Dual Surface Approach Research Articles

A Finite Plasticity Gradient-Damage Model for Sheet Metals during Forming and Clinching

In recent years, clinching has gathered popularity to join sheets of different materials in industrial applications. The manufacturing process has some advantages, as reduced joining time, reduced costs, and the joints show good fatigue properties. To ensure the joint strength, reliable simulations of the material behaviour accounting for process-induced damage are expected to be beneficial to obtain credible values for the ultimate joint strength and its fatigue limit. A finite plasticity gradient-damage material model is outlined to describe the plastic and damage evolutions during the forming of sheet metals, later applied to clinching. The utilised gradient-enhancement cures the damage-induced localisation by introducing a global damage variable as an additional finite element field. Both, plasticity and damage are strongly coupled, but can, due to a dual-surface approach, evolve independently. The ability of the material model to predict damage in strongly deformed sheets, its flexibility and its regularization properties are illustrated by numerical examples.

Evapotranspiration estimation performance of root zone water quality model: evaluation and improvement

Root zone water quality model (RZWQM) has proven to be useful in evaluating the agricultural systems. However, it has previously been recognized that its evapotranspiration estimates somewhat depart from the measured values indicating the potential for some improvement in this module. Estimation procedure is based on the dual surface approach of Shuttleworth and Wallace, that although having a solid theoretical basis, it is greatly empirical in its application due to the lack of accurate quantitative knowledge of the resistance terms that control the heat fluxes in the canopy. Analysis of the formulation used in RZWQM allowed to detect some weakness in the calculation of bulk surface resistance ( r s c), which is based on stomatal resistance averaged by effective leaf area index (LAI eff). In this work, an alteration to the definition of LAI eff is proposed, which leads to an improvement of the estimates. It is also discussed that a greater improvement could be obtained by introducing a model of stomatal resistance response to environmental conditions, which would increase the complexity of the model and be difficult to apply given the lack of adequate knowledge of the quantitative behavior of stomata. Alternatively, the simple crop coefficient approach could be incorporated to define an upper limit to evapotranspiration fluxes and avoid some otherwise unrealistically high estimates of the current version of the model.

Comparison of dual surface and tri‐surface approaches for acoustical holography measurements of radiated and scattered fields

Using near‐field measurements of the pressure field of an immersed structure, an estimate of the structure’s radiated field and scattered field can be made. With the pressure and the pressure gradient measured at discrete locations in the near field, a discretized solution of the exterior Helmholtz integral is solved to obtain the far‐field pressure. This approach has been shown to work for both acoustic radiation and acoustic scattering problems. Using two normal surfaces to approximate the pressure gradient, parametric simulations were performed to determine the scattered field predictability of the approach for an insonified rigid sphere. The results indicate that this dual surface approach better predicts the scattered field when the surfaces are very close, which implies that a good approximation of the gradient is needed. A different approximation of the pressure gradient uses three surfaces instead of two. Using a cylindrical shaped measurement surface, the radiated pressure field from a monopole and the scattered pressure field from a rigid sphere are simulated for the different cases of the approximated gradient, namely the dual surface approach and the tri‐surface approach. The effects on the predictability of the radiated field and the scattered field are discussed. [Work supported by ONR.]

A dual surface approach to near-field measurements of target strength

Using near-field measurements of the pressure field over dual surfaces surrounding an immersed and insonified structure, an estimate of the structure’s target strength can be made. Choosing a cylindrical shaped surface, two measurement arrays enclosing the structure are formed with one array slightly larger than the other. At discrete points over the dual cylindrical shaped surfaces, two pressures are measured, and the pressure and the pressure gradient at these locations result. With the pressure and the pressure gradient, a discretized solution of the exterior Helmholtz integral is solved for the far-field pressure. Using the near-field measurements and this approach, far-field estimations of both radiation problems and target strength problems are possible. Using a rigid sphere as the insonified structure, the target strength problem is modeled and numerically solved using MATLAB. In this paper, an error analysis of this target strength application is presented. The effects on the estimated target strength due to different variables of the dual surface approach will be discussed, some of which include the following: open and closed dual surfaces; differences in the size of the dual surface; and variations on the density of the scanned dual surface. [Work supported by DARPA.]