Christoffer Cromvik

GEOMETRY ASSURANCE INTEGRATING PROCESS VARIATION WITH SIMULATION OF SPRING-IN FOR COMPOSITE PARTS AND ASSEMBLIES

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Welding Simulation of Non-Nominal Structures With Clamps

In any industrial assembly process, there are a number of sources of variation. Variation in the manufacturing process leads to component variation, which, together with fixture variation and variation stemming from the joining process, propagates to the final product. In order to analyze and diminish the effect of variation, it is important to identify and be able to simulate the phenomena contributing to final variation. In this paper, the focus is variation in welding distortion arising from non-nominal components that are joined. In the welding process, it has been shown that variation in components and in fixtures influences the size and distribution of weld-induced distortion. Hence, in order to accurately simulate geometric variation of an assembly joined by weld joints, variation simulation and welding simulation need to be performed in combination. Previous research that focused on the combination of variation simulation and welding simulation has not considered components that are clamped. Instead the components were treated as rigid bodies at non-nominal positions prior to welding. In many industrial applications, clamps are used when assemblies are welded, and it is therefore important to quantify the influence that clamping has on welding of non-nominal components. In this paper, we simulate the combination of variation in components and fixtures with welding, considering that the components are clamped prior to welding. Although clamps will force the components closer to their nominal positions along the weld joint, they also introduce a stress field in the structure, which together with the welding process may cause additional distortion. Two case studies are performed and analyzed: a T-weld joint and a butt-weld joint. The results show that welding distortion depends on fixture error even in the presence of clamps.

Simulation of non-nominal welds by resolving the melted zone and its implication to variation simulation

The prediction of geometric variation and its consequences is one important aspect of product development. For welded assemblies it has been shown that positioning errors of the parts prior to welding affects the weld-induced distortion. Therefore, to accurately predict geometric variation in welded assemblies, variation simulation and welding simulation need to be performed in combination. This is usually a very time consuming task, and therefore, the relatively fast SCV-method is utilized. This method is used to calculate welding distortion when positioning errors are present and it consists of the fol-lowing three steps: 1) a steady state computation of the thermal distribution during welding, 2) the melted zone along the full joint is encapsulated by sweeping a two-dimensional convex hull along the weld gun path, and 3) a uniform temperature is applied to all nodes inside this zone. The two-dimensional convex hull is computed so that when swept along the weld path, it will encapsulate the melted zone from the steady state temperature computation. The weld-induced distortion is obtained from the elastic volumetric shrinkage. In this article the focus is on the first step in this method; the temperature distribution computation. The positioning error can cause the connecting parts to have varying distances to each other at the joint, which cause the melted region to vary along the weld path. Therefore, it is not sufficient to capture the steady state temperature distribution at only one location. Depending on the geometry surrounding the weld path, several locations may be needed. In this new approach, the two-dimensional convex hull that is to be swept along the weld path, can vary along the weld path, and is computed from an interpolation of the multiple two-dimensional convex hulls obtained from the multiple steady state temperature computations. A comparison of the melted region using transient temperature calculation, a single steady state temperature calculation and this new approach has been made. Furthermore, the implication on distortion calculation has been studied.

On the Robustness of the Volumetric Shrinkage Method in the Context of Variation Simulation

Welding induces high temperatures that cause residual stresses and strains in the welded structure. With a welding simulation, these stresses and strains may be predicted. A full simulation implies performing a transient thermal and a quasi-static mechanical analysis. These analyses usually involve a large number of time steps that leads to long simulation times. For welding distortions, there are approximate methods that require considerably less time. This is useful when simulating large structures or for analyses that use an iterative approach common in optimization or variation simulation. One of these methods is volumetric shrinkage, which has been shown to give reasonable results. Here it is assumed that the driving force in welding distortion is the contraction of the region that has been melted by the weld. In volumetric shrinkage, the nodes that are inside the melted region are assigned a uniform temperature and the distortion is calculated using elastic volumetric shrinkage. Although this method has been shown to give reasonable predictions, we will show that it is sensitive to small perturbations, which is an essential part in variation simulation. We also propose a modification of the volumetric shrinkage method that addresses this lack of robustness; instead of defining the melted region by applying a uniform temperature to the nodes inside the zone, we formulate an optimization problem that finds a temperature distribution such that the local melted volume is preserved. A case study with application to variation simulation has been used to elicit the proposed method.

Geometry Assurance Integrating Process Variation with Simulation of Spring-in for Composite Parts and Assemblies

Geometrical variation and deviation in all manufacturing processes affect quality of the final product. Therefore geometry assurance is an important tool in the design phase of a new product. In the automotive and aviation industries where the use of composite parts is increasing drastically, new tools within variation simulations are needed. Composite parts tend to deviate more from nominal specification compared to metal parts. Methods to simulate the manufacturing process of composites have been developed before. In this paper we present how to combine the process variation simulation of composites with traditional variation simulations. The proposed method is demonstrated on a real complex subassembly, representing part of an aircraft wing-box. Since traditional variation simulation methods are not able to capture the spring-in and the special deviation behavior of composites,the proposed method adds a new feature and reliability to the geometry assurance process of composite assemblies.

Variation Simulation of Welded Assemblies Using a Thermo-Elastic Finite Element Model

Every series of manufactured products has geometric variation. Variation can lead to products that are difficult to assemble or products not fulfilling functional or aesthetical requirements. In this paper, we will consider the effects of welding in variation simulation. Earlier work that have been combining variation simulation with welding simulation have either applied distortion based on nominal welding conditions onto the variation simulation result, hence loosing combination effects, or have used transient thermo-elasto-plastic simulation, which can be very time consuming since the number of runs required for statistical accuracy can be high. Here, we will present a new method to include the effects of welding in variation simulation. It is based on a technique that uses a thermo-elastic model, which previously has been shown to give distortion prediction within reasonable accuracy. This technique is suited for variation simulations due to the relative short computation times compared to conventional transient thermo-elasto-plastic simulations of welding phenomena. In a case study, it is shown that the presented method is able to give good predictions of both welding distortion and variation of welding distortions compared to transient thermo-elasto-plastic simulations.