In this thesis, it is aimed to determine the ballistic limit thicknesses of ST-37 steel plates under oblique impact of bullets having hard steel core (DIN 100Cr6 at 61-62 HRc) by using both experimental and numerical methods. In experimental part, angles of attack of the bullets are changed from 0 to 70 degrees by 10 degrees increments. Bullet velocities are measured for each shot just before the impact and they are found to be between 790-830 m/s. The minimum plate thickness that is not perforated and the maximum plate thickness that is perforated are determined by conducting three shots for each angle of attack - plate thickness combination. After the monolithic case, for some angles, layered plate combinations are also investigated. In the numerical analysis part, Johnson Cook constitutive and failure models are used together with the data obtained from literature. Experiments are simulated numerically by using a commercial non-linear explicit hydrocode software package, ANSYS AUTODYN. Results of the numerical simulations and the experimental findings are presented in tabular and graphical forms and then compared to each other.

Over the past several decades increasing focus has been paid to the impact of structures where energy, during the impact event, needs to be absorbed in a controlled manner. This has led to considerable research being carried out on energy absorbers, devices designed to dissipate energy during an impact event and hence protect the structure under consideration. Energy absorbers have found common usage in applications such as vehicles, aircraft, highway barriers and at the base of lift shafts. A type of energy absorber which has received relatively limited attention in the open literature is the tapered rectangular tube. Such a structure is essentially a tube with a rectangular cross-section in which one or more of the sides are inclined to the tube's longitudinal axis. The aim of this thesis was to analyse the impact and energy absorption response of tapered and non-tapered (straight) rectangular tubes. The energy absorption response was quantified for both axial and oblique loading, representative of the loading conditions typically encountered in impact applications. Since energy absorbers are commonly used as components in energy absorbing systems, the response of such a system was analysed which contained either straight or tapered rectangular tubes as the energy absorbing components. This system could typically be used as the front bumper system of a vehicle. Detailed finite element models, validated using experiments and existing theoretical and numerical models, were used to assess the energy absorption response and deformation modes of straight and tapered tubes under the various loading conditions. The manner in which a thin-walled tube deforms is important since it governs its energy absorption response. The results show that the energy absorption response of straight and tapered rectangular tubes can be controlled using their various geometry parameters. In particular, the wall thickness, taper angle and the number of tapered sides can be effectively used as parameters to control the amount of absorbed energy. Tapered rectangular tubes display less sensitivity to inertia effects compared with straight rectangular tubes under impact loading. This is beneficial when the higher crush loads associated with inertia effects need to be reduced. Furthermore, though the energy absorption capacity of thin-walled rectangular tubes diminishes under oblique impact loading, the capacity is more maintained for tapered rectangular tubes compared with non-tapered rectangular tubes. Overall, the results highlight the advantages of using tapered rectangular tubes for absorbing impact energy under axial and oblique loading conditions. Understanding is gained as to how the geometry parameters of such structures can be used to control the absorbed energy. The thesis uses this knowledge to develop design guidelines for the use of straight and tapered rectangular tubes in energy absorbing systems such as for crashworthiness applications. Furthermore, the results highlight the importance of analysing thin-walled energy absorbers as part of an energy absorbing system, since the response of the absorbers may be different to when they are treated on their own.

Preservation and protection of the head segment is of upmost importance due to the criticality of the functions entailed in this section of the body by the brain and the nervous system. Numerous events in daily life situations such as transportation and sports pose threats of injuries that may end or change a person’s life. In the European Union, statistics report that almost 4.2 million of road users are injured non-fatally, out of which 18% is represented by motorcyclist and 40% by cyclists, being head injuries 34% for bicyclists, and 24% for two-wheeled motor vehicles. Not only vehicles, are a source of injuries for the human head according to the injury report, 6,1 million people are admitted in hospitals for sports related injuries, where sports such as hockey, swimming, cycling presented head injuries up to 28%, 25% and 16% respectively (European Association for Injury Prevention and Safety Promotion, 2013).  According to records the vast majority of head crashes result in an oblique impact (Thibault & Gennarelli, 1985). These types of impacts are characterized for involving a rotation of the head segment which is correlated with serious head injuries. Even though there is plenty of evidence suggesting the involvement of rotational forces current helmet development standards and regulations fail to recognize their importance and account only for translational impact tests. This thesis contains an evaluation for a different developed method for testing oblique impacts. In consequence a new test rig was constructed with basis on a guided free fall of a helmeted dummy head striking an oblique (angled) anvil which will induce rotation. The results obtained are intended to be subjected to a comparison with another oblique test rig that performs experiments utilizing a movable sliding plate which when impacted induces the rotation of a dropped helmeted dummy head. The outcome will solidify the presence of rotational forces at head-anvil impact and offer an alternative testing method. After setting up the new test rig; experiments were conducted utilizing bicycle helmets varying the velocities before impact from 5m/s to 6m/s crashing an angled anvil of 45°. Results showed higher peak resultant values for rotational accelerations and rotational velocities in the new test rig compared to the movable plate impact test, indicating that depending on the impact situation the “Normal Force” has a direct effect on the rotational components. On the other hand a performed finite element analysis predicted that the best correlation between both methods is when the new angled anvil impact test is submitted to crashes with a velocity before impact of 6 m/s at 45° and the movable sliding impact test to a resultant velocity vector of 7,6m/s with an angle of 30° . In conclusion the new test method is meant to provide a comparison between two different test rigs that will undoubtedly have a part in the analysis for helmet and head safety improvements.

Compared to traditional thermal spray, cold spray as a new emerging surface treatment eliminates or substantially reduces phase transformation of deposited material and reduces coating porosity. Therefore, the appearance of this new type of surface treatment and additive manufacturing process has attracted considerable attention from researchers. In this research, three-dimensional modeling of Al6061-T6 particle impact and cohesion process was simulated by utilizing commercial finite element analysis (FEA) software ABAQUS/Explicit. To guarantee that a stable bonding phenomenon can be realized in the scope of physical validity, a built-in cohesive contact behavior model was implemented in the simulation to understand the bonding phenomenon. A non-planar surface was introduced to replace the usual planar impacted surface to mimic micron-scale curvature of the sprayed target in the real condition. Simulation models of spraying particles impact on positions with spray angle corresponding to 90°, 80°, 70° were created to investigate the effect generated by the curvature for the residual stress after bonding. Curvature function was exploited to describe the non-planar surface wavy condition derived from optimized impacting angle for achieving bonding phenomenon. This numerical simulation work can provide further insights for the residual stress evolution status in the condition of realized cohesion between impactor and non-planar surface after a kinetic peening process. Beneficial suggestions toward cold spray technology utilization in additive manufacturing areas are concluded from the results of the numerical simulation.

No description available.

We use the three-dimensional finite element code DYNA3D to analyze two problems: (a) the normal impact of a cylindrical monolithic or composite rod against a smooth flat rigid target, (commonly known as the Taylor impact test), and (b) the penetration of composite and monolithic steel cylindrical rods into reinforced concrete targets. The composite rod is made of either a steel or copper shell enclosing a ceramic. The ceramic and the steel are assumed to fail at a critical value of the effective plastic strain, whereas no failure is considered in the copper. The thermoviscoplastic response of steel and copper is modeled by the Johnson-Cook relation and the ceramic and concrete are assumed to be elastic-plastic. Values of material parameters in the constitutive relation for the reinforced concrete (RC) are derived by the rule of mixtures. Failure of a material is simulated by the element erosion technique for ceramic and steel, and element erosion along with stiffness reduction for the RC. The effect of the angle of obliquity of impact on the damage induced in the target is ascertained. For the solid cylindrical copper rod impacting a smooth flat rigid target, the time history of the deformed length and the axial variation of the final diameter are found to match well with the experimental findings. For the composite rod, the diameter of the deformed impacted surface, the shape and size of the mushroomed region and the volume fraction of the failed ceramic material strongly depend upon the impact speed, the shell wall thickness and the thickness of the solid copper rod at the front end. Some composite cylindrical rods impacting at normal incidence RC targets were found to buckle during the penetration process in the sense that their outer diameter at a cross-section close to the impacted end increased by at least 20%. For steel penetrators, the damage experienced increased as the nose shape got blunter and the angle of obliquity became larger whereas the damage induced to the target only increased with penetrator bluntness. / Master of Science

Milling is a common unit operation in industry for the purpose of intentional size reduction. Considerable amount of energy is consumed during a grinding process and much of the energy is dissipated as heat and sound, which often makes grinding into an energy-intensive and highly inefficient operation. Despite many attempts to interpret particle breakage during a milling process, the grindability of a material in a milling operation remains aloof and the mechanisms of particle breakage are still poorly understood. Hence the optimisation and refinement in the design and operation of milling are in great need of an improved scientific understanding of the complex failure mechanisms. This thesis aims to provide an in-depth understanding of particle breakage associated with stressing events that occur during milling. A hybrid of experimental, theoretical and numerical methods has been adopted to elucidate the particle breakage mechanics. This study covers from single particle damage at micro-scale to bulk comminution during the whole milling process. The mechanical properties of two selected materials, i.e. alumina and zeolite were measured by indentation techniques. The breakage test of zeolite granules subjected to impact loading was carried out and it was found that tangential component velocity plays an increasingly important role in particle breakage with increasing impact velocity. Besides, single particle breakage via in-situ loading was conducted under X-ray microcomputed tomography (μCT) to study the microstructure of selected particles, visualize the progressive failure process and evaluate the progressive failure using the technique of digital image correlation (DIC). A new particle breakage model was proposed deploying a mechanical approach assuming that the subsurface lateral crack accounts for chipping mechanism. Considering the limitation of existing models in predicting breakage under oblique impact and the significance of tangential component velocity identified from experiment, the effect of impact angle is considered in the developed breakage model, which enables the contribution of the normal and tangential velocity component to be rationalized. The assessment of breakage models including chipping and fragmentation under oblique impact suggests that the equivalent normal velocity proposed in the new model is able to give close prediction with experimental results sourced from the public literature. Milling experiments were performed using the UPZ100 impact pin mill (courtesy by Hosokawa Micron Ltd. UK) to measure the comminution characteristics of the test solids. Several parameters were used to evaluate the milling performance including product size distribution, relative size span, grinding energy and size reduction ratio etc. The collective data from impact pin mill provides the basis for the validation of numerical simulation results. The Discrete Element Method (DEM) is first used to model single particle breakage subject to normal impact loading using a bonded contact model. A validation of the bonded contact model was conducted where the disparity with the experimental results is discussed. A parametric study of the most significant parameters e.g. bond Young’s modulus, the mean tensile bond strength, the coefficient of variation of the strength and particle & particle restitution coefficient in the DEM contact model was carried out to gain a further understanding of the effect of input parameters on the single particle breakage behavior. The upscaling from laboratory scale (single particle impact test) to industrial process scale (impact pin mill) is achieved using Population Balance Modelling (PBM). Two important functions in PBM, the selection function and breakage function are discussed based on the single particle impact from both experimental and numerical methods. An example of predicting product size reduction via PBM was given and compared to the milling results from impact pin mill. Finally, the DEM simulation of particle dynamics with emphasis on the impact energy distribution was presented and discussed, which sheds further insights into the coupling of PBM and DEM.