I think you misunderstood that there is only one type of bolster, installed between the bogies and the car body and it is not made of aluminum alloy. I think that is not the case. There are 2 types of bolsters. One is attached to the bogie via suspension and it can rotate. This type of bolsters are not made of aluminium alloy. There is another set of bolsters welded to the car body. These bolsters are made of aluminium alloy. They are aluminium beam welded across the bottom of the body to reinforce the car body and help to transfer the weight of the body to the bogie. They are called car body bolsters. Not the same as the bolsters of the bogies. The car body bolster sits on the bolsters of the bogie and have attachments for coupling them together.
The official statement said that cracks are found on the car body bolsters (not the bolster of the bogies) so my comments were based on their statement. Anyway, I think the bolsters of the bogies are made of steel and it is unthinkable to find cracks in steel which was invented to replace iron to overcome the brittle nature of iron.
Below is an extract from LTA'so statement :
"We did not send all of the trains back at once as they were still fit and safe for service and we wanted to ensure sufficient train-availability for commuters."
I felt that it takes a very experienced PE with vast exposure to conclude that the car bodies are still fit. He or she must have good knowledge of how aluminium alloy stress crack propagates and felt that it is still safe.
As to why the need to send back the car body with bogies, i believe that it is easier to return the whole thing rather than dismantling it. If I am not wrong, they also deliver the body with the bogies assembled together. Otherwise, they will need to assemble the bodies to the bogies.
Looks like you bought Govt story hook, line and sinker.
There were cracks on the bolster which is made of steel and not aluminium. There were cracks on car body made of aluminium. There are cracks and defects on the bogies itself. All 3 components can be separated easily at any service dept. These are modular in nature.
Yet the entire train is being returned. New car bodies will be fitted and all the bogies and bolsters will be replaced.
Govt well known for what is stated and what has been omitted. Common sense will tell you if its only the bolster, they should have been fitted in a matter of days. It only 2 per car and modular construction. New ones can be sent to Singapore.
Why change the entire car body. Why even sent the heavy bogies back? You got to learn to ask question to clear your own doubt.