Aerospace applications
A flight approved system for FBG interrogation for Structural Health Monitoring (SHM) will be developed and tested. The design of the new system is revolutionary (light weight, compact and solid-state). SHM includes all monitoring aspects which have a direct link to structure integrity. SHM offers reduced cost of ownership through condition-based maintenance, extended service life (Fatigue Life Assessment), and reduced risk of failure. Automated inspection means time savings and a reduction of the Human Factor. Fibre optical systems are lightweight, small, and easy to integrate into/onto composites and other materials. Due to the fact that many sensors can be integrated into one optical fibre, the sensor network becomes much simpler when compared to electrical sensor networks. Fibre optical sensors are EMC insensitive and safe. These sensor systems can be used for predictive health monitoring and realtime monitoring during flight.
Second test flight
TFT-FOS and the Dutch National Aerospace Laboratory (NLR) made a first successful flight test with the Deminsys C interrogator on October 24th 2008. This first flight was performed with the PH-NLZ test aircraft, a Fairchield Metro II twin turboprop aircraft with pressurised cabin. It demonstrate the initial feasibility, or Proof of Concept, of the FBG sensor technology and especially the Deminsys interrogator for use in an aircraft environment
In 2010 the electronics of Deminsys C was re-designed and re-engineered to move towards the flight safety standards for in-flight equipment sponsored by the province of Noord-Brabant, the province of Zeeland, the department of Economic Affairs and private investors. Prior to the second flight test a flight safety check was performed on ground on site at NLR and Deminsys D / Ultra showed full compliance with the flight safety requirements. Qualifying for this check was a prerequisite for carrying out a second flight test.
After the flight safety qualification the flight test was scheduled and finally on December 30th 2010 the Ultra made is maiden flight on the PH-LAB test aircraft, a Cessna Citation II jet aircraft with pressurised cabin. The aim of this flight was to validating Deminsys Ultra in-flight against a reference data acquisition system. Analysis of the inflight recorded data of both systems demonstrated that Deminsys successfully reached a Technology Readiness Level equal to five (TRL 5). According to NASA and ESA definition this means ‘Component and/or breadboard validated in a relevant environment’. The reports of flight safety qualification and the TRL5 validation are available on request.
Although TRL5 target is successfully realised, TFT-FOS already set its next goal “to develop and realise an airworthy system which will be fully qualified for fixed and rotary wing aircraft operation”.
Damage identification with optical fibres in composites
During the last year considerable effort has been put in the application of optical fibres with Fibre Bragg Grating sensors for damage identification in composite structures. This is a collaboration between TFT-FOS, the National Aerospace Laboratory (NLR) and the University of Twente.
Aircraft require regular costly inspections to guarantee their safety. This currently mainly relies on manual non-destructive inspection methods. During the last century, a lot of research has been dedicated to more automated systems called Structure Health Monitoring (SHM), which consists of a network of sensors to detect changes in the physical and/or geometric properties of a structure from data gathered at two different states, a reference state, considered as the undamaged state, and the current state. Changes can be caused by damage present in the structure. SHM techniques can be operated on-line during the flight or off-line on the ground and can be focused on global inspection of large surface areas or
on local inspection of highly critical areas (hot spots). The main objectives of SHM are to reduce the cost of ownership and to improve the system operational availability.
The objective was to examine vibration-based response characteristics of a structure, such as: natural frequencies, mode shapes, modal strain energy, and determine which of these can serve as a damage indicator in a SHM system based on optical fibres. Several vibration-based damage indicators were evaluated for damage types relevant for aerospace applications such as cracks in aluminium structures and impact damage and stringer debonding for composite structures.
This research focused on the methodology to extract damage information by comparing the measured response signals on the current state of a structure against an initial (undamaged) state of the structure. In general, four consecutive levels of damage identification can be distinguished of increasing complexity:
- Determination that damage is present in the structure
- Determination of the location of the damage
- Quantification of the severity of the damage
- Prediction of the remaining service life of the structure
Besides optical fibres various other sensors are available. Optical fibres have a number of advantages for application in an aerospace structure, of which the most important are:
- Light weight
- Tolerant for harsh environments, such as temperature and chemical components
- Long term stability and durable
- Completely passive
- No interference with other signals in the aircraft
Numerical results obtained for a cracked aluminium plate and a two-stiffener composite plate with impact damage and stringer debonding have demonstrated that the presence of various damage types can be detected and located with a limited number of sensors by means of the change in modal strain energy, yielding a level 2 structural health monitoring system. For a sufficiently large number of sensors the damage size (level 3) can be determined as well.
The experimental investigation has been performed by the University of Twente on a cantilever composite beam structure. Dynamic properties, like natural frequencies and strain mode shapes, were obtained by using a forced vibration set-up including optical fibre Bragg gratings and by applying Operational Modal Analysis. Currently, this work is extended to the two-stiffener composite plate structure.
Deminsys Ultra
Over the last 9 months, a complete redesign of Deminsys electronics and firmware has taken place. The first stage of the process involved collecting requirements from various sources. It soon became clear that it would not be feasible to incorporate all of the elements suggested, and so a clear set of requirements was selected. It was agreed that the remaining requirements would follow at a later stage, with a particular customer or clear roadmap as guideline.
The redesign focused on developing an industrial solution suitable for the majority of high-tech applications. Limited additional requirements were added to anticipate requests from the aviation sector and other branches of industry. This resulted in the following three interrogators: Industrial, Ultra and Python.
The complete specifications of the Industrial are also available in the Ultra and the Python. However the Ultra is smaller, lighter and satisfies various environmental requirements stated in the RTCA/DO-160-F standard (Environmental Conditions and Test Procedures for Airborne Equipment), which makes it suitable for research aircraft flights.
SHM and impact detection for primairy aircraft structures
High-speed, synchronous interrogation using multiple fibre bragg grating sensors enables design and delivery of robust inspection and analysis systems.
The increasing use of thermoplastic carbon fibre-reinforced plastic (CFRP) materials in the aerospace industry for primary aircraft structures, such as wing leading-edge surfaces and fuselage sections, has led to rapid growth in the field of structural health monitoring (SHM). Impact, vibration, and load can all cause failure, such as delamination and matrix cracking, in composite materials. Moreover, the internal material damage can occur without being visible to the human eye, making inspection of and clear insight into structural integrity difficult using currently available evaluation methods. Here, we describe the detection of impact, its localization, and its potential damaging effects on materials and structures by high-speed interrogation of multiple-fibre bragg grating (FBG) sensors mounted on acomposite aircraft component. Read more.
First test flight
Technobis Fibre Technologies and the Dutch National Aerospace Laboratory (NLR) made a first test flight with the Deminsys optical fibre measurement system using the NLR test aircraft PH-NLZ on October 24th 2008. The NLR’s PH-NLZ test aircraft is a Fairchield Metro II, a twin turboprop aircraft with pressurised cabin. The PH-NLZ is equipped with independent electrical circuits and has special adaptations and accommodations for testing of measurement equipment and sensors. Read more.