PSI - Issue 5

G. Lesiuk et al. / Procedia Structural Integrity 5 (2017) 912–919 Lesiuk et al./ Structural Integrity Procedia 00 (2017) 000 – 000

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2.2. Fatigue crack growth rate experiments The used tensile machine, apparatus (e.g. clevises, grips etc.) and specimens were prepared for experiments in accordance with the ASTM E647 (2015). General view on the measurement stand is shown in Fig. 4. For non reinforcement specimens, the tests were performed using constant amplitude loading ( R =0.1, F max =4.5kN). The stress intensity factor (SIF) for the CT specimen is specified using formula presented in ASTM E647 (2015):

W a

2



   

   

2

3

4

W a

W a

W a

W a

  

   

  

 

  

K = F

 0.886 4.64

13.32

14.72

     5.6



,

(1)

3/ 2

B W

W a

    1

  

where: − crack length, − specimen thickness, − specimen width, − applied force.

Fig. 4. The CT specimen during FCGR test: 1 – 50kN load cell, 2 – fracture mechanics clevis, 3 – CT specimen, 4 – clip gage (extensometer), 5 – displacement actuator

For non-reinforcement specimen, the crack length was monitored using compliance method, periodically controlled by visual observations. During survey following signals were registered: force, displacements, crack opening displacement (COD). Amid applying of monotonically arising loading, the crack length size was determined by compliance procedures ASTM E647 (2015). The function of plane stress elastic compliance for CT specimens is described by formula: = + + + + + . (2) Coefficients C 0 , C 1 , C 2 , C 3 , C 4 , C 5 are fully described by ASTM E647 (2015) depending on measurement localization of COD. The u x quantity is defined as: = √ + , (3) where:  represents crack opening displacement (COD) measured from clip gage and E is Young modulus. The proper tests were realized by constant amplitude of force range  F . Before the main investigation, the fatigue pre-crack was made preserving all condition of loading described in ASTM E647 (2015). Since, the described elastic-compliance method application of equation (2) is impossible for strengthened CT specimens, due to significantly changed stiffness of specimen. Therefore, it was decided to apply beach marking method, based on the following active and passive blocks of loading. The beach marks are macroscopic – unlike striation which are microscopic – fatigue feature with explicit difference of colour from area which was created under other loading character, so that man’s eye is able to distinguish both blocks of loading and assume number of cycles N (Fig. 5). The beach mark method is based on