# What is the “allowable number of axle-load to failure equation” used in Pavement ME Design?

How is the allowable number of axle-load to failure equation actually used in the software? Is there any documentation regarding the calculation of the tensile strains?

The allowable number of axle-load repetitions to failure, $$N_f$$ is used to calculate the incremental fatigue damage using Miner's law for each analysis interval (which is equal to one month, or two 15-day intervals if there is a freezing cycle in the month). ME pavement design procedure considers the following inputs in the damage calculation process:
• Temperature variation within the pavement structure as a function of time. This is used to calculate asphalt mixture stiffness $$E_{HMA}$$ which is used to calculate $$N_f$$.
Pavement ME calculates the horizontal tensile strain at the bottom of AC layer (for bottom-up fatigue cracking cracking) and at the AC surface (for top-down fatigue or longitudinal cracking) using JULEA (linear elastic analysis procedure). Within each damage computation interval, the strains are calculated for each $$i^{th}$$ group of axle loads (axle passes, $$n_i$$) at critical locations that are defined separately for different axle types. These strains are then used to calculate allowable load repetitions $$N_{f,i}$$ for the $$i^{th}$$ load group, and damage $$D_i = n_i / N_{f,i}$$. Cumulative damage due to all traffic is the sum of incremental damage caused by each load group in all analysis intervals.