As the traditional damage identification indicator is not sensitive to the early partial damage

a damage identification indicator that can identify the position and the degree of early damage was studied based on strain modal.This indicator measured strain measurements of a series of key points

and then the technique of the parameter identification of strain modal was used to get the strain modal shape

and the differences between the strain modal shapes was gained

thus to achieve the purpose of identifying the position of early damage and degree the damage.Digital simulation and experimental results show that the strain mode shape is very sensitive to the damage at the non-modal node

and the difference between the strain modes is very sensitive to the damage at the modal node.By using these two indicators

the identification of early damage can be realized.