Transmission mode procedure and test parameters

Impact damage

Impact damage is categorized into low-velocity impact damage (LVID) and high-velocity impact damage (HVID). The low-velocity impact is caused by quasi-static force, the higher limit of impactor velocity ranging from one to a few tens of ms−1 depending on the material’s and impactor’s properties. In HVID, boundary condition effects are negligible because the impact event is over before the stress waves have reached the edge of the structure. HVID is characterized by impactor penetration and induced fibre breakage, while LVID results in delamination and matrix cracking. Matrix cracks typically occur near the impact location. The cracks do not affect the laminate strength significantly, although they are usually the first failure mode to occur in delamination, Fig. 1-1 [6], [7]. The delamination pattern is dependent on the structure of the fabric. Typically, multiple butterfly-shaped delaminations are observed in the impacted area. They are extended through the axis parallel to the fibre direction. The delaminations are expanded in size, with an increase of the impact load.

They rotate from the depressed surface to the bottom layer [7], [8]. LVID on aircraft is due to either operational or maintenance events. In service, hailstone strikes, birds and runway debris are the primary causes of LVID. In maintenance operation, LVID is due to accidental tool drops. Such blunt impactors could cause little permanent indentations and barely visible impact damage (BVID), which may end-up contributing to large delaminations [9]. Boeing damage definition classifies BVID as small damage, which may not be found during heavy maintenance general visual inspections, using typical lighting conditions, from a distance of five feet. Damage dent depth is typically 0.01 to 0.02 inches [10]. This type of damage can have a significant effect on the strength and durability of the structure. Therefore it is a primary concern for the design and the maintenance of composite structures. A wide range of characterization techniques, both destructive and non-destructive, can be employed to improve our understanding of the damage mechanisms occurring in CFRP panels. However, destructive tests have a significant limitation, which is the partial or total loss of the test part [11].

Point-by-point NDT methods As a point-by-point NDT method, MIA measures the mechanical impedance of a point on the surface of a structure by applying a harmonic force to it at a single frequency (1 to 10 kHz). In fact, the stiffness of the part under test affects the mechanical loading of the probe that sits on it; going from good to flawed areas changes the phase and the amplitude of the mechanical vibration induced by the probe. Because they significantly reduce local stiffness of a part, debonds in adhesive joints as well as delamination and voids in laminated structures affect the dynamic properties of a part and are thus good candidates for MIA inspection. The sensitivity of the method changes with frequency, so selecting appropriate driving frequency is critical to obtain the greatest difference between good and bad parts [14]. In AET, sound wave pulses are triggered by an external loading which is often provided by regular operating conditions of the monitored structure. These acoustic pulses are caused by a sudden release of the energy stored in discontinuities that are growing under the applied stress [16]. In a typical test, a piezoelectric transducer is mounted on the surface of the test part. It converts the elastic wave into electrical signals that are amplified and digitized for further numerical processing. In most of the cases, data extracted from AET signals are more difficult to interpret than signals extracted from other NDT methods [12], [17]. ECT is based on electromagnetic induction.

A coil is driven by an alternating current so as to produce magnetic flux. Eddy currents are induced in an electrically conductive part exposed to this flux. Variation of eddy current intensity results in changes in the electrical impedance of the test probe. This method is broadly used for detecting cracks and corrosion in metals. Although electrical conductivity is lower in CFRP as compared to metallic parts, ECT has the potential to inspect them. However, it is challenging to apply conventional ECT methods to CFRP because of low conductivity and the anisotropic properties of the CFRPs [18]. UT is the most frequently used NDT method to detect internal flaws in CFRP laminates. An ultrasonic system consists of a transmitter and a receiver circuit, a transducer tool, and a display device. Based on the information carried by the signal, location, size and orientation of the flaws are characterized [19]. Pulse-echo is a conventional ultrasonic testing mode also named as a reflection method. It is based on an ultrasonic energy beam reflected from the medium interfaces. When ultrasonic waves encounter a defect, the reflected energy can be distinguished from a back wall echo of the sound area. For damage detection on a CFRP laminate, the pulseecho technique has limitations on distinguishing the layer-by-layer information signals. As a result, Time-of-Flight (TOF) scan has been developed to allow delamination or cracking to be identified and localized with better accuracy. UT has an excellent ability in sizing and revealing the location of flaws such as matrix crack and delamination in CFRP, although UT is timeconsuming and requires a contact medium [20]-[21].

Full-field NDT methods In today’s competitive aerospace industries, high-speed inspection technology is needed. Fullfield NDT methods are providing a faster inspecting speed for new aircraft both during the manufacturing process and in the field. For instance, a point-by-point inspection method, such as UT C-scan, have a slow-pace inspection of about 10 sq. ft ./hour. On the other hand, for a full-field method such as shearography and thermography, an inspection rate of 25 to 1200 sq. ft per hour is achievable. Therefore, a fast, full-field and real-time technique is essential to ensure quality and reliability at the lowest possible cost and time [22]. Among available full-field NDE methods, VT is, at the same time, the simplest and the fastest one. However, VT has an inherent disadvantage in the characterization of interior damages. In some cases, the internal failure mechanism absorbs impact energy without showing any external sign of the flaw. Although the visual inspection is a routinely performed task as part of an aerospace maintenance schedule, its definition varies notably from one source to another. In general, we can say that it is an evaluation method based on human sensory systems aided only by mechanical tools such as magnifiers and borescopes [6], [23].

RT is a widely-used NDT damage analysis method. An X-ray source transmits a beam of photons through the specimen and a sensitive film or sensor is used to image the intensity of the transmitted X-rays. The darker regions on the film received more X-ray photons than the brighter areas. The absorption of X-rays varies with the thickness and the density of the test material. Any defect that changes at least one of these two parameters will thus create a contrast in the radiographic image. Radiography shows excellent performance in detecting voids and inclusions. However, this technique is susceptible to the angle of the incident beam with respect to the defect orientation. Therefore, delamination is not detectable as the incident Xray beam is perpendicular to the plane defect [24]. Thermography and shearography are both full-field and real-time methods to evaluate the nature and the severity of defects such as delamination in laminar composites [15], [25]–[29]. Since delamination is the primary feature of impact damages, high performance and reliability are expected from these methods to assess this type of damage.

Infrared thermography

IRT (infrared thermography) is reported as an applicable method to evaluate the nature and the extent of defect in CFRPs [30]–[32]. It is conducted in aerospace industries mostly to detect delamination and moisture ingress in composites wherein the variation in thermal diffusivity between CFRP and the flaws produces excellent NDT indications. IRT techniques are based on thermal contrast, which is recorded from the surface of the test material. On top of a defect, parts’ surface temperature will be different in comparison to the sound area. In this method, a thermal excitation source heats the specimen while the surface temperature of the test part is monitored by an infrared camera. The heating process can vary in terms of application area (surface, line or point) or the type of modulation (step, pulse or periodic forms). The thermal response is captured for a determined period, deeper defects requiring a longer observation period. In fact, temperature variation is a function of the discontinuity depth and the thermal proprieties of the defect and the test part [33]. IRT techniques can be classified by the arrangement of the specimen, infrared camera, and thermal stimulation source. Practically, in most of the cases, the advantages of thermographic non-destructive testing are demonstrated in a one-side procedure, which is called the reflection mode. In this case, both camera and heating sources are located on the same side of the specimen. On the other hand, a two-sided, or transmission test, requires that the heat flow crosses the thickness of the part and, therefore, can not be applied to thick or thermallyinsulating materials [34].