Lamb waves

In this paper, a configuration for finding delamination in carbon-based plate like structureis presented and reviewed. Using
induced guided Lamb Wave in order to identify damages in a carbon plate by analyzing key distinct features and using
support vector machine (SVM) tools to compare data from a delaminated plate and reference training data. This research
resets a sensor circle configuration for identifying and scanning delamination in a panel by using the wavelet transform to
create a descriptive feature extraction method, then using the known sensor position to create a reliable and robust C-scan.
This method enables both post-production and ongoing monitoring of plate-like carbon-based composite structures. In the
form of a low-cost scan option that can detect even sub-surface delamination.

Structural Health Monitoring (SHM) has become, in recent years, one of the most important area of investigation, not only in the Aeronautical and Aerospace environments, but in a wide area of engineering fields, such as the Civil one.
Quality, Reliability and Safety are the main features that every artifact or service shall carry not only because of the norms, but also for making it the most attractive possible for the buyers. SHM is the answer to the new globalized market that asks, more than any other thing: “Optimization”!
Optimization doesn’t only mean maximizing the performances of a manufact with the lowest price as possible at the time of purchase, but must be also taken into account its whole operational life. The maintenance of a modern civilian aircraft absorbs about 20-25% of its entire operational cost, so it plays a definitely crucial part.
SHM aims to improve dramatically the parameters of Quality, Reliability and Safety, thus also reducing the maintenance costs! This can be achieved through a different design process that reduces the required number of scheduled light and heavy maintenances.

Low velocity impact induces barely visible damage in the form of matrix cracking or delamination that can grow under hydro-thermo-mechanical loading and possibly lead to catastrophic failure if not detected at an early stage. A network of piezoelectric transducers can be used to monitor a structure over time for life prognosis through generation and sensing of guided ultrasonic waves. The aim of this study is to design and develop such a sensing method for damage assessment in a composite T-joint subjected to mechanical impacts. In this context, monitoring of Lamb waves in a carbon fibre reinforced polymer (CFRP) T-joint has been completed where dispersion and tuning curves have been obtained. Guided waves are transmitted into the structure through different specified pairs of surface-bonded lead-zirconate-titanate (PZT) transducers in a pitch-catch active structural health monitoring (SHM) approach. With these experiments, Lamb wave fundamental modes (A0 and S0) are identified for monitoring impact damage by signal comparison with a prior obtained baseline. Detecting 4J and 10J inner impacts within the central region of the specimen is challenging when using conventional non-destructive techniques (NDT) because of the complex geometry and interference with the web. Signals are compared for the same selected sensing path; and amplitude differences have been observed in tuning curves after the 10J impact, which implies the occurrence of a structural change related to the impact.

We experimentally investigate a diagnostic technique for identifying a weak bond in composites using highly nonlinear solitary waves. We set up a one-dimensional chain of granular crystals, consisting of spherical particles with nonlinear interactions, to generate highly nonlinear solitary waves. These solitary wave packets are transmitted into an inspection area of composites by making a direct contact with the chain. We demonstrate that a strong type of solitary waves injected to the weak bond area can break the weak bond of laminates, thereby causing delamination. Then, to identify the creation of the delamination, we transmit a weak type of solitary waves by employing the same apparatus, and measure the solitary waves reflected from the specimens. By analyzing these reflected solitary waves, we differentiate the weak bond samples with the pristine bond ones in an efficient and fast manner. The diagnostic results based on the proposed method are compared with the strength and energy release rate at bond interfaces, which are measured via standard testing methods such as three point bending and end notched flexure tests. This study shows the potential of solitary wave-based detection of weak bonds for hot spot monitoring of composite-based structures. ‡ Equally contributed first author

Ultrasonic guided wave imaging with a sparse, or spatially distributed, array can detect and localize damage over large areas. Conventional delay-and-sum images from such an array typically have a relatively high noise floor, however, and contain artifacts that often cannot be discriminated from damage. Considered here is minimum variance distortionless response (MVDR) imaging, which is a variation of delay-and-sum imaging whereby weighting coefficients are adaptively computed at each pixel location. Utilization of MVDR significantly improves image quality compared with delay-and-sum imaging, and additional improvements are obtained from incorporation of a priori scattering information in the MVDR method, use of phase information, and instantaneous windowing. Simulated data from a through-hole scatterer are used to illustrate performance improvements, and a performance metric is proposed that allows for quantitative comparisons of images from a known scatterer. Experimental results from a through-hole scatterer are also provided that illustrate imaging efficacy.

Permanently attached, distributed sparse arrays of piezoelectric transducers have been proposed as a cost-effective solution for rapid interrogation of large, plate-like structures for structural health monitoring. Many proposed methods using these arrays rely on guided wave imaging techniques to interpret and graphically display information for damage detection and localization. Guided wave imaging algorithms, however, contain imaging artifacts that are due to reflections from boundaries and other structural features, even in a noise-free environment. The magnitude relationship between pixel values corresponding to actual damage and imaging artifacts is dependent on a number of variables, including imaging algorithm, number and location of transducers in the array, location and scattering behavior of damage, and structural geometry. This study proposes a metric to characterize the imaging performance of distributed sparse array systems and utilizes numerical simulations to analyze the impact of damage location, imaging algorithm, and physical array configuration on performance in the context of a single flaw in a rectangular aluminum plate.

Abstract. The ultrasonic detection of uniform corrosion in a thin sheet of aluminum alloy 2024-T3 was investigated at an early stage of corrosion damage. The artificial corrosion, similar to the corrosion in lap joints during operational service, was achieved using a corrosive solution as described in the literature. The corrosion damage intended was relatively weak when it has not yet caused significant changes of thickness and fatigue strength. On the other hand, it has caused significant changes of the intensity of transmitted ultrasonic waves. Maybe this effect can be related to the influence of corrosion damage on the attenuation of waves at the surface layer. The main results of the research shows that even small corrosion damage can be reliably detected by the integrated system (structural health monitoring) at an early stage of its development.

An acoustical-ultrasonic technique was used to demonstrate relationships existing between changes in attenuation of stress waves and tensile stress on an eight ply 0 degree graphite-epoxy fiber reinforced composite. All tests were conducted in the linear range of the material for which no mechanical or macroscopic damage was evident. Changes in attenuation were measured as a function of tensile stress in the frequency domain and in the time domain. Stress wave propagation in these specimens was dispersive, i.e., the wave speed depends on frequency. Wave speeds varied from 267,400 cm/sec to 680,000 cm/sec as the frequency of the signal was varied from 150 kHz to 1.9 MHz which strongly suggests that flexural/lamb wave modes of propagation exist. The magnitude of the attenuation changes depended strongly on tensile stress. It was further observed that the wave speeds increased slightly for all tested frequencies as the stress was increased.