Predicting 3D Fatigue Cracks without a Crystal Ball

Turbochargers increase the power and boost the fuel efficiency of internal combustion engines, but engineering teams find they pose unique design challenges. For example, because the turbine is driven by the engine’s own hot exhaust gases, components must withstand widely varying thermal stresses as temperatures cycle between 120 and 1,050 degrees Celsius for engine speed variations relating to idle, acceleration and braking.

In particular, components such as the cast-iron housing that directs hot gases into the turbine are subject to thermomechanical fatigue cracking - a problem that often is not discovered until parts fail in qualification tests. To replicate four to five years of severe thermal shock loading - far greater than parts would  experience in normal operation - engineers perform rounds of tests that each can be very expensive and take weeks to complete.

Several of these rounds generally must be performed before arriving at a workable design that passes scrutiny. Many stress intensity factor formulas are available in handbooks for predicting fatigue crack growth with simplified 2-D geometries; typically, though, these formulas are not applicable for complex part geometries under elastic-plastic conditions in high-temperature environments with multi-axial loading. As a result, many part designs are based on modifying previous geometries, trial -and- error testing cycles and, in many cases, “crystal ball” best-guess predictions based partly on conjecture and simplified assumptions.

Honeywell Turbo Technologies overcomes these limitations by using ANSYS Mechanical software together with the ANSYS Parametric Design Language (APDL) scripting tool to calculate the probability of a crack initiating as well as its most likely growth rate, length and 3-D path.

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