Chwan-Huei Tsai

Fracture Mechanism of Laser Cutting With Controlled Fracture

Laser cutting using the controlled fracture technique has great potential to be used for the machining of brittle materials. In this technique, the applied laser energy produces a mechanical stress that causes the material to separate along the moving path of the laser beam. The material separation is similar to a crack extension and the fracture growth is controllable. The fracture mechanism of laser cutting with controlled fracture is studied in this paper. The temperature and stress distributions are obtained by using the finite element software ANSYS. The laser heat first induces compressive stress around the laser spot. After the passage of the laser beam, the compressive stress is relaxed, and then a residual tensile stress is induced, which makes the fracture grow from upper surface to lower surface of the substrate. The stable separation of the brittle material is due to the local residual tensile stress. However, if the tensile stress is distributed throughout the thickness around the crack tip, the crack will extend unstably. The experimental materials in this study are alumina ceramic and the laser source is CO 2 laser. It is found that the crack propagation is non-uniform and the speed is variable during the cutting process. The relationships between laser power cutting speed, diameter of laser spot, and specimen geometry are obtained from the experimental analysis, and the phenomena are also explained from the results of stress analysis.

Laser milling of cavity in ceramic substrate by fracture-machining element technique

Abstract A new laser-machining technique based on the concept of fracture-machining element is proposed to shape a closed cavity for advanced ceramic. The material removal of the three-dimensional milling can be composed of the removal of a series of elements. The proposed fracture-machining elements are the triangular element and quadrilateral element. A focused laser is used to scribe a series of groove-cracks at the surfaces of the material. The fracture-machining elements are enclosed with the scribed lines. After the completion of the laser scribing, a defocused laser beam is applied throughout the scribed lines. The defocused laser heat generates the tensile stresses that are concentrated at the tip of groove-crack and induces the extension of the groove-cracks. The material removal is due to the linkage of the groove-cracks. The technique of the conventional laser milling requires high laser power which results in many cracks during the grooving process. The proposed technique can be employed not only for the edge milling, but also for the central cavity milling, and the laser power required is much less than that in the conventional method. The experimental material is alumina ceramic, and the laser sources are CO 2 laser and Nd:YAG laser. The SEM photographs of the removed elements are obtained to analyze the micro-mechanism of the fracture process. The temperature and stress distributions are analyzed by using the finite element software ANSYS. The relationship between laser power and element size is obtained from the experimental analysis, and the phenomena are also explained from the results of stress analysis.

Weight Functions and Stress Intensity Factors for Axial Cracks in Hollow Cylinders

In this study, stress intensity factors for axial cracks in hollow cylinders subjected to mechanical and thermal loadings are determined by using the weight function method. The weight function is a universal function for a given cracked body and can be obtained from any arbitrary loading system. The weight function may be thought of as Green’s function for the stress intensity factor of cracked bodies. Once the weight function for a cracked body is determined, the stress intensity factor for any arbitrary loading can be simply and efficiently evaluated through the integration of the product of the loading and weight function. A numerical method for the determination of weight functions relevant to cracked bodies with finite dimensions is used. Results for weight functions covering a wide range of hollow cylinder geometries are presented in functional or graphical form. The explicit crack face weight functions for applying mechanical loadings are obtained by using the least-squares fitting procedure. As a demonstration, some examples of special loading problems are solved by the weight function method, and the results are compared with available results in the published literature.

Theoretical Transient Analysis of the Interaction Between a Dynamically Propagating In-Plane Crack and Traction-Free Boundaries

In this study, the transient response of a propagating in-plane crack interacting with half-plane boundaries is investigated in detail. The reflected waves which are generated from traction-free boundaries will interact with the propagating crack and make the problem extremely difficult to analyze. The complete transient solutions are constructed by superimposing fundamental solutions in the Laplace transform domain. The fundamental solutions represent the responses of applying exponentially distributed loadings in the Laplace transform domain on the surface of half-plane or the propagating crack faces. We focus our attention on the determination of the dynamic stress intensity factor. The dynamic stress intensity factors of a propagating crack in a configuration with boundaries and subjected to dynamic loadings are obtained in an explicit closed form. The transient solutions obtained in this study are in agreement with the experimental results from the literature. Some interesting phenomena observed in the published experimental works are also identified and discussed. it is concluded that the reflected waves generated from the boundary parallel to the crack have much stronger influence on the propagating crack than those generated from the boundary perpendicular to the crack. When the reflected waves generated from the boundary parallel to the crack return to the moving crack tip, the stress intensity factor will increase rapidly.

Transient Analysis of a Propagating In-Plane Crack in a Finite Geometry Body Subjected to Static Loadings

In this study, a cracked body with finite boundaries subjected to static loading and the crack propagating with a constant speed are analyzed. The interaction of the propagating crack with reflected waves generated .from traction-free boundaries is investigated in detail. The methodology for constructing the scattered field by superimposing the fundamental solution in the Laplace transform domain is proposed. The fundamental solutions represent the responses of applying exponentially distributed loadings in the Laplace transform domain on the surface of a half-plane or a crack. The dynamic stress intensity factors of a propagating crack induced from the interaction with the first few reflected waves generated from the traction-free boundary are obtained in an explicit closed form. The analytical solutions of dynamic stress intensity factors are compared with available numerical and experimental results and the agreement is quite good. We find one thing very interesting: the dynamic stress intensity factor for a long time period is a universal function of the instantaneous extending rate of a crack tip times the static stress intensity factor for an equivalent stationary crack for the finite strip problem. It was also found that the reflected waves generated from free boundaries always increase the stress intensity factor, and the influence from reflected waves generated from the boundary, which is perpendicular to the crack, are weaker than those generated from the boundary, which is parallel to the crack.

Elastodynamic Fracture Analysis of Multiple Cracks by Laplace Finite Element Alternating Method

The Laplace finite element alternating method, which combines the Laplace transform technique and the finite element alternating method, is developed to deal with the elastodynamic analysis of a finite plate with multiple cracks. By the Laplace transform technique, the complicated elastodynamic fracture problem is first transformed into an equivalent static fracture problem in the Laplace transform domain and then solved by the finite element alternating method developed. To do this, an analytical solution by Tsai and Ma for an infinite plate with a semi-infinite crack subjected to exponentially distributed loadings on crack surfaces in the Laplace transform domain is adopted. Finally, the real-time response can be computed by a numerical Laplace inversion algorithm. The technique established is applicable to the calculation of dynamic stress intensity factors of a finite plate with arbitrarily distributed edge cracks or symmetrically distributed central cracks. Only a simple finite element mesh with very limited number of regular elements is necessary. Since the solutions are independent of the size of time increment taken, the dynamic stress intensity factors at any specific instant can even be computed by a single time-step instead of step-by-step computations. The interaction among the cracks and finite geometrical boundaries on the dynamic stress intensity factors is also discussed in detail.

Pulsed laser breaking technique for glass substrates

The aim of this paper is to present the pulsed laser breaking technique, a technique that further builds on the unstable fracture technique. In this study, a diamond-point scoring tool was used to scribe a groove and create a median crack (grooved-crack) along the cutting path in a glass substrate. A pulsed CO2 laser was then applied at the cutting path to cut the glass substrate. A crack developed in the grooved-crack after the first laser pulse, increased in length after the second pulse, and then extended unstably after the third. The scribed grooved-crack determined the direction of crack propagation. The glass substrate separated along the scribed path. The surface of the separated pieces was free of microcracks common to mechanical breaking. The scribe force and groove depth were established and the laser parameters were set. Photographs of the glass substrate surface were obtained to analyze the cutting quality. An image processing system of crack detection was employed to obtain the crack image continuously during the laser breaking process. The stress analyses via ANSYS were performed to explain the mechanism of the breaking process that has been successfully developed in this study on cutting LCD glass substrates.

Laser cutting ceramics using an unstable fracture technique

This study proposes a new laser cutting method that uses the unstable fracture technique. In this method, a diamond blade is used to scribe a groove along the desired cutting path in an alumina substrate. A defocused CO2 laser is applied to the substrate surface. As the laser moves a short distance, a short crack is generated through the edge of the substrate. When the laser direction is reversed, a great thermal stress is induced at the crack tip, causing the substrate to break along the scribed line. The acoustic emission signals produced during the fracture process were obtained to analyze the crack growth phenomenon. SEM photographs of the breaking surface were obtained to analyze the cutting quality. The relationship between the laser scanning speed and groove depth is discussed based on findings from the experimental results. The stress analyses via ANSYS were performed to characterize these complicated phenomena. It was found that the laser-cutting surface was free of micro-cracks and the surface q...

The interaction of two inclined cracks with dynamic stress wave loadings

To gain insight into the phenomenon of the interaction of stress waves with material defects and the linkage of two cracks, the transient response of two semi-infinite inclined cracks subjected to dynamic loading is examined. The solutions are obtained by the linear superposition of fundamental solutions in the Laplace transform domain. The fundamental solution is the exponentially distributed traction on crack faces proposed by Tsai and Ma [1]. The exact closed form solutions of stress intensity factor histories for these two inclined cracks subjected to incident plane waves and diffracted waves are obtained explicitly. These solutions are valid for the time interval from initial loading until the first wave scattered at one crack tip returns to the same crack tip after being diffracted by another crack tip. The result shows that the contribution of diffracted waves to stress intensity factors is much less than the incident waves. The probable crack propagation direction is predicted from the fracture criterion of maximum circumferential tensile stress. The linkage of these two cracks is also investigated in detail.