Frequent asked questions about stress analysis

What is the difference between finite elements stress analysis and strain gages?
These analyses are used in different situations and, in a way, they can be considered complementary. The strain gage is useful when some data necessary for the stress analysis is unknown, or to confirm the values found in the finite element stress analysis.
The advantages of the finite element stress analysis are: there is no limitation (for example the lack of access for the gage reading, high temperatures, dynamic loads etc).  In several cases its cost is lower than the strain gaging. 
It is important to consider that strain gage readings made in a complex component, without the previous knowledge of the strain distribution will not get good results and for that matter, a finite element stress analysis must be done in order to dtermine the correct locations for the gages.

Does the ASME Code predict the use of finite elements stress analysis for the pressure vessel design?
Yes, the ASME Code, Section VIII, Division 1, Paragraph UG-23 (c ) and (e); Section VIII, Division 2,Part 5 and Section VIII, Division 3, Paragraph KD-230, show the criteria for the evaluation of the stresses found in the stress analysis.

What is the precision of the finite element analysis?
If the supplied data are correct and the model is adequate, the precision is extremely high. In cases where the results of the finite element stress analysis were compared with the high precision strain gage readings on load cells, the difference found were only 0,3%. Also comparing finite element stress analysis with known formulas for stresses in cylindrical pressure vessel, the difference found varies from 0,022% to 0,45%.

Is an elastic stress analysis valid when there is plastic deformation?
Yes, as long as the plastic deformation is localized and caused by secondary stresses. In these cases, the ASME Code criteria evaluates the pseudo-elastic stress, obtained in an elastic analysis.

What is the software used by TRESCA for the stress analysis?
TRESCA uses the ANSYS software.

Should a stress analysis be done to evaluate the stress corrosion cracking phenomenon?
The stress corrosion cracking phenomenon depends on many variables, such as temperature, concentration, chemical environment, besides the stress. And that is the reason why is difficult to determine a criteria for the evaluation of the stress found. Also, as the welding residual stresses are very high in comparison with the working stresses, the stress analysis may not give all the required informations for the evaluation of this damage mechanism.