The expressions for various load carrying capacities given above were derived by examining the geometry of the joint and the marine surveyor must see that in each problem he understands the geometry so that the expressions for the forces may be written.
The Failure of Riveted Joints At
The above is fine where design is concerned, but the marine surveyor is usually more concerned with the failure of riveted joints in service rather than the design of new riveted structures. A riveted joint may fail in several ways but occurs as soon as failure takes place in any one mode. The following is a general discussion of the possible modes of failures of a riveted joint. These modes are described by considering a single riveted lap joint, which is subjected to tensile load of P newtons. In general, the description will be applicable to any other type of joint. The tensile and shear stresses in a rivet are analysed and calculated in the same way as a bolted joint. Despite their apparent inherent strength, marine surveyors should know that riveted joints can and do fail in any one of five ways and they are:
• shearing of the rivets.
• tearing of the plates between the rivets.
• crushing of the rivets or that part of the plate in contact with them.
• breaking of the plate between its edge and the nearest rivet hole by splitting.
• breaking of the plate between its edge and the nearest rivet hole by shearing.
Tearing of the plates between the rivet holes is sometimes called postage stamp failure. The two types of plate edge failure are sometimes found in combination. It is found with experience that, provided the rivets are at least one and a half shank diameters from the plate edge these types of failure, are rare. Because of that rarity it really is only necessary to investigate analytically the first three type of failure. With aluminium alloy plates, the rivets should be set back at least two shank diameters.
50 | The Report • June 2021 • Issue 96 Figure 10 Rivet Failure Mode 1 Figure 10 shows how a plate can shear from hole to hole as indicated by the red line.
Failure Mode 2 – The Shearing of the Rivet The failure will occur when all the rivets in a row shear off simultaneously. Considering the strength provided by the rivet against this mode of failure, it is necessary to think of the number of rivets in a pitch length which is obviously one. Further, in a lap joint failure due to shear may occur only along one section of rivet as shown in Figure 11 (a). However, in case of a double butt strap joint, failure may take place along two sections in the manner shown in the Figure 11 (b). So, in the case of single shear, the area resisting shearing of a rivet is:
As = πdh2 /4 mm2 (23)
[As the difference between the diameter of the hole and the diameter of the rivet is very small, the diameter of hole is used for the diameter of the rivet in Formula (5)].
If the allowable shearing stress in single shear of rivet is τs in newtons per mm2 then the shearing strength or the shearing load carrying capacity of the joint is:
, = (p – dh )tp If the allowable stress for a plate in tension is σt = σt (p – dh )tp mm2 in N/mm2 N
strength of the joint or tensile load carrying capacity of the joint is: - Pt
(21) , then the tensile (22)
If P is the applied tensile force in newtons per pitch length, then the joint will not fail in this mode if P is less than the figure calculated from the Formula.
Failure Mode 1 – The Tearing of the Plate at the Section Weakened by the Holes
The plate at any other section other than that indicated in Figure 10 below is obviously stronger, and hence will not fail first. If an applied tensile force P is to cause tearing, it will occur along weakest section, which is that carrying the row of rivets. Such a failure is often called a Postage Stamp tear and is occasionally found in barges built of poor quality wrought iron. Considering one pitch length p it is weakened by one hole diameter dh and the area that resists the tensile force is:
P
P
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