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Item type:Publication, Access status: with files , Lifetime prediction of high-performance fibre-reinforced polymer composites using Vibro-acoustic Modulation(2025); Fibre-reinforced polymer (FRP) composites are characterised by high density-specific stiffness and strength as well as high durability and corrosion resistance. However, the multi-scale and multi-phase structure of FRP also leads to a complex damage mechanism during operation. Any production deviation, such as voids or delaminations, negatively affects residual strength and service life. Monitoring the structural integrity during operation and quality assurance prior to operation is essential for reliable and safe use. Various approaches exist to detect damage at an early stage before failure-critical loads occur. This thesis investigates the applicability of vibro-acoustic modulation (VAM) as a damage detection method for carbon fibre reinforced polymer (CFRP) structures. VAM is a vibration-based non-destructive testing (NDT) method that employs high-frequency ultrasonic waves modulated as an introduced carrier signal by low-frequency vibrations with high amplitude. These modulations can be quantified and used to assess damage. In principle, the type of initiation of the low-frequency vibration can vary depending on the application, whereby the carrier signal is usually initiated by piezoceramic actuators. VAM offers great potential in large structures, especially due to its high sensitivity to cracks, simple measuring equipment, and long transmission paths. The complex – and thus well-known – material and damage behaviour of FRP is used in this work to develop a new test method using VAM. Various vibro-acoustic damage indicators are employed to predict the service life and degradation of FRP and to investigate the following research hypothesis: Damage detection and determination of CFRP structures can be ensured through vibro-acoustic in-situ measurements, even retrospectively. The VAM technique permits the monitoring of localised, concentrated damage in notched and impacted CFRP as well as statistically distributed damage in unnotched CFRP laminates. As evidenced by tensile and fatigue tests conducted under controlled conditions, the type and extent of damage could be identified from the VAM signal. Conventional measurement methods such as Acoustic Emission (AE), Ultrasonic testing (US), Digital Image Correlation (DIC) and X-ray analysis (XR) were also used to evaluate the VAM technique. In general, inter-fibre failures lead to an increase in modulation, accumulating over the service life to global delamination, which reduces modulation. Characteristic points, such as resulting damage-related VAM amplitude maxima, can be used for lifetime prediction. The sensitivity of the VAM method increases with increasing crack size. The linear correlation between the projected delamination area and the MI indicates that an estimation of the impact damage has been achieved. Artificially introduced defects such as PTFE films or contamination with release agent can not initially be detected in scarfed(Sc) joints and single-lap shear(SLS) joints using VAM. To monitor the damage progression of the adhesive film due to permittivity changes, localised capacitance measurements of the adhesive film area can be used. An analytical model for determining crack length correlates with the VAM signal. The combination of both measurement methods demonstrated that, defects in Sc specimens with homogeneous stress distribution have a significant influence on the service life due to the resulting stress concentrations. In contrast, in SLS specimens, the stress concentrations at the overlap ends dominate the development of damage compared to the damage introduced, and thus do not affect the service life. The results of this study demonstrate that the VAM method is a cost-effective in-situ technique for monitoring the structural integrity of CFRP structures. Implementation of the piezoceramic sensors required to introduce the carrier signal is simple and can be done on the surface, independent of the stress state of the structures. The influencing parameters, layer structure, and mechanical test procedures were adjusted, thereby facilitating enhanced comprehension of the measured signal and further advancement towards its implementation as a reliable NDT methodology.Publicationtype: Doctoral Thesis60 45 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Vibro-acoustic modulation based measurements in CFRP laminates for damage detection in Open-Hole structures(2023-10-01); ; ; The occurrence of matrix cracks, fibre failures and delaminations results in a critical degradation of the structural integrity of composites. A reliable early detection of these damages is mandatory for a safe operation. In this paper, the application of Vibro-Acoustic Modulation measurement in notched carbon fibre reinforced polymers (CFRP) for damage detection is demonstrated. A piezoceramic actuator introduces an ultrasonic signal into the specimen, which is modulated by a low-frequency vibration excited by a servo-hydraulic testing system. The specimens tested are open hole tensile specimens according to ASTM D5766, made from a quasi-isotropic CFRP layup. Degradation in the form of matrix cracks or delaminations localised along the drill hole significantly reduces the stiffness and can be detected as a modulation increase. Taking into account the extent of the crack size and the acoustic emission (AE), a magnitude estimate of the modulation increase is possible. In consequence, the presented method can be used for damage monitoring in CFRP.Publicationtype: Journal Article25 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Synthetic generation of vibroacoustic modulation signals for structural health monitoring(Elsevier, 2023-06-16); ; ; ; The vibroacoustic modulation (VAM) is a nonlinear ultrasonic testing method that utilizes the modulation of a high-frequency/low-amplitude probe wave with a low-frequency/high-amplitude pumping vibration, resulting in high sensitivities to damages in the structure. However, applying the method outside of the laboratory as a structural health monitoring system for actual structures is challenging, since the amplitude and frequency of the ambient vibrations – which is ideally utilized as pumping vibration – fluctuates over time. To circumvent this, we present a synthetic generation of the VAM signal using only the probe measurements , acquired at two (or more) stress levels of the structure when a steady state is reached. We could show that only 16 values (8 measured values of only two stress levels with a sampling frequency of 1/20 of the Nyquist frequency) are required to generate the often calculated Modulation Index with a mean deviation of 0.97% to the expected dynamic measured values for glass fiber reinforced composites and 1.86% for the aluminum specimens, which is negligible compared to a typical increase of the Modulation Index of 10–30 dB in case of severe damage. Even undersampled measurements at each stress level can be used without sacrificing accuracy, which reduces the sensing requirements for the sensor nodes. Moreover, this method decouples VAM from the actual need for a constant recurrent frequency and amplitude of the natural vibration in order to reliably compare measurements throughout the lifetime. Hence, this work aims to open the possibility of ultimately applying VAM to assess the structural health of complex structures.Publicationtype: Journal Article79 29 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Health monitoring of CFRP laminates under cyclic loading via vibro-acoustic modulation based measurements(Elsevier, 2023-01-18); ; ; ; Estimating the damage state of composites is essential since carbon fibre reinforced polymers (CFRP) have a complex damage and propagation behaviour. Accordingly, a comprehensive study of the effects of matrix cracks – e.g. their size and crack density – and the influence of different damage types and their interaction on the vibro-acoustic modulation (VAM) behaviour of CFRP specimens under fatigue loading was carried out. These effects were evaluated in two sets of commonly employed laminate layups. Firstly, in a cross-ply (CP) layup, the number of 90°layers was varied, with a constant areal weight (268 g/m2). Secondly, quasi-isotropic (QI) specimens were tested, where the areal weight was reduced from thick-ply to thin-ply (30–268 g/m2), changing the failure mode from a delamination-dominated to a brittle failure. After damage introduction under tension–tension loading (R=0.1), high-frequency Lamb waves are introduced via piezoceramic actuators and modulated by a low-frequency vibration using a servo-hydraulic testing machine. VAM correlates with the different damage behaviour and damage conditions in CFRP. For this purpose, the evolution of the sideband amplitudes, the modulation index, the signal in time domain and X-ray scans are used. Regardless of the layup, the degree of stiffness-reducing inter-fibre fractures and large-scale delaminations can be detected.Publicationtype: Journal Article42 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Vibro-acoustic modulation measurements for a lifetime evaluation of composite materials(International Committee on Composite Materials, 2023); ; ; ; This paper investigates the influence of different damage types and their interaction on the vibroacoustic behaviour of glass fibre and carbon fibre reinforced polymers (GFRP and CFRP) under stepwise tensile and fatigue loading. For this purpose, different GFRP layups were used to correlate visible matrix cracks with VAM signal. Additionally, for CFRP the layer thicknesses are varied for cross-ply laminates with constant areal fibre weight (268 g/m2) and quasi-isotropic laminates with different areal fibre weights (30, 60 and 120 g/m2). After damage introduction under tension-tension loading (R=0.1), high-frequency Lamb waves are introduced via bonded piezo ceramic actuator and modulated by a low-frequency longitudinal wave using a servo-hydraulic testing machine. In this study, the analysis and interpretation of the vibro-acoustic response signal can be used to demonstrably identify different damage types and conditions of the CFRP specimens. The sideband amplitudes and the modulation index are used as VAM characteristics in the frequency domain and the shape of the modulation envelope in time domain as well as X-ray scans to prove the damage progress after various cycles. It is shown that regardless of the lay-up, both inter-fibre failures, as well as extensive delaminations, can be reliably and sensitively detected using a simple vibration-based test setup, providing a promising nondestructive testing methodology. Finally, it is demonstrated that for certain GFRP specimens the modulation index and lifetime can be predicted after only a few loading cycles.Publicationtype: Conference Paper54 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Weak bond detection in single-lap shear bonds by evaluating vibroacoustic modulations with artificial neural networks(Composite Construction Laboratory, 2022-06); ; ; Adhesive bonding is an essential method for joining composite materials. However, the occurrence of contaminations, resulting in a not detectable weakened adhesion, persists. This study aims to uncover weak bonds with the vibroacoustic modulation method, a nonlinear ultrasonic testing method, where ultrasonic guided waves are modulated by a simultaneously applied, high amplitude pump wave. Afterwards, the measurements are evaluated by a deep learning approach. A previous dataset of 40 single-lap shear specimens (ASTM D5868-01), in which artificial interfaces in the form of circular PTFE films or release agent contaminations were introduced, was extended by a second dataset with 14 specimens of a different laminate to evaluate the robustness and transferability of the method. The proposed neural network approach can reliably recognize the bonding flaws in the training dataset and even has high accuracies on the transfer dataset, demonstrating the tremendous potential for the nondestructive evaluation of adhesive joints.Publicationtype: Conference PaperScopus© Citations 1 123 230 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Weak adhesion detection – enhancing the analysis of vibroacoustic modulation by machine learningAdhesive bonding is a well-established technique for composite materials. Despite advanced surface treatments and preparations, surface contamination and application errors still occur, resulting in localised areas with a reduced adhesion. The dramatic reduction of the bond strength limits the applicability of adhesive bonds and hampers further industrial adaptation. This study aims to detect weak-bonds due to manufacturing errors or contamination by analysing and interpreting the vibroacoustic modulation signals with the aid of machine learning. An ultrasonic signal is introduced into the specimen by a piezoceramic actuator and modulated through a low frequency vibration excited by a servo-hydraulic testing system. Tested samples are single-lap shear specimens, according to ASTM D5868-01, with artificial circular debonding areas introduced as PTFE-films or a release agent contamination. It is shown that an artificial neural network can identify various defects in the bonded joint robustly and is able to predict residual strengths and hence demonstrates great potential for non-destructive testing of adhesive joints.Publicationtype: Journal ArticleScopus© Citations 15 229 343