破壊力学 ぜい性薄板の熱線割断と寸法効果

Fracture of brittle and ductile crystals in the generalized stress state. Strength and strain fracture criteria

Brittle materials like semiconductor wafers and ceramic substrates are usually cut by diamond tools, where the generation of microcracks cannot be avoided. Recently, thermal stress cleaving of a thin strip using a point heat source is studied to cut brittle materials without leaving any microcracks on the cleaved surfaces. However, this method often causes thermal damages to the workpieces because of the very high temperature reached around the heating point. It is desirable to cut the electronic substrates, on which precise circuits were already formed, with minimizing the temperature rise. In the present paper, based on the plane thermoelasticity, the time dependent thermal stress intensity factor of a line crack has been obtained for two heating ways : by two point heat sources and a circular zone heat source located symmetrically with respect to the crack line. It was found that a zone heating is better than a point heating in the case of a thin strip cleaving under the restriction of the temperature rise. The most successful cleaving is generally achieved when a radius of a heating region is expanded up to the crack tip. The present calculation was compared with the experimental results and good agreement was found.

The transient analysis of a subsurface inclined crack subjected to a buried dynamic dilatational source

Application of fracture mechanics methods in safety analysis of piping components in subcreep and creep behavior

The objective of the present research is to build a modeling method for delayed hydride cracking (DHC) of zirconium alloys. DHC tests were carried out on Zircaloy-2 cladding tubes in the chamber of a scanning electron microscope to directly observe the crack propagation and measure the crack velocity in the radial direction. These in situ observations showed that a sharply tipped crack propagated at a relatively high rate, while the velocity decreased when the crack tip was blunted, supporting the occur-rence of intermittent crack propagation that could be expected from the DHC mechanism. V-KI curves or diagrams of crack velocity, V, versus stress intensity factor at a crack tip, KI, were obtained as a function of 0.2 % offset yield stress, hydride orientation, and pre-crack depth. The steady state crack velocity and the threshold stress intensity factor for the onset of the crack propagation tended to increase or decrease, respectively, with an increase in the 0.2 % offset yield stress. Analyses of stress distribution and hydrogen diffusion around a crack tip were made using a finite element computer code. The analyses showed that a strong hydrostatic pressure field was generated concentrically around the crack tip and hydrogen diffused towards the crack tip according to the hydrostatic pressure gradient. The crack velocity was estimated from the calculated hydrogen flux rate assuming the critical hydrogen quantity for the crack propagation. There was good agreement between the experiments and the calculations regarding the crack velocity and its dependency on KI. Calculations showed that the increase in the 0.2 % offset yield stress would accelerate the crack propagation by increasing the hydro-static pressure at the crack tip.

A boundary element procedure is formulated to treat frictional contact problems of thermally stressed structures. The aim is the computation of steady‐state and time‐dependent thermal stress intensity factors of interfacial cracks in two‐dimensional bimaterial structures when crack closure conditions are present. The effect of friction between the crack faces is taken into consideration. The problems considered are formulated in an incremental and iterative fashion because of their non‐linear nature. Near crack tip singularities of temperature and displacement fields are modelled through appropriate quarter‐point singular elements. Fracture parameters are evaluated from nodal displacements of singular elements utilizing proper formulas. Numerical results are compared with available solutions from the literature, where possible. Good agreement between them can be found. Copyright © 2005 John Wiley & Sons, Ltd.

An imaging and a strain mapping in the vicinity of a crack tip in material were investigated using a high energy white X-ray obtained from BL28B2 beam line at SPring-8 in Japan. Low-alloy and high-tensile steel (JIS G3128 SHY685) was used as a specimen prepared in the G-type geometry. A fatigue crack was introduced into the specimen by a cyclic loading. The imaging of the crack in the specimen was carried out by using the X-ray CCD camera that can detect the X-ray transmitted through the specimen. To measure the strain, the synchrotron white X-ray beam, which had a height of 80μm and a width of 300μm, was incident on the specimen with the Bragg angle θ of 5 degrees using the energy dispersive X-ray diffraction technique. The internal strain in the vicinity of the crack tip was mapped out by scanning the irradiated X-ray position around it. As the results, the imaging of the crack, with about 1mm length, in the specimen under the loading of crack opening was practicable by using the synchrotron white X-ray. The map of the internal strain near the crack tip of the steel of 5mm thickness could be obtained using the white X-ray with energy ranging from 50keV to 150keV. The plastic region estimated from the distribution of the FWHM of diffracted X-ray profile almost agreed with the theoretical value calculated by fracture mechanics. It was confirmed that the synchrotron white X-ray is useful for the imaging of the internal crack and the strain mapping near it.

Analysis of subsurface crack propagation and implications for wear of elastically deforming materials

DeterminingK and related stress-field parameters from displacement fields

Modeling surface pressure, interfacial stresses and stress intensity factors for layered materials containing multiple cracks and inhomogeneous inclusions under contact loading

Real-time measurement of shear fatigue crack propagation at high-temperature using the potential drop technique

: Crack tip deformations and fracture parameters in functionally graded glass-filled epoxy beams are experimentally evaluated under static and dynamic loading conditions. Beams with unidirectional, monotonic elastic gradients and cracks along the gradient are examined. SEN samples with increasing or decreasing Young's modulus ahead of the crack tip are studied in symmetric four-point bending and one-point impact loading configurations. Optical method of Coherent Gradient Sensing (CGS) is used to measure crack tip deformations prior to crack initiation. For impact loading experiments, CGS is used in conjunction with high-speed photography for recording instantaneous deformation fields. Stress intensity factors (SIF) or SIF-histories in functionally graded materials (FGM) based on locally homogeneous material descriptions in the immediate crack tip vicinity are evaluated and compared with companion finite element simulations. The influence of elastic gradients in FGM samples with cracks on the compliant and stiff sides of the beam are quantified relative to their homogeneous counterparts and with each other. Under static loading conditions, the crack tip located on the compliant side of the beam is elastically shielded when compared to the situation when the crack is on the stiffer side of the same FGM beam. Under dynamic conditions, however, elastic gradients affect crack initiation differently. Crack initiation in an FGM with a crack on the stiff side of the beam and impact occurring on the compliant edge is delayed when compared to the opposite configuration. Independent finite element simulations of FGMs with idealized elastic gradients with identical crack tip elastic properties suggest that lower crack tip loading rate in the former is responsible for the differences.

This paper presents the effect of eccentric cracks on the behavior of stress intensity factors (SIF) of single edge crack in bi-material plates. According to literature, it is found that most of the research conducted previously more on central single edge crack and it is well understood. However, not many research conducted on the eccentric stress intensity factor of single edge crack in bi-material plates. In order to evaluate the SIFs of eccentric edge cracks, ANSYS finite element software is used to model plain strain single edge crack in a plate subjected mode I loadings. The present SIFs are then validated with the existing central crack and it is well agreed to each others. According to the present results, it is found that mode I SIFs decreased insignificantly and mode II SIFs decreased asymptotically when the crack situated away from the central line. As expected all types of SIFs increased when crack length is increased.

Monitoring the generation and expansion of fatigue cracks in mechanical structures is critical to structural safety. To solve this problem, an optical sensing method for identifying crack propagation in mechanical structures is proposed. On-line monitoring of crack location, length, and expansion direction during crack propagation is achieved by combining micro-cavity array (MCA) fiber and optical frequency domain reflection (OFDR) system. Two adjacent ultra-short FBGs are used as a micro-cavity (MC) sensing element to obtain the strain distribution near the crack tip through a high spatial resolution distributed strain detection system. The crack state is obtained by combining the classical theoretical model, and a near real-time detection is achieved. Thereby, the system can perform an online monitoring and timely alarms on cracks. In this paper, we show the monitoring of the crack state during the process of preset crack length of 20 mm and crack propagation to 50 mm. An MCA fiber with 2542 MC elements with a spatial resolution of 1 mm is densely laid perpendicular to the crack tip direction. The crack propagation process is realized by using fatigue machine to apply cyclic load on aluminum alloy specimen, the distribution of non-uniform strain field of aluminum alloy specimen is obtained by detecting the wavelength drift of each MC element in the MCA fiber. In the test result, the distribution of the non-uniform strain field of the aluminum alloy specimen measured by the MC element is consistent with the simulation results. Consistently, the location of the crack tip and the detection of the crack length can be realized according to the distribution of the non-uniform strain field, and the feasibility of the aluminum alloy crack extension recognition system based on the MCA fiber is verified.

Purpose: Carefully investigate the stress-strain state of the side grooved I-beam specimen with edge crack and determine the effect of crack length and crack faces friction on stress intensity factor at transverse shear. Design/methodology/approach: The finite element method was used to estimate the stress-strain state of I-beam specimen at transverse shear. For this purpose, a fullscale, three-dimensional model of the specimen was created, which precisely reproduces its geometry and fatigue crack faces contact. For the correct reproduction of the stress singularity at the crack tip, a special sub-model was used, which has been tested earlier in solving similar problems of fracture mechanics. In order to improve the accuracy of the calculations, for crack plane and cross-section of the specimen on the crack extension modeling, an algorithm for changing the crack length without changing the total number of elements in the model was developed and applied. Young's modulus and Poisson's ratio of structural steels were specified for the model material. The static loading of the model was realized assuming small scale yielding condition. The stress intensity factor was found through the displacement of nodes in the prismatic elements adjacent to the plane and the front of the crack. Findings: Mathematical dependences, which show an increase of stress intensity factor in the I-beam specimen with an increase in the crack length, and its decrease with an increase of crack faces friction factor at transverse shear, were established. The results are compared with the partial cases known from the literature and their good convergence was shown. Research limitations/implications: By analyzing the obtained graphical dependences, it is established that for relative crack lengths less than 0.4 there is a significant influence of the initial notch on the stress-strain state of the specimen, and for the lengths greater than 0.9 an influence of constrained gripping part took place. For this reason, all subsequent calculations were carried out in the range of relative crack length from 0.4 to 0.9, which represents the applicability range of the final calculation formula. Increasing of the crack faces friction factor from 0 to 1 monotonically reduces the stress at the crack tip. For a short crack, this effect is 1.5 times greater than for a long one, which is reflected by the calculation formula. Practical implications: Using the proposed calculation formula, one can calculate the stress intensity factor in the I-beam specimen, and to determine the crack growth resistance characteristics of structural steels at transverse shear. Originality/value: A new, easy-to-use in engineering calculations formula is proposed for stress intensity factor determination in the I-beam specimen at transverse shear. The formula takes into account crack faces friction for various crack lengths.