Newsletter #22 - Structural Health Monitoring Test Validation

a) Board Only	b) LED, External Antenna
Figure 1. Wireless strain gauge sensor with multiple source of power input

a) Pristine - undamaged panel	a) Damaged panel (diamond cut in center)
Figure 2. Fabricated composite stiffened panels

Background: AlphaSTAR Corp. developed and validated wireless integrated strain monitoring and simulation system for Structural Health Monitoring (SHM) of composite structures [1]. The system is made up of hardware and software components for use in damage detection, maintenance, and repair of critical structures. The hardware consists of wireless strain gauge sensors (Figure 1) and Zigbee network. The software is an extension to an existing suite of an SHM system based on a diagnostic-prognostic system (DPS) methodology [2].  The SHM-DPS system uses multi-scale nonlinear physics-based progressive failure analysis to determine residual strength, remaining service life, and inspection intervals and maintenance procedures. The system was validated by applying it to monitor the performance of pristine and damaged composite aircraft panel structures (Figure 2).   

Objective: The newsletter describes a test verified wireless strain monitoring system for structural health monitoring of composite structures. The capability is demonstrated on aircraft composite stiffened panels that were manufactured, instrumented, and tested under compressive loading. The objective here is demonstrate the SHM-DPS system and to evaluate the "as-is" state of the structure and validate the wireless strain measurement.  

Figure 3. DPS interactions for SHM - strain data from wireless sensor: red and green light threshold strain (red means strain limit is exceeded)

 Approach:  The integrated structural health monitoring system transmits, stores, and processes strain data from a network of wireless sensors to monitor damage events at single or multi-site critical locations. Damage events that are captured include matrix cracking, delamination, and fiber tensile and compressive failure[3,4]. The multi-scale damage analysis uses commercial finite element software and is capable of handling tape, 2D, and 3D composite architectures. It comes with built-in strength and strain based failure mechanisms.  The sensor monitors the structure's local deformation and transmits strain data in real time for display over a network to allow remote monitoring of the process status as shown in Figure 3.  When damage is detected, the system provides a range of material options for repair and overall properties, life, and residual strength are recalculated.     

 Application:  A building block strategy was used to validate the DPS-SHM system. ASTM coupons, single stiffener (element level), and stiffened panels (sub-component level) made from carbon fiber reinforced polymer material were manufactured, instrumented, and tested to failure as described in reference [5]. The ASTM coupon tests were used to characterize the composite material by deriving a validated set of fiber/matrix/ply properties for use in the SHM of the panels. The first panel was pristine ("as-is" and damage free) while the second panel sustained some damage in the form of a diamond cut in the center.  Figure 4 shows the stiffened panels that were

Figure 4. Instrumentation and Testing of Composite Panel

instrumented with wired and wireless strain gauges. Figure 5 shows the damage in the panel with a diamond cut after testing. The analysis simulated properly the debonding of the stiffeners observed in the test.  The diamond cut contributed to a reduction in the maximum load of 42% as compared to that of the pristine panel.   The load displacement from analysis and validated with test is shown in Figure 6.  The wireless strain data collected as the panel was loaded in compression is shown in Figure 7. Changes detected in the strain pattern are indication of changes in material properties and in boundary conditions of the structure.   Strain collected from test wirelessly is stored and compared to threshold values from multi-scale simulation to determine potential risk to the structure. The technology is generalized and is applicable to all types of structures including those used in aerospace vehicles, automotive (pressure vessels), and infrastructure (bridges).

Figure 5.  Damage sustained in the panel with diamond cut

Figure 6. Stiffened panel with diamond cut - load displacement from test and analysis

Figure 7. Wireless Strain - Panel with Diamond Cut

References:

1.       Hiroshi Ide, Frank Abdi, Chau Dang, Tatsuya Takahashi, Bruce Sauer," Wireless-Zigbee Strain Gage Sensor System For Structural Health Monitoring",  SPIE DEFENSE SECURITY + SENSING Photonics in the Transportation Industry: Auto to Aerospace II Conference DS203 13 - 17 April 2009, Orlando, FL USA.

2.       A. Mossallam, F. Abdi, J. Qian, R. Miraj "Development of a Diagnostic/prognostic System (DPS) For Monitoring the Performance of a repaired Composite Military Bridges", proceeding of SPIE, Volume 6178.

3.       GENOA Durability and Damage Tolerance of Composite Structures, AlphaSTAR Corp., www.alphastarcorp.com, Long Beach, CA 2013.

4.       M. Garg, F. Abdi, and E. Ravey, "Multi-Physics And Multi-Scale Progressive Failure Analysis Approach to Predict Thickness Effect on Compressive Strength of Carbon/Epoxy Laminates".  SAMPE 2011 Long Beach, CA, May 23-26, 2011.

5.       M. Talagani, F. Abdi, D. Saravanos; N. Chrysohoidis, K. Nikbin, R. Ragalini, and I. Rodov; "Damage Tolerance Modeling And Validation Of A Wireless Sensory Composite Panel For A Structural Health Monitoring System", SPIE Defense, security and Sensing 2013 Conference DS207, Baltimore, MD.