Ultrasonic uses the concept of waves, which implies that an incident wave is reflected back after hitting a reflection surface. Ultrasonic detects the presence of flaws in the material, using frequency much above that the human ear can detect, as sound with a frequency exceeding 18 kHz is used. In the inspection of microscopically homogeneous components, non-composites, ultrasound in the range of 20 kHz to 20MHz is usually used. For composite materials, this frequency is reduced substantially to gather increased attenuation in composites. The frequency of operation in ultrasonic testing for composites is normally less than 5 MHz. The reduction in the frequency reduces the ability of the process to resolve small flaws (Blitz & Simpson 1996, p.5).
Pulse-echo, back-scattering, ultrasonic spectroscopy, and through-transmission techniques involve passing of short pulses in the range of a few microseconds to the composite material and sensed after interrogating the component or structure. These methods work in a similar principle of emitting and receiving an ultrasonic sound.
The choice of frequency for this method is critical in establishing the process integrity, since frequencies at which resonance may occur in between laminate interfaces are avoided. Unidirectional plies with 8 piles per millimetre have a resonance frequency of 12 MHz. Therefore, using ultrasound, in this case, is inappropriate.
This method uses two ultrasonic transducers whereby one of them generates an acoustic pulse inserting an elastic wave into the material. The released elastic wave is propagated across the material thickness under test until it hits a reflection surface, which could be the boundary of the material or the edge of an internal flaw. The second transducer has a specialized low-noise amplifier and receives the reflected wave by sensing the acoustic response from the component under inspection (Chang, Zheng, & Ni 2006, p. 451).
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Pulse-echo technique is well established and used widely in Non-Destructive Testing. A pulse of ultra sound is transmitted perpendicularly into the specimen. There are two instances where this pulse will be reflected to a receiver probe. The first is after hitting good boundary of the matrix, while the second one is after its incidence on the flaws boundary.
After hitting a reflecting surface, the reflected signals are detected by the probe. This allows for defect size and position determination through the total time of travel and received amplitude. A sound beam that takes a short time to be echoed off a boundary implies that the reflecting surface is inside the material and not the usual matrix boundary. Each echo is examined for its amplitude.
Applications of Ultrasonic Testing
A single element transducer is the most applied technique in the inspection of small and medium size composite parts. In ultrasonic technique, inspection is carried out by scanning the transducers in a raster pattern on the specimen surface. The image of this process comprises of recorded acoustic signals from reflected wave. The strength of this acoustic signal is directly proportional to materials properties. The pattern and shape of recorded signal indicates the composite material uniformity. A place of deformity inside the material attenuates the propagation of elastic pulse. The analytical results inspection helps in detection of flaws (Schnars & Henrich 2006, p. 4).
There are several techniques applying ultrasonic technology; their applications are closely related. Ultrasonic is applicable in the determination of delamination defect in the composites. The presence, size, shape and location of delamination can be ascertained through ultrasonic testing. The method is applicable in the determination of foreign inclusions and voids presence, since they result in back-wall signal loss.
A conventional pulse-echo set up as shown above is capable of detecting velocity and attenuation related effects. This application relies on the ability to measure the level of degradation in the pulse amplitude.
Advantages of Ultrasonic Testing
- Ultrasonic can be used to detect surface and subsurface defects.
- UT is superior means of determining the depth of flaw.
- Single-sided access is sufficient to detect a flaw.
- Shape and size can be described with high precision.
- There is little specimen preparation required.
- Incorporation of electronic equipment offers instant results.
Limitations of Ultrasonic Testing
- At least one access to the surface is needed for acoustic wave transmission.
- The method requires suitably trained personnel.
- The method cannot test material with rough and irregular surface.
- Defects lying parallel to incident beam cannot be detected.
- There has to be a standard reference for accurate characterization of flaws and equipment calibration.
Radiography is the process of deploying a beam of ionising radiation to a specimen under inspection. It works on the principle that a composite specimen has different magnitude of absorption abilities, which can be reflected in an image developed after passing the beam through the specimen. Neurons and X-rays are found to be relevant for inspection.
X-ray is the most common form of radiography. The convectional type of X-ray inspection involves the use of the radiation beam alone. Another form of X-ray inspection is the enhanced X-radiography, which involves the use of specialized liquid formulation, amplifying radiographic images contrast. Since polymer matrix is characterised by low absorption cross-sections, suitable contrast can be hardly achieved with high energy X-rays. Low-energy X-rays sin the range of 10kV to 50kV must be used to affirm that there is absorption taking place (Cartz 1999, p. 45).
In the conventional X-ray method, X-rays beams of suitable energy bombards the specimen. Some beams are absorbed by the material; others penetrate through the specimen. The radiation sensitive target is a film with thin layer emulsion gelatine comprising of silver halide crystals, such as silver chloride or silver bromide. The film is qualities allow speedy development, fixation, and drying.
Once the film has been developed, it is subjected to technical examination by qualified personnel. The examination objective is to ascertain the presence of flaws in the composite material. The variations in the concentration of unabsorbed radiation due to some of them being absorbed in the specimen are reflected in the film. The evaluation of these radiographs is based on the disparity in the density generated by the specimen under test. There is a benchmark material, in which any other part of the composite must have similar shade of greyness. Fluorescent light is used to generate real-time images through fluoroscopy technique (Hellier 2003, p. 1.27).
In a suitably processed composite, the density of greyness in the developed film must be the same for a uniform cross-section. A non-uniform part indicates that there is a flaw, or inclusion in the material. For these flaws to be identified, they must be having an appreciable depth to the beam direction for it to be detected. It is compulsory to carry out the inspection, while the test specimen is oriented in different directions. It is not possible to detect cracks, which are not lying parallel to radiation beam direction and delaminations, which are perpendicular to the radiation beam. Another weakness is inherent in its difficulty to detect variations in fibre volume fractions, in CFRP composites.
Application of Radiography
The conventional radiography can be used in the identification of volumetric flaws like voids in composites only if they portray 2% absorption or more. Its weaknesses are allowing the detection of cracks and delaminations as long as they are exposed. Enhanced radiography is capable of determining fibre alignment and fibre volume fraction on composites through incorporation of absorptive additives, like glass and boron (Hellier 2003, p. 1.24). Compton Backscatter is applied in cases, which demand one-sided examination.
Advantages of Radiography
- Radiography is capable of detecting any flaw in the composite.
- Visual representation of the flaws makes their interpretation easy.
- The method is not selective in material under test.
Limitations of Radiography
- The highly hazardous radiations call for close control.
- The operator must access both sides of the test object.
- The shape of the component may make it impossible to generate useful radiographs.
- The actual location of the defect cannot be ascertained.
- Radiography requires suitably skilled personnel.
Shearography is one of the interferometric techniques, providing a whole-field visualization of isoclines related to surface strains, based on uneven strain concentrations. Through shearography, defects from the irregularities in interferometric fringes can be established.
A flaw, located near the surface, reduces the local strength of a composite and depends on subjected loading if flaw exists in some sections. An optical interference technique is suitable to identify the disparities. In shearography, divergent laser beam illuminates the surface of the object and the scattered light is directed to a shearing lens. The specialized lens shears the specimen’s image in its plane, enabling interference between images. The pattern, formed in the process, is a representation of the manner how local surface strain is distributed. The new stress distribution is either on or outside the plane. Subtraction or superimposition of images, produced after the imposition of stress, produces a fringed pattern. Each fringe represents a line of strain. The areas of the composite with elevated strain display a high concentration of these lines.
Phase stepping optics allows the recognition of sub-fringe defects, resulting in better details. The latest developments utilize twin angle laser beams, capable of separating out of the plane and in-plane components.
Four techniques have been identified to help in comparing the patterns from the specimen in distorted and undistorted states. The interference patterns may be classified using Time Integrated Technique, Sandwich Technique, Real-Time Technique, or Digital Radiography.
The invention of video camera and considerably fast and cheap microprocessor technology has made the digital shearography a common method of Non-Destructive Testing. It provides Real Time images, eliminating time-consuming and expensive film development processes. The reflected light emanating from the surface moves through a shearing interferometer before entering CCD camera. Image improvement software may be applied to enhance the quality of the picture. The ability to integrate digital applications in the shearography technique makes it the best option of all Non-Destructive Testing methods (Shull 2002, p. 34).
Advantages of Shearography
- Carrying out non-contact inspection using shearography is possible.
- Shearography can be used in field inspection due to rigid body motion.
- The equipment for this method can be easily assembled.
Limitations of Shearography
- Since the test sample must be loaded during the process, the core essence of Non-Destructive Testing may be lost due to the possible damage to the component.
- Shearography is the restriction of surface requirements, in which the method may be applied. It is limited to such specimens whose surface roughness is in the range of one light wavelength or more. Test specimens, having smooth surfaces will generate random interferences, resulting in masking the effects on fringes caused by distortion.
- The interpretation of the results is complicated and requires that the experienced analyst. Light influence and flaws location affects the resultant image.
Application of Shearography
Shearography is widely applied in the inspection of composites, e.g. the examination of aerospace structures, where the whole areas of wings and fuselages are scanned from tripod position. Either of out-of-plane or in-plane stresses may be employed to deform the specimen. In-plane stress is helpful in the inspection of repairs’ quality. Surface-breaking cracks are usually repaired through boron patch bonding over the flaw in aircraft panels. The patch absorbs the in-plane strain so that the resultant map, showing strain distribution, indicates that the concentration of strain is not on the crack ends, but they have been distributed all over the patch and surrounding area (Toh, et al. 1990, p. 268).
Shearography may be employed in the inspection of GRP pressure vessels whereby the application of pressure is through internal pressurization, where shearography may detect inclusions. Sub-surface flaws and surface breaking have been detected through the application of out of plane stressing. These defects weaken the integrity of a normal surface. The distortion generated through out-of-plane pressure reduction has hemispherical shape leading to two bulls-eye interference outlines as illustrated on the diagram below (Summerscales 1987, p. 109).
In summary, this article has articulated three commonly used techniques of Non-Destructive Testing in the inspection of composite materials. Apart from outlining the peculiarities of each technique, constraints of each method were highlighted, implying that the selection of a technique should be based on the weaknesses and the nature of inspection. The three methods reviewed in the light of their application in composites inspection are Ultrasonic, Radiography biased to X-ray and Shearography.
The suitability of each method depends on prevailing conditions and functional requirements desired. Ultrasonic testing is the most versatile and may be used in various applications. The precision and ease for use makes Ultrasonic Testing a dependable and widely used technique in aircraft inspection. Radiography generates better resolution images, since the wavelength is comparatively shorter in X-ray than ultrasonic waves. For more reliable results, an inspector may find it beneficial to use more than one method in examining one component.