Wireless/Integrated Strain Monitoring and Simulation System

Executive Summary:

Rapidly increasing use of advanced composite materials in primary aerospace structure as well as aging of current aircraft has resulted in structural health monitoring (SHM) becoming a major concern in the aerospace community. Considerable research is being directed towards development of SHM sensors and systems. The majority of the research and development effort has been in the diagnostic phase of SHM; determining what the stress / strain states are including damage events and locations. The prognostic phase of a future SHM will couple sensor-based diagnostic systems with computational structural mechanics-based predictive systems. This will result in self-sufficient systems that use built-in, distributed sensor / actuator networks that not only detect structural discrepancies and determine the extent of damage but also monitor the effects of structural usage and determine cause-based inspection. The WISMOS project was aimed at developing and testing such a diagnostic-prognostic SHM system (DPS).

A DPS application case study used a realistic composite stiffened panel representative of fuselage / wing component. The three final objectives of the project were:

1. to develop and test small cell phone size radio frequency (ZigBee) wireless strain measurement hardware unit;

2. to develop and test a data-linked full-field stress/strain mapping software module;

3. to develop and test a multi-scale (micro-macro) Progressive Failure Analysis of a composite (PFA)  and damage tolerance (D&DT) software.

Finally, these three main objectives were integrated to create a new diagnostic-prognostic SHM system.

The initial phase of the project focussed on further defining the structural elements to be tested based on Finite Element Analysis (FEA), the development of a test plan and an analysis of the exact requirements for the wireless sensor system. This resulted in a detailed design of the stiffened panels used for the study case; both an undisturbed and a disturbed panel were designed, where the disturbed panel had a diamond shaped cut in the middle. A test plan was written that used the building block approach having tests from coupon to component level. In this building block approach the tests were verified using FEA and vice versa to assure compatibility between test and analysis at the component level. The defined requirements on the wireless sensor system were the starting position for the ZigBee sensor design process.

After the initial phase of the project was finished, the second phase aimed at preparing the different aspects of the SHM system. The small cell phone size radio frequency (ZigBee) wireless strain sensors were designed and manufactured and the test specimens were produced as well. FEA was used to assess the best locations for strain measurements in the undisturbed and disturbed panels and Far Field Strain Mapping (FFSM) software was developed in order to be able to match physical sensor locations on the test specimens to nodal or element locations in the numerical analysis model. At the end of this phase all required elements of the SHM system were finished and could be brought together to test the complete system in the final phase.

In the final phase, the complete building block test program was carried out with accompanying FEA. The tests on coupon level were used to characterise the material properties used in the FEA. During the tests both the newly developed ZigBee sensors were used for strain measurements in combination with conventional strain measurement equipment in order to assess the correctness of the wireless system. The comparison between strain measurement systems was very good, which validated the use of the wireless ZigBee sensor. For the tests on component level (stiffened panels) FEA were also performed in order to verify the multi-scale (micro-macro) PFA composite D&DT software and agreement between the tests and analyses were satisfactory.

In conclusion, the case study performed in the WISMOS project has shown that it is possible to develop a SHM system that incorporates the diagnostic-prognostic approach by coupling newly developed wireless ZigBee sensor hardware with PFA composite D&DT software. Future research in this field could focus on performing in-flight tests and/or the use of embedded sensors.

Project context and objectives:

Rapidly increasing use of advanced composite materials in primary aerospace structure as well as aging of current aircraft has resulted in SHM becoming a major concern in the aerospace community. Considerable research is being directed towards development of SHM sensors and systems. The majority of the research and development effort has been in the diagnostic phase of SHM; determining what the stress / strain states are including damage events and locations.

The prognostic phase of a future SHM will couple sensor-based diagnostic systems with computational structural mechanics-based predictive systems. This will result in self-sufficient systems that use built-in, distributed sensor / actuator networks that not only detect structural discrepancies and determine the extent of damage but also monitor the effects of structural usage and determine cause-based inspection. SHM will provide earlier warnings of physical damage, which can be used to define remedial strategies before the damage compromises airworthiness. Such a tool will allow accurate monitoring and life prediction of difficult-to-inspect areas in complex structures, eliminating the need for costly disassembly.

This project addressed the development and real time test validation of an integrated hardware and software environment that was able to measure real-time in-situ strain and deformation fields using a state-of-the-art wireless sensor system to enhance structural D&DT, reliability via real-time SHM for sensorised aerospace structures. The tool was a vital added extension of existing suite of SHM and DPS.

The goal of the extended SHM-DPS was to apply a multi-scale nonlinear physics-based FEA to the 'as-is' structural configuration to determine multi-site damage evolution, residual strength, remaining service life and future inspection intervals and procedures. Information from a distributed system of wireless sensors was used to determine the 'as-is' state of the structure versus the 'as-designed' target. The proposed approach will enable active monitoring of aerospace structural component performance and realisation of DPS-based conditioned based maintenance. Software enhancements incorporated information from a sensor network system that is distributed over an aerospace structural component.

As case study DPS application a realistic composite stiffened panel representative of fuselage/wing components was selected. Two stiffened panels were manufactured and instrumented:

1. an undisturbed healthy panel; and
2. a disturbed panel with a diamond cut in the middle.

The panels were tested in compression following low-velocity impact. The sensor system output was routed and integrated with a FEA tool to determine the panel's multi-site damage locations and associated failure mechanisms, residual strength, remaining service life and future inspection interval. The FEA utilised the web / internet based GENOA progressive failure analysis commercial software suite, D&DT and reliability software capable of evaluating both metallic and advanced composite structural panels under service loading conditions.

The three main objectives of the project were the following:

1. to develop and test small cell phone size radio frequency (ZigBee) wireless strain measurement hardware unit;
2. to develop and test a data-linked full-field stress / strain mapping software module;
3. to develop and test a multi-scale (micro-macro) PFA D&DT software.

Finally, these three main objectives were integrated to create a new diagnostic-prognostic SHM system.

Project results:

The technological results of the WISMOS project were threefold. The first result was the development of the wireless ZigBee sensors. By successfully reducing the size of the sensors and verifying them by comparing the sensor output to conventional strain sensor systems, a step forward has been made in the possibility of implementing structural sensor and sensor acquisition / transmission in existing structures. Historically, flight qualified structural sensors and sensor acquisition / transmission modules required a large number of shielded wires (i.e. large diameter wire bundles) and modifications to existing internal aircraft structure (cut outs, routing holes and attachment brackets). Installation issues came up due to a posteriori installation of the sensor systems. Application of the wireless ZigBee sensor system internal to an airframe will minimise these problems.

The ZigBee sensor system (WN128 sensor hardware) uses an off-the-shelf strain gauge, sensor data transducer and ZigBee wireless chip to communicate the strain data wirelessly to a PC. The design and parts selection for the system were kept relatively simple in order to keep the manufacturing cost at a minimum. The ZigBee chip already supports MCU and memory within the ZigBee chip so additional parts are not needed for the system. The ZigBee sensor nodes are combined with a USB receiver to collect sensor data and WN128 control software (Java and Windows). Consequently, the firmware to perform numerical analysis is tailored to fit into the spec.

The second main technological result of the WISMOS project was the development of FFSM software. This software can be used to assess the state of a structure based on the strain values at the locations of the wireless sensors by combining the FFSM software with advanced PFA composite D&DT numerical analysis. This combination of advanced predictive methods in numerical analysis with the miniaturised in-situ real-time measurement of loading history and damage development in aero structures under operating conditions is a goal that the industry seeks.

The third result of WISMOS was the validation of the complete diagnostic-prognostic SHM system by performing the tests in the building block approach defined in the test plan (going from coupon to component level). Testing of the stiffened panels was performed using a 250 kN capacitance testing frame with a special head for compression testing. During testing the strain generated at various locations was measured using both wired and wireless strain monitoring devices. Both panels failed as expected in different load levels, the first one at 160 kN and the second under 106 kN. The curves of the applied displacement versus the generated strain as well as the generated strain were recorded and all of them are presented as a function of the developed load on the specimen.

In general, good agreement between the wired and the wirelessly acquired strain signals was found. Especially in the case of the healthy panel the recorded strain from both setups appears to be quite similar. However, in the case of the panel with the diamond cut the strain recorded from the ZigBee strain sensors is significantly higher. This is due to the fact that the large delamination crack appeared at the side where the ZigBee sensors were located. This was an important index revealing the drastic reduction of the stiffness on the skin due to the presence of delamination. In addition, ZigBee sensors have been evaluated independently with a three-point bending test and the wireless system was compared to a conventional strain measuring system during a test that was carried out at Imperial College in London. The wireless sensor system did perform as expected and the simplicity of use of the system was clearly evident.

Another part of the project was not only to carry out tests, but to also perform numerical analyses to see whether the two methods are in good agreement. Based on the tests on coupon level the calibrated material properties of the material used in the stiffened panels were found. After the material characterisation was the calibrated data was used to perform progressive failure analysis of the undisturbed and disturbed (diamond cut) stiffened panels using GENOA in combination with Abaqus FEA. Delamination behaviour of the stiffeners was analysed with the help of linear elastic fracture mechanics in Abaqus FEA. The analysis results showed good comparison with the experimental analyses results both in terms of qualitative comparison (failure / fracture mode) as well as quantitative comparison (load versus displacement and strain comparison).

In conclusion, the objectives of the WISMOS project have all been achieved. Crucial steps in the development of diagnostic-prognostic SHM systems have been taken. Future research could focus either on testing the current system in-flight or further develop the sensor system by embedding the strain sensors.

Potential impact:

Flight qualified structural sensors and sensor acquisition/transmission modules have historically required a large number of shielded wires (i.e. large diameter wire bundles) and modifications to existing internal aircraft structure (cutouts, routing holes and attachment brackets). Installation issues came up due to a posteriori installation of the sensor systems. Application of a wireless sensor system internal to an airframe would minimise these problems.

The combination of advanced predictive methods in numerical analysis with the miniaturised in-situ real-time measurement of loading history and damage development in aero structures under operating conditions is a goal that the industry seeks. The deliverables of the WISMOS project can be used by industry users and suppliers to optimise their methods and improve life assessment.

Development of a wireless structural health sensor network integrated with structural life prediction software has many commercial, military and space applications. It can also be used for commercial civilian legacy airframe applications such as determining the durability and damage tolerance of aging aircraft, both commercial and military. The proposed SHM system can greatly benefit new commercial aircraft by converting inspections to need-based rather than calendar-based with specificity of inspection sites. Minimised inspections result in cost and time savings for the operator. In space applications, such as satellites or deep space probes, the proposed system will give engineers early warning of potential problem areas allowing early remedial action prior to structural failure. The proposed system can also be applied to non- aerospace products. Buildings, infrastructure, ground vehicles, ships and marine structures can also benefit from the proposed SHM system. Identification of potential structural problems as well as remaining life and residual strength will enable owners/ operators to alter operating conditions to a safe level, perform need-based inspection and repairs prior to catastrophic failure. It will effectively improve the safety and reliability of many different types structures by quantifying risks associated with missed defects.