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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