CompositesWorld

OCT 2018

CompositesWorld

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OCTOBER 2018 8 CompositesWorld DESIGN & TESTING ยป Polymer matrix composites (PMCs) exhibit impressive stiffness and strength properties that are commonly attributed to the rein- forcing fibers. However, for many important mechanical properties, the polymer matrix also plays an important role. For example, the fiber direction compression strength of a PMC is highly dependent on the polymer matrix material's ability to support the reinforcing fibers and resist buckling under compression loading. e role of the polymer matrix material becomes apparent when evaluating "matrix-dominated" mechanical properties of PMCs at elevated temperature conditions, at which the stiffness and strength properties of the polymer matrix are reduced. ese mechanical properties gradually decrease as the test tempera- ture increases, due to reductions in the polymer matrix material properties. As the temperature of thermosetting PMCs are further increased, the polymer matrix material undergoes a transition from the glassy state present at lower temperatures to a more rubbery state. e thermosetting polymer does not melt, but a two to three order of magnitude reduction in stiffness is produced by this transi- tion from the glassy state 1 . Although this dramatic transition takes place over a temperature range, a single temperature typically is specified for a given polymer matrix and referred to as the glass transition temperature (T g ). e glass transition temperature of PMCs can be determined by performing a series of tests at increasing temperatures until a sudden drop in the measured property is observed. Note, however, that a matrix-dominated property of the PMC must be measured in which a significant change occurs when T g is reached. Since performing a series of tests at increasing temperatures is time- consuming and expensive, test procedures typically are used in which single specimens are subjected to repeated or continuous loading as the test temperature is slowly increased. For thermosetting polymers typically used in PMCs, three material properties are commonly measured to determine the glass transition temperature: heat capacity, coefficient of thermal expan- sion and flexural stiffness. Standardized test methods exist for T g measurement of polymers based on these properties. But how well do these test methods work for fiber-reinforced PMCs? e first method of T g determination focuses on changes in the heat required to increase the temperature, or heat capacity, of the sample. e use of differential scanning calorimetry (DSC) provides a relatively simple and cost-effective method for determining T g based on heat capacity changes. is method, described in ASTM D7426 2 , is commonly used for polymeric materials because of the significant increase in measured heat capacity associated with their glass transition. Since PMCs typically are less than half polymeric material based on volume, however, the change in heat capacity associated with their glass transition is significantly smaller and more difficult to detect using DSC. erefore, T g determination using DSC is not a preferred method for use with PMCs. A second property used to determine the T g of polymeric mate- rials is the coefficient of thermal expansion (CTE), a measure of the dimensional change of a polymer sample associated with a temperature increase. Changes in CTE are used for T g determi- nation of polymeric materials due to the dramatic CTE increase during their glass transition. ermomechanical analysis (TMA) methods are used to measure the increase in length of a polymer sample with increasing temperature as described in ASTM E1545 3 . Although expansion TMA is most commonly used for T g determi- nations, other TMA methods exist by which T g determinations can be made while slowly increasing the sample temperature 1 . Penetra- tion TMA measures the hardness of the sample, whereas flexural TMA measures the center deflection of a beam-like sample loaded in three-point flexure. In both methods, a significant increase in displacement is measured by the TMA probe associated with their glass transition. However, for PMCs, the reinforcing fibers signifi- cantly reduce the change in CTE associated with the polymer's glass transition. Additionally, neither penetration TMA nor flexural TMA have received much attention for PMCs. As a result, TMA methods are not commonly used for T g determination of PMCs. e third property used to determine the T g of polymeric materials is the stiffness of a sample under a specified mechanical loading, a logical choice since unreinforced polymers exhibit large reductions in stiffness at their glass transition. Dynamic mechanical analysis (DMA) methods are used to apply an oscillating force to the polymer specimen and measure the resulting displacement as the test temper- ature is slowly increased. From the measured force and displace- ment, the specimen stiffness is determined and used to identify the glass transition. Standardized test methods exist for T g determina- tion of PMCs using DMA for a variety of loading methods, including tension (ASTM D5026 4 ), compression (ASTM D5024 5 ), torsion (ASTM D5279 6 ), three-point bending (ASTM D5023 7 ) and dual cantilever beam flexure (ASTM D5418 8 ). Procedures calculating T g values based on DMA testing of polymeric materials are provided in ASTM D4065 9 . For PMCs, the magnitudes of stiffness reductions at T g are reduced significantly for many of these loading methods due to the reinforcing fibers. However, significant flexural stiffness reduc- tions are still produced, and thus DMA testing of PMCs focuses on flexural loading configurations. ASTM D7028 10 recommends the use of either a double cantilever beam or three-point bend flexural loading. e use of DMA under flexure loading is the most commonly used method for T g determination of PMCs. Although the measured T g establishes a limiting use The glass transition temperature of PMCs is determined by a series of tests at increasing temperatures. Glass transition temperature testing of composites

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