Page 76 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
54
(d) Either static loads or fluctuating loads can generate fluctuating stresses.
4 Complete the statement with the correct phrase. _________ of material is chosen
for evaluating the fatigue strength of machine elements.
(a) yield strength
(b) endurance limit
(c) ultimate strength
(d) elastic limit

5 Each of the four elements A, B, C, D carries the same maximum stress ,with
max
stress ratios of +1, 0, −0.5, −1, respectively. Fatigue is most likely to occur on ele-
ment ______.
(a) A
(b) B
(c) C
(d) D

Calculation Questions

1 An alloy steel element has a mean stress of = 250 MPa at its critical section.
m
The stress ratio is r = 0.25. Decide the maximum, minimum and stress amplitude
of the fluctuating stress. Draw a −t curve.

2 The endurance limit of a material is = 200 MPa. Assuming the critical num-
−1
6
ber of cycles N = 5 × 10 , m = 9. Estimate endurance strength when N is 60 000,
0
600 000 and 6 000 000, respectively.
3 A shaft shown in Figure P2.2 carries a maximum working stress = 280 MPa
max
and minimum stress min =−80 MPa at the shoulder. The mechanical properties of
the material are = 800 MPa, −1 = 450 MPa, = 0.3. Assuming the combined
s

influence factor is K = 1.62. If the allowable safety factor is selected as [S] = 1.3.

(1) Draw a simplified − diagram of the material and the element, and find out
a
m
the safety factor from the diagram.
(2) Calculate the safety factor of the shaft.
(3) To increase the fatigue strength of the shaft, how should a designer revise the
design of the fillet radius?
Figure P2.2 Illustration for Calculation Question 3.

4 A shaft carries a fluctuating load, with the maximum stress max = 300 MPa,
the minimum stress min =−100 MPa and constant stress ratio r = C.The
mechanical properties of the shaft material are = 800 MPa, −1 = 450 MPa
s