<title>Damage assessment in hybrid laminates using an array of embedded fiber optic sensors</title>

Abstract

Hybrid laminates typically consist of alternate layers of fibre-reinforced polymer and aluminium alloy. Developed primarily for fatigue critical aerospace applications, the hybrid laminates are orthotropic materials with lower density and higher strength compared to the aluminium alloy monolith. One of the damage mechanisms of particular interest is that of fatigue crack growth, which for hybrid laminates is a relatively complex process that includes a combination of delamination and fibre bridging. To facilitate the development of a unified model for both crack and damage growth processes, a remote sensing system, reliant upon fibre optic sensor technology, has been utilised to monitor strain within the composite layer. The fibre optic system, with capacity for sub microstrain resolution, combines time domain multiplexing with line switching to monitor continuously an array of Bragg grating sensors. Herein are detailed the findings from a study performed using an array of 40 sensors distributed across a small area of a test piece containing a fatigue crack initiated at a through-thickness fastener hole. Together with details of system operation, sensor measurements of the strain profiles associated with the developing delamination zone are reported.

Self-monitoring Properties of Concrete Columns with Embedded Cement-based Strain Sensors

“…SHM is normally performed by measuring the strain/stress of a structure's critical zone with sensors; the health of the structure can be evaluated based on the measured information, which can be used when deciding whether or not to repair the structure (Scott et al, 1996). Traditional strain sensors (such as optical fiber sensors, electrical resistance strain gages, and piezoelectric ceramics) have been widely used in SHM (Claus et al, 1988;Chen, 1995;Austin et al, 1999;Li et al, 2006a). Recently, a new kind of strainsensing material, smart concrete, appeals more and more for long-term health monitoring due to its advantages: durability, low cost, and compatibility with concrete matrix (Chung, 2000).…”

<title>Development of sensor technology to facilitate in-situ measurement of damgae in composite materials for spacecraft applications</title>

“…As first test samples, CARALL specimens used in earlier fatigue measurements [15] were recycled by removal of one or more aluminum layers, in order to more closely represent materials used in space. The remaining 4 ply unidirectional CFRP (913HTA) composite sheet (0.5mm thick) still had a fully-intact array of forty Bragg grating strain sensors embedded in its center, with 8 sensors in each of 5 fibers.…”

“…This signal facilitates re-tuning of the AOTF center wavelength to lock in to each grating in turn (as described in more detail in [13] Once this lock-in is achieved, the mean AOTF drive frequency, and hence wavelength, is measured. In previous studies [15], the interrogation system repeatedly poled a single sensor at low frequency, to allow measurement of the amplitude of the sinusoidally varying strain field during accelerated fatigue experiments. For this study, a software revision facilitated the measurement of residual strains and dynamic strain at a sample rate of 200Hz.…”

Characterisation of fatigue crack growth and related damage mechanisms in FRP–metal hybrid laminates

“…Each fibre-optic line was interrogated in turn, and for each grating the maximum and minimum radio frequency driving the AOTF were recorded from 224 samples (taken over about 20 load cycles) [30,31]. The radio frequency was proportional to the wavelength of the light reflected by the grating.…”

“…It was based on the use of a broadband source and an acousto-optic tuneable filter (AOTF) to determine the peak grating wavelength [24][25][26][27][28][29]. By embedding a two-dimensional array of sensors within the composite, a method was developed to map the strain field within the FRP layer of a CARALL panel subjected to fatigue damage [30,31]. The purpose of this paper is to illustrate how such strain mapping may quantify the redistribution of stress within the laminate as crack growth proceeds.…”