ASM Metals Handbook - Desk Edition (ASM_ 1998) Episode 12 pot

Stereo Views Stereo-pair photographs of fracture surfaces provide a means of viewing the fracture contours in simulated three dimensions. The basic technique for preparing such photographs entails taking two pictures of a subject area, the second from an angle slightly different from the first. The photographs are then examined under a visual condition in which, for example, the viewer's left eye focuses on the first picture and his right eye focuses on the second. Stereo viewers are available, and in most cases are necessary, to ease the task of viewing stereo pairs. The effect is to convince the brain that the eyes are indeed seeing a three-dimensional scene. If the angular displacement between the two pictures is appropriate (an included angle of 12° or 14° is desirable), the illusion is very vivid. Stereo images by light microscopy have been used only to a limited extent because of the restricted depth of field. They provide a useful means of studying fractures at magnifications generally not greater than 200×. Stereo-pair photographs can be taken using a single-lens camera, if it is provided with a mount that will pivot about a horizontal axis through the subject. After the subject is properly aligned beneath the camera with the mount vertical, the camera is swung to an angle of 6° to 7° from vertical and an exposure is made. The second exposure is made with the camera at the same angle as for the first exposure, but swung to the other side of the vertical, or zero, position. With the stereomicroscope shown in Fig. 3, it is possible to observe the voids that are a part of ductile dimpled fracture, and, in brittle fractures, cleavage facets and some of the river marks can be discerned. It is most useful for preliminary examination of fracture surfaces, leaving final documentation of fine details for scanning electron microscopy. Fig. 3 Optical stereomicroscope with camera box partly removed for direct viewing. The light source is two 40 watt incandescent tubes above the specimen. An example of a light-microscope stereo pair is shown in Fig. 4. The subject is a fracture surface in a low-carbon steel casting that cracked along prior austenite grain boundaries during a straightening operation. The bold relief contours of the columnar fracture are evident when viewed in stereo. Fig. 4 Stereo view of a fracture surface of cast experimental low- carbon steel. The smooth columnar contours in the lower portions of the photographs were the result of cracking along prior austenite grain boundaries during straightening. The upper portions of the photographs also show i ntergranular fracture, but along ferrite grain junctions, produced by impact in the laboratory to expose the original crack surface. Light fractograph (stereo pair). 3× If a tool such as a parallax bar is used, quantitative measurements of topographic depths and elevations can be obtained. Fractographic Features Revealed by Light Microscopy THE MACROSCOPIC APPEARANCE of a fracture surface has often been used to appraise the degree of ductility and of toughness present in a metal. According to the concepts of fracture mechanics, toughness is the critical material property associated with overload or rapid fracture. The fracture surface contains vestigial marks that indicate the amounts of high-energy (ductile, or tough) and low-energy (brittle) crack extension that produced the fracture. Tensile-Fracture Surface Marks in Unnotched Specimens Tensile-fracture surface marks have been classified into three zones by configuration: the fibrous zone, the radial zone, and the shear-lip zone. This shear-lip zone is the highest-energy portion of the fracture. The relative amount of shear lip provides an indicator of the toughness of the material. The three zones are shown in Fig. 5. Fractures consisting solely of one zone occur only under conditions of extreme ductility or brittleness. Fig. 5 Tensile- fracture surface marks. (a) Schematic representation of zones within a typical tensile fracture of an unnotched cylindrical specimen. The surfaces of the fibrous and radial zones are usually normal to the tensile axis, as shown. The shear- lip surface is always at about 45° to the tensile axis. (b) Fracture surface of a 4340 steel tensile specimen showing fibrous zone (A), radial zone (B), and shear lip (C). 3× Fatigue Marks The formation of cracks under conditions of repeated or cyclic stress has been denoted as fatigue cracking. Zones of crack propagation on fatigue fractures exhibit several types of surface marks, such as beach marks, striations, and ratchet marks. Beach Marks. The term "beach mark" describes the macroscopic features present on the fracture surface as illustrated in Fig. 6. Beach marks indicate a local region of variation in crack-growth rate. Such beach marks may, but do not always, indicate fatigue as the mode of cracking. Stress-corrosion fractures may also show beach marks. Fig. 6 Fatigue- fracture surface of an AISI 1050 shaft (35 HRC) subjected to rotating bending. Numerous ratchet marks (small shiny areas at surface) indicate that fatigue cracks were initiated at many locations along a sharp snap- ring groove. The eccentric pattern of oval beachmarks indicates that the load on the shaft was not balanced. The final rupture area is near the left (low stress) side, where there may have been no fatigue action. The presence of beach marks is fortuitous, at least for the investigator, because beach marks permit the origin to be easily determined and provide the analyst with other information concerning the manner of loading, the relative magnitude of the stresses, and the importance of stress concentration (see Fig. 6). Striations. The term "striation" refers to a "line" on the fracture surface indicating the position of a crack front after an increment of crack propagation has occurred. Each increment of propagation is due to a cycle of stress, i.e., a cyclic load. The distance between striations indicates the advance of the crack front during each succeeding cycle. Fatigue striations are not readily resolvable with the light microscope and are best viewed using a scanning electron microscope (see the section "Interpretation of Scanning-Electron Microscope Fractographs" ). Ratchet marks are macroscopic features that may be seen on fatigue fractures in shafts and flat leaf springs, and they may also occur in ductile fractures in overtorqued fasteners. In fatigue fractures, ratchet marks are the result of multiple fatigue-crack origins, each producing a separate fatigue-crack zone (Fig. 6). As two approaching cracks meet, a small step is formed. The small steps are the ratchet marks. Although ratchet marks are most apparent on the peripheries of fractures in shafts, the stepped appearance is characteristic whenever fatigue cracks emanate from several origins and subsequently meet to form one principal crack front. Discontinuities Fractures originate from a broad variety of discontinuities within the metal structure, such as laps and seams from primary metal forming, shrinkage and gas cavities in cast structures, hot tears, inclusions, segregation of impurities, and imperfections in welds. Many of these features are illustrated in the section "Discontinuities Leading to Fracture" in the article "Use of Fractography for Failure Analysis." Interpretation of Optical Fractographs A RUDIMENTARY KNOWLEDGE of how to "read" a fracture surface must be gained so that meaningful fractographs can be taken from which to describe the fracture process. Fractography can provide information about the conditions of stress, the effects of temperature and chemical environment, and the origin of the fracture and how the crack progressed to final rupture. States of Stress Information regarding the stresses that caused a fracture can be learned from a casual examination of the fractured part. In many types of fracture, the general plane of fracture is perpendicular to the maximum principal tensile stress. These types, called group I fractures in the following discussion, include cleavage and other brittle fractures, ductile fractures (also called microvoid coalescence and dimpled rupture) under plane-strain conditions (in thick sections), fatigue fractures (at least in the intermediate stages), and stress-corrosion cracks. Other types of fracture propagate along planes of maximum shear stress. These types, called group II fractures here, include ductile fractures under plane stress (that is, in thin sections or near free surfaces), shear fractures, and the very early stages of fatigue fractures in pure or relatively impurity-free metals. In a ductile material, the shear stresses cause considerable deformation prior to fracture, although the deformation is not always obvious because the shape of the part is not changed except for flow on the surface. Figure 7(a) is a photograph showing deformation in a fractured shaft. Torsional single-overload fractures (group II) of a ductile material usually occur on the transverse shear plane, straight across the cylinder, and exhibit a telltale swirled appearance (Fig. 7b). The final-fracture area will be at the center of the bar. A brittle material in pure torsion will fracture in a plane perpendicular to the tensile-stress component, which is 45° to the specimen axis (group I fracture). A spiral-type fracture is one characteristic of this type of loading and material and can be demonstrated by twisting a piece of chalk to fracture (Fig. 8). The elastic-stress distribution in pure torsion is maximum at the surface and zero at the center. Thus, fracture normally originates at the highest-stressed region (the surface) in pure torsion. Longitudinal torsional fractures are sometimes observed (for example, Fig. 9) because longitudinal planes have the same magnitude of shear stress as transverse planes, and longitudinal planes usually have lower toughness, due to the shape and distribution of inclusions. Fig. 7 Splined shaft of 6118 steel that fractured from a single torsional overload. (a) Photograph ( 2×) of the shaft showing the deformation of the splines in the reg ion of fracture, which would not occur if the fracture were caused by fatigue. The shaft, 28 mm (1 in.) in diameter, was made of 6118 steel and had a hardness of 23 HRC. Being made of a ductile metal, it was twisted in pure torsion with a single overload, yielding the fracture on the transverse shear plane shown in (b). (b) Fracture surface of the shaft, showing the rotary deformation characteristic of a single-torsion- overload fracture in a ductile metal. If there is a combined bending component, the region of final, fast fracture will be offset from the center of the section. This type of fracture should not be confused with one resulting from rotating- bending fatigue, which does not have the gross distortion seen here. Light fractograph. 2× Fig. 8 Torsional brittle fracture of chalk. Fracture follows the 45° direction of maximum tensile stress. Fig. 9 A torsional-fatigue fracture in an induction- hardened 1037 steel shaft 25 mm (1 in.) in diameter that finally fractured in longitudinal shear. No clear point of origin is visible because the surfaces rubbed as the crack propagated. Light fractograph. 0.95× Crack Origins An interest in the exact location of the point of origin of a fracture derives from the importance of determining what initiated the fracture. The initial examination of a fracture is concerned with the recognition of all features that may point to the crack origin. Gross Aspects of Fractures. Some indications of crack-propagation direction can be seen by examination of the gross aspects of a broken part. They relate to the order in which events occured, sometimes called "fracture sequencing." The fragments of a fractured structure can be reassembled in approximate juxtaposition, without allowing the fracture surfaces to touch, and then the telltale indications should be sought. First, a fast-running crack in sheet or plate will frequently branch as it propagates but will almost never join another crack to continue as a single crack. Second, if a running crack joins a pre-existing fracture, it will usually meet it at approximately a 90° angle, not at a shallow angle. Third, it is almost impossible for an intersecting crack to cross and propagate beyond a pre-existing fracture. These considerations lead to the following useful guidelines concerning crack origins: • The direction to the crack origin is always opposite to that of crack branching, as shown in Fig. 10. • If a crack meets another at about 90°, it occurred later and the origin should not be sought in it but in the earlier crack. This is known as the T-junction method of crack-origin location (Fig. 11). Fig. 10 Schematic representation of the information conveyed by crack branching with regard to the location of the crack origin Fig. 11 Schematic representation of the T- junction method of determining which fracture surface to search to locate the crack origin. Because B does not cross A but meets it at about 90°, B occurred later and cannot contain the crack origin. The initial section of fracture (containing the crack origin) transfers its original load to adjoining sections, in all probability overstressing them. If these sections do not contain imperfections, succeeding fractures (assuming a normally ductile material) will be preceded by a certain amount of plastic deformation. Fibrous marks, tear ridges, and beach marks can also indicate the location of a crack origin. Chevron markings also can be used. Where the curvature of such marks is slight, the origin is generally on the concave side of the crack-front curve. In general, the region of crack initiation will be flat and will lack any free-surface shear-lip zone. The shear-lip zone appears only at some distance from the origin and becomes larger as the distance increases (see Fig. 12). Fig. 12 Fracture in a welded pressure vessel of 4340 steel displaying a flat origin at the top with a shear lip be ginning on either side of it. The shear lip increases in width with increasing distance from the origin. The radial marks below the origin and the chevron patterns to the left and right also indicate the directions of fracture. Light fractograph. 6× Location of Origins in Impact-Overload Fractures. Figure 13 shows the fracture of a 12% Cr steel bar that was notched and then struck with a hammer. Two blows were necessary to complete the fracture. The fracture marks are radial. They may be traced downward to a common intersection. Also present are crack arrests, one at A and a second at B, where fracture progress came to a complete halt before the second blow was struck. The arrest marks are parallel to the crack front, and lines drawn normal to them should intersect at or near the origin. The contours of the final fracture marks, at C and D, also point to the general location of the beginning of fracture. Fig. 13 Locating the origin in an impact fra cture, produced by two hammer blows, in a notched bar of 12% Cr steel. Fracture origin can be found in three ways: by tracing the radial marks in the lower portion of the fracture to their point of convergence (the arrows on the curved lines indicate the d irection of crack propagation); by drawing normals to the crack- arrest fronts labeled A and B; and by projecting the tangents to the final radial marks at C and D toward the bottom. The crack came to a full stop at B with the first hammer blow and resumed motion at the second hammer blow. Light fractograph. 3× Fracture Progress Many types of fractures, including most service fractures, occur by a sequence involving crack initiation, subcritical crack propagation (due to ductile crack extension, fatigue, corrosion fatigue, stress-corrosion cracking or hydrogen embrittlement), and fast fracture, which occurs when the remaining cross section can no longer support the applied load. The fracture processes leave telltale marks on the fracture surfaces, which enable a trained investigator to locate the initiation sites, to discern the propagation direction and crack-front shape, and to distinguish the fast-fracture zone. This information can lead to an understanding of the stress levels and conditions leading to fracture Fracture Changes During Crack Propagation Several influences may affect the growth of a crack, causing it to progress thereafter by a mechanism of fracture different from that in effect when cracking started. These influences include local differences in microstructure; changes in stress- intensity factor, K; changes in chemical or thermal environment: differences in stress state. Changes Caused by Local Differences in Structure. Microstructure exerts a pronounced influence on local fracture appearance. The presence of two or more types of microstructure may result in different fracture mechanisms being involved and a different fracture appearance. A simple example is a fracture in a chilled white iron part. Fracture is by cleavage through the chilled zone and is fibrous in the pearlitic zone. Another structure difference is that of case and core in carburized, flame-hardened, and induction-hardened parts. The difference in properties between such structures can cause a crack to proceed by quite different fracture mechanisms in adjacent regions. Changes Caused by Altered Environments. A fracture-mechanism change as a result of different chemical and stress environmental conditions is shown in Fig. 14. A corrodent generated small pits below a layer of chromium plate and provided the environment for the growth of stress-corrosion cracks, which originated at the pits. The stress may have been residual or applied. As the stress-corrosion cracks grew, the stress intensity at the crack tip increased for the applied cyclic loads. At some critical level of environment and cyclic-stress intensity, the fracture mechanism changed to one of fatigue. The fatigue cracks propagated until the critical crack-tip stress-intensity values were reached, and then unstable fracture occurred in an essentially ductile manner. Fig. 14 Changes in fracture mechanism and appearance that were caused by changes in chemical and stress environment for a chromium-plated aluminum alloy 7079- T6 forging. Small corrosion pits formed beneath the layer of chromium plate, as at A, and generated stress- corrosion cracks B. Growth of these cracks altered the stress intensity at the crack tips, leading to propagation of fatigue cracks C. Fin al, fast fracture D occurred when the critical crack-tip stress-intensity value was reached. Light fractograph. 5.7× Interpretation of Scanning-Electron Microscope Fractographs AT LOW MAGNIFICATIONS, the features in scanning electron microscope (SEM) fractographs strongly resemble the aspects of the fracture apparent to the naked eye; but at high magnifications, more detail is visible which needs to be categorized and interpreted if the fractograph is to be related to the micromechanisms of fracture that were active. It is important to realize that microscopic features of fractures ordinarily differ widely within a small area. The principal categories of fracture features (or fracture modes) are as follows: • Cleavage features (tongues, microtwins, and location of cleavage-crack origins) • Quasicleavage features • Dimples from microvoid coalescence • Tear ridges • Fatigue striations • Separated-grain facets (intergranular fracture) • Mixed fracture features, including binary combinations of cleavage features, dim ples, tears, fatigue striations, and intergranular-fracture features • Features of fractures resulting from chemical and thermal environments Transgranular Cleavage Features In cleavage fracture, the fracture path follows a transgranular plane that is usually a well-defined crystallographic plane. This plane of fracture is one of the {100} planes in most body-centered-cubic metals. Cleavage fracture is produced, usually at low temperature, under a condition of high triaxial stress that is, at the root of a notch or at a high deformation rate, as, for example, by impact loading, or as a result of environmental factors. Figure 15 provides three views, at increasing magnification, of an area in an impact fracture exhibiting features that are typical of cleavage. It is apparent that the fracture plane changes orientation from grain to grain. As a result, the average grain size can be measured on the fractograph and related to grain-size measurements on a metallographic section. The change of orientation from grain to grain leads to a branching of the crack along different planes and to a very chaotic overall appearance of the fracture surface. At higher magnification, many features typical of cleavage can be identified. In Fig. 15(b), the evidence of change in orientation between grain A and grain B is particularly clear because of the river patterns that begin in grain B at the interface. The river patterns, which represent steps between different local cleavage facets of the same general cleavage plane, are well defined. Fig. 15 Cleavage fracture in a notched impact specimen of hot-rolled 1040 steel broken at -196 °C (- 321 °F), shown at three magnifications. The specimen was tilted in the scanning electron microscope at an angle of 40° to the electron beam. The cleavage pl anes followed by the crack show various alignments, as influenced by the orientations of the individual grains. Grain A, at the center in fractograph (a), shows two sets of tongues (see arrowheads in fractograph b) as the result of local cleavage along the {112} planes of microtwins created by plastic deformation at the tip of the main crack on {100} planes. Grain B and many other facets show the cleavage steps of river patterns. The junctions of the steps point in the direction of crack propagation from gr ain A through grain B, at an angle of about 22° to the horizontal plane. The details of these forks are clear in fractograph (c). Quasicleavage Features In steels that have been quenched to form martensite and then tempered to precipitate a fine network of carbide particles, the size and orientation of the available cleavage planes within a grain of prior austenite may be poorly defined. True cleavage planes have been replaced by smaller, ill-defined cleavage facets, which usually are initiated at carbide particles or large inclusions. The small cleavage facets have been referred to as quasicleavage planes, because, although they look like cleavage planes with river patterns radiating from the initiation sites, until recently they have not been clearly identified as crystallographic planes. Quasicleavage features tend to be more rounded, indicating a somewhat higher energy absorption than that of true cleavage. Quasicleavage facets on a fracture surface of a quenched and tempered 4340 steel specimen broken by impact at -196 °C (-321 °F) are shown in Fig. 16. The poorly defined cleavage facets are connected by tear ridges and shallow dimples. [...]... Nondestructive Evaluation and Quality Control, Vol 17, ASM Handbook, ASM International, 1989, p 159163 12 W.L Rollwitz, Magabsorption NDE, Nondestructive Evaluation and Quality Control, Vol 17, ASM Handbook, ASM International, 1989, p 14 3-1 58 13 R Mason and R.L Lessard, Rapid Identification of Metals and Alloys, Metals Handbook: Desk Edition, 1985, p 2 2-4 6, 3 3-5 1 Leak Testing Introduction LEAK TESTING is used... Die castings Metal-permanent mold casting Carbide-tipped cutting tools Cutting tools Type of defect Heat-treatment cracks Grinding Cracks Cracks Porosity Cracks and pores Cracks and pores Cracks Laps Seams Surface porosity Cold shuts Shrinkage porosity Poor braze Cracks in steel Cracks in tip Penetration time, min 2 10 10 2-5 2-5 1 0-2 0 1 0-2 0 20 20 1 0-2 0 3-1 0 1 0-2 0 3-1 0 2-1 0 2-1 0 2-1 0 Removal of Excess... Control, Vol 17, ASM Handbook, ASM International, 1989, p 2 9-4 5 7 A.R Marder, Replication Microscopy Techniques for NDE, Nondestructive Evaluation and Quality Control, Vol 17, ASM Handbook, ASM International, 1989, p 5 2-5 6 8 L.D Lineback, Strain Measurement for Stress Analysis, Nondestructive Evaluation and Quality Control, Vol 17, ASM Handbook, ASM International, 1989, p 44 8-4 53 9 P.S Prevey, X-Ray Diffraction... the Section "Recycling and Life-Cycle Analysis" in this Handbook References cited in this section 4 C Bixby, Laser Inspection, Nondestructive Evaluation and Quality Control, Vol 17, ASM Handbook, ASM International, 1989, p 1 2-1 7 5 D.H Genest, Coordinate Measuring Machines, Nondestructive Evaluation and Quality Control, Vol 17, ASM Handbook, ASM International, 1989, p 1 8-2 8 6 J.D Meyer, Machine Vision... instances in which environment caused a specific fracture response that can be characterized through electron-microscope fractography are illustrated in Fig 21 and 22 Fig 21 Intermingled cleavage facets and dimples in two views of a stress-corrosion fracture in a step-cooled two-phase Ti-6Al-2Sn-4Zr-6Mo alloy exposed to a 3 % NaCl aqueous solution Cleavage facets formed in the alpha phase, and poorly developed... Panhuise et al., Quantitative Nondestructive Evaluation, Nondestructive Evaluation and Quality Control, Vol 17, ASM Handbook, ASM International, 1989, p 66 1-7 15 3 M.P Kaplan et al, Damage Tolerance of Aircraft Systems, Fatigue and Fracture, Vol 19, ASM Handbook, ASM International, 1996, p 55 7-5 88 Uses of NDT Although flaw detection is usually considered the most important aspect of NDT, there are also... suitable for use in many different information-processing applications, including image sensing in television-camera technology Charge-coupled devices offer an advantage over vacuum-tube image sensors because of the reliability of their solid-state technology, their operation at low voltage and low power dissipation, extensive dynamic range, visible and near-infrared response, and geometric reproducibility... postemulsifiable and solvent-removable penetrants Water-soluble developers are not recommended for use with water-washable penetrants, because of the potential to wash the penetrant from within the flaw if the developer is not very carefully controlled Water-soluble developers are supplied as a dry powder concentrate, which is then dispersed in water in recommended proportions, usually from 0 .12 to 0.24 kg/L (1... Low-energy electrons emitted by the tritium are collected on the central probe by means of a polarizing voltage maintained between the probe and the cell wall The resulting electric current is amplified and displayed on a conventional meter The display meter is a taut band-suspension microammeter reading 0 to 50, and response time of the instruments one second Leak-rate sensitivity is 1 0-8 mL/s Mass-Spectrometer... and a tungsten-rhenium alloy in Fig 20 Fig 20 Intergranular brittle fractures in tungsten, iridium, and a tungsten-3 wt% rhenium alloy (a) Sintered tungsten rod drawn to 1.5 mm (0.060 in.) diam, recrystallized for 100 h at 1 0-6 torr and 2600 °C (4 712 °F), and fractured in tension (b) Iridium sheet annealed for 50 h in purified helium at 1700 °C (3092 °F) and broken by bending (c) Tungsten-3 wt% rhenium . 17, ASM Handbook, ASM International, 1989, p 66 1-7 15 3. M.P. Kaplan et al, Damage Tolerance of Aircraft Systems, Fatigue and Fracture, Vol 19, ASM Handbook, ASM International, 1996, p 55 7-5 88. Control, Vol 17, ASM Handbook, ASM International, 1989, p 1 2-1 7 5. D.H. Genest, Coordinate Measuring Machines, Nondestructive Evaluation and Quality Control, Vol 17, ASM Handbook, ASM International,. Vol 17, ASM Handbook, ASM International, 1989, p 5 2-5 6 8. L.D. Lineback, Strain Measurement for Stress Analysis, Nondestructive Evaluation and Quality Control, Vol 17, ASM Handbook, ASM International,