Non destructive Evaluation (NDE) Techniques
NDE technology refers to an array of non destructive techniques (NDT) and processes to monitor, probe and measure material response. The measured response is related to a desired material property or test object attribute by interpretation. The main NDT methods are:
Visual Inspection
Visual inspection is the most common form of inspection for composites and other structural systems. This is sometimes called “enhanced” or “close visual” inspection if assisted by magnifying glasses, special lighting or other tools. Still or video cameras are commonly used to provide a permanent record of the inspection.
The best quality visual inspection for transparent/translucent composite materials, such as uncoated E-glass laminates, is when access is possible from both sides with backlighting. Under these conditions, internal defects such as delaminations, fabrication defects and cracking may be seen. The effectiveness depends on fiber architecture, laminate thickness, resin type and coating. If access is limited to one side, then only apparent or obvious defects from one side will be seen.
X-Radiography (X-Ray)
Radiography uses localized differences in attenuation under X-ray illumination to provide a cross-sectional picture of the density of a structure. Images are typically recorded on film. Increasingly, digital or real-time recording systems are used. The method is well suited to volumetric defects and to complex components, which might be difficult to inspect by other methods. X-rays do not reveal surface defects. The method is not popular because of health and safety implications associated with the radiation source. However, portable low intensity systems that reduce the associated hazards are becoming available and are being used in the offshore industry.
In composites, X-ray inspection is typically limited to thinner walled laminates. Radiography is sensitive to major changes in density so it is good at checking adhesive joint assemblies. Identifiable defects include voids, delamination, cracks (dependant upon geometry), excesses of adhesive in joints, and improper joint assembly.
Eddy Current
In eddy current inspection, an electromagnetic coil or arrays of coils are passed over the surface being examined. This induces local eddy currents below the coil, which are sensed by detection coils. The presence of a defect will affect the flow of eddy-currents. By adjusting the frequency, it is possible to move from a surface specific technique to a lower frequency method with good depth penetration that allows inspection of sandwich structures and more complex components and materials systems, such as flexible risers in the offshore industry and concrete structures. The method cannot be used with E-glass laminates, as they are non-conductive. Some success with carbon fiber laminates has been reported.
It should be noted that eddy current NDE has very limited use in detecting composite damage and for inspecting repairs for integrity. It is most commonly used to detect cracks emanating from fastener holes in metal structures without removing the fasteners.
Ultrasonic Inspection
Ultrasonic testing (UT) technology has been around for more than 50 years and is the predominant non-destructive testing method for composites in the aviation and aerospace industries. It evolved from sound navigation ranging (SONAR) after World War II. [Gardiner 2010] UT uses high frequency sound energy to locate structural anomalies in composite laminates. A typical UT inspection system consists of a pulser/receiver, a transducer and a display device. The pulser/receiver is an electronic device that can produce a high voltage electrical pulse. Driven by the pulser, the transducer generates high frequency ultrasonic energy. Typical frequencies are in the range of 0.5 MHz to 15 MHz. Sound energy then travels through the structure. A couplant effectively transmits sound through the interface with the composite because sound is not transmitted well through air at frequencies usually employed for non-destructive testing; even a thin air gap between the transducer and the test piece will make typical UT inspection impossible. Discontinuities, such as cracks or delaminations, reflect the energy back from the location of the flaw surface. The reflected wave signal is then transformed into an electrical signal by the transducer and is displayed on a screen. A UT system measures what is called the time-of-flight and the amplitude of the received pulse. This information can be used to determine laminate mechanical properties such as density and elastic moduli.
An inspector using ultrasonic methods must interpret any differences found and, therefore, needs a thorough knowledge of the structure being inspected. There are generally two types of ultrasonic inspection:
• Through-transmission ultrasonics (TTU), which uses two transducers (one to send the ultrasonic wave and one to receive it after traveling through the part), is typically limited to the factory because access to both sides of the part is required.
• Pulse-echo (P/E) ultrasonics uses a single transducer and requires access to only one side of the part. This method is more predominant for inspections in the field.
