Alan MacBeath

Temperature Effects in Ultrasonic Cutting of Natural Materials

This paper investigates the relationship between ultrasonic cutting parameters and temperature around the cutting site for wood and bone, with the aim of reducing thermal damage in natural materials. Two ultrasonic cutting blades, tuned at two different frequencies but with identical cutting edge and surface profiles, are used in the experiments. Temperature is monitored close to the cutting site and the effects of applied load and blade tip vibration velocity are studied. Results indicate that ultrasonic cutting blade design can incorporate cutting parameters to reduce or eliminate thermal damage in these natural materials without using an additional blade cooling system.

Applications of Power Ultrasonics in Engineering

Applications of power ultrasonics in engineering are growing and now encompass a wide variety of industrial processes and medical procedures. In the field of power ultrasonics, ultrasonic vibrations are used to effect a physical change in a medium. However, the mechanism by which a process can benefit from power ultrasonics is not common for all applications and can include one or more of such diverse mechanisms as acoustic cavitation, heating, microfracture, surface agitation and chemical reactions. This paper presents two applications of power ultrasonics involving some of these different characteristics by concentrating on two case studies involving material failure (ultrasonic cutting) and acoustic cavitation (bacterial inactivation).

A Finite Element Model for Ultrasonic Cutting of Toffee

The performance of an ultrasonic cutting device critically relies on the interaction of the cutting tool and the material to be cut. A finite element (FE) model of ultrasonic cutting is developed to enable the design of the cutting blade to be influenced by the requirements of the toolmaterial interaction and to allow cutting parameters to be estimated as an integral part of designing the cutting blade. In this paper, an application in food processing is considered and FE models of cutting are demonstrated for toffee; a food product which is typically sticky, highly temperature dependent, and difficult to cut. Two different 2D coupled thermal stress FE models are considered, to simulate ultrasonic cutting. The first model utilises the debond option in ABAQUS standard and the second uses the element erosion model in ABAQUS explicit. Both models represent a single blade ultrasonic cutting device tuned to a longitudinal mode of vibration cutting a specimen of toffee. The model allows blade tip geometry, ultrasonic amplitude, cutting speed, frequency and cutting force to be adjusted, in particular to assess the effects of different cutting blade profiles. The validity of the model is highly dependent on the accuracy of the material data input and on the accuracy of the friction and temperature boundary condition at the blade-material interface. Uniaxial tensile tests are conducted on specimens of toffee for a range of temperatures. This provides temperature dependent stress-strain data, which characterises the material behaviour, to be included in the FE models. Due to the difficulty in gripping the tensile specimens in the test machine, special grips were manufactured to allow the material to be pulled without initiating cracks or causing the specimen to break at the grips. A Coulomb friction condition at the bladematerial interface is estimated from experiments, which study the change in the friction coefficient due to ultrasonic excitation of a surface, made from the same material as the blade, in contact with a specimen of toffee. A model of heat generation at the blade-toffee interface is also included to characterise contact during ultrasonic cutting. The failure criterion for the debond model assumes crack propagation will occur when the stress normal to the crack surface reaches the tensile failure stress of toffee and the element erosion model uses a shear failure criterion to initiate element removal. The validity of the models is discussed, providing some insights into the estimation of contact conditions and it is shown how these models can improve design of ultrasonic cutting devices.

Design of an Ultrasonic Blade for Cutting Bone

Damaging temperature effects observed during ultrasonic cutting operations are typically a result of friction between the vibrating blade and material, and combustion of debris. In order to prevent the high temperatures causing damage, the ultrasonic blade has to cut with a sufficient speed. This can be achieved either by applying a relatively high static load or by increasing the working vibration amplitude of the cutting edge, however, the result can be poor operational control and exceeding the fatigue limit of the blade, respectively. In this paper, the effect of blade tip profile is considered, particularly with reference to the influence of the cutting edge contact area on temperature under different static loading conditions. Titanium blades, with different cutting edge profiles are tested in a series of experiments that monitor cutting speed, static load, temperature around the cut site, and vibration amplitude at the cutting edge. The blades are tested cutting bovine femur and artificial bone material, and the cut surfaces are examined for signs of damage after each test. The experimental data reveal that blades with a small cutting edge contact area cut at a lower temperature, and that signs of thermal damage are less evident.

Ultrasonic Cutting with High-Gain Blades

The design of high power ultrasonic cutting devices is based on tuning a blade to a longitudinal mode of vibration at a low ultrasonic frequency, usually in the range 20-100 kHz. To achieve the required cutting amplitude, gain is designed into the blade via profiling. It is expected that the use of higher-gain blades could enable longitudinal-mode guillotine-type cutting of a range of materials traditionally difficult to cut using this technology. Using a conventional high-gain blade, a feasibility study of ultrasonic cutting of bone is conducted using compact tension specimens of bovine femur. Finite element (FE) models are created, based on the assumption that the ultrasonic blade causes a crack to propagate in a controlled mode 1 opening. The models are compared with the experimental data collected from ultrasonic bone cutting experiments. Although the proposed cutting mechanism is supported by the data, the blade gain is insufficient to enable through cutting of long bone or other difficult to cut materials. Consequently, the paper examines the relationship between gain, profile, stress and nodal position for a range of ultrasonic cutting blades with increased gain.

P2G-7 Effect of Ultrasonic Cutting Blade Orientation on Cutting Temperature

Ultrasonic cutting of bone offers advantages compared with orthopaedic devices that rely on a reciprocating action, including the elimination of swarf, improved cut quality and precision, and reduced reaction forces. The technology has become accepted as an alternative cutting procedure for use in surgical operations on soft tissue. Recent studies conducted on bovine bone and a bone substitute material have shown that ultrasonic cutting results in a precise and fast operation using relatively low forces. Previous work by the authors highlighted the significance of frictional heating during ultrasonic cutting, a phenomenon that can lead to material degradation and excessive burning of the cut surface. The work presented a method of reducing cutting temperature by controlling ultrasonic cutting parameters and also presented a method of further reducing cutting temperature by incorporating blade geometry modifications that reduce friction between the blade and the specimen. Such studies have been concerned with uni-axial cutting blade orientations, known as guillotine cutting, and opportunities exist to enhance orthopaedic ultrasonic cutting by developing blades that can operate in more than one cutting direction. This paper investigates the relationship between cutting parameters (such as cutting speed, applied load and blade tip vibration velocity) and temperature at locations around the cut site for a synthetic bone material during guillotine and slicing mode cutting