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Sensors 2016, 16(6), 777; doi:10.3390/s16060777

Monitoring Pre-Stressed Composites Using Optical Fibre Sensors

1
Engineering System Department, Royal Military College of Science, Shrivenham SN6 8LA, UK
2
School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UK
*
Author to whom correspondence should be addressed.
Academic Editor: Jandro L. Abot
Received: 19 March 2016 / Revised: 19 May 2016 / Accepted: 20 May 2016 / Published: 28 May 2016
(This article belongs to the Special Issue Integrated Structural Health Monitoring in Polymeric Composites)

Abstract

Residual stresses in fibre reinforced composites can give rise to a number of undesired effects such as loss of dimensional stability and premature fracture. Hence, there is significant merit in developing processing techniques to mitigate the development of residual stresses. However, tracking and quantifying the development of these fabrication-induced stresses in real-time using conventional non-destructive techniques is not straightforward. This article reports on the design and evaluation of a technique for manufacturing pre-stressed composite panels from unidirectional E-glass/epoxy prepregs. Here, the magnitude of the applied pre-stress was monitored using an integrated load-cell. The pre-stressing rig was based on a flat-bed design which enabled autoclave-based processing. A method was developed to end-tab the laminated prepregs prior to pre-stressing. The development of process-induced residual strain was monitored in-situ using embedded optical fibre sensors. Surface-mounted electrical resistance strain gauges were used to measure the strain when the composite was unloaded from the pre-stressing rig at room temperature. Four pre-stress levels were applied prior to processing the laminated preforms in an autoclave. The results showed that the application of a pre-stress of 108 MPa to a unidirectional [0]16 E-glass/913 epoxy preform, reduced the residual strain in the composite from −600 µε (conventional processing without pre-stress) to approximately zero. A good correlation was observed between the data obtained from the surface-mounted electrical resistance strain gauge and the embedded optical fibre sensors. In addition to “neutralising” the residual stresses, superior axial orientation of the reinforcement can be obtained from pre-stressed composites. A subsequent publication will highlight the consequences of pres-stressing on fibre alignment, the tensile, flexural, compressive and fatigue performance of unidirectional E-glass composites. View Full-Text
Keywords: pre-stress; composites; autoclave; extrinsic fibre Fabry-Perot; fibre Bragg gratings; Strain; temperature; residual stresses pre-stress; composites; autoclave; extrinsic fibre Fabry-Perot; fibre Bragg gratings; Strain; temperature; residual stresses
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Krishnamurthy, S.; Badcock, R.A.; Machavaram, V.R.; Fernando, G.F. Monitoring Pre-Stressed Composites Using Optical Fibre Sensors. Sensors 2016, 16, 777.

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