Mechanical APDL Structural Analysis Guide

a. Fr om E1,L1 to E1,L2: 50.05 ksi

b. Fr om E1,L1 to E2,L1: 0.05 ksi

c. Fr om E1,L1 to E2,L2: 50.00 ksi

d. Fr om E1,L2 to E2,L1: 50.10 ksi

e. Fr om E1,L2 to E2,L2: 0.05 ksi

f. Fr om E2,L1 to E2,L2: 50.05 ksi

Sorting these combinations by decreasing alternating stress intensity gives:

d. Fr om E1,L2 to E2,L1: 50.10 ksi

a. Fr om E1,L1 to E1,L2: 50.05 ksi

f. Fr om E2,L1 to E2,L2: 50.05 ksi

c. Fr om E1,L1 to E2,L2: 50.00 ksi

b. Fr om E1,L1 to E2,L1: 0.05 ksi

e. Fr om E1,L2 to E2,L2: 0.05 ksi

The range counting then proceeds as follows:

d. 500 cycles of E1,L2 to E2,L1 - this uses up 500 cycles of E1 and E2

a. 0 cycles of E1,L1 to E1,L2 - E1 is all used up

f. 500 cycles of E2,L1 to E2,L2 - this uses up 500 more cycles of E2

c. 0 cycles of E1,L1 to E2,L2 - both events are all used up

b. 0 cycles of E1,L1 to E2,L1 - both events are all used up

e 0 cycles of E1,L2 to E2,L2 - both events are all used up

Thus, only 1000 repetitions of about 50 ksi range would be counted, instead of the known 1500 cycles.
This error results solely from improper assembly of events.

If the loadings had each been described as separat e events (such that E1,L1 ≥ E1; E1,L2 ≥ E2; E2,L1 ≥

E3; and E2,L2 ≥ E4), then the following range counts would be obtained:

d. 500 cycles of E2 to E3 - this uses up 500 cycles of E2 and E3

a. 0 cycles of E1 to E2 - E2 is all used up

• uses up 500 more cycles of E3, and 500 of
  E4

f. 500 cycles of E3 to E4

c. 500 cycles of E1 to E4 - uses up 500 more cycles of E4

b. 0 cycles of E1 to E3 - E3 is all used up

e. 0 cycles of E2 to E4 - E2 and E4 are both all used up

Cumulative fatigue damage in this case would properly be calculated for 1500 repetitions of about 50
ksi range.

• Conversely, using separat e events for each maximum and each minimum stress condition could
  sometimes become too conservative. In such cases, carefully choose those loadings that should be
  counted together, and group them into the same events. The following example illustrat es how some
  events can appropriately contain multiple extreme stress conditions:

Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information

Evaluating Fatigue