Abstract:
A method of ultrasound inspection is provided. The method includes providing a composite first part, introducing ultrasound to the part, receiving reflections of the ultrasound introduced in the first part, and predicting a residual strength of the first part using an amplitude of the received reflections.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &amp; DEVELOPMENT 
   The government has rights in this invention pursuant to Contract No. N00019-98-C-0007 awarded by the Department of the Navy. 

   BACKGROUND OF INVENTION 
   This invention relates generally to non-destructive testing and, more particularly, to ultrasound inspection of composite parts. 
   Ultrasonic inspection techniques are used in many applications where non-destructive evaluation of a workpiece is required. One application of such ultrasonic inspection is in the inspection of composite fiber reinforced aircraft propeller blades. Such blades are typically formed from a plurality of layers of composite fibers (graphite, boron or S-glass, for example) laid over each other and adhesively bonded. Any separation of the fiber layers due to an incomplete bond or void in the blade may detrimentally affect blade strength. Ultrasonic inspection techniques can be used to identify and locate such flaws in a composite fiber reinforced blade. Additionally, ultrasound inspection techniques can be used to inspect composite aircraft engine parts, such as, but not limited to, composite ducts. 
   One technique of ultrasound inspection is sometimes referred to as bottom echo detection. By measuring the part thickness and knowing the attenuation characteristics of the part being inspected, one can compute an expected arrival time for the echo. If the echo is received earlier than expected then the ultrasound sound wave encountered an interface causing a reflection of the sound wave. This early reflection is typically caused by a delamination of the lamination layers indicating a flaw in the object. An echo received at the expected time is indicative of an absence of a delamination. This technique, however, relays on the absence or presence of the echoed sound wave and is therefore commonly called a “go/no go” type of inspection. At least one disadvantage of using a “go/no go” type of inspection is that small micro-cracks may be undetected. 
   SUMMARY OF INVENTION 
   In one aspect, a method of ultrasound inspection is provided. The method comprises providing a composite first part, introducing ultrasound to the part, receiving reflections of the ultrasound introduced in the first part, and predicting a residual strength of the part using an amplitude of the received reflections. 
   In another aspect, an ultrasound inspection system comprises a pulse echo transducer, and a processor operationally coupled to the transducer. The processor is configured to predict a residual strength of a first part using an amplitude of a received ultrasound reflection. 
   In a further aspect, an ultrasound inspection device comprises means for non-destructively testing a first part, and means for predicting a residual strength of the first part using a result from a non-destructive test of the first part with a plurality of destructive and non-destructive tests on second parts substantially similar to the first part. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic view of an ultrasound system. 
       FIG. 2  is a chart illustrating a correlation of the amplitude of the backside signal with a shear strength loss for a plurality of composite parts. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a schematic view of an ultrasound system  10 . System  10  includes a pulse echo transducer  12  coupled to a control unit  14  containing a processor  16 . Processor  16  is configured to perform the below described processes. As used herein, the term processor is not limited to just those integrated circuits referred to in the art as processors, but broadly refers to computers, processors, microcontrollers, microcomputers, programmable logic controllers, application specific integrated circuits, and other programmable circuits. 
   In use, transducer  12  is placed in acoustical conduct with a part  18  to be tested and ultrasound is introduced to part  18 . In one embodiment, a known acoustic gel is placed between part  18  and transducer  12  to facilitate sound transfer between part  18  and transducer  12 . In another embodiment, part  18  and transducer  12  are placed proximate each other submerged in a liquid that facilitates ultrasound wave travel through the liquid. In an exemplary embodiment using the liquid in an automated setting, system  10  includes a rotatable table (not shown) including at least one collet or mandrel (not shown). Part  18  is automatically chucked in the collet or onto the mandrel and the table is rotated such that part  18  is in close proximity to transducer  12 . Transducer  12  emits ultrasonic energy which is at least partially reflected when an interface is encountered within part  18  (such as a micro-crack) or at an interface on a back side of part  18  between part  18  and the liquid. Traditionally, it is the presence or absence of the reflection arising out of reaching the back side (commonly called a backside signal) that is used to determine if part  18  is structurally sound. Given the width of part  18 , a window of time for the expected return of the emitted ultrasound energy is determined and if the backside signal is received within that window then part  18  is deemed structurally sound. The table is rotated and part  18  is ejected into a good part bin or a bad part bin depending on the presence or absence of the backside signal. 
   However, determining the presence or absence of the backside signal only reveals the presence or absence of a delamination within part  18 . Accordingly, a part is kept in service until a delamination is detected. However, as explained in greater detail below, by examining an amplitude of the backside signal, a prediction of a residual shear strength can be made and the part removed from service when a likelihood of a material degradation is high regardless of whether a delamination is present. 
     FIG. 2  is a chart  30  illustrating a correlation of an amplitude  32  of the backside signal with a shear strength loss  34  for a plurality of composite parts such as composite aircraft engine parts. The parts are inspected using ultrasound system  10  (shown in  FIG. 1 ) and using another type of inspection method. In one embodiment, the parts are inspected using a destructive test such as a core sample test wherein a core sample is removed from the part and the core is examined for micro-cracks. The results of the ultrasound test and the core sample test are correlated to provide an ability to predict a residual shear strength in a part which is only tested using ultrasound system  10 . More specifically, the parts are tested ultrasonically and an amplitude of a received backside signal is correlated with results from the core-sample test. In one embodiment, the correlation is a linear least squares fit. In another embodiment, the correlation is other than a linear least squares fit. 
   After the correlation between the composite part&#39;s reflected backside signal and at least one non-ultrasound test of the residual strength of the parts, a part such as part  18  is tested and an amplitude of a backside signal of part  18  is obtained by using system  10 . The obtained amplitude is used with the correlation to predict the residual shear strength of part  18 . For example, when part  18  has an amplitude of about fifteen then, as seen in  FIG. 2 , the corresponding percent of shear strength loss is about sixty percent. If desired, a cut off percent is determined and when a part is predicted to have a shear strength loss greater than the cut off percent, then the part is tested further, such as performing a core sample on that part. In one embodiment, system  10  includes a memory (not shown) in which the correlation is stored, and processor  16  compares the amplitude to the stored correlation to predict a residual strength of part  18 . In an exemplary embodiment, the residual strength is a residual shear strength. 
   While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.