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  STRUCTURAL HEALTH MONITORING
  Tomography based MTMR Approach
  Phase Reconstruction based STMR Approach  
  Sensor Embedment Issues in Composites
  Smart Pipes-Long Range Magnetostriction Sensors
     
 
     
    Tomography based MTMR Approach
 
 
 
  Multi Transmitter Multi Receiver (MTMR) technique with iterative tomographic reconstruction algorithms were proposed to image low-velocity impact damages (Barely Visible Impact Damages, BVID) in fiber reinforced aerospace composite structures. Piezo sensors in SFPS patches were arranged such that conventional Cross-Hole Tomography (CHT) as well as Modified Cross-Hole

Tomography (MCHT) can be carried out on composite plate and structures. Using MCHT, the lateral extent of the impact damages on composite structures like wing and aileron were imaged. Damages with low impact energies, 11 joules on 2 mm thick graphite quasi-isotropic plate structure were imaged with reasonable accuracy. The conventional CHT was deployed in a region of large aspect-ratio such as stiffeners and bolted joints. It was observed that disbonds of stiffener in Elevon and other structures may be effectively monitored by this method.

 
Tomography based MTMR Approach
 
Composite Skin-Stiffener Joint
Composite Skin-Stiffener Joint
Ultrasonic C-scan of poor Joint

Ultrasonic C-scan of poor Joint

 
Lamb wave monitoring result
Lamb wave monitoring result
       
 

B. V. Soma Sekhar, K. Balasubramaniam, C.V. Krishnamurthy, Structural Health Monitoring of Fiber-Reinforced Composite Plates for Low-Velocity Impact damage using Ultrasonic Lamb Wave Tomography, ,Journal of Structural Health Monitoring 5(3) 243-53 (2006).

     

 

 

 
   
Phase Reconstruction based STMR Approach
 
 
   
A method for structural health monitoring SHM of anisotropic plate like structures using a STMR (single transmitter- multiple-receiver) array of transducers has been developed. The method uses a new phase reconstruction algorithm, which takes into account the directional dependence of the Lamb wave dispersion characteristics in anisotropic plates, for processing data obtained from the STMR array.

A system which is conceptually similar to the STMR array, but using only a single PZT transmitter and receiver, has been shown to be capable of locating damage of dimensions much smaller than the plate dimension with good accuracy. The phase reconstruction algorithm works well in the “far field,” but may show distortions/artifacts in the “near field” which is the region within approximately 2–3 times the sensor configuration diameter. Also, more experiments, with different flaws, need to be performed to assess the efficacy of this technique to image other types of defects.

It has been demonstrated elsewhere that a multiple transmitter-multiple-receiver array system provides a better signal to noise ratio, which results in images with improved resolution and clarity for plates made from isotropic materials.

But a STMR system was chosen here since it requires simpler electronics and a reduced footprint desired during the SHM of aerospace structures. Also, other types of piezo-materials like PVDF film sensors which have good reception capabilities but poor transduction capabilities can be used as receivers in an STMR system along with a PZT transmitter.
 
 


Architecture for the Lamb wave based SHM system.
Architecture for the Lamb wave based
SHM system.
 
Reconstruction of edges of composite plate within the 12 dB range.
Reconstruction of edges of composite
plate within the 12 dB range.

Reconstruction of defect _hole_ in the composite plate within the 20 dB range.
Reconstruction of defect _hole_ in the composite plate within the 20 dB range.

  1. Jagannathan Rajagopalan, Krishnan Balasubramaniam, and Krishnamurthy V. C., The Journal of the Acoustical
    Society of America (2006 )Volume 119, Issue 2, pp. 872-878.
  2. J. Rajagopalan,  K.Balasubramaniam, and C. V. Krishnamurthy, Smart Mater. Struct. 15 1190–1196 (2006)
    3. A. Muralidharan, C.V.Krishnamurthy, and K. Balasubramaniam, Smart Structures and Systems in press (2007)
 

 

 
    Sensor Embedment Issues in Composites
 
 
  
     Use of conventional non-destructive testing methods for large plate like structures is time consuming and therefore not cost effective. Structural Health Monitoring (SHM) of such structures is seen as a new paradigm that will reduce maintenance costs and increase safety. Center for Non-Destructive Evaluation (CNDE) Lab has developed an SHM process that involves the observation of a system over time using advanced sensing, processing, and integrated hardware with a novel data interrogation software package.

‘Smart Sensors’, thin Piezo Wafer Active Sensors (PWAS, 0.2 mm thick and Φ 10 mm dia) were used to make Smart Flexible Piezo Sensor (SFPS) Patches that could be embedded/bonded on the structures. For structures with multiple-curvatures and complex geometry, patches can be custom designed with special shapes to provide a perfect bonding. An innovative method of designing the SFPS patches, such that it can hold its shape upon fabrication has been developed at CNDE lab.

Mechanical tests on composite coupon specimens with and without embedded patches were conducted to assess the change in structural integrity due to inclusion of the SFPS patches. Static loading and impact tests were performed on graphite/epoxy coupon specimens with a lay-up [0/+45/-45/90]4 and [0/90]4.  The test results indicate that the presence of these Patches does neither noticeably affect the strength of the host composite structure nor promotes delamination. Electromechanical Impedance (EMI) technique and non-contact measurements using Laser Interferometer Vibrometer for the effective embedment and bonding of SFPS patches were carried.

 
 
 

Typical Patch for MCH
Typical Patch for MCH
 
A results obtained on the impacted composite laminate using the MCH based Tomorgraphy recounstruction using the PWAS Patch.
A results obtained on the impacted composite laminate using the MCH based Tomorgraphy recounstruction using the PWAS Patch.
 
 
 
 

 

 
    Smart Pipes-Long Range Magnetostriction Sensors
 
 
  
      Long-range, torsional guided waves generated in pipes using magnetostrictive sensors (MsS) have great potential for applications to the Structural Health Monitoring (SHM) of hard-to-inspect pipes. This paper reports an improved MsS technique (when compared to related techniques currently used for the NDT of pipes) that uses polymeric magnetic tape material that is suitable for use in a variety of industries as an SHM tool for pipes.

Improvements include increased efficiency, reduced cost and increased long-term survivability of the sensor system. Transduction efficiency was increased by reducing the sensor eddy current losses and by using a field concentrator strip. For long-term monitoring, a low-cost magnetic oxide based MsS material (video recording tape) having the required magnetic properties was used. The MsS strips were oriented to generate non-dispersive torsional guided ultrasonic waves that propagate long distances with minimal mode conversion.

Further, considering both safety and long-term survivability of the sensor, low-power ultrasonic instrumentation was developed and tested. Measurements reported here demonstrate the sensitivity of this sensor to both radial notches (saw cuts) and drilled holes. Results also show that magnetic anisotropy of the strip plays a role in generating torsional waves. It is envisioned that results obtained from the present study will significantly enhance the ability to monitor the long-term structural health of piping system.

 
 


Fig 1.Location of the transmitters and receiver on the Al pipe.
Fig 1.Location of the transmitters and receiver on the Al pipe.
 
Fig 2. Experimental arrangement of in-house built, low-power ultrasonic instrumentation
Fig 2. Experimental arrangement of in-house built, low-power ultrasonic instrumentation
 
Fig 3. signals from the 29% notch and a 4.9 mm drilled hole
Fig 3. signals from the 29% notch and
a 4.9 mm drilled hole
 

References :1. E.  Kannan, B. W. Maxfield and Krishnan Balasubramaniam, “E.  Kannan, B. W. Maxfield and Krishnan Balasubramaniam”,
Review of Progress in Quantitative NDE, July 22 – July 27, 2007 Colorado School of Mines, Golden, Colorado, USA.

 




















































































































































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