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Direct observation and analysis of crack front morphology during crack pinning by heterogeneous interface

Direct observation and analysis of crack front morphology during crack pinning by heterogeneous interface

Abstract. Adhesion of thin film multilayers deposited on glass is a crucial issue for many industrial applications. Thus, it becomes of great interest to measure and also to increase the adhesion. Many mechanisms of toughening a brittle solid can be found in literature; but few of them can be applied to thin film layer. By introducing a heterogeneous interfacial toughness field, it should be possible to increase adhesion. This toughness modification would be the consequence of the existence of a pinning regime due to a local change of the toughness. To experimentally validate this new approach of adhesion modification, we investigate the crack front pinning by performing cleavage tests on multilayer coated samples with a heterogeneous interfacial toughness. We have tested different patterns of pinning region. The crack front morphology was nicely described in the framework of the perturbative approach initially developed by Gao and Rice and allowed us to determine the local value of the energy release rate (~adhesion).
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On the modified monotonic loading concept for the calculation of the cyclic j-integral

On the modified monotonic loading concept for the calculation of the cyclic j-integral

This paper investigates an approach for calculating the cyclic J-Integral through a new industrial application. A previously proposed method is investigated further with the extension of this technique through a new application of a practical 3D notched component containing a semi-elliptical surface crack. Current methods of calculating the cyclic J-Integral are identified and their limitations discussed. A modified monotonic loading concept is adapted to calculate the cyclic J-integral of this 3D Semi Elliptical Surface Crack under cyclic loading conditions. Both the finite element method (FEM) and the Extended Finite Element Method (XFEM) are discussed as possible methods of calculating the cyclic J-Integral in this investigation. Different loading conditions including uniaxial tension and out of plane shear are applied, and the relationships between the applied loads and the cyclic J-integral are established. In addition, the variations of the cyclic J-integral along the crack front are investigated. This allows the critical load that can be applied before crack propagation occurs to be determined as well as the identification of the critical crack direction once propagation does occur.
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Simulation of Fatigue Life Behavior of Circular Cross Section Bar with Straight Fronted Crack

Simulation of Fatigue Life Behavior of Circular Cross Section Bar with Straight Fronted Crack

The behavior of a surface crack with straight front in a round bar under tension cyclic loading has been analyzed. The effects of supposed cracks on the fatigue behavior of round bar were simulated as well. The finite element method is employed in the present simulation technique because of its versatility and generality for complicated cracked structures. Also, the 1/4-point displacement method was used for evaluating the stress intensity factor by strain/stress singularity at the corner of a 20-node isoparametric element. It is noted that a few parameters, namely the initial crack aspect ratio has an influence on the crack front evolution, provide that the crack geome- try is represented by relative dimensions with respect to the bar diameter. Results were shown, under pure cyclic tension loading, it can be seen that the crack propagation paths differ with diverse initial flaw depths, but con- verge to the same configuration when the crack depth ratio b/D is larger than about 0.5. The functions of aspect ratio and relative crack depth are obtained and by means of the function, the crack front shape and crack growth rate can be predicted well. This observation is obtained by means of the computational model. By using extra- polate method equations were offered for K 1C and crack growth rate estimations. Based on these equations, the
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Master Curve Analysis of a German RPV Steel Considering Local Constraint

Master Curve Analysis of a German RPV Steel Considering Local Constraint

The material investigated is a German 22 NiMoCr 37 (A 508 Class 2) rector pressure vessel steel. The fracture toughness of the material is determined using a variety of fracture mechanics specimens including Charpy sized SE(B)10x10 specimens with three different crack depths (a/w ≈ 0.51, a/w ≈ 0.18 and a/w ≈ 0.13), C(T) specimens with different sizes (C(T)25 and C(T)50, both with a/w ≈ 0.51) as well as center cracked CC(T)100 specimens with 2a/w ≈ 0.51. All specimens are tested displacement controlled under quasi static loading. The test temperatures are varied between T = -120°C and T = 0°C. The fracture toughness is determined according to ASTM Standard E 1921-03 (2003). For the shallow cracked SE(B)10x10 specimens, the plastic correction factor in the approximate formulae given in ASTM E1921 is adjusted such that the obtained approximation for the average J-integral matches with the value obtained from a finite element analysis of the test (see Section 3) using its definition as a path independent integral around the crack front. The fracture toughness for the CC(T) specimens is obtained in a similar manner. All toughness results are normalized to a crack front length of B = 25 mm as required in ASTM E 1921 (2003).
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Fracture mechanics analysis for pressurized thermal shock of reactor pressure vessel based on the refined mesh division of crack tip

Fracture mechanics analysis for pressurized thermal shock of reactor pressure vessel based on the refined mesh division of crack tip

In order to calculate fracture mechanics parameters of the crack in the specific position of the reactor pressure vessel, the crack model is established by the finite element method. Due to the existence of the singular field at the crack tip, the fine meshing is the prerequisite for obtaining accurate stress intensity factor. In this paper, boundary release and mesh relaxation techniques are used to ensure the mesh quality of the crack front. The example of boundary release and mesh relaxation is shown in Figure 2.

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Understanding Avalanche Statistics on Crack Dynamics

Understanding Avalanche Statistics on Crack Dynamics

In brittle solids, stress concentration at crack tips makes the macroscopic fracture properties very sensitive to heterogeneities at the scale of the micro structure. So, the macroscopic resistance of a solid depends strongly on the resistance fluctuations at the microscopic scale. To describe quantitatively this phenomenon in disordered materials, we model first the behavior of the crack by a stochastic equation of motion taking into account the role of the microstructure. Our approach is first validated by comparing our theoretical predictions with recent experimental observations made on the dynamics and morphology of a crack front. We show how to use this approach to determine the e ff ective resistance of a brittle material from the characteristics of its micro structure.
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Use of the Master Curve Methodology for Real Three Dimensional Cracks

Use of the Master Curve Methodology for Real Three Dimensional Cracks

At VTT, development work has been in progress for 15 years to develop and validate testing and analysis methods applicable for fracture resistance determination from small material samples. The VTT approach is a holistic approach by which to determine static, dynamic and crack arrest fracture toughness properties either directly or by correlations from small material samples. The development work has evolved a testing standard for fracture toughness testing in the transition region. The standard, known as the Master Curve standard is in a way ”first of a kind”, since it includes guidelines on how to properly treat the test data for use in structural integrity assessment. No standard, so far, has done this. The standard is based on the VTT approach, but presently, the VTT approach goes beyond the standard. Key components in the standard are statistical expressions for describing the data scatter, and for predicting a specimen’s size (crack front length) effect and an expression (Master Curve) for the fracture toughness temperature dependence. The standard and the approach it is based upon can be considered to represent the state of the art of small specimen fracture toughness characterization. Normally, the Master Curve parameters are determined using test specimens with "straight" crack fronts and comparatively uniform stress state along the crack front. This enables the use of a single K I
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Finite Element Analysis of J-Integral for Surface Cracks in Round Bars under Combined Mode I Loading

Finite Element Analysis of J-Integral for Surface Cracks in Round Bars under Combined Mode I Loading

node, whereas the axial force is directly applied in the direction-x on the cross-sectional area of the bar. At the other end, the component is constrained appropriately. In order to obtain a suitable finite element model, it is necessary to compare the proposed model with other published models [11, 16, 17]. In this work SIFs results are used for the validation purposes. Since, it is hard to find the result of J-integral results for these particular Abstract: This paper numerically discusses the role of J-integral along the surface crack front in cylindrical bar under combined mode I loading. It is also verified the analytical model derived from the first part of this paper by comparing the results obtained numerically using ANSYS finite element program. It is found that the proposed model capable to predict the J-integral successfully along the crack front but not for the area away from the deepest crack depth. This is probably due to the fact that the problem of singularity.
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Detection of Through-Deck Type Fatigue Cracks in Steel Bridges by Self-Reference Lock-in Thermography

Detection of Through-Deck Type Fatigue Cracks in Steel Bridges by Self-Reference Lock-in Thermography

Abstract. A new remote nondestructive inspection technique, based on thermoelastic temperature measurement by infrared thermography, is developed for detection and evaluation of fatigue cracks propagating from welded joints in steel bridges. Fatigue cracks are detected from localized high thermoelastic temperature change at crack tips due to stress singularity under variable loading from traffics on the bridge. Self- reference lock-in data processing technique is developed for the improvement of signal/noise ratio in the crack detection process. The technique makes it possible to perform correlation processing without an external reference signal. It is very difficult to detect through-deck type fatigue cracks in steel decks by the conventional NDT technique, since they are not open to the inspection. In this paper, self-reference lock-in thermography is applied for detection of through-deck type fatigue cracks. Experiments are carried out to steel deck sample, which simulates an actual steel bridge, during crack propagation test. It is found that significant stress concentration zone can be observed near the crack front, which enabled us to detect through-deck type fatigue cracks and to estimate its size.
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Determination of the Fracture Parameters in a Stiffened Composite Panel

Determination of the Fracture Parameters in a Stiffened Composite Panel

* CEDI(i) Coefficients for each of the 6 stress-work density terms in the* * calculation of the equivalent domain integral. * * DSHAP(i,j) Array for the derivative of the i-th shape function w.r.t. the * * local coordinate j. j=1:x1; j=2:x2; j=3:x3. * * DSRST(i,j) Array for the derivatives of the s-function w.r.t. the natural * * coordinates j at the i-th integration point. j=1:xi; j=2:eta; * * j=3:zeta. * * DSX(i,j) Array for the derivatives of the s-function w.r.t. the local * * coordinates j at the i-th integration point. j=1:x1; j=2:x2; * * j=3:x3. * * DUX1(i,j) Array for the derivatives of the j-th displacement component * * w.r.t. the local x1 coordinate at the i-th integration point. * * j=1:u1; j=2:u2; j=3:u3. * * KE The element number in the ring. * * KR The ring number. * * KS The element layer(segment) number along the crack front. * * RX(i,j) Inverse of the Jacobian. i=1..3; j=1..3 * * SF(i) s-function at the i-th integration point, i=1..8. * * SIGMA(i,j) 3x3 array for the stress tensor of the element. * * TERMEDI(i,j) Array for the terms in the expression of the equivalent * * domain integral. i=1: 1st term; i=2: 2nd term. * * TERMI2(i,j) Array for the terms in the expression of the interaction * * integral I(2). i=1: 1st term; i=2: 2nd term. * * TERMII(i,j) Array for the terms in the expression of the interaction * * integral I(1). i=1: 1st term; i=2: 2nd term
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Estimation of the Onset of Crack Growth in Ductile Materials

Estimation of the Onset of Crack Growth in Ductile Materials

Abstract: In this paper, the ductile fracture mechanism is discussed. The results of the numerical and experimental analyses are used to estimate of the onset of the crack front growth . It is assumed that the ductile fracture in front of the crack starts at the location along the crack front where the accumulated effective plastic strain reaches a critical value. It is also assumed that the critical effective plastic strain depends on the stress triaxiality and the Lode angle. The experimental programme was performed using five different specimen geometries, three different materials and three different temperatures of +20°C, -20°C and -50°C. Using the experimental data and the results of the finite element computations, the critical effective plastic strains are determined for each material and each temperature. However, before the critical effective plastic strain is determined, a careful calibration of the stress–strain curves was performed after modification of the Bai– Wierzbicki procedure. Finally, by analysing the experimental results recorded during the interrupted fracture tests and scanning microscopy observations, the research hypothesis is verified.
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Investigation of Fracture Toughness Measurement for Pipeline Steels Based on SE(B) and SE(T) Specimens

Investigation of Fracture Toughness Measurement for Pipeline Steels Based on SE(B) and SE(T) Specimens

This thesis deals with issues related to the experimental determination of the fracture toughness resistance curves, i.e. the J-integral (J)-R curve and crack tip opening displacement (CTOD)-R curves, using the single-edge bend (SE(B)) and single-edge tension (SE(T)) specimens. First, the impact of the crack front curvature on the J-R curve measured from the SE(B) specimen is investigated through systematic linear-elastic and elastic-plastic three-dimensional (3D) finite element analyses (FEA) of SE(B) specimens containing both straight and curved crack fronts. Three average relative crack lengths are considered, namely 0.3, 0.5 and 0.7, and three specimen width-to-thickness ratios are considered: 0.25, 0.5 and 1. The curved crack fronts are characterized by a power-law expression. The analysis results suggest that the crack length evaluated from the CMOD compliance of the SE(B) specimen is insensitive to the crack front curvature and that the impact of the crack front curvature on the experimentally-evaluated J values varies with the specimen configurations. For a given specimen configuration, as the crack front curvature increases, the value of J evaluated based on the test standard ASTM E1820-11 without considering the crack front curvature becomes less conservative and tends to overestimate the actual J. New crack front straightness criteria that are in most cases less stringent than ASTM E1820-11, are recommended. The accuracy of the double clip-on gauge method for experimentally determining CTOD is examined through systematic 3D elastic-plastic large-strain FEA of clamped SE(T) specimens. The relative crack lengths of the specimens range from 0.3 to 0.7, and the thickness-to-width ratios are 0.5, 1 and 2. It is observed that the CTOD values determined from the double clip-on gauge method may involve significant errors. This error primarily depends on the crack length, the material property and the loading level. Based on the analysis results, a modified CTOD evaluation equation is developed to improve the accuracy of CTOD evaluated using the double-clip on gauge method.
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Semi-large Scale Experiments Performed on Specimens with  Underclad Cracks

Semi-large Scale Experiments Performed on Specimens with Underclad Cracks

The specimens were manufactured from artificially aged material of decommissioned (not operated) reactor pressure vessel of WWER 440 Type NPP Nord. Each of the specimens contained through-thickness crack embedded in the base material (with approx. 3 mm ligament separating the upper crack front from cladding), and was loaded by 4PB loading at room temperature. During loading, majority of specimens exhibited pop-ins followed by ductile tearing of cladding and final failure; only 3 specimens fractured through suddenly, without preceding pop-ins. Evaluation of the experiments concentrated on both process of cleavage fracture in base material and process of ductile tearing in cladding.
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Environmentally Assisted Cracking Behavior of Dissimilar Metal Weldments under High Temperature Water Conditions

Environmentally Assisted Cracking Behavior of Dissimilar Metal Weldments under High Temperature Water Conditions

fatigue tests were conducted on a closed-loop, servo-electric machine with a water circulation loop under load control and at a frequency of 0.02 Hz. The constant load amplitude was periodically reduced to maintain a constant stress intensity factor range to characterize the fatigue crack growth rates in the weld, interface and base metal. The load amplitude was set at an R ratio of 0.2 by an inner load cell control, which deducted the friction force between the pulling rod and the sealing material. The external load cell measurements including the friction force were also monitored for a comparison with the ones taken by the inner load cell. The filtered air was injected into the water tank to maintain a saturated oxygen level in the water environment. The conditions of the water environment are summarized in Table 4. The crack length was measured by an ACPD technique. The AC current leads and PD probes were made of SS 316L wires 0.5 mm in diameter. The wires were spot-welded to the specimens. ACPD signals were measured by a Matelect model CGM5R crack growth monitor. In the present study, an alternating current of 230 mA was applied to the specimens at a frequency of 3 kHz. An amplification gain of 70 dB was selected for all tests. 6) The final fatigue crack length measurement was further calibrated against the average value of five measure- ments taken along the crack front on the fracture surface by a microscope at a magnification of 20 according to ASTM E 647.
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RULES REDUCTION AND OPTIMIZATION OF FUZZY LOGIC MEMBERSHIP FUNCTIONS FOR 
INDUCTION MOTOR SPEED CONTROLLER

RULES REDUCTION AND OPTIMIZATION OF FUZZY LOGIC MEMBERSHIP FUNCTIONS FOR INDUCTION MOTOR SPEED CONTROLLER

As a crack model, this proposed model has adopted the physical characteristic of crack. It is similar to the idea in other existing researches in crack modeling. In this proposed model, the higher value of crack initial energy makes the crack grows larger. In the other hand, the crack resistance factor reduces the crack growth probability [3]. The energy reduction variable has the same function with the energy release rate terminology that is used in the existing research [3]. The crack segment length adopts stochastic approach while in some existing research, crack segment length adopted deterministic approach [3]. The crack propagation direction uses continuous approach and it is similar to other existing researches [3,4]. In this proposed model, material surface can be viewed as solid material while other existing research viewed materials as a set of disjoint elements [19].
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The Influence Of Crack On The Buckling Of Axially Compressed Truncated Cones

The Influence Of Crack On The Buckling Of Axially Compressed Truncated Cones

From the literature study we found that although geometric imperfection plays dominant role in load carrying capacity, but there are few studies conducted on imperfect cones with crack introduced. This experimental project work aims at examining the effect of crack orientation (i.e., axial crack, circumferential crack and angle crack) with different crack length on the load carrying capacity of conical shells subjected to axial compression.

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Two Dimensional Crack Analysis of AL7075 7651 Under Various Tensile Load

Two Dimensional Crack Analysis of AL7075 7651 Under Various Tensile Load

PLATE MODEL: The plate model as shown in Figure 2and3 is modelled using ANSYS with two holes at the centre of the place. The plate is found to have symmetry along the centre of the hole and the crack is modelled in the horizontal plane at the ends of the hole. The symmetry condition means that only one half of the plate is modelled to introduce the crack and find the stress intensity factor for the whole model. The visibility of the crack at the centre of the model is more in the symmetric modelling of the plate. The plate shown in Figure 2 has two clamped portions on either ends of the model, these portions will not be modelled in ANSYS as it is used to clamp the plate while experimental analysis are carried out.The dimensions of the plate are determined by the following relationship. r/t = 0.075, 0.1, 0.2, 0.333, 0.5, 1.0, 2.0, 3.0, 6.0
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ALTERNATIVE SOLUTION TO THE DETECTION OF CRACK LOCATION AND CRACK DEPTH IN STRUCTURE BY USING SOFTWARE ANALYSIS METHOD

ALTERNATIVE SOLUTION TO THE DETECTION OF CRACK LOCATION AND CRACK DEPTH IN STRUCTURE BY USING SOFTWARE ANALYSIS METHOD

 An analytical and experimental approach by H. Nahvi and M. Jabbari et al. to the crack detection in cantilever beams by vibration analysis. Sensibility analysis of the inverse problem of the crack parameters (location and depth) determined by M. B. Rosales, C P Filipich and F S Buezas et al. An efficient numerical technique is necessary to obtain significant results.

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Experimental Study and Reactive Transport Modeling of Boric Acid Leaching of Concrete

Experimental Study and Reactive Transport Modeling of Boric Acid Leaching of Concrete

1-D and 2-D reactive transport simulations were conducted to determine the degree of concrete dissolution and pH change that may occur as boric acid solution diffuses into the matrix or flows in the crack of a reinforced concrete structure. The depth of concrete leaching by boric acid solution derived from the 1-D model agree relatively well with the measured leaching depths in concrete specimens that were immersed in boric acid solutions for up to 300 days. The 1-D simulation results indicate that leaching by boric acid solution diffusing into concrete is mitigated by the acid- neutralizing capacity of the cement minerals such that reinforcement steel with a 5.1-cm [2-in] concrete cover is unlikely to undergo corrosion for at least 77 years. The 2-D simulation results indicate that concrete provides significant chemical reactivity to neutralize the acidic pH of borated water that may leak from an SFP, flow into a crack in the SFP concrete structure, and diffuse into the concrete matrix. However, reinforcement steel close to the crack inlet, whether exposed in the crack or covered by concrete, can become susceptible to corrosion depending on the crack aperture, solution flow rate and duration, and boric acid concentration.
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Finite Element Analysis of Fatigue Life Assessment of Butt & Fillet Welded Joint with Cracks

Finite Element Analysis of Fatigue Life Assessment of Butt & Fillet Welded Joint with Cracks

The S-N curves based fatigue designs of welded components and structures do not fully represent the weld details like effects of geometry, welding process and material defects. Experimental fatigue life prediction of welded joints is time consuming and costly. These limitations have prompted the need for adopting other ways for modeling the fatigue process rather than simply relating applied stress and fatigue life as in the S-N curves. A recent trend in this regard is the use of local approaches like fracture mechanics based attempts to model the whole fatigue process by considering the above effects. The stress at the crack tip can be accurately evaluated by means of linear elastic fracture mechanics by using suitable singular crack tip elements and proper mesh density. This can be further used to predict the fatigue life of a welded joint by using Paris’s law. The final crack length of a welded joint can be found out by virtual crack extension method (VCEM), where the final crack length is that crack length at which for a given loading the stress intensity factor of a crack approaches or exceeds an upper limit known as critical stress intensity factor.
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