Hi Tony,
I have just had my exhaust repaired by one of our clients. After repairing, he also gave me a report into why he thinks so many of them are cracking. I was not aware of the 3-minute warm-up procedure, but I was naturally doing it as I would run my engine and then go set up my wing. My motor has roughly 30 - 35 hours.
I am relaying his words as I remember them, I am no expert: The exhaust is made in two halves by pressing/forming "thin" sheets of carbon steel into a press. When steel is formed into a mould, you send up with parts of the steel that are stretched more than others. He said that the main cause of the crack is due to the type of metal used (there are better metals, but this increases cost) and that there are parts close to the exhaust of the engine that has been stretch so much that they are wafer thin.
He said that what they could have done was to make the exhaust in more parts, reducing the amount of stretch up at the neck of the exhaust. Vortex Aero has made an aftermarket exhaust that implements this. https://www.vortexaero.com/product-page/vittorazi-moster-engine-exhaust
In my case, I had a big crack like yours and one of the springs had ripped off. He has added a new plate over the top to strengthen the neck. Before and after shots attached.
He charged me 3 hours of labour.
Richard