Virtual Durability Process: Bringing Together MBD, FE and Fatigue

Simulation is indispensable within any product development organization. We at Altair believe this outright and you see that in the vision statement from our CEO, to radically change the way organizations design products and make decisions. Simulation driven design is applied at different stages of development, from concept stage to the final design tuning and validation. Optimization is an integral part of it, so your starting point is an optimal design.

As practitioners of this sentiment, what do you encounter in your surroundings? Can simulation reach its full potential? An interesting observation in the article is that a cultural shift is needed. I experienced this first hand a few months ago in a phone call with a customer who would only rely on physical testing and wouldn’t trust the results of simulation for fatigue design. Do you run into skeptics within your organization who don’t think simulation adds value?

When designing for durability through simulation, there are many factors like loads, convergence of results, material properties, modeling practices, etc. that can affect the prediction of life. While it’s a general rule of thumb that 10% variation in stress results affects fatigue results by 2X – in the realm of fatigue life, is it a satisfactory result if the predictions are within 2X of the actual values?

The long test cycles, costs associated with test set up and fixing of issues late in the cycle makes it imminent that durability considerations start early in design. Virtual durability analysis emphasizes simulation driven process bringing together the best of analysis methods of MBD (multi-body dynamics), FE, and fatigue. As written in an excellent article by my colleague, virtual durability process doesn’t have to replace testing, but minimize it or augment it to enable a rapid turnaround process as well as provide a deeper insight into the system in a cost effective manner.

MBD analysis plays a significant role in fatigue evaluation. Component loads can be extracted from a system level MBD analysis, which coupled with the FE methods to obtain detailed stresses feed into the fatigue methods thereby enabling the cycle of a virtual durability process. MotionSolve, the MBD solver, and OptiStruct, the FE solver, work in tandem with fatigue tools in the Altair Partner Alliance (APA) products nCode DesignLife, ECS FEMFAT, and CAEFatigue Vibration to accomplish this goal.


Dynamic systems can be evaluated in time domain or frequency domain. A simplified process for a typical time domain approach is shown below. Since the event durations are longer, instead of solving as a direct transient problem, a common approach is to solve in the modal space and combine the modal stresses with modal participation factors (MPFs) to get the transient data. Depending on the number of modes captured, the required number of time steps, and the size of the model, the calculations can be quite compute intensive, which is done in the fatigue tools (or a postprocessor like HyperView for general transient animations and contours).

In OptiStruct 13.0 or newer version, an option to recover transient stress history from the MBD run is available which greatly speeds up the calculation (at the expense of file size on disk, particularly when analyzing multiple events).

A typical time domain approach for fatigue, stress calculation done in fatigue tools.

A typical time domain approach for fatigue, stress calculation done in fatigue tools.

Time domain approach for fatigue, stress recovery done by OptiStruct.

Time domain approach for fatigue, stress recovery done by OptiStruct.

Shown below is a typical process for frequency domain fatigue evaluation. The load history from MotionSolve is converted to a PSD load matrix in the preprocessing stage of the fatigue tool and multiplied with the FRF stresses from OptiStruct. The process is much more efficient when evaluating large systems with multiple inputs across a wide number of events.

Frequency domain approach for fatigue.

Frequency domain approach for fatigue.

Our partner Neil Bishop of CAEFatigue, who is one of the leading researchers in the domain of vibration fatigue, gives a succinct comparison of the two approaches as below. There are basically 2 areas where benefits occur – related to performance and information generated.

In terms of performance, there can be a huge benefit because in the time domain it is necessary (for dynamic systems) to perform a full transient dynamic analysis for every event applied to the structure in order to obtain the loading Modal Participation Factors (MPF’s). Typically up to 100 such events might be specified – meaning 100 full transient dynamic analyses have to be done. In the frequency domain the system properties (transfer functions) are independent of the loads applied. Therefore, only one structural analysis is necessary.

In terms of information generated, the results from a transient dynamic analysis can be processed to obtain fatigue damage at every point on the structure. This provides fringe plots of damage and absolute values. So we will know if the structure survives. In the frequency domain we get the same fringe data (as well as numerous other statistics like the irregularity factor). We can also easily see, for each event individually, the damage done, the response statistics like rms, the irregularity factor and the linear peak stress and non-linear elastic-plastic strain. Crucially, at any point, we can obtain the response PSD. This will indicate the frequencies where resonances occurred. So, not only do we see what the fatigue life and damage that occurred were, but we also see why that damage occurred. Then, if we combine that with the event by event histogram for damage, we see what the damage was, what caused it, and what to do about it (which event to focus on).


More detailed publications available here for those interested.

The advantages in the frequency domain approach made it possible to undertake a BIW gauge optimization project that was presented at the recently concluded European Altair Technology Conference, which impressively demonstrates many of the HyperWorks products in loop with the fatigue tool accomplishing a virtual durability process with a weight reduction goal.


Questions or comments? Email

To learn more about CAEFatigue VIBRATION, FEMFAT or nCode DesignLife, visit

Sridhar Ravikoti
Sridhar Ravikoti

About Sridhar Ravikoti

Sridhar Ravikoti is the Technical Director of Global Partner Programs at Altair. He has been with Altair since 2000, gaining experience in engineering product development and software program management. In his current role as a technical lead for the Altair Partner Alliance, Sridhar drives a synergetic relationship between Altair offering and its Partners. He holds a Bachelor’s degree from Osmania University in India, and a Master’s degree from the University of Nebraska-Lincoln, with a major in Mechanical Engineering and a minor degree in Applied Mechanics.