Fatigue Crack Paths 2003

permitting to neglect the presence of a third material. A n appropriate grip-head was

realised in test specimens to avoid non homogeneous load distribution. Material

properties are reported in Table 1.

Table 1. Elastic properties

Material

Youngmodule [Mpa]

Poisson ratio

6061-T6

64000

0.33

PSM-l

3220

0.35

Tests have been performed by using an tensile test machine together with a classical

white light circular polariscophe. Photoelastical images have been recorded with a

NikonD1 digital camera which was fully driven by a computer.

P H O T O E L A SMT EI CT H O D

The fotoelastic fringes where then analysed with the frange 3.0 software, which was

developed by the authors [10], [11], providing a rapid, simple, full photoelastic analysis.

This software, which implements a Sanford and Dally least square method with the

overdeterministic Newton-Rapsonapproach, permits to evaluate stress intensity factor

value(SIF). This methodis based on the assumption to express the stress function with a

Cartesian formulation:

6 X( K l : K I T 2 6 O :r a 9 ) 6 y( K 1 2 KIT: 6 0 ’r a 9 ) Txy ( K l : KIT: 6 02 r29 ) we can write the m a x i m u imn plain shear stress as:

( G y -G X)2 + ( Z T X y)2 = ( 2 T m a )x2

the fundamental photoelastic relation is:

L ’

(8)

2 * ’ : : m a x

t

T h e nit canbe written:

2

fi ( K 1 , K 1 1 , O - o ) I ( G y - G X ) Z + ( Z T X y ) Z - L ¥ ] : 0

if w e give an estimation of K1, K2, 60 w e are sure to obtain j§ (K1, K2, 0', ) I 0 but w e

can evaluate the correction with expansion of this relation to a Taylor series:

m e o w5f JIAIQJLJIAKMQ‘ jag,

(10)

6K1 ,

6K1] ,

0 ,