Majmaz:
That’s a pretty tough and broad question. Thickness, t </= .125" (3mm) vs. t > .125" and b/t ratio approx. constant for various pieces; as this relates to ductility of the material of the various pieces? How are the edges of these pieces prep’ed, and how are they loaded?
There are so many things which might affect ductility, that thickness and/or a constant b/t ratio may not even be the most critical ones in my mind. What text books, codes, parts of those codes, or commentaries prompt you to ask this question this way? Could you post that so we could see what prompted the question?
Thin rolled materials in the .125" range, and less, usually show some cold working and hardening due to the manufacturing process which will adversely affect ductility, but also increases strength prior to initial yielding. While thicker rolled materials start to show slightly reduced material properties in the through thickness direction, or across the rolling grain. They also might have (used to be slightly more prone to have) inclusions and/or laminations parallel to the plane of the plate. Thus, the manufacturing process of the piece or pieces in the detail might be important. With processes like continuous casting, hot rolling, cold rolling, casting and forging all having some pros and cons. This whole issue can be drastically affected by a number of different anomalies/irregularities, such as hard spots, stress raisers, residual stresses, etc.
Shape and geometry of the detail is very important, how do the stresses flow? Are there potential stress raisers, etc. Can ductility develop sufficiently and in a gradual way? Orientation of the rolling grain is important, with the primary stresses parallel to the grain being the preferred orientation. Does the configuration of the detail allow the ductility to occur where you want it, and in sufficient quantity for your detail to work properly? Is your detail and/or the pieces compact enough so they yield rather than buckle or deform in some other fashion?
And, of course, it also depends on the type of material. Some materials have a yield point and then a long plastic plateau before their strain hardening range. These materials are generally pretty ductile, but you want toughness too. While some other materials have a yield strength (not a well defined point), without a well defined plastic range, and go right into strain hardening, up-slope on the stress-strain curve, prior to their tensile failure.
