C.M.A. Silva

Innovative Testing Machines and Methodologies for the Mechanical Characterization of Materials

This paper is focused on the development of innovative testing machines and methodologies for the characterization of materials in working conditions similar to those found in mechanical processing of materials. Special emphasis is given to the design, fabrication, and instrumentation of a flexible drop weight testing machine, an electromagnetic compressive Hopkinson bar, and an electromagnetic cam-driven compression testing machine that are capable of performing the mechanical characterization of materials under mediumand high rates of loading. The fundamentals of plastic flow in selected forming and cutting processes are utilized to assess the validity and reliability of the proposed testing machines and associated experimental methodologies. Experimental data with technical pure Lead allows understanding the combined influence of strain, strain rate, and strain rate versus strain loading paths in the overall stress response of the materials. Results also show that the new proposed electromagnetic cam-driven compression testing machine equipped with a root type or logistic cam profile can successfully replicate the operating conditions of the two other testing machines.

Mechanical characterization of materials for bulk forming using a drop weight testing machine

Abstract The main limitation of mechanical testing equipments is nowadays centred in the characterization of materials at medium loading rates. This is particularly important in bulk forming because strain rate can easily reach values within the aforesaid range. The aim of this article is twofold: (a) to present the development of a low-cost, flexible drop weight testing equipment that can easily and effectively replicate the kinematic behaviour of presses and hammers and (b) to provide a new level of understanding about the mechanical characterization of materials for bulk forming at medium rates of loading. Special emphasis is placed on the adequacy of test operating conditions to the functional characteristics of the presses and hammers where bulk forming takes place and to its influence on the flow stress. This is needed because non-proportional loading paths during bulk forming are found to have significant influence on material response in terms of flow stress. The quality of the flow curves that were experimentally determined is evaluated through its implementation in a finite-element computer program and assessment is performed by means of axisymmetric upset compression with friction. Results show that mechanical characterization of materials under test operating conditions that are similar to real bulk forming conditions is capable of meeting the increasing demand of accurate and reliable flow stress data for the benefit of those who apply numerical modelling of process design in daily practice.

Pressure-assisted forming of non-concentric tubular cross sections with solid medium

Pressure-assisted forming of tubes allows producing a wide variety of tubular components that are difficult or impossible to fabricate by means of conventional tube forming. In contrast to previous investigations in the field that were almost exclusively focused on the utilization of fluids (tube hydroforming) or elastomers (tube rubber forming) as pressuring medium, the subject matter of this article is centred in the utilization of low melting point, recyclable, metallic alloys as solid pressurizing medium. The aims and scope of the article are centred on the feasibility of forming straight carbon steel tubes into complex gooseneck geometries with non-concentric cross sections using lead as a solid pressuring medium and employing a double-action cam-driven tool system. The presentation is focused on the tool system, on its adequacy to produce customized tubular components, on the required forming forces and on the typical modes of deformation that result from the different movements provided by the vertical and horizontal actuators of the double-action tool system. Results and observations confirm that the utilization of a double-action tool system with a solid pressurizing medium to assist plastic deformation and prevent collapse can be successfully and effectively employed to fabricate non-concentric tubular cross sections for prototypes and small batches of lightweight components.

An electromagnetic testing machine for determining fracture toughness under different loading rate and superimposed pressure

This article presents an innovative testing machine for determining fracture toughness in double-notched cylindrical and prismatic test specimens loaded in shear over a wide range of applied strain rates and superimposed pressures. The equipment makes use of an electromagnetic actuator that is capable of accelerating the shearing punch against the specimens, with excellent repeatability and very precise control of the impact velocity, by means of the high pressure that is generated in a transient magnetic field produced by passing a pulse of electric current through a series of coils. Experiments performed in technically pure lead and aluminium AA1050-O give support to the presentation and allow understanding the influence of the loading rate and pressure on fracture toughness of ductile metals.

Joining by Forming of Tubes to Sheets with Counterbored Holes

This paper presents a new joining by forming process for connecting tubes to sheets. The process consists of forming an annular flange with rectangular cross section by partial sheet-bulk of the tube wall thickness and performing the mechanical interlock by upsetting the free tube end against a flat-bottomed (counterbored) sheet hole. The presentation identifies the variables and the workability limits of the process and includes an analytical model to assist readers in the design of the new joints. The new proposed joining by forming process and the corresponding analytical model are validated by experimentation and numerical simulation using finite element analysis. The process allows connecting tubes to sheets made from dissimilar materials at room temperature, avoids the utilization of addition materials or adhesives and produces joints that are easy to disassembly at the end of live, allowing recyclability of the tubes and sheets.

Joining by Plastic Deformation

This paper draws from the existing processes and applications of joining by plastic deformation to a comprehensive overview of a new set of processes that have been recently developed by the authors. The presentation includes solutions for connecting tubes, sheets and tubes to sheets and provides information on the tooling systems, operating variables, deformation mechanics and workability limits. Results from analytical modelling, finite element analysis and experimentation give support to the presentation and prove the feasibility of the new joining by plastic deformation processes for connecting tubes, sheets and tubes to sheets made from dissimilar materials, at room temperature, without having to use addition materials or adhesives. The resulting joints are easy to disassembly at the end of live, thereby allowing recyclability of the individual parts.

Joining by forming of additive manufactured ‘mortise-and-tenon’ joints

This article is aimed at extending the ‘mortise-and-tenon’ joining concept commonly utilized in corner or tee joints to lap joints in which one sheet is partially placed over another without any change in their shape. The approach makes use of wire arc additive manufacturing to fabricate the tenons and allows various shapes and thicknesses to be made from a wide range of metallic materials. Upset compression of the tenons is utilized to mechanically lock the two sheets being joined. Experimental and finite element simulation works performed with monolithic (aluminium–aluminium) and hybrid (aluminium–polymer) ‘unit cells’ consisting of a single lap joint are utilized to investigate the deformation mechanics and the feasibility of the new proposed joining process. Tensile-shear loading tests were carried out to determine the maximum force that the new proposed joints are capable to withstand without failure. Pull-out forces of approximately 8 and 6 kN for the monolithic and hybrid joints allow concluding on the potential of additive manufactured ‘mortise-and-tenon’ lap joints to connect sheets made from similar and dissimilar materials.

Cutting Under Gas Shields: Phenomenological Concepts Versus Industrial Applications

This chapter is focused on the interaction between cutting medium and freshly formed surfaces with the aim of providing a new level of understanding on the mechanics of chip flow in orthogonal metal cutting and analyzing its influence on the cutting forces and tool life in conventional milling. The first part of the chapter presents experimental results from an investigation performed with a specially-designed orthogonal metal cutting apparatus and a model material under dry conditions, with the objective of analyzing the influence of active and inert gas shields in the kinematics of chip flow, the friction coefficient, the chip-compression factor and the cutting forces. In particular, authors provide a correlation between surrounding medium, tribological conditions, surface roughness, freshly cut surfaces and chip curling at the tool-chip contact interface. The second part of the chapter extends the investigation to conventional milling of an engineering material in order to study the influence of cutting in the presence of air or under an argon atmosphere in the overall cutting forces and tool wear. Results from the investigation based on orthogonal metal cutting show that cutting in the presence of oxygen leads to higher values of friction, chip compression factor and chip curl radius, and to lower values of the shear plane angle. The presence of oxygen is also responsible for increasing the cutting forces and tool wear in conventional milling.

Innovative Joining Technologies Based on Tube Forming

This chapter is focused on innovative and environmental friendly joining technologies for connecting tubes and fixing tubes to sheets in situations where the axis of the branch tube or sheet is perpendicular or inclined to the axis of the main body tube. In case of connecting tubes, the chapter will also cover the challenge of joining two tubes by their ends. The proposed joining technologies are based on the utilization of plastic instability waves in thin-walled tubes subjected to axial compression and may be seen as an alternative to conventional joining based on mechanical fixing with fasteners, welding and structural adhesive bonding. Besides allowing connecting dissimilar materials (e.g. metals and polymers) and being successfully employed in fixture conditions that are difficult and costly to achieve by means of conventional joining the new proposed technologies also cope with the growing concerns on the demand, lifecycle and recycling of materials.

Cam-driven electromagnetic mechanical testing machine

Abstract: This chapter describes an innovative testing machine for the mechanical characterization of materials under high rates of loading. The machine consists of an electromagnetic actuator, a fixed housing containing two flat compression platens, a translating cam and a follower. The electromagnetic actuator enables high strain rates to be reached with very precise control of the impact velocity and of the energy transmitted to the translating cam. The cam profile enables compression testing to be performed under the strain rate vs. strain loading paths that are commonly found in metalworking in order to meet the combined specifications of the machine-tool and process. Emphasis is placed on giving constructive details for those readers who are interested in developing a low-cost machine for conducting material tests at high rates of loading, as well as on demonstrating the flexibility and adequacy of its operative conditions by determining the flow curves of aluminium AA1050-O and technically pure lead (99.9 per cent) under different testing conditions. These are implemented in a finite-element computer program and applied in the numerical simulation of two different basic metalworking operations.