Common design mistakes and how to avoid them #4
![[Translate to English:] merkle-partner-konstruktionsfehler-4](/fileadmin/_processed_/6/2/csm_Brennkammer-16x8_6a07234e9c.jpg.webp "[Translate to English:] merkle-partner-konstruktionsfehler-4")
This is a continuation of my blog on common construction mistakes.
Error #4:
Obstruction thermomechanical stresses
The manufacturers of engines, motors, exhaust systems and burners are well aware that thermal expansion in the high-temperature range must only be hindered to a limited extent, otherwise cracks will occur.
From school physics, we are still familiar with the experiment in which a solid bolt breaks when a rod is heated and then the contraction is hindered by the bolt.
Almost every student has seen the enclosed experiment in physics class.
Moderate temperature increases
But what about when the temperature increases are comparatively small?
Here, there are usually few problems with assemblies made of one material.
However, the situation is different if, for example, components such as impellers of fans or turbomachinery are made of aluminum and are connected to steel shafts or flanges.
Here, even small temperature increases of about 60°C are sufficient to generate high stresses if the thermal expansions are hindered.
The classic, which I have encountered several times in my professional career, consists of an impeller connection in which dowel pins are used in addition to bolts.
Steel impellers are replaced by aluminum / silumin as part of a redesign for increased requirements due to weight. The test shows that the shaft / impeller connection slips or imbalances occur after operation, although the system was perfectly balanced beforehand. Now you put bolts to stop the displacements and observe cracks in the holes.
The design flaw here is that an attempt is made to impede thermal expansion.
The coefficient of expansion of aluminum is about twice that of steel.
Solutions here can be, for example, to put a medium between the steel and aluminum that is able to compensate for the expansions without sliding movements. Reducing the diameter of the bolt circle also reduces the thermal stresses.
Thermal shock
Often, temperature shocks during heating or cooling of a machine also lead to cracks. This is the case, for example, when cold water flows through a hot component, e.g. during cleaning, or when a cold component comes into contact with a hot fluid.
Again, there is the classic school experiment in physics class where a heated stone cracks when cold water is poured over it. In this case, the thermal stresses are so high due to the strong temperature gradients within the stone that the fracture stress is exceeded.
Extension
A misconception that I have observed more often among designers is that a body that is heated uniformly (i.e., very slowly) exhibits stresses or expands unevenly. This is not the case until multiple materials with different coefficients of expansion are connected. Otherwise, a body simply becomes uniformly, i.e. geometrically similar, larger at higher temperatures. Stresses only occur here if the expansion is hindered, for example, by the connection environment.
Components with different wall thicknesses
If components with different wall thicknesses are heated or cooled, the thinner components reach the final temperature more quickly than the thicker ones, which can also result in high stresses.
Many cases of damage investigated by us could essentially be traced back to inadmissibly high thermomechanical stresses, in particular also during startup and shutdown of equipment.
The effects described above can all be estimated and evaluated via transient (time) FE simulations.
I look forward to your feedback.
Stefan Merkle