Fiber-reinforced polymer (FRP) materials usually have the benefits of a high strength-to-weight ratio, high durability, and corrosion resistance. As a result, the use of FRPs is rapidly increasing in a wide range of applications such as aerospace, power generation, and automobiles.
FRPs typically consist of a polymer matrix and filler materials, such as glass or carbon fibers. An important part of quality control in several safety-critical applications includes the analysis of FRP’s composite structure.
Analysis of Fiber Reinforced Polymers
Features that are usually of interest include: the percentage of reinforcing fibers; delamination or voids; and the degree of cure in the matrix and any additional filler materials in the matrix.
In order to obtain accurate measurements, reproducible preparation is an important consideration. Varying material removal rates and differences in hardness between the filler materials and matrix creates problems in preparation processes such as rounding or polishing relief. The problems can lead to incorrect measurements, create artificial damage, or disguise problems.
Sectioning needs to be done with minimal deformation and damage – aggressive sectioning can result in delaminated structure or deep brittle-fracture damage in fibers. It is recommended that cutting is carried out with a diamond wafering blade on a precision cutter.
Whenever it is possible, specimens can be mounted in a low viscosity, low shrinkage epoxy system. In order to avoid possible exothermic reactions influencing the sample during the mounting process, slow-curing low temperature epoxies are preferred.
Grinding and Polishing
Grinding and polishing are typically carried out in four steps. A standard preparation technique is available, suitable for many types of materials. Step 1 involves planarizing the specimen, and silicon carbide grinding discs can do this effectively and quickly.Care should be taken to avoid using a disk for prolonged periods as it can result in excessive fiber damage.
Steps 2 and 3 use of 9 µm and 3 µm diamond respectively to remove damage from the grinding process while preserving exceptional flatness. After step 3, the fiber must have a flat and undamaged appearance, although the polymer matrix can contain fine scratches.
Flatness is critical to accurately analyze fiber percentage and porosity, so it is preferable to carry out the analysis at this stage, before the final polishing is done. For some brittle and delicate fibers, diamond polishing can result in fracture damage. In such cases, step 3 must be modified to use 1 µm alumina on TexMet C.
Step 4 is the final polish, which must be long enough to remove the fine scratches that may remain without creating too much relief and rounding effect. After step 4, the polymer matrix and constituents should be completely damage-free.
This information has been sourced, reviewed and adapted from materials provided by Buehler.
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