ASM Metals HandBook Volume 12 - Fractography
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to the uniaxially fractured specimen (b). Source: Ref 196 Fig. 63 Effect of a triaxial state of stress on the fracture mode in 13-8 pH stainless steel heat treated to an ultimate tensile strength of 1634 MPa (237 ksi). (a) and (b) Equiaxed dimples on the fracture surface of an unnotched specimen. (c) and (d) The quasi-cleavage fracture appearance if a notched specimen Effect of Strain Rate. The strain rate is a variable that can range from the very low rates observed in creep to the extremely high strain rates recorded during impact or shock loading by explosive or electromagnetic impulse. Very low strain rates (about 10-9 to 10-7 s-1) can result in creep rupture, with the accompanying changes in fracture mode that have been presented in the section "Creep Rupture" in this article. At moderately high strain rates (about 102 s-1), such as experienced during Charpy impact testing, the effect of strain rate is generally similar to the effect of the state of stress, namely that the bcc metals are more affected by the strain rate than the fcc or the hcp metals. Because essentially all strain rate tests at these moderate strain rates are Charpy impact tests that use a notched specimen, the effect of strain rate is enhanced by the presence of the notch, especially in steels when they are tested below the transition temperature. A moderately high strain rate either alters the size and depth of the dimples or changes the mode of fracture from dimple rupture to quasi-cleavage or intergranular decohesion. For example, when an AISI 5140 H steel that was tempered at 500 °C (930 °F) was tested at Charpy impact rates, it exhibited a decrease in the width of the stretched zone adjacent to the precrack and an increase in the amount of intergranular decohesion facets (Fig. 64). The same steel tempered at 600 °C (1110 °F) showed no significant effect of the Charpy impact test (Ref 199). Fig. 64 Effect of Charpy impact strain rate on the fracture appearance of an AISI 5140 H steel tempered at 500 °C (930 °F) and tested at room temperature. (a) Fatigue-precracked specimen tested at a strain velocity of 5 × 10-2 mm/s (2 × 10-3 in./s) (b) Fatigue-precracked specimen tested at a strain velocity of 5400 mm/s (17.7 ft/s). The more rapid strain rate results in a reduction in the width of the stretched zone adjacent to the crack and the presence of some intergranular decohesion with ductile tearing on the facets. f, fatigue crack; s, stretched zone. Source: Ref 199 At very high strain rates, such as those observed during certain metal-shearing operations, high-velocity (100 to 3600 m/s, or 330 to 11,800 ft/s) projectile impacts or explosive rupture, materials exhibit a highly localized deformation known as adiabatic** shear (Ref 200-208). In adiabatic shear, the bulk of the plastic deformation of the material is concentrated in narrow bands within the relatively undeformed matrix (Fig. 65, 66, 67). Adiabatic shear has been observed in a variety of materials, including steels, aluminum and titanium alloys, and brass. Fig. 65 Micrograph (a) and schematic (b) of a shear band in a plate of rolled medium carbon steel produced by ballistic impact showing the transformed zone and the zone of strain localization. (D.A. Shockey, SRI International) Fig. 66 Appearance of adiabatic shear bands in an explosively ruptured Ti-6Al-4V STA alloy rocket motor. The material exhibits multiple, often intersecting, shear bands (open arrows). Slender arrow points to portion of shear band shown in more detail in (b). Note that the intense deformation has obliterated the \u3b1-\u3b2 microstructure within the band. The 1.9-mm (0.075-in.) sheet thickness direction is left to right. (V. Kerlins, McDonnell Douglas Astronautics Company Fig. 67 Low-magnification (a) and higher-magnification (b) views of a failure surface produced in vacuum-arc remelted AISI 4340 steel (40 HRC) by dynamically shearing in a split Hopkinson torsion bar at a nominal shear strain rate of 6000 s-1. The knobbly fracture surface suggests that local melting occurred. (J.H. Giovanola, SRI International) These shear bands are believed to occur along slip planes (Ref 201, 202), and it has been estimated that under certain conditions, such as from the explosive-driven projectile impact of a steel target, the local strain rate within the adiabatic shear bands in the steel can reach 9 × 105 s-1 and the total strain in the band can be as high as 532% (Ref 204). An estimated 3 × 106-s-1 strain rate has been reported for shear bands in a 2014-T6 aluminum alloy block impacted by a gun- fired (up to 900 m/s, or 2950 ft/s) steel projectile (Ref 205). The extremely high strain rates within the adiabatic shear bands result in a rapid increase in temperature as a large portion of the energy of deformation is converted to heat. It has been estimated that the temperature can go high enough to melt the material within the bands (Ref 205, 206). The heated material also cools very rapidly by being quenched by the large mass of the cool, surrounding matrix material; therefore, in quench-and-temper hardenable steels, the material within the bands can contain transformed untempered martensite. This transformed zone is shown schematically in Fig. 65(b). The hardness in the transformed bands is sometimes higher than can be obtained by conventional heat treating of the steel. This increase in hardness has been attributed to the additive effects of lattice hardening due to supersaturation by carbon on quenching and the extremely fine grain size within the band (Ref 203). However, for an AISI 1060 carbon steel, the hardness of the untempered martensite bands was no higher than that which could be obtained by conventional heat treating (Ref 206). In both cases, the hardness of the adiabatic shear bands was independent of the initial hardness of the steel. For a 7039 aluminum alloy, however, the hardness of the shear bands was dependent on the hardness of the base material. The adiabatic shear bands in an 80-HV material exhibited an average peak hardness of about 100 HV, while those in a 150-HV material had an average peak hardness of about 215 HV (Ref 208). For the Ti-6Al-4V STA alloy shown in Fig. 66, there was no significant difference in hardness between the shear bands and the matrix. In materials that do not exhibit a phase transformation, or if the temperature generated during deformation is not high enough for the transformation to occur, the final hardness of the adiabatic shear band is the net result of the competing effects of the increase in hardness due to the large deformation and the softening due to the increase in temperature. The width of the adiabatic shear bands depends on the hardness (strength) of the material (ref 206, 208). Generally, the harder the material, the narrower the shear bands. In a 7039 aluminum alloy aged to a hardness of 80 HV, the average band width resulting from projectile impact was 90 \u3bcm, while in a 150-HV material, the band width was only 20 \u3bcm (Ref 208). The average width of the shear bands observed in a Ti-6Al-4V STA alloy (average hardness, 375 HV1kg was 3 to 6 \u3bcm. When an adiabatic shear band cracks or separates during deformation, the fractured surfaces often exhibit a distinct topography referred to as knobbly structure (Ref 205, 206, 207, 208). The name is derived from the surface appearance, which resembles a mass of knoblike structures. The knobbly structure, which has been observed in 2014-T6 and 7039-T6 aluminum alloys, as well as in an AISI 4340 steel (Fig. 67) and AISI 1060 carbon steel, is believed to be the result of melting within the shear bands (Ref 205, 206). Although the cracked surfaces of adiabatic shear bands can exhibit a unique appearance, adiabatic shear failure is easiest to identify by metallographic, rather than fractographic, examination.