Installing Strain Gauges on Fiber-Reinforced Plastics

What Materials Do Composites Consist of?

Commonly used fibers include, for instance:

• Glass fiber (GFRP)
• Carbon fiber (CFRP)
• Aramid fiber (AFRP)
• Ceramic fiber
• Polymer fiber
• Mineral fiber
• Natural fiber (NFRP)

The resins used include epoxy resin, polyester resin, and polyurethane resin.

• Excellent strength-to-weight ratio and improved fuel efficiency
• High strength and elastic bending properties
• Free formability of materials (strength, stiffness, thermal, electrical resistances, form, function)

• Temperature resistance
• Chemical resistance
• High corrosion resistance

The characterization of composite materials and structures is extremely important to ensure their durability during use. Different tests are performed to achieve this. It is essential to measure component deformation. Strain in the material is a critical factor determining damage effect and durability.

1. Determination of durability parameters of components/structures in the test bench or in the field
2. Determination of material properties of standardized test samples. There are many different test standards for composite materials that involve the use of strain gauges. Typical tests include, for instance:

• Bending tests (3-point, 4-point)
• Tensile tests
• Shear tests (interlaminar)
• Lap shear (adhesive test)
• Open hole/Filled hole
• Compression after impact
• Compression tests
• Notched-bar impact-bending tests
• Hole-bearing tests

Previous calculation approaches for these materials can only be applied to specific cases (e.g. Tsai Wu). There is no universal calculation method and no standard for components similar to the FKM guideline for metallic components. Since these are laminate structures, this also applies for the use of quasi-isotropic laminates. Many methods for performing calculations on composite materials have already been developed.

Another challenge is to convert the strain signal into mechanical stress.

• Damage/failure mechanisms are complex

  • Intermediate-fiber breakage

  • Delamination

  • Cracks run in parallel to the fibers

• Manufacturing tolerances are, in general, more difficult to control

  • Fiber orientation

  • Matrix offset

  • Intermediate-fiber compounds

  • Resin accumulations

  • Foreign bodies

  • Porosities

  • Batch variations

• More expensive than conventional metallic materials

• Temperature-sensitive

• Sensitive to UV light

• Difficult to recycle

• High investment costs (production)

• Additionally, the thermoelastic response has to be considered:

• Reduced thermal conductivity: Composite materials have a lower thermal conductivity than conventional metals
• Differences in the thermal coefficient’s residual stresses (e.g. hybrid structures) and anisotropic material behavior

It depends on the test case:

• We recommend using the Y series (max. 5% strain) for static, high strain and coupon tests
• We recommend using the M series (max. 1% strain) for alternating load tests

We recommend using our pre-wired Y strain gauges for composites that show a critical response to typical soldering temperatures.

Some strain gauges for composite materials are available from stock.

• Linear strain gauges are often used in structural and sample testing

• T rosettes are used, for instance, to determine Poisson's ratio

• 3-measuring grid rosettes are also used; however, this is only recommended with homogeneous materials for determining the principal strain and stress directions

The graphic below shows the heating process of a 350-ohm strain gauge measuring grid on a slowly cooling material:

Heat in measuring points easily occurs with metals; particularly with aluminum, a high heat transfer is possible. Composites have a considerably lower thermal conductivity.

• Caution is advised when treating plastics with solvents, because they may cause expansion or stress corrosion (for instance, the use of acetone is critical). There is a risk of swelling due to humidity or stress corrosion.
• White gas and isopropyl alcohol may be considered largely uncritical, especially because of the short contact time.
• In critical cases, a preliminary test should always be made, because no clear predictions can be made due to the very large number of modified plastics. This also applies for the use of RMS1 cleaning agent.
• If possible, no solvent should be used to clean the surface. Alternative cleaning agents include:
  • De-ionized water
  • Petroleum ether
  • Soap

• We recommend preparing the measuring point as follows: Roughen with emery cloth (grain size 400), then use dishwater for cleaning and rinse with water (ideally: de-ionized water).
• The release agent and the epoxy filling material need to be removed (grain size 400)
• Slightly roughen the surface to activate the function (improve surface bonding properties)
• Surface plasma activation is also optionally possible to improve the bonding properties

Please note: The lower layer fibers must not be damaged by excessively deep roughening!

All cold-curing adhesives from HBM's range of products can be used to install strain gauges.

• Z70 for smooth surfaces
• X60 for rough composite surfaces
• X280 for high temperatures (Please note: Post-curing at temperature is recommended, see instructions)

Make sure to exactly align the strain gauge on the material:

Y series strain gauge, fixed before bonding:

Type 1-LY41-6-350 strain gauge, professionally installed on a CFRP material with X60 adhesive: