When modeling the reliability of a system, it is usual to use exponential distribution to model errors that occur randomly throughout the system's useful lifetime (the middle part of the well-known bathtub curve).
When modeling the wearout/aging of the system when it approaches the end of its lifetime, what distribution or processes can be used?
My intention is to use NHPP to model minimal repairs and then model imperfect repairs. However, it is not clear in the literature what kind of distributions I can use and how I can obtain my $w(t)$ ROCOF.
Thank you in advance.