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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changing stress ratio R (up to 0.6) by preserved F max value and there are always two values of R used for the test. For strengthened specimens, the active block loading ( F max =6kN, F min =0.6kN) duration was N a =75 000 cycles, and duration of passive block loading ( F max =6kN, F min =3.5kN) N p was equal 35 000 cycles.

2.3. Experimental results of FCGR

For non-strengthened specimen the fatigue lifetime was obtained direct from experiment. However, in case of the CTs specimens with CFRP patches, the obtained fatigue lifetime (based on active and passive blocks) was incorrect. In order to calculate equivalent fatigue lifetime (for comparison) it need to eliminate the passive blocks from lifetime calculation. As a proof for the “passive” nature of the blocks with R =0.6 we can consider the fracture surface of the CT specimen with CFRP after tests, where the crack growth increment during passive block (dark area) is very small in comparison with the active block (bright area).

Fig. 5. Fracture surface of the specimen 1 with CFRP after FCGR tests

The crack growth increment for active blocks was calculated based on Fig. 5 and measuring the crack length (average) using digital microscope. The fatigue lifetime for ( F max =4.5kN, no-reinforcement) and ( F max =6kN, CFRP patches) is presented in Fig. 6. For better comparison, with the same magnitude of loading, the numerical calculation (based on Paris law) was performed. According to raw data for non-reinforcement specimen, the Paris (1960) law was involved: = (∆ ) , (4) where C, m are experimentally determined constants. In the presented approach, C =4x10 -10 , m =3.66 for steel from Kluczbork (calculated in mm/cycle - MPa  m axis configuration). The fatigue lifetime N t (from initial a 0 =19mm to a 1 =31mm) under constant load amplitude ( F max =6kN, R=0.1) was calculated by integrating Paris law: = ∫ (∆ ) 1 0 . (5) As it was expected – only in initial stage of fatigue crack growth (a=19-21 mm) the kinetics of fatigue crack growth was similar in each cases. The strengthened Kluczbork steel presents higher resistance level against fatigue crack propagation. The required number of cycles (considering the same magnitude of loading F min =0.6kN, F max =6kN) for crack growth from a 0 =19mm to a 1 =31mm was equal N =450000 for strengthened specimen, and 168000 for non reinforcement specimen. It means that fatigue lifetime was improved with factor 2.67. The fracture surface of specimen (Fig. 5) indicate that the main mechanism of CFRP strengthening consist in significant reduction of  K with