LARGE—A Plasma Torch for Surface Chemistry Applications and CVD Processes—A Status Report

The LARGE (LONG ARG GENERATOR) is a new generation DC-plasma torch featuring an extended arc which is operated with a perpendicular gas flow to create a wide (up to 45 cm) plasma jet well suited for large area plasma processing. Using plasma diagnostic systems like high speed imaging, enthalpy probe, emission spectroscopy, and tomography, the LARGE produced plasma jet characteristics have been measured and sources of instability have been identified. With a simple model/simulation of the system LARGE III-150 and numerous experimental results, a new nozzle configuration and geometry (LARGE IV-150) has been designed, which produces a more homogenous plasma jet. These improvements enable the standard applications of the LARGE plasma torch (CVD coating process and surface activation process) to operate with higher efficiency.     

A COMMON FORMAT APPROACH FOR THE DUCTILE FRACTURE METHOD USING A DISPLACEMENT-BASED NORMALIZATION PARAMETER

Abstract— The Ductile Fracture Method (DFM) proposed by Ernst and Landes and further developed by Landes and coworkers is examined in conjunction with the Common Format Equation (CFE) proposed by Donoso and Landes. The DFM can be used to predict the load versus displacement behavior for a structure from the load versus displacement record for a fracture toughness test specimen. This involves a calibration function transformation procedure which is often laborious. But, it will be shown here that using the CFE approach with an adequate normalizing parameter for the deformation function, H , the transformation procedure can be virtually eliminated. As a consequence, the predictions done by the DFM can be completed in a shorter time. The equations which originated the CFE approach are reviewed to show that the modification proposed in this paper is necessary for H to get closer to a unique representation, independent of the geometry. Some examples are presented to demonstrate the applicability of the approach.     

Improvement of Coating Properties in Three-Cathode Atmospheric Plasma Spraying

The main aim of this study is to improve the coating properties of three-cathode atmospheric plasma-sprayed coatings with respect to porosity and residual stresses. This was done by means of numerical simulation coupled with advanced diagnostic methods. A numerical model for the triple injection of alumina feedstock, as well as acceleration and heating of the powder particles in the characteristic threefold symmetrical plasma jet cross section produced by a three-cathode-plasma torch, was developed. The modeling results for the standard injector’s position “0” were calculated and experimentally verified by laser Doppler anemometry. Based on the criteria defined for the concentrated feedstock transport and homogeneous thermal treatment of powder particles in the plasma jet, the optimal injection position was found. In the next step, a previously developed, coupled CFD-FEM-simulation model was used for simulations of the coating build-up, describing flattening, solidification, and deformation due to shrinkage for alumina particles on a rough substrate surface.     

THE BLUNTING LINE IN ELASTIC-PLASTIC FRACTURE

Abstract The blunting line evaluation procedure used in the ESIS standard fracture toughness test method “Procedure for Determining the Fracture Behaviour of Materials” is re-evaluated to see if a simpler format can be developed. An equation based on the ultimate tensile strength was found to represent the blunting line in a simple manner. This equation is in error at most ±5% from the analytical representation and is as accurate as the graphical procedure used to determine the blunting line. It is recommended that this equation be used for fracture toughness test standards which use the ESIS blunting line. A comparison of the ESIS blunting line and the ASTM blunting line is made using some J-R curve data generated with an elastic unloading compliance test procedure. These data do not favor one line over the other. Microscopic evaluation of the blunting line reported in the literature using the stretch zone width measured on the fracture surface suggests that the ESIS blunting line better represents the physical blunting process.     

Finite element simulation of nonlinear wave propagation in thermoviscous fluids including dissipation

A recently developed finite element method (FEM) for the numerical simulation of nonlinear sound wave propagation in thermoviscous fluids is presented. Based on the nonlinear wave equation as derived by Kuznetsov, typical effects associated with nonlinear acoustics, such as generation of higher harmonics and dissipation resulting from the propagation of a finite amplitude wave through a thermoviscous medium, are covered. An efficient time-stepping algorithm based on a modification of the standard Newmark method is used for solving the nonlinear semidiscrete equation system. The method is verified by comparison with the well-known Fubini and Fay solutions for plane wave problems, where good agreement is found. As a practical application, a high intensity focused ultrasound (HIFU) source is considered. Impedance simulations of the piezoelectric transducer and the complete HIFU source loaded with air and water are performed and compared with measured data. Measurements of radiated low and high amplitude pressure pulses are compared with corresponding simulation results. The obtained good agreement demonstrates validity and applicability of the nonlinear FEM     

Study of the load-separation principle and a procedure to estimate ηp-factor for structures with different geometries

The theoretical background of load separation principle has been reviewed. Based on dimensional analysis, experimental results and finite element analysis the conditions for the validity of this principle have been studied and a method has been suggested to estimate the load separation function, and hence to estimate the _p-factor and to determine the J-R resistance curve easily from records of a single specimen or structure.     

The load separation and η pl

Load separation is the theoretical basis for the single specimen J form and the incremental calculation of J-R and J _M -R curves. It is based on the assumption that the load can be represented as a multiplication of two separate functions; a crack geometry function and a material deformation function. Until recently, the main experimental basis for such an assumption was the approximate agreement between the experimental results of the single specimen J form and the energy rate interpretation of J in blunt notched bending geometries. The load separation assumption has been also implied in the growing crack records in order to develop the R-curve analysis. Both the crack geometry and material deformation functions were assumed to maintain their forms as the crack grows. Recently, an experimental study investigated the load separation in the test records of stationary crack specimens of different geometry, material, and constraint. The study showed that the load can be represented by a separable form for the entire plastic region except for a limited region at the early region of plastic behavior. Also, it was found that the load separation is not limited to a certain geometry, material, or constraint but it is a dominant property in the ductile fracture behavior of stationary crack specimens. The study also showed that the crack geometry function is a power law function. Hence _pl is a constant equal to the power law exponent of the geometry function.The objective of this study is to investigate the extension of load separation to growing crack records. Sets of test records from three different materials are used in this study. For each material three or four precracked specimen test records and one blunt notched record are analyzed for the compact specimen geometry. The study will discuss the main condition to have a separable behavior in a growing crack test record. It will also construct the geometry and deformation functions for the materials studied, these functions are compared with those obtained from stationary crack records.     

The role of constraint on the calibration functions for the prediction of ductile fracture behavior in structural components

The role of constraint in ductile fracture has been studied for its effect on fracture toughness. When toughness is characterized by the J–R curve, a lowering of constraint will usually increase the level of the J–R curve. In a ductile fracture methodology both the J–R curve and the calibration functions which relate the applied load to the plastic deflection of a structure, are required to predict structural behavior. Constraint also influences the calibration functions. In many cases the calibration functions play a more important role in the prediction of maximum load than does the J–R curve. Therefore in predicting the behavior of a structural component the effect of the constraint must be accounted for in determining the calibration functions to be used as input in the ductile fracture methodology.The calibration functions have been traditionally taken from the EPRI-GE Handbook [6]. Using the format of the Handbook calibration functions, a constraint factor is derived which can be used to evaluate the relative constraint in the calibration function. For prediction of the behavior of a structural component from a laboratory test specimen the constraint factor must be known for each component. This constraint factor for the test specimen can then be used to predict the calibration function that is appropriate for the geometry and constraint of the structural component.A number of examples taken from test specimens of various thicknesses are presented to show that the constraint factor for the calibration function can be reasonably predicted based on information given in the Handbook. This method of prediction is applied to develop calibration functions that can be used in a ductile fracture methodology for a new structural constraint. The methodology is then used to predict load versus displacement behavior for cases of different constraints. Results from a test with essentially plane strain constraint are used to predict behavior of a plane stress structure. Finally a list of general rules is developed to account for the constraint of an arbitrary structural component. Along with this, some indication is given of where additional information is needed to make the prediction of constraint more complete.     

Physical aging kinetics of syndiotactic polystyrene as determined from creep behavior

Creep experiments in uniaxial extension have been performed to explore the kinetics of the physical aging process in semicrystalline syndiotactic polystyrene (sPS) having two processing histories. Classical time-aging time superposition behavior was found for both materials at temperatures from 70 to 95°C, with the shift rate μ decreasing as temperature was increased. Virtually no aging was seen at 95°C, the DSC determined glass transition, T_g. This behavior was atypical for a semicrystalline polymer and reminiscent of the behavior of glassy amorphous thermoplastics. Some evidence for a separate crystalline aging mechanism > T_g, which manifests itself as only vertical shifts without timescale shifts, is seen in experiments at T > 100°C. Finally, the two different materials age differently, suggesting that some control of aging can be obtained by altering processing conditions or morphology.