The Concept of Fatigue in the Discipline of Materials Engineering - Assignment Example

MATERIALS ENGINEERING EXPLAIN THE CONCEPT OF FATIGUE. (HOW DOES IT AFFECT A COMPONENT AND HOW CAN IT BE PREVENTED DURING THE DESIGN STAGE?) The concept of fatigue in the discipline of materials engineering aims to recognize the occurrence of material flagging which is a consequence of constant load application. As stated by Schijve (2001), failures which are triggered by fatigue remain one of the most foremost issues specifically with regard to the development of metallic structures. It has been identified that the incidence of fatigue in structures is recorded due to the onset of cyclic loading of materials. However, it is intriguing to note that for a material to experience fatigue the emergence of the highest stress level maybe recorded at a much lower scale in comparison with the material’s static yield strength (Liu, 2005). According to Liu (2005), the damaged posed by fatigue is a consequence of three primary factors which can be defined as 1) concurrent action of cyclic stress 2) plastic strain and 3) tensile stress. Therefore, if the previously mentioned elements which trigger fatigue are absent then the commencement of what is termed as a ‘fatigue crack’ does not appear. The process by which a fatigue crack emerges is initially caused by the plastic strain which is a product of the cyclic stress itself, as the process progresses accordingly the tensile stress assumes responsibility for advancing the expansion of the crack (Liu, 2005). Consequently, the aspect of recurring loading and unloading of material is also a factor which can be recognized for promoting fatigue. The initiation of fatigue which is an outcome of this component is demonstrated by the appearance of infinitesimal cracks which eventually develop to an alarming size thereby, allowing crack propagation to occur and lead the way for breakage in structure. The concept of fatigue can be categorized as 1) high-cycle fatigue and 2) low-cycle fatigue. The application of these concepts essentially defines the variance in the behavior of material (Schijve, 2001). PREVENTION AGAINST FATIGUE DURING DESIGN STAGE The development of systematic frameworks to cope with the occurrence of fatigue in the structure’s life is a consideration which must not be overlooked during the design stage. The fundamental tenets which must be comprehensively explored by structural engineers during the design phase to ensure prevention against fatigue are classified as follows: 1) Damage tolerant design: Under this design technique, the part must be thoroughly examined, checked and scrutinized from time to time to recognize the occurrence of cracks and for also conducting a substitution of those parts which have surpassed a specified critical timeframe. 2) Fail-safe design: In comparison with the damage tolerant design concept whereby, the replacement of parts is only decided upon and conducted as a result of periodic examinations and evaluations, this approach directs the substitution of parts once they have failed. However, a fundamental and essential consideration which must not be disregarded during the execution of this design is to ensure that the structure does not comprise of a single source of malfunction as it could potentially comprise the functioning of the whole structure. 3) Safe-life design: This design approach recommends the user to assign the part with an expected life cycle during the phase of development. As implied by the name of this technique, the adoption of this approach is beneficial and pragmatic because it defines the stage at which the part is to become outdated and hence require immediate replacement to prevent fatigue. 2. EXPLAIN THE CONCEPT OF CREEP. (HOW DOES IT AFFECT A COMPONENT AND WHAT ARE THE CAUSES?) The concept of creep in the discipline of materials engineering is considered to be high-temperature occurrence as it depicts the pattern of crystalline solids to accept any lasting inelastic strains which take place in scenarios where a material is exposed to continuous phases of stress (Liu, 2005). The rate at which a material distorts because of creep is associated with several variables that comprise of the 1) complete time period of exposure 2) the temperature at which the material remained exposed 3) the characteristics and properties of the material and 4) the extent of the structural load which was exerted upon the material. According to Liu (2005) the specific variables which determine the process of creep deformation define it as a ‘kinetic process’. Henceforth, for the purposes of ensuring that any material is not subjected to ‘creep deformation’ at a later stage, it is imperative to identify the creep properties of a material which is being utilized for a specific purpose. Considering the role of temperature as a variable which promotes the occurrence of creep formation, it is important to establish that temperature as an integral creep characteristic is variable for a range of materials used for different purposes rather than being standard or uniform. However, to simplify the accumulation of data regarding a specific material’s creep minimum temperature, the common instruction which can be followed suggests that the impact of creep deformation usually become evident when the temperature has reached thirty per cent of the stage at which the part is supposed to melt. Accordingly, for identifying the creep minimum temperature of metals, the principle which can be applied states that creep is initiated at the stage during which the temperature surpasses 0.4Tm. (Fig 1. - Source: Liu, 2005) The diagram presented above demonstrates the characteristic creep curve which indicates the three primary phases of the creep. 3. EXPLAIN THE PROPERTIES OF HIGH STRENGTH ALUMINUM ALLOY AND PROCESSING METHODS High strength aluminum alloys can generally be found under the category of tempered grades including the 6061-T6. The characteristics of 6061-T6 comprise of the magnesium and silicon components of this particular tempered grade in addition with the fact that this alloy is also precipitation hardening. The concept of precipitation hardening is a crucial application which is executed for achieving the objective of enhancing the yield strength which is possessed by soft materials. THE PROCESS OF ALUMINUM DIE CASTING During the occurrence of aluminum die casting, the mould cavity is filled with molten aluminum by executing increased pressure. The purpose of the mould cavity which is developed to aid this specific process is to act as an insertion during the ongoing procedure. The primary benefits of aluminum die casting are largely associated with the feasibility and economic viability of the procedure because systematic and straightforward phases which are involved in the process allow makers to maintain a low average cost per unit throughout. Additionally, the quantity or outputs of castings which are produced through the execution of this technique are also greater than that of competing techniques thereby, promoting the benefits which are linked with the adoption of this procedure. THE PROCESS OF FORGING As stated previously, tempered grades such as the 6061-T6 are favorable for launching the process of forging. The execution of this technique is characterized by the adoption of what is termed as a closed die process. Similar to the process of aluminum die casting the adoption of this process is favored because it possesses a low average cost per unit while, maintaining a high level of output. However, the issue of expensive supplies and inputs including raw materials may reduce the impact of the aforementioned advantages. Nonetheless, forging remains a widely and extensively used procedure specifically for the development of parts and structures which are expected to cope with augmented levels of stress. 4. Explain the common causes of in service failure of ceramics In service failures of ceramics can be triggered by several common causes which may prove to be very costly and expensive for the user to bear. The most frequently reported issues that lead to the in service failures of ceramics comprise of selection of 1) inadequate materials and processing techniques 2) the adoption of a poor designing approach and 3) unsuitable usage of products. A common problem which maybe observed in ceramics is that of fracture which causes the division of ceramics into smaller parts because of various factors that can induce stress on the material. Ceramics undergo brittle fracture as compared to ductile fracture because the transmission of a crack occurs at a much more rapid speed in this case. The incidence of brittle facture in ceramics can be attributed to a rise in temperature and the said temperature is recorded at a higher scale in ceramics in comparison with metals. Consequently, the concept of fatigue is also applicable with regard to the in service failure of ceramics because of constant and forceful shifts in stresses. 5. EXPLAIN THE DIFFERENCE BETWEEN CREEP AND FATIGUE The identification of the critical difference between creep and fatigue lies in the examining the consequences of the application of each concept. The outcome of creep as compared to fatigue is that the former is attributable for the occurrence of lasting deformation while, the latter is responsible for the propagation of cracks over an extended period of time (Davis, 1997). While, it is evident that the emergence of both these concepts can transform into failure at a later stage as their source can be linked to the application of prolonged stresses and loads, the defining characteristic of these concepts also vary. For example, the process of determining creep involves viewing a specific sample and locating the elongation of the same over an extended period of time. However, when locating a particular material or part for evidence of fatigue a user is expected to identify any potential evidence which points towards the elongation or propagation of a crack. Moreover, the occurrence of creep is highly attributable to higher temperatures however, in case of fatigue periodic or cyclic loading and unloading activity can cause the material to demonstrate signs of failure. (Fig 2a. and Fig 2b. - Source: Schijve, 2005) The diagrams presented above demonstrated the variation and differences in the pattern of crack expansion in the scenarios of fatigue (Fig 2a. - located on right) and creep (Fig 2b. - located on left). In case of the fatigue crack growth, the trends which have been recorded indicate how variation in temperature can assist periodic transformation. On the other hand, the crack growth rate for creep is characterized by the presence of transgranular patterns regardless of the temperature changes involved during the process (Schijve, 2005). However, as time progressed the trend in crack growth for creep became essentially intergranular and depicted differing patterns in comparison with the fatigue sample. As noted by Schijve (2005) the execution of this experiment verifies and substantiates that the fracture mechanisms which have been noted in both cases are not comparable. References Davis, J. R. (Ed.). (1997). Heat-resistant materials. ASM International. Liu, A. F. (2005). Mechanics and mechanisms of fracture: an introduction. ASM International. Schijve, J. (2001). Fatigue of structures and materials (pp. 1-507). Dordrecht: Kluwer Academic. Read More