Fig 4 - uploaded by Russell Marc Kurtz
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Finite element analysis of a 0.1-inch aluminum plate with a 0.03-inch diameter defect on the back, using a 0.1-inch diameter collimated, 125-mW acoustic beam indicates that the surface profile expands 7.5 μ m at the center (and is held in place by bolts at the corners).
Source publication
Modern vehicles use modern materials, including multiple metallic layers, composites, and ceramics. This has led to significant improvements in quality, reliability, and lifetime, at the cost of significantly increased complexity. It is particularly difficult to test these modern materials for buried defects such as internal corrosion, glue/weld fa...
Contexts in source publication
Context 1
... This provided boundary conditions for modeling the surface deflection induced by the acoustic stress, using the finite element method. We found that a 0.1-in. diameter collimated beam, whose total acoustic power was 125 mW, would induce a surface deflection of >7.5 µm if it encountered a 1/32-in. defect on the back of a 0.1-in. thick plate of Al (Fig. 4). The surface deflection of 7.5 µm at the center is equivalent to a surface curvature change from flat to convex, with a curvature radius of 14.5 m. Although not a large curvature-it is difficult, for example, to purchase lenses whose radius is this large-this curvature is easy to detect with shearography. The curvature induced by the ...
Context 2
... assumed the test coupons would be mounted firmly by the 1/4-20 bolts. This provided boundary conditions for modeling the surface deflection induced by the acoustic stress, using the finite element method. We found that a 0.1-in. diameter collimated beam, whose total acoustic power was 125 mW, would induce a surface deflection of >7.5 μ m if it encountered a 1/32-in. defect on the back of a 0.1-in. thick plate of Al ( Fig. 4). The surface deflection of 7.5 μ m at the center is equivalent to a surface curvature change from flat to convex, with a curvature radius of 14.5 m. Although not a large curvature—it is difficult, for example, to purchase lenses whose radius is this large—this curvature is easy to detect with shearography. The curvature induced by the modeled 125-mW beam striking a defect on the back of test coupons is shown as a function of coupon thickness in Fig. 5. We have estimated that our prototype metrology system can detect curvature whose radius is <300 m. Finite element modeling also indicated that the radius of curvature was approximately inversely related to the acoustic power; greater depths can be probed if more power is used. To test the DAS concept, and our finite element modeling, we performed an experiment on a test coupon whose thickness was 0.1 in. Instead of a PA, we used a piezoelectric transducer mounted about 1.2 in. from the in-plane location of the defect (see Fig. 6). The transducer was run at a relatively low frequency, 20 kHz, due to limitations of the equipment (an ideal frequency would be closer to 500 kHz). We tested the system at two voltage levels, 0-9 V and 0-30 V to the piezo stack, which correspond to 450 mW and 1.5 W acoustic power. Propagating the acoustic signal through the aluminum plate, we find that the acoustic intensities are equivalent to 30 mW and 100 mW in our ...
Citations
... With respect to the difficulty of using digital shearography to measure flaws away from the object surface, a directed acoustic shearography system using a ring-shaped phased array transducer was developed to measure subsurface depth flaws. In 2010, Kurtz proposed the concept of directed acoustic shearography [53]. In 2017, Liu et al. demonstrated that it was feasible to use acoustic waves as stress loading for shearography testing with a piezoelectric disc transducer [54,55], thus demonstrating the feasibility of such a system. ...
Composite materials have seen widespread use in the aerospace industry and are becoming increasingly popular in the automotive industry due to their high strength and low weight characteristics. The increasing usage of composite materials has resulted in the need for more effective techniques for nondestructive testing (NDT) of composite structures. Of these techniques, digital shearography is one the most sensitive and accurate methods for NDT. Digital shearography can directly measure strain with high sensitivity when combined with different optical setups, phase-shift techniques, and algorithms. Its simple setup and less sensitivity to environmental disturbances make it particularly well suited for practical NDT applications. This paper provides a review of the phase measurement technique and recent developments in digital shearographic NDT. The introduction of new techniques has expanded the range of digital shearography applications and made it possible to measure larger fields and detect more directional or deeper defects. At the same time, shearography for different materials is also under research, including specular surface materials, metallic materials, etc. Through the discussion of recent developments, the future development trend of digital shearography is analyzed, and the potentials and limitations are demonstrated.
... Shearography with these conventional loading methods are not suitable for crack detection around fastener holes. Kurtz [27,28] proposed the concept of directed acoustic shearography where the acoustic waves from phased array transducers were to be used for providing stress loading in a shearography testing. However, the concept was not demonstrated with phased array transducers or transducers capable of controlling the wave directions. ...
... This is an important science, from the simple measurements of coins, to more difficult measurements on vehicles, to ultraprecise metrology of semiconductor surfaces. Metrology can be used in automobile manufacturing 1 , aircraft assembly 2 , high-resolution lithography 3 , and nondestructive evaluation 4 . In general, however, there is a tradeoff among various capabilities of a metrology system, particularly among the parameters of system complexity, measurement accuracy, and measurement speed. ...
There is a constant search for a more accurate measurement, which generally leads to higher cost, greater complexity, or devices that do not lend themselves to manufacturing environments. We present a method of using statistical sampling to improve metrological accuracy without these undesirable effects. For metrology of flat surfaces and steps between flat surfaces, this method demonstrated precision improvement up to a factor of 55, and accuracy increase of at least a factor of 10. The corresponding precision and accuracy improvements on a spherical surface were both factors of 8. Since this accuracy improvement can be implemented in software, it does not affect the speed of measurement or the complexity of the hardware, and it can be used to improve the accuracy of a wide range of metrology systems.
