Improved methods for characterising acoustoplasticity

The benefits of high-power ultrasonics to industrial metal forming processes have long been demonstrated in uniaxial mechanical tests. The astonishing reductions in flow stress observed have been linked to changes to surface friction and to an interaction of the excitation with the mechanisms of plastic deformation in metals. Many advanced techniques and material models have been brought to bear on the problem of the underlying physics of acoustoplasticity, and yet all rely fundamentally on accurate force and extension data. The effects of inertia and inhomogeneity in the loading distribution on the specimen have been largely ignored, and yet are incompatible with commonly used instrumentation. This thesis reports investigations which address the error introduced into force measurement in mechanical testing by ultrasonic excitation. After reviewing experimental mechanics techniques, it was found that the piezoelectric force transducer retained its central role in defining true flow stress reduction. An inertia-based barrier to vibration was introduced between the force transducer and test machine crosshead, to impose the rigid boundary condition desired to ensure the force transducer coincided with a displacement node. Lumped-parameter modelling indicated that the dynamic response of the piezoelectric force transducer’s structure could significantly distort the amplitude of an oscillatory force measurand. Either amplification or attenuation could result depending on the proximity of excitation frequency to natural frequency of the force transducer’s first longitudinal mode. Simple impulse experiments provided the natural frequency of the force transducer in the free-free condition, a parameter used in later finite element (FE) modelling of the ultrasonic tensile test structure. Experimental Modal Analysis (EMA) was used to investigate the dynamic response of the ultrasonic tensile test structure, and to map the mode shape of the first longitudinal mode, the mode utilised in ultrasonic tensile testing. A finite element model was constructed of the test apparatus, and subsequently solved in an eigenvalue analysis to extract the natural frequency and mode shape of the first longitudinal mode. When the numerically predicted waveform was compared with that found from EMA, a significant difference was discovered between the horn and specimen. The compliance of the joint was adjusted until the simulated mode shape converged on its experimental counterpart. Once experimentally calibrated, the FE model was used to predict the force experienced by the force transducer for increasing values of vibration amplitude. Comparison with experimental force measurements found good agreement. Of greatest importance to the investigation of flow stress, the FE model predicted the indicated value from the force transducer to be 1.91 times greater than the measurand at the specimen-force transducer interface. Strain gauges were attached to the gauge section of the specimen in the ultrasonic tensile test apparatus, and the vibration varied over a range of amplitudes. By converting the oscillatory strain measurement into force on the specimen cross-section, the loading experienced by the specimen at the strain gauge location was compared to force measurements made simultaneously by the piezoelectric force transducer. The ratio of force amplitude from the force transducer over the force amplitude calculated from the specimen strain measurement was found to vary from 3.13 to 3.50, with a mean of 3.32. Repeating the experiment within the FE model calculated an amplitude ratio of 3.33, constant over all vibration amplitudes. This value was used to develop a correction factor to extrapolate force on the specimen from piezoelectric force transducer measurement. The correction was applied to an ultrasonic tensile test on a soft aluminium. Though the mean stress was reduced during the periods of excitation, no real reduction in flow stress was observed, which is consistent with the theory of stress superposition. The evolution of plastic deformation was studied over the gauge section of an ultrasonically excited specimen, using an optical metrology system adapted for use on the ultrasonic tensile test. To eliminate oscillatory motion from images, a high-speed strobe lit the specimen in bursts of light synchronised with the ultrasonic excitation. Digital Image Correlation was used to process the image sequence to find strain and strain rate across the whole face of the specimen gauge length. It was observed that the application of ultrasonic excitation disrupted the usual distribution of plastic deformation along the specimen length, focussing deformation towards the location of peak stress amplitude. Again, observations were consistent with the theory of stress superposition. This thesis demonstrates how the dynamic response of the structure of the specimen and force transducer in an ultrasonic tensile test can significantly distort the force measurement, crucial for accurately identifying a real reduction in flow stress. This has implications for studies of acoustoplasticity aiming at determining underlying physical mechanisms. It is found that, when the effect of inertia is accounted for, the theory of stress superposition is sufficient to explain the stress-strain relationship observed

Characterising the effective material softening in ultrasonic forming of metals

This thesis has presented experimental and finite element (FE) analyses of the static and ultrasonic forming of two metals; aluminium 1050 and magnesium AM50. Aluminium and magnesium are considered to be soft metals and can be easily shaped by any of the main industrial metalworking processes. Frequently aluminium and magnesium have been the subject of research studies. These two metals most commonly chosen in manufacturing industry because of their cost, mechanical properties and flexibility in processing. In this research, simple compression and forming tests were designed and the effects of superimposed ultrasonic excitation on workpiece and die, which is tuned to a longitudinal mode at 20.8 kHz, were studied via stress-strain measurements. Research through experiments and finite element simulations studies in the application of ultrasonic excitation has been carried out to gain quantitative understanding of the mechanisms of improvement in ultrasonic forming characteristics, such as a reduction in material flow stress and oscillatory stress. This research study has shown these mechanisms by applying ultrasonic vibration to the tool and die in the forming test and, similarly, effects were measured and predicted in the experimental and numerical analysis. The development and application of high power ultrasonic techniques in forming processes required the use of specifically designed ultrasonic components to correctly transmit the energy from the transducer to the workpiece and die interface. The application along with the ultrasonic vibration amplitude required for the process, were considered in order to design the most suitable horn profile. In this study, a 20 kHz transducer was used to provide up to 10 µm of peak-to-peak vibration amplitude, depending on the generator setting. Therefore, the booster and horn were designed to provide a range of ultrasonic vibration amplitudes between 5 to 20 µm and also used as a tool and die in the study of ultrasonic metal forming. The horn was designed using finite element modelling (FEM), and modal frequencies and associated mode shapes were subsequently confirmed using experimental modal analysis (EMA). The ultrasonic system has been measured and calculated as having a longitudinal mode of vibration at 20.8 kHz and to provide an amplitude gain of four. In this study, a generator uses mains electricity to generate a high frequency ultrasonic signal to drive the transducer, which is tuned to a specific frequency of 20 kHz. The booster and horn were designed to meet the criteria of transducer, which is to provide a longitudinal vibration at tuned frequency of 20 kHz. However, the profile of booster and horn have been measured and calculated as having a longitudinal mode of vibration at 20.8 kHz, which is considered close to the transducer tuned frequency. The review of previous studies of superimposed ultrasonic excitation on upsetting showed that the most experimental characterisations of the volume effects mainly depended on an interpretation of measurements of the mean flow stress, and have neglected the oscillatory stress. In this study, the characteristics of oscillatory stress and the material behaviour in plastic deformation when superimposed ultrasonic excitation is applied on a static compression test under dry friction were considered. The effects were explained in terms of flow stress reduction, oscillatory stress, mean flow stress, maximum and minimum path of oscillatory stress in the stress-strain diagram. The results showed that the static flow stress of compressive deformation was lowered by the ultrasonic vibration superimposed on the static load and this phenomenon has been referred to as the material softening mechanism which is influenced by volume and surface effects. The volume effect is defined as a reduction in flow stress of the material being formed and the surface effect is defined as a reduction in frictional conditions at the interface between the vibrating device and the workpiece. Finite element models were used to investigate numerically the volume and surface effects during ultrasonically assisted compression. The finite element models were developed using material model parameters which were identified from the experimental analysis. The influence of volume and surface effects were investigated separately in the FE model and it was shown that the volume effect dominated the effective material softening results during ultrasonic excitation. The application of ultrasonic excitation on metals under plastic deformation conditions has been investigated previously. Most researchers have reported that superimposing ultrasonic excitation on metal working processes reduced the material flow stress. A further study of superimposed ultrasonic excitation on a static load during elastic deformation in metal working was not investigated, so it is not possible to determine the effect of ultrasonic excitation on the material. In this study, the investigation of oscillatory stress behaviour in the ultrasonic compression test of cylinder metal specimens during elastic deformation was carried out. In the stress-strain diagram, the ultrasonic vibration was shown to have lowered the static flow stress during elastic deformation under dry contact conditions and it was found that the reduction in static flow stress linearly increased with ultrasonic vibration amplitude. The stress reduction was influenced by volume and surface effects which occurred during the superimposed ultrasonic excitation. The results also showed that the maximum path of oscillatory stress exceeded the static flow stress, however, the mean flow stress is lower than the static flow stress at the onset of ultrasonic excitation. To investigate the influence that volume and surface effects have on material softening during experimental compression tests, a series of FE models were developed. As mentioned previously, the FE models were developed using material model parameters which were identified from the experimental analysis in Figure A1, however, the mechanism of flow stress reduction which is related to acoustic softening and friction reduction which is labelled as (i) cannot be predicted in FE models. The FE models adopted the material softening effects in order to simulate realistic stress reduction compared with experimental results. The significant stress reduction in the FE analysis was obtained by adjusting the yield stress and contact conditions parameter. It was concluded that the surface effect dominated the stress reduction during metal upsetting test in elastic deformation. The study continued to a simple forming test where samples of flat sheet metal were forced into a shaped die by a shaped plunger on a test machine. The results of this study illustrated how ultrasonically assisted metal forming resulted in a lowering of the static forming force during ultrasonic excitation of the die. As a result, the static forming force was seen to be reduced by ultrasonic excitation of the die and the path of the maximum oscillatory force was observed to be parallel to or below the path of the static forming force. Force reduction was measured in these experiments using a high power ultrasonic transducer and also by tuning the die and then the punch during the metal forming test. It was found that a good coupling between punch, specimen and die allowed ultrasonic energy to be effectively transferred into the materials during superimposed ultrasonic excitation in the static forming test. This thesis has concluded that evaluation of the benefits of ultrasonic excitation not only relied on measurements of the mean flow stress alone but also on measurement of the oscillatory stress during superimposed ultrasonic excitation on forming tests. Findings on the effectiveness of ultrasonic excitation led the study to recognize that the lack of understanding of the effects of ultrasonic excitation on the forming process has resulted in difficulties in maximising the benefits and applications of this technology

The application of oscillation to the deformation of an elastoviscoplastic material

The research reported in this thesis demonstrates the benefits of applying coaxial vibration to forming tools in soft solid forming processes using Plasticine as a model material. In the study of vibration assisted upsetting and indentation (conical and spherical), finite element models under kinematic loading were first developed to gain insight into interface mechanics. FE simulation of the model material included the effects of elasticity, viscoplasticity, strain rate, large strains and a coulombic stress boundary condition in the presence of a lubricant. Agreement was achieved between the FE results and those obtained from upsetting and indentation experiments with respect to the force-displacement curves and deformed configurations for a range of friction coefficients, specimen sizes and platen velocities. The FE models, subsequently developed to simulate processes under superimposed vibration loading of the forming tool, predicted an apparent reduction in the mean forming force. The reduction in mean force is largely dependent on the vibration amplitude and shows a weak dependence on frequency. The results illustrate the phenomenon of stress superposition, where a cyclic stress is superimposed on a non-oscillatory stress. However, a reduction in the mean force alone is not necessarily beneficial since the maximum stress under idealised superimposed vibration loading will follow the same stress-strain curve as under static loading, with both the mean and minimum stresses following paths parallel to the non-oscillatory stress-strain curve. In fact, in the case of strain rate dependent materials, the maximum stress can be greater under vibration loading, and this overstress is correctly predicted by the FE model. However, more importantly, experiments under vibration loading using Plasticine have shown both a reduction in the mean forming force, and a maximum stress which is less than the static stress. The reduction in maximum stress achieved is related to the friction condition at the die/specimen interface. The relationship between vibration condition and soft solid material flow is investigated. It is established that vibration assisted forming can result in a significant reduction in resistance of the forming material to deformation, by a combination of stress superposition effect and a reduction in interface friction

Experimental and finite element modelling of ultrasonic cutting of food

In recent years ultrasonic cutting has become an established technology in a variety of industries including the food processing industry to cut a variety of materials. An ultrasonic cutting system consists of a generator, transducer and either a single or multiple blade cutting devices tuned to a specific mode of vibration, commonly the longitudinal mode, between 20 and 100 kHz. High power ultrasonic cutting device design has traditionally relied on the cut requirements of the product, the use of empirical approaches where ultrasonic cutting system parameters such as cutting speed, frequency of vibration, mode of vibration, blade tip amplitude, gain and cutting orientation are determined from experimental and experience of the tool designers. Finite element (FE) models have also been used to predict the vibrational behaviour of the cutting tool. However, the performance of an ultrasonic device critically relies on the interaction of the cutting tool and material to be cut. Currently the interaction between the resonant blade and the material to be cut is neglected but the cutting mechanism at the interface is of significant importance and knowledge of this mechanism would be of considerable benefit to designers when developing ultrasonic cutting blade concepts and processing requirements. Simulations of the cutting process would also enable designers to conduct parametric studies quickly using computational methods instead of conducting lengthy, laborious experimental tests. The research reported in this thesis provides an insight into the requirements of the tool-material interaction to allow optimal cutting parameters to be estimated as an integral part of designing cutting blades for use in the food industry. A methodology is proposed for modelling the interaction between the resonant blade and the material to be cut using the finite element method, to gain an understanding of the cutting mechanism. The effect of ultrasonic cutting parameters, such as resonant frequency, mode of vibration, blade tip sharpness, cutting force, cutting speed, blade tip amplitude and are also investigated. Knowledge of the temperature distribution at the interface between the resonant blade and the substrate material would also be of benefit as currently experimental determination of the temperature at the interface is impractical using current measuring systems. Two thermo-mechanical FE models of ultrasonic cutting are developed which simulate the cutting tool and material interaction to allow cutting parameters to be derived numerically to enhance cutting blade design. The FE models incorporate experimentally derived mechanical and thermal properties of the common engineering thermoset Perspex and also of the following food materials; toffee, cheese, chocolate and jelly. The combined thermo-stress FE model allows the temperature at the cut interface to be determined under various loading conditions and provides a method for investigating the effects of blade design on temperature at the blade-material interface. Estimations of accurate mechanical and thermal properties of foodstuffs for inclusion in the FE models are determined experimentally using materials testing techniques such as tension and compression tests. Ultrasonic cutting blades are designed using finite element analysis and experimental investigations are performed on an ultrasonic cutting rig to validate the FE models. Two different generic 2D modelling approaches to simulate ultrasonic cutting are presented. One uses the debond method in ABAQUS standard and the alternative uses the element erosion method in ABAQUS explicit. Progression of the element erosion method into a 3D model is also presented with the intension of improving the accuracy of the modelling technique and to offer the flexibility to model complex geometries or cutting orientations. The models are presented and validated experimentally against a common engineering material, Perspex, and parametric studies are presented and discussed for the food materials; toffee, cheese, chocolate and jelly. For accurate modelling of any process, accurate material data is required and for common engineering materials such as Perspex accurate data is readily available in the literature. For food products however, the mechanical and thermal properties are not readily available and are often batch dependent. Methodologies for testing and determining the mechanical and thermal properties of two selected food materials, toffee and cheese, are also presented and the results from these experimental tests are incorporated in the finite element models to simulate the food materials during ultrasonic cutting. Models of ultrasonic cutting are for both single layer materials and also for multi layer material architectures

Bone Healing Evaluation Following Different Osteotomic Techniques in Animal Models: A Suitable Method for Clinical Insights

Osteotomy is a common step in oncological, reconstructive, and trauma surgery. Drilling and elevated temperature during osteotomy produce thermal osteonecrosis. Heat and associated mechanical damage during osteotomy can impair bone healing, with consequent failure of fracture fixation or dental implants. Several ex vivo studies on animal bone were recently focused on heating production during osteotomy with conventional drill and piezoelectric devices, particularly in endosseous dental implant sites. The current literature on bone drilling and osteotomic surface analysis is here reviewed and the dynamics of bone healing after osteotomy with traditional and piezoelectric devices are discussed. Moreover, the methodologies involved in the experimental osteotomy and clinical studies are compared, focusing on ex vivo and in vivo findings

Adipic Acid Sonocrystallization in Continuous Flow Microchannels

Crystallization is widely employed in the manufacture of pharmaceuticals during the intermediate and final stages of purification and separation. The process defines drug chemical purity and physical properties: crystal morphology, size distribution, habit and degree of perfection. Particulate pharmaceuticals are typically manufactured in conventional batch stirred tank crystallizers that are still inadequate with regard to process controllability and reproducibility of the final crystalline product. Variations in crystal characteristics are responsible for a wide range of pharmaceutical formulation problems, related for instance to bioavailability and the chemical and physical stability of drugs in their final dosage forms. This thesis explores the design of a novel crystallization approach which combines in an integrated unit continuous flow, microreactor technology, and ultrasound engineering. By exploiting the various benefits deriving from each technology, the thesis focuses on the experimental characterization of two different nucleation systems: a droplet-based system and a single-phase system. In the former, channel fouling is avoided using a carrier fluid to segment the crystallizing solution in droplets, thus avoiding the contact with the walls. In the latter channel blockage is prevented using larger channel geometries and employing higher flow rates. The flexibility of the developed setup also allows performing stochastic nucleation studies to estimate the nucleation kinetics under silent and sonicated conditions. The experiments reveal that very high nucleation rates, small crystal sizes, narrow size distributions and high crystal yields can be obtained with both setups when the crystallizing solution is exposed to high pressure field as compared to silent condition. It is concluded that transient cavitation of bubbles and its consequences are a significant mechanism for enhancing nucleation of crystals among several proposed in the literature. A preliminary study towards the development and design of a growth stage is finally performed. Flow pulsation is identified as a potential method to enhance radial mixing and narrow residence time distribution therefore achieving optimal conditions for uniform crystal growth. The results suggest that increasing values of Strouhal number as well as amplitude ratio improve axial dispersion. Helically coiled tubes are identified as potential structures to further improve fluid dynamic dispersion

Metal Micro-forming

The miniaturization of industrial products is a global trend. Metal forming technology is not only suitable for mass production and excellent in productivity and cost reduction, but it is also a key processing method that is essential for products that utilize advantage of the mechanical and functional properties of metals. However, it is not easy to realize the processing even if the conventional metal forming technology is directly scaled down. This is because the characteristics of materials, processing methods, die and tools, etc., vary greatly with miniaturization. In metal micro forming technology, the size effect of major issues for micro forming have also been clarified academically. New processing methods for metal micro forming have also been developed by introducing new special processing techniques, and it is a new wave of innovation toward high precision, high degree of processing, and high flexibility. To date, several special issues and books have been published on micro-forming technology. This book contains 11 of the latest research results on metal micro forming technology. The editor believes that it will be very useful for understanding the state-of-the-art of metal micro forming technology and for understanding future trends

VISUALIZATION AND CHARACTERIZATION OF ULTRASONIC CAVITATING ATOMIZER AND OTHER AUTOMOTIVE PAINT SPRAYERS USING INFRARED THERMOGRAPHY

The disintegration of a liquid jet emerging from a nozzle has been under investigation for several decades. A direct consequence of the liquid jet disintegration process is droplet formation. The breakup of a liquid jet into discrete droplets can be brought about by the use of a diverse forcing mechanism. Cavitation has been thought to assist the atomization process. Previous experimental studies, however, have dealt with cavitation as a secondary phenomenon assisting the primary atomization mechanism. In this dissertation, the role of the energy created by the collapse of cavitation bubbles, together with the liquid pressure perturbation is explicitly configured as a principal mechanism for the disintegration of the liquid jet. A prototype of an atomizer that uses this concept as a primary atomization mechanism was developed and experimentally tested using water as working fluid. The atomizer fabrication process and the experimental characterization results are presented. The parameters tested include liquid injection pressure, ultrasonic horn tip frequency, and the liquid flow rate. The experimental results obtained demonstrate improvement in the atomization of water. To fully characterize the new atomizer, a novel infrared thermography-based technique for the characterization and visualization of liquid sprays was developed. The technique was tested on the new atomizer and two automotive paint applicators. The technique uses an infrared thermography-based measurement in which a uniformly heated background acts as a thermal radiation source, and an infrared camera as the receiver. The infrared energy emitted by the source in traveling through the spray is attenuated by the presence of the droplets. The infrared intensity is captured by the receiver showing the attenuation in the image as a result of the presence of the spray. The captured thermal image is used to study detailed macroscopic features of the spray flow field and the evolution of the droplets as they are transferred from the applicator to the target surface. In addition, the thermal image is post-processed using theoretical and empirical equations to extract information from which the liquid volume fraction and number density within the spray are estimated

Understanding nanoscale material behaviour for improved precision machining of shape memory alloys; testbed study on elliptical vibration assisted cutting of CuZr SMA.

The field of ultra-precision machining has gained significant importance in the manufacture of components for the electronic, optical and medical industry. Two crucial factors that play a key role in the machinability of materials are the machining parameters and the material’s physical properties. Certain materials such as hardened steel or nickel-based superalloys are difficult-to-machine but innovations in the field of precision machining have developed a technique known as elliptical vibration assisted machining, which enables to improve the machinability of these materials. CuZr high-temperature shape memory alloy is categorized as a difficult-to-cut material and although EVAM has been applied to a wide range of metals it hasn’t yet been studied in CuZr HTSMA. In this context, the purpose of this thesis is twofold: On the one hand, to characterise the mechanical properties of CuZr SMA using Molecular Dynamics and, on the other hand, to explore the nanoscale mechanism of material removal of CuZr shape memory alloy (SMA) during elliptical vibration assisted machining (EVAM). The conclusions of this thesis can be summarized as follows. To characterise the mechanical properties of Cu₅₀Zr₅₀, Cu₂Zr and Cu₅Zr, a tensile and shear test were carried out using MD. Tensile test was done with crystal orientation and direction of tensile pulling as . The results showed that Cu₅₀Zr₅₀ and Cu₂Zr exhibited a phase transformation (pseudoelasticity) during loading. However, Cu₅Zr showed dislocation nucleation as the main plastic deformation mechanism followed by fracture. Shear tests were done in the same phases with crystal orientation and direction of shear pulling as . Interestingly, the shear test results showed no phase transformation for Cu₅₀Zr₅₀ and Cu₂Zr but the Cu₅Zr composition did show phase transformation during loading. It is important to highlight that all three phases of CuZr binary alloy that we have tested showed a different plastic response during the tensile test and the shear test. As far as machining is concerned, we observed indications that EVAM shows improved machinability compared with conventional machining. Although cutting forces were lower in EVAM, the stresses on the workpiece were slightly higher and both techniques showed the same mechanism of plasticity during machining. Neither dislocation nucleation or martensitic transformation was exhibited in either of the two machining techniques and instead, amorphisation was observed as the main plastic deformation mechanism in both cases. Interestingly, amorphisation has been previously observed by Saitoh and Kubota (2010) during loading NiTi SMA [1]; however, it didn’t show up in every crystal orientation confirming that NiTi shows significant changes in response to loading in different lattice directions. One of the main outcomes from this thesis is that CuZr SMA exhibits different modes of plastic deformation; namely amorphisation, dislocation nucleation and martensitic transformation during loading. The governing mechanism that arises during loading highly depends in the lattice direction in which the load is being applied. These findings can potentially enable reliable predictions and provide guidelines of the microstructural design of CuZr SMA systemsPhD in Manufacturin