Since true strain and engineering strain are not equal quantities, obviously the material must fracture at a point with different values of engineering and true strain.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

True strain or engineering strain quantities are used for specific engineering applications, and both are considered acceptable for use.

An additional clarification to my post above is that a true engineering stress/strain curve may be used for specific engineering applications, while an engineering stress strain curve is probably more widely used for comparison of mechanical properties in technical literature. Both curves are considered equally acceptable for use, one just has to remember that the quantities are not the same for comparison.

why the fracture point in ture curve happen at strain lower than engineering curve

I would go back and re-evaluate your plotted data.

By definition  

true strain = ln (1 + engineering strain)

similarly;

true stress = engineering stress (1 + engineering strain)

True strain is an additive quantity based on the above definition, so I don't see how the engineering strain could exceed true strain in a standard tensile test for metals.