October 2017 Calendar Image Carbon Fiber in DIC

Challenges in Fiber Reinforced Polymers

Fiber-reinforced polymer materials (FRP) typically have the advantage of a high strength to weight ratio, great durability and corrosion resistance. As a result their use is rapidly increasing in a wide variety of applications including automotive, aerospace and power generation. They typically comprise of a polymer matrix and filler materials such as glass or carbon fibers. Analysis of the composite structure is a vital part of quality control in many safety critical applications.

Features that are typically of interest include: percentage of reinforcing fibers; degree of cure in the matrix and any additional filler materials in the matrix; voids or delamination.

Reproducible preparation is important for getting accurate measurements. Differences in material removal rates and hardness between the matrix and filler materials create difficulties in preparation such as polishing relief or rounding. These problems can cause incorrect measurements, disguise problems or create artificial damage.

Sectioning should be done with minimal damage and deformation – aggressive sectioning can leave deep brittle-fracture damage in fibers, or cause delamination in the structure. Cutting with a diamond wafering blade on a precision cutter is recommended.

Mount specimens whenever possible in a low shrinkage, low viscosity epoxy system. Slow-curing low temperature epoxies are preferred, to avoid the possibility of exothermic reactions during the mounting process effecting the sample.

Grinding and Polishing can typically be achieved in four steps. A standard preparation route can be found here that is suitable for a wide range of materials. The first step should planarize the specimen, and Silicon Carbide grinding discs can do this quickly and effectively. However, avoid using any one disc for too long as this can cause excess fiber damage.

Steps 2 and 3 use 9 µm and 3 µm diamond respectively to remove damage from grinding while retaining exceptional flatness. After step 3 the fiber should be flat and undamaged, although fine scratches may remain in the polymer matrix. Flatness is vital to analyzing fiber percent and porosity accurately and so it can be preferable to analyze at this stage, prior to the final polish. For some very delicate and brittle fibers, diamond polishing can leave fracture damage. In these cases, step 3 can be modified to use 1um alumina on TexMet C

The final polish (step 4) should be just long enough to remove the few fine scratches that remain without generating too much relief and rounding. After this stage, the polymer matrix and constituents should be completely damage free.

Carbon Fiber in Aerospace

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