On the feasibility of optical fibre sensors for strain monitoring in thermoplastic composites under fatigue loading conditions

Abstract Fibre reinforced composites materials offer a pathway to produce passive shape adaptive smart marine propellers, which have improved performance characteristics over traditional metallic alloys. Automated Fibre Placement (AFP) technology can provide a leap forward in Cyber-Physical automated manufacturing, which is essential for the implementation and operation of smart factories in the marine propeller industry towards Industry 4.0 readiness. In this paper, a comprehensive structural health monitoring (SHM) routine was performed on an AFP full-scale composite hydrofoil to gain confidence in its dynamic and structural performances through a number of active and passive sensors. The hydrofoil was subjected to constant amplitude flexural fatigue loading in a purpose-built test rig for 105 cycles. The hydrofoil was embedded with distributed optical fibre sensors (DOFS), traditional electrical strain gauges and linear variable displacement transducers (LVDTs). Both microelectromechanical system (MEMS) and piezoelectric (PZT) accelerometers were used to conduct experimental modal analyses (EMA) to observe changes in the modal response of the hydrofoil at regular intervals throughout the fatigue program. The hydrofoils modal response, as well as the stiffness measured using both displacements and strains, remained unchanged over the fatigue loading regime demonstrating the structural integrity of the hydrofoil. The optical fibre sensors endured the fatigue test cycles showing their robustness under fatigue loads. Furthermore, the sensing systems demonstrated the potential of being utilised as a useful maintenance tool combining their adaptability with automated manufacturing during manufacturing through integration within the hydrofoil, a structural test framework for performance measurement, data acquisition and analytics for visualization, and the prospect of decision making for maintenance requirement during any onset in structural performance.

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High Strain Monitoring during Fatigue Loading of Thermoplastic Composites Using Imbedded Draw Tower Fibre Bragg Grating Sensors

Advances in Science and Technology ◽

10.4028/www.scientific.net/ast.56.441 ◽

2008 ◽

Vol 56 ◽

pp. 441-446 ◽

Cited By ~ 1

Author(s):

Eli Voet ◽

Geert Luyckx ◽

Ives De Baere ◽

Joris Degrieck ◽

J. Vlekken ◽

...

Keyword(s):

Composite Laminates ◽

High Strength ◽

Fatigue Loading ◽

Strain Sensors ◽

Thermoplastic Composites ◽

Bragg Grating ◽

Fibre Bragg Grating ◽

Strain Measurements ◽

Strain Monitoring

This paper presents the experimental study of fibre Bragg grating sensors for measuring strain inside composite laminates during fatigue loading. The optical fibres are imbedded inside thermoplastic CFRP test-coupons which have an ultimate strain of about 1.1%. Tension – tension fatigue cycling at a rate of 5Hz is been carried out at 314MPa with a maximum strain of 0.51%. At such extreme strain levels the use of high strength sensors becomes inevitable. Neither the sensor nor the composite test-coupons showed any significant degradation even after more than 500000 cycles. Fibre optic Bragg grating sensors are known to be very accurate strain sensors but one should be very careful interpreting their response once they are imbedded inside composite materials. In this study high strength fibre Bragg grating sensors with coating are imbedded in composite test coupons and a pretty good correlation was found between the strain measurements of an electrical extensometer and the imbedded sensor during the complete cycling. The high strength sensor show to be very feasible for extreme and long term strain measurements.

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