Failure Criteria of Aluminium - Essay Example

Failure Criteria of Aluminium Introduction People have always used aluminium alloys since the early years of the twentieth centuryfor mechanical engineering purposes, in civil engineering and transportation industries. Today, novel aluminium alloys are extensively developed to suit the needs of the civil engineering and transportation industries for improved resistance to damage, high strength, and reduction of cost of production. Aluminium alloys falling under the 2xxx, 6xxx, and 7xxx series that can undergo welding have been developed to serve these requirements. The aerospace industries employ one type of these aluminium alloys in the wing skin and other plate applications. 6xxx series aluminium alloys have an application in thin sheet material in the fuselage. Improved fracture hardness, high strength, and good corrosion characteristics are some of the principles to be met by nascent alloys, which normally present some disadvantages such as poor fatigue performance and higher weight. A new aluminium alloy needs to meet a set of criteria before qualifying for application. Recently, studies looking into the mechanical properties, corrosion resistance and damage tolerance have been done. Examinations of residual strength of aerospace structures have been in the limelight since the unveiling of the Airframe Structural Integrity Program of NASA. In this regard, myriad authors have contributed to the advancement of models, procedures and methods to classify and foretell the crack extension in aluminium alloys used in aircrafts. Classification of alloys, including those of aluminium, against failure or ductile crack extension, stands as a key part of the concept of damage tolerance, which takes into account structural damage and the presence of cracks. In this paper, an analysis of the failure criteria for aluminium is given. Diagnostic tools in failure analysis Fractography is applied in the assessment of in-service botches, associated with fracture, of a wide array of products. These vary from comparatively trivial products, such as toys, false teeth, tools, and ladders, to huge engineering structures like ships, bridges, and aircrafts. It is important to measure the reason of failure in order to make sure similar products are reliable and safe. Additionally, it ensures liability in a situation where personal injury, loss of life, losses and loss of property take place. At times, a brief assessment of the fracture surface is enough to recognize the reason of failure, without requiring intricate techniques. In other instances, a wide array of methods and in-depth analysis is needed. Here the experience and skill of the fractographer is of high significance. It is pertinent to have a deep understanding of the elements that impact the topography and analysis of pictures of fracture surfaces (Hull, 1999). In failure assessment, fracture surface analysis is hardly used in seclusion. An in-depth investigation may be: the assembly and evaluation of the history of the application of the component. It also involves the review of the magnitude of the application of the component in conformity to terms, stress assessment, non-destructive assessment of the botched parts, chemical review, and fractographic analysis. The analysis begins with visual investigation and goes on progressively to procedures that offer information at successively finer resolutions. Additionally, it is essential to have thorough knowledge of the failed materials microstructure and some insights into the ideal features of the materials failure under a variety of experimental conditions. Without any prior experience of the appearance of the fracture surfaces of the material, it is not hard to confuse one failure for another (Hull, 1999). Fractography Fractography refers to the interpretation of characteristics seen on fracture surfaces and, albeit its simplicity; it proves to be a daunting process in practice. Usually, this is the case on tempered and high quenched metals or in alloys where there is an influence of the microstructure on the crack path. There exist significant references designed and documented in fractograph atlases for use with metal alloys. Once the mechanism of fracture has been determined at low magnification, fractographs of high magnification can be applied to make confirmation and develop a ‘reference library of the same (Gdoutos, 2008). In general, there exist three basic crack growth mechanics: intergranular along the boundaries of grain (interdendritic fracture), transgranular ductile crack through microvoid coalescence, and brittle fracture through cleavage along crystallographic planes. In defining the fatigue, we can state that it involves ductile cracks, which are normally transgranular, although there are possibilities of intergranular fatigue in unique situations. In some instances, it may be daunting to distinguish between stress corrosion, fatigue, or fast fracture through fractographs under some circumstances. Therefore, general consideration of the circumstances and facts of a special case normally allow correct elucidations of the evidence (Gdoutos, 2008). The determination of fracture growth proportions in metallic materials is important for the assessment of damage tolerance of exhaustion cracking in airframe elements. By identifying the deformations given on the fracture exterior of a metal fatigue botch, also known as quantitative fractography (QF), one can depict information concerning the rate of growth of the fatigue crack under the specified flight loading. This is conducted by destructively disclosing the cracked component and associating the deformations on the fracture surface with distinct manoeuvre events. Therefore, this makes it possible to plot the crack against expected flight hours (Irisarri & Atxaga, 2013). Quantitative fractography is not easy to retrieve all the time as different sequences of flight load and metals have differing propensities for developing visible deformations of fracture surface. The application unique ‘marker load arrangement, introduced among the normal flight loads, is a comparatively mundane procedure for enhancing the reliability of the quantitative fractography. While aluminium markers have occasionally been applied in tests of airframe fatigue, titanium markers are not fully developed. This is partially because of titaniums microstructure, for which it can be quite difficult to present distinguishable marks (Roy & Nadot, 2012). Von Mises failure Criterion Von Mises failure criterion, also referred to as Huber Stress, is a method that takes into account all the six components of stress of a general three-dimensional state of stress (Kurowski, 2012). The following stress components express Von Mises: Importantly, von Mises stands as a nonnegative, scalar stress ratio. The stress is usually used to present results as the structural safety of many materials in engineering showing properties of elastic-plastic.For instance, aluminium or steel alloy can be assessed with the used of von Mises stress. The highest von Mises stress failure criterion is grounded on the von Mises-Hencky theory, also referred to as the maximum distortion energy model or the scalar-energy model. The model asserts that a ductile material begins to yield at a point when the von Mises stress equals the stress limit. In most instances, the strength of the yield is applied as the stress limit (Kurowski, 2012). Failure criteria in Al 2198 In one study assessing the failure criteria of aluminium, alloys made from the Al-Cu-Li aluminium family were considered, which are developed in perspective of application in the aviation industry. The alloy is a member of the 2xxx aluminium series. This alloy is uniquely designed to have excellent mechanical characteristics in order to apply them for mechanical components in aircrafts. The alloy has copper (Cu) as the main alloying component, and aging allows the alloy to achieve hardening. The novel alloy Al 2198 also has a lithium as an element used to minimize the density and thus make the aluminium light. The chemical composition of the alloy provides the particular aluminium with higher yield strength than other established aluminium alloys such as Al 2024. The producer also claims that the particular has improved tolerance to damage, thermal stability, fatigue resistance, and higher corrosion (Al-dheylan & Hafeez, 2006). If proven to be true, Al 2198 will be considered ideal for fuselage of aircrafts among other similar uses. This input envisages the placement-dependent distortion under stationary loading situations, damage, and failure mechanisms as well as their extrapolation by numerical models. Criteria used to determine the microstructure of the aluminium is called quantitative image analysis, which depicts the morphology of the particles and grains in the material. Further, fractography elucidates the elementary mechanisms of failure that are to be modelled with the use of a damage model along with an explanation of anisotropic deformation of plastic. This model of materials was identified in the finite elements framework. The parameters involved are calibrated from U-notched, and smooth tensile specimen tests machined for myriad orientations of the aluminium metal sheet (Al-dheylan & Hafeez, 2006). The parameters generated can be applied to examine the residual durability of constructions with and without faults as the initiation and extension of cracks are included in this particular constitutive model. Failure Criteria in Al 6061 Fatigue behaviour and torsional deformation of both thin-walled and solid-walled aluminium alloy - 6061 matters as they are widely applied in aerospace, automobile, aircraft, and other structural applications. The superior mechanical properties of the aluminium alloy 6061 make it perfect for application in the various industries. For instance, they have great ductility, optimal strength-weight ratio, and other desirable properties. Tresca criteria and Von Mises criteria have been applied in assessing the cyclic and monotonic deformation curve. The data acquired from the cyclic and monotonic deformation curves is then matched to torsional data. Both Von Mises and Tresca fit the investigational curve. However, Tresca prediction depicts a curve that fits the investigational curve better than the fatigue life depicted by Von Mises. It also exhibits that T6 alloys that have been treated have higher fatigue strength and fatigue life compared to normal alloys. Fractography analysis is also used in this instance to analyse the failure criteria of the cracked surfaces of the aluminium (Marini & Ismail, 2011). In an experiment to test the tensile failure micromechanism of aluminium alloy – 6061 that has submicron Al2O3 composites as reinforcements, scientists found that with the increase in volume fraction, both tensile strength and modulus elasticity increased. Researchers prepared composites by powder metallurgy. Al2O3 power and aluminium alloy 6061 powder were blended with an average size of 0.7 µm and compacted using uniaxial pressing machines. Instron 8801 testing machines then helped in making uniaxial tensile tests, and procedures laid down in ASTM E 8m were followed in measuring tensile property. As the amount of reinforcement went up, the clustering degree also rose in the 6061 alloy (Marini & Ismail, 2011). Factographic analysis of 7050 Aluminium Alloy The 7050 Aluminium alloy corrosion fatigue characteristics have gained popularity owing to its extensive application in plate forms in myriad components of primary load bearing structures of aircrafts that navigate marine atmosphere. Especially, army aircrafts may be exposed to splashing water over and over and thereby exposing the plate cover to salt water from the sea. Additionally, condensed sea water exposes the skin of the aircraft to aggressive water from the sea even further. The assessment of the feature exteriors of the alloy by low power magnifiers and eye view show a high dependency of surface textures on orientation. The fibrous look of the aluminium surfaces in the progressing direction is apparent. The assessment of fracture exteriors through a scanning electron microscope (SEM) exhibit clearly that the propagation of cracks in salt water and air is of transgranular manner. Propagation also occurs longitudinally on the planes – with the striations of fatigue on them – conjoined by tear crests. However, the mixed type of fracture with intergranular cracks together with the transgranular crack planes with markings occur only on some parts of the aluminium alloy. The furrows and tear ridges parallel to the crack development direction seen on almost all 7050 aluminium alloy show that cracks move with myriad local fronts on various growth planes (Irisarri & Atxaga, 2013). Conclusion Aluminium has become highly important in the past decades, and it is ubiquitously applied in automotive, aerospace, airplane, and other application. Because of the increased application and importance of aluminium, it is essential to ensure that there is maximum safety while using the material. Measuring the reason for failure presents as highly important as it ensures that similar engineering products do not experience failures as well. Diagnostic tools such as fractography, von Mises criterion, and other measures prove useful when it comes to measuring failure. An analysis of various aluminium alloys shows the myriad factors causing failure and the methodologies considered when measuring for the same. References Al-dheylan, K., & Hafeez, S. (2006). Tensile Failure Micromechanisms Of 6061 Aluminum Reinforced With Submicron Al 2 O 3 Metal – Matrix Composites Tensile Failure Micromechanisms Of 6061 Aluminum Reinforced With Submicron Al 2 O 3 Metal – Matrix Composites. The Arabian Journal for Science and Engineering, 31(2), 89–98. Gdoutos, E. E. (2008). Fracture of Nano and Engineering Materials and Structures: Proceedings of the 16th European Conference of Fracture, Alexandroupolis, Greece, July 3-7, 2006. Springer Science & Business Media. Hull, D. (1999). Fractography: Observing, Measuring and Interpreting Fracture Surface Topography. Cambridge University Press. Irisarri, A., & Atxaga, G. (2013). Fractographic study of two damage tolerant aluminium alloys. ECF13, San Sebastian 2000. Retrieved from http://www.gruppofrattura.it/ocs/index.php/esis/ECF13/paper/download/8505/4947 Kurowski, P. M. (2012). What is calculated in FEA?, 1–3. Marini, M., & Ismail, A. (2011). Torsional Deformation And Fatigue Behaviour Of 6061 Aluminium Alloy. IIUM Engineering Journal, 12(6), 21–32. Retrieved from http://journals.iium.edu.my/ejournal/index.php/iiumej/article/viewArticle/145 Roy, M., & Nadot, Y. (2012, June 4). Multiaxial fatigue behaviour of A356‐T6. British Columbia. Retrieved from http://arxiv.org/abs/1406.1204v1 Read More