PFLEX
INOD
Q: INOD, would there be results differences depending on which INOD is selected?
A: The user can select the position of the spiral. This is to be able to choose a critical position along for example a bend-stiffener. So yes, changing inod may mean different curvature histories and hence difference in the boundary and pflex stresses.
Transverse curvature stresses in -boundary vs. -pflex model
Q: The Axial stress and Normal curvature stress are observed to be identical for the -boundary and -pflex model part in Xpost. However the Transverse curvature stress differs. Why is this?
A: The Axial stress, Normal curvature stress and Transverse curvature stress are populated by PFLEX both for the -boundary and -pflex part of the model. For Transverse curvature results the -boundary model results only contain the dynamic bending induced part of the stresses, excluding the axi-symmetric part.
Total longitudinal stresses in -boundary vs. -pflex model
Q: The Total longitudinal stress differs from the -boundary to the -pflex model part in Xpost. Why is this?
A: The BOUNDARY and PFLEX procedures are different analysis models with different underlying assumptions. The obtained stress components may hence differ. PFLEX is responsible for populating the Axial stress sigma_ax, Normal curvature stress sigma_nx and Transverse curvature stress sigma_tx result types both for the -boundary and -pflex part of the model. For the -pflex model results sigma_ax + sigma_nx + sigma_tx = sigma_xx, Total longitudinal stress. However BOUNDARY also calculates its own axial and transverse stresses in the analysis step before PFLEX has been executed. The Total longitudinal stress reported in the -boundary part of the model is based on these results. It includes the axial stress, and the transverse curvature stress arising from tilting of the components. It will normally differ slightly from the one reported in the -pflex part of the model. As PFLEX has populated the separate stress components, sigma_ax + sigma_nx + sigma_tx is not equal sigma_xx for the -boundary results.
Stresses in fatigue assessment
Q: Which stress components are relevant to include in the fatigue assessment?
A: For fatigue assessment the -pflex model results stress results can be applied for transverse cracking of the wires.
For longitudinal cracking, a combination of the results from BOUNDARY and PFLEX must be applied. As the BOUNDARY procedure does not include stresses due to ovalization, PFLEX will calculate and report this. PFLEX will also report the local bending stresses due to global bending, both into the -boundary model. These normal and transverse curvature stresses must be added to the Total longitudinal stress from the -boundary model. The Normal, Transverse and Shear stress from the -boundary model must also be included in the von Mises stress calculation.
To obtain consistent results with LIFETIME the following approach must be followed:
-
Axial stress sigma_ax results from the -boundary model, PFLEX result
-
Normal curvature stress sigma_nx from the -boundary model, PFLEX result
-
Transverse curvature stress by sigma_tx + (sigma_xx - sigma_ax) from the -boundary model, to capture transverse curvature stress results obtained by both PFLEX and BOUNDARY
-
Transverse stress sigma_yy from the -boundary model, BOUNDARY result
-
Normal stress sigma_zz from the -boundary model, BOUNDARY result
-
Shear stress sigma_yz from the -boundary model, BOUNDARY result