Deformation - Orthopaedic Biomaterials in Research and Practice - page 49

D. False. See E below.

E. True. Bending stiffness of a beam increases as the cube of its
depth. See Table 2.4.

PROBLEM 2.14
Figure 2.22 is a stress-strain diagram. Point P is the proportional limit,
point A is the yield point, and point U represents the fracture conditions.
Which of the following statements are true?

A. This material is not linearly elastic.
B. Stresses exceeding the yield stress produce large plastic
deformations.
C. Young's modulus of the material is 26.7 × 10 9 N/m.
D. U represents brittle failure.

E. This material is not ductile owing to the small difference between
the yield stress and the ultimate stress.

ANSWER:

A. True. The proportional limit is less than the yield stress.

B. True.

C. False. At P , Y = stress/strain = 400 MPa/0.015 = 26.7 GPa (1 GPa =
10 9 N/m 2 , see Table 2.2). Right number, wrong units!

D. False. Plastic/elastic deformation ≈ 3.5.

E. False. Ductility is related to the difference between yield and ulti-
mate strains, not stresses.

PROBLEM 2.15
Consider the load-unload cycle that passes through points A , B , and C in
Figure 2.22. Which of the following statements is true?

600 U

A

500

B

P

400

σ
(MPa)

300

200

100

0.01 C 0.05 0.09

ε

FIGUre 2.22 stress-strain curve (Problems 2.14 and 2.15).