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Indian Journal of Engineering & Materials SciencesVol. 18, August 2011, pp. 283-292

Comparison of bolted joints with two different clearance types

mran Esendemir*&Aye ndrcDepartment of Mechanical Engineering, Sleyman Demirel University, 32260 Isparta, Turkey

Received 27 July 2010; accepted 11 April 2011

In this study, bearing strengths of bolted joints with different clearances are compared. Woven-glass-epoxy prepregcomposites are tested under two different bolt-bearing conditions. The geometry of the samples for bolted joints is suitablyvaried in order to find the limit for width-to-diameter (W/d) and edge distance-to-diameter ratios (E/d), which are necessaryto avoid unsafe failure modes. Damage progression is examined using scanning electron microscope (SEM) on specimenswith mixed (bearing+net tension) mode for different percentages of their ultimate failure load. It is observed that theclearance between the bolt and hole has an important influence on the failure load of mechanically fastened joints.

Keywords: Bolted-joints, Failure analysis, Scanning electron microscopy (SEM)

An understanding of bolted joint behaviour isessential to the design of efficient aerospace structuresfrom carbon-fibre reinforced polymer materials.Maximum joint efficiencies in composite structurestend to be less than for metals, so poorly designedjoints detract significantly from the weight advantageof composites over metals. In a typical manufacturingenvironment, the diameter of fasteners and holes willvary within certain allowed tolerances. The

combination of bolt and hole tolerances will result ina range of allowable bolt-hole fits, which incomposites are generally clearance rather thaninterference fits, due to concerns over damage causedto the composite during insertion of the fastener, andalso possibly removal of the fastener duringinspections1. The strength and failure modes of boltedjoints have been shown to be significantly affected byrelations between geometrical parameters such as thebolt diameter (d) bolt-hole diameter (D) laminatethickness (t) width (W) and edge distance (E)2-5. Otherfactors such as the stacking sequence2,6, clamping

moment4-10

have been investigated and shown to beimportant for the joint strength. Studies on the effectsof clearance have been performed in boltedjoints1,3,4,11-18. A review of the investigations that havebeen made on the stress and strength analysis ofmechanically fastened joints in fibre-reinforcedplastics (FRP) was presented by Camanho andMatthews19. The experimental observations of theeffects of joint geometry, ply-orientation, lay-up and

through-thickness pressure on the joint behaviour aredescribed briefly for both single and multi-fastenerjoints. Xiao and Ishikawa20 conducted theexperimental investigations in order to study thestrength and failure of mechanically fastenedcomposite joints. Their results show that the bearingfailure can be outlined as a process of compressivedamage accumulation, and can be macroscopicallydivided into the following four stages: damage onset,

damage growth, local fracture and structural fracture.An experimental investigation was carried out on afibreglass/aluminium (FGA) laminate in order tocharacterise its behaviour under pin and bolt-bearingconditions. Bearing tests were carried out on afibreglass/aluminium laminate, varying the jointgeometry and clamping pressure, in order to find thedesign conditions to be fulfilled for a safe failuremode21. Additionally, a review of the investigationsthat have been made on the mechanics of mechanicallyfastened joints in polymer-matrix composite structureswas presented by Thoppul et al.22

In this study, bearing strengths of bolted joints withdifferent clearances have been compared. Twodifferent clearance types have been formed toinvestigate the effect of the clearance in compositebolted joints. All specimens were torqued to finger-tight (T = 0 Nm) conditions. For each type, load-displacement curves and bearing strengths wereobtained according to different geometricalparameters. Furthermore, failure modes of bolted-joint specimen with different clearance werepresented. The damage progression of specimens with

_____________*Corresponding author (E-mail:[email protected])

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INDIAN J ENG. MATER. SCI., AUGUST 2011284

clearance was examined using scanning electronmicroscope (SEM).

Bolted Joint Configuration

The geometry of composite specimen is shown inFig. 1. The dimensions of the specimen were chosenas W= 10, 15, 20, 25 mm, E= 5, 10, 15, 20, 25 mm,L= 80 mm, t= 1.45 mm. Two different clearance typeswere formed, as shown in Fig. 2.

The hole diameter of the composite specimen was5 mm for Type-I; however, the hole diameter of thecomposite specimen was 5.2 mm for Type-II. Forboth types, the diameter of the hole of test fixture was5.2 mm. A metric 5 bolt was used in the experimentalstudy. The preload moment was obtained by finger-tightening (see Fig. 3).

As seen in Fig. 4, there are failure modes related tothe geometry of specimen: net tension, shear-out,

cleavage and bearing3. Although joint failure with thenet-tension, shear-out and cleavage modes iscatastrophic and immediate, bearing failure is

progressive. For this reason, bearing failure is preferredin applications.Bearing stress b is defined by the following

equation,

max

. =b

P

D t ... (1)

Where maxP is the applied maximum load, D is the

hole diameter and tis the specimen thickness.

Experimental ProcedureThe mechanical properties of woven prepreg glass-

fibre composed of eight-layer composite blanks aregiven in Table 15.All tests were conducted at ambient temperature

by means of an Instron Testing Machine in Pamukkale

Fig. 2Two different clearance types

Fig. 1Geometry of the composite specimen Fig. 3Test fixture of bolted joints

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ESENDEMIR & ONDURUCU: BOLTED JOINTS 285

University Mechanics Laboratory. The test specimenswere rigidly clamped into testing machine. They werefixed into the fixture so that no initial bending moment

was caused. The bolt was tightened with a torque of 0Nm to represent finger tight. The tests were run indisplacement control at a rate of 1 mm/min. Eachsample was loaded to ultimate failure. During statictests, load-displacement response was recorded in orderto examine the failure process of joint specimens.

Results and DiscussionAn experimental study on the effects of two

different bolt-hole clearances in bolt joints wasperformed. Load-displacement curves of compositebolted-joints with two different clearances wereobtained. The failure loads and the failure modes

obtained in this experimental study have beenpresented in Table 2, for each types. As seen in theTable 2, when W/d=2 is kept constant and the ratio ofE/dis changed from 1 to 5, failure modes become nettension. For W/d=3, 4 and 5, failure modes becomenet tension for only E/d=1. However, for the otherratios of E/d, failure modes become bearing + nettension. Failure modes obtained from experiments arethe same for Type-I and Type-II. When failure loadsare compared, failure load of the Type-I is bigger thanthat of the Type-II. In both of the types, it wasobserved that there are three different failure modes.

These failure modes are net tension, cleavage+nettension and bearing+net tension as shown in Fig. 5.Additionally; load-displacement curves have beengiven according to failure modes. Figure 6 showsload-displacement curves in different geometries forboth of the types.

For each type, bearing strengths are given in Fig. 7.As seen in the figure, the bearing strength of jointswith Type-I is higher than that of joints with Type-II.

According to McCarthy et al.23, clearance was amore significant factor in determining load distributionthan plate thickness, plate width, bolt diameter, or bolt

pitch. The results indicate that variable clearances inmulti-bolt joints significantly alter the loaddistribution11. According to Rosales-Iriarte et al. 24, ifbolt-hole clearance is included in the joint, the bolt-hole contact area is reduced, hence the stress is higherand the load carrying capacity of the joint is decreased.The results which involved experimental studies onbolt joints agree with findings in literature. It has beenfound that bolt-hole clearances have a significant effecton failure, therefore clearance should be taken intoconsideration when designing composite joints.

Fig. 4Common failure modes (a) net tension, (b) cleavage, (c)shear-out and (d) bearing

Table 1Mechanical properties of prepreg composite materials5

Properties Symbols Magnitudes

Longitudinal modulus E1(GPa) 27.80Transverse modulus E2(GPa) 27.80Shear modulus G12(GPa) 4.66Poissons ratio 12 0.16

Longitudinal tensile strength Xt(MPa) 343Longitudinal compressive strength Xc(MPa) 280Transverse tensile strength Yt(MPa) 343Transverse compressive strength Yc(MPa) 280Shear strength S(MPa) 94

Table 2Failure loads and failure modes for Type I and Type II

Clearance type I Clearance type IIW/d E/d

Failure load(N)

Failuremode

Failure load(N)

Failuremode

2

12345

9201472155616171667

NNNNN

7941219140712591243

NNNNN

3

12345

10411694198319342055

C-NB-NB-NB-NB-N

9101649196818391859

C-NB-NB-NB-NB-N

4

12345

9841760248223912520

C-NB-NB-NB-NB-N

8721712182921332144

C-NB-NB-NB-NB-N

5

12345

10812050256025472840

C-NB-NB-NB-NB-N

9271763229120802595

C-NB-NB-NB-NB-N

N: Net Tension, C-N: Cleavage-Net Tension, B-N: Bearing-NetTension

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Fig. 5Load-displacemet curves for different failure modes in experimental study

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Fig. 6Load-displacemet curves according to different geometrical parameters for (a) clearance type-I and (b) clearance type-II

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Fig. 8SEM micrograph of the bearing surface for 80% of the failure load

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AcknowledgmentsThe authors wish to express a special thank to

Izorell Firm, zmir, Turkey, for their collaboration in

producing of the woven glass-epoxy prepregs and toPamukkale University Mechanical EngineeringDepartment.

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