We Claim:
1. A fracture analysis method for a spot welded portion comprising the steps of:
calculating a maximum allowable load value, the maximum allowable load value being a maximum load value at which a crack occurs, of a welded portion in a load fracture mode, a moment fracture mode, and a nugget interior fracture mode based on at least one of a sheet thickness t, a tensile strength TS, an elongation El, and a chemical composition of a nugget portion (1) in each of spot-welded steel sheets, a nugget diameter d of the welded portion, an effective width B of the welded portion determined by a distance between adjacent welded portions, edges or ridge lines, and a sectional height H; and finding the fracture mode where the maximum allowable load value is reached first in the load fracture mode, the moment fracture mode, and the nugget interior fracture mode;
calculating an allowable load value at every moment after the maximum allowable load value of the welded portion is reached, in the fracture mode where the maximum allowable load value is reached first in the fracture modes, and finding a displacement or a time at which the allowable load value becomes 0 where a complete fracture occurs; wherein the maximum allowable load value Fs of the welded portion in the load fracture mode is calculated as
Fs=TS'W-f/a -(3)
where a is a stress concentration factor obtained by conducting a shear tensile test or a cross tensile test, TS is a tensile strength, W(mm) is a width W(mm) of the test piece and T(mm)is a thickness of the test piece; the maximum allowable load value Mlim of the welded portion in the moment fracture mode is calculated as
Y=Mp/M -"(4)
Milim^Mp'/i - (5)
where M is a bending moment M obtained by conducting a flange tensile test, Mp is a full plastic moment of the test piece and Mp' is a full plastic moment to arbitrary materials, and
the maximum allowable load value Fs of the welded portion in the nugget interior fracture mode is calculated as
Fs=Gxn(d/2)2x(fxCta+g) "-(6)
c^x^-ivc.;}/^^! -(7)

wnere 1 is a Kina or a Lest piece, a is a nugget diameter, Ceq is a thickness-direction weighted average of a nugget portion carbon equivalent, and e, f and g are coefficients.
2. The fracture analysis method for the spot welded portion according to claim 1, wherein the maximum allowable load value of the moment fracture mode is corrected by decreasing the effective width B as the sectional height H of a structure becomes larger.
3. The fracture analysis method for the spot welded portion according to claim 1, wherein in the step of finding the maximum allowable load value, when the number of the welded steel sheets is three or more, two welded portions or more where the three steel sheets or more are joined are separately subjected to the determination, and in the determination, as the sheet thickness of the steel sheet stacked on a rear surface side, a total sheet thickness of the steel sheets stacked on the rear surface side is adopted.
4. The fracture analysis method for the spot welded portion according to claim 1, wherein detailed fracture information is output.
5. A fracture analysis device for a spot welded portion comprising:
an input means (101) for inputting at least one of a sheet thickness t, a tensile strength TS, an elongation El, and a chemical composition of a nugget portion in each of spot-welded steel sheets, a nugget diameter d of the welded portion, an effective width B of the welded portion determined by a distance between adjacent welded portions, edges or ridge lines, and a sectional height H; a first calculating means (102a) for calculating a maximum allowable load value of the welded portion in the load fracture mode by using information inputted by the input means;
a second calculating means (102b) for calculating a maximum allowable load value of the welded portion in the moment fracture mode by using information inputted by the input means;
a third calculating means (102c) for calculating a maximum allowable load value of the welded portion in the nugget interior fracture mode by using information inputted by the input means;
wherein the maximum allowable load value is a maximum load value at which a crack occurs;

a means for finding the fracture mode where the maximum allowable load value is reached first in the load fracture mode, the moment fracture mode, and the nugget interior fracture mode;
a means (102d) for calculating an allowable load value at every moment after the maximum allowable load value of the welded portion is reached, in the fracture mode where the maximum allowable load value is reached first in the fracture modes, and for finding a displacement or a time at which the allowable load value becomes 0 at which a complete fracture occurs,
wherein the first calculating means (102a) for calculating a maximum allowable load value of the welded portion in the load fracture mode is adapted to calculate a maximum allowable load value Fs by using Equation (3);
Fs=TS*W-t/a •••(3)
where a is a stress concentration factor obtained by conducting a shear tensile test or a cross tensile test, TS is a tensile strength, W(mm) is a width W(mm) of the test piece and T(mm) is a thickness of the test piece; the second calculating means(102b)for calculating a maximum allowable load value of the welded portion in the moment fracture mode is adapted to calculate a maximum allowable load value Mlim by using Equations(4) and (5);
y= Mp/M - - • (4)
Mlim^Mp'^y ""(5)
where M is a bending moment M obtained by conducting a flange tensile test, Mp is a full plastic moment of the test piece and Mp' is a full plastic moment to arbitrary materials, and
the third calculating means (102c) for calculating a maximum allowable load value of the welded portion in the nugget interior fracture mode is adapted to calculate a maximum allowable load value Fs by using Equations (6) and (7);
Fs=-e*IT(d/2)2M(f*Ce<|+g) -(6)
c„,«i;-1Mvcj}/