Why does something break down? Cause effect analysis

For cooling a generator (airflow) casted fans are made out of aluminum. Due to fatigue one fan broke of and  demolished an expensive generator. The problem was in the design of the aluminum fan. During clotting of the aluminum in the mold, air bubbles arise at the thickness transition between the thick stubshaft and thin vane.  This happens due to the effect that the thin vane will be solid but the thick stubshaft not. Due to this suction will occur pulling air in th eproduct just at the trasition from vane to stubshaft. That is also exactly the place where the highest stresses are present in the vane. The air bubbles were a starting point for fatigue cracks. Read more about this on the page fields / generators……………..

In what order does the molded ridge triangle break down during testing? Does the handle break first or does the molded triangle break first?

On the top foto we see the failure of the ridge construction of a greenhouse when the ends of the handles are moved to each other (buckling of the ridge stiffener triangle). The bottom photo shows the failure mechanisms caused by a high snow load. The people at work did not know what product failed at first due to a high snowload. Did the triangle break out first or did the handle break first. The ridge stiffener consists of two triangular aluminum castings clamped to the two handles with 3 screws.  Due to windloads the two handles can creep out of the clamping triangular. That is why both the handles have a small cut out to overcome this problem. This saw cut can be seen if you click om the link on the page disciplines/kassenbouw ……………….

The question was if the triangular shape breaks down first or the handle? The handle is made by extrusion. The triangular shape is made by molding. Both production technics require a different aluminum quality. The aluminum used for extrusion is very flexible. The aluminum used for molding is much stiffer and brittle.  Now we can conclude which part breaks down first. For the handle to break a lot of bending should take place due to its flexibility. So if the triangular shape was intact the handle cannot bend that much. Only when part of the triangular shape breaks off (position A in the picture) the handle can bend far and will break. This means that at first the triangular end breaks off and after that the handle breaks appart (position B). Nowing this gives you the possibility to improve the strength of the combination. For me it was strange that the people did not know which part breaks first. It is logical reasoning to get this information.

Breaking of glass at specific location in greenhouse with a ridge that can open from the top. There are several types of greenhouses. One of them is a greenhouse at which the ridge can open. This greenhouse was invented to let the heat escape very easy and fast. This greenhouse consists of a steel load carrying structure and aluminum profiles. A motor and long shaft are connected with the windows through pinions to open or close the windows. This can be seen in this drawing. Due to the weight of the opened windows a steel C profile is positioned just underneath the rotation point of this window. At this position just under the rotating glasses the fixt glass breaks. This glass is inserted into the not rotating part of the hinge. It seems like this hinge might rotate too much breaking the glass it is holding in place.

Looking at the drawing and knowing where the glass breaks directs us to the main problem of this part of the greenhouse. No torsional stiffness of the supporting structure near the hinge of the window. In three ways this torsional stiffness can be a problem. There is one solution overcoming all these problems.

At first should be noted that the shear center of the most torsionally rigid structural element near the hinge, the steel C-profile, is point P (see bottom left of the first page of the PDF). A force exerted on the C-profile going through point P (forces F1 and F2) will not twist the C profile. The force F3 does twist the C-profile much much more when applied loads are in the same range. The torsional rigidity of the aluminum has only a minor contribution. Material thickness, shape, way of connection and type of material determine the torsional stiffness of a construction.

When the windows are in open position te weight of the window pushing down on the C-profile will probably not go through point P. This torsion moment can rotate the C-profile with attached hinge. It is the glass that is inserted in this not rotating part of the hinge that is breaking. A more likely cause can be the windload on the windows. This windload on opened windows is diverted into the greenhouse by a force in the rack and a force near the hinge. In the PDF we see that a force due to a windload does not go through point P as well. In the PDF we see that the distance B is bigger as distance A. Assume the windload is as high as the weight of the window then the effect on the twistangle of the C-profile is much higher due to a longer working arm to point P. This means that the windload will give a much higher torsional force on the C-profile compared to the force of the weight of the windows if both windload and weight of the window are in the same range.  

Possible causes of broken windows are;

  • Too low torsional stiffness of the hinge, the C-section and their connections
  • The drive motor pulls on the rack too long when the window is closed 
  • To high torsional forces on the hinge and C-profile due to wind loads

                               Too low torsional stifness of the load carrying structure

The used steel C-profile and how this profile with a strip on its end is attached to the main steel support structure, does not excel in torsional stiffness. Moreover, the fixed and rotating aluminum hinges are very torsional flexible due to its thin shape and the way the profiles are connected. The horizontal forces due to a windload can easily rotate the C-profile. This rotates the whole hinge system. When the windload is in the same range as the weight of the window the rotation of the hinge due to a windload is much bigger. Compare the length of the torque arm A for the weight of the window and torque arm B for wind force acting on open windows.

The underlying glass that often breaks near the hinge is like all the glass in a greenhouse lightly clamped by aluminum profiles. If one of these profiles will rotate, in this case the fixed hinge profile, the underlying glass will bend and break near the clamping.

                           Drive motor for closing windows is on too long

If the engine is still pulling on the pinion when the window is closed, the engine (motor) will pull the window towards the ridge. This can also be the cause of a rotating hinge profile. When the hinge that is connected to the C-profile rotates too much breaking of the clamped glass can be possible. For the glass it is impossible to rotate due to the fact the glass is partly clamped around its 4 edges by aluminum profiles.

                                         Wind loads and twisting of the C-profile

The windows of a Cabrio greenhouse are in moderate wind conditions almost vertical orientated when in open position. The wind generates horizontal forces on the window. Part of this force is taken by the pinion. Another part will be taken by the hinge. A horizontal force acting on the hinge can be able to rotate the fixed hinge profile. The underlying glass clamped around its edges can not rotate so it breaks. You can see that the torque arm in the PDF associated with the wind load (distance B) is three times bigger as the arm for the weight of the window. So this windload can be the main cause for breaking of glass in Cabrio greenhouses.

The solution can be in exchanging the weak torsion C-section with a torsionally more rigid hollow closed section. This hollow section must be connected to the main steel structure in a tosional stiff way. This means that we need two strips welded on the main steelstructure to ceonnect with the hollow section. By using two steel stips also the connectionbecomes more torsional rigid. This lowers the torsional rotation. Off course the aluminum hinge profile should be connected torsional rigid to the new hollow section. So more bolt are needed. 

The simplest solution is to add a rejuvenation on the clamping lips of the fixed hinge profile. Due to the rejuvenatin the lips can bend elastically more easy. This rejuvenation can be included in the extrusion process which forms these aluminum profiles. The steel C-profile and its attached hinge can partly rotate without causing ruptures in the clamped windows.

The funny thing is that a solution can go both ways. You can increase torsional rigidity or you can lower the bending stiffness. Increasing torsional rigidity of th esupport structure can be done bij chaging the weak C-profile with a tosional more stiff closed hollow section. This will not negitively influence the entry of light inside the greenhouse.

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