Bolt-hole clearance

This chapter presents both experimaital and finite element approaches that are commonly used to assist in the design and failure analysis of composite bolted joints. The work presented mostly stems from the authors work in the field over the past decade. Results from both local (i.e. detailed) finite element and global (Le. efficient) closed form and finite element approaches are presented, and expmmental results from the open literature are used to both calibrate and validate the modelling procedures. The primary variable under consideration is bolt-hole clearance, which is chosen as it induces significant three-dimensional stresses into the joint, and significandy alters the bolt-load distribution in multi-bolt joints, and so provides a rigorous test case for analysis. 

Table iiJi Bolt-hole clearances used... 

Table 11.3 Reduction in joint stiffness as a function of bolt-hole clearance — simulations versus experiments...

All tests initially show fairly linear behaviour up to about 2 kN appUed load. The slope (marked Slope V in Figure 11.10(b) and (c)) is similar for both clearances. This is most likely because this region is dominated by static friction forces which would be independent of bolt-hole clearance and therefore the same for all joints. 

McCarthy MA, Lawlor VP, Stanley WF, McCarthy CT. Bolt-hole clearance effects and strength criteria in single-bolt, single-lap, composite bolted joints. Compos Sci Technol 2002 62(10-11) 1415-31. 

McCarthy CT, McCarthy MA. Three-dimensional finite element analysis of single-bolt, single-lap composite bolted joints part II — effects of bolt-hole clearance. Compos Struct 2005 71(2) 159-75. 

Lawlor VP. An experimental study of the effects of variable bolt-hole clearance on the mechanical behaviour of composite bolted joints, Ph.D. thesis. Ireland University of Limerick 2004. 

McCarthy MA, McCarthy CT, Padhi GS. A simple method for determining the effects of bolt-hole clearance on load distribution in single-column, multi-bolt composite joints. Compos Stmct 2006 73(l) 78-87. 

McCarthy MA, Lawlor VP, Stanley WF. An experimental study of bolt-hole clearance effects in single-lap, multi-bolt composite joints. J Compos Mater 2005 39(9) 799—825. Hart-Smith LJ. Design and empirical analysis of bolted or riveted joints. In Joining fibre reinforced plastics. Elsevier Science 1987. pp. 227—69. 

Supporting beams are used to stiffen the trays, and the trays must be fastened securely to prevent movement caused by gas surges. To allow for thermal expansion and to facilitate installation, slotted bolt holes should be provided in the supporting rings, and there should be adequate clearance between the tray and shell wall. 

Above 2 kN the joints begin to slip and the behaviour transitions to a new region of quasilinearity with a reduced slope Slope 2 ). In the transition region, the clearance is taken up in the 240 pm clearance joints because the bolts were initially centred in the holes, no bolt-hole contact is made imtil the end of the transition region. 

As expected, the 240 pm clearance joints are particularly interesting because there are three distinct regions of behaviour, as the load increases the first dominated by static friction, then the transition region dominated by kinetic friction, then a final region in which the stiffness is due to bolt-hole contact forces as well as friction. 

For this study, two different joint configurations were considered. The first was a C1 C1 C1 joint with neat-fit clearances at each bolt-hole, and the other (C4 C1 C1) had a 240 pm clearance at bolt-hole No. 1, as shown in Table 11.4. 

First fibre failure was detected in this joint at 17 kN, which is 55.3% less than the neat-fit control case (i.e. the C1 C1 C1 joint). This highlights that joints with variable clearance incur significant joint damage at much lower loads than joints with all neat-fit bolt-holes. 

McCarthy CT, McCarthy MA. Progressive damage analysis of multi-bolt composite joints with variahle holt-hole clearances. Compos Part B 2005 36 290-305. 

Three connections (Table 1) had mechanical fastening by M16 grade 8.8 steel bolts with 30 mm diameter standard size washers. These were tightened to a torque of 23.8 N m, as recommended in reference 1 for FRP bolts. To aid buildability in connections Wmj bt, Wmj bt+bd and Wmj bd, bolt holes had a clearance of 2 mm. After test Wmj bt was conducted, the specimen was modified such that the holes in the members and cleats (Figure 3) were re-drilled to provide a tight fit for M20 steel bolts. This additional test, Wmj bt.2, was conducted for a number of reasons (reference 10). 

Bolt size Clearance hole diameter (imi Washer outside diameter (mm) Spot facing diameter (mm) Eiix Back Spot facing diameter (mm)... 

Piece A - Base Cap - Bottom Section " 3.25"0Dx1" steel plate Machine to dimensions shown. Drill a. 653 diameter center hole and tap for a 3/ x10 TP I bolt. Grind or machine the two 30 bevels as shown. Top bevel is for fin clearance. 

There is good repeatabihty between the tests with virtually no distinction between the initial slopes (Slope 1) in any test, while Slope 2 in the neat-fit clearance cases had an average value of 50.37 kN/mm with a standard deviation of 0.99 (2%) and in the 240 pm clearance cases had an average value of 37.06 kN/mm with a standard deviation of 1.74 (4.7%). The difference in Slope 2 for the two clearance cases was due to differences in the developing contact area between the bolt and the hole, as described in Section 11.3.1 above. 

Figure 11.18 shows the load distribution from the instrumented bolts for the C1 C1 C1 (all holes neat-fit) case and the C1 C1 C2 case (80 pm clearance in Hole 3 — see Figure 11.4 for hole numbering). Shown also are results from the three-dimensional finite element model where the agreement between the experimental and numerical results in terms of load distribution between the bolts is considered very good. 

The bolt-load distribution for the C3 C3 C1 joint (i.e. 160 pm clearances at hole 1 and 2, neat-fit at hole 3) is shown in Figure 11.24. Because of the 160 pm clearance at holes 1 and 2, bolt 3 carries the entire applied load until a joint load of approximately 10 kN. At this point, bolt 1 followed by bolt 2 starts to pick up load but bolt 3 still remains the most highly loaded. At a joint load of approximately 27 kN (37 kN in the... 

To increase the rotation at first failure of the bolted connection there should be bolt clearance holes of 2 mm and a gap of 6 to 12 mm between the beam end and column face, as recommended in the 1995 addendum to the MMFG Design Manual (reference 1). 

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