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.