Ductile Fracture
Ductile fracture refers to the process by which a material undergoes significant plastic deformation before breaking. This type of fracture is characterized by the stretching and thinning of the material before it ultimately fails. Ductile fractures often occur in materials like metals and alloys, and are associated with the ability of these materials to deform under stress before rupturing.
7 Key excerpts on "Ductile Fracture"
eBook - ePub- H.K.D.H. Bhadeshia, R.W.K. Honeycombe(Authors)
- 2017(Publication Date)
- Butterworth-Heinemann(Publisher)
...However, a detailed knowledge of structure and of the distribution of impurities in steels is gradually leading to a much better understanding of the origins and mechanisms of the various types of cracks encountered. Furthermore, the now well-established science of fracture mechanics allows the quantitative assessment of growth of cracks in various stress situations, to an extent that it is now frequently possible to estimate the tolerance of engineering structures to avoid the risk of sudden failure. There are essentially three fracture modes in steels at ambient temperature. Ductile failure is accompanied by gross plastic deformation so that the work done in fracture is orders of magnitude greater than purely attributable to the creation of new surfaces as the steel is parted. Cleavage fracture, and the failure of the material by the separation of grain surfaces, involve minimal absorption of energy and hence are regarded as brittle fracture mechanisms. For pure metals, the transition from ductile to brittle fracture is described by the ratio of the modulus μ which is a measure of the resistance to shear, and the bulk modulus K because the stress field at a brittle crack tip is similar to pure dilatation [1]. A small value of the ratio μ / K favours ductile failure because plastic deformation by shear is relatively easy. For pure α -iron made using electron beam zone-melting under high vacuum, the ratio is 0.33 which is less than the critical value associated with brittle failure. Indeed, high-purity iron is found to be ductile at temperatures as low as 4.2 K [2]...
eBook - ePub- John Moalli(Author)
- 2001(Publication Date)
- William Andrew(Publisher)
...Ductile Fractures are characterized by material tearing and exhibit gross plastic deformation. Brittle fractures display little or no macroscopically visible plastic deformation and require less energy to form. Ductile Fractures occur as the result of applied stresses exceeding the material yield or flow stress. Brittle fractures generally occur well below the material yield stress. In practice, Ductile Fractures occur due to overloading or under-designing. They are rarely the subject of a failure analysis. Fracture analysis usually involves the unexpected brittle failure of normally ductile materials. Many macroscopically visible fractographic features serve to identify the fracture origin(s) and direction of crack propagation. Fractographic features common to metals and plastics are radial marks and chevron patterns. Radial marks (Figure 4) are lines on a fracture surface that radiate outward from the origin and are formed by the intersection of brittle fractures propagating at different levels. Chevron patterns or herringbone patterns are actually radial marks resembling nested letters “V” and pointing towards the origin. Figure 4 Beach and radial marks emanate from the origin (“O”) of this torsional fatigue fracture. (0.5X). Fatigue failures in metals display beach marks and ratchet marks that serve to identify the origin and the failure mode. Beach marks (Figure 4) are macroscopically visible semi-elliptical lines running perpendicular to the overall direction of fatigue crack propagation and marking successive positions of the advancing crack front. Ratchet marks are macroscopically visible lines running parallel to the overall direction of crack propagation and formed by the intersection of fatigue cracks propagating from multiple origins. Brittle fractures in plastics exhibit characteristic features, several of which are macroscopically visible (Figure 5). These may include a mirror zone at the origin, mist region, and rib marks...
eBook - ePubMachinery Failure Analysis Handbook
Sustain Your Operations and Maximize Uptime
- Luiz Octavio Amaral Affonso(Author)
- 2013(Publication Date)
- Gulf Publishing Company(Publisher)
...The fracture surface shows no macroscopic plastic deformation, the separation occurs due to cleavage between the crystalline planes caused by large enough tension stress. 2. Ductile Fracture, in which the cross section of the component is reduced due to the macroscopic plastic deformation caused by the slippage between the crystalline planes. The slippage is caused by large enough shear stress. The fracture propagation is stable, which means that propagation stops if the load is reduced. The fracture surface shows macroscopic signs of plastic deformation. A Ductile Fracture can be understood as the ultimate plastic deformation of a component. A component may suffer a brittle or Ductile Fracture, depending on the circumstances. A reduction in temperature, for example, can be enough to modify the fracture behavior of a material. 4.1 Ductile Fracture Morphology A Ductile Fracture surface normally presents three distinct regions: 1. A fibrous zone, which corresponds to the initiation site of the fracture; the fracture propagates stably in this zone. This fibrous zone is formed on the region with the highest stress triaxiality, which means it is close to the centerline of the component or near stress concentrators. 2. A radial zone, which corresponds to an unstable propagation region. This region shows a rough surface and radial marks diverging from the fibrous zone. Its appearance is similar to a brittle fracture. 3. A shear lip zone, which shows an inclination of approximately 45° to the external load direction. This is the region where stress triaxiality is reduced and shear slip of the crystalline planes is possible...
eBook - ePubMaterials
Engineering, Science, Processing and Design
- Michael F. Ashby, Hugh Shercliff, David Cebon(Authors)
- 2009(Publication Date)
- Butterworth-Heinemann(Publisher)
...Within it voids nucleate, grow and link, advancing the crack in a ductile mode, absorbing energy in the process. The ductile-to-brittle transition A cleavage fracture is much more dangerous than one that is ductile: it occurs without warning or any prior plastic deformation. At low temperatures some metals and all polymers become brittle and the fracture mode switches from one that is ductile to one of cleavage—in fact only those metals with an FCC structure (copper, aluminum, nickel and stainless steel, for example) remain ductile to the lowest temperatures. All others have yield strengths that increase as the temperature falls, with the result that the plastic zone at any crack they contain shrinks until it becomes so small that the fracture mode switches, giving a ductile-to-brittle transition. For some steels that transition temperature is as high as 0 °C (though for most it is considerably lower), with the result that steel ships, bridges and oil rigs are more likely to fail in winter than in summer. Polymers, too, have a ductile-to-brittle transition, a consideration in selecting those that are to be used in freezers and fridges. Embrittlement of other kinds Change of temperature can lead to brittleness; so, too, can chemical segregation. When metals solidify, the grains start as tiny solid crystals suspended in the melt, and grow outward until they impinge to form grain boundaries. The boundaries, being the last bit to solidify, end up as the repository for the impurities in the alloy. This grain boundary segregation can create a network of low-toughness paths through the material so that, although the bulk of the grains is tough, the material as a whole fails by brittle intergranular fracture (Figure 8.14)...
- Shashanka Rajendrachari, Orhan Uzun(Authors)
- 2021(Publication Date)
- Bentham Science Publishers(Publisher)
...(9) represents the load-extension diagram showing the ductility of materials [ 4 ]. Examples of ductile materials are copper, aluminum, steel, and some more metals. Ductility is a physical property, and it is not having any unit. 5.11. Malleability Malleability is just opposite to that of ductility. In the case of the ductility test, tensile forces are used, but here compressive forces are used. It is defined as the ability of a material to undergo plastic deformation before fracturing under compressive stress. It can also be defined as the ability of a material to undergo rolling or flattening into thin sheets. Most of the metals with high ductility also possess greater malleability, and it can be measured by % reduction in the cross-sectional area of the material under study. Fig. (9)) Load-extension diagram showing ductility [ 4 ]. 6. TYPES OF ENGINEERING STRESS-STRAIN CURVES FOR DIFFERENT TYPES OF MATERIALS 6.1. Ductile Materials without Yield Point The engineering stress-strain diagram for ductile materials without a yield point is represented in Fig. (10) [ 4 ]. Due to strain hardening, the below curve shows the ascending trend of stress during plastic deformation. As we see from the figure, the curve starts dropping after the ultimate tensile stress because of the necking phenomenon. The distance between ultimate tensile stress and proof stress, as well as ultimate tensile stress and breaking stress, increases with an increase in the ductility of a material. Some of the metals and alloys which can undergo plastic deformation without yield point are nickel, copper, aluminum, austenitic stainless steel, silver, gold, platinum, etc. 6.2. Ductile Materials with a Yield Point Fig. (11) represents the engineering stress-strain curve for ductile materials with a yield point [ 4 ]. The curve is almost similar to Fig. (10), but the only difference is the presence of a yield point near the elastic deformation region in Fig. (11), as shown...
eBook - ePubMetallurgy and Mechanics of Welding
Processes and Industrial Applications
- Regis Blondeau, Regis Blondeau(Authors)
- 2013(Publication Date)
- Wiley-ISTE(Publisher)
...Chapter 7 Fracture Toughness of Welded Joints 1 7.1. Ductile Fracture and brittle fracture In general, manufacturers dimension structures to avoid the risks of failure in service, by avoiding in particular any going beyond, even local, of the yield strength of the material. In spite of that, failures do occur, for various reasons: – exceptional load conditions; – progressive cracking related to fatigue, stress corrosion, hydrogen embrittlement; – behavioral change related to a transition of the ductile-brittle type, which originates in a temperature fall. Brittle fracture, due to the loss of plasticity, was first studied as early as the second half of the 19 th century. With the development of welding and its application to the construction of large-scale structures, a certain number of spectacular, often catastrophic, failures have occurred. Among the most famous examples we might recall bridge failures (at Berlin Zoo in 1936, in Rudesdorf and Hasselt in 1938) and storage tank failures. Of nearly 5,000 ships built by mechanized welding during World War II, a quarter of them presented important failures and more than 200 suffered catastrophic failure. The Schenectady oil tanker at anchor, without any particular excess loading, one night in winter 1942 is the classic example (see Figure 7.1). From immediately after the War, considerable work was undertaken in order to understand the phenomenon and to reduce the risks. The teams of the Naval Research Laboratory in Washington have been at the center of a great deal of work, particularly with W.S. Pellini and G.R. Irwin, which are still valid points of reference today...
eBook - ePub- Raul D. S. G. Campilho, Raul D. S. G. Campilho(Authors)
- 2017(Publication Date)
- CRC Press(Publisher)
...Also, this approach doesn’t predict the sequence of events leading to failure or behaviour post failure, which is useful for many reasons, such as in designing in-service monitoring or modelling energy absorption in an impact. In order to predict this, a progressive damage modelling approach is required. In ductile materials, a progressive damage approach can be based on plastic yielding, with failure defined when a complete path of yielded material has developed between load points. This may be defined by plastic collapse of the structure and may play a part in a limit state design approach. In a brittle material, a fracture mechanics approach may be more applicable where the propagation of a macro-crack through otherwise (assumed) undamaged material is predicted by modelling the conditions for crack growth. However, with many modern engineering materials, particularly those exhibiting a variety of concurrent micro-mechanical failure mechanisms, neither of these methods can fully capture the sequence of progressive damage under loading in a mechanistically accurate way. For example, a modern polymeric adhesive is a complex multi-phase material, typically comprised of a viscoelastic matrix with rubber particles for toughening, some form of filler particle and a carrier mat. Failure in such a material is complex and can involve the initiation and propagation of a macro-crack, accompanied by a region ahead and/or around the crack exhibiting numerous forms of micro- damage such as particle debonding and cavitation, carrier mat debonding and micro-cracking and yielding of the matrix material. In such a system, the state of the material after undergoing loading induced damage may be better represented by some form of damage mechanics...
