Accepted Manuscripts

Benjamin Eckstein, Maria Moix Bonet, Martin Bach and Claus-Peter Fritzen
ASME J Nondestructive Evaluation   doi: 10.1115/1.4039692
The increased usage of Carbon Fiber Reinforced Plastics (CFRP) for primary aerospace structures involves dealing with the principal susceptibility of composite laminates to impact loads as well as the occurrence of barely visible impact damages. One special case among the variety of impact sources is the so called blunt impact, which may cause primarily damage to the internal structure. Therefore, the assessment of debonding of stiffening elements in CFRP structures poses an attractive application case for Structural Health Monitoring by Guided Ultrasonic Waves. Wave propagation phenomena at impact damages as well as the utilized signal processing to extract a damage related feature (i.e. damage index) contribute to the sensitivity and thus to the reliability of SHM systems. This work is based on data from the EU-funded project SARISTU, where a generic CFRP door surrounding fuselage panel with an integrated sensor network has been built and tested by introducing a large number of impact damages. Wave interaction of delaminations and stringer debondings of different size and morphology in omega-stringer stiffened structures are examined to highlight the factors contributing to the sensitivity. Common damage indicator formulations for use with imaging algorithms, such as the Reconstruction Algorithm for the Probabilistic Inspection of Damage (RAPID), are applied on data from various damage cases. Furthermore, the difference in detectability of delaminations and debondings as well as the implications on imaging algorithms are examined.
TOPICS: Nondestructive evaluation, Carbon reinforced plastics, Waves, Delamination, Damage, Algorithms, Structural health monitoring, Imaging, Plastics, Ultrasonic waves, Signal processing, Sensor networks, Aerospace industry, Stress, Wave propagation, Doors, Composite materials, Inspection, Laminates, Reliability, Carbon fibers
Edward Angus, Yuntao An and Gary S. Schajer
ASME J Nondestructive Evaluation   doi: 10.1115/1.4039691
X-ray Computed Tomography is a powerful tool for industrial inspection. However, the harsh conditions encountered in production environments make accurate motion control difficult, leading to motion artifacts in CT applications. A technique is demonstrated that removes motion artifacts by using an iterative-solver CT reconstruction method that includes a bulk Radon Transform shifting step to align radiographic data before reconstruction. The paper uses log scanning in a sawmill as an example application. We show how for a known nominal object density distribution (circular prismatic in the case of a log), the geometric center and radius of the log may be approximated from its radiographs and any motion compensated for. This may then be fed into a previously developed iterative reconstruction CT scheme based on a polar voxel geometry and useful for describing logs. The method is validated by taking the known density distribution of a physical phantom and producing synthetic radiographs in which the axis of object rotation does not coincide with the center of field of view for a hypothetical scanner geometry. Reconstructions could then be made on radiographs that had been corrected and compared to those that had not. This was done for progressively larger offsets between these two axes and the reduction in voxel density vector error studied. For CT applications in industrial settings in which precise motion control is impractical of too costly, radiographic data shifting and scaling based on predictive models for the Radon Transform appears to be a simple but effective technique.
TOPICS: Nondestructive evaluation, Computerized tomography, Motion compensation, Density, Motion control, Geometry, Phantoms, Rotation, Inspection, Errors
Rohan Soman, Katarzyna Majewska, Magdalena Mieloszyk and Wieslaw Ostachowicz
ASME J Nondestructive Evaluation   doi: 10.1115/1.4039359
Composite materials find wide ranging applications due to their high strength-to-weight ratio. Due to this increasing dependence on composite materials there is a need to study their mechanical behavior in case of damage. There are several ENDT and SHM methods for the assessment of the mechanical properties each with their set of advantages and disadvantages. The paper presents a comparative study of three distinct damage detection methods (infrared thermography, neutral axis method based on optical strain sensor measurements and terahertz spectroscopy) for the detection of delamination and temperature induced damage in a simple GFRP beam-like structure. The terahertz spectroscopy is a specialized technique suitable for detecting deterioration inside the structure, but has limited application for in-service 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.
TOPICS: Sensors, Thermography, Composite building materials, Nondestructive evaluation, Damage, Composite materials, Terahertz spectroscopy, Mechanical behavior, Visualization, Fiber Bragg gratings, Calibration, Fiber optic sensors, Strain measurement, Strain sensors, Structural health monitoring, Delamination, Mechanical properties, Weight (Mass), Temperature, Glass reinforced plastics

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