Charles Ume

Non-contact optical fibre phased array generation of ultrasound for non-destructive evaluation of materials and processes

Abstract The use of non-contact laser techniques for the generation of ultrasound has extended the limits of the application of traditional ultrasonic techniques. This paper focusses on the use of one such non-contact laser technique, known as ‘optical fibre array’, to generate shear and surface waves. The shear wave experimental directivity pattern results are presented and compared with the theoretical results of a single source and an array source. The experimental directivity results for the surface wave are also presented, and compared with the theoretical results. The data show that the array enhances signal generation in the forward direction for both shear and surface waves. The array gain is also discussed. The receiver for the directivity measurements was a contact piezoelectric transducer.

In-process board warpage measurement in a lab scale wave soldering oven

An automated on-line warpage measurement system for printed wiring boards (PWBs) and printed wiring board assemblies (PWBAs) has been developed. The system is capable of simulating a variety of soldering processes, including the wave soldering process, and performing real-time PWB/PWBA warpage measurements using the shadow moire technique. The system can be used to characterize the warpage behaviour of virtually any PWB/PWBA during the soldering process. Using this system, warpage of PWB test vehicles was measured during simulated wave soldering. The measured warpage varied significantly during wave soldering from that observed both before and after wave soldering. These results help us to understand how the board deforms at every stage of the soldering process.

Longitudinal wave generation in laser ultrasonics

The purpose of this work is to conduct a detailed investigation of wave generation by thermal dipoles in laser-generated ultrasound. A simplified description of ultrasound generation is used to provide a clear explanation as to how longitudinal waves are generated by thermal dipoles. The effects of the shape and time dependence of the temperature distribution on the waveforms are illustrated. Studies such as this are needed in order to explain the shapes of laser-generated ultrasound waves for nondestructive materials testing, and to enable researchers to select optimum laser pulse parameters for specific applications. In particular, this work identifies the physical origins of the precursor.

Laser generated ultrasound: a thermoelastic analysis of the source

Abstract The purpose of this work is to study the behavior of laser ultrasound sources in isotropic metals using ultra-short laser pulses in the thermoelastic regime. Temporally Gaussian and high frequency modulated laser pulses are investigated, and numerical results show that the thermal response rates of both steel and aluminum are much faster than their mechanical response rates. This indicates that the temporal point source limit in laser ultrasonics is a mechanical rather than thermal limitation. Two temporal point source limits are identified. For waves generated by the SCOE (surface center of expansion), it is shown that the temporal point source limit occurs at a Gaussian rise time of 1 ns in both aluminum and steel. However, the precursor is a result of thermal diffusion and does not display a temporal point source limit within the range of parameters considered here. A dimensional analysis of the laser source is conducted, and it is found that a single dimensionless parameter defines thermal similarity for laser sources, but no single parameter can define acoustic similarity. The physical dimensions of the laser source are estimated for steel and aluminum, and it is found that for temporally and spatially Gaussian laser pulses the outer edge of the source is 1.65 times the Gaussian spot radius over a wide range of rise times and spot sizes. Also, a dimensionless parameter is given for three dimensional problems which predicts minimum rise times and maximum modulation frequencies for which hyperbolic heat conduction effects will be of no significance in the ultrasonic displacements.

Hyperbolic heat equations in laser generated ultrasound models

The generation of ultrasound by pulsed lasers is a thermoelastic process, and in the literature on general thermoelasticity there are presently three different forms of the heat equation in popular use, the classical, Lord-Schulman (LS), and Green-Lindsay (GL) heat equations. The question may thus arise as to which heat equation should be used to model laser generated ultrasound. The purpose of this work is to summarize the current rationale for using these different heat equations, in order to provide a basis for choosing one of the forms. A review of the classical, LS and GL theories is given, and the potential advantages of hyperbolic heat flow theories in laser ultrasonics are discussed. A numerical example is given that clearly shows the small time differences these theories predict, and also points out potential problems with using hyperbolic heat equations on small time scales.

Mechatronics instruction in the Mechanical Engineering curriculum at Georgia Tech

The increasing use of microcontrollers and microprocessors in a wide variety of consumer and commercial products, laboratory test instruments and equipment, and industrial applications has created a need for Mechatronics education in all engineering disciplines. The subject of Mechatronics is broad, encompassing, and interdisciplinary. How to teach Mechatronics in the various Engineering disciplines is still an open area for discussion. This presentation focuses on the way Mechatronics is taught in George Woodruff School of Mechanical Engineering at Georgia Tech. The courses are structured to teach students design at the micro-chip level, and to teach them how to write assembly language programs for measurement and control. They learn by hands-on experience with interfacing sensors, actuators, and passive and active devices with microprocessors and microcomputers in laboratory exercises. The class includes a final group project and a group class lecture. Each group of three students is required to complete a design project that integrates hardware and software design with electronic interfacing design and mechanical systems analysis. Several such projects and all the laboratory exercises are presented.

Temporal modulation of a laser source for the generation of ultrasonic waves

Abstract Ultrasound is generated in an aluminium test block using a pulsed Nd:YAG laser. The laser beam is modulated at high frequencies using an acousto-optic modulator. Ultrasound is detected using piezoelectric transducers. Frequency spectra of ultrasound generated by modulated and unmodulated laser beams are compared. Signals generated by modulated beams are found to have greater amplitude and a narrowed bandwidth around the modulation frequency.

Time integration procedures for a cyclic thermoviscoplasticity model for Pb-Sn solder applications

A semi-implicit time integration scheme has been developed for a cyclic thermoviscoplastic constitutive model with tensorial internal state variables for Pb-Sn solder, a common metallic constituent in electronic packaging applications. The procedure has been implemented numerically into the commercial finite element (FE) code ABAQUS (1995) by user-defined material subroutines. Several simulations are conducted to compare the numerical implementation to experiments including monotonic uniaxial tests at different temperatures, creep tests at four stress levels, and a test with two-step load-controlled cyclic loading for 62Sn36Pb2Ag solder. An explicit time integration scheme is used as well to compare with the semi-implicit scheme. The accuracy as well as the stability of the solutions are considered. Several suggestions are made for using the material constitutive model and the semi-implicit integration scheme for modeling solder connections. This work provides guidelines to implement user-defined material behavior into FE analyses to perform more sophisticated thermomechanical simulations for solder connections in electronic packaging applications.

Hyperbolic heat conduction effects caused by temporally modulated laser pulses

Abstract In non-destructive testing (NDT), the intensity of laser pulses may be temporally modulated to improve the signal-to-noise ratio of ultrasonic signals. The purpose of this work is to determine the time scale on which hyperbolic heat conduction effects are significant in ultrasonic displacements and stresses for temporally modulated laser pulses, under the assumption that the heat wave speed equals the longitudinal wave speed. A one-dimensional model with a finite train of heat flux pulses is investigated. Using the Green and Lindsay model, it is found that the critical modulation frequency in steel, above which hyperbolic heat conduction effects will become signficant, is 2.26 GHz. A dimensionless factor is given to calculate critical modulation frequencies in other isotropic materials. The signal enhancement potential of temporal intensity modulation is apparent in the numerical results.

Characterizing the temperature dependence of electronic packaging-material properties

A computer-controlled, temperature-dependent material characterization system has been developed for thermal deformation analysis in electronic packaging applications, especially for printed wiring assembly warpage study. For fiberglass-reinforced epoxy (FR-4 type) material, the Youngs moduli decrease to as low as 20–30% of the room-temperature values, while the shear moduli decrease to as low as 60–70% of the room-temperature values. The electrical resistance strain gage technique was used in this research. The test results produced overestimated values in property measurements, and this was shown in a case study. A noncontact strau]n measurement technique (laser extensometer) is now being used to measure these properties. Discrepancies of finite-element warpage predictions using different property values increase as the temperature increases from the stress-free temperature.