Page 40 - Mechanical Behavior of Materials

Problems and Questions 39

(c) Plastic scissors develop a small crack just in front of one of the finger rings.
(d) A copper water pipe freezes and develops a lengthwise split that causes a leak.
(e) The steel radiator fan blades in an automobile develop small cracks near the base of the
blades.
1.2 Repeat Prob. 1.1 for the following failures:
(a) A child’s plastic tricycle, used in rough play to make skidding turns, develops cracks
where the handlebars join the frame.
(b) An aluminum baseball bat develops a crack.
(c) A large steel artillery tube (barrel), which previously had cracks emanating from the
rifling, suddenly bursts into pieces. Classify both the cracks and the final fracture.
(d) The fuselage (body) of a passenger airliner breaks into two pieces, with the fracture
starting from cracks that had previously initiated at the corners of window cutouts in
the aluminum-alloy material. Classify both the cracks and the final fracture.
(e) The nickel-alloy blades in an aircraft turbine engine lengthen during service and
rub the casing.
1.3 Think of four deformation or fracture failures that have actually occurred, either from your
personal experience or from items that you have read about in newspapers, magazines, or
books. Classify each according to a category in Fig. 1.1, and briefly explain the reason for
your classification.

Section 1.3
1.4 As an engineer, you work for a company that makes mountain bicycles. Some bicycles that
have been in use for several years have had handlebars that failed by completely breaking off
where the handlebar is clamped into the stem that connects it to the rest of the bicycle. What
is the most likely cause of these failures? Describe some of the steps that you might take to
redesign this part and to verify that your new design will solve this problem.
1.5 Repeat Prob. 1.4 for failures in the cast aluminum bracket used to attach the rudder of a small
recreational sailboat.
1.6 Repeat Prob. 1.4 for failures of leaf springs in small boat trailers.
1.7 A plate with a width change is subjected to a tension load as in Fig. A.11(c). The tension
load is P = 3800 N, and the dimensions are w 2 = 30, w 1 = 14, and t = 6 mm. It is made of
a polycarbonate plastic with yield strength σ o = 65 MPa. In a tension test, as in Fig. 1.3,
this material exhibits quite ductile behavior, finally breaking at a strain around ε f = 110
to 150%. What is the safety factor against large amounts of deformation occurring in the
plate due to yielding? Does the value seem adequate? (Comment: Note that the stress units
2
MPa = N/mm .)
1.8 A shaft with a circumferential groove is subjected to bending, as in Fig. A.12(c). The bending
moment is M = 150 N·m, and the dimensions are d 2 = 22, d 1 = 14, and ρ = 3 mm. It
is made of a titanium alloy with yield strength σ o = 900 MPa. In a tension test, as in
Fig. 1.3, this material exhibits reasonably ductile behavior, finally breaking at a strain around
ε f = 14%. What is the safety factor against large amounts of deformation occurring in the
shaft due to yielding? Does the value seem adequate? (Comment: Note that the stress units
2
MPa = N/mm .)