fiber. Sub-fiber tow resolution of strain and low strain error are achieved. The fiber weave architecture is seen to exert a significant influence over strain heterogeneity within the composite. Moreover, strain concentrations at tow crossovers lead to the formation of macroscopic cracks in adjacent longitudinal tows. Such cracks initially grow stably, subject to increasing app lied stress, but ultimately lead to composite rupture. Once cracking is evident, the composite response is couched in terms of displacements, since the computed strains lack physical meaning in the vicinity of cracks. DIC is used to identify the locations of these cracks (via displacement discontinuities) and to measure the crack opening displacement profiles as a function of applied stress.

Polygon scaled boundary finite elements for crack propagation modelling

SUMMARYAn automatic crack propagation modelling technique using polygon elements is presented. A simple algorithm to generate a polygon mesh from a Delaunay triangulated mesh is implemented. The polygon element formulation is constructed from the scaled boundary finite element method (SBFEM), treating each polygon as a SBFEM subdomain and is very efficient in modelling singular stress fields in the vicinity of cracks. Stress intensity factors are computed directly from their definitions without any nodal enrichment functions. An automatic remeshing algorithm capable of handling any n‐sided polygon is developed to accommodate crack propagation. The algorithm is simple yet flexible because remeshing involves minimal changes to the global mesh and is limited to only polygons on the crack paths. The efficiency of the polygon SBFEM in computing accurate stress intensity factors is first demonstrated for a problem with a stationary crack. Four crack propagation benchmarks are then modelled to validate the developed technique and demonstrate its salient features. The predicted crack paths show good agreement with experimental observations and numerical simulations reported in the literature. Copyright © 2012 John Wiley & Sons, Ltd.

Full-Thickness Decarburization of the Steel Shell of an Annealing Furnace

After 35 years of service, the steel shell of a continuous annealing furnace developed cracking problems, even after careful repairs. A metallurgical evaluation was performed on sections of the ASTM A36 steel furnace shell. Testing included chemical analysis, light metallography, mechanical testing, and hardness measurements. The evaluation found through-thickness decarburization of the 6 mm thick shell in the vicinity of open cracks. The strength of the shell decreased significantly in these areas. Laboratory decarburization tests performed on unaffected samples of ASTM A36 steel shell confirmed that field conditions existed that led to rapid through-thickness decarburization of the shell. High ambient humidity is believed to have contributed to rapid decarburization.

Microstructure and stray electric fields at surface cracks in ferroelectrics

Ferroelectric perovskites are widely used in transducer, memory and optical applications due to their attractive electromechanical and optical properties. In these brittle materials, reliability and failure of devices is dominated by the behavior of cracks. The electromechanical coupling causes cracks to interact strongly with both mechanical as well as electrical fields. Additionally, cracks and domain patterns interact strongly with each other. Hence, an understanding of the electromechanics of cracks requires an accounting of all these interactions. In this work, we apply a real-space phase-field method to compute the stresses, domain patterns, and stray electric fields in the vicinity of a stationary crack, defined here as a geometric feature that causes large but bounded stress. We investigate the effects of charge compensation on the crack face, crack orientation with respect to the crystal lattice, and applied far-field stress and electric fields.

Damage Study on AZ31 Mg Alloy under Bending Load

The microstructure, mechanical properties and damage morphology of as-cast and as-extruded AZ31 Mg alloy under different annealing treatments were investigated. The result of three-point bending test showed that the as-extruded magnesium alloy after annealing treatment followed by 300°C×30min can obtain a good mechanical properties and fine uniform grains. There are clear surface macro-cracks and emboss, the micro-cracks and cavities generated in the vicinity of the main crack, the main cracks of as-extruded samples arises from the maximum compressive stress position ,but as-cast samples arises from the maximum tensile stress position. The main crack of magnesium alloys is extended from the edge to the center of the specimen. This is because the crack propagates along grain boundaries or twin boundaries, and the cracking process will be obstructed by dislocation and/or the second phase particles

Impact of Micro-Cracks on the Degradation of Solar Cell Performance Based On Two-Diode Model Parameters

Electroluminescence (EL) imaging technique has become a powerful and fast characterization tool providing spatially-resolved information about electrical, optical and material properties of solar cells. When applying a forward bias the solar cell emits infrared radiation whose intensity is governed by the minority carrier lifetime in the bulk and thus decreases considerably in the vicinity of intrinsic defects and cracks. In this work the influence of micro cracks and the progression of already existing cracks in multicrystalline silicon solar cells are investigated. The impact of these defects on the electrical solar cell parameters is studied in the frame of the two-diode model. Statistically proven, it was found that beside a decrease in EL intensity the short circuit current Jsc and the efficiency η also will be reduced, whereas a strong increase in the recombination current density j02 is observed. This work on crack detection and the research on crack-forming will help to explain the reasons behind the loss of efficiency seen in long-term performance analyses

Study on Fatigue Crack Propagation of AZ31B Magnesium Alloy Based on Infrared Thermographic Technology

Fatigue crack propagation of AZ31B magnesium alloy in room temperature is studied.And the temperature on the surface of the samples is tested by infrared thermographic technology.The mechanism of magnesium crack propagation is studied according to the relationship between the temperature of crack tip and crack length,and the relationship between temperature distribution on the surface and the trend of crack propagation in this test.Results show that the temperature on the surface of the samples increases in the beginning and then decreases during the process of fatigue crack propagation,the temperature changes a little in the stabilize phase and obviously in the rapid phase.The highest temperature rise of three group samples is sample A 10.89 ℃,sample B 15.19 ℃ and sample C 12.37 ℃.By observing the crack tip and vicinity region of crack tip,the orientation of crack tip changed,most of the crack is transgranular fracture with some intergranular fracture.There is some plastic deformation in the vicinity region of crack tip.The maximum temperature area on the surface of samples is related to the damage mechanisms of materials.This area corresponding to the stress concentration zone,it is the site of which the fatigue micro-crack formation and expansion,the temperature change and the final section of the specimen is consistent.

Combined Visibility and Surrounding Triangles Method for Simulation of Crack Discontinuities in Meshless Methods

In this paper a combined node searching algorithm for simulation of crack discontinuities in meshless methods called combined visibility and surrounding triangles (CVT) is proposed. The element free Galerkin (EFG) method is employed for stress analysis of cracked bodies. The proposed node searching algorithm is based on the combination of surrounding triangles and visibility methods; the surrounding triangles method is used for support domains of nodes and quadrature points generated at the vicinity of crack faces and the visibility method is used for points located on the crack faces. In comparison with the conventional methods, such as the visibility, the transparency, and the diffraction method, this method is simpler with reasonable efficiency. To show the performance of this method, linear elastic fracture mechanics analyses are performed on number of standard test specimens and stress intensity factors are calculated. It is shown that the results are in good agreement with the exact solution and with those generated by the finite element method (FEM).

Coupling of the BEM with the MFS for the numerical simulation of frequency domain 2-D elastic wave propagation in the presence of elastic inclusions and cracks

This paper proposes a coupling formulation between the boundary element method (BEM displacement and TBEM traction formulations) and the method of fundamental solutions (MFS) for the transient analysis of elastic wave propagation in the presence of multiple elastic inclusions to overcome the specific limitations of each of these methods. The full domain of the original problem is divided into sub-domains, which are handled separately by the BEM or the MFS. The coupling is enforced by imposing the required boundary conditions.The accuracy, efficiency and stability of the proposed algorithms, using different combinations of BEM and MFS, are verified by comparing the solutions against reference solutions. The computational efficiency of the proposed coupling formulation is illustrated by computing the CPU time and the error at high frequencies.The potential of the proposed procedures is illustrated by simulating the propagation of elastic waves in the vicinity of an empty crack, with null thickness placed close to an elastic inclusion.

Formation and Growth of the Crack in Bonded Joints Under Mode I Fracture: Substrate Deflection at Crack Vicinity

Adhesive bonding is now commonly used in aircraft, cars, boats, etc. In these applications, thin panels are often bonded. In such thin structures, heterogeneous mechanical loading along the bondline edge (or potential crack front) is likely to arise due to 3D structural effects. The crack front and its vicinity is a special region, in that it is where structural properties of the adherend material meet those of the adhesive (discontinuity). To investigate the stress distribution in this region, we have observed the deflection of a flexible adherend in an asymmetric wedge bonded joint loaded in Mode I. A sensitive laser profilometry technique was used to observe the main vertical beam displacement and curvature along the length, as well as the resulting transverse, or anticlastic effect, due to Poisson's ratio. From this analysis is evaluated the heterogeneous tensile stress distribution in the adhesive in the vicinity of the crack front.

Intermediate Debonding Failure of RC Beams Retrofitted in Flexure with FRP: Experimental Results versus Prediction of Codes of Practice

An experimental investigation has been conducted with the aim to investigate on the midspan debonding failure of reinforced concrete (RC) beams retrofitted in flexure by means of the application of a fiber-reinforced polymer (FRP) lamina externally applied to concrete substratum. Experimental tests on a series of RC beams with different geometries and type of internal steel reinforcing bars have been carried out in four-point bending up to failure to evaluate the influence of flexural/shear cracks on the debonding of FRP reinforcement from concrete substratum. It is widely known that the overall performance of a standard RC element is primarily influenced by the interaction between FRP and concrete substratum rather than by the strength of the FRP. The failure of such innovative retrofitting techniques may be attributed to an inadequate anchorage length; in other cases, when the FRP is correctly applied at the ends of beam, debonding starts in the vicinity of cracks and propagates toward the supports. The results of the experimental tests are used to investigate on the effectiveness of design procedures currently recommended for such debonding failure mode by the codes of practice. The results of the analytical calculations based on such models are presented in this paper and will provide an element of comparison and discussion with the experimental results. The codes of practice propose different analytical models based on the extensive research done in the last two decades, which involve different types of approximation and often yield quite different results.

Visualization of Hydrogen Diffusion in a Hydrogen-Enhanced Fatigue Crack Growth in Type 304 Stainless Steel

To study the influence of hydrogen on the fatigue strength of AISI type 304 metastable austenitic stainless steel, specimens were cathodically charged with hydrogen. Using tension-compression fatigue tests, the behavior of fatigue crack growth from a small drill hole in the hydrogen-charged specimen was compared with that of noncharged specimen. Hydrogen charging led to a marked increase in the crack growth rate. Typical characteristics of hydrogen effect were observed in the slip band morphology and fatigue striation. To elucidate the behavior of hydrogen diffusion microscopically in the fatigue process, the hydrogen emission from the specimens was visualized using the hydrogen microprint technique (HMT). In the hydrogen-charged specimen, hydrogen emissions were mainly observed in the vicinity of the fatigue crack. Comparison between the HMT image and the etched microstructure image revealed that the slip bands worked as a pathway for hydrogen to move preferentially. Hydrogen-charging resulted in a significant change in the phase transformation behavior in the fatigue process. In the noncharged specimen, a massive type α′ martensite was observed in the vicinity of the fatigue crack. On the other hand, in the hydrogen-charged specimen, large amounts of e martensite and a smaller amount of α′ martensite were observed along the slip bands. The results indicated that solute hydrogen facilitated the e martensitic transformation in the fatigue process. Comparison between the results of HMT and EBSD inferred that martensitic transformations as well as plastic deformation itself can enhance the mobility of hydrogen.

Mathematical modelling and thermal stress intensity factors evaluation for an interface crack in the presence of a system of cracks in functionally graded/homogeneous bimaterials

The work is devoted to mathematical modeling of the fracture processes in the vicinity of an interface crack in functionally graded/homogeneous bimaterials with internal defects subjected to a thermal flux. A previously obtained solution (Petrova and Schmauder, 2009, 2011) [7,8] is used and supplemented with the additional possibility to take into account the crack closure. The solution is based on the integral equation method and it is assumed that thermal properties of the functionally graded material (FGM) have exponential form. For a special case where an interface crack length is much larger than the internal cracks in the FGM an asymptotic analytical solution of the problem is obtained as series of a small parameter (the ratio between sizes of the internal and interface cracks). Analyses of the effects of the location and orientation of the cracks, the material non-homogeneity parameters and the crack closure effects on the thermal stress intensity factors of the interface crack in FGM/homogeneous bimaterials are performed. Examples of some FGM/homogeneous bimaterial combinations (i.e., metal/metal, ceramic/metal) are discussed.

Nonaxisymmetric problem of the stress-strain state of an elastic half-space with a near-surface circular crack under the action of loads along it

The nonaxisymmetric problem of the influence of a free surface of an elastic semifinite body on the distribution of stresses in the vicinity of a near-surface disk-shaped crack is considered within the framework of linearized mechanics of deformable solids. A joint analysis of two nonclassic mechanisms of fracture, namely, fracture of a material with initial stresses that act parallel to the plane of the crack and fracture of a body in compression along a crack, is performed. Using representations of the general solutions of linearized equilibrium equations in terms of harmonic potential functions and the apparatus of integral Fourier–Hankel transformations, we reduce the problem separately for each harmonic in the angular coordinate to resolving systems of Fredholm integral equations of the second kind. Expressions for stress intensity factors in the vicinity of the crack contour are obtained, and their dependence on the initial stresses and geometrical parameters of the problem is analyzed. For some highly elastic materials, critical compression parameters that correspond to nonaxisymmetric forms of local loss of stability of a material in compression along a near-surface crack are determined.

OS05-4-4 Observation of crack path in a dual phase steel and crystallographic orientation in the vicinity of the crack in 3D

OS05-4-4 Observation of crack path in a dual phase steel and crystallographic orientation in the vicinity of the crack in 3D

OSO204 鉛フリーはんだにおけるクリープ疲労き裂進展のEBSDによる研究(放射光およびEBSD法による材料評価,OS-2 回折法による材料評価)

Crack propagation tests of lead-free solder were conducted using center-notched plates under cyclic tension-compression of three stress waveforms: pp waveform having fast loading and unloading, cch waveform with tension and compression holds, cph waveform having tension hold. Fatigue and creep components of the J integral were evaluated from the load displacements relations using simple estimates. For fatigue loading of pp waveform, the crack propagation rate was expressed as a power function of the fatigue J integral. The creep component due to the hold time greatly accelerated the crack propagation rate. The creep crack propagation rate was expressed as a power function of the creep J integral for each case of cph and cch waveforms. The crack propagation rate for cph waveform was higher than that for cch waveform. SEM observation after the removal of the surface layer showed many tiny cracks in the vicinity of the main crack for cph waveform, suggesting crack propagation acceleration. The grain fragmentation was observed near the crack by EBSD and its amount increased with increasing crack propagation rate.

The electric field modulation by three-dimensional crack on fused silica subsurface

The main factor of laser-induced damage is the modulation to electromagnetic field of laser by the crack on the subsurface. In this paper, a three-dimensional crack model on the exit surface is presented. Three-dimensional finite-difference time-domain (FDTD) method is employed to simulate the electric field intensity distribution in the vicinity of crack on fused silica subsurface. The roles of the crack width, depth, length and the gradient angle in the modulation to the incident light field are analyzed in detail. Results show that the crack size plays an important role in the electric modulation. With the increasing depth and width, the peak value of maximal electric field intensity appears in fused silica. However, the maximal electric field intensity tends to be a constant when the crack length reaches 1 μm. Besides, the enhancement of light intensity becomes obvious when total internal reflection occurs in fused silica. Our calculated results provide an advisable theoretical criterion to the corresponding experiment.

Deformation localization at the tips of shear fractures: An analytical approach

Mechanical heterogeneities are important features in rocks which trigger deformation localization in brittle, ductile or brittle–ductile modes during deformation. In a recent study Misra et al. (2009) have investigated these different processes of deformation localization at the tips of pre-existing planar shear fractures. The authors identified four mechanisms of deformation, ranging from brittle to ductile, operating at the crack tips. Mechanism A: brittle deformation is the dominant process that forms a pair of long tensile fractures at the two crack tips. Mechanism B: nature of deformation is mixed where the tensile fractures grow to a finite length with incipient plastic deformation at the tips. Mechanism C: mixed mode deformation characterized by dominating macro-scale shear bands, and short, opened-out tensile fissures. Mechanism D: localization of plastic bands in the form of a pair of shear bands at each tip without any discernible brittle fracturing. The transition of the mechanisms is a function of orientation ( α) of the crack with respect to the bulk compression direction and the finite length ( l) of the crack. The aim of this study is to present a theoretical analysis to account for the variability of deformation localization in the vicinity of pre-existing shear cracks considering an elastic-plastic rheological model. The analysis calculates the principal tensile stress ( σ 1) and the second stress invariant ( I 2) of the stress field at the fracture tip to explain the transition from Mechanism A (tensile fracturing) to Mechanism D (ductile strain). The results show that σ 1 at the fracture tip increases non-linearly with increasing α and A r (aspect ratio of the shear crack), and assumes a large value when α > 50 o, promoting tensile fractures. On the other hand, I 2 is a maximum at α < 45°, resulting in nucleation of ductile shear bands.

Towards the Derivation of Stress Intensity Factors by Parametric Modelling of Full-Field Thermoelastic Data

Thermoelastic Stress Analysis (TSA) is a well-established full-field technique for experimental stress analysis that has proved to be extremely effective for studying stress fields in the vicinity of cracks. Recently, work has focused on the observation that the stress-sum contours (isopachics) obtained from TSA take the form of a cardioid. Genetic Algorithms (GAs) and Differential Evolution (DE) have proved successful for accurate parameter estimation of the cardioids, thus allowing the SIFs to be calculated. Originally, some curve-fits indicated that a pure cardioid form is inappropriate for the base model, especially for mixed-mode cracks. The deviation from the cardioid form has been shown to be due to higher-order terms within the stress function. The objective of the current paper is to use a modified version of the original methodology (that fitted parameters to a single isopachic) to find the higher-order parameters from the entire data field obtained from the TSA.

Numerical modeling of vibrothermography based on plastic deformation

This paper investigates vibrothermography for the detection of fatigue cracks in aluminum beams using combined experimental and finite element (FE) analyses. First, a FE modal analysis is carried out to predict the optimal excitation parameters to be employed in experimental investigations performed with an infrared camera. A coupled thermo-mechanical model involving plastic deformation heating is then built and used to simulate the thermographic inspection process. The model shows that the stress at the crack faces exceeds the material’s yield stress confirming that the heat generated during plastic deformation leads to crack detection. The model also predicts the detection of cracks as short as 1 mm that is confirmed experimentally with a maximum error of 0.46% on the temperature evolution. The Fourier transform applied on the numerical thermal response shows that the specimen’s temperature at the crack vicinity changes according to the excitation frequency and presents harmonics due to the nonlinearity induced by the crack.