Failure of metals under long term static loadsIntroductionMetals are an important factor in almost every aspect of contemporary life, as such it is essential that designers have sufficient knowledge of the behaviour of these materials when subjected to different types of loads in a variety of environments (Gere and Goodno 2012). In practical terms a metal component is designed to withstand static and imposed loads over the service life of that component, with the designer taking account of the environment in which the metal is located and the type of load, as well as other factors such as impacts and corrosion, which could affect the performance of the material. The design will also include a factor of safety to optimise the achievement of the design life in safe manner (Bhaduri 2018).Madia et al.,(2017) suggests that the failure of a metal is dependent on the static and imposed loads on the material over the life of that material, with Zerbst et al.,(2019) adding that approximately 90% of the incidents are due to fatigue. In many cases, such failure is linked to fatigue cracks started by at geometrical defects such as holes and notches. Failure can also be due to irregularities in the metal such cavities, voids, corrosion pits and inclusions, which are often the root cause of component failure. Forrest (1970) points out that failure of metal by fatigue is typically the result of loads that are either repeated or varied, and where the maximum amount load that causes failure is significantly lower than the static breaking load. The fact is that in service life, many metal structures and components are subjected to varying loads and whilst the average induced stresses are generally low and do not decrease the static strength, failure may still occur due to fatigue. Forrest (1970) therefore argues that there are a greater number of failures in service due to fatigue than due to static failure.However Martin and Vladimír (2017) point out that the conditions assumed in the design stage, may alter over the long term life of the metal, as such it is essential to understand the potential environmental and operational conditions which affect both static and dynamic loading on materials. Panin et al.,(2017) agree, adding that it is important to understand the impact of long term static loads on metals as this can have a major effect on structures and pipelines over time, citing the example of long term static loading on steel gas pipelines.The aim of this essay is to explore failure in metals under long term static loads, focusing on static loading on alloys. This essay asks how long term static loading can affect the mechanical properties of these materials. The essay continues with a brief  resume of the key mechanical properties of metals before discussing studies on static loading on alloys.Mechanical Properties of MetalsLascoe (1998) maintains that metal structures can be manufactured and constructed using a variety of metals ranging from carbon steel to cold-formed steel to stainless steel and aluminium. The stress-strain curves of each of these materials is different, with different yield values and post-yield criteria, as shown in Figure 1. It is evident form the above figure, that for example high strength steel and stainless steel have a rounded behaviour with no yield plateau compare to mild steel (Ellobody et al.,(014). Macaulay (2012) agrees, pointing out that some materials have a well-defined yield stress, exhibiting linear elastic behaviour, prior to plasticity and failure. However there are also other materials where the end of the elastic behaviour is not clear-cut. Furthermore, time-dependent behaviour in the elastic-plastic region is negligible for metals under static loading, however behaviour is dependent on a range of factors including temperature, which will induce creep. The point being that structural performance of a metal is dependent on the micro-structure and macro-structure of the material, as well as properties such as strength under static and dynamic loading, hardness and ductility.Figure 1. Stress-strain curves of different metals (Ellobody et al., 2014, p.4, Figure 1.1)Hosseini et al.,(2015) point out that during service, a metal structure can be damaged through a number of events, and loading condition. Once a structure is even partly damaged this can affect the energy absorption, ductility and strength of the metal. These load scenarios include high strain, static, seismic or thermal loadings. Static loading can occur due to factors such as foundation settlement which can occur over time and which is a common cause of damage to the structures causing large relative displacements in a metal which ultimately push the material beyond its yield point. Other forms of large static loads include lateral ground water pressure in relation to temporary loads and bridge piers imposed in repair processes. The point being that these loads result in strain ageing, and it is submitted that static load- induced strain ageing can considerably change the mechanical properties of metals such as steel material. It is acknowledged that static loads and strain ageing are considered at the design stage, both in the manufacturing of the metal and in the choice of the materials.Hosseini et al.,(2015) explain that the place that the strain aging effect takes place in metal starts with the movement and formation of linear defects within the microstructure of the material, referred to as dislocations. It is noted that the strength of a metal is related to the microstructure of the metal and is essentially managed by the stress that is needed to make dislocations move in the microstructures for considerable distance. For example in steels, the presence of carbon and nitrogen are key factors in mechanical properties and behaviour of the material, from which it follows that these substances also have a role in strain ageing over the long term static loading of steel. In other words, the behaviour of the metal in ageing depends on time, with short term aging and long term aging, where the latter relates to the time because of a phenomenon like creep in the material which takes place in a very long time. The problems of long term ageing is that it is difficult to predict the changing conditions to which a metal is subjected, as the mechanical properties of the material can be altered by external forces such as a corrosive environment which makes it difficult to state the exact impact of long term static loads on failure.Harris (2014) makes a similar point, noting that the microstructure of the material, has a profound effect on behaviour under long-term static loading. This is in turn is influenced by the method production, and the processes which are used to manufacture the metal. These process are essentially deformation processes seeking to transform solid materials from one shape into another. The original shape might by simple such as a billet or a sheet, which is then plastically deformed using heating processes, tooling and dies, to obtain the desired final geometry. In addition the deformation process typically involves additional operations such as  casting and machining, grinding and heat treating, to transform the raw material into the finished metal part or component (National Academy Press 1995). The point being that each of these processes affects the mechanical properties of the metal and ultimately the behaviour of that material under long term static loading (Harris 2014).Whilst there has been significant research on the impacts of long term static loads on conventional metals it is argued that there has been little research on alloys. Yet Winzer et al., (2005) point out that alloys play an increasingly important role in industry, with for example magnesium (Mg) alloys being used in a diverse range of applications from the automobile industry (Matta et al., 2018) to medical implants (Witte 2010). For example Mišovi? et al., (2016) point out that alloys such as aluminium alloys do not have a long tradition in engineering however the elastic and yield limits of these materials can greater than those of ordinary structural steel elements. In addition alloys are typically corrosion resistant and are lighter than conventional metals, facilitating transportation and assembly. These materials also exhibit resistance to brittle fracture within low temperature ranges and have minimal  susceptibility to temperature gradient and residual stress. In short these materials could be useful in improving performance under long term static loading. It is prudent therefore to explore the impacts of long term static loading on these materials.Long Term Static Loading on AlloysFedirko, et al., (2014) point out that titanium-based alloys are structural materials that exhibit high physico-mechanical characteristics, including specific strength and resistance to corrosion in aggressive environments. However as with conventional metals, a key issue in the strength and durability under loading is the method of production. For instance a  current area of concern is the formation of interstitial impurities in solid solutions which is caused by the gas saturation around the surface of the metal, which affects the serviceability of the products. There are several proposed solutions to this problem including the method of surface hardening that can potentially enhance the corrosion resistance and to improve the wear resistance of the metal. However some research suggests that this process will increase the brittleness of the alloy, thereby decreasing the ability to resist deformation and fatigue. In short there is no consensus on the optimal method for improving the serviceability of titanium alloys in service of static loading and fatigue, prompting Fedirko, et al., (2014, p.415) to research gradient hardening and how it can potentially influence on the durability of titanium alloys under a variety of load conditions.The study involved the use of industrial ? titanium alloys including OT4-1 pseudo-?-alloy (Ti–2.0Al–1.5Mn), PT-7M (?i–2.5Al–3.0Zr),VT1-0 (commercially pure titanium), and VT5 (Ti–5Al),  all of which were subjected to a series of tests including cyclic tension, rotating bending and pure bending under a uniform load, as shown in Figure 2 (Fedirko, et al., 2014, p.415).Figure 2. Mechanical test of specimens: (a) cyclic pure bending, (b) rotating bending, (c) cyclic tension, (d) and long-term static loading (Fedirko, et al.,2014, p.416, Figure 1).The experiments included using a series of specimens with 35-70 µm gas saturation layers deep from the subsurface being treated with solid solution hardening process within different levels, 0% [accessed 17th February 2019].Panin, S.V., Vlasov, I.V., Marushchak, P.O., Eremin, A.V., Byakov, A.V., Berto, F., Vinogradov, A.Y., Syromyatnikova, A.S. and Stankevich, R., 2017. Influence of long-term operation on structure, fatigue durability and impact toughness of 09Mn2Si pipe steel. Procedia Structural Integrity, 5, pp.401-408.Wang, X.J., Xu, D.K., Wu, R.Z., Chen, X.B., Peng, Q.M., Jin, L., Xin, Y.C., Zhang, Z.Q., Liu, Y., Chen, X.H. and Chen, G., 2018. 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