Failure modes bolted joints

The principal failure modes of bolted joints are (1) bearing failure of the material as in the elongated bolt hole of Figure 7-44, (2) tension failure of the material in the reduced cross section through the bolt hole, (3) shear-out or cleavage failure of the material (actually transverse tension failure of the material), and (4) bolt failures (mainly shear failures). Of course, combinations of these failures do occur. 

Bonded-bolted joints generally have better performance than either bonded or bolted joints. The bonding results in reduction of the usual tendency of a bolted joint to shear out. The bolting decreases the likelihood of a bonded joint debonding in an interfacial shear mode. The usual mode of failure for a bonded-bolted joint is either a tension failure through a section including a fastener or an interlaminar shear failure in the composite material or a combination of both. 

Bonded-bolted joints have good load distribution and are generally designed so that the bolts take all the load. Then, the bolts would take all the load after the bond breaks (because the bolts do not receive load until the bond slips). The bond provides a change in failure mode and a sizable margin against fatigue failure. 

Figure 113 Failure modes in composite bolted joints (a) bearing failure, (b) net-tension failure, (c) shear-out failure, (d) cleavage failure, (e) fastener puU-through, (f) bolt failure.

In this section, the effects of clearance on the damage response of the C1 C1 C1 double-lap joint (i.e. a control case) and the C3 C3 C1 double-lap joint (i.e. a worst case scenario in the sense that all the load is initially carried by one bolt) are investigated. The progression of damage in the joints is shown at applied load levels of 10,30 and 50 kN for the four different failure modes considered, i.e. tensile matrix failure compressive matrix failure tensile fibre failure and compressive fibre failure. These load levels were chosen so that the damage could be tracked fiom initiafion up to the point of extensive damage. 

Figure 15.3 Composite damage due to fastener holes, (a) Tjfpical bolted junction between a composite wing skin and metallic connection (b) failure modes in a bolted joint under tension [6] and (c) compressive failure modes in a plate with an open hole.

Fibre orientation The strength of bolted joints is significantly affected by fibre orientation. To avoid matrix-controlled failure modes, such as shearout, there should, wherever possible, be three basic fibre orientations of 0°, 90° and +45° in a bolted laminate (reference 5.8) (the 0° direction being parallel to the load). 

Like mechanically fastened metal structures, composites exhibit failure modes in tension, shear and bearing but, because of the complex failure mechanisms of composites, two further modes are possible, namely cleavage and puUout. Environmental degradation of a bolted joint, after exposure to hot, wet environment is most likely to occur in the shear and bearing strength properties. The evidence shows that for fiber reinforced epoxies, temperature has a more significant effect than moisture, but in the presence of both at 127°C, a strength loss of up to 40 percent is possible. 

Bolted and welded construction A first distinction must be made according to the form of joint that is used in metal silo constmction. Many smaller steel silos have bolted joints, and where these are present, every stress developing in the wall, at every point, must be transmitted through a joint. The joints are lines of weakness, so they should be made stronger than is strictly necessary. Careful attention should be paid to edge distances, and it is most desirable that the weakest failure mode of the joint should be by bearing rather than bolt shear, since the latter is not very ductile and lack of fit in the joints may cause... 

Bursting of the vertical wall Bursting failures are very uncommon and are almost all found in bolted silos where a joint detail has failed. A careful analysis of the loads and strengths in different modes shows that this failure mode is only critical near the surface, or in squat silos. 

Figure 4.36 Failure modes for bolted joints. (From Nelson, W.D., Bunin, B.L, and Hart-Smith, L.J., in Proc. 4th Conf. Fibrous Composites in Structural Design, Army Materials and Mechanics Research Center, Manuscript Report AMMRC MS 83-2,1983, pp. U-2 through 11-38)...

Three tests to failure on the C1 C1 C1 joint configuration were carried out in [41]. The study found that one joint failed in net-tension at hole 1 in the lower laminate at a joint load of 80 kN, while another failed by net-tension at hole 3 in the top laminate at 84 kN. The third joint tested failed by bolt failure at 76 kN. It should be noted that bearing damage was evident at holes where bolt failure occurred. However, as this damage mode is non-catastrophic, high loads are transferred through the bolt until... 

Consider the web-flange splice shown in Figure 5.14. The fasteners are subjected to shear. Assume that the flange splice transfers the bending moment through tensile and compressive forces and that the web splice carries the transverse shear. The composite beams are made from fibre reinforced composite material. As the joint is loaded, the flanges and web move and the bolts contact portions of the holes. As the load is increased, failure of the joint may occur in different modes, i.e. net-section, bearing, and shear out. 

The resistance of connection WmJ bt+bd (Table 2) had been increased by 16% by having a Joint with combined bolting and bonding. The mode of ultimate failure was gross delamination Just below the surface of the inside face of the left column flange. 

Various failure and degradation mechanism have been identified that adversely impact the integrity of bolts used in safety related applications and in applications important to safety. Depending on the nature of the degradation mechanism a potential for common mode failures exists for same system or redundant system components. In addition, leaks from flanged joints represent a significant part of total number of leaks in primary circuit. 

---