Page 279 - Failure Analysis Case Studies II

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Table 2
Equivalent reversed stresses and cycles to failure for each weight stack setting

Weight stack setting Fully reversed equivalent stress
(Ib) (MPa) Cycles to failure
~
15 23 1,337,234,019,763
30 47 6,465,350,374
45 70 285,787,954
60 93 31,259,118
75 1 I6 5,617,138
90 140 I ,38 1,747
105 163 422,137
120 186 151,134
135 210 6 1,077
150 233 27,158
165 256 13,047
180 279 668 1
195 303 3609
210 326 204 1
225 349 1200
240 373 73 1
255 396 458
270 419 295
285 442 195

1
Urn = - (crnax + cmin). (7)
2
The ultimate stress of the pin used in Goodman’s equation (5) was estimated to be 324 MPa based
on a Rockwell hardness test and equation (1). The gmin was zero for all weight levels while gmax
was taken from Table 1 for each weight level. The Goodman equation (5) was used to calculate
an equivalent fully reversed stress for each weight level. These new equivalent stresses take into
account the effect of the mean stress on the fatigue life. Once the equivalent fully reversed stresses
were calculated, each was substituted into Basquin’s law (5) to determine the number of cycles to
failure at each weight level. Using the following constants for AIS1 1005 steel [5]: A E 878 MPa,
b r -0.13 and equation (5), the number of cycles to failure were calculated at each weight level.
Table 2 summarizes the equivalent fully reversed stresses using the Goodman equation and the
corresponding cycles to failure according to Basquin’s law for each weight setting.