Ultrasonic testing for composites with the dolphicam2

Ultrasonic testing for composites with the dolphicam2

Published article in NDT Magazine

May 9, 2021

Dolphitech was founded in 2009 in Gjøvik, Norway, with the mission of pushing forward the boundaries of ultrasonic testing (UT). In 2018, this resulted in the launch of the dolphicam2 as a general-purpose 2D matrix array UT system. The dolphicam2 is unique among UT devices with its architecture of 128 x 128 crossing electrodes, giving a transducer with an active aperture of 16,384 elements. With this unparalleled resolution, the system can produce detailed images in 3-dimensions. As a result, it is well-suited to a wide variety of straight-beam ultrasonic inspections, including composite materials, corrosion mapping, and inspection of adhesive bonding. With a range of transducer frequencies from 1.5MHz to 10MHz, the dolphicam2 provides a volumetric NDT solution for materials ranging from thick marine and wind turbine blade glass fibre reinforced polymer (GFRP), to thin carbon fibre reinforced polymer (CFRP) skins in aerospace sandwich structures.

For fibre reinforced composites, the dolpihcam2 adds value throughout a product lifecycle. During production, defects such as porosity, resin starvation and wrinkling can be flagged during the quality assurance process. Such indications, which may individually beyond the resolution limits of the system, combine to present as a local reduction in back wall signal amplitude (Figure 1). This in turn requires a known reference standard against which to compare inspection results. The dolphicam2 facilitates this with its in-built tools, meaning that statistical measurements, notably the mean signal amplitude, can be measured within a defined area and compared to existing references.

Figure 1. Distributed porosity in 5mm thick CFRP. Left: Low porosity content and high amplitude backwall response. Middle: Moderate porosity content with a slight reduction in backwall response. Right: High porosity content with no backwall response. Samples courtesy of Research Institutes of Sweden (Ri.Se)

During service, indications such as water ingress, impact damage, and adhesive disbonding can be quantified for confident go/no-go decision making. Each damage mode has a characteristic ultrasonic signature. Water ingress into a sandwich structure acts as an absorber of the ultrasound at the interface between the skin and the core, causing a reduction in the reflected signal amplitude (Figure 2).

Figure 2. Example of water ingress into a carbon-fibre composite skin to aluminium honeycomb. Left: Undamaged material showing bright locations of high amplitude reflection at the contact points between the composite skin and honeycomb core. Right: Water ingress into the honeycomb on the right hand side of the image, damping the reflected signal.

Impact damage presents as successive delaminations propagating outwards and downwards from the central impact point. Adhesive disbonds reflect the ultrasound, whereas successful bonding transmits the signal through the interface. Adhesive disbonding is more easily detected in composites than in metals, since the acoustic impedance mismatch between a composite and adhesive is typically much lower than between a metal and adhesive. This means that in a well bonded composite, the majority of the sound is transmitted through the interface, and deviations from this due to disbonds are readily apparent. This is useful for inspection for skin-to-spar disbonds on wind turbine blades (Figure 3).

Figure 3. C-scan overlay of a wind turbine blade inspection, showing the regions of adhesive bonding with the internal spars (red lines)

At the end of design life, material condition can be assessed for its continued structural integrity. This is valuable when life extension is being considered, which in turn may enable substantial cost saving, particularly for installations with low typical interventions such as wind farms. Damage modes that can be assessed by ultrasonic testing to make life extension decisions include erosion, impact damage, and internal cracks and delaminations resulting from fatigue or overload.

Attaching all such inspection results to a digital twin model will become more commonplace in the future for composite parts and assemblies. Digital twins will enable periodic inspections of an asset to be stored and combined with other information to help inform maintenance, repair and life extension decisions. This digital twin approach is facilitated with the dolphicam2 since the data is in the open HDF5 format and can thus provide compatibility with a range of external software. It is also capable of fully remote analysis, and permits simultaneous instances to be run, enabling live acquisitions to be compared side-by-side with previously acquired data from the same part. This can be performed using the same system by a single user, or with data being analysed remotely by a second individual via a virtual session. This unlocks the ability to quickly measure damage progression such as delamination propagation or wall thickness loss.

Overall, the dolphicam2 is well-suited to inspection of composites throughout their lifecycle, with various damage modes having distinct ultrasonic signatures that can be readily resolved to help inform engineering decisions. The inspection landscape is evolving rapidly, with digital considerations that have traditionally been secondary within the NDT sector, poised to become critical for asset integrity management.

Click here to view the Norwegian article in NDT magazine.