Graham H. Thomas

Overview of nondestructive evaluation technologies

The infrastructure in the United States and the world is aging. There is an increasing awareness o the need to assess the severity of the damage occurring to our infrastructure. Limited resources preclude the replacement of all structures that need repairs or have exceeded their lifetimes. Methods to assess the amount and severity of damage are crucial to implementing a systematic, cost effective approach to repair and/or replace the damaged structures. The challenges of inspecting aging structures without impairing their usefulness rely on a variety of technologies and techniques for nondestructive evaluation. This paper will briefly describe several nondestructive evaluation technologies that re required for inspecting a variety of systems and structures.

Laser ultrasonic signal processing: A model‐reference approach

A model‐reference approach is developed to solve the signal enhancement problem of a laser ultrasonics application for nondestructive evaluation. In this problem a sophisticated laser thermoelastic propagation model is used to synthesize the surface displacement of the specimen under test. Once synthesized, this model response is used as the reference signal in an optimal (minimum error variance) signal enhancement scheme. Both fixed and adaptive processors are considered in this application where it is shown that a significant improvement in signal levels can be achieved over the usual methods to enhance noisy data acquired from a Michelson interferometric measurement system and increase its overall sensitivity.

Signal analysis approach to ultrasonic evaluation of diffusion bond quality

Solid state bonds like the diffusion bond are attractive techniques for joining dissimilar materials since they are not prone to the defects that occur with fusion welding. Ultrasonic methods can detect the presence of totally unbonded regions but have difficulty sensing poor bonded areas where the substrates are in intimate contact. Standard ultrasonic imaging is based on amplitude changes in the signal reflected from the bond interface. Unfortunately, amplitude alone is not sensitive to bond quality. We demonstrated that there is additional information in the ultrasonic signal that correlates with bond quality. In our approach, we interrogated a set of dissimilar diffusion bonded samples with broad band ultrasonic signals. The signals were digitally processed and the characteristics of the signals that corresponded to bond quality were determined. These characteristics or features were processed with pattern recognition algorithms to produce predictions of bond quality. The predicted bond quality was then compared with the destructive measurement to assess the classification capability of the ultrasonic technique.

Three dimensional ultrasonic imaging

Ultrasonic nondestructive evaluation techniques interrogate components with high frequency acoustic energy. A transducer generates the acoustic energy and converts acoustic energy to electrical signals. The acoustic energy is reflected by abrupt changes in modulus and/or density which can be caused by a defect. Thus defects reflect the ultrasonic energy which is converted into electrical signals. Ultrasonic evaluation typically provides a two dimensional image of internal defects. These images are either planar views (C-scans) or cross-sectional views (B-scans). The planar view is generated by raster scanning an ultrasonic transducer over the component and capturing the amplitude of internal reflections. Depth information is generally ignored. Examples of potential ultrasonic imaging applications are: inside liquid filled tanks, inside the human body, and underwater.

Ultrasonic monitoring of laser damage in fused silica

The growth of a laser induced, surface damage site in a fused silica window was monitored by the ultrasonic pulse-echo technique. The laser damage was grown using 15 ns pulses of 1.053 μm wavelength light at a fluence of ∼25 J/cm2. The ultrasonic signal amplitude exhibited variations with the damage size which are attributable to the changing subsurface morphology of the damage site. The sensitivity to subsurface morphology makes the ultrasonic methodology a promising tool for monitoring laser damage in fused silica optics. This type of diagnostic capability may facilitate the safe deployment of large, high powered laser systems used in high energy and fusion research facilities.

A spatio‐temporal approach to acoustical imaging of laser‐generated ultrasound

In this paper an application of spatio‐temporal array signal‐processing techniques applied to broadband ultrasonic data gathered from a pulsed laser system is discussed. Using a laser source to heat a material specimen under test for flaws, a spatio‐temporal processor capable of estimating the displacement field of the specimen is applied. The peak surface displacement is displayed as an image showing the initial source (displacement field) propagating throughout the material as well as any flaws (scatterers) that may be present within the specimen. Clearly, this method of imaging enables a unique methodology for nondestructive evaluation (NDE). Here, a pulsed laser generates an acoustic (ultrasonic) wave by heating the material and causing thermoelastic expansion. The resulting ultrasonic wave propagates throughout the material and is receied by an array of interferometers created synthetically. Assuming a spherically propagating wave field, the processor creates an image of the field by estimating the p...

An application of laser‐based ultrasonic nondestructive evaluation using a fiber‐optics‐based Fabry–Perot interferometer

Fiber optics lend increased flexibility to laser‐based ultrasonic nondestructive evaluation (NDE). In this work, fiber‐optic cables are used to transmit light from a laser to the detection site, and then from the detection site to a Fabry–Perot interferometer. The use of fibers allows both the detection laser and interferometer to be placed at a considerable distance from the object under test. A direct line‐of‐sight of the object from the main equipment is not required, since the fibers may be fed through walls and around obstacles. In addition, by containing the laser light in the fibers, the chance of accidental exposure to powerful laser beams that may otherwise be transmitted through air is decreased. Laser‐based ultrasonics is generally less sensitive to traditional contact ultrasonics, and in addition, some light is lost in the coupling of laser light energy into optical fibers, further decreasing the sensitivity; thus the need for signal processing of the received signals is of great importance. In this work, the waveforms obtained using the Fabry–Perot interferometer and the corresponding signal processing performed on the data to enhance the resulting image for NDE are discussed.

Acoustic scattering from laser damage spots

High energy laser systems sustain damage to critical optical components over their lifetime. This damage is manifested as localized pitting and cracking in the glass components. The damage pits have complicated morphology consisting of a hemispherical pit lined with crushed material, and surrounded by an outer zone with cracks. Monitoring of damage is necessary in some applications to prevent catastrophic failure of critical components. An acoustic system for monitoring optical damage spots was investigated to determine its applicability for the National Ignition Facility (NIF) at LLNL. Experiments and simulations of acoustic scattering from damage spots were performed to assess the ability to detect and size optical damage before failure becomes imminent. Pulses with a center frequency of 5 MHz were scattered from damage spots with nominal sizes of 0.5 mm to 7 mm. The amplitude of the measured return showed evidence of resonant scattering from the complicated morphology of the pits. This was confirmed wi...

Acoustic Characterization of Prosthetic Heart Valves

Prosthetic heart valves are a blessing for people with defective heart valves. One type of mechanical heart valve that was manufactured between 1979 and 1985 has been implanted in approximately 86,000 people. Between 500 and 600 of these valves are known to have failed. The failure occurs when a thin wire strut breaks free, see Figure 1. The strut has two legs that are attached to the main body of the heart valve. Typically one of the legs breaks first, leaving the other leg intact and the heart valve still functioning. This condition is called a single leg separation. A technique that analyzes the sound generated by the heart valve was developed to detect this single leg separation. Acoustic data was acquired from implanted heart valves prior to their being explanted. These signals were processed and distinguishing characteristics have been identified that correlated the condition of the heart valve strut with its acoustic signature. An automated classification algorithm was developed and trained to predict the heart valve’s condition.

Matched‐field imaging of laser ultrasound using a novel correlation cancelling approach

Matched‐field imaging (MFI), which has evolved from various underwater acoustic applications, provides a solution to the inverse problem through forward modeling. In terms of investigating parts for flaws during nondestructive evaluation (NDE), the matched‐field approach offers a reasonable technique for imaging provided the necessary preprocessing and parameter estimation can be accomplished. In this paper the results of controlled NDE experiments are described which are aimed at detecting a well‐defined flaw in an aluminum part. By heating with a pulsed laser an ultrasonic wave is generated and received by a synthetic array of interferometers. It is demonstrated how these displacement measurements coupled to MFI can be used to detect the flaw. Preprocessing is accomplished by a correlation cancelling scheme designed to separate correlated (reference) from uncorrelated (flaw) measurements. The reference signals are generated by using a known flawless part of the same dimensions and material. The cancelle...