Abstract: According to the bottleneck of metal magnetic memory (MMM) critical characteristic extracting for early hidden damage of welded joints, the biorthogonal method is presented, that is, the orthogonal testing path and the MMM orthogonal vector analysis based on experiments. The experimental materials are Q235B welded plate specimens. By measuring the normal component and the tangential component signals, the magnetic field vector space figure and the Lissajous figure are introduced. By contrasting the results of scanning electron microscope and X ray detection, the MMM changing laws and the critical characteristics of early hidden damage are obtained. The Lissajous figure becomes partial closure when the early hidden damage occurs. And the partial closure area becomes larger and larger with the expansion of the hidden damage. Furthermore, the Lissajous figure becomes completely closed till the specimen is broken. At last two new MMM parameters are presented, that is, the integral of orthogonal gradient vector (S(K)) and the ratio of orthogonal gradient vector sum (Kr). And the quantitative relational model between Kr and the residual life ratio Nr is built which provides a new quantitative evaluation tool of early hidden damage for welded joints in practical engineering.

Key words: biorthogonal, hidden damage, metal magnetic memory, welded joints