TEST SAMPLE PREPARATION OF PULTRUDED UNIDIRECTIONAL CARBON PARTS - ISSUES IN MACHINING PRACTICE AND ALTERNATIVES

摘要

The advantages of carbon fiber composites are well known - lightweight, corrosion resistant, high strength and high modulus. The economic advantages of producing composites by a continuous pultrusion process are also well understood. But testing these unidirectional carbon composite parts can be problematic. Most composite testing methods call out the use of thin flat rectangular strips or in some cases cylindrical sections. Unfortunately, shapes produced by pultrusion are not limited to cross-sectional areas or geometries that are easily or directly tested. Unlike isotropic metals or quasi-isotropic prepreg lay-ups, the unidirectional parts typically made in pultrusion are very anisotropic, not perfectly symmetrical in the Z-axis, and have a propensity to fail in shear during a variety of testing. It is often the case that full scale sample testing is required to meet consumer or regulatory compliance. Some end market specifications require testing that can be challenging or nearly impossible to perform based on composite structural shape or testing limitations. In order to complete testing, samples are manipulated through traditional machining methods to create a sample that can be tested. One such test is tensile strength as measured by ISO 527-5 (ASTM D3039). For carbon composites made of a standard modulus carbon, this test is particularly difficult to perform if the part tensile strength exceeds the shear strength of the tabbing adhesive and shear area combination. Traditional practice would dictate that the part thickness be ground down to achieve a breakable cross-sectional area for this carbon composite. However, grinding unidirectional pultruded composites can terminate fibers producing a false material failure point. This paper will highlight the challenges of using the tensile test to qualify carbon unidirectional composites and discuss the merits of producing and testing a thinner representative sample in place of a ground sample for qualifications.