When modeling problems that involve temperature changes, it may be necessary to consider how the material properties change with temperature and how thermal residual stresses can influence the stresses, and ultimately the failure in the model. If the temperature of the model becomes much greater than that of the experimental temperature for material properties and warrants a consideration of material property changes or residual stress inclusion to the model, Helius PFA can be used to account for these considerations.
Generally speaking, the stiffness of the lamina in the fiber direction is relatively insensitive to temperature while matrix dominated (transverse) stiffnesses are typically more sensitive to temperature. Strength values do not show a consistent trend other than matrix dominated strengths tend to decrease at elevated temperatures. If material data is available at multiple temperatures and the variation in values is significant, Helius PFA can be used to account for property variation with temperature. The procedure for creating a multi-temperature material definition is demonstrated in the next section.
Thermal residual stresses occur in composite parts as a result of mismatches in the coefficients of thermal expansion (CTE) at the laminate level and at the constituent level. At the laminate level, the mismatch in CTE between adjacent plies in the laminate creates ply level thermal stresses. At the constituent (fiber/matrix) level, the mismatch in CTE is between the fiber and the matrix material. It is typically assumed that these stresses are inherently accounted for in the experimental material properties determined at a given temperature.
For ply-level finite element analyses, these stresses are accounted for with different methods. Ply level thermal stresses can be determined by the finite element solver given a temperature change in the model. The constituent level thermal stresses, however, are not explicitly accounted for the by the solver because the solver does not distinguish between the fiber and the matrix. Since Helius PFA has access to the fiber and matrix stiffnesses and stresses, it can compute the constituent level thermal stresses. To include the constituent level thermal stresses in the analysis, add the *CURE STRESS keyword to the HIN file (refer to the Helius PFA User's Guide). By default, constituent level thermal stresses are not included in the Helius PFA jobs.
In what follows, a temperature dependent material file is created, four methods for analyzing the same thermal problem are described and results from the models are compared. For further information related to temperature dependent material properties and thermal residual stresses, refer to the Helius PFA User's Guide and Theory Manual.