Fatigue failure prediction and prevention
.
1.
Which statement is untrue?
The likely cause of machine elements that are found to have failed under the action of fluctuating stresses far below the yield strength of the material is fatigue.
Once a crack is initiated at a hot spot where tensile stresses are maximum the stress concentration may further increase despite of a constant load.
Finally, when the remaining material becomes overstressed sudden fraction occurs.
The zone of sudden fraction is quite similar to the fracture of a ductile material.
2.
Which statement is untrue?
The ratio of maximum local stress to the nominal stress in a notched cross section area is called the "Geometric stress concentration factor Kt".
The "Fatigue stress concentration factor Kf" is smaller or equal to the "Geometrical stress concentration factor Kt".
The Kf factor of ductile materials is larger than for high strength and brittle materials.
It is always save to use Kt=Kf.
3.
A notched beam having a rectangular cross section is subjected to cyclic bending. The thickness in the notched area is reduced from h to 0.8h. The local stress in the notched area is calculated with finite element modeling and appears 2 times the nominal stress in the notched area.
The geometrical stress concentration Kt=1/0.8=1.25
The geometrical stress concentration Kt=1/0.8^2=1.56
The geometrical stress concentration Kt=2
The geometrical stress concentration Kt=2·1.56=3.12
4.
Which statement is untrue?
A small crack might induce high local stresses at the tip of the crack, large enough to result in sudden catastrophic failure.
Especially in aircraft parts with small cracks must be replaced.
The critical value of stresses at which the crack propagate is studied in the field of fracture mechanics.
Crack growth is due to cyclic tensile stress.
5.
Which statement is untrue?
In the "Low Cycle Fatigue" LCF-regime the fatigue strength is only smaller that the tensile strength of the material.
The number of cycles to failure in the LCF-regime is quite low due to cyclic plastic strains.
The "High Cycle Fatigue" HCF-regime is generally between 10e3 and 10e6 cycles.
Steels exhibit infinite fatigue live below the so called endurance limit. of stress.
6.
A notched shaft of an electric motor drive is subjected to bending by belt pulley forces. The shaft rotates with 2800 rpm and the service life of the drive is 2000 hours. Which statement is true
The fatigue strength of the shaft lies in the LCF-regime (below 10e3 cycles).
The fatigue strength of the shaft lies between the endurance limit σe and the elastic limit σm
The fatigue strength of the shaft lies in the infinite life regime.
It's not possible to determine the fatigue strength while the mean value of the stress is still unknown.
7.
Determine the endurance limit of a pull bar with threaded end subjected to a fluctuating load. The tensile strength of the pull bar Rm=1000MPa, the diameter of the threaded part 12 mm.
Fatigue strength σa=50 MPa
Fatigue strength σa=250 MPa
Fatigue strength σa=500 MPa
Fatigue strength σa=800 MPa
8.
A frame is welded from steel 1.0036 pipes. The fatigue strength of the butt welds are classified by weld detail 103 and a fatigue strength parameter K=45 [Eurocode 3, NEN 2063]. The weld is subjected to a constant fully reversed load of Δσi=100 MPa. Calculate the fatigue life.
Fatigue life Ni=1e4 load cycles
Fatigue life Ni=1e5 load cycles
Fatigue life Ni=1e6 load cycles
Fatigue life Ni=1e7 load cycles
9.
The fatigue strength of butt welds can be improved by smoothening the weld toe from K=45 to K=60 [Eurocode 3, NEN 2063]. To what factor will the fatigue life be improved if the weld is loaded in the LCF regime.
Improved fatigue life by a factor 1.3
Improved fatigue life by a factor 2.4
Improved fatigue life by a factor 4.2
Improved fatigue life by a factor >6
10.
Which statement is untrue? The fatigue strength can be improved by
smoothing stress concentrations
moving welded joints to low stressed areas.
Hammer peening and shot peening
single side butt welds in stead of double sided butt welds