Distributed Strain Sensing In Composites

What Lies Within…How Distributed Strain Sensing Plays a Role in Lifecycle Testing for Composites

Composite materials are taking over the aerospace, automotive, marine, aviation, civil engineering and sports/leisure industries. And with good reason.

Composites include materials which are usually stronger, lighter, or less expensive than traditional manufactured materials. They usually consist of a reinforcement (often a high-performance fiber such as carbon or glass) and a matrix (such as epoxy polymer).

Typical engineered composites find their place in reinforced concrete and masonry, wood, plastics, ceramics, and metals. They are generally used for buildings, bridges, and structures such as boat hulls, swimming pool panels, racing car bodies, storage tanks, and even bathroom showers and countertops.

The most advanced examples perform routinely on spacecraft and aircraft in demanding environments. Researchers have even begun to create “smart composites” known as robotic materials that mimic the human central nervous system.

What’s more, design engineers like the material for its ability to mold into a wide range of shapes and a surface texture that can be altered to mimic any finish, from smooth to textured.

Despite their strength, composite material structures are frequently subjected to harsh environmental conditions and are still vulnerable to fatigue damage and failures (buckling, splitting, cracking, fracturing, and bending).

It is for that reason they must be monitored, preferably in real time, for the duration of their lifecycle. Historically, this diagnostic and condition monitoring was performed with discrete strain gauges or several different types of single point sensors.

However, these relatively inexpensive solutions have shown their drawbacks. Single sensors are point solutions that do not measure along the entire length of the structure. Therefore, a lack of cracks or deformations where the sensor is placed does not mean that the entire composite structure is stable.

The same goes for individual strain gauge installations which also have other limitations, namely:

  • Strain gauges are generally more susceptible to installation faults such as glue voids, inclusions, and glue line thickness. On top of this, strain gauge accuracy can suffer with incorrect selection of excitation voltage, gauge metal selection, self-heating, gauge size selection, as well as incorrect amplification and filtering methods.
  • Strain gauges are susceptible to fatigue and plastic deformation which results in hysteresis and inaccurate measurements.
  • Strain gauges are susceptible to electromagnetic interference and are difficult or impossible to use in underwater, corrosive, or explosive environments.
  • They can exhibit loss of repeatability and reduction in accuracy with prolonged use.

Distributed strain sensing using fiber optics is an excellent solution for structural health monitoring of composite materials because it is very compact and can be embedded within the material, actively taking diagnostics and condition monitoring during operation of the structure.

Distributed strain sensing technology is also immune to electromagnetic interference, does not corrode and, because it does not require much labor to install, is extremely cost effective. Finally, true distributed strain sensing systems can take multiple measurements (such as strain and temperature) simultaneously.

One example is the fiber embedded within a composite panel of an automobile that sends an alert if the fiber demonstrates sudden strain, movement, or if the temperature of the fiber puts the structure at risk of damage or failure.

According to the Fiber Optic Sensing Association (FOSA) “Fiber optic sensing is not constrained by line of sight or remote power access and, depending on system configuration, can be deployed in continuous lengths…with detection at every point along its path. Cost per sensing point over great distances cannot be matched by competing technologies…”.

As Aristotle said, “the whole is greater than the sum of its parts” and that sentiment certainly applies to the use of composites as a building tool.

As costs come down and design flexibility improves, fiber-reinforced composites like carbon fiber and fiberglass are opening new design opportunities and gaining popularity with manufacturers who rely on fiber optic distributed strain sensing to keep these new structures functioning and safe.

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