Issue 46

V. Rizov, Frattura ed Integrità Strutturale, 46 (2018) 158-177; DOI: 10.3221/IGF-ESIS.46.16 159 functionally graded materials plays an important role in the design of various structural members and devices made by these novel un-homogeneous materials. Understanding the fracture behavior of functionally graded structures under various loading conditions is vital for the further development of the methods for safety design. The present paper deals with analyses of a cylindrical delamination crack in a multilayered functionally graded non-linear elastic circular shaft under combined loads. It should be mentioned that in one of the previous works of the author, non- linear analyses of cylindrical delamination cracks in circular shafts have been developed assuming that the shafts are loaded in pure torsion only [10]. However, in reality, the circular shafts usually are under various load combinations which include torsion (this fact is the basic motive for writing the present paper). F RACTURE ANALYSIS IN TERMS OF THE STRAIN ENERGY RELEASE RATE Shaft under centric tension and torsion he multilayered functionally graded circular shaft, shown schematically in Fig. 1, is under consideration. The shaft is made of adhesively bonded concentric longitudinal layers. In each layer, the material is functionally graded in radial direction. Besides, the functionally graded material exhibits non-linear mechanical behavior. Figure 1 : Multilayered functionally graded circular shaft loaded in centric tension and torsion. The number of layers is arbitrary. Also, each layer has individual thickness and material properties. The shaft cross-section is a circle of radius, R . The length of the shaft is 2 l . The shaft is loaded in centric tension and torsion, respectively, by longitudinal forces, F , and torsion moments, T , applied at the end sections of the shaft as shown in Fig. 1. A circular notch is cut-out in the middle of the shaft in order to generate conditions for delamination fracture. It is assumed that a cylindrical delamination crack of length, 2 a , is located symmetrically with respect to the middle of the shaft. The delamination crack represents a cylindrical surface (the crack front is a circle of radius, b r ). Thus, the internal crack arm is a shaft of length, 2 a , and circular cross-section of radius, b r . The external crack arm is a shaft of length, 2 a , and ring- shaped cross-section of internal radius, b r , and external radius, R . The delamination crack is located arbitrary between layers. The circular notch divides the external crack arm in two symmetric segments of length, a , each. Apparently, the two segments of the external crack arm are free of stresses (Fig. 1). Due to the symmetry, only half of the shaft, 2 l x l   , is analyzed. In the present paper, the delamination fracture is studied in terms of the strain energy release rate. It is obvious that the longitudinal force, F , induces mode II crack loading conditions. The mode II component of the strain energy release rate, II G , is determined by analyzing the energy balance. By assuming an increase of the crack length, a  , the energy balance is written as F II C U F u a G l a a        (1) T