| When machine working, it comes into being tittle crack for fatigue, destroy of environment(such as high temperature, low temperature, causticity gas) metallurgy lard, shrink loose, shrink hole, bug of machining and assemble. For circle load, stress convergence comes into being by these cracks, and they extend gradually, result in abate of part or wheel, then lead to severe event.Truss is important part of all kinds of engineering equipment and groundwork establishment. And damage of the truss harm the safety of whole structure .If the damage cannot be found, such severe events as project collapse, facility attaint, even personnel casualty happen, intermits production.Though existing means of damage identification have many kinds of, early damage identification means have few. Very a few of literature involves, but not lucubrate.In this paper a technique about early damage identification basing mode analyzing is presented, and stress curves are used to verify the method. Finite element model of truss in ANSYS is built, quadruplet bars are selected, in every group one bar is designated randomly and early damage is simulated through receding its elasticity; mode of truss is analyzed before and behind damage, mode data of four groups of truss are picked up. These data contain damage message of bars, such math method as difference, square errand , etc are used to show damage message, similar mode shape of every node change relation is denoted, the node of damage bars is found, accordingly damage bars found. Then stress of truss is analyzed before and behind damage, the data are processed through Matlab and selected four groups of bars stress curve is obtained, every node stress value's change validates the accurate of the early damage identification.The technique about early damage identification in this paper is based on computer imitating. Before it works in real project there is such work as real mode data measuring, intact mode databank constituting, noise eliminating, etc.
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