Monitoring Pre-Stressed Composites Using Optical Fibre Sensors

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.

A Review of Prestressed Fibre-Reinforced Polymer Matrix Composites

“…Prestressing levels are applied through a loading pin by making use of a mechanical test machine. The prestressed composite is clamped and tightened with a bolt to the frame, vacuum-bagged and cured in an autoclave [ 42 ]. As a result of the sharp edge of the frame, it is difficult to prepare the assembly for vacuum-bagging [ 43 ].…”

On the Sensing and Calibration of Residual Stresses Measurements in the Incremental Hole-Drilling Method

“…Thus, much research has been conducted in order to measure and predict the induced RSs inside materials, as this is considered an important stage for designing the structural components and estimating their reliability [2][3][4][5]. The stresses inside the material are usually estimated in two common ways, optical methods [6][7][8][9] or using physical sensors [10][11][12]. Different experimental and numerical techniques are utilized to precisely determine the magnitude and types of the stresses, such as the hole-drilling method (HDM) [13,14], X-ray diffraction (XRD) [15], the neutron diffraction method [16], the slitting method [17], and the curvature method [18].…”

Electro-spinning of highly-aligned polyacrylonitrile nano-fibres with continuous spooling

“…Applying uniform tension to the fibres will be simpler if they are oriented in one direction and plane. Moreover, aligned fibre stacks can be subjected axial stretching to control the residual fabrication stresses 46 . The mechanical testing of carbonised and graphitised fibres using conventional bundle-testing will be significantly simpler if the filaments are oriented in one direction 47 – 50 .…”