Crack Paths 2009

where E and ν are Young’s modulus and Poisson’ ratio, respectively. In this equation,

coefficient

1 A is proportional to mode I stress intensity factor KI and A2 is proportional

θ angles (θ=0, π± , ± 3/2π) A1 factor is eliminated from the

to the T-stress. For three

)ε

(

ε

θ π ±=,0 cannot be chosen for practical reason as

difference

. The angles

rr

θ θ −

gauge directions. Taking the values °±=120 θ , Eq. 5 leads to

2/1

1

2 3

4 r A

E

2 − ≈ + yy

3 A rA

−

xx

(6)

ε ν

ε

+

(

)

4

.

If r is small, an approximation of Eq. 6 gives

E

yy xx − ≈

(

)

ν ε ε

2

A

.

(7)

2

1

+

E

(

)

ε ε

− ≈

, A 2

Y

yy

xx

yy

2

ν

1

+

T- gauge

xx

Line (LL) to calculate

θ

T andK

θ

120°

r

r

3

m

m

X

3

m

m

Distance from notch-tip

Notch-tip

K- gauge

()r3/8EKrr I π ε =

Figure 3. Positions and directions used to determine the T-stress and the notch stress

intensity factor using strain gauges.

The T-stress is measured by this experimental method at a point located at 3

millimetres from the notch tip and is calledTρ* . Whenthe T-stress is measured for 3,mm fracture load, a subscript c is added. Computed values are generally higher than

experimental values (average increase is 15%). In the following, the computed T value

is called Tef and keeps as results.

Material properties and specimen geometries

The material used in this study is an X52 steel meeting requirements of API 5L

standard. In table 2, the mechanical properties of API X52 have been presented.

E, y σ , u σ , A%, n, k and KIc are the Young’s modulus, yield stress, ultimate stress,

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