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Threaded Joints:

History:   The Greek mathematician Apollonius (200 B.C. ?) set forth the theory of the spiral helix based upon prior scientific art known simply as the spiral inclined plane. The inclined plane forms the foundation for two critical machine elements used today, that being the wedge and the screw. The work of another Greek mathematician, Archimedes (287-212 B.C) inspired Apollonius to refine his theories of the spiral inclined plane. Few scientific changes or improvements have been made these past two millennium to the theory of the spiral helix or screw as its known today. Mathematical theorems have been put forth relatively recently by Isaac Newton, James Watt, and Jerome Cardan concerning screw thread theory, such that today the principal of the screw thread provides civilization with the ability to not only fasten, but also efficiently transmit power and motion.

Mechanics:   As noted above, the screw or more precisely the screw thread, is a spiral inclined plane or spiral helix. The machine element known as the wedge comes into play forming the basis of the mechanical force capable of being applied by screw threads, with the thread flanks in essence helical wedges. This helical wedge action results in lengthwise displacement (spiral) along the length of the helix. Since this wedge action is balanced across the diameter of the cylindrical helix, a screw thread will naturally try and center itself within a mating thread under longitudinal load. The lowest energy stress state within the screw thread is achieved with radial wedge displacement (RWD) or self centering. The pitch diameter, located on the thread flanks, is the point of maximum load transfer.

Application to Notch Tensile Testing:

As detailed in specification ASTM E292, Conducting Time-For-Rupture Notch Tension Tests of Materials, ref para. 5.1.2 through 5.1.4, the rupture strength of notched samples may be reduced by bending stresses produced by eccentricity of loading or a lack of coincidence between the loading axis and the longitudinal axis of the sample. In essence, the rupture strength goes down as the bending stresses go up due to eccentricity of loading. ASTM E292 shows threaded joints as the preferred gripping method due to the self centering ability of a spiral helix to minimize eccentricity of loading. ASTM E292 limits the bending strains to 10%, but in reality for hydrogen embrittlement testing, the bending strains should be less. (Ref General Electric Aircraft Engines requirement of 8% max) Metallurgically, during an embrittlement test, samples exposed to higher bending strains can exhibit "pseudo" embrittlement failures or greater scatter in a sample population. Bending stresses are additive on top of normal tensile stresses and are governed by the bending stress relationship of f= mc/I. It can be shown that a nominal 0.005" eccentricity during loading can result in an additional +65,000psi localized tensile stress on one side of the notch. Good machine design practice by equipment manufacturers has shown that spherical ball seat attachments, top and bottom in the load chain, in conjunction with thread grip attachment will produce excellent low bending strain tests.


Other Joint Configurations:

ASTM E292, para. 6.5 and Figure 1, show threaded joints as the preferred embodiment for sample attachment; however, other methods such as button head or pins can be used. However, as noted above in the Mechanics discussion, the self centering capability of threaded joints is superior in minimizing eccentricity of loading within the test machine. All known tensile testing equipment available in the world today utilize threaded joints in the machine load chain - threaded joints inherent to the applied and reacted load members in the equipment. Button head gripping devices have shown a slight edge with high elevated temperature creep notched testing, due to threaded joint degradation at the higher temperatures. This is not an issue and the reverse is true with the lower room temperature hydrogen embrittlement testing. Button head notched samples rely on precision machined tight tolerance split collar grips, with dial indicator run-out measurements usually required on each individual test sample to ensure low eccentricity of loading during test.

Often, measurements & adjustments during the test are needed with button head grip samples due to time-under-load movement of load chain components. Button head samples were designed for, and are mainly used in tension-compression type tests, i.e. fatigue tests, where zero axial backlash is needed to prevent "hammering" during cyclic loading. Threaded joints inherently have backlash built into the pitch diameter tolerance of the mating components, and thus are not suitable for tension-compression fatigue testing without jam-nut provision.


B. Effect of Sample Geometry


Sample geometry has an effect on notch tensile testing. (Ref: ASTM E399 and E292) Sample manufacture also is very important, (ref. Menasco Specification TSK 502). Many different sample configurations have been proposed over the years for use in monitoring potential hydrogen embrittlement and the reader is referred to ASTM F519 for a brief description of some. This Technical Commentary is primarily concerned with notched round tensile tests, by far the prevailing method in use today.


Sample Geometry:

It is a given that parts or samples contain defects. These defects can be metallurgical such as porosity, brittle phases, or included material both micro and macro etc. They can also be physical such as scratches, machining artifacts and/or damage such as grinding burns. The science of statistics when applied to fracture mechanics shows that larger parts or samples will contain more defects. When probability is applied to fracture processes, it is easy to see that only one defect is necessary to initiate a failure within a sample. This was proven by Leonardo da Vinci in 1503, with his classic experiments on tensile testing of iron wires of varying lengths. He proved then, as now, that longer sections statistically exhibit physical and mechanical properties inferior to smaller samples, i.e. longer iron wires had a lower breaking strength than short ones.

Oversize embrittlement samples have the potential to exhibit:

1) Less sensitivity to embrittlement, due to sample bulk effects, i.e. a realized dilution of nascent hydrogen within a fixed notched sample area.

or

2) More sensitivity to embrittlement, due to potentially more sites of metallurgical instability. These can be:

   a) air-melt inclusions, primarily of the oxide category in the microstructure,

   b) higher incidence of notch grinding burns due to upwards of a 50% greater notch circumference.

More sensitivity can also occur from higher bending strains due to a greater moment of inertia from a larger sample diameter. This issue of moment induced higher bending strains is why Menasco Landing Gear Corp. (Ref Dwg # TSK 502) and Lockheed-Georgia (Reference LCP77-2033 specification) allows only the standard sample with a 0.175" diameter notch. Multiple self centering joints (threads) within a load chain provide the ability to subtract other bending strains, i.e. self-centering joint compliance has added benefits here. Longer samples with 90 deg. button head shoulders have very limited capability to cancel out other induced load chain bending moments.

Some effort has been made through the years to utilize the ASTM E399 fracture toughness sample for embrittlement tests. However, these have been found wanting, as much longer test times (>500 hours) have been found necessary for equivalent results to the round notched tension samples. Round notched tension samples inherently produce a tri-axial state of stress under load, accelerating or enhancing the test protocol. A good treatise on tri-axial states of stress can be found in Peterson's book on Stress Intensity Factors or Hertzberg's book on Fracture Mechanics.

Standard size samples provide a good compromise with the above. Grinding burns of untempered martensite are always a possibility during manufacture. Untempered martensite grinding burns act as hydrogen sinks, drawing up massive amounts of hydrogen if present. The potential for hydrogen embrittlement initiating at grinding burns is virtually guaranteed. Smaller notch configurations involving smaller amounts of metal removal minimize occurrences of grinding burns. Rogue failures due to bulk effects such as non-metallic inclusions, or other metallurgical defects are inherently reduced with minimized sample sizes.

Whatever sample geometry is chosen, the important goal of any experimentalist is to ensure consistency. Consistency should not be viewed as skewed results, but rather good physics. Test conditions or configurations that produce greater scatter in a sample population allow questions to be raised as to the validity of a test and reduce the confidence of the test's outcome.

W.Craig Willan, P.E. Chief Engineer

Southlake, Texas November 1998



 
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