Account for ply-level and constituent-level thermal residual stresses of a composite part.
At room temperature, an unloaded laminated composite structure already has non-zero self-equilibrating stresses at both the composite ply level and the constituent material level. This is caused by the initial cooling of the structure from its elevated cure temperature to room temperature. At the composite ply level, these thermal residual stresses are caused entirely by differences in the thermal expansion characteristics of adjacent plies. At the constituent material level (fiber/matrix), the thermal residual stresses are caused in part by the previously mentioned ply level thermal residual stresses and in part by differences in the thermal expansion characteristics of the fiber and matrix materials. Simulation Composite Analysis can explicitly account for these ply-level and constituent-level thermal residual stresses that exist prior to any externally applied loads or temperature changes. In this case, the thermal residual stresses contribute to the total stress state of the composite material and thus influence the mechanical load level at which the material fails. If you wish to include the effects of thermal residual stresses in the analysis, the following keyword must be included in the HIN file:
*CURE STRESS
When the *CURE STRESS keyword is included in the HIN file, Simulation Composite Analysis explicitly accounts for thermal residual stresses in the response of the unidirectional composite material. To do so, it computes the ply-level and constituent-level thermal residual stresses caused by the post-cure cool down from the stress-free temperature (i.e. cure temperature) to ambient temperature. In this case, the stress free temperature is read from the material data file (mdata file) and ambient temperature corresponds to 72.5 °F, 22.5 °C or 295.65 °K. When this feature is active, ply-level and constituent-level thermal residual stresses are present in the composite material prior to the application of any external mechanical and/or thermal loads imposed during the actual simulation. If you choose to explicitly account for thermal residual stresses in the analysis, you should verify the material data file (mdata file) actually contains a defined stress free temperature; otherwise, the stress free temperature will default to 0° and the predicted thermal residual stresses will be erroneous.
If the *CURE STRESS keyword is not included in the HIN file, thermal residual stresses are not included in the response of that particular composite material during the simulation. In this case, the stress free temperature of the composite material defaults to Tsf =0° (regardless of the system of units employed), and the temperature change that is used in the constitutive relations [σ = C(ε - αΔT)] is simply computed as ΔT = T - Tsf = T. Several points should be emphasized here. First, the stress free temperature Tsf defaults to 0° even if the composite material data file (mdata file) explicitly defines a non-zero stress free temperature. Second, regardless of the system of units employed by the finite element model, the current temperature T completely defines the temperature change ΔT used in the constitutive relations. Third, for composite materials characterized at multiple temperatures, the current temperature T is used to interpolate the various material properties that contribute to the constitutive relations. Consequently, it is recommended that a single-temperature characterization (i.e., a single-temperature mdata file) be used for the composite material in question. In summary, if you do not include the *CURE STRESS keyword in the HIN file, the current temperature T influences the constitutive relations in two different ways: 1) the temperature change used in the constitutive relations simply becomes ΔT =T, and 2) T is used to interpolate the temperature-dependent material properties that contribute to the constitutive relations.
It should be emphasized that the default temperature in Abaqus/Standard is 0°. This default temperature is completely compatible with the default stress free temperature of 0° that is assumed when the seventh user material constant is specified as 0. In this case, the model can still be subjected to temperature changes by imposing a temperature other than 0°. However, these thermal stresses develop over the course of the analysis, as opposed to being present at the start of the analysis.
For a comprehensive theoretical discussion on thermal residual stresses, refer to the Theory Manual and for a demonstration of this feature, refer to Example Problem 3.