3.7 m Fig 11 Mud cracks on the surface of an intergranular fracture in 7079-T651 aluminum that failed under SCC conditions in a 3.5% chloride solution. TEM replica 1.5 Fig. 12 Mud cracks on the fracture surface of a quenched and tempered 4340 steel exposed to a marine environment. TEM replica References cited in this section 7. Electron Fractography, STP 436, ASTM, 1968 25. R W. Hertzberg, Deformation and fracture Mechanics of Engineering Materials, 4th ed, John Wiley, 1996
Fig. 11 Mud cracks on the surface of an intergranular fracture in 7079-T651 aluminum that failed under SCC conditions in a 3.5% chloride solution. TEM replica Fig. 12 Mud cracks on the fracture surface of a quenched and tempered 4340 steel exposed to a marine environment. TEM replica References cited in this section 7. Electron Fractography, STP 436, ASTM, 1968 25. R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 4th ed., John Wiley, 1996
26. H. Rogers, The Fundamental Aspects of Fracture in Deformation Processing, Deformation Processing, Syracuse University Press, 1964, p 199 27. G. Fett, Testing for Fatigue, Adv. Mater. Process, May 1999, p 37 28. D. Broek, Fatigue Design Handbook, AE-10, 2nd ed, Society of Automotive Engineers, 1988 29. D.J. Ulpi, Understanding How Components Fail, 2nd ed, ASM International, 1999 30. W.A. Spitzig, G.E. Pellister, C D. Beachem, A.J. Brothers, M. Hill, and W.R. Warke, A Fractographic Analysis of the Relationships Between Fracture Toughness and Surface Topography in Ultrahigh Strength Steels, Electron Fractography, STP 436, ASTM, 1968, p 17 31. W. Gerberich, Microstructure and Fracture, Mechanical Testing, Vol 8, Metals Handbook, 9th ed Mechanical Testing, American Society for Metals, 1985, p 476-491 32. H L. Ewalds and R.J.H. Wanhill. fracture Mechanics. Edward Arnold Ltd. London. 1984 Fracture Appearance and Mechanisms of Deformation and Fracture W.T. Becker, University of Tennessee, Emeritus; S Lampman, ASM International Ductile and brittle behavior Perhaps most importantly, the question of whether a fracture is ductile or brittle is almost always addressed in a failure analysis. Ductile and brittle are terms often used to describe the amount of macroscale plastic deformation that precedes fracture. The presence of brittle fracture is a concern, because catastrophic brittle fracture occurs due to the elastic stress that is present and usually propagates at high speed, sometimes with little associated absorbed energy. Fracture occurring in a brittle manner cannot be anticipated by the onset of prior macroscale visible permanent distortion to cause shut down of operating equipment, nor can it be arrested by a removal of the load except for very special circumstances It must be pointed out, however, that the terms ductile and brittle also can be and are applied to fracture on a microscopic level. At the macroscale, ductile fracture by the microscale ductile process of microvoid formation and coalescence is characterized by plastic deformation and expenditure of considerable energy, while microscale brittle fractures by cleavage are characterized by rapid crack propagation with less expenditure of energy than with ductile fractures and without macroscale evidence of plastic deformation. The point is that the terms ductile and brittle are used to describe both appearance(macroscale behavior) and mechanism (microscale behavior ) The macroscale view of ductility is neither more nor less correct than the microscale definition for the fracture mechanism The specific meaning of ductile and brittle may carry different connotations depending on background, context and perspective of the reader. It is therefore important to clearly identify whether a ductile or brittle fracture is being described in terms of macroscale appearance or microscale mechanisms. It is also important to note that there is no universally accepted dividing line for macroscale ductile and brittle behavior in terms of strain at fracture nor in terms of energy absorption. For example, large fracture strain is desirable for forming operations, and material selection may be based on the relative ductility observed during tensile testing Materials that do not show obvious necking in a tensile test are sometimes described as brittle. but that is not a generally accepted or valid meaning of the term. For example, the absence of obvious necking may be due to the geometry of the specimen elative ductility observed during tensile testing also is an arbitrary basis for defining macroscale ductility. For example, that a material has adequate" ductility when the reduction in area(RA)is between 26 and 18% and Thefileisdownloadedfromwww.bzfxw.com
26. H. Rogers, The Fundamental Aspects of Fracture in Deformation Processing, Deformation Processing, Syracuse University Press, 1964, p 199 27. G. Fett, Testing for Fatigue, Adv. Mater. Process., May 1999, p 37 28. D. Broek, Fatigue Design Handbook, AE-10, 2nd ed., Society of Automotive Engineers, 1988 29. D.J. Wulpi, Understanding How Components Fail, 2nd ed., ASM International, 1999 30. W.A. Spitzig, G.E. Pellister, C.D. Beachem, A.J. Brothers, M. Hill, and W.R. Warke, A Fractographic Analysis of the Relationships Between Fracture Toughness and Surface Topography in UltrahighStrength Steels, Electron Fractography, STP 436, ASTM, 1968, p 17 31. W. Gerberich, Microstructure and Fracture, Mechanical Testing, Vol 8, Metals Handbook, 9th ed., Mechanical Testing, American Society for Metals, 1985, p 476–491 32. H.L. Ewalds and R.J.H. Wanhill, Fracture Mechanics, Edward Arnold Ltd., London, 1984 Fracture Appearance and Mechanisms of Deformation and Fracture W.T. Becker, University of Tennessee, Emeritus; S. Lampman, ASM International Ductile and Brittle Behavior Perhaps most importantly, the question of whether a fracture is ductile or brittle is almost always addressed in a failure analysis. Ductile and brittle are terms often used to describe the amount of macroscale plastic deformation that precedes fracture. The presence of brittle fracture is a concern, because catastrophic brittle fracture occurs due to the elastic stress that is present and usually propagates at high speed, sometimes with little associated absorbed energy. Fracture occurring in a brittle manner cannot be anticipated by the onset of prior macroscale visible permanent distortion to cause shut down of operating equipment, nor can it be arrested by a removal of the load except for very special circumstances. It must be pointed out, however, that the terms ductile and brittle also can be and are applied to fracture on a microscopic level. At the macroscale, ductile fracture by the microscale ductile process of microvoid formation and coalescence is characterized by plastic deformation and expenditure of considerable energy, while microscale brittle fractures by cleavage are characterized by rapid crack propagation with less expenditure of energy than with ductile fractures and without macroscale evidence of plastic deformation. The point is that the terms ductile and brittle are used to describe both appearance (macroscale behavior) and mechanism (microscale behavior). The macroscale view of ductility is neither more nor less correct than the microscale definition for the fracture mechanism. The specific meaning of ductile and brittle may carry different connotations depending on background, context, and perspective of the reader. It is therefore important to clearly identify whether a ductile or brittle fracture is being described in terms of macroscale appearance or microscale mechanisms. It is also important to note that there is no universally accepted dividing line for macroscale ductile and brittle behavior in terms of strain at fracture nor in terms of energy absorption. For example, large fracture strain is desirable for forming operations, and material selection may be based on the relative ductility observed during tensile testing. Materials that do not show obvious necking in a tensile test are sometimes described as brittle, but that is not a generally accepted or valid meaning of the term. For example, the absence of obvious necking may be due to the geometry of the specimen. Relative ductility observed during tensile testing also is an arbitrary basis for defining macroscale ductility. For example, that a material has “adequate” ductility when the reduction in area (RA) is between 26 and 18% and The file is downloaded from www.bzfxw.com
"limited"ductility when the Ra is between 18 and 2% and is brittle when the Ra is below 2% has been suggested(Ref 33). Strain hardening exponents(n) for most structural alloys are typically in the range of 0.05 to 0.2, which translates to a Ra in the range of 5 to 22% before necking instability is attained. Thus, few materials that are not cold worked would neck before 2% strain and would be considered brittle in this criterion for metal forming operation Another set of criteria may apply in structural design, where analytical expressions to determine allowable loads are based on whether failure is ductile or brittle. Some(arbitrary) value of tensile elongation or reduction in area(RA)is required to define whether a( ductile) distortion energy yield criterion or a(brittle)maximum normal stress or maximum shear criterion(perhaps modified by a normal stress term, as the Coulomb-Mohr model) is used in design. Ductile behavior also is often associated with high energy absorption at fracture, and adequate toughness or ductility may be evaluated and defined by impact data, where criteria to determine whether the fracture is ductile or brittle involves some minimum level of absorbed energy at the service temperature of interest, say 14 or 20 J(10 or 15 ft. Ibf) The macroscale definition of ductile versus brittle behavior also may be misleading about material behavior For example, when subjected to large compressive hydrostatic loads, "brittle" materials may behave in a ductile manner. The fracture strain of ductile materials increases with an increase in loading conditions containing large compressive hydrostatic component relative to the deviatoric component of stress and decreases with an increase in the tensile-hydrostatic stress component. It is also possible for ductile fracture to require little energy for initiation or propagation if strain-hardening capacity is low From the perspective of"safe"design, materials that are inherently ductile but can behave in a brittle manner in service require the most caution. Many engineering materials are inherently ductile and some are inherently brittle. but those behaviors can be altered. Possible reasons for brittle behavior of ductile materials include loading conditions and the internal state of stress created by the part geometry and the geometry of any imperfections in conjunction with the operating environment(chemically reactive and/or high or low temperature). The inherent ductile behavior of metallic material also can be drastically reduced by improper heat treatment(e.g, incipient melting, temper embrittlement, improper age hardening) by processing(hydrogen embrittlement due to plating baths Therefore, it is necessary to understand Why some materials are inherently ductile or brittle, and How part geometry, operating conditions or mechanical/thermal processing may alter that behavior The inherent ductility or brittleness of materials is addressed later, in the section"Structure and behavior"in this article, after the discussion of the macroscale appearances of ductile and brittle fracture Observation of Plastic Strain. Smaller amounts of plastic deformation might be determined via careful measurement if the surfaces of the component are relatively smooth. The ability to see a neck in a tensile specimen depends on the amount of strain hardening and to some extent, the amount of strain-rate hardening. If there is no hardening to force the neck to grow along the length of the specimen, plastic flow via slip can occur without visual evidence. That is, there may be microstructural evidence or microscale fractographic evidence of plastic deformation, but it occurs over a sufficiently small volume that it is not visually apparent In some instances, small amounts of plastic deformation may be visible at the macroscale, such as the twisting of extrusion marks around the axis of the component(torsion loading). Two halves of a bending fracture can often be brought into close proximity to determine if a small amount of plastic bending has occurred(for example, by placing the two components on a flat surface). This is a helpful technique in the examination of threaded cylindrical sections. However, it is of extreme importance that two fracture surfaces not be brought into actual physical contact. Doing so can destroy microscale fractographic information Sometimes plastic strain can also be seen in such an instance by examination of the surface of the component adjacent to the fracture. Plastic strain will result in a roughening of the surface if the grain size is very large Conversely, the presence of a large grain size may be visible(detected) by roughening of the surface for a component with a distortion of the original geometry. In extreme cases, the roughening occurs over the complete section, not just the area immediately adjacent to the fracture surface, and is then described as"orange It is also possible to use the SEM or atomic force microscopy to obtain three-dimensional surface maps showing surface profiles as discussed in other sections of ASM Handbook. Hull(Ref ll)describes a technique
“limited” ductility when the RA is between 18 and 2% and is brittle when the RA is below 2% has been suggested (Ref 33). Strain hardening exponents (n) for most structural alloys are typically in the range of 0.05 to 0.2, which translates to a RA in the range of 5 to 22% before necking instability is attained. Thus, few materials that are not cold worked would neck before 2% strain and would be considered brittle in this criterion for metal forming operations. Another set of criteria may apply in structural design, where analytical expressions to determine allowable loads are based on whether failure is ductile or brittle. Some (arbitrary) value of tensile elongation or reduction in area (RA) is required to define whether a (ductile) distortion energy yield criterion or a (brittle) maximum normal stress or maximum shear criterion (perhaps modified by a normal stress term, as the Coulomb-Mohr model) is used in design. Ductile behavior also is often associated with high energy absorption at fracture, and adequate toughness or ductility may be evaluated and defined by impact data, where criteria to determine whether the fracture is ductile or brittle involves some minimum level of absorbed energy at the service temperature of interest, say 14 or 20 J (10 or 15 ft · lbf). The macroscale definition of ductile versus brittle behavior also may be misleading about material behavior. For example, when subjected to large compressive hydrostatic loads, “brittle” materials may behave in a ductile manner. The fracture strain of ductile materials increases with an increase in loading conditions containing a large compressive hydrostatic component relative to the deviatoric component of stress and decreases with an increase in the tensile-hydrostatic stress component. It is also possible for ductile fracture to require little energy for initiation or propagation if strain-hardening capacity is low. From the perspective of “safe” design, materials that are inherently ductile but can behave in a brittle manner in service require the most caution. Many engineering materials are inherently ductile and some are inherently brittle, but those behaviors can be altered. Possible reasons for brittle behavior of ductile materials include loading conditions and the internal state of stress created by the part geometry and the geometry of any imperfections in conjunction with the operating environment (chemically reactive and/or high or low temperature). The inherent ductile behavior of metallic material also can be drastically reduced by improper heat treatment (e.g., incipient melting, temper embrittlement, improper age hardening) by processing (hydrogen embrittlement due to plating baths). Therefore, it is necessary to understand: · Why some materials are inherently ductile or brittle, and · How part geometry, operating conditions or mechanical/thermal processing may alter that behavior The inherent ductility or brittleness of materials is addressed later, in the section “Structure and Behavior” in this article, after the discussion of the macroscale appearances of ductile and brittle fracture. Observation of Plastic Strain. Smaller amounts of plastic deformation might be determined via careful measurement if the surfaces of the component are relatively smooth. The ability to see a neck in a tensile specimen depends on the amount of strain hardening and to some extent, the amount of strain-rate hardening. If there is no hardening to force the neck to grow along the length of the specimen, plastic flow via slip can occur without visual evidence. That is, there may be microstructural evidence or microscale fractographic evidence of plastic deformation, but it occurs over a sufficiently small volume that it is not visually apparent. In some instances, small amounts of plastic deformation may be visible at the macroscale, such as the twisting of extrusion marks around the axis of the component (torsion loading). Two halves of a bending fracture can often be brought into close proximity to determine if a small amount of plastic bending has occurred (for example, by placing the two components on a flat surface). This is a helpful technique in the examination of threaded cylindrical sections. However, it is of extreme importance that two fracture surfaces not be brought into actual physical contact. Doing so can destroy microscale fractographic information. Sometimes plastic strain can also be seen in such an instance by examination of the surface of the component adjacent to the fracture. Plastic strain will result in a roughening of the surface if the grain size is very large. Conversely, the presence of a large grain size may be visible (detected) by roughening of the surface for a component with a distortion of the original geometry. In extreme cases, the roughening occurs over the complete section, not just the area immediately adjacent to the fracture surface, and is then described as “orange peel.” It is also possible to use the SEM or atomic force microscopy to obtain three-dimensional surface maps showing surface profiles as discussed in other sections of ASM Handbook. Hull (Ref 11) describes a technique
using surface profiles that can be used to identify small-scale plastic strain. Profilometer traces are obtained from matching regions on each half of the fracture surface. If the two traces cannot then be brought into alignment, it is likely that there has been some plastic deformation associated with fracture. Obviously, if a piece has dropped out of the surface, there may be no matching but neither has there been any plastic deformation It is also important to clarify whether the term ductile refers to lastic strain accumulated prior to the nucleation and growth of a crack The process of crack nucleation The process of crack growth As an example of the necessity for careful description, consider two tensile specimens fabricated from the same alloy, but one is in the annealed condition and the other prepared from cold rolled material. The annealed material is expected to show fracture in the necked region, but no or minimal necking is expected in the cold worked material. The microscopic mechanism of fracture in the annealed material is a ductile mechanism (microvoid coalescence), and macroscopic deformation preceded fracture. Thus, there is little confusion in describing the fracture as ductile on both the macroscopic and microscopic scales of observation However, what about the cold worked material? There was plastic deformation(presumably compressive) during manufacturing, and the presence of prior cold work may explain the absence of the macroscopic necking. Metallographic observation and/or hardness testing would determine the material condition and clarify the effect of previous cold working on the fracture appearance, which could be either ductile or brittle at the microscale References cited in this section 11. D. Hull, Fractography, Cambridge University Press, 1999 33. R. Paprino, Ductility in Structural Design, Ductility, American Society for Metals, 1968 Fracture Appearance and Mechanisms of Deformation and Fracture W.T. Becker, University of Tennessee, Emeritus; S. Lampman, ASM International Macroscopic ductile and Brittle Fracture Surfaces As noted in the previous section, there is no universally accepted dividing line between ductile and brittle behavior at the macroscale in terms of strain at fracture, nor is there a defined dividing line in terms of energy absorption. The terms ductile and brittle also can be and are applied to fracture on a microscopic level Therefore, it is desirable to first provide a working definition of macroscopic ductile and brittle fracture In the context of this article, a fracture is brittle at the macroscale if it is on a plane normal to the maximum normal stress(condition 4 in Fig. 13). A fracture is considered to be macroscopically ductile when the fracture surfaces are inclined to an imposed load(slant fracture or plane-stress ). Toughness is higher under conditions of plane stress, as the additional work expended in work-hardening deformation contributes to fracture resistance under load. A fracture surface displaying both types of planes can be described as a mixed mode fracture or alternatively, by indicating the presence of shear lips on the fracture surface Thefileisdownloadedfromwww.bzfxw.com
using surface profiles that can be used to identify small-scale plastic strain. Profilometer traces are obtained from matching regions on each half of the fracture surface. If the two traces cannot then be brought into alignment, it is likely that there has been some plastic deformation associated with fracture. Obviously, if a piece has dropped out of the surface, there may be no matching but neither has there been any plastic deformation. It is also important to clarify whether the term ductile refers to: · Plastic strain accumulated prior to the nucleation and growth of a crack · The process of crack nucleation · The process of crack growth As an example of the necessity for careful description, consider two tensile specimens fabricated from the same alloy, but one is in the annealed condition and the other prepared from cold rolled material. The annealed material is expected to show fracture in the necked region, but no or minimal necking is expected in the cold worked material. The microscopic mechanism of fracture in the annealed material is a ductile mechanism (microvoid coalescence), and macroscopic deformation preceded fracture. Thus, there is little confusion in describing the fracture as ductile on both the macroscopic and microscopic scales of observation. However, what about the cold worked material? There was plastic deformation (presumably compressive) during manufacturing, and the presence of prior cold work may explain the absence of the macroscopic necking. Metallographic observation and/or hardness testing would determine the material condition and clarify the effect of previous cold working on the fracture appearance, which could be either ductile or brittle at the microscale. References cited in this section 11. D. Hull, Fractography, Cambridge University Press, 1999 33. R. Paprino, Ductility in Structural Design, Ductility, American Society for Metals, 1968 Fracture Appearance and Mechanisms of Deformation and Fracture W.T. Becker, University of Tennessee, Emeritus; S. Lampman, ASM International Macroscopic Ductile and Brittle Fracture Surfaces As noted in the previous section, there is no universally accepted dividing line between ductile and brittle behavior at the macroscale in terms of strain at fracture, nor is there a defined dividing line in terms of energy absorption. The terms ductile and brittle also can be and are applied to fracture on a microscopic level. Therefore, it is desirable to first provide a working definition of macroscopic ductile and brittle fracture. In the context of this article, a fracture is brittle at the macroscale if it is on a plane normal to the maximum normal stress (condition 4 in Fig. 13). A fracture is considered to be macroscopically ductile when the fracture surfaces are inclined to an imposed load (slant fracture or plane-stress). Toughness is higher under conditions of plane stress, as the additional work expended in work-hardening deformation contributes to fracture resistance under load. A fracture surface displaying both types of planes can be described as a mixed mode fracture or alternatively, by indicating the presence of shear lips on the fracture surface. The file is downloaded from www.bzfxw.com
Kic a. B Fig. 13 Schematic of variation in fracture toughness and macro-scale features of fracture surfaces for an nherently ductile material. As section thickness(B)or preexisting crack length(a) increases, plane strain conditions develop first along the centerline and result in a flat fracture surface With further increases section thickness or crack size, the flat region spreads to the outside of the specimen, decreasing the widths of the shear lips. When the minimum value of plane-strain toughness(Klc) is reached, the shear lips have very small width. The local state of stress created by a load on a component geometry may cause crack propagation (i.e, critical fracture)that results in a fracture surface with a macroscale appearance; that is Totally ductile 2. Totally brittle 3. Initially brittle, then ductile 4. Initially ductile, then brittle 5. Mixed mode(ductile and brittle) In the latter two cases(4 and 5), the ductile appearance may not be directly visible at the macroscale. Initially, ductile fractures(case 4) are usually associated with rising-load ductile tearing, or the initial ductility may be inferred by transverse strain at the crack tip. The size of the plastic zone may be microscale in this case. Mixed- mode ductile and brittle cracking(case 5) would be inferred due to the presence of an intimate mixture of cleavage and microvoid coalescence at the microscale or by the presence of shear lips at the macroscale The fracture appearance that occurs depends on the microstructure(strength and ductility) of the material and the degree of constraint associated with the presence of a cracklike imperfection. Constraint and fracture appearances are discussed further in following paragraphs, and the macroscopic conditions associated with the onset of critical fracture (i.e, stress and crack size) are also briefly described in terms of fracture mechanics However, it also must be noted that some of the above criteria are based on macroscopic conditions or appearances and do not consider the microscopic mechanisms (i.e, slip, twinning, viscous flow, cleavage) that cause fracture. A fracture may appear to be macroscopically brittle, but the cracking process may occur by a ductile mechanism. Examples in which the cracking mechanism is ductile but for which there is no or little visual macroscopic distortion include: monotonic loading of a component containing a cracklike imperfection (plane-strain microvoid coalescence fracture induced by part and crack geometry), long- life cyclic loading, and elevated temperature failure(intergranular creep fracture). These examples are discussed in subsequent sectio of this article, but the major point here is that the terms ductile and brittle should be used carefully with respect to the scale of observation or the description of fracture mechanisms. The distinction is important, because macroscopic brittle fractures can occur from the microscopic mechanism of ductile cracking Constraint and Macroscopic Fracture Appearance. Constraint is created by longer cracks, thicker sections, and a decreased crack tip radius. If the material is inherently brittle(say a steel below the ductile-brittle transition
Fig. 13 Schematic of variation in fracture toughness and macro-scale features of fracture surfaces for an inherently ductile material. As section thickness (B) or preexisting crack length (a) increases, plane strain conditions develop first along the centerline and result in a flat fracture surface. With further increases in section thickness or crack size, the flat region spreads to the outside of the specimen, decreasing the widths of the shear lips. When the minimum value of plane-strain toughness (KIc) is reached, the shear lips have very small width. The local state of stress created by a load on a component geometry may cause crack propagation (i.e., critical fracture) that results in a fracture surface with a macroscale appearance; that is: 1. Totally ductile 2. Totally brittle 3. Initially brittle, then ductile 4. Initially ductile, then brittle 5. Mixed mode (ductile and brittle) In the latter two cases (4 and 5), the ductile appearance may not be directly visible at the macroscale. Initially, ductile fractures (case 4) are usually associated with rising-load ductile tearing, or the initial ductility may be inferred by transverse strain at the crack tip. The size of the plastic zone may be microscale in this case. Mixedmode ductile and brittle cracking (case 5) would be inferred due to the presence of an intimate mixture of cleavage and microvoid coalescence at the microscale or by the presence of shear lips at the macroscale. The fracture appearance that occurs depends on the microstructure (strength and ductility) of the material and the degree of constraint associated with the presence of a cracklike imperfection. Constraint and fracture appearances are discussed further in following paragraphs, and the macroscopic conditions associated with the onset of critical fracture (i.e., stress and crack size) are also briefly described in terms of fracture mechanics. However, it also must be noted that some of the above criteria are based on macroscopic conditions or appearances and do not consider the microscopic mechanisms (i.e., slip, twinning, viscous flow, cleavage) that cause fracture. A fracture may appear to be macroscopically brittle, but the cracking process may occur by a ductile mechanism. Examples in which the cracking mechanism is ductile but for which there is no or little visual macroscopic distortion include: monotonic loading of a component containing a cracklike imperfection (plane-strain microvoid coalescence fracture induced by part and crack geometry), long-life cyclic loading, and elevated temperature failure (intergranular creep fracture). These examples are discussed in subsequent sections of this article, but the major point here is that the terms ductile and brittle should be used carefully with respect to the scale of observation or the description of fracture mechanisms. The distinction is important, because macroscopic brittle fractures can occur from the microscopic mechanism of ductile cracking. Constraint and Macroscopic Fracture Appearance. Constraint is created by longer cracks, thicker sections, and a decreased crack tip radius. If the material is inherently brittle (say a steel below the ductile-brittle transition