User Manual

Page 53
NonDestructive Testing (NDT)
Advanced composite materials are very desirable as structural members. Light weight, high strength,
corrosion resistance, and non-magnetic are among the advantages for these materials. The need for very
routine nondestructive testing of such structures to prevent catastrophic failure due to delamination, is
one of technology's greatest concerns. Flexible sheets of piezoelectric polymer transducer arrays,
acoustically well matched to the composites, are desirable for use for non-destructive testing. One
example is as an NDT array for testing rocket motor housings prior to launch. These arrays can be
applied to the surfaces of composite fuel housings, and each element sequentially activated to provide a
pulse-echo response. An array element size of about 0.5 to 1 square inch is sufficient for this
application, as well as most large area NDT. Center frequencies of 3-10 MHz and -6 dB fractional
bandwidths exceeding 100% are typical with such transducer arrays.
Systems and Instrumentation, Ltd. personnel use piezo film for NDT of aerospace engine parts. NDT
transducers capable of detecting flaws down to 1/64th inch are now required. Further, the frequency
response range of these new materials are broader than the bandwidth of conventional transducers.
S&I, Ltd. find that a single broadband transducer covers the bandwidth of interest. Their transducers
are also used in near-surface NDT applications, where high resolution and short pulse duration are
required. Defects of 0.8 mm in size, lying within 1 mm of the surface, have been detected with the S&I
probes.
Critical points or inaccessible test areas within a composite structure, like support strut mounts, where
delamination or other damage is especially likely, can have custom fabricated NDT arrays permanently
affixed for in-service testing and monitoring. It is possible to achieve uniformity of ±1 dB between
the elements of a multi-element transducer array. Special shaped transducers, providing special focal
characteristics, have also been built with these polymer transducers.
Acoustic Emission
Acoustic emission of materials including fiber-reinforced composites, aluminum, steel and glass can be
performed with contact microphones of piezo film, or, as with NDT, by large area arrays. These arrays
can continuously monitor structures for 0.1 - 1.0 MHz acoustic emission, the precursor to structural
failure. Piezo film, being broad band, responds well at these frequencies. This capability is especially
necessary for critical application like tank rail cars carrying toxic products, underground fuel storage
tanks, nuclear plants, etc.
Fluid Level Sensor
There are a variety of fluid level sensing transducer technologies available to the designer. A float arm,
attached to a sliding potentiometric device is still widely used in automobiles. Ultrasonic pulse-echo
devices that measure the distance from a fixed transducer to the fluid surface from above through air, or
from below through the fluid, are popular. Newer capacitance types, where the fluid becomes the
dielectric, are also used. Each of these technologies represent tradeoffs in system cost, performance and
reliability. A new ultrasound level sensor, in development by MSI, holds promise as a digital, solid state
ultrasonic level sensor.
The novel construction is a level sensor with ultrasonic through-transmission with multiple transmitters
and a single, common receiver. The sensor is fabricated by attaching an unmetallized strip of piezo film
to a printed circuit board containing electrode patterns, conductors, and interconnections to circuitry on
the opposite side of the board. The electrode patterns are capacitively coupled to the piezo film layer,
becoming the multiple transmitter elements. A second conductor bar, parallel to the patterned elements
becomes the common receiver.