Comparison of fracture toughness of a 7XXX aluminum alloy under mode I, mode II and mode III loading

Abstract

A crack in a body can be subjected to three distinct modes of loading: mode I (tensile), mode II (in-plane shear) or mode III (anti-plane shear). In most practical situations, structures are subjected to complex loading and can have flaws or cracks oriented in arbitrary directions. Hence, it is essential to evaluate the fracture toughness of engineering materials under different loading modes to correctly assess their damage tolerance. However, most of the published literature is concerned with only mode I fracture toughness and very limited experimental data are available on the fracture toughness under mode II or mode III loading [1][2][3][4][5][6][7][8][9][10]. The studies have yielded contradictory results with no clear picture emerging on whether mode II and mode III fracture toughnesses are higher or lower than mode I fracture toughness. This confusion can partly be attributed to the lack of standard procedures for the determination of mode II or mode III fracture toughness. The specimen designs proposed to date permit the evaluation of only pure mode II [11][12][13][14][15] or pure mode III [16,17] which makes the comparison of fracture toughness data under different modes of loading extremely difficult. In a recent publication Richard and Kuna [18] have proposed an 'All Fracture Mode (AFM)' specimen and loading fixture wherein it is possible to conduct fracture toughness tests for pure mode I, pure mode II, pure mode III and for all possible combinations of the above-mentioned using the same specimen design and loading fixture. This testing procedure should minimize the errors associated with using different specimen geometries for the evaluation of fracture toughness under different loading modes.The objective of this study was to evaluate the fracture toughness of an experimental 7XXX alloy under mode I, mode II and mode III loading using the AFM specimen so as to determine the lowest fracture toughness of this alloy and to assess the feasibility of using the AFM specimen proposed by Richard and Kuna [ 18].The experimental 7XXX aluminum alloy was of the nominal composition: 5.2 Zn, 2.2 Mg, 0.067 Mn, balance A1. The alloy was received in the as forged, natural aged condition. The tensile properties of the alloy were as follows: yield strength (~y) = 230MPa, ultimate tensile strength (~) = 360MPa, uniform plastic strain (e o)-= 16 percent. Mode I, mode II and moore III tests were conducted using the AFM specimen, illustrated in Fig. 1, with w = 30mm, a = 15mm, c = 22.5mm and t = 13.5mm. The loading device which was used is shown in Fig. 2. The device consists of two force introduction elements (1) and (2) which are Int Journ of Fracture 63 (1993)R8 diametrically connected to one another by the specimen (3), forming an octantal sphere. The load was applied by means of an Instron 8500 servohydraulic machine through connecting rods linked to the force introduction elements by threaded holes. The appropriate mode of loading was chosen by correctly selecting the diametrically opposite holes. The detailed pr...

Mode Ii Stable Crack Growth

“…The mode I1 toughness is therefore less than that of mode I. This is an observation contrary to the observation of Cowie and Tuler [16], Banks-Sills and Sherman [23], and Aoki et al [25], but in agreement with that of Kamat and Eswara Prasad [9] and Mageed and Pandey [45].…”

“…However, there has been no study reported in the literature on mode I1 stable crack growth. A number of specimens geometries are designed or considered for measurements of fracture toughness in mode I1 [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Results based on elastic-plastic fracture mechanics have been presented recently concerning the mode I1 crack growth initiation [21-251.…”