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Occasionally, we are asked the question,".... My notched tensile samples appear to be over 350,000 psi, I thought they were required to be 260,000 -280,000 psi? .... "
This is called notch strengthening and is a normal phenomenon with hydrogen embrittlement samples
Notch strengthening falls in the field of Fracture Mechanics, and can be present in any ductile metal. When your hydrogen embrittlement samples are manufactured they are first blanked out, and then heat treated to the required 260,000 - 280,000 psi tensile strength range. This is the required tensile strength for the base steel alloy, e.g. AISI 4340. A heat treat tensile strength test is then performed on these smooth, un-notched samples to verify the proper heat treat. Then the samples are finish machined and ground, with notch placement the last manufacturing operation to occur. During the final testing and certification of the samples, now with a notch present, a final notched tensile strength test is performed. (In the field of mechanical metallurgy, always remember the difference between a tensile load, in pounds and tensile strength / stress in pounds per square inch or psi )
Do not confuse the results of the heat treat tensile strength test with the notch tensile strength tests. The heat treat tensile strength test establishes proper heat treat - The notched tensile strength test simply provides the notched tensile strength necessary for the test laboratories to evaluate your plated test samples.
Even though the cross sectional area at the base of the notch is less than the cross sectional area of a smooth sample, an increase in the tensile strength/stress happens due to this phenomenon called notch strengthening. For our 4340 low alloy steel, an increase in the tensile strength/stress approaching 50% is possible. This phenomenon is not present in brittle materials, but only in materials exhibiting ductility. Even high heat treat 4340 low alloy steel at 260,000 - 280,000 psi tensile strength will exhibit upwards of 10-14% ductility.
The rest of this Technical Commentary is reprinted portions of section 7.5 of "Deformation and Fracture Mechanics of Engineering Materials", by Richard W. Hertzberg, John Wiley & Sons, New York 1976 which goes into more detail concerning this phenomenon called notch strengthening.
7.5 NOTCH STRENGTHENING
When an appreciable amount of plastic deformation is possible, an interesting turn of events may occur with regard to the fracture behavior of notched components. We saw in Chapter One that plastic constraint is developed in the necked region of a tensile bar as a result of a triaxial stress state; the unnecked regions of the sample experience a lower true stress than the necked section and, therefore, restrict the lateral contraction of the material in the neck. Similar stress conditions exist in the vicinity of a notch in a round bar. When the net section stress reaches the yield strength level, the material in the reduced section attempts to stretch plastically in the direction parallel to the loading axis. Since conservation of volume is central to the plastic deformation process, the notch root material seeks to contract also, but is constrained by the bulk of the sample still experiencing an elastic stress. The development of tensile stresses in the other two principal directions--the constraining stresses--makes it necessary to raise the axial stress to initiate plastic deformation. The deeper the notch, the greater is the plastic constraint and the higher the axial stress must be to deform the sample. Consequently, the yield strength of a notched sample may be greater than the yield strength found in a smooth bar tensile test. The data shown in Table 7.2 demonstrate the "notch strengthening" effect in 1018 steel bars, notched to reduce the cross-sectional area by up to 70%.
TABLE 7.2 Notch Strengthening in 1018 Steel
In this case note that the net section stress will increase with notch depth as a result of the increased plastic constraint. In this manner, you may prove to yourself that materials with limited deformation capacity will notch weaken, and highly ductile materials will notch strengthen.
Two factors need to be emphasized when discussing the observed notch strengthening effect. First, even though the notched component may have a higher net section stress, it requires a lower load for fracture than does the smooth sample when based on the gross cross-sectional area. I trust that this should temper the enthusiasm of any overzealous student who might otherwise race about, hacksaw in hand, with the intent of "notch strengthening" all the bridges in town. Second, there is a limit to the amount of notch strengthening that a material may exhibit. From theory of plasticity considerations, it is shown that the net section stress in a deformable material may be elevated to 2~ to 3 times the smooth bar yield strength value.
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