Adjustable acoustic mirror

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.

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.

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.