CIRAS News, Summer 2006, Vol.41, no.4

Center for Industrial Research and Service: CIRAS partners with Iowa manufacturing companies to enhance the performance of Iowa industries with education and technology

Dynamics of Ultrasonic Consolidation

Ultrasonic consolidation (UC) is a solid state rapid manufacturing process derived from ultrasonic welding of thin metal foils coupled with contour milling to achieve functional accurate components. The bonding of metal is accomplished by the local application of high frequency vibration energy under pressure producing a metallurgical bond without melting the base material. Its unique nature allows the design and fabrication of structural panels for satellites, production of injection molding tools, functionally graded structures, metal-matrix composites, embedded sensors, armor, and fiber embedded adaptive structures. It is commonly theorized that interfacial motion and friction at the bonding interface play a prominent role in the bonding process by removing surface contaminants, allowing direct metal to metal contact, and producing sufficient stress to induce plastic flow. The substrate\u27s geometry is also crucial in the bonding process. Researchers have experimentally observed that as the height of build specimen approaches its width, the bonding process degrades, and no further foils may be welded. This work explores the process as the dimensions of the build specimen modeled as a standard parallelepiped, approaches the critical geometry through a combination of numerical, analytical and experimental analysis. We examine the resonances of a build feature due to a change in geometry and material properties using a three dimensional Rayleigh-Ritz model. A simple nonlinear dynamic model of the Ultrasonic Consolidation Process examines how the geometry change may influence the overall process dynamics. This simple model is use to provide estimates of how changes is substrate geometry affect the differential motion at the bonding interface and the amount of changing friction force due to build height. The trends of changes in natural frequency, and differential motion, are compared to experimental limits on build height. These analyses lead to several predictions on build height that are verified experimentally. Finally, the work examines the effectiveness of using support material to extend the build height limit of the process. The results show that a proximity to a resonance excitation is clearly responsible for bonding degradation at features built with the nominal tape width of 0.9375 inches. However, for small widths other factors such as surface topography, and contact area may play an important role in bonding degradation

The measurement of the deformation properties of Cowden Till at small strains.

The work described in this thesis was firstly concerned with developing and evaluating automated soil testing equipment and associated instrumentation. The equipment consists principally of a triaxial stress path cell of the Bishop-Wesley type, a microcomputer and two pressure controllers. Inductive displacement transducers have been mounted inside the cell to measure axial and radial strains locally on the specimen boundary and axial strains between the end caps. The local axial strain measurements have proved superior to the end cap measurements which can be adversely affected by bedding errors and misalignment of the transducers relative to the loading axis. Following the development, the system was used to investigate the stress-strain behaviour of Cowden Till, particularly at small strains (0.01 - 0.10%). Cylindrical blocks of 250mm diameter were retrieved from the site and stored under isotropic stress. Eight specimens of 100mm diameter were trimmed from these blocks and subjected to either a drained or undrained compression test under load-controlled conditions. Cowden Till has been shown to exhibit strongly non-linear stress strain behaviour, even at small strains, and most of the shear strain is irreversible. The stress-strain characteristics were in acceptable agreement with those derived from a 865mm diameter plate loading test with under-plate instrumentation. Although the interpretation of the plate test is still being investigated, it is concluded that plate tests provide no better information about the stiffness of the material than triaxial tests of the type described in this thesis. The experimental stress-strain behaviour during compressive loading has been compared with the predictions of some mathematical models. The nonlinear elastic model of Atkinson (1973) appears to be applicable to Cowden Till, for which the behaviour is approximately isotropic. Simple stiffness predictions on the basis of critical state soil mechanics are inadequate at small strains. However, the model of Pender (1978) predicts the behaviour reasonably well. (ii) An attempt has been made to analyse the compression (bedding error) which occurs at the end of a triaxial specimen as the axial strain is increased. A quantification of the compression is hindered by the random nature of surface variations and by the limitations of present theories

Acoustics and manufacture of Caribbean steelpans

The Caribbean steelpan is a pitched percussion instrument that originated in Trinidad and Tobago during the Second World War. Despite several research initiatives to improve the making of this relatively new instrument, several areas remain unaddressed. This thesis presents new approaches to help improve the making of the instrument. These approaches are situated in the production, vibration and material aspect of the steelpan. A novel sheet forming technology termed Incremental Sheet Forming (ISF) is applied to the production of miniature steelpan dishes. The thickness distribution in the wall of the ISF dishes is compared to the wall thickness distribution in a traditionally formed steelpan dish and a wheeled dish. Unlike traditional forming and wheeling, ISF produces stretching in only a portion of the walls of the formed dishes. Multi-pass ISF is used to extend the stretched zone but this extension is minimal. A break even analysis is also applied to investigate the fiscal viability of ISF application to the production of miniature and full size steelpan dishes. The application of ISF to steelpan making is found to be commercially profitable but could be jeopardised by the tuning stage of the steelpan making process. A preliminary study on the effect of impact on tone stability is conducted on a pair of notes on a full size steelpan and detuning is found more likely to occur by repeated impact of the note at its centre. Mode confinement in test-pans is also investigated. ISF is used to produce miniature test-pans with test-notes that are geometrically identical to notes on full size pans. It is possible to confine modes by varying the curvature of the bowl surrounding the test-note. The number of localised modes in the test-note increases as the radius of curvature of the surrounding bowl increases. The natural frequency of the first confined mode in the test-notes is sensitive to material springback in ISF and the mechanism of confinement appears to be due to the change in geometry that occurs between the flat test-note region and the bowl wall. This control of mode confinement may find use in future efforts to completely or partially automate the steelpan making process. Material damping and mechanical properties in low-carbon steel used to produce steelpans are researched. Damping and mechanical properties are extracted from low-carbon steel that is subjected to identical stages to the steelpan production process. Material damping trends suggest that an annealing temperature between 300°C and 400°C would be appropriate for the heat treatment of steelpans. Air-cooled and water-quenched low-carbon specimens exhibit comparable damping trends. Hardness increases in cold formed low-carbon specimens is attributed to strain hardening and not strain ageing. Investigation of damping trends and mechanical properties in ultralow bake-hardenable and interstitial-free steels reveals that a wider range of low-carbon steels maybe suitable for steelpan making.This work was supported by the University of Trinidad and Tobag

Machining of hybrid composites

Tese de doutoramento. Engenharia Mecânica. Faculdade de Engenharia. Universidade do Porto. 200

Ultrasonically-assisted drilling of carbon fibre-reinforced plastics

Carbon fibre-reinforced plastics (CFRP) are widely used in aerospace, automobile and other structural applications due to their superior mechanical and physical properties. CFRP outperform conventional metals in high strength-to-weight ratio. Usually, CFRP parts are manufactured near to net-shape;however,machining is unavoidable when it comes to assembly. Drilling the holes are essential to facilitate riveting and bolting of the components. However, conventional drilling (CD) induces different types of damages such as cracking, fibre pull-out, sprintling and delamination due to the abrasive nature, inhomogeneity and anisotropy of CFRP. A novel technique, ultrasonically-assisted drilling (UAD) is hybrid machining technique in which highfrequency (typically above 20 kHz) vibration are superimposed on a standard twist drill bit in axial direction using ultrasonic transducer. UAD has shown several advantages such as thrust force reduction, improving surface quality and lower bur-formation in drilling of conventional metals. UAD has also effectively been used for drilling brittle materials. [Continues.