Numerical modeling and experimental study of thermally induced residual stress in the direct diode laser heat treatment of dual-phase 980 steel

The direct diode laser application has been found useful in the localized heat treatment of metal parts because of its wider beam and more uniform energy distribution with respect to other lasers with Gaussian-like energy distribution. In this study, an uncoupled thermomechanical finite element model is developed to study the temperature field and thermally induced stress evolution in high-strength dual phase (DP) 980 steel during its direct diode laser heat treatment. Thermal analysis results are experimentally validated through thermocouples and then input into a mechanical model as transient temperature loading in order to acquire the thermally induced stresses and strains. The effect of martensite phase transformation on residual stress distribution in heat-treated DP980 steel is considered. An X-ray diffraction technique is used to measure the residual stress distribution at the top surface of the heat-treated coupons of DP980 steel. The numerical results show that compressive stresses are located at the laser–material interaction zone. After heat treatment, tensile stresses are retained at the heat-treated DP980 steel coupons. There is qualitative agreement between the numerically predicted and experimentally measured residual stresses. The effect of the overlapping ratio on the residual stress and hardness of the heat-treated DP980 steel is also experimentally and numerically investigated.     

Optimization of geometry for lateral buckling process of a cantilever beam in the elastic region

Using the large displacement theory (theory of the third order according to Chwalla), the paper deals with lateral buckling process of a slender, elastic cantilever beam with a changeable cross-sectional area and represents it with a system of nonlinear differential equations. Based on a mathematical model of the lateral buckling process which considers the geometric and boundary conditions, an optimal geometry of a cantilever beam is obtained using the calculus of variation. A comparison between the properties of the beam with optimized geometry and those of a referential beam with a constant cross section is shown. The main feature of the optimized geometry beam is a constant maximal reference stress, obtained by the deformation energy theory, along the whole length of the beam in a deflected form which means that in terms of stability the material is completely exploited. The result of this feature is, besides higher critical load, also higher carrying capacity of the optimal geometry beam in the postbuckling region.     

Thermo-mechanical model with adaptive boundary conditions for friction stir welding of Al 6061

Thermo-mechanical simulation of friction stir welding can predict the transient temperature field, active stresses developed, forces in all the three dimensions and may be extended to determine the residual stress. The thermal stresses constitute a major portion of the total stress developed during the process. Boundary conditions in the thermal modeling of process play a vital role in the final temperature profile. The heating and cooling rates with the peak temperature attained by the workpiece determine the thermal stress. Also, predicting realistic peak temperature becomes important as the operating temperature at the interface of tool-workpiece is very close to the solidus temperature of the aluminum workpiece.The convection heat-transfer coefficients of the surfaces exposed to air can be theoretically determined using Newton's law of cooling. Contact conductance depends on the pressure at the interface and has a non-uniform variation. The actual pressure distribution along the interface is dependent on the thermal stress from local temperature and non-linear stress–strain state. Therefore, applying an adaptive contact conductance can make the model more robust for process parameter variations.A finite element thermo-mechanical model with mechanical tool loading was developed considering a uniform value for contact conductance and used for predicting the stress at the workpiece and backplate interface. This pressure distribution contours are used for defining the non-uniform adaptive contact conductance used in the thermal model for predicting the thermal history in the workpiece. The thermo-mechanical model was then used in predict stress development in friction stir welding.     

Statistical character of the failure of multiphase materials due to high pressure water jet impingement

In this study, fracture experiments on multiphase material samples have been carried out using high speed water jets. Based on fracture geometry measurements and on grain analyses it was found that the fracture of this type of materials by water jet impingement is a highly localized process at low pressure ranges. Beyond a critical pressure range of about 30 times the material's tensile strength a change in the material behaviour was observed. This result is in agreement with a theory suggested by Powell and Simpson. To explain the local character of the failure process, a simplified fracture model is introduced which resulted in a relation between a fracture probability parameter and the fracture width in the damaged materials.     

Observer parameterization for simultaneous observation

The stable inverse approach is used to obtain the observers for the simultaneous observation of a given set of plants. A parameterization in terms of a stable inverse and a stable null space is proposed for all simultaneous observers. To verify the effectiveness of the proposed method, a design example is also given     

Vortex Dynamics in Heavy Fermion CeCoIn5

Local magnetic measurements utilizing a micro-Hall probe were performed on a superconducting heavy fermion CeCoIn₅ single crystal. We show that the critical current follows a power law as predicted by Ginzburg–Landau theory. This behavior is found to be universal in different heavy fermion and high-T _c superconducting materials. Furthermore, we report on remanent magnetization relaxation showing a high relaxation rate with approximately linear temperature dependence. Although qualitatively similar to another undoped heavy fermion, UBe₁₃, this relaxation rate is significantly higher, providing evidence that CeCoIn₅ can be grown in the clean limit.     

A three dimensional model for direct laser metal powder deposition and rapid prototyping

A three-dimensional model for direct laser metal powder deposition process and rapid prototyping is developed. Both numerical and analytical models are addressed. In the case of numerical modeling, the capabilities of ANSYS parametric design language were employed. The model calculates transient temperature profiles, dimensions of the fusion zone and residual stresses. Model simulations are compared with experimental results acquired on line using an ultra-high shutter speed camera which is able to acquire well-contrasted images of the molten pool, and off-line using metallographical and x-ray diffraction analyses. The experiments showed good agreement with the modeling. The results are discussed to provide suggestions for feedback control and reduction of residual stresses.     

Filter bank frame expansions with erasures

We study frames for robust transmission over the Internet. In our previous work, we used quantized finite-dimensional frames to achieve resilience to packet losses; here, we allow the input to be a sequence in l₂(Z) and focus on a filter-bank implementation of the system. We present results in parallel, R^N or C^N versus l₂(Z), and show that uniform tight frames, as well as newly introduced strongly uniform tight frames, provide the best performance     

Local cosine bases in two dimensions

We construct two-dimensional (2-D) local cosine bases in discrete time. Solutions are offered both for rectangular and nonrectangular lattices. In the case of nonrectangular lattices, the problem is solved by mapping it into a one-dimensional (1-D) equivalent problem.