PSI - Issue 59

Bernadett Spisák et al. / Procedia Structural Integrity 59 (2024) 3–10 B. Spisa ´ k et al. / Structural Integrity Procedia 00 (2019) 000 – 000

6

4

references mentioned above contain the newly generated specimen geometry; it was therefore necessary to determine how the stressintensity factor could be calculated analytically.

Table 1. Dimensions of hybrid specimen. Ratios

1T [mm]

0.5T [mm]

0.25T [mm]

0.16T [mm]

1.5X

75

37.5

18.75 15.63

12 10

1.25X

62.5

31.25

1.2X

60 50 40 25

30 25 20

15

9.6

X

12.5

8

0.8X 0.5X

10

6.4

12.5

6.25

4 3 2

0.375X

18.75

9.4

4.7 3.2 2.5

0.25X

12.5

6.25

0.2X

10

5

1.6 1.5

0.188X

9.4

4.7

2.35

3. Determination of the equation to describe the stress intensity factor The definition of the relation to describe the stress intensity factor applicable to the hybrid specimen consists of several steps, which are the following.  Choose a basic formula shape for the determination of the stress intensity factor.  Perform finite element simulations for different a/W ratios and specimen sizes to plot the stress intensity coefficient as a function of force.  Fit a surface to the defined curves and determine the unknown parameters of the given shape formula for different specimen dimensions.  Unification of parameters so that the relationship applicable to the specified stress intensity factor can be used for all specimen sizes.  Check the validity of the formula with the help of the simulation results. The detailed description of the steps listed above follows. 3.1. Correlation of stress intensity factor for CT and DCB specimens Four load values (15, 20, 30, 35 MPa m 1/2 ) were selected as pre-loading for the measurement. In order to set the required load forces for these values correctly, it was necessary to establish a relationship to determine the stress intensity factor for the hybrid specimen. For this firstly, the formulas for the CT and DCB specimens were analysed. The equation for the determination of the stress intensity factor for the CT specimen can be written according to the ISO 7539-6 (2018) standard as follows:

W a

(1)

2



   

   

2

3

4

P

W a

W a

W a

W a

  

  

  

   

  

   

  

  

0.886 4.64 

13.32

14.72

5.6

,

K



1

3/2

1/2

BW

W a

  

  

1



where P is the magnitude of the load, B is the thickness of the CT specimen, a is the effective crack length and W is the net width of the CT specimen. The imprecision of this expression in the range 0.2 ≤ a/W ≤ 1.0 is up to ±0.5%.