Fracturing Characteristics of Aluminum-Alloy Plate

Commercial hot-rolled V/z-'m. 24ST aluminum plate exhibited large variations in fracture stress and reduction in area for different orientations of the test specimens. These variations were attributed to mechanical anisotropy in the plate. Measurement of the angles at which the specimens fractured indicated the presence of a plane of weakness, but no fracture function could be found to correlate this with the fracture stresses and ductilities. INTRODUCTION T TYPES OF ANISOTROPY may be encountered in annealed metal . "Crystallographic anisotropy" results from a preferred orientation or " texture" in the crystal structure of the metal, while "mechanical anisotropy" results from a geometrically directional arrangement of certain phases, inclusions, porosities, cracks, etc. In addition to these two, any cold-worked metal develops a third type of anisotropy, which may be termed "anelastic anisotropy." This type appears to be caused by a certain kind of residual stress set up during the cold working. I t can be reduced and seemingly eliminated by a stress relief anneal. A paper has been published recently which discusses this type of anisotropy. This paper will deal with the effect of the mechanical anisotropy on the fracturing characteristics of hotrolled aluminum plate. A previous paper dealt with the effect of the crystallographic anisotropy on the plastic flow properties of the plate. Mechanical anisotropy has been recognized as the probable cause for the variation with direction of the fracturing characteristics of some wrought metal p roducts . 3 7 In processing the metal, certain phases, inclusions, and cavities may be elongated in the direction of the principal strain and thereby constitute a mechanical fibering or mechanical anisotropy. This condition may be retained after annealing and heattreatment and may also contribute to the directional properties of sheet. The mechanical properties of forgings are probably more dependent upon the mechanical fibering than on Received June 25, 1948. * This paper is one of a series of reports in a research program conducted at the Research Laboratory for Mechanical Metallurgy, Case Institute of Technology, Cleveland, in cooperation with the Office of Naval Research, U.S. Navy. The authors are indebted to the Office of Naval Research, U.S. Navy, for permission to publish this report. The authors also wish to thank E. L. Aul for his assistance in the experimental work. f Research Laboratory for Mechanical Metallurgy. the crystallographic fibering. I t has generally been observed 5 7 t ha t the yield strength and tensile strength of forged carbon steel billets and bars vary little with the (per cent) reduction in area by forging and with the direction of the test specimen in relation to the fiber. However, the elongation and contraction in area in tension tests and the impact strength in notched bar impact tests increase with increasing reduction in the direction of the fiber and may decrease simultaneously in the transverse direction. Hard aluminumand magnesium-alloy forgings also have lower transverse strength than longitudinal strength because of the decrease in contraction in area in the test. No data are available for plate materials, bu t it would be expected that , as in forging, the mechanical fibering would cause a weakening in one or bo th of the transverse directions, and, for rolling, the normal direction would be expected to be the least strong. This paper will report on the fracturing characteristics of tensile specimens taken a t various orientations in commercial 24ST aluminum-alloy plate. M A T E R I A L AND PROCEDURE The. distribution of fracturing characteristics was made on a commercial hot-rolled l ^ i n . 24ST aluminum plate. All tests were made on P/Vin . threaded-end tensile specimens as shown in Fig. 1. The orientations of the specimens cut out of the plate are shown on the stereographic projection, Fig. 2. Points on this projection represent the intersection of straight lines from the center of the stereographic sphere with the surface of the sphere. The points may then be considered to represent the longitudinal axes of the specimens. The positions of the specimens can be more easily identified by passing planes normal to the rolled surface of the plate a t increments of 15° to the rolling direction. 731 732 J O U R N A L O F T H E A E R O N A U T I C A L S C I E N C E S — D E C E M B E R , 1 9 4 8 Thus, in this system plane A is defined by the rolling direction and the normal direction, plane B is defined by a line in the rolling plane a t 15° from the rolling direction and by the normal direction, and so forth, to plane G, which is defined by the transverse and normal directions. Any specimen can then be located by giving the plane designation and the angle of elevation of the specimen axis from the rolling plane. A number was stamped and a line was scribed on the end of each specimen blank after it was sawed from the plate, so t ha t a record of its original position ih= the plate could be kept. All specimens were made in duplicate, the test section in all cases being taken in the middle of the plate. Some tests were discarded because of faulty technique, and some additional tests were made as checks. All specimens were reheat-treated after machining in the following manner : solution t rea tment—920°F. for 45 min.; cold-water quench; aged a t least 4 days at room temperature. This t rea tment was carried out to standardize the heatt reatment of all the specimens and to remove any residual effects of machining. The specimens were tested in a Riehle hydraulic testing machine. The specimens were measured after fracture to determine the final area, and from this the reduction in area was calculated. Fracture stress was defined as the breaking load divided by the final area.* Almost all of the specimens fractured on a single plane (cleavage type) a t an angle to the specimen axis. The angle between the normal to this fracture plane and the specimen axis was defined as the fracture angle. This angle was measured on a Wilder microprojector a t a magnification of 20 X . However, a few of the specimens which fractured on what seemed to be a series of parallel planes could not be measured accurately and have not been reported. A special series of tests was made to check the general symmetry of the plate— namely, tha t the anisotropy in each quadrant of the system of coordinates formed by the rolling direction, the transverse direction, and the normal direction was the same. Only four quadrants needed to be checked since the test sections were all taken from the center plane of the plate. These tests were made on specimens located in the four possible C planes a t an angle of 45° to the rolling plane. The data for these tests are given in Table 1. I t can be seen from these data t ha t the values of each property measured were equal for all four quadrants , within the limits of accuracy and scatter. Therefore, only one quadrant as represented in Fig. 2 need be considered. TABLE 1 Properties of Specimens from the Four Possible C Planes at an Angle of 45 ° from the Rolling Plane Quadrant 1 1 2 2 C O C O 4 4 Fracture Stress, Lbs. per Sq.ln. 78,600 73,500 76,500 73,100 73,600 71,200 77,900 78,100 Reduction in Area, Per Cent 14 11 13 10 11 11 12 14 Fracture Angle, Deg. 36 41 39 37 35 39 42 * This fracture stress is actually the average stress at fracture, since most of the specimens necked. This caused a nonuniform distribution of stress over the cross section. R E S U L T S AND DISCUSSION The fracture characteristics of the aluminum-alloy plate were found according to Figs. 3 and 4 to be highly directional. While da ta for sheet and plate for directions other than within the rolling plane have not been previously determined, i t was to be expected from investigations on flat-forged products t ha t normal specimens would be considerably less strong and less ductile than those within the rolling plane. The data in Figs. 3 and 4 confirm these expectations. However, they reveal a rather complicated dependence of the fracture characteristics upon the direction of testing. These should be correlated in some way with the type of break, which was also found to be directional. Therefore, the relation between the fracture appearance and the orientation may be discussed first. The fracture surface of all specimens consisted nearly of a single plane.f This plane was generally inclined to the specimen axis, the angle between the testing direction and the normal to the fracture surface varying between approximately 38° and zero, depending upon the orientation, as shown in Fig. 5. Furthermore, all these fracture plane normals were found to be located in the plane defined by the specimen axis and the plate normal, as can be seen in Fig. 6. This picture illustrates the fractures for different specimens in the A f As contrasted with cup and cone fractures. F R A C T U R I N G C H A R A C T E R I S T I C S O F A L U M I N U M A L L O Y P L A T E 733 ^sY6/£ JtfWf* J/VTf/r^A'stS/tf stsVJ SXJXf /Z0*M4/ S0° 00° 70° 60° jo* 40* <J0* w /0 0° X X I I 40 60 40 00 60 40 00 60 40 00 60 40