Crack Paths 2012

much smaller effect on KS. The effect predominantly occurs at a ratio of applied

nominal K/Kmax < 0.43. A secondary effect on KR is also visible at a ratio of applied

nominal K/Kmax > 0.86 and this presumably reflects the greater peak stress applied in the

overload cycle.

]

. 5

2.0

KI (a=31.74mm) R 4 m)

0

P a .m

[M 1.6

KS(a=31.74mm) I 2 41 )

r s iFty a c t o

1.2

KR (a=32.41mm) S

n s

0.8

TheoreticalKI

e n t a l S t r e s s In te

0.4

0.0

xp -0.4 e r i m

E

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

Theoretical KI [MPa.m0.5]

Figure 10. Stress intensity data calculated through a loading half-cycle and obtained

using equation 1; immediately before (a = 31.7 m m )and immediately after (a = 32.4

m m )application of a single 15%spike overload.

C O N C L U S I O N S

This short paper has endeavoured to show that advanced microscopy techniques are

capable of providing deformation and crack path information for amorphous polymers

that can be used to support emerging models of the underlying physics of deformation

and fracture. Equally, the birefringent properties of polycarbonate make it an excellent

model material to explore improved models of crack tip stresses that take account of

shear stresses and crack wake contact. The paper also briefly outlines a crack tip stress

field model that has been published [21] by the present authors and which leads to a

new set of stress intensity parameters which offer the possibility of characterising the

retarding influences of a crazed region on fatigue crack growth. Further work is

required to explore this possibility.

72