Immersion
Contact
A number of techniques are available for the non-destructive inspection of adhesive joints:
There is no NDE technique that can provide a quantitative assessment of joint strength.
Ultrasonic inspection techniques are widely used for the non-destructive evaluation of materials. The techniques can be used to detect, measure and characterise a wide range of manufacturing and in-service defects in metallic and polymeric structures. Ultrasonic inspection is particularly suited to the detection of planar type defects (e.g. debonds and delaminations) normal to the incident beam. Voids and porosity in the adhesive and adherends are also detectable. The primary advantage of ultrasonic C-scan inspection is the ability to detect flaws deep within solid objects, whether the material is homogeneous or heterogeneous on either a micro- or macro- structural scale. For metal structures, ultrasonic inspection is done routinely on material upto ~6 metres thick. The maximum inspection depth for fibre-reinforced laminates is typically 40–50 mm. Planar defects as small as 0.3 mm in size can be detected and accurately located. Planar resolution is limited by the ultrasonic transducer diameter. Although a 0.3 mm spatial resolution is possible with many of the high resolution imaging systems, technical expertise is required to obtain this degree of accuracy. Discontinuities that are present immediately beneath the top surface are difficult to detect. This region is called the “dead zone” and is typically 0.1 to 0.25 mm thick. Hence, the technique is not suitable for detecting surface contaminants (e.g. oils and grease).
Polymeric materials, such as adhesives, can be highly attenuative to ultrasound, and thus a higher degree of signal amplification is required compared with metals. Attenuation of the ultrasonic signal occurs as a result of visco-elastic effects in the polymer and dispersion due to fillers, porosity, and damage or defects within the material. Ultrasonic signals are scattered or reflected from any interface that separates regions of differing acoustic impedance. A large range of ultrasonic transducers is used with operating frequencies between 0.5 to 75 MHz, and higher. Improved spatial resolution is achieved by using high frequency transducers. Higher frequency signals, however, are more sensitive to surface anomalies and surface roughness and are subject to high signal attenuation (i.e. signal-to-noise ratio decreases with increasing frequency).
Ultrasonic transducer beam diameters range from 6 to 25 mm, with the most commonly used being 10 mm. Increased spatial resolution can be achieved by the introduction of a small circular aperture (known as a collimator) in front of a parallel transducer (i.e. unfocused), although at the expense of a loss in beam power. The introduction of a collimator also improves near-surface resolution and increases penetration depth for use in inspecting thick honeycomb structures.
Of the many ultrasonic methods that exist, three predominate in their use for inspection purposes:
Pulse-Echo Method: In this inspection mode, a single transmitter-receiver transducer scans along the material surface capturing signals that have been reflected from the back surface, or from discontinuities (interfaces or defects) in the material. Regions free of discontinuities return echoes from only the near and back surfaces. Additional echoes are produced due to the presence of discontinuities within the region being interrogated. In the presence of a defect, the incident pulse is almost totally reflected at the interface with little or no ultrasonic signal transmitted to the material below the defect. The arrival time of these echoes provides information as to the through-thickness location of the associated defect.
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Pulse-echo method
This method is effective in interrogating the bondline between a uniform thickness skin and a non-metallic core substructure. The principle is to monitor a signal in a pre-set time window, or gate. The gate may be positioned between the front surface echo and before the back surface reflection, or actually on the back-wall echo. The amplitude of the signal in this gate indicates the level of acoustic impedance discontinuity in the material at that location. This method of operation can be carried out in an immersion tank with deionised water as the ultrasonic couplant or by using a contact transducer. For the contact mode, water is replaced by gel, oil or grease couplant.
In some circumstances exposure to water may be detrimental to the product (e.g. wood or paper based products, or where subsequent bonding is intended). Water may also enter the structure (e.g. honeycomb structure) and act as a block to ultrasonic signals, thus inhibiting the detection of flaws. One solution is to employ a contact probe. This requires considerable pressure to maintain good coupling between the ultrasonic transducer and the specimen surface, which is not particularly suitable for the inspection of delicate structures. Alternatively, air-coupling ultrasonic inspection could be used. For these systems, acoustic power output from the transmitter and sensitivity of the receiver have been maximised to partially overcome the inherent signal losses in air. Air-coupling systems, however, are less sensitive than immersion ultrasonic methods.
A strong reflection from the back surface means the specimen can be readily inspected from one side. This is particularly advantageous where access, as often the case, is limited to one side of a structure or component, hence the propensity of users to operate systems in the pulse-echo mode in preference to through-transmission. The pulse-echo mode is most sensitive to planar defects aligned normal to the interrogating beam. Pulse-echo is used for measuring amplitude attenuation and material thickness (time-of-flight). Measuring the amplitude of reflected signals by this method is preferred when inspecting thin or varying thickness composite skins bonded to a substructure (e.g. sandwich structures). To effectively use the amplitude mode, the skin must be sufficiently thick to isolate the bondline interface response.
Single Through-Transmission Method: This method of inspection involves two ultrasonic transducers (i.e. transmitter and receiver) facing directly opposite each other and separated by the specimen. The principle of operation is the measurement of the transmission of ultrasound through the material. To avoid spurious multiple reflections a short pulse is generally used. The transmitted pulse is received, amplified and displayed on an oscilloscope as well as the amplitude being measured and recorded. Discontinuities are detected by comparing the ultrasonic signal transmitted through the test specimen with the intensity transmitted through a reference standard made of the same material. Water couplant is generally used to transmit ultrasound from the transmitter to the specimen and from the specimen to the receiver. This can be accomplished either by fully immersing the specimen and transducers in a water bath (i.e. immersion method), by water jets (squirters) or by a water film. Defects will ether block or attenuate the transmitted ultrasonic signal, thus a reduction in the signal amplitude or a total loss of signal usually occurs in regions containing internal flaws.
This method is more suitable for large components (where water jets or squirters are used instead of a water bath), honeycomb structures and thick sections where multiple reflections occur due to the presence of numerous interfaces (composite laminates), often prevent the use of other methods. Single through-transmission is often superior to pulse-echo for detecting near-surface discontinuities, the reflections from which can often emerge from the front-surface signal. The main disadvantages are that access is required to both sides of the test material and that the method provides no information about through-thickness location of defects.
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Through-transmission method
Double Through-Transmission Method: In this inspection mode, a single transmitter-receiver transducer scans along the material surface capturing signals that have propagated through the specimen twice. The specimen is supported above a flat glass or metal reflector plate and the inspection area, transducer and reflector plate are fully immersed in water. A short ultrasonic pulse passes through the specimen, normal to the surface, is reflected by the reflector plate and travels back through the specimen again to the transducer. The reflected signal is capture, amplified and displayed on an oscilloscope and the amplitude is measured and recorded.
Using the double-through transmission approach enhances the detection of near-surface flaws by directly monitoring the amplitude of the back-surface reflection rather than monitoring intermediate signals between the front and back reflections. The presence of a near-surface discontinuity will result in a reflected signal, and thus a reduction in energy of the transmitted pulse that propagates to the reflector and back. This effectively reduces the amplitude of the reflection.
Next: Display Modes
