Aluminium Design and Construction John Dwight

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supplied in a coil and of typical diameter about 1.5 mm. It is fed automatically through the torch and into the arc while the weld is being made. The torch, which is water-cooled, is much more complicated than that used for ordinary stick welding, since the supply tube has to convey four things simultaneously: current, wire, argon and water. The MIG process is suitable for welds in material down to a minimum thickness of 4 or 5 mm. Normally the electrode wire is pushed down the tube leading to the torch. This can occasionally cause problems with the wire kinking and jamming in the tube. The equipment for making the smallest MIG welds, using 0.8 mm wire, avoids this problem by pulling the wire through the tube. This adds further to the complexity of the torch. The MIG process has two special attributes. First, it is suitable for posi-tional welding, including overhead. Second, the arc-length is selfadjust-ing. In other words, when the welder accidentally moves the torch nearer or further away from the job, the burn-off rate momentarily changes until the right amount of wire is sticking out again, thus maintaining the correct arc-length automatically. This ‘semi-automatic’ feature makes it a relatively easy process to use. When welding in the downhand position, the section area of the maximum possible size of MIG deposit is some 40 mm 2 per pass. Much higher welding speeds are possible than with stick welding of steel. For a weld on 6 mm plate, a speed of 1.5 m/s would be reasonable, compared with, say, 0.5 m/s on steel. 3.3.3 TIG welding TIG is an alternating current (AC) process. The water-cooled torch has a non-consumable tungsten electrode, and as with MIG it delivers a flow of argon. The filler wire is held in the other hand and fed in separately. TIG requires more skill than MIG, both to control the arc length and to feed in the filler. One disadvantage is that the operator may inadvertently let the torch dwell in one position, causing a local build-up of heat and hence an enlarged HAZ. The TIG process is suitable for the welding of sheet thicknesses, rather than plate. At the top end of its thickness range (say 6 mm), the penetration can be improved by using a helium shield instead of argon. It is possible to adapt the TIG process to autogenous welding, i.e. dispense with the use of filler wire. The necessary filler metal is provided by ribs on the parts being joined, which melt into the pool. Such a technique enables TIG to be set up for automatic welding, with machinecontrolled traversing of the torch. A useful variant of TIG is the pulsed-arc process. This employs a DC supply with current modulation, causing the weld to be produced as a series of nuggets, which fuse together if the parameters are suitably Copyright 1999 by Taylor & Francis Group. All Rights Reserved.
adjusted. The technique is claimed to be superior to ordinary TIG when welding thin sheet, where it provides easier control of penetration. Also there is reduced build-up of heat. 3.3.4 Filler metal Specification of the electrode/filler wire for MIG or TIG is a design decision and should not be left to the fabricator. It is mainly a function of the alloy of the parts being joined, known as the ‘parent metal’. Sometimes there is a choice between more than one possible filler alloy, and the selection then depends on which of the following factors is the most important: weld metal strength; corrosion resistance; or crack prevention. We divide possible filler alloys into four types, numbered to correspond with the alloy series to which they belong: Type 1 pure aluminium Type 3 Al-Mn Type 4 Al-Si Type 5 Al-Mg Table 3.1 (based on BS.8118) indicates the appropriate filler type to select, depending on the parent alloy. Compositions of actual filler alloys within each type are given in Chapter 4 (Table 4.8). When the parts to be connected are of differing alloy type, the choice of filler may be arrived at with the aid of Table 3.2, which is again based on the British Standard. Type 1 fillers The purity of the filler wire should match that of the parent metal. Table 3.1 Selection of filler wire type for MIG and TIG welding Copyright 1999 by Taylor & Francis Group. All Rights Reserved.
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Aluminium Design and Construction C
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First published 1999 by E & FN Spon
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2.2.1 Rolling mill practice 2.2.2 P
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5 Limit state design and limiting s
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8.2.6 Use of interpolation for semi
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10.3.2 Inertias for a section with
Page 13 and 14: Preface Aluminium is easily the sec
Page 15 and 16: List of symbols The following symbo
Page 17 and 18: uu, vv principal axes w effective s
Page 19 and 20: 1.1.3 The industrial metal It is on
Page 21 and 22: where A is the section area (mm 2 )
Page 23 and 24: 1.3.1. The good points about alumin
Page 25 and 26: Deflection Because of the lower mod
Page 27 and 28: 1.4.4 Establishment of the alloys I
Page 29 and 30: 1.5.2 New technology Most of alumin
Page 31 and 32: large span, where self-weight is a
Page 33 and 34: These tend to be in all-aluminium c
Page 35 and 36: Close-up of the M-dec system, which
Page 37 and 38: Aluminium glazing system for commer
Page 39 and 40: Nat West Media Centre, Lords Cricke
Page 41 and 42: The main requirements for the produ
Page 43 and 44: in which t and w are in mm. These a
Page 45 and 46: Figure 2.1 Extrusion process (direc
Page 47 and 48: Figure 2.2 Typical extrusion die. t
Page 49 and 50: Figure 2.6 ‘Semi-hollow’ profil
Page 51 and 52: especially the stronger alloys in t
Page 53 and 54: Figure 2.9 Conventional profiles. 2
Page 55 and 56: 2.4 TUBES By tubes we mean hollow s
Page 57 and 58: CHAPTER 3 Fabrication 3.1 PREPARATI
Page 59 and 60: Figure 3.1 Alternative designs for
Page 61 and 62: Figure 3.2 Thread insert. a coil-sp
Page 63: 3.3 ARC WELDING 3.3.1 Use of arc we
Page 67 and 68: For welds of minimum or fatigue qua
Page 69 and 70: Figure 3.5 Friction-stir welding: s
Page 71 and 72: are, and the designer/fabricator mu
Page 73 and 74: cartridge, and mixing takes place a
Page 75 and 76: 3.7.4 Painting Alloys having durabi
Page 77 and 78: Where a range is given, as for the
Page 79 and 80: will be only slightly above that fo
Page 81 and 82: heating the metal to a temperature
Page 83 and 84: Table 4.4 Characteristics of differ
Page 85 and 86: softening in the heat-affected zone
Page 87 and 88: 60 alloys, are: BSEN.485 plate and
Page 89 and 90: Firure 4.3 Nominal composition and
Page 91 and 92: popular alloys in the series are: 6
Page 93 and 94: Figure 4.6 Variation of tensile str
Page 95 and 96: undercarriages of aircraft. They ar
Page 97 and 98: Figure 4.11 Minimum stress-strain c
Page 99 and 100: AC.51400 This is another non-heat-t
Page 101 and 102: times that of the actual pits) prod
Page 103 and 104: The severity of bimetallic corrosio
Page 105 and 106: Nominal loading. Nominal loads are
Page 107 and 108: 2. Material factor (� m ). In the
Page 109 and 110: design does not, because stress at
Page 111 and 112: Table 5.2 Summary of limiting stres
Page 113 and 114: Figure 5.4 Plastic strain at workin
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example, a reduced value might be t
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The parameter � depends purely on
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CHAPTER 6 Heat-affected zone soften
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area of softening. With 7xxx-type m
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Figure 6.3 Typical hardness plots a
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6.4 SEVERITY OF HAZ SOFTENING 6.4.1
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Figure 6.6 Categories of welded joi
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Figure 6.10 One-inch rule, extent o
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e allowed for in design by replacin
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Figure 6.13 Predicted area (A z ) o
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Figure 6.15 Overlapping HAZs. nomin
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In determining A zp , an appropriat
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failure plane in the HAZ, close to
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With MIG welding, an electrode-posi
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CHAPTER 7 Plate elements in compres
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(a) Compression member elements The
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Table 7.1 Classification of element
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Figure 7.4 Slender internal element
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Figure 7.7 shows curves of � ° p
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(7.9a) (7.9b) The strain gradient c
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(through the centroid) for ß S . F
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Figure 7.13 Outstands under strain-
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Figure 7.15 Reinforced elements. Th
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Figure 7.18 Stiffener location for
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Figure 7.20 Slender reinforced inte
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CHAPTER 8 Beams 8.1 GENERAL APPROAC
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8.2.2 Section classification The di
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that for the gross section. It woul
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the usual manner. Line 1 in the fig
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1. Fully compact sections. The limi
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Figure 8.8 Transverse and longitudi
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8.3.4 Shear resistance of bars and
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Figure 8.12 Tension-field action. i
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Figure 8.14 Moment/shear interactio
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depending on the estimated degree o
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where a is the stiffener spacing an
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plates (if fitted), p v =limiting m
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ending moment diagram. Two cases ar
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Figure 8.23 Sections covered in Tab
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where: I yy , I vv =inertia about m
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post (Section 8.6.4) or by some oth
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Figure 8.26 Components of deflectio
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9.1.2 Classification of the cross-s
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hole. Alternatively, it might fail
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Figure 9.2 Limiting stress p b for
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The determination of l involves a c
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Figure 9.6 Monosymmetric section. I
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Figure 9.9 Limiting stress p b for
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where s=� s /� t s =slenderness
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Figure 9.11 Type-R sections covered
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Figure 9.14 Four standardized profi
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9.7.2 Secondary bending in trusses
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Table 9.5 Necessary checks for memb
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Figure 9.18 Axial load with biaxial
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1. Tension members. The localized f
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Figure 10.1 Symmetric plastic bendi
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We now turn to monosymmetric sectio
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distance of its centroid from the n
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Figure 10.6 Elements of sections. I
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10.3.3 Product of inertia The produ
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Usually the conditions are such as
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Table 10.2 Torsion constant. Factor
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Figure 10.12 Warping. Conventional
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where b, t=element width and thickn
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10.5.3 Evaluation of warping When t
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For sections where the position of
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(10.31) where: e=distance that S li
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10.5.9 Asymmetric sections Before s
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CHAPTER 11 Joints This chapter cons
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the use of limiting stresses taken
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and the summation is made for all t
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Figure 11.3 Interaction diagram for
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k in order to give a fair compariso
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3. The design is satisfactory if fo
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When the method of surface preparat
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11.3.3 Weld force arising In many c
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Table 11.6 Limiting stress p w (N/m
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where p a =limiting stress for unwe
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Figure 11.11 Interaction diagram fo
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Manufacturers of structural adhesiv
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11.4.6 Creep Shear strength figures
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Figure 11.15 Effect of glue-line th
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Table 11.7 and Figure 11.17 provide
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11.4.11 Resistance calculations for
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high value reflects the various unc
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therefore usually presented in term
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For example: The loading spectrum i
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act on the structure. However, BS.8
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Figure 12.4 Crack propagation: (a)
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12.5 CLASSIFICATION OF DETAILS 12.5
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i.e. a joint extending in the same
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when the design life is low or when
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Methods (2) and (3) are difficult t
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it reverts to line 1. The slope s o
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Aluminium Design and Construction John Dwight

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