Materials for engineering, 3rd Edition - (Malestrom)

62
Materials for engineering
10 4
Endurance limit/strength
σ
σ
e
y
= 10
1
Endurance limit σ e (MPa)
10 3
10 2
10
1
10 –1
Fatigue insensitive
materials
(unnotched)
10
Elastomers
I
PI
SQR
NR
CH
PU
Filled
10 –1 PH 10
–2
Engineering
ceramics Ti
Alloy S-steels Mgalloys
and C-
Ni
steels
Cu
AL alloys
alloys Mg Znalloys
Al alloys alloy
alloys
EP
ABST
PPO
PMMA
Nylon
PC PS
PTFE
Engineering
polymers
Woods
⊥ grain
Polymer
foams
Tool
steel
10 –1
Engineering
composites
Wood II to
grain
10 –2
Fatigue
sensitive
Materials
(un-notched)
10 –2 10 –1 1 10 10 2 10 3 10 4
Strength σ y (MPa)
2.19 Endurance limit versus yield strength for engineering materials.
material class, which emphasizes the importance of employing fatigue
properties rather than monotonic properties in design for cyclic loading.
2.7.4 Environment-assisted cracking
Materials are known to fail under stress when the initial stress intensity level
is considerably below K Ic . This is because cracks are able to grow to a
critical size until the stress intensity level is increased to the critical value.
There are a number of processes which may give rise to such crack extension
in the presence of an applied static stress in an aggressive environment,
namely stress-corrosion cracking (SCC), hydrogen embrittlement (HE) and
liquid metal embrittlement (LME). These phenomena are referred to collectively
as environment-assisted cracking (EAC).
da/dt – K curves
The EAC effect is encountered in polymers, ceramics and metallic materials.
In order to quantify EAC processes, the rate of crack advance (da/dt) is