Noncontact Surface-Hardness Measurement Using Laser-Based Ultrasound
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MRS BULLETIN/OCTOBER 1996
is a key prerequisite for the nondestructive determination of surface hardness and case depth. Case hardening, a metallurgical process to increase the fatigue and wear resistance of steel components, is an integral part of many manufacturing processes. The case refers to the hardened layer that is formed in this process, and the depth of the case is critical in determining component performance. To successfully measure the SAW veloc-
Q-Switched Nd:YAG Generation Laser
ity dispersion, high accuracy is required to resolve variations of less than 2% in the SAW velocity. Currently to inspect the quality of a batch of material that has gone through the hardening process, one or more of the parts are sectioned with an abrasive wheel, polished flat, and the hardness profile measured using a microhardness indenter. This method of inspection can take several hours, often with the production line stopped, until the results are known. It does have the advantage of measuring directly the desired material properties but has the obvious disadvantages of costly manufacturing downtime, unnecessary scrappage of a production item, and the assumption that the material properties of other samples in the batch are similar to those inspected. As a promising alternative approach, nondestructive inspection (NDI) permits a 100% inspection of the batch, which is not economically viable using destructive-inspection methods. Generally NDI methods are based on inferring case depth indirectly through measuring electromagnetic or mechanical properties of the part using eddy current or ultrasonic probes.6"9 Eddy-current systems are commonly used for case-depth measurements and are known to be reliable for many applications. However they lack sensitivity if the case depth is deep (e.g., greater than 5 mm in steel parts), and custom probes are required for inspection of components with different geometries.
Argon-Ion Detection Laser
Figure 1. Experimental configuration for laser-based ultrasonic measurement of surface hardness in steel-axle shafts. The generation laser is linearly scanned while the detection laser remains stationary.
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Noncontact Surface-Hardness Measurement Using Laser-Based Ultrasound
Laser-Based Ultrasound System Laser-based ultrasound relies on the transient thermal expansion at a material surface that results from the sudden heating caused by illumination with a high-power, short-duration laser pulse.10 Relief of the surface-induced thermal stresses results in the propagation of elastic waves that emanate from the heated region. Unlike conventional piezoelectric transducers, the laser source is capable of generating a number of elastic-wave modes simultaneously. In the bulk of a material, both longitudinal and shear waves are generated with SAWs propagating across the surface of the material. For platelike materials, Lamb waves may also be generated. Although many elastic modes are generated simultaneously with the laser, the studies described here used only the surface waves. Figure 1 illustr
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