Saw this advice on the Brompton site which means they are not immune from fatigue issues because some of their parts or upgrade parts are aluminium and so they have recommendations for mileage for replacing those parts.
https://brompton.zendesk.com/hc/en-u...r-rider-safety
Many components on a bike are highly stressed, and with high mileage, heavy loads or hard riding, will eventually reach the end of their design life; in particular, aluminium alloy has a limited fatigue life. Failure in use can cause injury. You should check all load-bearing parts for any signs of damage, corrosion, or cracking, and replace if necessary. Please visit an authorised Brompton dealer for expert advice if you are uncertain.
Like all lightweight machines, the Brompton features aluminium alloy parts which have a finite design life before failure; in normal use, the risk of aluminium fatigue failure is remote, even after many thousands of miles, but the risk of failure increases with use, especially with hard riding or severe loading.
As such a failure could cause injury, we recommend that these items are checked regularly and that the folding pedals, aluminium seatpost, hinge clamp plates, handlebar and chainset be replaced every 5,000 miles (8,000 kilometres), or more frequently on a bike subjected to particularly hard use.