Extrapolating Accelerated Test TTF to Predict Use Level Lifetime

Collect TTF at multiple accelerated stress levels.
Fit TTF data to life-distribution (for eg. 2P Weibull) for each stress level, and estimate parameters - each distribution with different eta but same beta (for the same failure mechanism).

Life distribution characteristic (eta) is a function of stress & is equated to stress level using a given Life-Stress-Relationship (LSR) - for eg Eyering Model
Estimate model constants

Now, knowing model constants, estimate eta at various stress levels, including at use level. And knowing beta (for the given failure mechanism) use-level life distribution can be predicted.