How to Design Components for Thermo-Mechanical Fatigue

Thermal simulation of an exhaust manifold

Industry trends are demanding that components become lighter, smaller and more efficient.

For engineers that translates to: “Did you get my report on increasing the thermal-mechanical loads while making our part smaller? Yeah, I’m going to need you to do that.”

The challenge is that these loads influence the overall durability of the component thanks to thermal-mechanical fatigue (TMF). As a result, keeping the loads constant, let alone increasing them, while designing a smaller part is no easy task.

Unfortunately, you can’t fight city hall as numerous industries are following this trend. For instance, look at the electronics industry that is always increasing the power density of their ever-shrinking products. Meanwhile, the transportation industry is demanding more efficiency from their turbochargers while maintaining that their vehicles get lighter.

Fortunately, simulation can play a crucial role in speeding up the design and testing of components that are at risk of TMF. ANSYS offers a high-fidelity solution for predicting the inherent multiphysics associated with TMF.

Let’s dig a little deeper into the thermal power industry where the trends are pointing toward ultra-supercritical power plants. These power plants are being designed to, you guessed it, increase efficiency and reduce emissions.

Thermal stress simulation of an exhaust manifold.

The IEA’s Technology Roadmap: High-Efficiency, Low Emissions Coal-Fired Power Generation tells us that these new plants need to operate at peak steam temperatures, perhaps higher than 700 C (1290 F). On top of that, these plants are also expected to operate with a load factor that can vary significantly due to the increased contribution of renewable energy to the grid. The moral of the story is that these requirements are also associated with an increased risk for TMF.

John Shingledecker’s thesis Metallurgical Effects on Long-Term Creep Rupture in a New Nickel-based Alloy shows that conventional alloys are not able to meet the design requirements of these thermal-mechanical conditions. Instead, heat-resistant alloys, like those based on nickel, are used in critical regions to meet the design requirements. Unfortunately, these alloys can get quite pricy.

The engineering challenge then becomes designing components (like boilers, turbines, valves and piping) that minimize the use of these expensive materials without sacrificing function under the equipment’s normal operating conditions. The engineer also needs to determine how the components will respond to conditions outside of normal operation, such as at startup or with potential extreme conditions.

ANSYS simulations enable you to solve for a high-fidelity temperature distribution by considering all the mechanisms of heat flow. For instance, with our computational fluid dynamics (CFD) products you can analyze a detailed conjugate heat transfer where all the fluid effects and thermal effects are considered. Users will also have access to models that can assess combustion, turbulence, multiphase, radiation and more.

Alternatively, you can also perform a simpler thermal analysis in ANSYS Mechanical. In this case, you don’t solve for the fluid flow. Instead, you assume heat transfer coefficients at the fluid interfaces. To increase accuracy, however, you can determine the heat transfer coefficients from CFD simulations.

The final transient temperature behavior is then mapped onto the structural analysis to see how the component behaves in response to the thermal and mechanical loads. The stress and strain that are developed for a set of representative cycles (as shown in Figure 1 and 2) are then used to calculate where local damage occurs and the service life of the component.

This image outlines the workflow to perform a thermal-mechanical fatigue analysis from CAD to transient thermal, stress and fatigue simulations.

ANSYS high-accuracy and streamlined solutions for TMF, as seen in Figure 3 are used across industry segments to develop innovative designs using new materials under harsh conditions in less time. To learn more about how these simulations are made, check out this Thermo-Mechanical Fatigue White Paper.