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Each of the methods have their own set of advantages and disadvantages. The terahertz spectroscopy is a specialized technique suitable for detecting deterioration inside the structure, but is not suitable for in performance monitoring. Similarly the infrared thermography technique in the active domain may be used for in situ monitoring but not in in-service assessment. Both methods allow the visualization of the internal structure and hence allow identification of the type and the extent of damage. Fibre optic sensors (especially FBG) due to their small diameter and no need of calibration can be permanently integrated within the sample and applied for continuous dynamic strain measurements. The measured strain is treated as an input for neutral axis (NA) method, which as a damage sensitive feature may be used for in-service monitoring but gives absolutely no information about the type and extent of damage.
The results for damage detection based on proposed comparative studies give a complete description of the analyzed structure.
The experimental and numerical investigations were performed on two rectangular samples of two glass fibre reinforced composite elements bonded together using an adhesive commonly used in the bonding or repair of aircraft elements. Moisture contamination due to diffusion process changes the volumetric properties of the material induced strain. This strain was measured by FBG sensors embedded in the adhesive layer parallel to the main axis of the sample. The behaviour of the adhesive layer in the analysed sample was also modelled using the finite element commercial code ABAQUS.
Numerical and experimental results confirm the utility of FBG sensors for moisture detection in the adhesive layer even when the amount of moisture is around 2% of the sample weight.
Investigated damage is in the form of mechanical failures as cracks, delaminations, debonding, voids. Also methods dedicated to thermal degradation, moisture and chemical contamination are shown. Presented methods are also suitable for performance of bonded joints assessment. Problem of external factor (temperature, load) on investigated methods is also discussed in this paper. The characteristic of each method is summarized by a critical look. Promising combination of selected techniques should lead to an innovative approach to ensure safety operation of structures. All problems have been dealt with a hybrid experimental – numerical approach.
Elastic waves were excited using a piezoelectric transducer. Full wave-field measurements using scanning Laser Doppler vibrometry have been performed. This approach allows the detailed analysis of elastic wave propagation in composite specimen with linearly changing thickness. It will be demonstrated here experimentally, that the wave velocity changes significantly due to the tapered geometry of the structure. Hence, this work motivates the theoretical and experimental analysis of adiabatic mode propagation for the purpose of Non-Destructive Testing and Structural Health Monitoring.
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