Vipul Ranatunga

Use of UBET for design of flash gap in closed-die forging

Abstract An upper bound elemental technique (UBET) has been proposed for the design of the flash gap in closed-die forging operations of axisymmetric shapes. The developed method has been applied to analyze the forging load, die-cavity filling, and effective strain and strain rate distributions in axisymmetric closed-die forging with a rib-web type of cavity. The forging process has been superimposed with two sub-stages called the extrusion and the back-fill stages. The total die load required for extrusion and back fill are calculated separately and a maximum allowable extrusion gap is proposed for a given percentage die-fill. FEM simulations have been carried out with different die geometries and the results are in good agreement with UBET predictions.

Cohesive modeling of damage growth in z-pinned laminates under mode-I loading

In this paper, a traction-separation-based cohesive modeling approach is proposed to predict the effect of z-pinning on laminated composites. A detailed experimental characterization of the z-pin pullout process using the flatwise tension test is presented. Utilizing these flatwise tension results, numerical simulation of the progressive damage due to delaminations in a double cantilever beam with z-pinning has been performed. Experimental details of the z-pinned double cantilever beams are presented for IM7/977-3 graphite/epoxy. The approach taken in this study utilizing the cohesive elements within the Abaqus® finite element software has proven that the models can predict the behavior of z-pinned composites close to experimental observations. It was found that the discretization of the fracture resistance curve along the z-pin field is essential to capture the dynamics of the delamination accurately.

UBET-based numerical modeling of bulk deformation processes

The paper summarizes the development of numerical procedures for modeling bulk deformation process and preform designing techniques based on the upper bound elemental technique (UBET). UBET has a unique place where an approximate, but faster solution is needed for decision making. In designing and optimizing multistage forging and profile ring-rolling processes, an approximate solution can be used to identify the most influential process parameters. Once an optimum combination of process conditions are determined, computationally intensive, but more accurate finite element analysis can be used to verify and refine results. In this paper, UBET procedures for closed-die forging and profile ring rolling are high-lighted. Experimental investigations are used to validate the model predictions. Also, the UBET-based preform design tool is presented as a process and die design tool for multistage forging processes. Application of these techniques is presented with evidence of effective material usage and extended overall die-life.

Three-dimensional UBET simulation tool for seamless ring rolling of complex profiles

Simulation of complex ring profiles using an improved Upper Bound Elemental Technique (UBET) is presented in this paper. The profile of the ring is approximated with a collection of triangular-prismatic and rectangular-brick elements. A generalized solution procedure is proposed to find the optimized kinematically admissible velocity field for the unique three-dimensional material flow experienced in shape rolling. Close agreement between numerical results obtained from simulations and available data for profile ring rolling has been observed for high-temperature aerospace materials. It has been demonstrated that the complex ring profiles used in aerospace industry can effectively be simulated using the current software tool.

Internally Reinforced Adhesively Bonded Metal to Composite Joints

A methodical scientific study was completed to develop and demonstrate a fail-safe advanced structural concept for joining metallic and composite parts without mechanical fasteners. The approach was to internally reinforce an adhesively bonded joint. The primary reinforcement concepts that were investigated included a titanium mesh and titanium wire bent into a U-shape. Of these, the more successful was the titanium wire bent into a U shape. Observations from the wire mesh concept include difficulties in welding and bending the mesh due to the lack of ductility and high temperature processing characteristics. Presence of the mesh generally degraded the mechanical properties. The primary benefit of the mesh was that it significantly improved the strength of the joint in the presence of a flaw. The Ushaped pin reinforcement concept was very successful. The effect of pin aerial density on the failure characteristics of single overlap shear specimens was determined. The highest pin density increased the overall toughness of the joint by over 4X.

Low-Velocity Impact Damage and Delamination Crack Arrestment with Translaminar Reinforcements

The process of z-pinning has been used to improve the delamination strength of laminated composites. Improvements in Mode-I and Mode-II fracture toughness and the degradation of in-plane properties have been studied in the past and documented in the literature. This paper presents a preliminary study on the use of z-pinning to improve the delamination resistance against barely visible impact damage. Experimental results indicated that the use of pinning can considerably improve the compression after impact strength. Non-destructive observations revealed a lower delamination area after impact when the samples were fully covered with z-pins. Additionally, samples with a surrounding z-pin boundary were able to provide a containment mechanism for an area with sub-surface delamination.

A volume-based mapping method for parameter estimation in multi-stage material processing

Abstract Current emphasis on affordability and sustainability of manufacturing systems creates a need for new material process design methods that consider alternative materials and processes. The advent of low-cost, high-performance computing has enabled a generic acceptance of powerful computational tools such as finite element analysis (FEA) for analysis purposes. The large number of computations required by a high accuracy FEA simulation makes an optimization-based design technique that is built around FEA simulations almost prohibitive. Alternative simpler techniques are needed at the initial design stages in order to predict the most important characteristics of the material behavior during processing in a small fraction of time taken by an FEA run. The technique presented in this paper describes a volume-based mapping method for the parameter estimation in forging operations. The method can produce the deformed geometry and important process parameters such as strain and strain rate in matter of seconds. The results have been compared with a widely used FEA tool and the predictions are found to be in good agreement with FEA results.

Process Modeling of Shape Rolling for Aerospace Industry

Shape rolling of seamless rings constitutes an efficient manufacturing process offering excellent material yield, energy conservation, and component production, which require a minimum of subsequent machining operations. An increasing number of rings are being produced from high temperature Titanium and Nickel based super alloy materials for gas turbine engine parts such as vane and fan casings, exhaust casings, turbine shrouds, and combustion liners. With the increasing cost of super alloy raw materials and growing demand for cost-competitive parts, the importance of ring rolling to contoured shape becomes an increasingly important factor. This paper describes a new process modeling technique based on Upper Bound Elemental Technique (UBET) for shape rolling of super alloys. This tool provides a new design paradigm for an industry relying to heavily on designer experience and cut-and-try methods. As a rapid software tool to aid designers in developing ring-rolling process schedules thereby helping in reducing the design and analysis cycle time, the potential to capture the unique 3-D flow situation experienced in shape rolling of seamless rings is being explored. Numerical results have been compared with data available for high temperature alloys such as IN718 and Ti-6Al-4V.Copyright