Abstract:
An ultrasonic inspection element and method are provided for improved ultrasonic inspection of curved entry surface parts. The transducer element may be spherically focused, or have a flat surface. The transducer/mirror element combination is used to inspect through a concave or convex surface. A mirror element shapes the sound beam relative to the shape of the curved surface of the part being inspected. Curvature of the mirror is adjusted with a screw, rod, voltage modulator, or other suitable adjustment mechanism. An alternative mechanism includes multiple “quick disconnect” interchangeable curved mirror elements.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to ultrasonic inspection and particularly to an adjustable acoustic mirror for improving ultrasonic inspection through curved surfaces. 
     When an ultrasonic inspection is performed, a transducer is calibrated on a flat-top block made from the same material as that being inspected, and containing flat bottomed holes of known diameter and known depth from the surface. A set of inspection parameters, such as energy level, operating frequency and water-path, are set and calibrated to a flat-top block calibration standard. The inspection parameters are used to inspect production hardware. In many cases, the same block inspection parameters are used to inspect through curved entry surfaces. Conventional procedure provides that certain curved surface parts with entry surface curvature larger than about 38 cm radius are inspected just like flat-top parts. For radii less than 38 cm, typically the operator will increase the gain (energy level). This compensates for losses due to the curved entry surface. Increasing the gain, however, also increases both the system noise (electronic noise) and the material noise, and for a large number of aircraft engine materials the parts become “uninspectable” (the part fails inspection because of “high noise”, i.e. “noise rejects”). The problem is escalated when trying to focus a sound beam below the curved entry surface (subsurface focusing), and, is more pronounced when going through a concave surface than through a convex surface. 
     Passing a sound beam through a curved surface decreases the effective sensitivity. A concave surface will cause the beam to focus much shorter than the operator expects. A convex surface will defocus the beam, yielding a less sensitive sound beam than the operator expects and that may not ever focus. The severity of each case is dependent on the radius of curvature of each, the smaller (or tighter) the radius the greater the effect on the sound beam. Also, for surfaces with curvature in just one direction, such as a bore or hole for example, the sound beam will not focus since the surface is not symmetric about the center of the transducer beam. The effect of a curved surface on inspection sensitivity is very complex. It is therefore difficult to compensate for the effect of curved surfaces without some form of correction. Such a correction would keep the same sensitivities and beam properties as those of the flat entry surface. Currently, curved parts receive additional inspection gain to compensate for the energy loss at the material boundary. The inspection gain is determined for each radius and inspection depth combination. 
     Improving ultrasonic inspection capabilities through curved surfaces has been an insurmountable obstacle for many years. Curved surfaces redirect the sound beam, often in an undesirable direction, resulting in loss of energy and resolution. The severity of the incorrect focusing and energy loss is dependent on the magnitude of the surface curvature. The more curvature there is, the more incorrect focusing, energy loss, and resolution loss there will be. Hence, even providing a fixed curvature acoustic mirror for inspection will not be wholly accurate for all curvatures. 
     An adjustable or precisely interchangeable device is desired, capable of accurately inspecting any curved surface, regardless of its curvature. 
     BRIEF SUMMARY OF THE INVENTION 
     Mathematical calculation shows that concave surfaces, not just convex, need compensation to get inspection results like a flat surface. However, concave surfaces present complications in that intensity of the sound beam increases then, after a certain depth, sharply decreases. The radius of the surface determines the change in both intensity and depth. For example, in a bore (one concave radius of curvature) the sound energy is split. The portion of the beam entering the plane of the curved surface is focused shallower than the portion entering the plane of the flat surface. Hence, an adjustable acoustic mirror or a fixed shape interchangeable mirror is provided. The mirror improves ultrasonic inspection through concave and convex curved surfaces, regardless of the curvature. 
     An ultrasonic inspection element and method are provided for ultrasonic inspection. The system for use with a transducer inspects through a curved surface. The transducer has a spherically focused lens. A mirror element is provided for shaping the sound beam. Curvature of the mirror is adjusted with an adjustment means such as a screw, rod, or voltage modulator. An alternative means to change the mirror curvature is to interchange fixed curvature mirror elements. All segments of the mirror elements can be fashioned as “quick disconnect” units. This allows efficient, repeatable assembly of a complete inspection system. 
     Accordingly, the present invention provides an effective technique for performing ultrasonic inspection, particularly of curved surface parts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an adjustable curvature structure of a mirror, by translational movement of an adjustment screw in a three-point bend configuration; 
     FIG. 2 illustrates an adjustable curvature structure of a mirror, by horizontal bi-directional movement of an adjustment rod in a compression mode; 
     FIG. 3 illustrates an adjustable curvature structure of a mirror, by modulating voltage to a reflective piezoelectric element; 
     FIG. 4 illustrates one example of the relative positioning of an adjustable curvature mirror to a curved surface part with curvature in just one direction; 
     FIGS. 5,  6  and  7  illustrate a quick-coupler, detachable structure that can support both adjustable and nonadjustable (fixed) curvature mirrors; and 
     FIGS. 8 and 9 illustrate a top and side view, respectively, of the interchangeable fixed curvature mirror elements. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention proposes an acoustic mirror for ultrasonic inspection through any curved surface. The mirror will inspect through concave radii of virtually any dimension, including rotating parts inspected with ultrasound. The mirror can also be used for subsurface-focus ultrasonic inspection in materials having a preferred ultrasonic direction. Such materials exhibit beam steering phenomena. For example, single crystalline materials and laminate composite materials are such materials. These materials steer sound energy in a direction that is dependent on the structure of the material. In a single crystalline material, the sound energy is directed, or steered, in a path along the primary crystallographic axis. In a laminate composite material, the sound energy is directed in a path along the fiber axis. The adjustable acoustic mirror, or the fixed curvature interchangeable mirror elements, can compensate for the natural steering effects in materials. 
     Referring to the drawings, an acoustic mirror  10  improves ultrasonic inspection through curved surfaces. The mirror  10  comprises a flexible mirror element  12  in a mirror frame structure  14 . The mirror element  12  may be any suitable thickness as determined by transducer frequency, acoustic impedance and flexibility of the mirror material. For example, the mirror element  12  would need to be around 0.038 cm thick piece of stainless steel for transducers that are 5-50 MHz. For transducers less than 5 MHz, the mirror element  12  would need to be thicker than 0.038 cm if stainless steel is to be used. A means  16 , such as an adjustment screw, adjusts the flexible mirror element  12  depending on the curvature being inspected. This adjustability allows the mirror  10  to focus sound energy through a variety of concave and convex surfaces. 
     Curvature of the mirror element  12  is controlled in FIG. 1 using a mechanical three-point bend configuration. The three points are the two ends  18  where the mirror element is attached, and the screw/beam contact point  20 . The means  16  may also be a voltage source, and the mirror element can be a piezoelectric material, as in FIG.  3 . In this case, curvature is induced from the voltage along  22 . That is, the mirror element  12  changes shape with voltage. Modulating the voltage can control the mirror  12  curvature. The mirror element  12  is constrained in the vertical direction, and is allowed to rotate and translate in the horizontal direction about the fixed ends,  18 , attached to mirror frame  14 . The adjustment means  16  is used to regulate mirror curvature, and the beam insures uniform curvature to the mirror element. The actual adjustment of means  16  can be manual or motorized. And, if the mirror element  12  is made from a piezoelectric material, as shown in FIG. 3, a voltage is applied. Then the mirror element becomes the means as well, capable of adjusting the mirror curvature. 
     As stated, the mirror element may be a single adjustable mirror, or multiple interchangeable units. With the adjustable mirror element  12 , as illustrated in FIGS. 1 and 2, the adjustment means  16  can be any suitable means. For example, in FIG. 2, the adjustment is a rod  32 , which can comprise one or multiple rods. The adjustment rod  32  controls mirror curvature with movement between the ends, compressing the distance. The type of mirror, concave or convex, is determined by whether the element is deflected up (convex mirror) or down (concave mirror). A convex mirror would most likely be used on a concave surface. A concave mirror would most likely be used on a convex surface. In both cases, the adjustment rod  32  compresses the distance. Each end  18  can be moved simultaneously; or differently to create the desired curvature of mirror  12 . Movement may also be in both directions simultaneously or each direction independently along an approximate horizontal axis of mirror  12 . An advantage of the configuration of FIG. 2 is that each turn buckle rod  32  may be adjusted independently. This allows for more flexible mirror shapes, such as cone, tapered holes, and some compound curvatures, where there is a different radius 90 degrees apart. The embodiment in FIG. 1 allows curvature adjustment in just one plane (hence, the name cylindrical mirror). The embodiment in FIG. 2 allows for a cone shape as well as a cylindrical shape. 
     As illustrated in FIG. 4, the mirror element  12  is oriented at around 45 degrees with respect to transducer  24  axis. The transducer may be placed generally one inch or more above the mirror surface. In FIG. 4, the transducer-mirror apparatus  12 ,  24 , is being used to inspect curved surface  26  of part  28 , a step-block. The entry surface cone  30  resembles an ellipse  30 . The curvature of mirror  12  is adjusted to shape the sound beam. 
     FIG. 4 shows the relative location and orientation of the mirror/transducer apparatus. The step block is an example of a “calibration block”. This block has Flat Bottom Holes (FBHs) drilled at specific depths below the surface. Calibration is made off of these holes, then the production part is inspected to that sensitivity. The step block is the same shape (as to radius and acoustic properties) as a production part. It is used to develop/setup/measure an inspection. 
     FIGS. 5,  6  and  7  illustrate a quick-coupler, detachable structure that can support both adjustable and nonadjustable (fixed) curvature mirrors. These drawings show how the interchangeable mirror element sits relative to the transducer  24  and a manipulator head  34 . In FIG. 5, a quick-coupler mirror collar  36  can use dowel pins to insure alignment. The collar  36  is aligned once during installation and is tightened around an UHF connector  38  associated with manipulator head  34 . 
     In FIG. 6, a quick-coupler, detachable mirror holder  40  can support the mirror element  12 , whether flat or curved, interchangeable or adjustable. It may be used in any ultrasonic inspection where a flat or curved 45-degree mirror is needed. The mirror holder  40  is preferably stainless steel or PVC. Guide slots (not shown) in the holder can be fitted to dowel pins on the collar  36 . As illustrated in FIG. 7, the beam focal properties, indicated by lines  42 , are affected by the mirror element curvature. 
     Representative fixed mirror element examples are illustrated in FIGS. 8 and 9. FIG. 8 illustrates a top view of the fixed mirror element  12 , and FIG. 9 illustrates a side view. The interchangeable mirror elements  12  can be flat or curved. Curved mirror elements will change the beam focal properties. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. For example, this process can be applied in various environments such as turbine blades with laser drilled holes. The process can also be applied to any part that has laser expulsion on its surface and around the hole. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.