Materials for engineering, 3rd Edition - (Malestrom)

Determination of mechanical properties 63
Fracture threshold ∆K th (MPa m 1/2 )
10 2
10
1
10 –1
10 –2
Fatigue threshold/fracture toughness
Fatigue
insensitive
materials
(notched)
Polymer
foams
Woods II
to grain
Engineering
ceramics
Al 2 O 3
Nylons
PSI
PS EP PVC PC
HIPS
PMMA HDPE
Woods
⊥ to
grain
Mo
alloys
Tool
MgO steel
-PSZ Zn
alloys Al
Mg
alloys
∆K th =1
Klc
alloys
Engineering
polymers
Ni
alloys
C- S-
steels Ti steels
alloy
Cu
alloy
steels
Alloy
PU
10 –1 PH NR
Filled
BR
Elastomers
10 –2
Fatigue
sensitive
materials
(notched)
10 –1
Engineering
alloys
10 –2
10 –3 10 –2 10 –1 1 10 10 2 10 1
Fracture toughness K fe (MPa m 1/2 )
2.20 Threshold stress intensity range versus fracture toughness of
engineering materials.
monitored as a function of the instantaneous value of the stress intensity.
Cracked specimens are tested in Mode I (crack opening mode) and, in general,
the data are represented on plots of log da/dt versus K in the form shown
schematically in Fig. 2.21.
Three distinct crack growth regimes are shown (though all may not be
present in a given material). In region I, da/dt is strongly dependent on K
and, in some materials, the slope of this part of the curve is so steep that a
value K I EAC may be defined, i.e. a level of K I below which da/dt becomes
vanishingly small.
In region II, the rate of crack growth appears to be relatively independent
of the prevailing level of K. The crack extension rate is still strongly dependent
on the temperature, pressure and the environment, however. Finally, in region
III, da/dt is strongly K-dependent and, eventually, the crack grows unstably
as K approaches K Ic .