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*:
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.
Commonly used fibers include, for instance:
The resins used include epoxy resin, polyester resin, and polyurethane resin.
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.
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)
It depends on the test case:
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
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.
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.
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:
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
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:
Please note: The lower layer fibers must not be damaged by excessively deep roughening!
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:
This will bring together HBM, Brüel & Kjær, nCode, ReliaSoft, and Discom brands, helping you innovate faster for a cleaner, healthier, and more productive world.
This will bring together HBM, Brüel & Kjær, nCode, ReliaSoft, and Discom brands, helping you innovate faster for a cleaner, healthier, and more productive world.
This will bring together HBM, Brüel & Kjær, nCode, ReliaSoft, and Discom brands, helping you innovate faster for a cleaner, healthier, and more productive world.
This will bring together HBM, Brüel & Kjær, nCode, ReliaSoft, and Discom brands, helping you innovate faster for a cleaner, healthier, and more productive world.
This will bring together HBM, Brüel & Kjær, nCode, ReliaSoft, and Discom brands, helping you innovate faster for a cleaner, healthier, and more productive world.