Effect Bolt Torque (Applied Clamping Load)

The applied clamping force and the distribution of such forces affect the performance of PFRP bolted joints.

2.3.9.1 Applied Clamping Force. Bolt bearing failure modes in com-posite materials are a mixture of compressive and delamination failure at the bolt hole. The applied clamping force prevents the expected delamina-tion with a consequent increase in failure load (Stockdale and Matthews 1976). Similar to bolted steel joints, increasing the through-the-thickness clamping force provided by tightening the bolt improves the joint strength of PFRP bolted joints. It is possible that a grossly overtightened bolt could result in damage to the composite laminate by forcing the washer into the surface (Matthews 1988). However, for normal volume fractions this local damage is less likely to develop. Instead, for a reasonable hole-diameter-to-thickness ratio (d/t), it is more likely that damage by stripping of the bolt threads will occur before laminate damage occurs. This process was observed for PFRP threaded rods in several full-scale studies (Mosallam et al. 1993; Mosallam and Schmitz 1996). The bearing strength of a fully tightened bolt is up to four times that for a pin-joint. On the other hand, the fi nger-tight bolted joint has twice the strength of the corresponding pin-bearing strength (Matthews 1988).

Nassar et al. (2007) conducted an experimental and analytical investi-gation of the behavior of a double-bolted single-lap shear composite joint.

Several bolt tightness scenarios were considered for composite-to-com-posite and comcomposite-to-com-posite-to-aluminum bolted joints. Based on both analytical and experimental results, it was concluded that increasing the bolt tight-ness without exceeding the joint strength would signifi cantly reduce the potential for delamination around the bolt hole when a tensile load is applied. It has been also concluded that tightening of at least one bolt in a two-bolt composite joint has increases the joint bearing strength.

Also, joint stiffness is increased only when both bolts are suffi ciently tightened.

The study by Doyle referenced earlier (J. R. Doyle, “Behavior of Bolt and Adhesive Connections in Glass Fiber-Reinforced Plastic Members.”

Master’s thesis, Department of Civil Engineering, West Virginia Univer-sity, Morgantown, WV, 1991) indicates that an increase of up to 31% in PFRP bolted joint strength is achieved by tightening the steel bolts by the maximum permissible tension, as compared to similar “fi nger-tightening”

the bolted joints. The applied axial force can be calculated from the fol-lowing expression, from Doyle:

W T

W = applied axial force, lb (N) Ts = applied torque, ft-lb (N·m) r = radius of bolt bet area, in. (mm) α = angle of inclination of bolt threads θs = angle of static friction.

Matthews et al. (1982) observed losses in the bolt tension when the bolt was fully tightened and the joint was not loaded. However, under the same condition but with the joint loaded, the bolt tension increased as the applied tensile force on the joint increased. Cooper and Turvey (1995) reported the results of their study on the effect of bolt torque and joint geometry. The material used in that study was 1/4-in. (6.35-mm)-thick E-glass/polyester EXTREN 500 Series (manufactured by Strongwell).

Tables 2-2, 2-3, and 2-4 summarize the experimental pin-joint, lightly clamped joint, and fully clamped joint, respectively. Based on this study, the following conclusions were drawn:

• The joint strength increased by 45% and 80% for lightly-clamped (3 N-m) and fully clamped (30 N-m) joints [2.21 ft-lb (3 N·m)], respectively, as compared to pin-joint strength.

• Increasing bolt torque signifi cantly increases the critical values of e/d and w/d ratios.

• Bolt torque does not affect the initial bolt displacement.

• Bolt-clamping torque and e/d ratio have a small effect on the joint initial stiffness.

Klett et al. (2004) and Sun et al. (2004) reported results of several tests on both steel-to-steel and several bolted pultruded composite joints with different thicknesses. The applied torque of all bolted joints was 50 ft-lb (67.8 N-m). Test results showed that the loss of preload with time was relatively low for steel-to-steel bolted joints and as compared to pultruded bolted joints with different thicknesses. The test results further indicated that major preload loss occurred for bolted pultruded joints with largest thickness [t = 1/4 in. (6.35 mm)]. It was also reported that a signifi cant portion of preload loss occurred during the fi rst 5 min after the bolt has been tightened, and a very rapid decrease in preload occurred within the fi rst 30 sec after application of the torque. Based on the results of the initial tests, it was suggested that retightening the bolts within a few minutes of the initial assembly may assist in reducing the bolt’s preload loss rate with time, as well as the overall loss of preload.

DESIGN GUIDE FOR FRP COMPOSITE CONNECTIONS

Table 2-2. Hole Clearance Tests Summary Results

Test Group

Hole Clearance [in. (mm)]

Load at Incipient Failure [kips (kN)]

Ultimate Joint Load [kips (kN)]

Failure Mode

D8/16 0 (0) 6.65 (29.60) 9.24 (41.12) Bearing

D9/16 0.03125 (0.794) 6.1 (27.15) 9.1 (40.50) Bearing

D10/16 0.0625 (1.5875) 6.0 (26.70) 8.86 (39.43) Bearing

D11/16 0.09375 (2.38125) 4.75 (21.14) 8.74 (38.94) Bearing

D12/16 0.125 (3.175) 4.20 (18.69) 8.43 (37.51) Bearing

Source: Cooper and Turvey (1995).

COMPOSITE JOINTS 49 Table 2-3. Average Failure Loads and Joint Stiffnesses of Pin-Bearing Single-Bolt Joints

in EXTREN 500 Series 6.35-mm-Thick Flat Plates^a Member

Width-to-Hole Diameter, w/d

Edge Distance-to-Hole Diameter

Ratio, e/d

Average Failure Load (kN)

Average Damage Load (kN)

Typical Failure Mode

Average Initial Joint Stiffness

(kN/mm)

5.0 1.5 11.2 — Shear 20.3

5.0 2.0 15.2 12.9 Cleavage 28.6

5.0 3.0 19.7 16.8 Bearing 29.9

5.0 4.0 19.3 16.2 Bearing 28.5

5.0 5.0 17.2 13.8 Bearing 29.6

2.0 4.0 8.7 — Tension 15.6

3.0 4.0 16.8 14.8 Tension 23.2

4.0 4.0 18.6 17.7 Bearing 27.9

7.0 4.0 17.5 15.2 Bearing 30.2

a d = 10 mm.

Source: Cooper and Turvey (1995).

DESIGN GUIDE FOR FRP COMPOSITE CONNECTIONS Table 2-4. Average Failure Loads and Joint Stiffness of Lightly Clamped (3 N·m)

Single-Bolt Joints in EXTREN 500 Series 6.35-mm-Thick Flat Plates^a Member

Width-to-Hole Diameter, w/d

Edge Distance-to-Hole Diameter

Ratio, e/d

Average Failure Load (kN)

Average Damage Load (kN)

Typical Failure Mode

Average Initial Joint Stiffness

(kN/mm)

10.0 2.0 16.8 — Shear 34.1

10.0 3.0 23.6 21.4 Shear 37.6

10.0 4.0 27.5 21.0 Cleavage 37.8

10.0 5.0 28.9 21.5 Bearing 41.8

10.0 6.0 27.5 18.2 Bearing 36.9

2.0 5.0 10.1 — Tension 17.2

4.0 5.0 21.4 19.5 Tension 29.4

6.0 5.0 25.5 17.7 Bearing 32.0

8.0 5.0 29.3 19.4 Bearing 35.7

a d = 10 mm.

Source: Cooper and Turvey (1995).

2.3.9.2 Effect of Using Washers. The torque is transferred to the specimen in the form of lateral pressure (σz) exerted by the washer onto the area around the bolt. The use of washers is important to prevent the composite from splitting through the thickness on the hole’s loaded side.

Assuming that the width, w, and the edge distance, e, are large compared to the hole diameter, d (i.e., w > 6d, and e > 6d), the failure area is confi ned to within one or two diameters of distance from the loaded half of the bolt hole, i.e., a bearing failure (Kretsis and Matthews 1985). If the laminate is restrained laterally, the part of the laminate under the washers develops shear cracks but is not allowed to expand under compression, thus the lateral expansion; hence, the delamination is spread into a wider area outside the washer boundary. The ultimate load is therefore expected to increase since the easiest failure modes are suppressed.

The bearing stress increases asymptotically; however, the bearing stresses are expected to decrease when the magnitude of σz becomes high enough to cause the washer to punch through the laminate. Matthews et al. (1982) reported that increasing the contact area by replacing a washer with a composite plate resulted in a slight increase in failure load. W. S.

Arnold, in his Ph.D. dissertation (Dept. of Mechanical Engineering, Paisley College of Technology, Paisley, UK, 1989) titled “The Behavior of Mechani-cally Fastened Joints in Composite Structures,” reported that the use of recessed washers results in signifi cant damage compared to the use of plain and beveled washers. Khashabaa et al. (2006) studied the effect of washer size and tightening torque on the performance of bolted compos-ite joints. The experimental results showed that (in the range of the inves-tigated tightening torques) the slope of the load–displacement curve (stiffness) increased with increasing the tightening torque as a result of increasing the contact pressure. Also, bolt bearing strength increased as the tightening torque increased and, under the same tightening torque, joint stiffness increased with the decreasing washer size. The results also indicated that the load–displacement curve of bolted joint specimens fi nger-tightened, T = 0 Nm, had the lowest stiffness, with several knees (infl ection points in the load-displacemet curve), which indicated unsta-ble development of internal damage.

For bolted pultruded composite joints, Abd-El-Naby (unpublished Ph.D. dissertation, “Experimental and Theoretical Investigations of Bolted Joints for Pultruded Composite Materials,” Dept. of Civil Engineering, University of Surrey, Surrey, UK, 1992) and Abd-El-Naby and Hollaway (1992) conducted a study to investigate the effect of clamping area and the materials used for clamping on the bearing strength and extensibility of the single-bolt pultruded composite joint. The pultruded material used in this study was E-glass/polyester manufactured by the Strongwell Company. The bolt diameter, db, was 3/8 in. (9.5 mm), and the washers were tight-fi tting with an outer diameter, dw, equal to 2.2 times the bolt

diameter [i.e., 0.825 in. (20.90 mm)]. The small clamping torque used for all specimens was equivalent to the fi nger-tight condition. The test was conducted on three specimen groups, each being subjected to a different clamping condition. It was also reported that parts of the joint specimens with steel washers were separated from the rest of the PFRP plate by two cracks that extended from the hole parallel to the specimen axis. The material between these two cracks was not damaged because of the pres-ence of the steel washer suppressing the bearing failure mechanism.

It should be noted that in most of the current reported works on frame connections (e.g., Mosallam 1999; Mosallam and Schmitz 1996), the use of washers did not affect the mode of failure of the pultruded connections.

This can be attributed to the relatively large area of the FRP molded square nuts as compared to the bolt diameters that were used in these investiga-tions. However, and due to the sharp corners of the molded FRP nuts, a high stress concentration is likely to develop, which was observed to result in localized punching shear damage. In these cases, the sharp corners of the FRP molded nuts should be rounded, and an FRP thin composite plate is recommended. It should be noted that other FRP bolt-and-nut systems are manufactured by the Strongwell Company, are avail-able commercially, and not only help to avoid stress concentration at the square nut sharp edges but also have built-in washers, as shown in Fig. 2-17.

Klett et al. (2004) and Sun et al. (2004) performed an experimental study to evaluate the effect of washer size on the strength of pultruded bolted

Figure 2-17. Fibrebolt threaded rods and molded nut system.

Courtesy of Strongwell Company, Bristol, Va.

joints made of EXTREN profi les. Several pultruded composite lap shear joint specimens with a nominal size washer (15.9 mm diameter), an over-size washer [1 in. (25.4 mm) diameter], and a steel plate washer (50 mm

× 35 mm × 6 mm) were evaluated. Test results indicated that the static strength increased approximately 10% with an oversize washer and approximately 20% with a plate, compared with the nominal washer. It was further observed that joint specimens with nominal and large washers showed the typical characteristics of a bearing type of failure. However, bolted joints with a constraining plate did not show clear strength degra-dation prior to reaching the peak joint strength. In all three washer types, a common cleavage tension failure mode was observed and the damage was initiated in all cases at the border of the bolt hole.