Analysis Type: Linear Elastic Static Model, Fatigue Analysis with NX Nastran and Fatigue Essentials
A static pressure and fatigue analysis was performed for a four-cell stacked simulated moving bed (SSMB) column (see Figure 1) to evaluate whether it would meet ASME Section VIII, Div. 2 requirements. The structure was modeled in half symmetry with the most conservative side of the structure in the finite element model (see Figure 2) using a combination of plate and hexahedral solid elements. Fillet welds in the structure were modeled using rigid elements constraining only the translational degrees of freedom (DOFs). Releasing the rotational DOFs represent the physics of a single sided fillet weld and allow weld forces to be extracted quickly and easily. The analysis is linear elastic which allowed the stresses to be scaled linearly with pressure.
A 75 psi pressure load was applied to the inside of the structure (see Figure 3) to evaluate the weld stress at the maximum possible pressure. The stress was normalized to a 1 psi load and then scaled to find the weld stress at each pressure in the operating cycle. Fatigue Essentials was leveraged to count the fatigue cycles using the rainflow-counting method and calculate the cumulative damage over the lifetime of the pressure vessel. The operating cycle of the SSMB pressure vessel was a series of repeating patterns which were stored as Fatigue Essentials Spectrums (see Figure 4). The S-N curve for the material was added to Fatigue Essentials from ASME Section VIII, Division 2. With all of the data input into a Fatigue Essentials project file, the cumulative fatigue damage in the welds was calculated at the click of a button.
The combination of linear analysis in NX Nastran and Fatigue Essentials provided an efficient, robust method to evaluate the fatigue life of a production pressure vessel. Not only were we able to confidently report to our client that their pressure vessel would not fail during the production lifespan due to fatigue damage, the analysis allowed the client to save money by switching to stitch welds in low stress areas of their structure.