Unmanned Systems Technology 018 | CES show report | ASV Global C-Cat 3 USV | Test centres | UUVs insight | Limbach L 275 EF | Lidar systems | Heliceo DroneBox | Composites

94 Inspection Detecting any structural flaws is critical to ensuring the safety of an unmanned vehicle, particularly as many of them can occur during the manufacturing process. For example, if curing is handled imperfectly, with a non-optimal temperature, pressure or duration, then the composite can become porous, as voids inside the resin will adversely affect inter-laminar shear stress. Impregnation by hand can introduce impurities into the prepreg, or incorrect resin-to-fibre ratios in parts of it. Additionally, the lay-up process can lead to misalignment of fibres in the material, leading to weaknesses and inconsistencies in its ability to handle loads along a wing, say. And once in service, the composite can experience other issues as a result of the fatigue or impacts inflicted by the vehicle’s mission and environment. Delamination, ingress of moisture or breakage of joints are among the issues that must be detected before they pose a critical risk to the vehicle or its surroundings. There are various ways of identifying the types of defects and damage in a composite part before it is integrated into an unmanned vehicle. Geometric inspection, for example, can be carried out using 3D laser scanning or a coordinate measuring machine (CMM), which will collect the x , y and z coordinates of a component and measure them against its designated parameters. Software tools for laminate analysis can help here, and even check each ply and record its orientation and position at each stage of the manufacturing process. Other techniques are also available, with ultrasonic inspection remaining the most common. Consisting of a transmitter, receiver and handheld ultrasound transducer, this method can produce fast, high-resolution images indicating the size, location and orientation of flaws in a material, as the ‘pulse-echo’ transducer sends waves into the part and receives back their reflection for analysis. For parts with more complicated geometries that prevent analysis with simple assemblies of transmitters and receivers, ‘through transmission’ ultrasound – in which the transmitter and receiver are situated on either side of the part – can be used. Thermographic inspection can also be critical for pre-identifying where damage might occur. Here, the surface of a thin composite material is first heated slightly, the temperature varying according to the inspector’s choice. Greater heating means longer cooling times and thus more data, but it must not exceed the upper temperature limit of the composite or damage will occur. Then a thermal imaging camera records the part as it cools, allowing visual identification of where minor defects are taking place and therefore where future damage may occur, as they are highlighted by points of difference in the cooling pattern across the part. Large parts can be rapidly inspected by this method. However, thick composites pose an issue as this technique cannot identify flaws deep below the surface; flipping over a panel and testing from the other side may not resolve this. Laser shearography or full-field strain measuring can also be used to validate the structure of a complex component. Following an interferometric scan of the part using a laser or some other kind of electromagnetic wave, it will have some form of stress – vibration, heat, placement in a vacuum and so on – applied to it and then scanned again. Any variations between the two sets of data will highlight deformations or discontinuities that may be caused or made worse as a result of that stress. The application of heat is most suited for carbon, glass or metal composite inspection, whereas placement in a vacuum is considered best for core materials such as foams or honeycombs. Measuring the deformations and stiffness of a part under a load can also be compared against the requirements and tolerances specified before construction. February/March 2018 | Unmanned Systems Technology Focus | Composites Applying a load to a composite part and inspecting the resulting deformations is essential to ensure it meets the required strength and stiffness (Courtesy of SHD Composites)