Stress Concentrations in Bolt-thread Roots

Frozen-stress three-dimensional stress-analysis techniques were used in f i rst phase of th is investigation; the results of these tests were augmented later by fatigue tests and by metallographic inspection of cracked bolts by James D. Chalupnik ~ s s r ~ ~ c r- ~- T h e allparent geometric origin of fatigue fractures in oversize shank holts is considered in this report. Bolt-nut interaction hetweeu the holt and both a standard nut and a special nut are investigated. The "frozen stress " method of three-dimensional stress analysis was used to deter~nine the magnitudes and loca-tions of points of stress conretltrat.ion in plastic models of both the oversize shank holt and a standard holt in cornhinations with standard and special nuts, The results indicate that. for instal lat io~~s in which the special nut is used, bolt geometry may he the cnll-trolling factor in determining the fa~igue life of ' the bolt nut cornbillation, hrcausr the stress-cil>nce~rtratio~r factors due to the nut loading are of the sameorder of magnitudeas thestress-co~rcrntratio~if';~cLorsdue ol ly to holtgeo~netry. On the other hand. fur install;rtionsi~l which standard nuts are used, the nut-induced stress-cntrrrr~tratio~i factors are n~uch larger than those due In holt geometry; so. holt gronretry will have less effect on the fatigue per-form;mcr of the cornhination. If the n u t is threaded onto the bolt so far that the first loaded thread of the nut is in the th read- runm~ ~ area of the holt, fatigue per-formance of the installation will! w greatly reduced due to the coml)ination of s~rt,ss-cil~ticetitratioli factors fronr thread loadin # iirrcl section ch;tn#r and. again, 111)lt gewn-etry may he a prominent causative factor