STRAINMON aimed to develop and establish a methodology for the strain monitoring of composite fuselage stiffened panels using embedded and/or bonded Fiber-Optic Bragg Grating (FOBG) sensors and strain gauges. The proposed work-programme of STRAINMON is summarised as follows:
- Definition of the FOBG system requirements and specifications based on the nature of the problem and the available technology using progressive damage modelling. The number of sensors will be decided, and the network of sensors will be designed.
- Description of the methodology for integration of sensors related to the autoclave manufacturing process by ensuring measurement capability of the sensors, integrity of sensors, and integrity of the parent composite material.
- Design of the test plan and design and fabrication of the test-rig,
- Execution of the series of the following tests to establish the proposed SHM methodology:
- Compression tests to failure on undamaged panels to characterize their reference compressive behaviour,
- Low velocity impact tests on selected critical locations of the panel in order to create impact damage,
- Compression tests to failure on the impact-damaged panels to evaluate the residual stiffness and strength of the panels,
- Fatigue of impact-damage panels up to a certain number of cycles in order to create additional fatigue damage,
- Compression tests to failure of panels subjected to impact and fatigue to evaluate the residual stiffness and strength data of the impacted and fatigued panels.
After each test series, NDT inspection of the panels will be conducted in order to detect the type, extent and location of damage induced.
Each of the above tasks met an activity of the topic.
A reliable and effective data acquisition system was built to take sensor measurements at appropriate rates during the whole duration of the testing scheme and from all sensors placed on the panels simultaneously.
STRAINMON's consortium comprises VZLU (CZ) and UNIPatras (GR).
The aim of the STRAINMON project was to develop methodology procedures needed for the implementation of an SHM system, based on FOBG sensors and/or standard strain gauges (SGs), onto/into a CFRP fuselage stiffened panel as well as to implement the system. The role of the FOBG sensor system is to monitor the structural integrity of impacted and fatigued panels under compression loading. The FOBG and/or SG sensors were placed (embedded or bonded) at various locations into and/or onto the structure.
A methodology for integration (embedding) of the selected sensors (FOBGs) in the structure, related to the selected manufacturing process (autoclave process), was developed by HAI. In this procedure, special attention was given to the ingress/egress points as well as to the definition of appropriate interfaces (connectors) of the testing equipment. An appropriate testing plan (static & fatigue loading scheme) for demonstrators of a typical fuselage panel was defined in order to fully capture the structural integrity of the panel. Associated test rigs/jigs/tools, necessary for the correct application of loading of the panels, were designed and built. Two panels were impacted by low velocity impact with various energies corresponding to BVID (Barely Visible Impact Damage) and VID (Visible Impact Damage) by UNIPatras. The extent of impact damage progression under fatigue and compression loading was evaluated during the panels mechanical testing. The static and fatigue tests including data measurement and evaluation were conducted at VZLU. The NDI was conducted before the mechanical testing by UNIPatras and HAI and during and after mechanical testing by VZLU. Both ultrasonic and laser shearography method for NDI were used.
The comparison between the predicted and actual behaviour of panels were performed. The strength of reference panel without any defects was highest in comparison with other results. The lowest strength was obtained for panel containing the manufacturing defects and BVID. The FOBG data is in good agreement with the SG and optical measurements.