Rapid Inspection of Complex Geometries Using Edge-Guided Ultrasonic Waves

Ultrasonic waves in thin plates have a number of interesting properties, such as velocities that vary with frequency and multiple possible modes of vibration at a given frequency. This project will use one particular group of modes, which follow the edges of thin plates, to create a method of monitoring the edges of carbon fibre reinforced polymer (CFRP) aircraft components for damage.

Structural health monitoring, whereby engineering structures are continually tested for damage, allows significant improvements in the way that the useful lifetime of engineering structures is managed. Methods based on designing structures to be viable long beyond their planned working life are being replaced by approaches that rely on monitoring for the first signs of deterioration and then repairing or replacing appropriately. This allows lighter structures to be used safely resulting in significant savings in construction materials and, for structures such as aircraft, ships and automobiles, improved efficiency throughout their working life. Ultrasonic waves have been successfully applied to structural health monitoring of plate-like structures and pipes, but structures with complicated geometries and physical properties that vary with direction (anisotropic materials) present particular challenges for ultrasonic structural health monitoring. This work will generate understanding of edge guided waves in anisotropic materials as a method of testing important sections of complicated structures.

Ultrasonic waves in thin, plate-like, structures have more complicated behaviour than waves travelling through bulk materials due to the effect of the surfaces of the restricting the possible shapes (or modes) through the thickness of the structure as the wave propagates. These guided waves can travel large distances (up to tens of metres) and are scattered by defects, allowing them to be used to detect damage. They can also have multiple modes at any given frequency, each with a different frequency-dependent velocity and this complicates their use. Substantial work has been done to find methods of applying them to damage detection. The behaviour of ultrasonic waves at the edges of thin structures is further complicated by the edge also acting as a guide to the wave. This leads to modes that propagate along the plate edges, but decay rapidly away from the edge. In addition to representing an interesting physical problem, these modes, collectively referred to as edge waves, are a candidate solution to the problem of inspecting important parts of complicated geometry structures. In particularly they are ideally suited to inspecting for damage on the edges of thin structures such as: the stiffeners of wing panels or control surfaces of aircraft, turbine blades or exposed steel girders.

The inspection of wing-panel stiffeners (small plates perpendicular to the panel to prevent it bending) is of interest as they are particularly susceptible to damage and carry significant loads. The following objectives will need to be achieved for this application to be realized: creating numerical models of edge waves in anisotropic materials, designing methods of generating and measuring edge waves, and performing experiments on damaged structures to determine the effect of defects on edge waves. A method of inspecting a specific structure (wing panel stiffeners) will be created and techniques generated to allow application to inspecting the edges of any thin structure for damage. A demonstrator system will be produced that showcases this technique.

The UK has a significant high-value carbon-fibre reinforced polymer (CFRP) manufacturing industry, which is seeking to increase its market share. In order to fully exploit the potential weight savings of CFRP, and the resulting improvements in performance and fuel economy, improved methods of inspecting CFRP structures are required. Structural health monitoring and the design decisions it allows will be vital to exploiting CFRP and maintaining the competitive nature of British manufacturing in this area.

The proposed research will provide a method of rapidly inspecting the edges of thin CFRP structures. A system for the structural health monitoring of stiffeners on CFRP wing panels will be demonstrated and the knowledge and methods created will enable further applications to the edges of other thin components, such as control surfaces of CFRP wings, metal turbine blades, beams and rails.

Impact damage to CFRP structures leads to internal cracking and delamination of the laminated layers used for construction. This leads to a reduction in strength of the structure. In order to remove the possibility of failure, components are designed so that they can withstand in-service loads even when they are damaged. The extent of damage that needs to be designed for is determined by the smallest detectable damage: the presence of damage which cannot be detected must not reduce the strength of the component such that in service loads lead to failure. Currently visual inspection is the most common method of inspecting CFRP parts during manufacture and service, with some ultrasonics used to inspect small regions of particular interest.

Barely visible impact damage (BVID) is the limit of damage that can be detected visually and hence components have to be designed to maintain strength even where damage of severity up to and including BVID is present. For example, Boeing's current design criteria for damage tolerant CFRP define BVID as small damage that may not be found during a visual inspection from five feet and requires that such damage does not reduce the component strength below the ultimate design strength and must not grow appreciably during the service life. The restriction on growth is the more restrictive of the two and is due to the need to ensure that at the time of inspection there is no damage that could grow to become problematic before the next inspection. Improving sensitivity and allowing more frequent inspections means that the severity of damage that will grow under service loads can be reduced without increasing the possibility of damage growing to dangerous levels between inspections.

By providing sensitivity to damage that cannot be detected by visual inspection, the maximum allowable stress can be increased and hence, for given in-service loads, the thickness of parts reduced. The strain that leads to BVID growth is determined experimentally for a particular CFRP component and is typically around 4500 microstrain. Assuming that the loads are fixed, the cross sectional area required to keep the strain below the threshold for growth is inversely proportional to that threshold strain. As BVID can lead to a reduction in strength of up to 60% there is significant opportunity to reduce the cross section of load bearing CFRP parts. The ultrasonic technique proposed here will allow rapid inspection of wing panel stiffeners, which are hard to access for visual inspection and subject to substantial loading.

GKN is a leader in the field of manufacturing highly complex composite and metallic aerostructures for both military and civilian applications. The proposal was discussed with representatives of GKN at an early stage and they have agreed to engage in further meetings to ensure that the project aligns with their requirements in this sphere. They have also agreed to provide access to appropriate test samples and to their engineers.