An Introduction to Fiber-Reinforced Composites

What Are Composites?

Composites or fiber-reinforced composites consist of at least two macroscopically differentiable materials that are combined with the basic aim of improving the material properties. A fiber structure is usually embedded in a resin (matrix material) and then cured.

To achieve this, fibers and fiber bundles are processed into a textile or fabric. Most methods to manufacture fabrics from fibers have originated from the textile industry, hence, most of the terminology used in this field is also used in the context of processing reinforcing fibers into textiles. The fibers determine the composite’s strength and stiffness. A material into which aligned fibers have been incorporated can be much stronger in the fiber direction than the same material without fibers. The increase in stiffness is less pronounced when force is exerted perpendicular to the orientation of the fibers. The strength in this direction is lower since the fibers act as concentrators of stress. In practice, fibers aligned in different directions are often incorporated.

There are many possible designs*:

Unidirectional fibers Bidirectional fibers Short fibers

The graph below shows the fiber’s contribution to a composite’s strength:

*Nanocomposites use very small fibers in the nanometer range as the reinforcing material.

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.

What Are the Fields of Application of Composites?

Aerospace industry (fuselage, drive components, aerodynamic components, etc.)
Automotive (chassis components, aerodynamic components)
Large vehicle bodies (trains, trucks, and buses)
Marine (hull structures)

Wind power turbines (rotor blades)
Sports equipment
Infrastructures and buildings (repair of buildings, GFRP bridges)
Medical engineering (prosthetics, X-ray tables)

Why Are Composites Used?

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

Why Is It Necessary to Perform Strain Measurements on Composites?

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.

Determination of durability parameters of components/structures in the test bench or in the field
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

Challenges of Composite Testing

Sophisticated methods/tools are required to calculate structural behavior. The mechanical properties are direction-dependent (strength, modulus of elasticity, Poisson's ratio, etc.), and many fiber composites behave contrary to metallic materials: The materials have differing stiffness properties in different directions (orthotropy).

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

Which Strain Gauges Does HBM Recommend for Measurements on Composites?

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

Selecting the Measuring Grid Length

A strain gauge integrates the strain below the surface, and an average strain is measured.

The right measuring grid length depends on the testing objective. Grid lengths of 6 mm and 10 mm are popular solutions for strain measurements on composites.

On principle, the same rule applies for selecting the strain gauges as for concrete: The strain gauge length should exceed the fiber distance by at least factor 5. The strain gauge width should also cover several fibers.

Local strain peaks can occur due to material inhomogeneities. In this case, strain gauge chains can be used to determine the strain gradient.

Often, the stress peaks between the fibers are a multiple of the average strain. As a consequence, the strain gauge may be overloaded at some points, its maximum elongation being reached or exceeded, although the amplifier displays a far smaller strain. Thus, there is a risk of the SG being overloaded (permanently damaged) at individual points or of failure of the whole installation. This problem can be eliminated by inserting a thin Polyimide film between the strain gauge and the workpiece. The film is glued between the component and the strain gauge and performs preliminary integration, i.e., it "distributes" the stress peaks under the strain gauge measuring grid. Because of the resulting thicker layers, the film should only be used if a high strain is expected.

Strain Gauge Resistance

HBM recommends using 1000-ohm strain gauges on slowly cooling materials. 350-ohm strain gauges can also be alternatively used. It is, however, recommended to check whether there is an impermissible temperature increase of the strain gauge or composite.

Excitation Voltage

The voltage at every strain gauge is converted into heat. Poorly conducting materials such as fiber composites show a heating-up of the sensor and component on the surface. To ensure a stable measurement, the heat flow Q must correspond to the applied power P.

P = Q

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.

Make sure to start measuring on composites only after a certain heat-up phase, when the measurement system has reached a stable state.

The following values can be used for quarter bridge applications with an excitation voltage of 5 V:

The heat-up phase is approximately 3 to 4 minutes for 1000-ohm measuring instruments
The heat-up phase is approximately 5 to 6 minutes for 120/230-ohm measuring instruments

With poorly cooling materials such as composites, HBM recommends using a low excitation voltage < 2.5 V. Higher excitation voltages result in a significant and constant heating up of the strain gauge. This heat will possibly build up in the material. The graphic below shows the differences between 0.5, 2.5, 5, and 10 V excitation voltage (DC) for a 350-ohm strain gauge grid:

Recommendation for composite materials (experience):

0.5 V for poorly conducting materials with poor cooling
1 V to 2.5 V for usual composite tests

Temperature Response Matching in Quarter-Bridge Applications

Quarter-bridge applications require optimal temperature response matching of the strain gauge due to temperature variations occurring during long-term measurements. In this case, the temperature response matching of the strain gauge should best fit the thermal expansion coefficient to minimize thermal strain signals.

It should be noted, however, that, due to manufacturing tolerances (fiber winding, layer production, fiber orientation, manufacturing method (automated or manual)), the material properties could differ too, and thus only an approximate temperature response matching can possibly be achieved, depending on the fiber composite.

It is generally recommended to use strain gauges with code number 6 for measurements on composites (α = 0.5 · 10-6/K). This may vary in some cases:

Surface Cleaning

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

Surface Roughening

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!

Selecting the Adhesive and Bonding

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)

With directed fibers, it is essential to correctly align the strain gauge owing to the orthotropic material behavior:

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: