Many of the nondestructive testing (NDT) methods available for testing of thermoplastic composite base materials can be used for welds in thermoplastic composites as well. In some cases, modifications are necessary.[1] Some national and international standards exist[2][3][4][5] for the inspection of the base materials. While these standards were not necessarily developed specifically for the welds in said materials, the physical principles are often still applicable.
The methods include:
Visual Inspection (VT) is typically the first option for any attempt at NDT, being the least expensive, as it requires the least specialized training and usually few if any special tools. Defects on the surfaces of thermoplastic composite welds can be detected visually if they are of sufficient size. Weld defects such as misalignment, porosity, lack of fusion and degradation of the matrix and/or fibers may be visually apparent. Subsurface defects may not be visible, unless the composite matrix was nearly transparent and the embedded fibers did not obscure them.
Radiographic testing (RT) can be performed in several ways. Typically low energies are required[6] for testing of composites in order to see any detail, which restricts the radiation sources to be used to x-ray types rather than gamma sources like Ir-192 or Cobalt-60, which tend to have higher energy levels. Data may be recorded either on film or digitally, using specially developed screens for detecting and saving an image than can be manipulated later with the proper software and hardware. Because radiographic testing relies on differences in material density to provide an image, resolution of fibers like carbon from the thermoplastic matrix is not always very high, since the density of the plastic does not differ much from that of the carbon or glass filaments. For digital imaging, the lack of contrast may be partially addressed after the radiographic images are taken, using digital imaging software. Radiography can detect porosity, voids and possibly differences in fiber density or orientation in the composite matrix due to the welding process. Lack of fusion may not be visible by RT unless it is perpendicular to the direction of the source of radiation.
Computed Tomography (CT), a subset of radiographic testing, is proving useful for the inspection of thermoplastic composite welds. CT involves the computerized building of a 3-D image using X-rays taken from numerous, incremental angles. It is particularly useful for the determination of fiber orientation in welds of glass reinforced composites. [7]
Ultrasonic A-scan display showing in indication of reflected ultrasound. Ultrasonic testing (UT) can offer detailed NDT information for welded thermoplastic composites. Tests can be done with shear wave or transverse waves, though the composite materials often attenuate the signals significantly and care must be taken to account for this. Contact methods using either manual or automated transducers coupled to the part being inspected or non-contact methods using water immersion or a bubbler (i.e a continuous stream of water through which the ultrasound passes) can be effective if designed and calibrated properly. Amplitude of reflection data may be used to generate B-scan or C-scan images, which can show the materials being welded at various, discrete depths or cross sections, a capability not available with traditional radiographic methods. Ultrasound can detect delaminations, lack of fusion, porosity, voids, inclusions and other defects mostly regardless of their orientation.
Deterring factors include that the method is time consuming and the data are open to some interpretation, requiring skilled technician to perform and interpret the test.
Thermography[8] involves testing the part for discontinuities that can be seen by an infrared camera when the part is heated or cooled. It offers a significant improvement on some of the more traditional NDT methods in that it can be used on large areas of, for example, airplane parts or storage tanks.
Eddy Current testing (ET) has been found to be useful for characterizing the nature of fibers and their orientation in certain composite materials, particularly those with conductive reinforcing fibers. It would not be useful for composites reinforced with glass or aramid fibers, for example, as no currents can be induced in these insulating materials. Much higher magnetic field frequencies are used to generate the eddy currents used for testing plastic composites than are typically used for metals. [9] Though delaminations in the material were either undetectable or nearly so, more recent research has found that by induction heating the part in addition to exciting an alternating magnetic field, some delaminations could also be detected in CFRP.
Shearography output image of CFRP/honeycomb part with artificial defects[10]
Laser Shearography[11] involves accurately measuring perturbations in the surfaces of a (usually thin) part under load or strain with the aid of lasers scanning across the surface being evaluated. Voids, pores, delaminations and other defects in composite welds can be detected by this method
Acoustic Emission[12] testing provides qualitative information on the presence and potential growth of defects such as cracks and delaminations in welded composite materials. Typically this method is used to help narrow down the locations(s) of defects in large structures before using a more precise NDT method such as radiography or ultrasonic testing to help localize and characterize the nature of the defect.
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