Issue 35

L. Songsong et alii, Frattura ed Integrità Strutturale, 35 (2016) 74-81; DOI: 10.3221/IGF-ESIS.35.09 80 The FE calculated plastic zone sizes at θ =90° for the three specimens are listed in Tab. 1, together with the results calculated by Eq. (1). The distances from the surface secondary cracks to the main crack tips are also measured from the optical photographs and listed in Tab. 1. The comparison in Tab. 1 indicates that: 1) the FE simulation results are about 10% higher than the results obtained by empirical equation, that is because the elastic-plastic material model is used in the FE model; 2) the calculated plastic zone sizes at θ =90° by both Eq. (1) and FE model match the measured distances from the surface secondary cracks to the main crack tips. That is to say, the secondary cracks initiation positions are near the plastic zone boundaries of the main crack. specimen Measured Distance between the secondary crack and the main crack Calculated plastic zone size of plane stress state at θ =90° by Eq. (1) Calculated plastic zone size of plane stress state at θ =90° by FE model L-T-1 0.241 0.209 0.23 L-T-2 0.185 0.177 0.19 L-T-3 0.083 0.074 0.08 Table 1 : Comparison of the plastic zone size of plane stress state at θ =90° with the measuring distance from the secondary crack to the main crack (mm). C ONCLUSIONS he following conclusions can be drawn from the experimental investigations and crack tip analysis: 1) The macroscopic crack branching in AA 2324-T39 results from two different types of mechanism. The uncommon kind of branching is resulted from the linking up of the secondary crack with the main crack, which is different from the intergranular crack branching. 2) The secondary cracks usually locate near the plastic zone boundaries of the main cracks and show transgranular growth characteristics. The initiation sites of the secondary cracks were deduced at the sub-surfaces of the specimen. 3) The grain shapes characteristics in different orientations of this alloy and their interaction with the plastic zone size contribute to the appearance of the second type of crack branching. A CKNOWLEDGEMENTS he National Natural Science Foundation of China is acknowledged for supporting the project (10802003). R EFERENCES [1] http://www.alcoa.com/mill_products/catalog/ pdf/alloy2324-t39techsheet. (2009) [2] Johnston, W. M., Newman, Jr. J. C., Fracture Test Results for 0.5, 0.7 and 0.9 Inch Thick 2324-T39 Aluminium Alloy Material, (2001). [3] Dawicke, D. S., Fracture testing of 2324-T39 aluminium alloy, NASA TM-109183, (1995). [4] Stoychev, S., Kujawski, D., Mallory, J., Fatigue crack growth in 2324 aluminium alloy. MAE-05-01, (2005). [5] Hailing, Tian, et al. Influence of low load truncation level on crack growth for Al 2324-T39 and Al 7050-T7451. Chinese Journal of Aeronautics, 22(4) (2009) 401-406. [6] Rui, B., Zhang, X., Fatigue crack growth behaviour and life prediction for 2324-T39 and 7050-T7451 aluminium alloys under truncated load spectra, International Journal of Fatigue, 32(7) (2010) 1180-1189. [7] Zhang, T., Rui, B., Binjun, F., Load effects on macroscopic scale fatigue crack growth path in 2324-T39 aluminium alloy thin plates, International Journal of Fatigue, 58 (2014) 193-201. T T