Melhem GN; Bandyopadhyay S; Sorrell CC

Short Communication

Ann J Materials Sci Eng. 2014;1(4): 5.

Use of Aerospace Fasteners in Mechanical and Structural Applications

Melhem GN^1,2*, Bandyopadhyay S¹ and Sorrell CC¹

¹School of Materials Science and Engineering, University of New South Wales, Australia

²Perfect Engineering Pty. Ltd, Australia

*Corresponding author: Melhem GN, School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia

Received: September 16, 2014; Accepted: November 15, 2014; Published: November 20, 2014

Abstract

The intention of the present short review is to introduce to the non-specialist reader the feasibility of the use of alternative materials not generally considered by engineers working in mechanical and structural applications. That is, the use of specialised aerospace rivets in the more general area of construction is considered. To this end, the text briefly overviews the different types of fasteners used in the construction industry and their common mechanisms of failure. The most common types of fasteners used for conventional mechanical and structural applications are all-steel rivets and pop rivets consisting of aluminium shank and mandrel of a higher strength alloy. In contrast, the aerospace industry makes universal use of pop rivets consisting of high-strength aluminium alloys, the design and installation of which are illustrated. These more specialised rivets are suitable for implementation because the aluminium alloys used exhibit superior mechanical properties and corrosion resistance compared to those of other rivets. For comparison, the mechanical properties of the aluminium alloys used in both conventional and aerospace rivets are surveyed in tabular form.

Since environmental failure by galvanic corrosion owing to exposure to seaspray is very common, the factors that affect galvanic corrosion are discussed. A relatively comprehensive graphic survey of the galvanic series for corrosion of metals and alloys in seawater, drawn from a variety of sources, is provided. While this provides a well known ranking of the susceptibility to corrosion, this version of the series is uncommon in that it illustrates the series generically for alloys and it differentiates the metals and alloys into four ranges of corrosion resistance rather than as a continuous series. More specifically, since the susceptibility to corrosion of chemically similar alloys can be subtly shaded and hence difficult to rank, the galvanic series for corrosion in seawater of an extended range of aluminium alloys also is provided. Finally, an example of the successful 10-year performance of aluminium alloy aerospace rivets for the rectification of the failure of a major rooftop structure, which failed rapidly owing to steel shank-aluminium alloy workpiece corrosion from seaspray, is briefly mentioned.

Fasteners

In the construction industry, which regularly involves mechanical and structural applications, metallic materials are utilised heavily. One major area of application is mechanical fasteners or connections, which include rivets, bolts/nuts, lock bolts, and pins [1,2]. More broadly, fasteners are categorised generally as follows:

Threaded fasteners

Bolt/nut systems are designed with threads, which allow this fastener to be removed without damage to the system.

Rivets

Rivets are permanent fasteners that constrain the joint with a head (factory-head) and an expanded tail (shop-head or buck-tail) on the opposite end of the shank.

Blind fasteners

Blind fasteners are those that are installed and can be accessed on one side of the joint only, such as pop rivets.

Pin fasteners

Pin fasteners typically are of a single elongated piece (solid or tubular), although they may include a malleable collar.

Special-purpose fasteners

Specialised fasteners often are designed for quick removal and replacement and may include studs, latches, slotted springs, and retaining rings.

Fasteners for composites

In the joining of composites, specialised design considerations often are required for joints subject to high stresses, tight tolerance requirements, thermal expansion mismatch, galvanic corrosion, and/ or leakage.

Failure

Failure of fastening systems usually is from static loading (overload in tension, bending, shear, or torsion), dynamic fatigue (from cyclic loading or repeated impact), or corrosion (galvanic, chemical, or stress) [1]. Typical locations of failure of the most common mechanical fasteners are directly beneath the head(s) of rivets, at the thread-shank transition (in bolts), at the first inner thread (in nuts), and at microstructural imperfections. Alternatively, failure of the plates or sheets being joined regularly is by dynamic fatigue [3].

Although it usually is straightforward to design for static and dynamic loads in mechanical and structural systems, the potential effects of corrosion are more difficult to predict. This is partly because they depend on factors that are intrinsic to the product, such as the chemical composition and the microstructure, which is dependent on the processing. It also is because they depend on factors that are extrinsic to the product, particularly the environmental conditions to which it is exposed. Consequently, these effects often can be overlooked when specifying materials for such applications.

Most conventional threaded fasteners are fabricated from various grades of alloy steel, which may have protective coatings, such as zinc, tin, cadmium, or aluminium [1]. In contrast, pop rivets are constructed from alloys in all-steel, all-aluminium, and aluminium shank/steel mandrel configurations. In most construction applications, the joined plates and sheets also consist of alloys of steel and aluminium. Therefore, the potential for galvanic corrosion resulting from the opposition of dissimilar metals is clear [4].

The principles of galvanic corrosion are well known [5-8]. Since many mechanical and structural applications are exposed to rainwater, condensed humidity, seawater, and seaspray, the metals used in these systems are subject to anodic corrosion by electrochemical reactions during which at least one of the metals is altered from the metallic to the non-metallic state. In terms of galvanic corrosion, there are five general issues of consideration:

Galvanic series

In galvanic reactions, dissimilar metals act as cathode and anode while the water acts as electrolyte. This configuration is sufficient to establish an electrical circuit involving a potential (voltage) difference between the electrodes and associated current (amperage) flow. The electromotive force (EMF) series [5], which ranks the potential for corrosion between pure bimetallic couples in water in terms of electrochemical cell voltages, is well known. Another, perhaps more practical, variant is the galvanic series, which provides the same ranking for commercial metals and alloys in seawater, which is a more conductive electrolyte than water. This is shown in Figure 1. These data, which are drawn from a range of sources, can be used to determine the probable location of corrosion (i.e., oxidation). For example, if structural steel members (i.e., mild steel) are fastened with a zinc-plated bolt and nut, their relative vertical locations in Figure 1 indicate that the former acts as the cathode (higher in Figure 1 → decelerated corrosion) and the latter acts as the anode (lower in Figure 1 → accelerated corrosion). The greater the separation of the two in Figure 1, the more severe the corrosion. Since electrons are conducted to the anode and cause reaction, then the zinc-plated bolt and nut will corrode (where the contact surface areas of both electrodes are identical) and hence form zinc oxide (ZnO).

Figure 1: Generalised galvanic series in seawater [9-13,22,28].



Figure 1:  Generalised galvanic series in seawater [9-13,22,28].

Similar EMF ranges

Since commercial metals and alloys exhibit a range of EMF values, rather than specific consistent values, when two metals in contact have EMF ranges that overlap, it becomes uncertain which acts as cathode and which acts as anode.

Surface area effect

When the surface areas of the electrodes are dissimilar, then an area effect becomes important. That is, when the anode is small relative to the cathode, then the concentration of electrons being conducted to the anode becomes high, which enhances reaction. Therefore, it is desirable to ensure that the anodic metal surface area is large compared to that of the cathodic metal surface area. The greater the difference in areas, the more severe the corrosion. So, when a coating is pitted or scratched and the underlying metal is exposed, the latter becomes an electrode of very small surface area. In this case, it is essential for the underlying metal to be cathodic relative to the anodic surface coating. Figure 1 shows that zinc-plated bolts and nuts, the coatings of which can be damaged relatively easily, meet this criterion.

Biocorrosion

Marine slimes or biofilms often form, which can facilitate bacterial corrosion [14]. As the films grow, bacteria can release corrosive species or establish conditions conducive to corrosion, both of which can increase corrosion rates significantly.

Contact resistance

Consideration of the effect of the galvanic series on the probability of corrosion relies on the assumption of two dissimilar metals in contact and subsequent active corrosion [15]. However, the resultant electrical circuit may be broken through the formation of an electrically insulating layer consisting of an oxide or another corrosion product [16], which typically results in passive corrosion [15]. This has the effect of hindering further corrosion and so reducing its severity and/or rate.

Rivets

There are five main types of conventional rivets, which are bifurcated or split, compression, full tubular, solid, and semitubular [1]. All of these rivets require access to both sides of the joint and, with the exception of compression rivets, they require the use of a bucking bar, which is a specially shaped metal piece that expands and work hardens the tail upon impact to the head (bucking or upsetting). Compression rivets have two heads and they form the join from radial compressive stress and deformation.

In contrast, blind rivets require access to only the head side of the join. There are four main types of blind rivets, which are chemically expanded, drive-pin, pull-mandrel, and threaded [1]. Of these, the most commonly used and the most convenient are known as the pop rivet, which consists of a tubular shank (sleeve) and contains an interior mandrel (pin). When the mandrel is drawn into the rivet shank with the appropriate tool, the mandrel causes the exposed shank tail to expand, after which the mandrel snaps off, leaving the mandrel head (or head + part of the mandrel) locked into the tail (or tail + shank). The design and installation of an open-end break mandrel is illustrated in Figure 2 [17].

Figure 2: Schematic of operation of Huck Magna-Lok® [17] rivet system.



Figure 2:  Schematic of operation of Huck Magna-Lok® [17] rivet system.

There are two main considerations concerning the failure of rivets in the environment:

Corrosion

Since the mandrels are made from aluminium alloy, low-carbon steel, stainless steel, copper, and Monel (63Ni31Cu) alloy, the potential for galvanic corrosion with what typically is an aluminium alloy tubular shank within the rivet itself is clear. Consequently, two common problems in mechanical and structural applications exposed to the environment near the ocean are shank-workpiece corrosion in the joint and mandrel-shank corrosion in the rivet.

Mechanical properties

Stresses deriving from periodic wind loading, cyclic thermal expansion/contraction, and continuous static loading can be significant since the mechanical strengths of the aluminium alloys typically used as the shank are relatively low, as shown in Table 1.

Table 1: Mechanical Data for Aluminium Alloys Commonly Used in Pop Rivet Shanks [18].



  
    Aluminium 
      Alloy Grade 
    Temper 
    Tensile 
      Strength 
      (MPa) 
    Yield 
      Strength 
      (MPa) 
    Shear 
      Strength 
      (MPa) 
    Fatigue 
      Strength 
      (MPa) 
  
  
    5050
    O
    145
    55
    105
    83
  
  
    H32
    170
    145
    115
    90
  
  
    H38
    220
    200
    138
    97
  
  
    5052
    O
    195
    90
    125
    110
  
  
    H32
    230
    195
    140
    115
  
  
    H38
    290
    255
    165
    140
  
  
    5056
    O
    290
    152
    179
    138
  
  
    H18
    434
    407
    234
    152
  
  
    H38
    414
    345
    221
    152



Table 1:  Mechanical Data for Aluminium Alloys Commonly Used in Pop Rivet Shanks [18].

Aluminium in Aerospace Applications

History

Aluminium metal was first isolated by Hans Christian Ørsted in Denmark in 1825 [19] and a commercial process for its manufacture was developed simultaneously by Charles Martin Hall in America and Paul Héroult in France in 1886 [20]. Aluminium in aerospace applications goes back to the earliest days of successful flight, where the crankcase of the engine used in the Wright Brothers’ inaugural flight of 1903 was fabricated using an aluminium alloy [21]. The first mass-produced aeroplane to make extensive use of aluminium was the Bréguet 14 bomber of 1916 [22]. The first all-aluminium aircraft was produced in the following year in the form of the Junkers J7 fighter [23]. In 1936, aluminium rivets were used in aircraft construction for the first time in both the US by Cherry Aerospace [24] and the UK by Aviation Developments (now Avdel) [25].

Applications of Aerospace Rivets in Mechanical and Structural Applications

The use of rivets in aerospace construction is well established [26-29]. This high-performance application requires superior performance in terms of corrosion resistance and mechanical stability. However, the use of aerospace rivets in more conventional mechanical and structural applications has remained very limited probably owing to lack of familiarity and higher costs. Consequently, the main purpose of the present work is to introduce to the reader the potential to use these more specialised rivets in conventional applications for which they may not have been considered. Table 2 gives some of the mechanical properties of aluminium alloys that are used commonly in aerospace rivets.

Aluminium Alloy Grade	Temper	Tensile Strength (MPa)	Yield Strength (MPa)	Shear Strength (MPa)	Fatigue Strength (MPa)	References
1100	O	90	34	62	34	14
	H14	124	117	76	48
	H18	165	152	90	62
2017	T4	427	276	262	124	30
2024	T3	483	345	283	138	30
2117	T4	296	165	193	97	31
2219	T851	455	352	285	103	30
5056	O	290	152	179	138	14
	H18	434	407	234	152
	H38	414	345	221	152
7050	T7451	524	469	303	240	30,32
7075	T6	572	503	331	159	29

Aluminium Alloy Grade	Temper	Tensile Strength (MPa)	Yield Strength (MPa)	Shear Strength (MPa)	Fatigue Strength (MPa)	References
6060	T5	220	185	140	90	38,39
6063	T6	241	214	152	69	30
6351	T6	310	283	200	90	31

Use of Aerospace Fasteners in Mechanical and Structural Applications

Abstract

Fasteners

Threaded fasteners

Rivets

Blind fasteners

Pin fasteners

Special-purpose fasteners

Fasteners for composites

Failure

Galvanic series

Similar EMF ranges

Surface area effect

Biocorrosion

Contact resistance

Rivets

Corrosion

Mechanical properties

Aluminium in Aerospace Applications

History

Applications of Aerospace Rivets in Mechanical and Structural Applications

References