9/10/2015 Omega Research Inc. | NOTCHED TENSILE STRENGTH

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Occasionally, we are asked the question,".... My notched tensile samples appear to be over 350,000 psi, Ithought they were required to be 260,000 280,000 psi? .... "

This is called notch strengthening and is a normal phenomenon with hydrogen embrittlementsamples

Notch strengthening falls in the field of Fracture Mechanics, and can be present in any ductile metal. Whenyour hydrogen embrittlement samples are manufactured they are first blanked out, and then heat treated tothe required 260,000 280,000 psi tensile strength range. This is the required tensile strength for the basesteel alloy, e.g. AISI 4340. A heat treat tensile strength test is then performed on these smooth, unnotchedsamples to verify the proper heat treat. Then the samples are finish machined and ground, with notchplacement 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 mechanicalmetallurgy, always remember the difference between a tensile load, in pounds and tensile strength / stress inpounds per square inch or psi )

Do not confuse the results of the heat treat tensile strength test with the notch tensile strength tests. Theheat treat tensile strength test establishes proper heat treat The notched tensile strength test simplyprovides 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 smoothsample, an increase in the tensile strength/stress happens due to this phenomenon called notchstrengthening. For our 4340 low alloy steel, an increase in the tensile strength/stress approaching 50% ispossible. This phenomenon is not present in brittle materials, but only in materials exhibiting ductility. Evenhigh heat treat 4340 low alloy steel at 260,000 280,000 psi tensile strength will exhibit upwards of 1014%ductility.

The rest of this Technical Commentary is reprinted portions of section 7.5 of "Deformation and FractureMechanics of Engineering Materials", by Richard W. Hertzberg, John Wiley & Sons, New York 1976 which goesinto more detail concerning this phenomenon called notch strengthening.

7.5 NOTCH STRENGTHENINGWhen an appreciable amount of plastic deformation is possible, an interesting turn of events may occur withregard to the fracture behavior of notched components. We saw in Chapter One that plastic constraint isdeveloped in the necked region of a tensile bar as a result of a triaxial stress state; the unnecked regions of thesample experience a lower true stress than the necked section and, therefore, restrict the lateral contraction ofthe material in the neck. Similar stress conditions exist in the vicinity of a notch in a round bar. When the netsection stress reaches the yield strength level, the material in the reduced section attempts to stretchplastically in the direction parallel to the loading axis. Since conservation of volume is central to the plastic

9/10/2015 Omega Research Inc. | NOTCHED TENSILE STRENGTH

http://www.omegaresearchinc.com/Publications/getpub.cfm?pub_id=18 2/3

deformation process, the notch root material seeks to contract also, but is constrained by the bulk of thesample still experiencing an elastic stress. The development of tensile stresses in the other two principaldirectionsthe constraining stressesmakes 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 deformthe sample. Consequently, the yield strength of a notched sample may be greater than the yield strengthfound in a smooth bar tensile test. The data shown in Table 7.2 demonstrate the "notch strengthening" effectin 1018 steel bars, notched to reduce the crosssectional area by up to 70%.

TABLE 7.2 Notch Strengthening in 1018 Steel

Reduction of Areain Notch Sample

yield strength,notched barsmooth bar

0 1.00

20 1.22

30 1.36

40 1.45

50 1.64

60 1.85

70 2.00

In this case note that the net section stress will increase with notch depth as a result of the increased plasticconstraint. In this manner, you may prove to yourself that materials with limited deformation capacity willnotch weaken, and highly ductile materials will notch strengthen.

Two factors need to be emphasized when discussing the observed notch strengthening effect. First, eventhough the notched component may have a higher net section stress, it requires a lower load for fracture thandoes the smooth sample when based on the gross crosssectional area. I trust that this should temper theenthusiasm 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 strengtheningthat a material may exhibit. From theory of plasticity considerations, it is shown that the net section stress in adeformable material may be elevated to 2~ to 3 times the smooth bar yield strength value.

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