Hey guys, when you run a direct shear test and you tighten the load bar down so that there is a small negative horizontal load before shearing, do you the correct for that load and add it to the peak? Say you begin shearing at -3 lbf and then reach a peak of 44 lbf, is this peak resistance then 47 lbf?
Not unless that would cause the force on the load cell or load ring to be different after the specimen has yielded and slid (after it's out of any sort of linear elastic response). I'm not sure exactly what your DS machine looks like, but draw yourself a freebody diagram, and I think it will tell you that the only horizontal forces are the amount indicated by the load cell and the shearing resistance in the specimen, with no effect from whatever load eccentricity existed before you started cranking the machine.
