Page 144 - Plastics Engineering

Mechanical Behaviour of Plastics 127

Table 2.2
spital fracture toughness parameters for a range of materials (at 20°C)
Ductility
Factor
KI~ (5) (in mm)
GI, ($)'I2
Material (kJ/mz) (m/m3I2)
ABS 5 2-4 0.13 17
Acetal 1.2-2 4 0.08 6
Acrylic 0.35-1.6 0.9-1.6 0.014-0.023 0.2-0.5
EPOXY 0.1 -0.3 0.3-0.5 0.005-0.008 0.02-0.06
Glass reinforced polyester 5-7 5-7 0.12 14
LDPE 6.5 1 0.125 16
MDPEMDPE 3.5-6.5 0.5-5 0.025-0.25 5-100
Nylon 66 0.25-4 3 0.06 3.6
Polycarbonate 0.4-5 1-2.6 0.02-0.5 0.4-2.7
Polypropylene copolymer 8 3-4.5 0.15-0.2 22-40
Polystyrene 0.3-0.8 0.7-1.1 0.02 0.4
UPVC 1.3-1.4 1-4 0.03-0.13 1.1-18
Glass 0.01 -0.02 0.75 0.01 0.1
Mild Steel 100 140 0.5 250

2.18 Stress Intensity Factor Approach to Fracture
Although Griffith put forward the original concept of linear elastic fracture
mechanics (LEFM), it was Irwin who developed the technique for engineering
materials. He examined the equations that had been developed for the stresses
in the vicinity of an elliptical crack in a large plate as illustrated in Fig. 2.66.
The equations for the elastic stress distribution at the crack tip are as follows.
cos (f) { 1 -sin (f) sin (y)}
B --
- (2nr)l/2
K cos (4) { 1 + sin (:) sin ( ) }
By = -
(2nr) 112

txy =- sin (!) cos (i) cos ( y)
(2nr) 1 12
and for plane strain
fH\
u -- cos (2)
- (2Irr)1/2
or for plane stress, a, = 0.
Irwin observed that the stresses are proportional to (nu)'/2 where 'u' is the
half length of the crack. On this basis, a Stress Intensity Factor, K, was