A key longstanding objective of the Structural Health Monitoring (SHM) research community is to enable the embedment of SHM systems in high value assets like aircraft to provide on-demand damage detection and evaluation. As against traditional non-destructive inspection hardware, embedded SHM systems must be compact, lightweight, low-power and sufficiently robust to survive exposure to severe in-flight operating conditions. Typical Commercial-Off-The-Shelf (COTS) systems can be bulky, costly and are often inflexible in their configuration and/or scalability, which militates against in-service deployment. Advances in electronics have resulted in ever smaller, cheaper and more reliable components that facilitate the development of compact and robust embedded SHM systems, including for Acousto-Ultrasonics (AU), a guided plate-wave inspection modality that has attracted strong interest due mainly to its capacity to furnish wide-area diagnostic coverage with a relatively low sensor density. This article provides a detailed description of the development, testing and demonstration of a new AU interrogation system called the Acousto Ultrasonic Structural health monitoring Array Module+ (AUSAM+). This system provides independent actuation and sensing on four Piezoelectric Wafer Active Sensor (PWAS) elements with further sensing on four Positive Intrinsic Negative (PIN) photodiodes for intensity-based interrogation of Fiber Bragg Gratings (FBG). The paper details the development of a novel piezoelectric excitation amplifier, which, in conjunction with flexible acquisition-system architecture, seamlessly provides electromechanical impedance spectroscopy for PWAS diagnostics over the full instrument bandwidth of 50 KHz–5 MHz. The AUSAM+ functionality is accessed via a simple hardware object providing a myriad of custom software interfaces that can be adapted to suit the specific requirements of each individual application.

Lap joints are widely used across many critical structures such as aircraft and bridges. Lamb waves have long been proposed to monitor lap joints against defects such as disbonds. However, there are many challenges which must be answered to make use of Lamb wave technology. Frequency selection is often overlooked, and many authors will select a single frequency without knowing if other frequencies will result in better sensitivity. Another challenge is the features (mode conversion, attenuation, reflection) associated with damage are also inherent in a lap joint. This sharing of features can lead to confusion (false positive/negative) depending on the chosen damage detection strategy. Furthermore, almost all proposed methods require a baseline reading of the structure in its flawless state. Relying on a baseline reading can result in false positives due to shifts in sensor outputs caused by ageing and inconsistent environmental conditions. Instead of a baseline, this article proposes a technique which uses strategically positioned sensors to detect Lamb wave modes generated only in the presence of a disbond. The technique is first developed using a numerical study and then verified with an experimental study. Several frequencies are trialled and detailed in this article which shed light on the ideal frequency selection when using this method.

In-fibre Bragg gratings (FBGs) are now well established for applications in acoustic sensing. The upper frequency response limit of the Bragg grating is determined by its gauge length, which has typically been limited to about 1 mm for commercially available Type 1 gratings. This paper investigates the effect of FBG gauge length on frequency response for sensing of acoustic waves. The investigation shows that the ratio of wavelength to FBG length must be at least 8.8 in order to reliably resolve the strain response without significant gain roll-off. Bragg gratings with a gauge length of 200 µm have been fabricated and their capacity to measure low amplitude high frequency acoustic strain fields in excess of 2 MHz is experimentally demonstrated. The ultimate goal of this work is to enhance the sensitivity of acoustic damage detection techniques by extending the frequency range over which acoustic waves may be reliably measured using FBGs.

No abstract

Typically, numerical simulations of Lamb wave propagation are done using material properties which originate from tensile testing. This approach is well established in relation to isotropic homogenous structures such as aluminium plates. However if this approach is used for woven composites such as carbon fibre reinforced plastics (CFRP), inaccuracies can arise that stem from vastly different stress distributions, strain rates and amplitudes during Lamb wave propagation. In order to account for this, an approach is presented where the elastic properties in a numerical Lamb wave model are optimised to achieve good correlation between model predictions and experimental observations. Since the material properties are determined under a Lamb wave propagation regime, the strain rates and amplitudes are consistent with the intended modelling application. The approach is validated with an experimental case study involving a M18/G939 carbon-epoxy system. The methodology is shown to yield property estimates that furnish simulations that closely match observed behaviours. The optimised properties were significantly different to those supplied by the manufacturer, as much as 52% for the in-plane stiffness. The findings demonstrate that large errors are possible if elastic properties determined using conventional quasi-static testing are used in Lamb wave simulations pertaining to woven composite materials.