Note that because of the axial symmetry of the

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wave calibration. Note that because of the axial symmetry of the through-pulse calibration, only normal displacement exists at the location of the transducer under test. Step Function Force Calibration The basis for the step function force calibration is that known, well characterized displacements can be generated on a plane surface of a test block. 56 A step function force applied to a point on one surface of the test block initiates an elastic disturbance that travels through the block. The transducer under test is located either on the same surface (surface calibration) or on the opposite surface at the epicenter of the source (through-pulse calibration). Given the step function source, the free displacement of the test block at the location of the transducer can be calculated by elasticity theory in both cases. 57-59 The calculated block displacement function is the transfer function (mechanical transfer admittance, when expressed in the frequency domain) or the Green’s function for the block. The free displacement of the test block surface can also be measured using a capacitive transducer with a known absolute sensitivity. 60 It is essential to the calibration that the calculated displacement and capacitive transducer measurement agree. The calibration facility at the National Institute of Standards and Technology has used a cylindrical steel test block 0.9 m (36 in.) in diameter by 0.43 m (17 in.) long with optically polished end faces. 51 The step function force is made by breaking a glass capillary (see Fig. 16). In U t J ka ka B t ( ) = ( ) 2 1 cos ω U t B a kx t a x dx a a ( ) = × ( ) π ω 2 2 2 cos 57 Fundamentals of Acoustic Emission Testing F IGURE 15. Results of the calculation of Eq. 10 compared with experimental results from a capacitive disk transducer. Source-to-receiver distance d = 0.1 m (4 in.); transducer radius a = 10 mm (0.4 in.); surface pulse is generated by a capillary break on a steel block. Output (relative scale) 1.0 0.8 0.6 0.4 0.2 0 Frequency (MHz) Legend = theory = experiment 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 F IGURE 16. Schematic diagram of the surface pulse apparatus. Steel block Transducer under test Capillary source Lead zirconate titanate disk Loading screw Charge amplifier Storage oscilloscope Transient recorder Computer Capacitive transducer Transient recorder
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the case of surface calibration, free normal displacement of the surface is measured by a capacitive sensor at a location symmetrical to that of the transducer under test with respect to the source location. The displacement is redundantly determined by elasticity theory from a measurement of the force at which the capillary broke. Source and receiver are 0.1 m (4 in.) apart. For through-pulse calibration, the free normal displacement is determined only by the elasticity theory calculation. Both calibrations are absolute: the results are in output volts per meter of displacement of the (free) block surface.
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  • Fall '19
  • Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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