In the presentation we will focus on characterization of resistive crack proapagation sensors, eddy current sensors as well as PZT sensors in terms of their sensitivity to damage, also in varying measurement conditions. For efficiency assessment, a methodology closed to MIL-HDBK-1823A for probability of damage detection will be followed. For PZT sensors, as they are widely used in Structural Health Monitoring (SHM) due to their ability to excite guided waves for damage detection propagating in any type of material, also results of structure evaluation based on guided waves excited by PZT transducers and full wavefield measurements obtained by laser scanning vibrometer will be presented. The measurements were acquired for pristine state of test structure and with introduced damage, therefore it is possible to calculate spatial distribution of various signal characteristics and estimate effective range of PZT sensors for damage detection based on a given damage index. Various parameters affecting the damage detection range will be compared, in particular: extent of damage, type of signal characteristic used, parameters of the excitation signal (e.g. frequency, duration), as well time interval used for damage index calculation. The obtained results may be useful for estimating the efficient range of PZT sensors sensitivity as well as for refining algorithms for sensor placement optimization and damage localization techniques calibration, e.g. based on RAPID imaging algorithm.

For resistance crack gauges, their sensitivity to fatigue crack propagation, both for open and closed-crack condition will be discussed. For eddy current sensors two damage scenarios are considered, i.e.: cracks and material erosion. For PZT sensors, in addition to those types of defects, also sensitivity to impact damage will be presented and discussed as well. Influence of environmental conditions on damage detection capabilities, such as: temperature, humidity or load, will be also discussed.

Simulation is playing an increasing role in industrial applications, as it enables to optimize design and perform sensitivity analyses without requiring all the costly manufacturing and corresponding test phases. However, there are limitations to what can be simulated, mainly due to inherent uncertainties and unmodeled effects. Uncertainties require to have efficient computational tools, to enable large parametric studies. As for unmodeled effects, efforts can be undergone to enhance models, but this may lead to additional computational costs that will limit the affordable parametric studies. It is the case in particular for SHM, where many parameters such as static load, temperature or aging of the sensors.

We will show in this talk how simulation can help to identify relevant parameters and effects of these parameters to consider in a large parametric study by studying separately each of them and comparing their influence. This will be exemplified on the case of two plates riveted together through a third plate, known as a butt splice. In this case, the effect of two environmental conditions on wave propagation, namely a static load and temperature variation, will be compared to determine which needs to be taken into account in the end. The static load is modeled through the definition of a linearized stiffness operator evaluated for the static load, which is used in the wave propagation problem [1]. Temperature effect is considered through the definition of effective material parameters [2]. Other modeling options as well as ongoing work to speedup computations will also be discussed.

[1] DALMORA, André, IMPERIALE, Alexandre, IMPERIALE, Sébastien, et al. A time-domain spectral finite element method for acoustoelasticity: modeling the effect of mechanical loading on guided wave propagation. Wave Motion, 2024, vol. 129, p. 103328.

[2] GINZEL, Ed et GINZEL, Robert. Approximate dV/dT values for some materials. e-Journal of Nondestructive Testing (eJNDT), 2017.

The damage tolerance approach enables the design of lightweight composite structures, provided that damage progression remains controlled throughout their operational life. However, delamination growth under fatigue loads, particularly for low-velocity impact (LVI)-induced damage, remains poorly understood. The no-growth design philosophy assumes that delaminations in the high-cycle fatigue regime exhibit an extended plateau phase, during which no significant propagation is observed. Nevertheless, recent studies suggest that this plateau phase may result from the limitations of conventional inspection techniques (i.e. C-SCAN), rather than from an actual absence of damage evolution. In particular, C-SCAN-based ultrasonic inspections, typically used for the scope, may fail to detect internal delaminations encapsulated within outer ones due to the shadowing effect, leading to an underestimation of fatigue-driven damage progression. This raises concerns regarding the reliability of traditional non-destructive evaluation (NDE) methods for monitoring damage in critical composite structures. As part of the TU-LEARN (Structural Life Extension Enhanced by Artificial Intelligence) project, funded by Unione Europea – Next Generation EU, under the PRIN 2022 PNRR – D.D. n. 1409 del 14-09-2022 program, this study aims to enhance the understanding of LVI damage propagation mechanisms under fatigue loading. An experimental campaign was conducted on Carbon Fibre Reinforced Polymer (CFRP) coupons with a stacking sequence of [(45,-45,90,0)]_2s, representative of aerospace-grade laminates. The test protocol included LVI tests in accordance with ASTM D7136 standard, followed by compression after impact (CAI) fatigue tests as per ASTM D7137 standard, to assess the residual strength and progressive damage evolution under cyclic loading. Impact tests were performed using an energy level of 15 J, with a drop mass of 4.567 kg, ensuring a representative damage state. Subsequently, fatigue tests were conducted under compressive-compressive loading with a stress ratio of R = 0.1 and a fatigue load frequency of f = 4 Hz. To monitor damage evolution at multiple fatigue stages, an ultrasonic guided wave (UGW)-based Structural Health Monitoring (SHM) system was integrated into the test campaign, complementing C-SCAN ultrasonic inspections. UGW signals were acquired in a pitch–catch configurationusing surface-bonded piezoelectric (PZT) transducers: at each inspection stop, a repeatable tone-burst excitation was applied to the actuator, and the received waveforms were recorded on selected sensing paths (with signal averaging to improve the signal-to-noise ratio). Damage Indexes (DIs) were then computed by comparing each acquired signal to an undamaged baseline to quantify damage-induced changes. This approach provided a more detailed assessment of delamination growth throughout the fatigue life, revealing potential limitations in the conventional no-growth assumption, Figure 1. The use of UGW-SHM appeared capable of tracking the internal damage progression, even in regions where C-SCAN technique was insufficient.

The widespread adoption of Structural Health Monitoring (SHM) is contingent upon the availability of a competent and standardized workforce. Building upon the foundational work that led to the establishment of the Testia SHM Academy, this paper details the Academy's current offerings and proposes a formalized, multi-tiered qualification system for SHM personnel. The foundational training provided via the SHM academy currently focuses on a 2 day SHM short course providing general SHM theory, regulations and procedures for deploying SHM, hands on experience with some of the mature SHM technologies and a structured overview of relevant industry standards. This course serves as the necessary stepping stone toward professional SHM qualification.

To further address the industry’s needs to have a methodical approach for standardizing SHM qualifications and training, the coursework will include an expansion to the foundational training provided today. Unlike the general introductory format, the proposed SHM Long Course has enhanced classroom lectures, designed to deliver more comprehensive training focused on quality assurance and performance/procedural compliance for SHM systems. Additional, focused, method-dependent training will address the detailed installation, monitoring and airworthiness assurance of individual SHM disciplines. To further enhance the experience of the trainees, real parts and structures will be used for the hands-on training segments with customized sensor kits and technique sheets. Furthermore, this detailed training covers advanced topics essential for SHM deployment, including necessary performance assessments, logistics integration into routine aircraft maintenance programs and detailed statistical analysis for ensuring suitable damage detection.

This work culminates with a proposal of a three-tiered grade approach for SHM personnel, mirroring the established structure of NDT certification to ensure global recognition and career progression. Grade 1 (SHM User/Inspector) qualifies personnel to perform system setup, basic troubleshooting, and execute inspections to record overall data results. Grade 2 (SHM Installer) focuses on the practical, method-dependent skill set required to install SHM systems, comprehend installation technique documents. Finally, Grade 3 (SHM Trainer) represents the highest level of mastery, adding advanced theoretical and conceptual knowledge, sophisticated troubleshooting abilities, and advanced data analysis proficiency, thus enabling them to train and certify lower-grade personnel. The implementation of this robust, extended curriculum and standardized grading framework is essential for accelerating the safe, effective, and widespread deployment of SHM technologies worldwide.