Control how thermal residual stresses are handled in an analysis.
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. Helius PFA 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 STRESSWhen the *CURE STRESS keyword is included in the HIN file, Helius PFA 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:
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 ANSYS is 0°. This default temperature is completely compatible with the default stress free temperature of 0° that is assumed when the 9th user argument is specified as 0. In this case, the model can still be subjected to temperature changes by simply 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.
When the *CURE STRESS keyword is used in the HIN file, a default value of 0.5 is used for the cure ratio (Rc) that appears in the calculation of the temperature change (see Eq. 33a in the Thermal Residual Stresses section of the Theory Manual for more information). This default value agrees well with research and is described further in the Theory Manual (refer to Reference 13 of the Theory Manual for additional details). To override the default values of the cure ratio value, you may specify Rc using the optional RATIO parameter.
*CURE STRESS, [RATIO=Rc]Consider the following as an example:
*CURE STRESS, RATIO=0.7In this example, the cure ratio used to determine the temperature change is set to 0.7. This value applies to all materials used in the analysis. The cure ratio must lie in the range 0 < RATIO ≤ 1.
By default, the *CURE STRESS keyword determines the thermal residual stresses using the current temperature and moisture content. In reality, these thermal residual stresses are a function of the temperature change from the cure temperature to ambient temperature and any potential change from ambient temperature to the testing temperature. To account for these different temperature changes, you may use the optional parameters with the *CURE STRESS keyword:
*CURE STRESS, [METHOD=SPECIFY ENV], [TEMPERATURE=TAMB]
RCA, RCBwhere the parameters contained inside the brackets [ ] are optional. When METHOD=SPECIFY ENV, you may define the material property environment at which the thermal residual stresses are computed. TAMB defines the temperature environment for the material properties used in Eqs. 32 of the Thermal Residual Stresses section in the Theory Manual. RcA and RcC default to 0.5.
If METHOD=SPECIFY ENV is not used, the default behavior of METHOD=CURRENT ENV is used. In this case, the current moisture and temperature are used to determine the set of material properties used to calculate thermal residual stresses.
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 2.