Application of Optical Fibre Sensors to Measuring the Mechanical Properties of Composite Materials and Structures

Wojciech Blazejewski, Pawel G^sior and Jerzy Kaleta

Wroclaw University of Technology

Poland

1. Introduction

The role of polymer composites in mechanical structures has been steadily increasing. The key factor here is the coefficient of specific strength, i. e. a ratio of tensile strength to specific gravity. As compared with metals, this coefficient assumes a high value for composites, including the polymer ones. Other major factors determining the choice of materials are: the product price, mass manufacturability, high strength and durability, corrosion resistance, material production and machining energy consumption and a few more. Also the latter criteria increasingly favour composites. Materials engineering as applied to composites offers the possibility of creating entirely new materials and modifying the traditional materials, which in both cases better meet designers’ expectations.

The ever increasing popularity of composite structures is first of all due to the considerable reduction in weight in comparison with metal structures, at a sufficiently high mechanical durability. Nevertheless, the safety requirements require that the stress-strain state of composite structure be monitored. Standard NDE methods such as radiography, interferometric holography, ultrasonic scanning or visual inspection are usually not effective in on-line monitoring. Even if they were used for periodic checks, they would not be able to detect promptly defects critical to the condition of the monitored structure. Moreover, conventional measurement devices, such as electrical resistance strain gauges, often get damaged in adverse environmental conditions.

State-of-the-art measuring methods based on optical fibre technology are increasingly often used to monitor the structural health of industrial objects. Optical fibre sensors have many advantages over the conventional devices. The obvious one is that they can work at high levels of electromagnetic interference and in other adverse conditions (high dust concentration, high temperature, high pressure, significant deformations). Moreover, such sensors are characterized by high (deformation and temperature) measuring sensitivity in a wide measuring range. Owing to their small geometric dimensions and a relatively small weight they can be installed inside a structure (e. g. by embedding them in the composite material) or on its surface. Thanks to their high potential for multiplexing it is possible to create a nervous system of the object being monitored, enabling health and damage assessment. In addition, optical fibre based sensors ensure spark-proof safety which is an important consideration for inflammable applications.

Because of the wide range of problems connected with the measurement of mechanical quantities in composites by means of optical fibre sensors it becomes necessary to concentrate on major problems. The authors decided to focus on the structural health monitoring (SHM) aspects for all kinds of composite structures. Solutions being mainly the result of the authors’ own research, with reference to the literature on the subject, will be highlighted.

Considering the above, the objective is to present the fundamentals of the measurement method based on optical fibre sensors as applied to:

– monitoring the behaviour of, among others, laminates and pultruded composites used as reinforcement in building structures and aeronautical structures;

– measuring (static and cyclic) strains in Smart composites where strong magnetic fields limit or exclude the use of other measuring techniques;

– monitoring the structural health of composite structures subjected to extreme strain, such as composite pressure vessels for storing compressed hydrogen and methane car fuels.

The focus is on the experimental aspect of measurements, and the numerical modelling of composite materials and structures is deliberately neglected. However, one should note that modelling plays an important role in many measurement situations (e. g. it is used to determine the number and location of sensors, the theoretical state of strain on a meso – and macroscale, the distribution of temperature, etc.).