Issue 42

D. Rozumek et alii, Frattura ed Integrità Strutturale, 42 (2017) 23-29; DOI: 10.3221/IGF-ESIS.42.03



















h a a 

/ 2 a a h  

Y

(6)

2 /

tan

III

0

0

where h – height of the specimen. Fig. 7 allows observing that change of ratio R = -1 to R = 0 results in increasing fatigue crack growth rate da/dN (dc/dN). For instance, in case of constant value of stress intensity factor range ΔK eq = 33 MPa·m 1/2 , crack growth rate along the length increases from da/dN = 1.69·10 -8 m/cycle (R = -1) to da/dN = 2.30·10 -7 m/cycle (R = 0). The increase is more than 13 times. Whereas, for other example of constant parameter ΔK eq = 68 MPa·m 1/2 , crack growth rate along the depth increases from dc/dN = 5.93·10 -8 m/cycle (R = -1) to dc/dN = 6.06·10 -7 m/cycle (R = 0). The increase exceeds 10 times. Coefficients C and m occurring in formula (1), determined on the basis of experimental tests, were computed using the least squares method. They are shown in Tab. 2, which also specifies correlation coefficients r. Test results for proportional bending with torsion are burdened with relative error not exceeding 13% at significance level α = 0.05. In all cases, correlation coefficients r, have values close to 1, which proves significant correlation of experimental test results and the approved Eq. (1). Moreover, Tab. 2 allows finding that coefficients C and exponents m for R = -1 and R = 0 (da/dN and dc/dN) have different values. Differences in the values of coefficients C and m prove that load ratio R affects shift C and inclination m of the curve.

C, m(MPa·m 1/2 ) -m /cycle

Figures

curves

R

m

r

7a 7a 7b 7b

1 (depth) 2 (length) 1 (depth) 2 (length)

-1 -1

5.784·10 -12 1.299·10 -10 1.131·10 -10 7.183·10 -9

2.201 1.409 2.026 0.988

0.995 0.985

0 0 0.991 0.993 Table 2 : Coefficients C and m of Eq. (1) and correlation coefficients r for the curves shown in Fig. 7.

C ONCLUSIONS

T

he following conclusions have been made on the basis of the obtained results fatigue cracks growth:  Under proportional bending with torsion was observed two stage fatigue cracks growth, quarter elliptic edge cracks, which then passes through the crack.  It was noticed that the difference in lengths of cracks on both specimen sides is higher for load ratio R = -1 than for R = 0.  The fatigue crack growth rate is higher for dc/dN compared with da/dN under the same value  K eq .  It has been confirmed that the change of ratio R = 0 to R = -1 results in fatigue life increase exceeding 14 times, while at the same time crack growth rate drops by more than 10 times. [1] Kocańda, S., Fatigue failure of metals, WNT, Warszawa, (1985). [2] Pyrzanowski, P., Estimation and consequences of the crack thickness parameter in the assessment of crack growth behaviour of ‘‘squat’’ type cracks in the rail–wheel contact zone, Engineering Fracture Mechanics, 74 (2007) 2574 2584. [3] Rozumek, D., Marciniak, Z., Fatigue properties of notched specimens made of FeP04 steel. Materials Science, 47(4) (2012) 462-469. [4] Gadomski, J., Pyrzanowski, P., Experimental investigation of fatigue destruction of CFRP using the electrical resistance change method, Measurement, 87 (2016) 236-245. [5] Martins, R. F., Ferreira, L., Reis, L., Chambel, P., Fatigue crack growth under cyclic torsional loading, Theoretical and Applied Fracture Mechanics, 85 (2016) 56-66. R EFERENCES

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