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A NASA effort is underway to develop a nondestructive inspection method to evaluate the bonding condition for heat shield blocks that are bonded to a composite substrate. The initial effort is reported in this paper. The goal is to demonstrate a proof of concept for a reliable detection method using a single-sided scan capability applied from the outside of the heat shield blocks. Two representative samples with known defects are made available for the initial proof of concept; the results of one sample are reported here. The two specimens had been scanned using various nondestructive testing methods including immersion ultrasound, handheld ultrasound, air-coupled ultrasound, microwave, Terahertz, computed radiography, and shearography. These efforts were performed by industry experts and academia. All were limited in their detectability of the known flawed regions in the specimens and suffered high levels of uncertainty in the good regions. Most nondestructive evaluation methods are sensitive to detecting changes in the material property. This means that an air pocket is easy to detect in an unbounded interface but when the air pocket is absent and the two surfaces are in intimate contact (kissing bond) most methods fails to detect the difference. Electromagnetic methods (radiography, thermography, microwave, Terahertz) will not detect a change in material property. Shearography is not successful in attempting to detect changes in stiffness through the highly damping thick heat shield layer. As a mechanical wave, ultrasound is the primary method that may be able to detect a kissing bond. However, it was quickly recognized that ultrasound has to overcome a number of difficult challenges related to the nonhomogeneous and highly damping nature of the thermal shield material. Inhomogeneity causes pronounced changes in signal strength that make the distinction between good and bad areas very difficult. At the same time, scatter and attenuation become detrimental to high frequency ultrasonic signals, which are necessary to resolve echoes returning from the bond line. Conversely, low frequency sound waves provide poor spatial and temporal resolution due to the broader wavelengths associated with the signal.

The change in properties of a propagating ultrasonic wave has been a mainstay characterization tool of the nondestructive evaluation (NDE) industry for identifying subsurface defects (e.g. damage). A variant of this concept could be applicable to 3D additive manufacturing where the existence of defects (e.g. pores) within a sub-layer could mark a product as non-qualifying. We have been exploring the utility of pulsed laser ultrasonic excitation coupled with CW laser heterodyne detection as an all optical scheme for characterizing sub surface layer properties. The all-optical approach permits a straight forward integration into a laser additive processing tool. To test the concept, we have developed an experimental system that generates pulsed ultrasonic waves (the probe) with high bandwidth ( >30-200 microns) beams. Current tests include characterizing properties of spot weld joints between two thin stainless steel plates. The long term objective is to transition the technique into a laser additive manufacturing tool.

A formed laser source, using a four-element lenticular array, is used in the ablative regime to generate select, narrowband, acoustic plate waves. The arrangement of the array produces acoustical signals that have frequencies compatible with the response of the broadband capacitive air-coupled transducer used in this study. A simplified concept is presented to explain the effect of a line array source on the frequency content of acoustic waves. The analytical model for a point pulse surface displacement is derived from the point load solution to Lamb's problem. The point pulse displacement elements of a line array source are summed mathematically, taking into account all applicable propagation modes and dispersion of plate waves. The model considers only the out-of-plane displacement of the antisymmetric plate modes to represent the detection capability of the broadband receiver. The distribution function of the laser beam energy profile is modified to depict the actual energy distribution that illuminates the surface of the plate. Filtering functions are made compatible with the sensitivity of the broadband receiver so as to retain only the detected frequencies in the model. The theoretical model showed good agreement with experimental results.

Non-destructive inspection of railroad wheels is performed in maintenance shops, where wheels are removed and inspected individually. No technique is yet available to the railroad industry to perform wayside inspections of wheels on a moving train. The characteristics of laser and air-coupled ultrasound are discussed to justify the use of a laser-air hybrid ultrasonic technique. Laser generation of ultrasound is combined with air-coupled detection to provide a flexible non-contact and remote technique that would enable the railroad industry to perform wayside inspections of moving railroad wheels. The present paper describes Proof of Concept set-up and results of the experiments performed at the Transportation Technology Center facilities, in Pueblo, Colorado. Sensors are used to detect the position of the wheel, drive a two-position mirror to direct the laser beam to one of two test stations and trigger the laser when the wheel is in position for detection. Wheels are inspected for subsurface shattered rim cracks, thermal fatigue cracks along the tread and the flange areas of the wheel. Analysis is based on the time domain radio frequency (RF) signal and time-frequency wavelet transforms. The experimental results are promising, indicating that dynamic wayside inspections are possible with the laser-air hybrid ultrasonic technique.

Laser-Air Hybrid Ultrasonic Technique (LAHUT) combines laser generation with air-coupled detection of ultrasound. The technique is non-contact and has the characteristic of operating from remote distances. Acoustic wave laser-generation apparatus can be metres away from the interrogated surface while air-coupled detection standoff can be on the order of several centimetres. The technique has the unique capability of interrogating structural materials in their true industrial environment. Dynamic tests are performed on parts with complex geometry, limited accessibility and curved surfaces. Also, dark and rough finish surfaces, which significantly reduce the efficiency of optical detection techniques, can be interrogated successfully. These characteristics make the LAHUT ideal for many industrial applications including the railroad industry. It was developed for railroad inspections targeting the most critical cracks in rails and wheels. State-of-the-art inspection techniques available to the railroad industry often miss Vertical Split Head (VSH) and Transverse Detail Defect (TDD), which lie in unfavourable positions and orientations in the rail head. No method exists to perform dynamic inspections of the rail base or any part of the railroad wheel. Laboratory experiments were performed for the detection of TDD and Proof Of Concept (POC)field tests were performed for VSH, rail base cracks, thermal fatigue cracks along the wheel flange and tread and subsurface Shattered Rim Cracks (SRC) along the wheel tread. The results were successful and highly repeatable. The technique lends itself for digital collection and automated processing of data making the LAHUT a very strong candidate,for next-generation railroad inspection technique.

A brief overview is provided on three common methods of nondestructive evaluation (NDE), Ultrasonic Testing (UT), Radiographic Testing (RT) and Eddy Current (EC). These methods vary in the physics applied to the testing or evaluation process while specific techniques vary in the way each method may be applied. Understanding the physics behind each method and constraints of each technique helps us understand the capabilities and limitations of the inspection. Recognizing the capabilities and limitation of these NDE Techniques helps us properly design the inspection, collect and process data and draw appropriate conclusions.

The Laser-Air Hybrid Ultrasonic Technique (LAHUT) combines laser generation with air-coupled detection of ultrasound. The technique is non-contact and has the characteristic of operating from remote distances. Acoustic wave laser-generation apparatus can be metres away from the interrogated surface, while air-coupled detection stand-off can be on the order of several centimetres. The technique has the unique capability of interrogating structural materials in their true industrial environment. Dynamic tests are performed on parts with complex geometry, limited accessibility and curved surfaces. Also, dark and rough finish surfaces, which significantly reduce the efficiency of optical detection techniques, can be interrogated successfully. These characteristics make the LAHUT ideal for many industrial applications including the rail industry. It was developed for railroad inspections targeting the most critical cracks in rails and wheels. State-of-the-art inspection techniques available to the rail industry often miss Vertical Split Head (VSH) and Transverse Detail Defect (TDD), which lie in unfavourable positions and orientations in the rail head. No method exists to perform dynamic inspections of the rail base or any part of the railroad wheel. Laboratory experiments were performed for the detection of TDD and Proof Of Concept (POC) field tests were performed for VSH, rail base cracks, thermal fatigue cracks along the wheel flange and tread and subsurface Shattered Rim Cracks (SRC) along the wheel tread. The results were successful and highly repeatable. The technique lennds itself for digital collection anal automated processing of data, making the LAHUT a very strong candidate for next-generation rail inspection technique.

Long- and short-range acoustic-signal interaction with surface-breaking cracks and geometric boundaries of the rail-wheel tread area are presented in this article. Ultrasonic signals are generated using a laser-line source of varying length and distance from a crack and are detected with a 1-MHz contact transducer to map the sound field behind the crack in the near, intermediate, and far fields of the insonified region. The factors that affect the behavior of a laser-generated surface acoustic wave propagating along the tread area of a rail wheel are discussed. A signal normalization method is proposed to help in sensing the presence of a crack from the transmitted signal unaffected by the boundary effect of the complicated wheel geometry, diffraction of the acoustic wave around the crack tip, and source-to-crack length ratio and separation distance.

Laser generation and air-coupled detection were combined as a hybrid ultrasonic technique for the inspection of surface flaws in rails. Narrowband acoustic signals were generated using a formed laser source by focusing the laser light to a point and to a line on the surface of the rail. The pulse energy, and therefore the intensity of the laser source, varied such that the generated signal transitioned from the weak thermoelastic to the strong ablative regime. The detection of flaws using a laser-generated surface acoustic wave, in the presence of surface flaws, was compared between both point and line laser sources operating under different pulse energy levels. The line source was found to be more sensitive to the presence of surface flaws than a point source. The sensitivity of the laser-generated acoustic signal appeared to be independent of the severity of the flaw and, within the ablative regime, independent of the laser-pulse energy. Theoretical analysis is provided to explain the underlying cause that influences the interaction of a formed laser-generated surface acoustic wave to surface flaws and how this sensitivity may vary between the thermoelastic and ablative regimes.

A narrow band laser-generated acoustic signal was created using a 4-element lenticular array. This arrangement of the array produces an acoustical signal with frequency content that is compatible with the response of a noncontact and remote broadband receiver, such as a capacitive air-coupled transducer. To support the experimental observations, a simplified concept is presented to explain the effect of a line array source on the frequency content of a surface acoustic wave. The analytical model solution for the wave front shape is derived from the point load solution of Lamb's problem that represents the displacement of a surface acoustic wave generated by an ablative line array. The distribution function, which was used for the model to represent the laser light energy, was tailored to depict the actual energy distribution that illuminates the lenticular array. Filtering functions are applied to the resultant surface displacement function to retain frequencies similar to those detected by the broadband 50 kHz-2.25 MHz receiver. The theoretical model showed good agreement with experimental results.