Brittle and Ductile Fracture of Materials - Coursework Example

Mechanics of Materials Student Name: Student No: Course: Faculty: University: Lecturer: Date of Submission: Introduction The study of the properties of materials is very important in structural engineering right from decision making on which material is suitable for a particular function. This article aims to discuss the main properties such as brittleness and ductility in relation to fracture. Further, the fatigue of steel will also be studied. Metals are very common in many engineering structures because they are ductile, non-porous, and weldable, have the ability to conduct electricity, have high compressive & tensile strength, and are also associated with few problems such as high cost, and corrosion in the moist conditions of the atmosphere. The main metallic materials include aluminum, iron, steel, nickel, refractory materials, titanium, and copper. Brittle and Ductile Fracture of Materials The ability of a material to deform before it brakes is called ductility. It is expressed as percentage maximum elongation. The deformation of ductile materials before fracture is large. If the material is not ductile it is said to be brittle, and, therefore, if a non-ductile or brittle material fractures, there is no plastic deformation before such failure. In order to change the ductility of some metals, some conditions should be altered. These will include decreasing or increasing the temperature. If the temperature is decreased, ductility will also decrease; thus the material will change from ductile to brittle in terms of its behavior. Further, increasing the temperature of the metal will mean increasing its ductility. Fracture & Fracture Mechanisms Fracture mechanics is concerned with the study of the mechanical behaviors of a cracked material whenever a load is applied to it (David, 1986). This field of study was developed by Irwin (1958) using the early work of Griffith, Westergaard and Inglis. Facture mechanics is related to the irreversible process of rupturing as a result of the development of cracks. When the cracks form, it leads to complex fracturing process that mostly depends on the microstructure of some crystalline or the amorphous solids, environmental conditions or applied loading. Microstructure plays a very crucial role in the fracture process as a result of precipitates, dislocation motion, grain size, and type of phases, inclusions and the composition of the microstructure. There are two mechanisms through which metals can fail (James, William, & John 1982). They can fail by ductile fracture or brittle fracture. The metals which sustain considerable plastic deformation or plastic strain before they fracture exhibit ductile fracture. A large portion of plastic flow is usually concentrated near the fracturing faces. Brittle fracture is exhibited by metals that fracture with relatively small or even negligible amounts of plastic strain. It results from splitting along definite places, a condition called cleavage. Brittle Fracture Brittle failure mode mainly depends on the following factors: Loading speed, thickness of the material, material toughness, steel strength grade, type the structural element, and the toughness of the material (Ravi-Chandar & Salama, 1989). The condition of brittle fracture is critical at ultimate limit state. Some of the evidences of brittle fracture are the small cracks that may or may not be visible. The cracks may extend rapidly to form sudden failure with some few signs of plastic deformation. Such form of fracture is common is the welded structures (Mittemeijer, 2010). The important conditions that lead to brittle fracture include the presence of tensile stress in the material, the existence of a defect or notch, or hole in the material, and a temperature level below transition temperature (John & Volker, 1979). Generally, the tensile stress essential for brittle fracture must not be too high. It can even be residual stress out of welding. A defect, hole or notch is responsible for stress concentration and cracks are initiated at low temperatures, whereas propagation is more probable when there is lower ductility. Therefore, the entire fracture mechanism works in the sense that a notch will raise local tensile stress to high level that are three times more than the average tensile stress. A material that usually fails due to shearing mechanisms will tend to fail as a result of brittle fracture cleavage mechanism that shows reasonably less plastic deformations. Temperature decrease encourages cleavage failure. However, ductile fracture is better as compared to brittle fracture. It takes place over a long period of time whereas brittle fracture is capable of occurring with flaws very fast, and as long as the stress levels are maintained below the ductile fracture. Ductile Fracture Tearing & gross plastic deformation, initiation of micro-voids and their growth into cracks characterize ductile fracture. Further, the fracture surface is also characterized by dimples and it takes place when the applied stress exceeds the material’s yield strength. The materials that undergo this form of fracture have got a high level of roughness (William & David, 2012). Ductile fracture involves a lot of energies whereby a large amount of the energy dissipated is associated with plastic deformation before a crack becomes stable. In such a process, the crack grows slowly because the hardening of the strain occurs at the tip of the crack region. Structural steel undergoes permanent strains before it fails. Further, ductility is a major property that explains as to why a steel bar can be bent to form a circular arc or be drawn into a wire without being broken. The materials that have the property of ductility can absorb large amounts of strain energy before they can fracture. The relationship for load applied to a material against displacement is represented in the graph below: Fundamentally, Hook’s Law can be used to determine the relationship between stress & strain. When such a relation is linear, a material is said to be linearly elastic. The table below shows the relationship between a ductile and a brittle material Ductile Material Brittle Material Deformation Extensive Little Necking It necks It does not neck Fracturing of the surface It is rough It is smoother Warning Undergoes permanent elongation None A comparison of the a ductile and a brittle material Fatigue of Steel Material Repeated application of strain and stress on a metallic material leads to some property changes of the material, and this is what is referred to as fatigue. Fatigue is one of the cases of failure where a material fails when subjected to cyclic loading before its ultimate stress. If the external stress exceeds a particular value of the maximum tensile stress, maximum shear stress or maximum compressive stress, then the component will fail. However, in the situation of fatigue failure, failure will occur early before the maximum value of the design stress is reached. Fracture Mechanics of Materials & its Applications in Examining & Analyzing Material Failures The study of fracture mechanics is a crucial method of characterizing fatigue crack growth, fracture toughness, and the stress-corrosion crack growth behaviors as far as structural design parameters are concerned in engineering (Alexander, 1996). Therefore, fracture mechanics is useful in the following main areas: Design, selection of material and development of alloys, and also in determining the importance of the defects. For example, when choosing steel for some functions, the following have to be considered: The tensile yield strength that is required, ductility, toughness, availability, cost, some arbitrary local conditions that are imposed by some codes of practice and specifications. Structural engineers will use fracture mechanics to characterize fatigue crack growth, fracture toughness, and stress-corrosion crack growth behaviors (Anderson, 2005). For instance, the conventional design procedures are normally based on yield strength, and this approach is considered to be safe if appropriate factors are used. Chell (1979) also argues that safety factors are based on the findings of fracture mechanics. They can be used for working out stress, the anticipated number of cyclic loading and the initial defect sizes. References Alexander, B 1996, Practical Fracture Mechanic in Design, Marcel Dekker Ltd, New York. Anderson, TL, 2005, Fracture Mechanics: Fundamentals & Applications, 3rd Ed., Taylor & Francis Group, LLC, Boca Raton. Chell, GG 1979, Development in Fracture Mechanics Vol I and II, Applied Science Publishers Inc., London. David, B 1986, Elementary Engineering Fracture Mechanics, Kluwer Academic Publishers, Dordrecht. Irwin, GR 1958, Fracture I, Springer-Verlag, New York. James, EC, William, WG, and John, HU, 1982, Application of Fracture Mechanics for Selection of Metallic Structural materials, American Society of Metals, NY. John, JB, & Volker, W 1979, Application of Fracture Mechanics to Design, Plenum Press, London. Mittemeijer, EJ 2010, Fundamentals of Materials, Pergamon Press, London. Ravi-Chandar, K & Salama, K, 1989, Brittle Fracture, Ductile Fracture, Dynamic Fracture, Springer, London. William, DC, & David, GR, 2012, Fundamentals of Materials Science & Engineering, John Wiley & Sons, New Jersey. Read More