(2011) Plastic Deformation Of Metals By Honeycombe Free Ebook.12
Giselda Sasao
This chapter provides an overview of the common types of defects found in various structural materials and joints in aircraft. Materials manufacturing methods (including large-scale production) have been established in the aircraft industry. However, as will be seen in this chapter, manufacturing defects and defects during in-service conditions are very common across all material types. The structural material types include metals, composites, coatings, adhesively bonded and stir-welded joints. This chapter describes the defect types as a baseline for the description of their detection with the methods of Chap. 5 to 8. Based on the understanding of the defect types, there is great expectation for a technical breakthrough for the application of structural health monitoring (SHM) damage detection systems, where continuous monitoring and assessment with high throughput and yield will produce the desired structural integrity.
(2011) plastic deformation of metals by honeycombe free ebook.12
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Both magnesium and beryllium alloys as extremely lightweight materials (competitive on specific strength and specific modulus) are considered for applications. In the case of magnesium alloys, the biggest obstacle to use them is their extremely poor corrosion resistance; hence, the products require special solutions for protection. Beryllium alloys represent an attractive combination of properties, but they must be processed using powder metallurgy technology with the requirement for controlled manufacturing environments and the concern for safety during the repair/service of deployed structures (Śliwa et al. 2016; Peel and Gregson 1995; Campbell 2006; Śliwa et al. 2017).
Note that safety-critical aircraft structure demands metallic materials that are both durable and lightweight, as well as being able to withstand severe structural stress at various altitudes and temperatures, including fatigue and wear resistance. High-quality material requirements for aeronautical applications make the defect detection and inspection techniques of prime importance, both in manufacturing and in-service operation. The following subsections describe the major defects encountered in metallic materials.
Metallic materials or their alloys are a class of elementary materials, such as aluminium, steel, titanium and nickel alloys, all of which are crystalline when solid. Given pure metals, some of the important defect types can be point defect, line defect and plane defect (Gilbert 2020). A point defect involves only a single particle (called a lattice point). A line defect is limited to a row of lattice points. A plane defect involves an entire plane of lattice points in a crystal. A vacancy occurs where an atom is missing from the crystalline array, constituting a tiny void in the middle of a solid.
There are four fundamental mechanisms for introducing a point defect into the structure of a solid (Hiroshi 2014; Fang 2018), such as (a) when a particle is missing at one or more lattice sites, a vacancy is attained; (b) when a particle forces its way into a hole between lattice sites, interstitial impurity is attained; (c) substitutional impurities result from replacing the particle that should occupy a lattice site with a different particle and (d) dislocations are unidirectional defects caused by holes that are not large enough to be a vacancy.
When a fraction of the original materials are replaced by impurities, a solid solution can be attained. Alloys are examples of solid solutions. Lattice distortions of the crystalline materials often occur when impurities are added to a solid. Thus, point defects often determine the properties of a material. Point defects can change the mechanical properties, such as strength, malleability or ductility. Dissolving a small percentage of carbon in pure iron (i.e. making it a steel) makes it stronger than iron; however, higher percentages of carbon can make the steel harder and more brittle.
A dislocation mechanism (screw or edge dislocation types) can weaken a metal, as it allows planes of atoms in a solid to move one row at a time. Interestingly, they can also strengthen a metal when work hardened during heating, hammering, cooling, reheating and reworking. In the course of the work hardening process, intersecting dislocations (i.e. when planes of atoms move one row at a time) that impede the movement of planes of atoms are created.
Most metallic materials are polycrystalline in nature (i.e. structure with many crystallites of varying size and orientation), whereas a group of crystals is called grains. Crystal grains in polycrystalline metallic materials deform by slips on specific slip systems. The place where two grains meet is called a grain boundary. The movement of a deformation through a solid polycrystalline tends to stop at a grain boundary. Therefore, managing the grain size in solids is necessary to obtain a desirable mechanical property, and fine-grained polycrystalline materials are usually stronger than coarse-grained ones.
Defects and its prevention in aerospace materials are uttermost concerns since undetectable flaws can cause catastrophic consequences for aircraft and passengers. The defects can be categorized, from the origin point of view, under four headings: (a) due to manufacturing, (b) during assembly, (c) during transport and (d) during service. This subsection is intended to shed light on the in-service related defects of aerospace materials. Defects during in-service mainly occur because of either inadequate material specification; in other words, inappropriate material choice and operation beyond the intended design parameters (Archer and McIlhagger 2015).
The common characteristic of in-service damage is that they occur unexpectedly, and it might be difficult to predict and diagnose it. Table 3.1 shows the most common causes of failure. The following subsections describe the major defects or failure types encountered in metallic structures.
Fatigue is the primary reason for failure in aerospace metals that occurs under repeated loads leading to premature failure of structural parts. If it is not detected in the early stages, it can cause catastrophic failures. It is usually characterized as the initiation and propagation of cracks to an unaccepted size. Fatigue is mostly controlled with stress history, material properties, chemical environment and manufacturing quality (Arrieta and Striz 2005). Table 3.2 shows a summary of the common fatigue causes observed in aircraft that have led to accidents, whereas Fig. 3.2 shows the structural areas prone to fatigue damage in early Airbus A300 design.
More recently, Rebhi et al. examined the reason for the fracturing of the ADF antenna placed just behind the cockpit of a military aircraft (Rebhi et al. 2018). Figure 3.4a shows the ADF antenna location on the aircraft while Fig. 3.4b shows where it breaks. Note that the upper portion of the antenna was fractured because of the fatigue initiated by the corrosion pits. The crack origin was found to be at the outer surface on the antenna by tracing back the beach mark Fig. 3.5.
Any metallic part in an aircraft is prone to corrosion. Corrosion, generally, can be defined as deterioration of metals by electrochemical reaction with surrounding environment and gradual material loss. It is one of the serious concerns especially for older aircraft and responsible for 25% of the metallic component failures. Corrosion-related expenses are estimated as big as 2.2 billion USD (Mouritz 2012a). It is commonly agreed that if corrosion issues are eliminated, maintenance of aircraft can be simplified. Many sources are available for corrosion during in-service phase of aircraft as illustrated in Fig. 3.6.
Three conditions should be available for corrosion: (i) availability of a reactive metal anode that corrodes and a passive metal cathode (does not corrode), (ii) a metal connector between cathode and anode and (iii) an electrode such as water. Preventing these conditions is quite challenging as it may not be practical, functional and hence feasible to eliminate them. For example, dissimilar metal contact cannot be prevented due to lightweighting, cost and functionality problems. Nevertheless, corrosion potential can be reduced by using surface enhancements such as painting, plating and sealing (Banis et al. 1999). Corrosion types can be categorized as follows:
Concentration cell (or crevice, deposit) corrosion: In this type of corrosion, water, moisture or any other pollutant trapped in between two surfaces (e.g. under loose pain, within a delaminated bond line or in an unsealed joint) may lead to pitting or exfoliation corrosion, depending on the alloy, temper and corroded material. Lapped skin joints or rivets on an oil-stained belly are primary spots to notice this type of corrosion.
Pitting corrosion: It occurs due to local loss of material. Although small amount of metal is removed, the pits can act as stress concentrators that may result in fatigue failure in critical load paths. Aluminium, magnesium and steel used in aircraft are vulnerable to this type of corrosion.
Stress corrosion: This is also referred to as stress corrosion cracking (SCC) or environmentally assisted stress corrosion that occurs rapidly and follows the grain boundaries in aluminium alloys. SCC arises from three factors: susceptible metals and alloys, corrosive environment and residual tensile stress. It is observed on highly stressed parts such as engine crankshafts or landing gears and may originate from a scratch or surface corrosion. SSC occurs in a variety of aerospace metals with the presence of corrosive environment. High-strength steels, heat-treated steels and aluminum alloys are known to be affected by the salt solutions and sea water, and these can cause stress corrosion cracking. Methyl alcohol-hydrochloric acid solutions are reported to cause stress corrosion cracking for some titanium alloys. Magnesium alloys, on the other hand, may stress corrode with moisture in air. It is also reported that sulfur from surrounding environment (e.g., air, dust, or lubricant) can initiate the SCC especially in hot parts (Rossman, 2020). Fig. 3.7f shows SCC failure in 7XXX alloy aircraft wing structure. Reducing the residual and assembly stresses and application of protective coatings are suggested to increase the corrosion resistance and to delay the initiation of SCC for aluminum alloys (Wanhill and Amsterdam, 2010).
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