A Self-Learning Manual - Institution of Engineers Mauritius

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A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU force and it is not until the external force has exceeded the preloading force when a substantial increase of internal force of the bolt will occur. 6. Why are insulating washers installed between steel bolts and connecting aluminium plates? (BF4) Corrosion of aluminium can be triggered by putting it in contact with another metal in the presence of water. This is known as bimetallic corrosion or galvanic corrosion. The mechanism of such corrosion is the formation of a cell in moist condition so that an electric current is generated to flow between the two metals in direct contact. The degree of corrosion is influenced by the nature of connecting metals, their electrode potential, their areas, conductivity of fluid etc. When aluminium plates are connected together by means of steel bolts, bimetallic corrosion may occur. Where there is presence of a good electrolyte like seawater, there may be local attack on aluminium. Therefore, some jointing compound or insulating insert and washer are adopted to insulate electrically the dissimilar metals from one another [1]. 388
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU Level One (Core FAQs) Part II: Welding 1. There is a general rule in fillet weld that “the leg should be equal to the thickness of metals.” Why? (DW1) Let’s take an example of 6mm thick plates to illustrate the rule. In case 12mm leg is adopted in the fillet weld, the weld volume would be 3-4 times more than required. It would result in waste of weld metal and welder’s time. Worse still, over-welding may weaken the structure and result in distortion owing to the formation of residue stress. As such, the resulting weld could support less stress than fillet weld with “the leg equal to the thickness of metals.” On the other hand, for welding the same 6mm thick plates, if 3mm leg is used instead, it is under-welded. The resulting weld may break through the leg of the weld. 2. Convex fillet welds are sometimes used in welding. Why? (DW2) There are three types of fillet weld cross section profile, namely, flat, convex and concave. The convexity in convex fillet welds serve as reinforcement, which is believed to provide additional strength. However, care should be taken in not introducing excessive convexity to fillet welds. Excess convexity leads to an increase in stress in weld toes which may subsequently fails by cracking. Fig. Different types of fillet welds 389
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A Self-Learning Manual - Institution of Engineers Mauritius

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