Handbook Fractography (1998) WW Part 13 ppsx

Wrought Aluminum Alloys: Atlas of Fractographs Wrought Aluminum Alloys Fig. 965 Fig. 964 Photograph and SEM fractograph showing the fracture surface of a small portable cylinder used for storage of helium gas under pressure that exploded while at rest in storage. The cylinder was approximately 70 mm (2 3 4 in.) in diameter by 250 mm (10 in.) long and had been formed from a 0.75 mm (0.03-in.) thick sheet of aluminum alloy 1100. Normal pressure in the cylinder, when full of helium, was 2 MPa (300 psi); maximum pressure, 4 MPa (600 psi). The cylinder broke when the bottom separated from the sidewall with explosive force, leaving a very even fracture surface; separation was probably at the very bottom of the sidewall. Figure 964 shows the entire fracture surface of the sidewall at actual size. Figure 965 is a magnified view (40×) of a portion of that fracture surface, showing several fracture levels, separated by offsets, containing crack arrests. Rupture began at the inside of the cylinder wall and terminated in a pronounced ductile shear lip (at top in Fig. 965). Note the secondary stress-corrosion cracks in the inside surface of the wall (at bottom in Fig. 965), which are parallel to the fracture surface. See also Fig. 966 and 967. Fig. 966 Fig. 967 SEM views of two portions of the fracture surface of the cylinder in Fig. 964. These views, at higher magnification than Fig. 965, also show the crack arrests, the offets between adjoining stress-corrosion-crack surfaces, and the numerous secondary cracks in the inside surface of the cylinder wall (at bottom) that are parallel to the fracture. In Fig. 967, numerous corrosion pits are also visible. These pits are probably due to condensation of water vapor at the bottom of the cylinder that was carried by the helium gas. (Light- microscope examination revealed that cracks formed at bottoms of corrosion pits.) It is likely also that the forming operation created large stress concentrations at the junction between the sidewall and the bottom of the cylinder. Fig. 966: 83×. Fig. 967: 108× Fig. 968 Pieces of the hub of a forged aircraft main-landing- gear wheel half, which broke by fatigue. The material is aluminum alloy 2014- T6. Tensile specimens from elsewhere in the wheel had tensile strength of 493.7 MPa (71.6 ksi) and 8.9% elongation in the transverse direction, and tensile strength of 466.1 MPa (67.6 ksi) and 8.0% elongation in the longitudinal direction. See also Fig. 969. ~0.1× Fig. 969 View of an area of one fracture surface of the broken hub in Fig. 968, showing the fatigue- crack origin. Visible are beach marks, which suggest that the origin is at the location marked by the arrow, presumably at one of several corrosion pits that were found on the surface of the hub. After having penetrated a short distanc e, the fatigue crack developed a step (dark facet); a second fatigue crack originated at one end of the step. 1.8× Fig. 971 Fig. 970 Figure 970 shows the hub of a forged aluminum alloy 2014-T6 aircraft main-landing-gear wheel half, which broke in fatigue. A tensile specimen machined from the hub had tensile strength of 499.2 MPa (72.4 ksi), 12.1% elongation, and hardness of 143 to 150 HB, which are acceptable. The fatigue crack originated at the inside surface of the hub. Figure 971 shows a fracture surface of the broken hub, showing the fatigue-crack origin. Clearly visible are beach marks, which indicate that the fracture began as a radial fatigue crack in a plane containing the axis of the wheel. Later, the crack turned to form a circumferential separation between the hub and web of the wheel half, as shown in Fig. 970. See also Fig. 972 and 973. Fig. 970: ~0.17×. Fig. Fig. 972 Enlarged view of the fatigue-crack origin in Fig. 971 , which plainly shows the region of initial penetration (light area). At arrow is a forging defect usually known as a bright flake. Note that grain flow is approximately parallel to the flake. ~5× Fig. 973 Fracture surface of the broken hub in Fig. 970 , showing an area of the circumferential separation between the hub and web. Bright flakes, similar to the defect that initiated the fatigue crack in Fig. 972 , are visible at the arrows. These defects have been attributed to hydrogen damage. Actual size Fig. 974 Top surface of an extruded aluminum alloy 2014-T6 bottom cap of an aircraft wing spar, showin g a fatigue fracture (center) that intersected one of the rivet holes indicated by the arrows. Hardness tests near the fracture gave an average value of 85 HRB, which is acceptable. See also Fig. 975, 976, 977, 978, and 979 . ~0.25× Fig. 975 View of the fracture surface in Fig. 974 . The rivet hole intersected by the fracture is abnormal, consisting of two overlapping holes (see Fig. 976 ). Beach marks, which are clearly visible, indicate that the fatigue crack began at the double-drilled rivet hole. 1.13× Fig. 976 Higher-magnification view of the rivet-hole area of the fracture surface shown in Fig. 975. The double- drilled nature of the rivet hole is shown quite clearly here. Two fatigue cracks originated at this hole one beginning at arrow A and growing to the left, and the other beginning at arrow B and growing to the right. 5× Fig. 977 Polished and etched section through the cap in Fig. 974 . At bottom is a fibrous interior structure typical of aluminum alloy extrusions. At top is one of the coarse- grained, recrystallized layers, 0.46 to 0.94 mm (0.018 to 0.037 in.) thick, at the top and bottom surfaces of the cap. Keller's reagent, 100× Fig. 979 Fig. 978 Views of the top surface (Fig. 978) and bottom surface (Fig. 979) of the wing-spar cap in Fig. 974, with the mating fracture surfaces fitted together. The segment at right in each view was deformed after fracture, causing the gap. The double drilling of the rivet hole, noted in Fig. 975 and 976, is clearly shown (arrows). The edge distance, "e," was measured for each hole (as shown in Fig. 979 for the hole closer to the edge). Edge distance for the hole farther from the edge is about 8 mm ( 5 16 in.) the minimum allowed for rivets of the size used here (4 mm, or 5 32 in. diam). For the closer hole, edge distance is about 75% of the minimum value, which increased the stresses in that section to excessive levels. Both at 5× Fig. 980 Fatigue fracture of an aluminum alloy 2014-T6 heat-treated forging. Details of the heat- treatment procedure were not available. Some machining was carried out on the forging prior to heat treatment. The aircraft structural component cracked in service. The horizontal lines on the fracture surface are grain boundaries. Fatigue striati ons are also visible, traversing the fracture face at roughly 60°. Note the absence of discontinuities at their intersections with the grain boundaries. SEM, 1800× (E. Neub, University of Toronto) Fig. 981 Fracture surface of a fatigue-test specimen of aluminum alloy 2024- T3, showing a portion of the region of final fast fracture. Stress-intensity range (∆K) was 21 MPa m (19 ksi in ); the stress was applied in an argon atmosphere at room temperature at a frequency of 10 cps. The area has voids that may be moderate- size dimples. The vertical face is apparently a very large tear rid ge or cleavage step joining two areas of dimpled rupture. See also Fig. 982 and 983. SEM, 270× Fig. 982 A different area of the fracture surface shown in Fig. 981 . This also exhibits vertical faces that are tear ridges. The surfac e is covered with voids, but at this low magnification it is not possible to decide whether or not they are dimples. The smooth central area outlined by the rectangle is shown enlarged in Fig. 983. SEM, 140× Fig. 983 A view of the rectangle-outlined area of the fracture surface in Fig. 982 , at higher magnification. With this enlargement, it is evident that the area is not truly smooth, but rather that it bears a uniform array of extremely fine fatigue striations. Near the right edge is a small area of minute dimples. SEM, 1400× Fig. 984 Fracture surface of a fatigue-test specimen of aluminum alloy 2024- T3 tested at 23 °C (73 °F) in argon. The fatigue crack, similar in appearance to the one in Fig. 982, was produced by a stress- intensity range (∆K) of 24.8 MPa m (22.6 ksi in ) at a frequency of 10 cps. Much of the surface shows features resembling dimples, but the vertical "cliffs" are probably delaminations along grain boundaries. See Fig. 985 for a higher- magnification view of the area in the rectangle. SEM, 170× Fig. 985 Area outlined by the rectangle in Fig. 984 , as seen at higher magnification. This view provides a much clearer delineation of the fine details of the fracture surface and shows a combination of dimpled rupture and grain-boundary separation. Intergranular secondary fissures such as those ma rked by the arrows at A led to the formation of the vertical "cliffs" shown here (arrows at B) and in Fig. 984. Cracked a nd broken inclusions are visible at many locations. SEM, 850× Fig. 986 Fracture surface of a fatigue-test specimen of aluminum alloy 2024- T3 that was tested in an environment of a 3.5% solution of NaCl in water. The stress-intensity range (∆K) was 19.8 MPa m (18 ksi in ) at 10 cps. The central region of this view contains patches of well- defined fatigue striations. In adjacent regions, there appear to be faintly defined striae that have been ob scured by corrosion. In other regions, it is uncertain whether fatigue or cleavage was active. See Fig. 987 for area in rectangle. SEM, 260× Fig. 987 View at higher magnification of the area in the rectangle in Fig. 986 , showing the fatigue striations in finer detail. Note that superimposed on the fine striations at somewhat irregular intervals is a system of fissu res, or perhaps more pronounced striations; the presence of these features may reflect either a repetitive variation in strain amplitude or stress, or periodic interruptions in the applied stress cycle (which allowed locally increased corrosion), or both. SEM, 1320× Fig. 988 View of the shank end of a fractured aircraft propeller blade fabricated of aluminum alloy 2025- T6. The b lade broke by fatigue, which originated at an interior cavity that was provided to contain a balance weight comprised of compacted lead wool. Chemical analysis established that the blade was within specified composition limits. Hardness measurements (500-k g load) yielded an average value of 107 HB, which was above the required minimum of 100 HB. See also Fig. 989. Actual size. Fig. 989 Fracture surface of the shank end of the broken aircraft propeller blade in Fig. 988. The balance- weight cavity is visible at center, with the fatigue- crack origin at the upper edge (arrow). The fracture originated at the beginning of the radius that formed one end of the cavity. Examination of the cavity surface revealed severe roughness caused by tool marks and by corrosion pits. The combined effect of these tool marks and corrosion pits was considered to be the cause of crack initiation. 1.75× Fig. 990 Fig. 991 Figure 990 shows the surface of a fatigue fracture near the hub of an aluminum alloy 2025-T6 aircraft propeller blade. The fracture originated in a shot-peened fillet. Small fatigue cracks joined to form the main crack at A, which propagated to B-B and C-C before final fast fracture occurred. Figure 991 shows a portion of the outside edge of the fracture surface in Fig. 990 between the arrows marked D, showing small, distinct fatigue cracks (at arrows) that had been present before final fast fracture. Figure 992 is a view of the shot- peened fillet of a companion propeller blade, showing small fatigue cracks. Depth of the cold-worked layer produced by shot peening was nonuniform and averaged about 0.038 mm (0.0015 in.), instead of the stipulated 0.14 mm (0.0055 in.) minimum, which afforded inadequate surface fatigue strength. See also Fig. [...]... characteristics of quasi-cleavage Note the large number of secondary cracks, possibly at grain boundaries, all of which are more or less parallel Area in rectangle is shown enlarged in Fig 1 013 SEM, 300× Fig 1 013 Higher-magnification view of the surface contours in the rectangle that is shown in Fig 1012 This reveals the cleavage river patterns quite clearly, note the changes in fracture direction... secondary cracking is intergranular, although the primary rupture is not Numerous pores contain alloy secondphase particles See also Fig 1020 1000× Fig 1020 Higher-magnification view of the area in the rectangle in Fig 1019 This fourfold enlargement makes visible the fine dimples surrounding the particle sites Note that the surfaces of the secondary grain-boundary cracks are remarkably smooth SEM, 4000×... in the rectangle in Fig 1032 The fine dimples can just be distinguished on the surfaces adjacent to the large dimples Many alloy second-phase particles are discernible, although some are so deep within the dimples as to be nearly invisible See also Fig 1034 SEM, 134 0× Fig 1034 Greatly magnified view of the fracture region in Fig 1031, showing the area in the rectangle in Fig 1033 The region at center... fatigue-precrack region (below arrows) to the tension-overload plane-strain fracture region (above arrows) Specimen was aged 24 h at 120 °C (250 °F); tensile strength was 593 MPa (86 ksi), and uniform elongation was 13% Note the appreciable number of vertical delaminations (as at A's), which probably are grain-boundary separations, caused by the transverse tensile stress in the plane-strain region Area in rectangle... enlarged in Fig 1006 SEM (gold shadowed), 50× Fig 1006 Area outlined by the rectangle in Fig 1005, as seen at ten times the magnification there It is apparent that the matrix contained many second-phase particles that have undergone brittle fracture (arrows) The surface is quite complex, with some regions that appear to show clusters of minute dimples (A) and other regions that strongly resemble intergranular... The as-received cylinder head is shown in the inset Fig 997 and 998: Close-ups of fatigue origins (arrows) in Fig 996 SEM, 26× and 23× (W.L Jensen, Lockheed-Georgia Company) Fig 999 Fig 1000 Fig 1001 Fractography of a laser beam weld in aluminum alloy 5456 The weld was made using a beam power of 11 kW, a speed of 15 mm/s (35 in./min), and a heat input of 0.74 kJ/mm (18.9 kJ/in.) Ductile fracture of... right This view also reveals the depth and continuity of the secondary cracks See also Fig 1014 SEM, 1000× Fig 1014 Another view, at still higher magnification, at the fracture surface in Fig 1012 and 1 013 At top is what appears to be a pocket holding a cracked inclusion The remainder of the surface shows cleavage facets plus projections that are somewhat similar to the tongues seen in cold fractures... 9× Fig 1016 SEM view of the central area of the fracture surface in Fig 1015 Although this is a ductile rupture, it contains very deep secondary cracks The major pores were sites of alloy second-phase particles that are no longer in place See Fig 1017 for an enlarged view of the area in the rectangle 1000× Fig 1017 Enlarged view of the fracture-surface area in the rectangle in Fig 1016 The higher magnification... 1003 Fatigue fracture of an aluminum alloy 7175-T736 forging The aircraft main landing gear component failed during a structural fatigue test Cracking initiated at a dross inclusion at the surface of the part Fig 1002: Portion of fracture surface of aluminum alloy 7075-T736 forging Dross inclusion is at top Note its spongy appearance SEM, 300× Fig 1003: High-magnification view of dross inclusion in Fig... aluminum alloy 7075-T736 aircraft main landing gear forging, similar to that described in Fig 1002 and 1003, which was shot peened on its inner-diameter surface to enhance fatigue resistance The shotpeened part withstood cycles far beyond the number required for acceptance One effect of peening was to drive the fracture-initiation site to a location well beneath the surface of the forging The dross inclusion . of which are more or less parallel. Area in rectangle is shown enlarged in Fig. 1 013. SEM, 300× Fig. 1 013 Higher-magnification view of the surface contours in the rectangle that is shown. interruptions in the applied stress cycle (which allowed locally increased corrosion), or both. SEM, 132 0× Fig. 988 View of the shank end of a fractured aircraft propeller blade fabricated of aluminum. 26× and 23× (W.L. Jensen, Lockheed-Georgia Company) Fig. 999 Fig. 1000 Fig. 1001 Fractography of a laser beam weld in aluminum alloy 5456. The weld was made using a beam power of