To find the true tensile strength of a weld first a coupon or several coupons are cut from a welded test sample then the surface of the weld is milled to match the surface of the base metal with no scratches or gouges in either side. the test shape profile is smooth and consistent on all sides The sample is often reduced dimensionally to match width to thickness at the weld the reduced area will have a 3 to 4 times length to thickness/ width ratio allowing for the reduces area to extend well beyond any possible heat effected zone.
A simple tensometer can be made with a lever& fulcrum and 2 clamps that are pivot mounted so as to not induce any lateral directional forces the just calculate the ratio of the force applied to the handle end of the lever against the length of the moment arm. sufficient travel of the pulling action of the moment arm needs to be long enough to create failure