Subash Jayaraman

Transient thermal deformation of alumina (A12O3) substrate during laser drilling

The cracking and failure in ceramic substrates during the laser drilling process has been acknowledged as a major problem by designers and manufacturers in the electronic component industries. The cracking and failure is due to large localized thermal stresses within the narrow heat-affected zone on the ceramics. Although the knowledge of the stress distribution in the ceramic substrate is important in understanding and solving the cracking/failure problem, it is impossible to measure the stress directly. The physical parameters of the laser drilling process such as temperatures or displacements, which can be directly related to stresses, can however be measured. That is why, in this research, an electronic speckle pattern interferometer (ESPI) system was designed and used to take speckle pattern images of the ceramic surface during the laser drilling process. Using commercial software, the speckle fringe images were image processed to quantify whole-field transient out-of-plane displacement measurements. A deformation history of the ceramic surface during the laser shaping process with millisecond temporal resolution was obtained, restricted only by the camera frame rate, camera resolution and laser power available. A finite difference model was developed to compare the deformation measurements with the predicted strain calculations. The experimental study and the analysis show that the designed in-situ electronic speckle pattern interferometer system provides an excellent experimental basis for whole- field, transient deformation measurements of ceramic substrates during the laser drilling process.

Simulation of acoustic emissions from delaminations and cracks in plates of aluminum and graphite

The structural health monitoring of structures during active use (in service) has long been of interest to the NDE community. One technique uses passive ultrasound or Acoustic Emission (AE). However, the interpretation of the AE signals is difficult especially when the operator tries to distinguish between the growth of harmless micro-cracks and the development of harmful delaminations. This paper focuses on two types of structures, i.e., aluminum plates such as used in wing structures in aircraft and graphite plates such as encountered in aircraft disc brakes where carbon-carbon composite is used. The objective in this work is to distinguish the acoustic emissions (AE) caused by delaminations from those associated with microcracking. The technical approach is to use finite element methods (FEM) to simulate AE from sources represented by piezoelectric wafers embedded in the composites. In flat panels of graphite and aluminum-alloy AE waveforms were modeled from transverse cracks and longitudinal delaminations. The results show distinct differences in the amplitudes, durations and frequency content creating a potential avenue for distinguishing between these two flaw types.

Progress in air-coupled ultrasound

A variety of industrial and everyday non-destructive inspection applications exist where the target material/product is inaccessible or, contact with the material is prohibited. In such cases, air-coupled ultrasonic techniques play a major role but commonly significant transmission loss is known to occur. Therefore, it becomes imperative to know the amount of absolute wave mechanical strain achieved in materials embedded in gaseous medium, for certain applications. Thus, the overall objective of this work was to establish simulated results and specific experimental verifications of the numerical modeling, and develop guidelines in the use of matching layers to maximize the wave mechanical strain imparted to materials. A Laser Doppler Vibrometer was used to obtain the displacements/strains induced in the materials. Coupled Acoustic Piezoelectric Analysis (CAPA), coupled field finite element method software was used to perform the simulations. The applications considered in this work include metallic targets inside an enclosed container, food products and also elastomeric composites such as automotive tires.

Measurement of Absolute Acoustic Strain by Non‐Contact Technique

Some ultrasonic applications require non‐contact techniques because the target material is not easily accessible. In such cases laser‐based and air‐coupled ultrasonic techniques play a major role but commonly significant transmission loss is known to occur especially at higher frequencies. Therefore, it becomes imperative to know the amount of absolute acoustic strain achieved for a given application. In this paper, we report on the use of laser‐based techniques to measure absolute strain on the face of vibrating rods excited under various scenarios. These include contact and air‐coupled excitation at frequencies at resonance, as well as a factor of 100 below and above the resonance. The limit of our out‐of‐plane displacement measurement appears to be about 5 nanometers. Strains as high as 10−6 have been obtained. The paper will describe the details of the ultrasonic techniques and some of the applications. The data are compared to theoretical and simulated strain calculations.

Laser pulse impact characterization using ultrasonic transducers

Experiments were conducted with an excimer laser (wavelength 248 nm) to cause damage to gelatinous materials, which have optical properties similar to skin tissue. The laser pulses, when they impact the specimen, generate an elastic wave that propagates through the material. A 150‐MHz broadband ultrasonic transducer was placed underneath the specimen to observe the material response and the pressure generated by the laser pulse. Different incident laser energies and laser beam spot sizes were utilized to obtain a range of input parameters, and the corresponding transducer responses were recorded. Fourier analysis of the signals was performed to identify the frequency response from the laser pulse. Additional tests were carried out to observe the effects due to a train of laser pulses. Earlier tests were performed with 200‐kHz hydrophone‐calibrated transducer but due to the shortness of the laser pulses (∼20 ns), a higher frequency transducer was used to accurately characterize the laser impact. The initia...

Non-intrusive non-destructive method to detect fissile material

The detection of fissile materials is of great interest to the National Homeland Security effort. Significant advantages of a technique using nuclear acoustic resonance (NAR) over the traditional detection methods are that it will not rely on nuclear radiation signatures, will be non-intrusive, and has the potential to identify individual components of composite substances including fractional isotope composition of the material under investigation. Technique uses the unique nuclear acoustic resonance signatures generated when materials are driven by high intensity resonant acoustic waves in the presence of a constant magnetic field. This would cause shifts in the nuclear and electronic spin energy levels of the material. Nuclear energy level shifts induce changes in the unique nuclear magnetic properties of the material which can then be quantified using sensitive instruments. This paper will discuss in detail, the physics and detection principles of NAR and also provide some preliminary results.

Investigation of mechanical impact effects on biological cells with scanned image microscopy

The objective of this study is to observe the behavior of the living cells after introducing impact, caused by an aluminum bullet shot out from an air gun, onto them via tungsten/polymer plate and culture liquid. An air gun type of apparatus shoots an aluminum bullet, wherein the shape of the bullet is substantially a sphere (diameter: 5 mm), and wherein the velocity of the bullet is controlled by the amount of air used for shooting. The aluminum bullet shot out from the air gun impacts onto the polymer/tungsten plate, located above the living cells grown on the bottom of the container (i.e., thin semi-transparent polymer membrane), which is located on the surface of a 200 kHz Panametrics transducer. The container is supported by a polymer member to prevent movement from shock caused by the bullet impact. The plate generates an acoustic wave (i.e., shock wave) by the mechanical impact (i.e., bullet impact) which is then converted into an electrical signal by the transducer. The amplitude of the electrical signal is measured and monitored by the digital oscilloscope. The transducer is calibrated by hydrophone with its peripheral equipment including computer software. The output voltage from transducer was monitored by the digital oscilloscope. The injury and recovery of the specimen are evaluated by scanned image microscopes. Furthermore, quantitative data showing the injury and recovery of the specimen can be obtained with the electromagnetic measurement.

Ultrasonic characterization of elastomers and elastometric composites

Understanding the elastic properties of the various types of rubber is important for many commercial and academic applications. A sample set consisting of generic elastomeric compounds was studied using non-destructive non-contact ultrasonic techniques. The longitudinal sound wave velocities in the sample and wave amplitude attenuation in the sample were measured using the Second Wave Inc. Non-Contact Analyzer 1000 (NCA1000). The Contact method was then used to corroborate the results obtained. The preliminary results suggest that the differences in attenuation are driven by polymer type and also to a lesser extent by the loading level of carbon black fillers.

Feasibility of healing delaminations in an AS4/PEEK composite plate

This paper describes the study carried out to determine the possibility of healing an AS4/PEEK composite plate that was impacted at a low velocity to create the delaminations. Images of the AS4/PEEK composite plates were obtained prior to the impact, to ensure it is free of any gross defects that could have been imparted during the manufacturing of the composite plate. A drop-weight was used to impact the composite plate at a low velocity and the incident energy of impact was maintained at 2.26 ft-lb. Interior images of the composite plates were obtained after the impact and prior to healing by a C-scan imaging system. The healing process was conducted at controlled temperature and pressure. The healed specimens were imaged again by the C-Scan imaging system. The results obtained by analyzing the images show a reduction in the size of the delaminations.

Long distance leaky surface ultrasonic waves

This paper contains a description of an unique wave that was created in a two plate system separated by a layer of water. First, a Lamb wave was created in the upper plate by placing a transducer on a wedge on top of the plate at an appropriate angle and frequency. This wave was created to act like quasi-Rayleigh (surface) waves on both surfaces of the plate. The wave on the bottom surface of the upper plate then leaked through the water into the upper surface of the lower plate. We will show both experimentally and theoretically that the wave on the surfaces of the plates in contact with water leak constructively to create a leaky-wave that can travel great distances.