Obtaining a Relationship Between Process Parameters and Fracture Characteristics for Hybrid CO2 Laser∕Waterjet Machining of Ceramics

A combined experimental and analytical approach is undertaken to identify the relationship between process parameters and fracture behavior in the cutting of a 1mm thick alumina samples by a hybrid CO2 laser∕waterjet (LWJ) manufacturing process. In LWJ machining, a 200W power laser was used for local heating followed by waterjet quenching of the sample surface leading to thermal shock fracture in the heated zone. Experimental results indicate three characteristic fracture responses: scribing, controlled separation, and uncontrolled fracture. A Green’s function based approach is used to develop an analytical solution for temperatures and stress fields generated in the workpiece during laser heating and subsequent waterjet quenching along the machining path. Temperature distribution was experimentally measured using thermocouples and compared with analytical predictions in order to validate the model assumptions. Computed thermal stress fields are utilized to determine the stress intensity factor and energy release rate for different configurations of cracks that caused scribing or separation of the workpiece. Calculated crack driving forces are compared with fracture toughness and critical energy release rates to predict the equilibrium crack length for scribed samples and the process parameters associated with transition from scribing to separation. Both of these predictions are in good agreement with experimental observations. An empirical parameter is developed to identify the transition from controlled separation to uncontrolled cracking because the equilibrium crack length based analysis is unable to predict this transition. Finally, the analytical model and empirical parameter are utilized to create a map that relates the process parameters to the fracture behavior of alumina samples.

Temperature-Dependent Micro-Crack Propagation

The crack-lattice trapping phenomenon introduce by R. Thomson et al[1] is studied for the conditions of the Frenkel-Kontrova-type experiment. By using a new method, which allows further model extension for a finite temperature case we are able to describe an equilibrium crack energetics for arbitrary externa conditions and ascertain the crack propagation conditions. Specifically, the system free energy F as a function of nonlinear bond displacement ul for an external forces P and for a finite temperature T is found. The equilibrium values for the displacement ul = ul* and for G* = G(ul*), are obtained. The free-energy barrier height G = Gmax − G* dependence upon P and T is determined. With the help of the exact solution of the equilibrium equations we obtained the free energy as function of crack length G(l,T,P). We found that local free energy barriers take place for every crack length l, which is in contrast to the Thomson model. From the microscopic viewpoint it means that crack advance is controlled by local free energy barriers. We found that near the equilibrium length the crack energy barrier is relatively high, while far from equilibrium crack position, energy barrier height decreases to a finite value. It is worth to note that the barrier height monotonically decrease with the increase of the environment temperature. On the basis of our model the temperature dependence of the crack surface energy will be found, the global energetics of the crack will be described.

On the Problem of Equilibrium Length of a Bridged Crack

A solution is given for a partially bridged straight crack in orthotropic elastic material in particular unidirectionally fiber-reinforced brittle composite. The problem of crack with constant bridging forces is solved by use the complex potentials. By use of Novogilov’s fracture criterion the estimation of the bridged part of crack and full length of equilibrium crack is obtained.

Interfacial energy states of moisture-exposed cracks in mica

Results of crack growth observations on mica in water-containing environments are described. The study focuses on equilibrium crack states for reversed loading cycles, i.e., for initial propagation through virgin solid and subsequent retraction-repropagation through healed or misoriented-healed interfaces. Departures from these equilibrium states are manifest as steady-state forward or backward crack velocities at specific applied loads. The equilibria are thereby interpreted as quiescent, threshold configurations G = WE, with G the Griffith mechanical-energy-release rate and WE the Dupré work of adhesion, on crack velocity (v-G) diagrams. Generally, WE is found to decrease with concentration of water, in accordance with a Gibbs formalism. Hysteresis is observed in the forward-backward-forward crack propagation cycle, signifying a reduction in the adhesion energy on exposure of the open interface to environmental species prior to healing. This hysteresis is especially marked for those interfaces that are misoriented before healing, indicating that the structure of the underlying solid substrate as well as of the intervening fluid is an important consideration in the interface energetics. The equilibrium states for different environments can be represented on a simple energy-level diagram, as differences between thermodynamic end-point states: initial, closed-interface states refer to crystallographic bonding configurations ahead of the crack-tip adhesion zone; final, open interface states refer to configurations behind the crack-tip zone. The significance of this diagram in relation to the fundamental atomic structure of interfaces in fracture and other adhesion geometries, including implications concerning kinetics, is discussed.

The nature of machining damage in brittle materials

The micromechanics of failure emanating from machining-induced cracks in brittle materials is investigated. In situ monitoring of crack response during breaking tests (with use of acoustic wave scattering), strength measurements and post-failure fractography all indicate that the crack response is dominated by residual stresses. Two components of residual stress have been identified: a crack-wedging force due to the plastic zone beneath the strength-controlling machining groove, and a compressive surface layer due to adjacent grooves. The wedging force dominates and causes stable equilibrium crack extension during a breaking test. The implications of the results for non-destructive evaluation of surface damage by acoustic wave scattering is discussed.