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This technique is suitable for detecting the presence of voids and solid inclusions (e.g. backing film) in the bondline. Thin debonds and delaminations are difficult to detect because the presence of these defects has minimal effect on the absorption characteristics of polymeric materials. The use of penetrant fluids can enhance the imaging process, however, these fluids can adversely affect the short-term properties and fatigue performance of polymeric materials. Penetrants should not be used to assist damage monitoring in those tests where the test data is to be used for design or quality assurance purposes. Small tensile loads or the use of a vacuum pump can be used to promote fluid penetration.
Thermal image showing a defect (debond) as a hot spot—cooling after heat soak
This non-contact technique can be used for rapid inspection of large bonded structures capable of detection and discrimination of gross defects and discontinuities close to the surface. The technique requires the inspected component to be heated to produce a surface temperature distribution that can be correlated with structural integrity or defect distribution. Heating of the bonded structure can be achieved either by:
Thermally soaking the entire structure (known as soak) to a constant temperature and then measuring the gradual dissipation of heat; or by
A thermal spike where the uptake and spread of thermal energy is measured.
Spatial and temporal temperature distribution is measured using infrared imaging CCTV cameras.
Acoustic emission monitoring (AE) involves detection of sound waves (usually inaudible to the human ear) made by a structure under load. The technique, which can be used for monitoring the “state of health” of a structure, involves attaching one or more ultrasonic microphones to the object and analysing the sounds using computer based instrumentation. AE may arise from friction (including bearing wear), crack growth and material changes such as corrosion. Microscopic events can be detected if sufficient energy is released and source location is also possible using multiple sensors. Large structures (e.g. pressure vessels) can be continuously monitored from a few locations, and proof and qualification tests for routine inspection purposes can be conducted whilst the structure is in service. Applications include testing pipelines and storage tanks (above and below the ground), fibreglass structures and weld monitoring. It can be used to monitor bonded joints for damage initiation and growth (e.g. debonds) during mechanical testing. The technique relies on the operator having sufficient experience to be able to identify particular defect types from the AE data.
Eddy current testing is routinely used in the aerospace industry (airframe inspection), and to a lesser degree in the automotive, marine and manufacturing industries for detecting cracks and subsurface damage, such as corrosion in bonded structures. The electromagnetic technique can only be used on conductive materials. Flaw size and material variations (e.g. thickness) can be determined using the eddy currents. The technique consists of an energising coil, through which AC current flows, for inducing currents into the metal component. The magnetic field of the coil when in close proximity of a conducting surface will induce circulating (eddy) currents in the surface. The magnitude and phase of the eddy currents will affect the loading on the coil, and thus its impedance. The presence of defects or variations in material conductivity will either interrupt or reduce the eddy current flow, thus decreasing the loading on the coil and increasing its effective impedance. The technique cannot detect cracks lying parallel to the current path. Changes in voltage are measured and displayed in a manner that indicates the type of flaw or material condition.
The conductivity and permeability (ease of magnetisation) of a material will have a direct effect on the eddy current flow. Eddy current increases with increasing conductivity. Conductivity is often measured by an eddy current technique. The eddy current density, and thus the strength of the response from a flaw, is greatest on the surface of the metal being tested and decreases with depth. Probes can be designed to fit the geometry of the component to be inspected. Penetration depths of 10 mm are achievable on aluminium structures using low frequency eddy probe currents, thus enabling detection of subsurface cracking, which is invisible from the surface, or thinning of any of subsurface layers.