In-Situ Observation of Propagating-Crack Front Geometry in Thermal Stress Cleaving of Thin Glass Plates

Geometry of propagating crack front and crack opening displacement were observed in situ in thermal stress cleaving of thin soda-lime glass plates using the interference fringe method. The crack front inclines to the propgaating direction advancing more at the heated surface due to the temperature gradient in the thickness. Mode I stress intensity factor was estimated from measured COD and its distribution along the curved crack front was found to be almost uniform. The uniform distribution was also confirmed from three-dimensional thermal stress FEM analysis. When a crack length at the middle plane of the plate thickness is used， two-dimensional analysis gives good approximation of K1 attained at the propagating crack fron

Thermal Stress Intensity Factor for a Semi-infinite Crack Parallel to a Straight Edge

A time-dependent thermal stress field in a semi-infinite plane having a semi-infinite crack parallel to the edge of the plate was analyzed when the vicinity of the crack tip was locally heated. The straight boundary of the semi-infinite plate were assumed to be kept at zero temperature or thermally insulated. The thermal stress field was analyzed after each stepwise time advance by the Body Force Method. It was found that a mixed mode deformation occurs at the crack tip independently of the heating time and the stress intensities of mode I and II vary with the heating location. Moreover, the increase of heat dissipation from the plate surfaces may be effective for the decrease of K_II

Analysis of the Stress Intensity Factors and the Shape of a Crack Growth on a Thermal Stress Cleaving

3D finite element analysis of thermo-elastic problem was performed to relate between the crack front geometry and the crack growing velocity in a thermal stress cleaving. In a stable crack growth, the crack front geometry through a plate thickness becomes so as for the stress intensity factors to distribute as much same value along a crack front. The faster a crack growing velocity, the wider a crack front leaves from a center of heat source to show stronger stress singularity. Analyzed result explains well the experiments

Control of a crack path by air blowing in the thermal stress cleaving

In the case of unsymmetrical thermal stress cleaving, cracks will not grow straight because mixed mode of stress intensity factors arises. To realize the thermal stress cleaving as a standard cutting process, this point has to be overcome. One solution may be cooling by the air blow combined with heating. Cooling reduced both of stress intensity factors, KI and KII. But the latter reduction is more than the former. In this paper, stress intensity factors are estimated for unsymmetrically edge-cracked rectangular glass plates by FEM and the variation of the ratio of KII/KI is discussed as a function of the cooling location, cooling area and initial plate temperature

Influence of Thermal Boundary on the Problems of Thermal Stress Cleaving

A closed-form expression of the stress intensity factor is derived for a problem of straight crack in an infinite plate under thermal stresses induced by a temperature rise at an infinitesimal area. The present solution is applicable to wide range of uncoupled thermoelastic problems and is especially useful to understand the mechanical behavior of cracks in thermal stress cleaving (TSC). For instance, the present solution is useful for the prediction of an optimal temperature distribution in TSC, which is a function of various conditions such as shape of the plate, crack path, cleaving velocity and so on. Some basic problems of TSC formerly analyzed by the use of thermoelastic fields induced by a heat source were examined to confirm the applicability of the present solution

Effect of Cooling by Airblow on the Thermal Stress Cleaving of a Thin Plate

The thermal stress field induced by a line heat source like an electric resistance wire causes crack extension. Taking advantage of this phenomenon, some glassy materials can be cleaved without any microcracks on the cleaving surface. On the cleaving process of glass plates, airblow has been comfirmed to be effective. In this paper, an optimum heating position and an effective airblow timing are investigated to increase the thermal stress intensity factors

Estimation of Stress Intensity Factors from Crack Opening Displacements Measured by Interference Fringes

Crack opening displacements were measured by the interference method, when a crack propagates in a transparent material like a glass. To determine the position of each order of the interference fringes, Discrete Fourier Transform and Maximum Entropy Method were applied to the digital image processing. Distribution of the stress intensity factors through a plate thickness and a shape of the crack front were successfully obtained