A new Approach for experimental evaluation of contact stresses
In metal forming operations, the intensity and distribution of stresses at the tool/material interface have great influence on the pattern and mode of deformation. This is due to the effect of frictional forces in being resistive to material movements and hence affects material flow and the directionality of volume distribution of material inside die cavities. This paper explores the use of the pressure pin technique for the measurement of tool stresses at the interface between the tool and the deforming workpiece material. It also introduces a new experimental methodology for the measurement of stresses where the measuring device is concealed inside the tool and not subjected to the severe conditions at the interface surface. Each tool containing measuring devices is split into two parts, one part has the shape to be formed (industrial die), while the second part includes the pressure pin measuring column. Friction resistance and friction coefficient are evaluated across the billet surface and at different stages of deformation.
Investigation into energy dissipation in equal channel angular extrusion
The field of energy absorption is definitely one the most important in engineering design, as many types of static and dynamic structures, designed and built for different purposes and tasks, require energy absorption capabilities under loading conditions. This thesis is aimed at the introducing experimental and theoretical analyses of a novel and revolutionary technique to dissipate unwanted energy in engineering systems. An extensive literature review on existing energy absorbers was undertaken in relevant application fields such as structural and personal protection. Hence, devices attached to buildings and designed to dissipate energy due to severe earthquakes have been discussed and compared. Types considered, in this review, are mainly based on friction, viscoelasticity and material yielding mechanisms. Furthermore, methodologies to strengthen structures against impacts such as those used in armoured walls are described, and their capabilities assessed. In addition techniques to protect the human body against dangerous loads were reviewed, and important issues for chest and head protection, leg defences in football and safety in motorcycles have been investigated. Experimental results about energy absorption in crash tests have been studied. Also, as an example the use of current technologies to dissipate energy during landing operations in aircrafts have been considered. A classified chart of energy absorption devices in different applications has been produced and referenced. In general most energy absorption devices were shown to be capable to eventually dissipate dangerous and unwanted energy, but poor reusability and predictability after impact were not part of the design process. The research base in this thesis is a novel energy dissipation technique capable of designing Universal Reusable Energy Absorption Devices (UREAD). This technique exploits the principles and working mechanisms that are used in extrusion of deformable materials through intersecting channels. Such mechanism of deformation is known in literature as Equal Channel Angular Extrusion (ECAE). ECAE is one of the severe plastic deformation processes. A theoretical analysis of internal pressure and stresses developed at the interface with the tools has been presented for channels of different geometrical parameters. In addition, energy absorption capabilities have been analysed by the Upper Bound ii method. Also, a numerical solution based on the implementation of the Finite Element Analysis, in ANSYS commercial package was obtained to show the intensity of stress distribution in the deforming material and the tools surrounding it. UREAD devices of different dimensions and geometries were designed, manufactured and tested using an experimental set up constructed for this work. Circular and square cross-sectional channels were tested using various deformable materials. Experimental results were compared with theoretical distributions, and several analogies were highlighted and discussed. Special tools were designed and manufactured to study experimentally the normal stresses at contact surfaces using the so called “Pressure Pin Technique”. Also, an experimental apparatus has been built to simulate the potential implementation of UREAD devices against the occurrence of heavy impacts and the effect of the energy absorber was experimentally measured at the instant of ground impact.
The use of the model technique in the prediction of the pressure distribution over the tool surfaces in cold forging
Abstract In today's practice, tool design and its optimization is accomplished using mostly empirical knowledge and extensive experience: considerable amounts of trial and error are thus necessary in order to be able to set up a tool for a reliable and reproducible process. It is therefore one of the major aims of cold-forging research and development to reduce this trial and error procedure — in tools which are finished, hardened, and polished: this can be done by means of model techniques combined with the necessary calculations. The parameters which limit the forging process are the maximum press force available and the contact pressure between the workpiece and the tool. Researchers can make use of this knowledge for a better understanding of the prevailing process mechanics and for improvement of the forging process, whilst the production engineer can use this knowledge for the selection and allocation of process steps. Furthermore, determination of contact pressure facilitates understanding of tool wear and fracture, of the plastic flow behaviour of the materials, and of the necessary radial and/or axial splitting and pre-stressing of the tooling. In recent times, many advances have been made in numerical analysis methods such as FEM, FDM and BEM, as a means of solving tooling design problems. However, the success of these methods depends very much on the availability of the input data, which very often have to be adjusted for a quick computer run. The different design parameters for a rod/cup component are discussed in this paper and experimental results are presented, particularly for the contact pressure on the die surface in forward/backward extrusion, using the model-material technique.