Lorenzo M. Smith

Double-sided high-pressure tubular hydroforming

Abstract It is generally understood that tensile stress loads lead to the shearing and/or expansion of micro voids in a metal specimen. Such deformation leads to stress concentrations that ultimately form the basis of the limiting strain capacity in a metal. Accordingly, a number of formability theories suggest that stress (not strain) is the key mechanism for ductile fracture. Motivated by such theories, an introduction to a special form of tubular hydroforming (THF) is put forth, where hydraulic pressure is introduced on both sides of the metal. Six different formability models are compared in light of their sensitivity to thickness normal stress (σ3) and other process variables associated with THF. It is shown that the models which capture the σ3 effect are most suitable for formability assessment for the double-sided high-pressure (DSHP) process. The stress space forming limit model is then employed in the context of a solid finite element model of a plane-strain THF expansion. The presence of the DSHP boundary condition is shown to lead to increased formability relative to that observed for the traditional single-sided high-pressure (SSHP) process. Research of the DSHP process may lead to discoveries of various avenues towards greater formability. Consequently, the design space currently available to those employing the SSHP process may be significantly increased through the DSHP process. Ultimately, such design space increase may result in lower product cost, greater customer enthusiasm and increased market share for those manufacturers who invest in the DSHP process. This work represents a preliminary numerical investigation into the DSHP process only. Issues of cost, time and fixture design are left untouched.

Deformable skin design to enhance response of a biomimetic tactile sensor

Grasping of objects by robotic hands in unstructured environments demands a sensor surface that is durable, compliant, and responsive to various force and slip conditions. A compliant and robust skin can be as critical to grasping objects as the sensor it protects. In an effort to combine compliant mechanics and robust sensing, a biomimetic tactile sensor is being developed. Deformations of its skin can be detected by displacing a conductive fluid from the vicinity of electrodes on a rigid core. In this study, we used simplified finite element models to understand the effects of various textures for the inner surface of the skin and then produced the more promising textures by molding the elastomeric skin material against negatives made by stereolithography. The impedance vs. force relationships obtained with these molded skins had the predicted and desired wide dynamic range. By selecting the appropriate materials for the skin and fluid, previously described problems with hysteresis and diffusion losses have been greatly reduced.

A two-dimensional approach for simulation of hydroforming expansion of tubular cross-sections without axial feed

Abstract A new two-dimensional numerical method is developed based on membrane analogy to tube-sheet deformation with the concept of finite elements for sheet discretization, while utilizing a kinematically admissible approach to derive explicit expressions for relating loads and deformation. Typical tube cross-sections can be modeled using this approach. A new concept of controlling region and direction is proposed to track the deformation in different regions of the cross-section. Sticking friction is considered and a power-law model is used to express the deformation behavior of material. The new method is verified using commercially available FEA packages. The results show that it is computationally efficient and would prove to be an important tool for the process designer to interactively study the parameters on the process. The current formulation can predict the pressure load requirements and final thickness distribution for most polygon shaped tubular cross-sections.

Design and fabrication of electro-thermally activated micro gripper with large tip opening and holding force

This paper reports on the design, fabrication, and characterization of a distinctive MEMS gripper electro-thermally driven jointly by a new metallic V-shape actuator (VSA), and a set of modified Guckel U-shape actuators (mUSA). The modification of the angle between the hot and cold arms in the mUSA facilitates desired unidirectional in-plane displacement and thus increases the opening of the gripper. This unique configuration distinguishes this MEMS gripper from other similar devices in the capability of generation of larger tip displacement and greater holding force. Tip opening of ∼173 µm and holding force of ∼5 mN have been measured at a low operating voltage of 1 V with consuming power of 0.85 W. MetalMUMPs is employed to fabricate the device. Electroplated nickel is used as the structural material. The metallic structure allows a low operating voltage and low overall power consumption.

A Numerical Investigation of Breast Compression: A Computer-Aided Design Approach for Prescribing Boundary Conditions

Prior to performing an MRI-guided breast biopsy, the radiologist has to locate the suspect lesion with the breast compressed between rigid plates. However, the suspect lesion is typically identified from a diagnostic MRI exam with the breast hanging freely under the force of gravity. There are several challenges associated with localizing suspect lesions including, patient positioning, the visibility of the lesion may fade after contrast injection, menstrual cycles, and lesion deformation. Researchers have developed finite element analysis (FEA) methodologies that simulate breast compression with the intent of reducing these challenges. In this paper, we constructed a patient-specific finite element (FE) breast model from diagnostic MR images. In addition, we constructed surfaces corresponding to the biopsy MR volume and used them to deform the FE breast mesh. The predicted results suggest that the FE breast model, in its uncompressed configuration, can be compressed to replicate the perimeter of the biopsy MR volume. The simulated lesion displacement was within 3 mm of its actual position.

Analysis of Dynamic Loads on the Dies in High Speed Sheet Metal Forming Processes

During high-speed sheet metal forming processes, the speed at which the work piece contacts the die tooling is on the order of hundreds of meters per second. When the impact is concentrated over a small contact area, the resulting contact stress can compromise the structural integrity of the die tooling. Therefore, it is not only important to model the behavior of the workpiece during the high-speed sheet metal forming process, but also important to predict accurately the associated workpiece/tooling interface loads so that engineers can more confidently propose robust die tooling designs. The foundation to accurate predictions of contact stress on die tooling is a reliable contact model within the context of a finite element simulation. In literature, however, there exists no comprehensive guideline for establishing a contact model for high-speed sheet metal forming processes using the finite element method. In this paper, mathematically justified contact model recommendations are offered for the electrohydraulic forming (EHF) process.

Development of a New Technology for Trimming of Dual Phase Steels

This paper describes the development and investigation of a new technology for trimming of dual phase steels and provides a comparison with other trimming processes. This technology utilizes an elastic support of the offal, sharp upper trim knife and angled lower trim knife. Such a configuration allows for a high quality trimmed surface with little or no burr and without slivers over a wide range of cutting clearances.

An Experimental Analysis Device for Obtaining Skid Line Limit Diagrams

A novel design for a machine intended to measure directly various in-plane and contact normal forces acting upon a sheet metal specimen during a stretch-bend-draw process is proposed, in order to gain insight into the formation of skid line defects in sheet metal. The new machine, called a Stretch-Bend-Draw Simulator (SBDS) is designed specifically to be integrated into a typical laboratory tensile testing machine, thereby making it accessible to those researchers lacking the resources to acquire expensive additional tooling. As the strip of sheet metal is pulled over a round tool radius, the SBDS is shown to be capable of collecting pulling force, back force, tool normal force, and the corresponding draw bead clamping force. Analysis of the force data in conjunction with visual observations of the actual pulled specimens allows researchers to ascertain the conditions under which so-called skid lines arise. Experimental results, including a newly unveiled Skid Line Limit Diagram (SLLD), are provided and discussed. The SBDS appears to be a promising new electro-mechanical laboratory device for improving researchers’ knowledge of the physical phenomena associated with skid lines in sheet metal products created in stamping dies.

A Transferable Education Model for Creating Work-Ready College Graduates

“Work-ready” college graduates, upon their first day at work, are able to make employee contributions which are comparable to those associated with a typical employee who is three years removed from his/her undergraduate graduation. The objective of CLIC-form (Chrysler Learning and Innovation Center for Sheet Metal Forming), a program recently implemented at Oakland University, is to deliver work-ready college graduates to the American sheet metal forming industry. Long-term fulfillment of this objective is expected to increase productivity and job satisfaction in the American work-force, enrich educational experiences for university students, and foster greater funded research opportunities for the university.CLIC-form is a proven educational model. Now in its second year, it is recruiting a third cohort of Oakland University students, hosting once-per-week educational workshops for both students and local industry professionals, and continuing to generate revenue through direct support from Chrysler Group, LLC and funded research activity from other sources. Presented in this paper is a description of CLIC-form’s structure, operation and financial outlook. A proposed form for extending the CLIC-form template to different fields of study is offered.Copyright

Pre-Forming Effects on AHSS Edge Cracking

Edge cracking in advanced high strength steels (AHSS) is a significant failure mode in many sheet metal stamping processes. Edge pre‐forming into a wave (or scallop) shape is a common technique used in conventional steels to gather material in high edge stretch regions in preparation for the subsequent edge stretch process. The pre‐forms designed for mild steels do not always apply to AHSS because the properties of AHSS can differ greatly from those of conventional steels. This work has studied the effects of pre‐forming on AHSS edge cracking. Experiments have been conducted to stretch pre‐formed steel strips to failure. Strain distributions of pre‐forms with various levels of stretch have been measured using digital image correlation (DIC) technology. Finite element analyses have been performed and compared with the experimental results. Different failure criteria have also been evaluated for use in this type of application.