Engineering Materials: Modes of Failure - Assignment Example

 Modes of Failure Ductile Fracture It is the fracture that occurs with a slow propagation of the crack while accompanied with a significant amount of plastic deformation. It involves: necking of the specimen and the formation of minute cavities in the necked region which has a high concentration of the plastic deformation, formation of micro cracks at the center of the specimen as a result of combination of dislocations, and the formed cracks grow outward at a direction of 45 degrees to the axis of tension. Source: (Mechanical Properties of Metals II Fracture and Failure, 2015) Figure showing the various stages in ductile fracture. Ductile fracture occurs through the tearing of the metal and energy is used during the tearing process. The initial cavities and crack form at foreign inclusions as in these regions glide dislocations pile up and give sufficient stress that can form a micro crack. A specimen that is subjected to increasing tensile load begin to work harden when the elastic point is exceeded. Increasing the load increases the permanent elongation and thus leads to a decrease in the area. This decrease in the cross section area leads to the necking of the specimen as shown in the diagram above. Source: (Mechanical Properties of Metals II Fracture and Failure, 2015) The Image showing the surface of the specimen after ductile fracture. There is a high dislocation density in the neck region leading to the material being subjected to complex stresses. Separation of dislocations from each other occur due to repulsive inter atomic forces at the neck region of the specimen. The dislocations then come closer due to the increase in the shear stress on the slip plane. High shear stress and low angle grain boundaries leads to the formation of cracks which then grow to result in failure. Brittle Fracture This is the failure of a material when subjected to stress with very little or no plastic deformation. If the broken pieces are bought back together after brittle fracture, the original shape and dimensions of the specimens are achieved. It occurs below or at the elastic limit of the material. This failure mode increases with: increasing rate of strain, the decrease in temperature, and stress concentration conditions. The fracture occurs due to a growth in the crack that exists in a material. The atoms at the surface form fever bonds as they do not have many neighboring atoms compared to those in the interior part of the material implying that surface atoms are at a higher energy. The adjacent parts of the material separate as a result of high stresses normal to the surface of the fracture. The fracture normally occurs along crystallographic planes referred to as cleavage planes. Brittle fracture requires less energy compared toductile fracture. Image showing the surface of a material after Brittle Fracture. Source: (Mechanical Properties of Metals II Fracture and Failure, 2015) Fatigue It is the fracture that occurs under repeated applied stresses in a material. The failure occurs at stresses that are below the tensile strength of the material. High rate of strain and increase in temperature results in an increase the tendency of fatigue occurring in a material. When micro cracks at the surface are subjected to motions of dislocation, fatigue fracture takes place as the micro cracks are the points of stress concentration. The excess stress aids in propagating the crack in each cycle of stress application. This failure mode is rapid in brittle materials as the crack propagates at a high rate through the material. There is little or no deformation in a specimen that has been subjected to fatigue fracture as it normally occurs below the tensile strength of the material. It occurs in three stages: crack initiation, stable crack growth, and rapid fracture. In the crack initiation stage, dislocations accumulate close to stress concentrations and form Persistent Slip Bands (PSB) after many loading cycles. PSB leave tiny steps in the material surface which serve as stress risers that initiate cracks. In stable crack growth, the micro cracks join and start to propagate in the material. In the loading cycles, the cracks will grow until the uncracked part of the material can no longer support the load. This then leads to rapid fracture of the specimen. Source: (Met-tech.com, 2015) Image illustrating how a surface fractured by fatigue. Creep It is the fracture that occurs due to creeping of the material subjected to constant loading. Creep occurs to metals like copper at high temperatures and it is accelerated by high straining rate and high temperatures in a material. The fracture takes place as a result of shearing of grain boundaries usually at moderate temperatures and stresses and the movement of dislocations at considerable higher temperatures and stresses from one slip to the next. Cracks along the boundaries are thus caused by movement of grain relation. The grain boundaries act as points of stress concentration. The cracks spread slowly across the material until the fracture occurs when little stresses are applied to the material for long periods of time. This failure mode is affected by heat treatment, grain size, alloying, and strain hardening. Figure showing the stages of creep. Source: (Doitpoms.ac.uk, 2015) Creep occurs in three stages: Primary creep in which the rate of creeping decreases with time due to strain hardening, Secondary creep in which the rate of creeping is constant as a result of simultaneous strain hardening and process of recovering, and Tertiary creep in which rate of creeping increases with time thus causing necking and fracture. Non Destructive Test Method Acoustic Emission Experiment to Detect Crack Initiation Material and Specimen The material used was an aluminium alloy supplied as compact tension specimen. Source: (Modarres&Keshtgar, n.d.) The figure showing the dimensions of the specimen used in the experiment. A DiSP-4 AE system was used to detect and record the acoustic emission signals. The specimens were instrumented by the use of AE sensor and placed on a fatigue testing machine which applied a stable amplititude cyclic loading. The minimum load used in the experiment was 4.5 kN and the maximum was 9 kN. The tests were carried out at ambient temperatures. AE signals were captured using an AE transducer. An optical microscope was used to monitor the fatigue cracks. The figure below show the optical microscope test set up. Source :(Modarres&Keshtgar, n.d.) A magnification of 50X was used in the optical microscope to focus the specimen. A gooseneck illuminator was used to light the specimen and a camera was used to take photos of the crack growth. The cracks were monitored until they exceeded a length of 0.5 mm. The image below shows the record of crack size as obtained from the tests. Source: (Modarres&Keshtgar, n.d.) The results showed that cumulative AE counts and AE amplitudes have a correlation with measured crack sizes. They have a significant similarity between them. Destructive Test. Tensile testing The objective was to determine the tensile strength of a material. The equipment used in the experiment were: tensile testing machine, dividers, steel rule, specimen, and extensometer. The procedure The specimen was placed between the jaws of the tensile testing machine. The extensometer was connected to the specimen and its locating points controlled to read gauge length. The specimen was subjected to a tensile load and the extension of the specimen was measured by the use of the extensometer. The load was increased gradually and the corresponding extension recorded. During the experiment, it was noted that at over 40kN the elongation was no longer proportional to the load. This is because the elastic limit of the material was exceeded. After the elastic limit was reached, the subsequent elongation measurements were taken by use of dividers and steel rule. At 80kN, necking was observed and it then fractured. Results Load at fracture = 80kN and elongation = 16.5 mm Diameter of the specimen = 15mm and gauge length = 75mm The graph of load against elongation. Source: (Shc-creo.co.jp, 2015) Tensile strength = 80000/ (3.142*7.5*7.5) = 452.65 N/mm2 Degradation Process: Corrosion Corrosion leads to the change of properties of materials which include: mechanical properties, physical properties, and the appearance of the material. Deterioration mechanism normally depends on the type of the material. Corrosion of a metal is the electrochemical of a metal and it begins at the surface of the metal. It is classified into dry corrosion which involves oxidation in gaseous atmosphere and wet corrosion which involves electrochemical corrosion and takes place in aqueous solutions. Oxidation is the process of the formation of an oxide layer on a metal for example in Iron it proceeds a follows: 2Fe + O2 -> 2FeO Image showing the corrosion of a metal due to Oxidation. Source: (Slideshare.net, 2015) In wet corrosion, metal atoms lose electrons and it takes place at the anode. In Ceramics, corrosion occurs by chemical dissolution. Ceramics are compound between metals and nonmetals and are thought to already have corroded thus they are very resistant to environmental corrosion as they are chemically inert in corrosive environment. In polymers, elastomers cause degradation of other plastics if they are placed in contact for long periods. The Ultra Violet light cause the weakening of plastics and produce a faded appearance on their surfaces. Cold cause some polymers to become brittle thus fracturing under pressure. Moulds that grow on plastics in humid conditions lead to their degradation in a process called bio-degradation. Source: (Slideshare.net, 2015) Image showing the degradation of a plastics which is a polymer due to exposure to Ultra Violet light. Task 4 Compressive Destructive Testing is used in testing materials that are known to resist tensile loads. Materials which are used in compressive applications for example concrete, brick, and ceramics are tested in compression tom determine their mechanical properties. Thus, compressive testing results are important as they help in determining whether a material is suitable for a compressive application or not. Liquid Penetrant Testing (PT) Non Destructive Testing Method is used to detect defects in materials to the surface or in detecting surface cracks. Red dye and fluorescent penetrants are employed in this method. This method is an effective method of detecting failure in materials as it is capable showing very tiny defects to a high accuracy(Met-tech.com, 2015). Solvent Attack Chemical attacks like sulfate attack, acid attack, and bacterial attacks have been known to affect the functioning of concrete which a very important engineering material. Sulfate attacks is degradation of a material due to presence of sulfate. Sulfates dissolved in ground water leads to deterioration of basements of structures. Internal sulfate attacks occurs in concrete as sulfates are within the mixture of concrete. An example of this attack occurred in Vallee de la Maurienne in France in which 15,000 cubic meters of concrete produced contained gypsum(). Source: (Expcep.com, 2015) Image showing the degradation of concrete due sulfate attacks Acid attacks occur when concrete is exposed to an acidic medium leading to the solid minerals becoming unstable and thus decomposing. For example the deterioration of concrete pipes when exposed to stagnant water which is acidic. Some bacteria produce corrosive by-byproducts. For example the desulfovibriodesulfuricans bacteria change sulfuric bearing compounds in concrete to hydrogen sulfides which when exposed to oxygen form sulfuric acid that has adverse effects on concrete. Source: (Expcep.com, 2015) The image above shows the collapsing of a concrete silo due to bacteria attacks. Solvents also affect some metals by weakening then thus reducing their functionality in the engineering structures. Chemical attacks cause polymers to dissolve, react chemically and even plasticize. This leads to reduction of mechanical properties and a change of weight of the polymer. They also lead to stress cracks which result to the failure of the affected part. Ageing Ageing degradation can lower the strength of the material, change physical properties in structures, thus leading to decrease on their capacity to sustain operation conditions and harsh natural events. The Brinell Hardness Test The Brinell Hardness test involves of indenting the specimen to be tested with a 10mm diameter hardened carbide or steel ball that is subjected to a 3000 kg load. The load in softer materials is reduced to avoid over indention of the specimen. The full load is applied for 10-15 seconds if the test material is iron or steel while for other materials, it takes at least 30 seconds. A low powered microscope is used to measure the diameter of the indentation made by the ball in the specimen. The hardness number is given by dividing the applied load by surface area. Source: (Gordonengland.co.uk, 2015) The image above shows the apparatus used in the Brinell Hardness test and formula used in calculation of the Brinell hardness number. The diameter of indention is the mean of two readings perpendicularly. The BHN table can be used to simplify the process of determining the Brinell hardness. The Brinell Hardness number reveals the conditions under which the test was carried out. For example “75 HB10/1500/30” reveals that the Brinell Hardness number of 75 was obtained after using a 10mm diameter indenter with a load of 1500 kg that was applied for 30 seconds. A tungsten-carbide ball was used instead of steel if the test materials are hard. The test averages hardness on a larger part of the specimen compared to other methods used to test hardness thus it is suitable for achieving hardness of heterogeneous materials. Bibliography Modarres, M., &Keshtgar, A. Detecting Crack Initiation Based on Acoustic Emission. Retrieved from http://www.aidic.it/cet/13/33/092.pdf Corrosion and Degradation of Materials. Retrieved from http://academic.uprm.edu/pcaceres/Courses/MatEng/MSE10-1.pdf Gordonengland.co.uk,. (2015). Brinell Hardness Test. Retrieved 24 May 2015, from http://www.gordonengland.co.uk/hardness/brinell.htm Expcep.com,. (2015). CONCRETE DEGRADATION - PART 2 : CHEMICAL ATTACKS | CEP. Retrieved 24 May 2015, from http://expcep.com/en/bulletin/concrete-degradation-part-2-chemical-attacks/ Mechanical Properties of Metals II Fracture and Failure. Retrieved from http://www.physics.uwo.ca/~lgonchar/courses/p2800/Chapter7_MechanicalII_Handouts.pdf Nde-ed.org,. (2015). Fatigue. Retrieved 24 May 2015, from https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Fatigue.htm Shc-creo.co.jp,. (2015). LabRep. Retrieved 24 May 2015, from http://www.shc-creo.co.jp/EigoNet/LabRep.html Met-tech.com,. (2015). Non Destructive Testing, Liquid Penetrant Testing (PT), Magnetic Particle Testing (MT). Retrieved 24 May 2015, from http://met-tech.com/ndt.html Slideshare.net,. (2015). Degradation of materials class. Retrieved 24 May 2015, from http://www.slideshare.net/nmacintoshwqsbqcca/degradation-of-materials-class-27412859 Met-tech.com,. (2015). Fractured Input Shaft - Nascar Engine Failure Metalurgical Analysis, Torsional Fatigue Stress Raiser. Retrieved 24 May 2015, from http://met-tech.com/fractured-input-shaft.html Doitpoms.ac.uk,. (2015). DoITPoMS - TLP Library Creep Deformation of Metals - Introduction. Retrieved 24 May 2015, from http://www.doitpoms.ac.uk/tlplib/creep/intro.php Read More