Thermal shrinkage estimating

Young and Day [95] used the shift in Raman band frequency of the 1580-cm band in carbon fibers to determine the compressive strain imposed on the carbon fibers by thermal shrinkage and crystallization of the matrix polymer, PEEK, in a carbon fiber-PEEK composite. This allows the estimation of the interfacial stress between the fiber and the composite resin due to processing. The lack of interfacial stress is considered critical to the performance of many fiber-reinforced composites because the interfacial stress adds to the applied stress and can lead to early fracture or debonding of the fiber from the matrix. 

What all of these figures show is the significant influence that the thermal shrinkage can have upon the prediction of the stress distribution within a lap joint. One conclusion which can be drawn from this is that if an accurate estimation of joint performance, be it a strength prediction or understanding of the joint behaviour, is to be achieved then it is imperative that the thermal effects be accounted for. 

Linear thermal expansion testing helps to determine if failure by thermal stress may occur in products and materials. Precise knowledge of the CTE can be utilized to estimate the thermal stresses. This aspect makes CTE to an important property of the used fiber for composite materials. A rule of mixtures is sufficient for calculating the CTE of polymers filled with powder or short fibers. In case of long libers, the rule of mixtures is valid perpendicular to the reinforcing fibers. Molecular orientation affects the thermal expansion of polymers. Processing also affects CTE, for semicrystalline polymers this fact is very important. For that reason, CTE measurements are often used to predict shrinkage in injection moulded parts. 

These TMA shrinkage measurements were carried out without the instrument furnace in place, that is, at ambient temperature. During a typical 60-min run an estimate of the ambient thermal fluctuations of the surroundings was made and found to be of the order of +0.2 °C. For a 0.6-mm-thick photopolymer, these temperature changes would result in only a 0.004% change in sample height and would be a negligible source of error in the experiments. 

A similar study carried out for the Snase molecule at full hydration has shown that the H-bonded water network envelopes Snase molecules permanently at temperatures below about 275 K [566]. A midpoint of the percolation transition was estimated atT k 295 K. So, the thermal break of a spanning water network occurs in a narrower temperature interval in the case of Snase molecule in comparison with ELP. The shrinkage of the temperature interval of the percolation transition should be attributed to the larger size of Snase molecule, which has about eight times more water molecules in the hydration shell than ELP. 

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