Abstract: Whether a structure is damaged under dynamic loading is judged commonly by damage value. There are two methods to transform equivalently to damage value from irregular dynamic stress: the first is based on its peak value, and the second is on its extrema. The later overcomes the shortage of the former, and it classes the extrema into stress groups according to whether the extremum sequence runs across the zero line. The max and the margin between two extrema of the stress couples in every extremum group are individually transformed to damage value. It is shown that the result obtained by the first method is a little bigger than that by the second, but in the structure parts where damage is high, the first method exaggerates the damage value to 30% or so. Enriching the fatigue theory, the second method is significant to structural design. It can be applied in geotechnical engineering.