fundamentals of engineering supplied-reference handbook - Ventech!

TESTING METHODS
Standard Tensile Test
Using the standard tensile test, one can determine elastic
modulus, yield strength, ultimate tensile strength, and
ductility (% elongation). (See Mechanics of Materials
section.)
Endurance Test
Endurance tests (fatigue tests to find endurance limit) apply
a cyclical loading of constant maximum amplitude. The plot
(usually semi-log or log-log) of the maximum stress (σ) and
the number (N) of cycles to failure is known as an S-N plot.
The figure below is typical of steel, but may not be true for
other metals; i.e., aluminum alloys, etc.
σ
KNEE
LOG N (CYCLES)
ENDURANCE LIMIT
The endurance stress (endurance limit or fatigue limit) is
the maximum stress which can be repeated indefinitely
without causing failure. The fatigue life is the number of
cycles required to cause failure for a given stress level.
Impact Test
The Charpy Impact Test is used to find energy required to
fracture and to identify ductile to brittle transition.
Impact tests determine the amount of energy required to
cause failure in standardized test samples. The tests are
repeated over a range of temperatures to determine the
ductile to brittle transition temperature.
Creep
Creep occurs under load at elevated temperatures. The
general equation describing creep is:
Q
e
( RT)
n dε
−
= Aσ
dt
83
MATERIALS SCIENCE/STRUCTURE OF MATTER (continued)
where:
ε = strain,
t = time,
A = pre-exponential constant,
σ = applied stress,
n = stress sensitivity,
For polymers below, the glass transition temperature, Tg, n
is typically between 2 and 4, and Q is ≥100 kJ/mol. Above
Tg, n is typically between 6 and 10 and Q is ~ 30 kJ/mol.
For metals and ceramics, n is typically between 3 and 10,
and Q is between 80 and 200 kJ/mol.
STRESS CONCENTRATION IN BRITTLE
MATERIALS
When a crack is present in a material loaded in tension, the
stress is intensified in the vicinity of the crack tip. This
phenomenon can cause significant loss in overall ability of a
member to support a tensile load.
K I
= yσ
πa
KI = the stress intensity in tension, MPa m (1/2) ,
y = is a geometric parameter,
y = 1 for interior crack
y = 1.1 for exterior crack
σ = is the nominal applied stress, and
a = is crack length as shown in the two diagrams below.
a
Exterior Crack (y = 1.1) Interior Crack (y = 1)
The critical value of stress intensity at which catastrophic
crack propagation occurs, KIc, is a material property.
Representative Values of Fracture Toughness
Material KIc (MPa•m 1/2 ) KIc (ksi•in 1/2 )
A1 2014-T651 24.2 22
A1 2024-T3 44 40
52100 Steel 14.3 13
4340 Steel 46 42
Alumina 4.5 4.1
Silicon Carbide 3.5 3.2
2a