Manufacturers in the aerospace industry aim to replace conventional materials with light-weight composites structures. These new materials need new concepts of reliability assurance and maintenance to ensure that damages do not go undetected. It is very important to get a real-time update of the structural integrity. Some of the questions like, Is there damage in the structure?
How can it be located? What is the intensity of the damage? Can it affect the structural integrity of the machine? How long can the structure function on its current condition? can be answered now by implementing Structural Health Monitoring (SHM).
According to the airbus: “SHM is the continuous, autonomous in-service monitoring of the physical condition of a structure by means of embedded or attached sensors with a minimum manual intervention, to monitor the structural integrity of the aircraft”.
SHM is an emerging technology, dealing with the development and implementation of damage detection strategy, monitoring, inspection, damage assessment and residual life prediction of smart structures for aerospace, civil, and mechanical engineering infrastructure.
The SHM process involves the observation of a system over time using periodically sampled dynamic response measurements from an array of sensors, the extraction of damage-sensitive features from these measurements, and the statistical analysis of these features to determine the current state of the system’s health.
For long term SHM, the output of this process is periodically updated information regarding the ability of the structure to perform its intended function in light of the inevitable aging and degradation resulting from operational environments.
After extreme events, such as 2015 Nepal earthquake, SHM has been used for rapid condition screening to gain reliable information regarding the integrity of the current structure without compromising the safety of the inspector and the building itself.
A way to illustrate a SHM system is to compare it to the human nervous system. The idea of SHM is to build a system that is similar to the human nervous system. It will work in the same manner but will differ in some cases: If we break our leg, it may be that we do not notice it or there would only be a slight pain.
This is because the bones have no nerves, i.e. no “SHM System”. In the case of aircraft, the sensors (that are the nerves) must be at the place where an area is to be monitored to ensure the structural integrity. These sensors monitor a structure over a period of time, extract damage sensitive features from the sensors and analyze these features to determine the current state of the structure.
In case of damage, the sensors directly identify the location and follow-up actions can be taken. SHM is the technology that will allow the current time based preventive maintenance philosophies to evolve into potentially more cost effective condition-based predictive maintenance philosophies. The concept of condition-based maintenance is that a sensing system on the structure will monitor the system response and notify the operator that damage has been detected.
Aircraft manufacturers are looking forward to reduce maintenance costs and that has been pushing researchers to integrate NDT technology in the aircraft structure itself.
By doing so, i.e. replacing conventional NDT with SHM systems, some major benefits that can be expected are:
- Increased availability of the aircraft
- Effective assessment of actual damage events
- Reduced costs of life-cycle and total ownership
- Eliminate unnecessary inspections
- Provide Accurate Information for Vehicle Life Extension
- Reduced logistics
- Increased safety and reliability
- Long-term cost savings and
- Extended fleet life.
Thus, SHM will enable maintenance concepts. The new ultimate goal of structural health monitoring Quality & Assurance Implementing SHM technology is damage prognosis, which estimates the remaining service life of a structure given the measurement and assessment of its current damaged state and accompanying its predicted performance in the anticipated future loading environments.
Every industry is interested in detecting degradation and deterioration in its structural and mechanical infrastructure at the earliest possible state and in predicting remaining useful life of the systems.
Damage diagnosis and prognosis solutions can be used to monitor systems to confirm system integrity in normal and extreme loading environments, to estimate the probability of mission completion and personnel survivability, to determine the optimal time needed for preventive maintenance, and to develop appropriate design modifications that present observed damage propagation. Many industries are moving towards selling maintenance service rather than final products.
This paradigm shift in business practice is imposing a heavy reliance on reducing maintenance cost through condition-based monitoring and performance prediction. However, On-board real-time sensing systems are a critical component of and SHM system. Such sensing systems will minimize the need for periodic Non Destructive Evaluation (NDE) inspections, or at least focus these inspections to specific areas where damage was indicated.
SHM sensors must be able to withstand harsh aerospace operating environments, while having minimal size, weight, and power requirements. A number of SHM sensor technologies are being researched. These include both active and passive ultrasonic methods, fibre-optic sensors, carbon nano-tube sensors, and wireless sensors. However, this apparently easy solution requires a quite complex research and implementation effort using well-coordinated collaborations of many disciplines and expertise.
A significant challenge for SHM is to develop the capability to define the required sensing system properties before field deployment and, if possible, to demonstrate that the sensor system itself will not be damaged when deployed in the field. If the possibility of sensor damage exists, it will be necessary to monitor the sensors themselves.
This monitoring can be accomplished either by developing appropriate self-validating sensors or by using the sensors to report on each other’s condition. Sensor networks should also be ‘fail-safe’. If a sensor fails, the damage identification algorithms must be able to adapt to the new network. This adaptive capability implies that a certain amount of redundancy must be built into the sensor network.
The development of robust SHM technology has many elements that make it a potential ‘grand challenge’ for the engineering community. Most of SHM systems for local area monitoring still face some challenges when they are applied on the commercial aircrafts in the real world. Some major challenges are as follows:
(1) Damage quantification, including the well-defined Probability Of Detection (POD);
(2) Robust and reliable system, including the survivability of sensors and miniaturized lightweight hardware;
(3) Environment compensation, including the practical procedures for obtaining baseline signals;
(4) Airworthiness compliance, including the integration SHM system with aircrafts.
(5) Self-diagnostics, including adaptive algorithms to compensate for damaged sensors. The use of SHM will revolutionize the current structural design practices and processes and is intended to be incorporated during any future aircraft design. The structural design is no longer considered as mechanical product design but a system design in which both mechanical and electronic components (sensors) interact. Both the structure and the health monitoring system have to be designed in parallel. Since, SHM can play an important role in increasing confidence in predicting the fatigue and damage tolerance characteristics of advanced materials, any increase in material allowable will have a direct impact on the overall structural weight reduction.
Conclusively, SHM as a concept is matured and now identified as one of the key enabling technologies to ensure the integrity of future aircraft structures.
It along with advanced alloys, composites and hybrid materials will revolutionize both airframe and engine structures of future aircrafts. It can help in increasing the structural allowable with higher confidence removing the conservatism in the current designs. This will reduce structural weight leading to reduced acquisition and maintenance costs.
SHM enabled structures need to be designed differently using integrated systems approach considering both mechanical aspects of structure and sensor technologies.
The sensor integration with structure is very critical and sensor locations should not become damage initiation locations. Use of SHM can translate to over 40% of reduction in the maintenance cost through inspection time and cost savings.
Thus SHM is one of the enabling technologies to revolutionize the future aircraft design, development and maintenance. However, significant future developments of this technology will, in all likelihood, come by way of multi-disciplinary research efforts encompassing fields such as structural dynamics, signal processing, motion and environmental sensing hardware, computational hardware, data telemetry, smart materials and statistical pattern recognition, as well as other fields yet to be defined.
