Bolted Joints

11
The strength of bolted joints in glass fibre/epoxy laminates G KRETSIS and F. L MA TTHEWS (Imperial College of Science and Technology, UK) The results obtained from an experimental study on glass fibre- reinforced epoxy laminates are described. Single-hole bolted joints were tested in a variety of lay-ups with two resin systems -- Fothergill Code 69 and Ciba-Geigy 91 3. A small number of tests carried out on carbon fibre laminates compared closely with data from other workers. The general behaviour of the two fibre systems was found to be similar, the optimum lay-ups for bearing strength being only slightly different. The failure modes seemed to be more dependent on the lay- up than the fibre/resin combination, although more delaminations were seen with the glass fibre/epoxy laminates which also showed stronger interaction between modes. Key words: composite materials; mechanically fastened joints; bearing strength; failure modes; laminate lay-up; glass fibres; carbon fibres; epoxy resins The problem of attachments and joints in composite materials has received great attention since the introduction of these materials to highly stressed structures for two main reasons: firstly, the anisotropy of the fibrous structure leads to high stress concentrations and, secondly, the non-yielding nature of composites does not permit large displacements that would otherwise relieve localized stresses. Bolted joints exhibit high stress concentration factors (SCFs) because of the inherent geometric discontinuities, but offer easy inspection, low cost, ease of manufacture and reliability compared with bonded joints. Furthermore, bolted structural parts can be easily removed for maintenance or replacement The development of a three-dimensional stress field around a bolt-loaded hole gives rise to interlaminar failure modes. Therefore, a satisfactory finite element model must be three-dimensional and iterative, making use of reduced stiffness methods for damaged areas. Obviously, such a model would be very expensive and the failure mechanisms very complicated to represent Because of this, past work on bolted joints has mostly been experimental. Although experimental work is quite costly, long-winded and very specific, it is the most reliable method to date. Most of the published experiments performed with bolted joints have been on CFRP (carbon fibres in epoxy resin), although some isolated results are quoted for GFRP (glass fibres in epoxy resin); considerable data 0010-4361/85/020092-11 $03.00 © 92 is also available for GRP (glass fibres in polyester resin). The differences between bolted joints in GFRP and the other two materials have not so far been fully invest- igate& The experiments described here concern GFRP, the objective being to analyse the factors affecting the static strength of single-hole bolted joints under standard conditions (ie no environmental effects). Some experiments have also been done with CFRP to compare the resin systems used here with those used in previously published work. Multi-hole bolted joints are not analysed but it is hoped that the present results can be used as a baseline reference for future investigations or even design. POTENTIAL FAILURE MODES OF BOLTED JOINTS IN COMPOSITE MATERIALS There are four possible joint failure modes in composites and these are shown diagramatically in Fi~ 1. They are generally similar to the failure modes observed in metals, despite the fact that metals exhibit considerable yielding prior to fracture. Yielding does not take place in composites, therefore the mechanisms of failure are completely different However, some form of pseudo-yielding can be experienced for certain lay- ups in which delamination and partial fibre breakage occur before final failure. It is geometric factors that usually render one of the failure modes predominant Such factors are the width of the specimen and the distance of the bolt from the 1985 Butterworth ~ Co (Publishers) Ltd COMPOSITES. VOLUME 16. NO 2. APRIL 1985

description

The Strength of Bolted Joints of Glassepoxy Material

Transcript of Bolted Joints

Page 1: Bolted Joints

The strength of bolted joints in glass fibre/epoxy laminates G KRETSIS and F. L MA TTHEWS (Imperial College of Science and Technology, UK)

The results obtained from an experimental study on glass fibre- reinforced epoxy laminates are described. Single-hole bolted joints were tested in a variety of lay-ups with two resin systems - - Fothergill Code 69 and Ciba-Geigy 91 3. A small number of tests carried out on carbon fibre laminates compared closely with data from other workers. The general behaviour of the two fibre systems was found to be similar, the optimum lay-ups for bearing strength being only slightly different. The failure modes seemed to be more dependent on the lay- up than the fibre/resin combination, although more delaminations were seen with the glass fibre/epoxy laminates which also showed stronger interaction between modes.

Key words: composite materials; mechanically fastened joints; bearing strength; failure modes; laminate lay-up; glass fibres; carbon fibres; epoxy resins

The problem of attachments and joints in composite materials has received great attention since the introduction of these materials to highly stressed structures for two main reasons: firstly, the anisotropy of the fibrous structure leads to high stress concentrations and, secondly, the non-yielding nature of composites does not permit large displacements that would otherwise relieve localized stresses.

Bolted joints exhibit high stress concentration factors (SCFs) because of the inherent geometric discontinuities, but offer easy inspection, low cost, ease of manufacture and reliability compared with bonded joints. Furthermore, bolted structural parts can be easily removed for maintenance or replacement

The development of a three-dimensional stress field around a bolt-loaded hole gives rise to interlaminar failure modes. Therefore, a satisfactory finite element model must be three-dimensional and iterative, making use of reduced stiffness methods for damaged areas. Obviously, such a model would be very expensive and the failure mechanisms very complicated to represent Because of this, past work on bolted joints has mostly been experimental. Although experimental work is quite costly, long-winded and very specific, it is the most reliable method to date.

Most of the published experiments performed with bolted joints have been on CFRP (carbon fibres in epoxy resin), although some isolated results are quoted for GFRP (glass fibres in epoxy resin); considerable data

0010-4361/85/020092-11 $03.00 ©

92

is also available for GRP (glass fibres in polyester resin). The differences between bolted joints in GFRP and the other two materials have not so far been fully invest- igate& The experiments described here concern GFRP, the objective being to analyse the factors affecting the static strength of single-hole bolted joints under standard conditions (ie no environmental effects). Some experiments have also been done with CFRP to compare the resin systems used here with those used in previously published work. Multi-hole bolted joints are not analysed but it is hoped that the present results can be used as a baseline reference for future investigations or even design.

POTENTIAL FAILURE MODES OF BOLTED JOINTS IN COMPOSITE MATERIALS There are four possible joint failure modes in composites and these are shown diagramatically in Fi~ 1. They are generally similar to the failure modes observed in metals, despite the fact that metals exhibit considerable yielding prior to fracture. Yielding does not take place in composites, therefore the mechanisms of failure are completely different However, some form of pseudo-yielding can be experienced for certain lay- ups in which delamination and partial fibre breakage occur before final failure.

It is geometric factors that usually render one of the failure modes predominant Such factors are the width of the specimen and the distance of the bolt from the

1985 Butterworth ~ Co (Publishers) Ltd

COMPOSITES. VOLUME 16. NO 2. APRIL 1985

Page 2: Bolted Joints

+

Beoring

C)

Concentration factors to describe tensile failure. These are defined as:

Shear out

Tension

I ¢___ I I

Cleavage or combined

Fig. 1 Failure modes in composite materials

I , i 200 mm

a p p r o x

I I I

Fig. 2 Typical specimen

'1

_l free end running normal to the loading axis. Another factor that affects the failure mode is the lay-up; cracks almost always propagate along the fibre direction thus, for example, making tensile failure in a unidirectional laminate practically impossible to achieve (in most fibre-reinforced plastics for which the interlaminar shear strength is very much less than the fibre tensile strength).

Often, failure modes are not clearly defined since combined modes may occur if the conditions are right In particular, some signs of bearing mode damage are almost always present after failure, since the pin damages the laminate area adjacent to the loaded half of the hole.

In order to manipulate the results concerning simple single-hole bolted joints, the following ultimate stresses are defined (refer to Fig, 2 for symbols other than Lult):

Lult Ultimate bearing strength, Ob(uk) - d t (1)

Zult Net ultimate tensile strength, anet(~) - (w-d) t (2)

Lull Ultimate shear strength, 7"xy(ult ) - 2et (3)

In every case Lul t is the failing load, taken as the max imum load attained during the test The above strength values are used for bearing, tensile and shear failure modes respectively. It is clear that in all three cases the failing load is divided by the nominal failure area. In the case of a combination of failure modes, lYb(ult) is usually used.

Work by Collings ~ suggests the use of two stress

kne t (4) Ox(ult)

i

Onet(ult)

and

kg~oss = Ox(~0 (5) Ogross(ult)

where

Lult Ogross - w t ( 6 )

and O'x(ult) is the theoretical ultimate tensile strength of the plain laminate. Collings uses a law of strain compatibility to calculate Crxtult) for a laminate containing plies at 0 ° and ± a ° to the direction of loading, and concludes that:

= {toeo + t .e. ) to Ox(uR ) \ t--oE-- ~ Of(ult) Vf t - (7)

where to and t a are the total thicknesses of the 0 ° and + a ° plies in the laminate respectively; E0 and E a are the corresponding Young's moduli; t is the total laminate thickness, equal to (to + ta); al~ult) is the tensile strength of the fibre; and Vf is the volume fraction of fibres in the laminate.

However, in the case of ___45 ° laminates, failure under tensile loading occurs by in-plane shear at 45 ° to the loading axis. In this case therefore, the equation:

Ox(ult) = 2rult (8)

is used, where Vul t is the in-plane shear strength of the material.

kne t and kgross are considered to be a measure of the tensile SCF in the jo int Similar factors exist for the bearing or shear failure modes but are not very representative of the respective SCFs. For example, a~ult) can be compared with the compressive strength of the plain laminate, but problems arise for the +45 ° lay-up, for which the compressive strength is very low, thus yielding a factor (0rx(ult)compr/O'b(ult)) considerably less than 1 Similar problems arise if v x Cult) is

• . . Y

compared with the m-plane shear strength of the laminate; in addition the latter property is very difficult to measure. Therefore, for the reasons stated above, only kne t and kgross are formulated and discussed in the present paper•

EXPERIMENTAL DETAILS

Materials

Experiments were conducted on both GFRP (E-glass fibre) and CFRP (XAS fibre), using two different resin systems - - Code 69 from Fothergill and 913 from Ciba-Geigy. The following four fibre/resin combinations were therefore possible: G/69, G/913, XAS/69 and XAS/913.

Most of the work was done on G/69, although a large number of tests were carried out on G/913. A few CFRP specimens were made for comparative purposes.

COMPOSITES. APRIL 1985 93

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Table 1. Mater ia l propert ies

Property G/69 G/913 XAS/913 and XAS/69

Longitudinal tensile strength (MPa) Longitudinal tensile modulus (GPa) Longitudinal compressive strength (M Pa) Longitudinal compressive modulus (GPa) I nterlaminar shear strength (M Pa) Shear modulus (GPa) Principal Poisson's ratio Transverse tensile strength (MPa) Transverse tensile modulus (GPa) Transverse compressive strength (MPa) Transverse compressive modulus (GPa) Tensile strength at 45 ° (MPa) Tensile modulus at 45 ° (GPa) Compressive strength at 45 ° (MPa) Compressive modulus at 45 ° (GPa) Average fibre volume fraction

(1100) (1170) (2100) (55) 42 150

(900) 750 1200 - - 90 126

(63) 67 100 5 m - -

0.230 0.227 0.263 (57) 73 57 (26) 15 9.5

- - 165 155 - - - - 9 .8

(126) 210 230 (24) 14 - -

- - 111 184 - - 178 17

(0.67) (0.60) (0.60)

The values in brackets were determined experimentally in the present work. The

The material was obtained in prepreg form, as unidirectional plies of nominal moulded thickness 0.250 m m (Code 69) or 0.125 m m (913), and fibre volume fraction 0.6 when moulded. The laminates were cured according to manufacturers ' specifications. Laminate thicknesses were nominally 1.5, 3, 4, 4.5 and 6 mm. The lay-ups used are discussed in the next section. Note, however, that because of the cure schedule used G/69 laminates were somewhat thinner than nominal, resulting in a fibre volume fraction of 0.67 on average, ie slightly higher than expected.

Initially, some tests were done to measure material properties. Manufacturer 's data are available for G/913 and XAS/913, therefore only longitudinal tensile tests were performed on these in order to confirm the choice of specimen dimensions. In the case of XAS/69 it was assumed that the properties are the same as for XAS/913 because there was not enough material available for a complete set of tests and no data were supplied. The experiments for G/69 were to measure Young's moduli parallel, normal and at 45 ° to the fibre direction, as well as the corresponding tensile strengths. Longitudinal compressive strength was also measured. The results obtained can be found in Table l, along with the data for the other three fibre/resin combinations.

Table 2. Lay-ups and stacking sequences

other data were supplied by the manufacturers

Testing variables The strength and the failure mode of a single-hole bolted specimen depend upon four geometric variables: the width, the edge distance, the hole diameter and the laminate thickness (see Fig. 2). Another very important variable is the degree of tightening of the bol t The torque is transferred to the specimen in the form of lateral pressure (trz) exerted by the washer onto the area around the bol t The size of the washer is also important but was not varied in the present work. The stacking sequence also plays a very important role and, in particular, its effects are coupled with those of the geometric variables. Here, 0 °, 00/__+45 °, +45 ° and 00/90 ° lay-ups were investigated, all balanced about the mid- plane. Detailed descriptions of these can be found in Table 2. Furthermore, for families of0°/+~t ° laminates, one can investigate the effects of sequence homogeneity and of proportion of + a ° layers.

It is evident that a consistent experimental programme should treat each one of the above variables as completely independent. In the present paper, however, only the most important aspects are covered, and an outline of the tests is provided below:

• effect of tr z o n O'b(ult) for 0°/___45 ° (1/3 0 °, 2/3 45 °) laminates;

1 Unidirectional:

2 0 ° /+45° (1 /3 0 °, 2 / 3 45°):

3 0 ° / 4 - 4 5 ° ( 1 / 3 0 °, 2 / 3 45°):

4 0 ° / + 4 5 ° ( 1 / 3 0 °, 2 / 3 45°):

5 +45° :

6 0° /90°:

7 0 ° / 4 - 4 5 ° ( 1 / 2 0 °, 1 / 2 45°):

8 0°/4-45°(2/3 0 °, 1 /3 45°):

9 0 ° / + 4 5 0 ( 2 / 3 0 °, 1 / 3 450), non-homogeneous:

12-ply, iet = 3 mm

( 0 / + 4 5 / - 4 5 / 0 / + 4 5 / - 4 5 ) s , t = 3 mm or 1.5 mm

(0/+45/-45/0/+45/-45/0/+45/--45)~, t = 4.5 mm

(0/+45/-45/0/+45/-45/0/+45/-45/0/+45/-45)~ t = 6 mm

( - t -45/ -45/+45/ - -45/ - I -45/ -45)~ t = 3 mm

( 0 / 9 0 / 0 / 9 0 / 0 / 9 0 ) s , t = 3 mm

(0/+45/-45/0/0/+45/-45/0)s, t = 4 mm

(0 / - I -45 /0 /0 / -45 /0 )s , t = 3 mm

(0/0/+45/-45/0/0)~ t = 3 mm

94 COMPOSITES. APRIL 1985

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I

I I I I

Load introduced

by the test machine grips

/ / L o a d cell

I , I

Loading mechanism (s tee l )

Specimen

Fig. 3 Loading mechanism (not to scale). Dimensions dependent on bolt size

• effects of width, edge distance, hole size and laminate thickness o n O'b(ul t ) , trne~(ul 0 and rxy(ul0 for various stacking sequences;

• effects of width and edge distance on failure mode; and

• effects of percentage of-+45 ° plies in a 00/+45 ° family of laminates on ab(ul0, O'net(ul0 and Vxy(ul0 (G/69 only).

A total of 450 specimens was prepared and tested.

Apparatus and procedure

The arrangement shown in Fig. 3 was used to fit the specimen in the 100 kN Instron test machine. The double-ended specimen configuration was chosen for ease and speed of manufacture. The distance between the holes was always long enough to rule out any stress field interactions. A pre-load ( ~ 100 N) was applied to the specimen before the bolts were tightened in order to avoid initial load eccentricity. The loading rate was kept constant during the test, ensuring failure in about 30 seconds. The failing load was taken as the mean value from two to four specimens and then normalized to a fibre volume fraction of 0.6. For each test, a load/ displacement curve was obtained, the displacement being the relative cross-head movement. The lateral pressure, tr z, applied through the washers was monitored by means of strain gauges fitted on small steel cylinders through which the bolts would pass, as shown in Fi~ 3. These 'load cells' had previously been calibrated in compression using the same test machine.

900 / , , ,o .~.~.- . . . . o "

8 0 0 J 7 / x ~ - - - - - - ~ - - ~ ~ T . . ~

0

---x-- G/69 b;6OO '- --o-- G/913

50O

40O

I I I I I 0 I0 2O 30 40 50

a" z (aPa)

Fig. 4 Variation of bearing strength of 0 ° /±45 * (~ 0", % 45") laminates with bolt clamping pressure, d = 6.35 mm, t = 3 mm

The washers used were tailor-made to ensure a good fit between the inside diameter of the washer and the bolts. The outside diameter was set at 2.2 times the inside diameter, so that results could be compared with those of Callings. ~ The significance of the size of the washer has been previously discussed, ~ where it was concluded that loose-fit washers gave inconsistent results although the bearing strength was slightly increased with increasing outside diameter.

The clearance between the specimen and the loading arrangement was minimized to reduce the possibility of pin bending.

DISCUSSION OF RESULTS Bearing 1. Effect of lateral pressure Provided the width, w, and the edge distance, e, of a specimen are large compared with the hole diameter d (>6d), the failure area is confined to within one or two diameters' distance from the loaded half of the hole, giving what is known as a bearing failure.

If the laminate is sufficiently restrained laterally, the part of the laminate under the washers develops shear cracks (see Figs 15 and 17) but is not allowed to expand under compression, therefore the lateral expansion - - and hence the delamination - - is spread into a wider area that lies outside the washer boundary. The ultimate load is therefore expected to increase since the easiest failure modes are suppressed. Indeed, as shown in Fig 4 for the case of 00/--+45 ° laminates of G/69 or G/913, the bearing stress increases asymptotically, but it is expected to start decreasing when tr z becomes high enough to cause the washers to dig into the laminate. Note that the a z = 0 points correspond to tests where the bolts were lightly 'finger tight', while a few 'pinned' tests were also carded out in which a reduction of 20-30% in strength (compared with finger-tight tests) was observed. The failure mode of the pinned specimens was brush-like, the damage being localized around the loaded half of the hole.

It should be noted that since the thickness of the 913 prepreg material was half that of the Code 69 prepreg, double layers were used when laying up the 913 laminates in order to achieve the same total thicknesses for similar stacking sequences of the two resin systems. Throughout the paper therefore, 'single layers' should be assumed to be 0.250 mm thick, except in one set of tests mentioned later, where the use of 0.125 mm layers is clearly stated.

COMPOSITES. APRIL 1985 95

Page 5: Bolted Joints

600

900[ 1.06 ~ , . . . . .

. . . . . . . - - - " 1.41 8 0 0 / ~ " - ~ 2.17-

4OO

3OO

1- 0

_,~/,' 3.18

d/t d(mm) t(mm)

1.06 6.35 6 1.41 6 3 5 4.5

2.12 6.35 3 3.18 9.53 3 4.23 12.70 3

. . ~ .

I I I I I I 0 20 30 40 50

o" z (MPa)

Fig. 5 Effect of d/t ratio on bearing strength of 0 ° / ± 4 5 ° (~ 0 °, % 45 °) laminates of G/6g. d/ t values as indicated

800

700

600

~ 5 0 0 b

400

300

1- 0

o 9 .55 zx 12.7

I 1 I I I I 2 3 4 5

d l t

Fig. 6 Variation of bearing strength of 0* /+45* (~ 0", % 45 °) G/69 laminates with d/ t ratio (finger-tight bolt), t = 3, 4.5 or 6 ram, az = 0

The above results compare well with those found previously, 1-3 although actual stress levels cannot be compared because of the different fibre/resin combinations and lay-ups used.

At this point, a value had to be chosen for (r z that would be used in all of the subsequent tests for reasons of consistency. The value chosen was limited to o" z = 12 MPa, corresponding to roughly 90% of the asymptotic strength (see Fig. 4), in order not to give unacceptably high torque levels for the 12.7 m m bolts.

2. Effect of thickness

The effects of thickness (t) were found to be best correlated if the ratio d/t was examined. In Fig. 5, tTb(ul0 is plotted against ~r z for different d/t ratios for a

o . 4 _ o • 0 / _ 4 5 laminate of G/69. It is clear that ~ult) increases considerably with decreasing d/t, as is also shown in Fig. 6 for the particular case of finger-tight bolts. Collings, 1 and Godwin and Matthews 3 presented similar results for CFRP and other materials, but Collings showed that for high lateral pressure values, the effects of d/t almost disappear. Fig" 5 suggests that this is not the case for GRE; it seems therefore that its low modulus favours instability effects that are independent of ~z. These effects may be occurring on a microscopic scale, but for values of d/t> 3 they become so pronounced that, during testing, the part of the specimen between the hole and the edge running normal to the length was observed to deform considerably out of the laminate plane, thus increasing the load eccentricity and introducing bending stresses.

This 'out-of-plane' buckling mode was noted for both G/69 and G/913 specimens of ' the 00/+45 ° (1/3 0 °, 2/3 45 °) lay-up, for all cr z used and for all d and t combinations giving a ratio of d/t>3. The details of high d/t values for the other lay-ups are not known because no comparative tests were carried out. It is interesting to note, however, that the out-of-plane buckling mode was also observed by Kutscha and Hofer 4 more than 20 years ago, while they were performing tests with Scotchply XP-251 S material in a 00/90 ° lay-up on double-lap bolted joints, using d/t = 3.13. This is the only reference found in which this buckling effect is mentioned.

It is apparent therefore that designers should try to use small d/t values for joints, noting that a lower limit exists below which fasteners would fail in shear. Naturally, this limit depends on the quality of the bolts, but it is recommended not to use values of d/t below 1-1.2.

3. Effect of width

As the width of the specimen decreases, there is a point where the mode of failure changes from one of bearing to one of tension, ie the specimen fails across the width at the net section, with cracks originating from the hole boundary. This mode change is associated with a considerable drop in load-carrying capacity, as shown in Fig. 7 for the case of 0o/+45 ° laminates. Since the transition points shown are not usually well defined it would be more appropriate to talk of transition 'regions', extending roughly half a unit of w/d either side of the transition points indicated.

It is important to note the more gradual asymptotic behaviour of the curves of the GFRP laminates, revealing a strong influence of the width on the bearing failure load. On the other hand, there is no apparent link between the point of transition and the type of fibre or resin used although, on a rough scale only, all fibre/ resin combinations seem to change-over at w/d = 3.2. Furthermore, all curves are expected to pass through the point Orb(ult) = 0, w/d = 1, but the two Code 69 curves are too steep for this to happen. Finally, CFRP bearing strength appears to be roughly 19% higher than

I100

100(3

900

800 Q_

~70o

50(3

4OO

" f i 0 I

T i rtgh _ens.o = . . _ _ ~ _ _ [ t~Bearing XAS/913

/ ,.-- _ . . . . . . . . . . . . G/913

/ / ~ f / ~ , .,...---" ~ - - - - - - XAS/69

/ ~ I I I I I I I

2 3 4 5 6 7 8 9 w/d

Fig. 7 Variation of bearing strength of 0° /+45 ° (~ 0 °, z~ 45 o) laminates with w/d ratio, d = 6.35 mm, t = 3 ram, ~r z = 12 MPa

96 COMPOSITES. APRIL 1985

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90O

8OO

7O(3 O

(3_ :E

b ~

400

30(

f(mm)

3 - - - 1 . 5

. .r l I I I I I I I I

0 I 2 3 4 5 6 7 8 w/d

Fig. 8 Effect of out-of-plane buckling on the tensile failure mode of 00 /±45 ° (½ 0 °, % 45 °) G /913 laminates, d = 6.35 ram, ~z = 12 MPa

IIOO

I000

900

~ 80o

.=

b 6O0~

500

4O0

T 0

Sheor.t,,.,~, - / - ~ -

/ '~-Beorinq

!

/ Z

I I I I I I 2 3 4 5

e/d

Fig, 9 Variation of bearing strength of 0 ° / ± 4 5 ° (½ 0 °, % 45 °) laminates with e/d ratio, d = 6.35 ram, t = 3 ram, (7 z = 12 MPa

XAS/913

G/9~ XAS/69

G/69

900

800

~ 7OO

400

300

/ / t (ram)

/ ~ L5

I [ I I I 0 I 2 3 4 5

e / d

Fig. 10 Effect of out-of-plane buckling on the shear-out failure mode of 0 ° / ± 4 5 ° (½ 0 °, % 45 =) G /913 laminates, d = 6.35 ram, (Tz= 12 MPa

that of GFRP, and laminates with 913 resin are 22% stronger in bearing than those with Code 69.

One of the 0o/+45 ° laminates was made using 0.125 m m thick layers of G/913 prepreg (12 layers used, i e t = 1.5 mm) to investigate the differences in the strength of single- and double-layered laminates. The results are presented in Fig. 8, using values of d/t of 2.12 and 4.23 (double and single layers respectively).

Where the two curves start to separate, out-of-plane buckling began to occur for the thin specimens, and this resulted in a reduction in full bearing strength of about 15%. No other differences were observed.

The above results of strength vs width are typical of most materials and compare well with similar data. 1,3,5.6 Numerical values, however, are difficult to compare because of the many variables involved.

4. Effect of edge distance

As the edge distance decreases, the bearing failure mode changes to one of shear-out. This is a combination of in-plane and interlaminar shear failures, and the load-carrying capacity decreases as shown in Fig, 9 for the case of 0°/_+45 ° laminates. The transition region for this lay-up lies around e/d = 3 and the decrease in strength is much more gradual for GFRP laminates. Apart from the latter, the shape of the curves does not depend on the fibre/resin combination. Again, results compare well with previous ones. 1,3,5

Similar tests were carried out on a 00/+45 ° G/913 laminate using 0.125 mm thick layers and the results compared with those of a double-layer laminate. As shown in Fig, 10, the bearing strength was 15% lower, as expected, but shear mode strength was increased by about 20%, while the transition region shifted to e/d = 2.8. The increase in shear strength could only be attributed to more uniform interlaminar shear stresses because of the reduced thickness (smaller three- dimensional influence). The decrease of the e/d value for transition agrees with the observation of Godwin and Matthews 3 that "min imum e/d ]in order to achieve full bearing strength] depends on diameter, being less for larger values of d". In this work, d was kept constant but t was halved, thus doubling the d/t ratio, effectively causing the same change.

5. Effect of lay-up

The lay-up has a great effect both on the joint strength and on the mechanism of failure. Figs 11 to 14 present the results of tTb(ult) vs w/d or e/d for 00/_+45 °, 0o/90 ° and _+45 ° laminates of G/69 and G/913. Note the close similarities between corresponding curves for the two fibre/resin combinations and the differences between the behaviour of the various lay-ups. For example, note the high w/d value needed for the +45 ° laminates to achieve full strength, and similarly the high e/d value needed for the 0°/90 ° laminates.

900

800

7O0

:5 I -=soo

b 500

4oc

3o(

.1. 0

Z ' ; p ' - ° o o / / , / - . - o v ~ _"2" (+ 0,-~45 I f / -~--o°/9o

• - -~x--+45 °

I I I ~ I I I I t I I I 2 3 4 5 6 7 8 9 I0

w/d

Fig. 11 Bearing strength vs w / d ratio for var ious G/69 laminates. d = 6.35 mm, t = 3 ram, (Tz= 12 M P a

C O M P O S I T E S . A P R I L 1 9 8 5 9 7

Page 7: Bolted Joints

°°° t 800

60o

b 50O

400

. ; : L - ' ~ ' z ' - " ~ - - - - -

,/ "/ e o I 0 2 o

--x--O l.-t. 45 (7 0,-~-45 ) / / f / . . . . 0°/90 °

/ " " ~ " + 4 5 0 ,y / ' Z/' / ;

"[ I I I I I I I I I I 0 I 2 5 4 5 6 7 8 9 I0

w/d

Fig. 12 Bearing strength vs w / d ratio for various G/913 laminates. d = 6,35 ram, t - 3 ram, ~rz-- 12 MPa

90(

80(

70(

60( g

b = 5O(

400

300

- ~ , ~ x ' - - - ' ' ~ - - ' - ~ - "~'----"

/ / ..fl--; ---'---°-- S Q ~ / / / ~ , / -

/ , / / - . - 0 % 45" ¢~ 0*,-}4s*~ --o--0"/90" ---~-+-45"

de' I I I I I I 2 5 4 5 6 7

e /d

Fig. 13 Bearing strength vs e / d ratio for various (3/69 laminates. d = 6.35 ram, t = 3 mm, ~z = 12 MPa

9OO

800

~_ 7oo

3 600

b 500

/I

/J

~ " W " . . . . - O " - -

o o I 0 2 o 400 / - - x - - O /:1: 45 (~.. 0,~.45 )

- . . o - -00 /90 °

30o ---~--+-45"

,,// I I I I I I 0 2 3 4 5 6 7

e/d

Fig. 14 Bearing strength vs e / d ratio for various G/913 laminates. d = 6.36 mm, t = 3 mm, % = 12 MPa

The crack initiation and propagation pattern, as well as the shape of the failure area, depends strongly on the lay-up used. Selected photographs of failed specimens are shown in Figs 15 to 18. It should be noted that the woven appearance of the laminates is due to the pattern left by the bleed cloth employed during curing, and also that the photographs were taken by placing a strong light source behind the specimens to enhance the through-thickness details. The damaged area was much larger in the 00/90 ° specimens. For all lay-ups the mode of failure under the washer appears to be one of compressiorL with many in-plane shear cracks originating from the hole boundary and propagating in a direction either parallel to the fibres (resin failure) or at 45 ° to them (fibre and resin failure). Outside the area restrained by the washer, the mode of failure changes to one of delamination followed by (or possibly caused by) fibre buckling.

Fig. 15 Bearing failure of a 00/±45 ° (½ 0 °, ~ 45 °) G/913 laminate. d = 6.35 mm, t - - 3 mm, cr z = 12 MPa. Load left to right

Fig. 16 Tensile failure of a 0 ° / ± 4 5 ° (~ 0 °. ~ 45 °) G/69 laminate. d = 6.35 ram, t = 3 ram, cr z = 12 MPa. Load left to right

Fig. 17 Bearing failure of a 0 ° / 9 0 ° G/913 laminate, d = 6.35 mm, t = 3 mm, ~r z = 12 MPa. Load left to right

Fig. 18 Shear failure of a +45 ° G / 6 9 laminate, d = 6 . 3 5 mm, t = 3 ram, ~r z = 12 MPa. Load left to right

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Fig. 19 Bearing failure of a 0 * / + 4 5 * (~ 0", % 45*) G / 9 1 3 laminate showing the ef fec ts of out-of-plane buckling, d = 6.35 ram, t = 1.5 turn, ~rz= 12 MPa. Load left to right

A specimen that failed in the out-of-plane buckling mode is shown in Fig, 19 where it can be seen how the washer 'dug' into the laminate, leaving the unconstrained area almost damage-free.

Tension

knet and kgross, as defined by Equations (4) and (5) respectively, can be treated as measures of the various

F " o + o o SC s. Fig. 20 presents kne t and/<gross for 0 / _ 4 5 0 /3 0 , 2/3 45 °) laminates of the four f ibre/resin combinations tested. It can be seen that k,et decreases and k-ross . . . . . E,

increases with decreasing width, suggesting that most of the load is transferred in the material close to the hole. Material away from the hole will be inefficient and therefore an increase in width will not give a pro-rata increase in strength. The magnitudes of kne t and kgross are not characteristic of the type of material used but instead of the width and of the lay- up. It was found that for _45 ° lay-ups of GFRP, approximate values for kne t and kgross were 1.1 and 1.5 respectively, indicating a more uniformly distributed stress at the net section. The specimen thickness was found not to influence the tensile strength, as shown in Fig. 8 for a 00/___45 ° laminate of G/913.

o ac

I--

~ G/69 m ~ XAS/69

" ~kxx kgm= . . . . G/913

\ ~ . . . . - . . . . XAS/915

I I I I 0 I0 20 30 40

w (mrn)

Fig. 20 Tensile SCFs for GFRP and CFRP 0 * / ± 4 5 * ( ~ 0", % 45*) laminates, d = 6 .35 mm, t = 3 ram, Gz = 12 MPa

As shown in Fig. 16, the tensile failure mechanism is a combination of transverse fibre breakage and delamination. Furthermore, it was observed during tests that it was almost impossible to achieve a neat tensile failure with Code 69 material, while CFRP was much more brittle than GFRP.

Shear

The shear-out strengths of 00/__-45 ° laminates of CFRP and GFRP are compared in Fig. 2, and it is apparent that the general trend is the same for both materials. The test results indicate that CFRP is stronger than GFRP for the same resin system and, except for the case of XAS/69, a peak in strength was encountered at roughly e = 13 mm.

Moreover, it was found that the shear strength of all lay-ups was almost constant with edge distance (for all materials), thus agreeing with the finite element results presented by Wilson and Pipes 7 that the shear SCF does not depend drastically on e/d, but instead stays almost constant at the relatively high value of 5.

The effect of thickness is shown in Fig. 10 and, as previously discussed, thinner specimens resulted in slightly higher shear strengths while change-over to bearing mode was at a slightly lower value of e/d.

Other factors affect ing strength

The amount of_+a ° layers present in a 0°/___e ° laminate has a great effect both on strength and mode of failure. In order to investigate this, tests were carried out on a number of 00/-----45 ° laminates of G/69, containing 0, 33, 50, 67 or 100 % __+45 ° plies. A detailed description of the lay-ups is given in Table 2. The strength test results are presented in Fig. 22, where ~b denotes the percentage of ___45 ° layers. It can be seen that bearing and tension strengths were both maximum for a value of ~b approximately equal to 30 while shear strength reached a peak at q~ = 50. However, the choice of the best laminate for use with bolted joints cannot be made on the basis of strength criteria alone, since the weight of the material used for jointing is of equal

2 0 0

150

Q .

~" I00

5 0 -

\ ' , XAS/913

/••-•--•--XAS/69 / \ G/915

, /

/ - ~ 6 / 6 9

I I I I 0 5 I0 15 20

e(nvn)

Fig. 21 Var ia t ion of u l t imate s h e a r strength with edge distance for GFRP and CFRPO°/+45 ° (~ 0", ~ 45 =) laminates, d = 6.35 mm, t = 3 mm, ~z = 12 MPa

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I100:

I000

800

A

~" 600, =E

2 ~ 5 ~

4OO

50G

200

I00

==-~-'Tensile strength of unidirectional G169

r,.~.-Compre~ive strength of unidirectionol G/69

/ / 1 " 4 " = ' ' " ' ~ Bearing

_ / /

/ /

!

~ . . . . " ~ Tension

I cu la ted N °'r°'net (ult) - using ~ w = 15ram

Lekhni tsk i N N ~

N - N

\ N

N N

N - N

Shear \ "rxy (ult) \ \

. . . . . .

I I I I I I I I I 0 10 20 30 40 50 60 70 80 90 100

~b (percentage of +-45*plies)

Fig. 22 Joint properties of 0*/+45* G/69 laminates containing various proportions of :t:45" plies d = 6.35 mm, t = 3 mm (except for ~ = 50%, t = 4 mm), trz= 12 MPa

importance. That is, in order to achieve full bearing strength, w/d and e/d have to lie above certain minimum values, and these are also dependent on ~b. For the case o f d = 6.35 mm and t = 3 mrn, these minima are plotted against ~b in Fig. 23 (these curves can only be approximate since the full strength is reached asymptotically). Hence, the ratio:

O b ( u a ) e = (9)

rr d2 Wmin emin --

can be calculated for all the 0o/4-45 ° laminates, thus providing a measure of efficiency. By plotting e against ~b (see Fig. 24), it becomes evident that the best results

80

70

60

50

E 40

E

20

I0

't

t

\ \ \ \ \ \ \ \ \

\ \ \ \ ~ \ \

- \ \

\ x

J ~ ~ ~ ~ ,,,,,. ~ I ~ ....~ ,.~- - ~ "

I I I I I I I I I 0 I0 20 30 40 50 60 70 80 90 1130

Fig. 23 Minimum values of width and edge distance required to obtain full bearing strength for 0"/:1:45" G/69 laminates, d-- 6.35 mm, az= 12 MPa

are achieved with 40% 0 °, 60% 4-45 ° laminates, justifying the fact that most of the present work was done using 33% 0 °, 67% 4-45 ° laminates.

The net tensile strength of a unidirectional specimen (as shown in Fig. 22) was predicted using an equation due to Lekhnitski, which is (quoted from Reference 1):

kgross=l+ 2 - v12 + ~-- (10)

where El is Young's modulus in the loading direction; E2 is Young's modulus normal to the loading direction; G is the in-plane shear modulus; and v,2 is the principal Poisson's ratio.

Equation (10) strictly applies to unloaded holes, but the variation of kgross with q~ should be similar for both loaded and unloaded cases.

Finally, the effect of stacking sequence homogeneity was investigated by testing two 2/3 0 °, 1/3 -t-45 ° laminates, laid as described by sequences (8) and (9) in Table 2. Sequence (9) was less homogeneous than (8) and resulted in a 19% lower bearing strength, while tensile and shear strengths remained constant (within experimental accuracy). Very similar results have been presented by Callings for CFRP) ,s

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0.8

0.7

0 . 6 -

: . 0 . 5 -

0 .4 -

~u

0 . 3 -

0 . 2 -

0 . 1 -

I I I I I I I I I 0 I0 20 3 0 4 0 50 6 0 7 0 BO 9 0

Fig. 2 4 Eff ic iency of 0 " / : t : 4 5 " G / 6 9 laminates in bearing

I00

Genera/remarks

There is a great amount of information contained in the load/displacement graphs drawn by the testing machine during loading. Typical curves are presented in Fig. 25, and it is interesting to note that bearing failures are a lot less catastrophic than the tensile or the shear modes. In addition, 00/90 ° laminates were observed to behave in a very 'noisy' fashion as the load approached maximum, as also indicated by the multiple drops on the load/displacement curve (see Fig~ 25(c)).

One of the most important aims of this work was to compare the behaviour of GFRP with that of CFRP in bolted joints. For the same specimen geometry, the

00/___45 ° (1/3 0 °, 2/3 45 °) laminates were found to be about 20% stronger for CFRP than for GFRP. Further- more, a stronger interaction between the different modes of failure was observed for GFRP as shown by the more 'spread out' transition regions. For the same geometry again, the bearing and tensile modes were similar for the two materials, provided the specimens were sufficiently clear of the transition regions, and also the shear mode was more easily obtained for GFRP. However, since only 0°/-t-45 ° CFRP lay-ups were examined here, additional tests to the present ones should be carried out to obtain a complete set of data for comparative purposes. Finally, GFRP showed a tendency for delaminations rather than in-plane shear or tensile cracking and the effect was most pronounced in the transition region between tensile and bearing failure modes.

CONCLUSIONS

The tests on single-hole bolted joints showed that the effects of width, edge distance, hole size, bolt clamping pressure and stacking sequence were in general similar for both GFRP and CFRP, while different resin systems only resulted in different strength levels. However, the effect of laminate thickness was found considerably more important for GFRP because its lower stiffness favoured instability effects which reduced its strength performance. The best lay-up was judged to be a 0°/-t-45 ° lay-up with 60% ___45 ° plies and a homogeneous stacking sequence.

Finally, the micromechanisms of failure for the bolted specimens were found to be heavily dependent on the lay-up. The material used was also significant, an increased number of delaminations being observed for GFRP compared with CFRP, together with a stronger interference between modes of failure.

,sL

IC

. X T e n s i o n

15

I I

5

I I

I ' a I I I I I I l I I I I 0 I 2 3 4 5 0 I 2 3 4 5 4

C r o s s h e Q d displacement (ram)

° I

5F

I

I I I o I 2 3

Fig. 2 5 Load /d isp lacement curves for G / 6 9 laminates, d = 6 . 3 5 mm, t = 3 mm, ~z = 12 MPa. (a) 0 ° / : t : 45 ° ( ~ 0 °, ~ 45=); bearing - - w = 4 0 mm, e = 3 0 ram; tens ion - - w = 17 ram, e = 2 5 mm; s h e a r - - w = 4 0 mm, e = 16 ram. (b) + 4 5 ° ; bearing; w = 6 0 ram; e = 4 0 ram. (c) 0 ° / 9 0 ° ; bearing; w = 4 5 ram; e = 4 0 m m

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ACKNOWLEDGEMENTS

This work was f inanced by the Minis t ry of Defence (P rocurement Executive). The authors would like to express thei r grat i tude to T.A. Col l ings for his he lp and guidance.

REFERENCES

l Collings, T.A. 'The strength of bolted joints in multidirectional CFRP laminates' RAE TR 75127 (Royal Aircraft Establishment, UK, 1975)

2 Stockdale, J.H. and Matthews, F.L 'The effect of clamping pressure on bolt bearing loads in glass fibre-reinforced plastics' Composites 7 No 1 (January 1976) pp 34-38

3 Godwin, E.W. and Matthews, F.L 'A review of the strength of joints in fibre-reinforced plastics, Part 1: Mechanically fastened joints" Composites 11 No 3 (July 1980) pp 155-160

4 Kutschn, D. and Hofer Jr, K.E. 'Feasibility of joining advanced composite flight vehicle structures' AD 690616 (lIT Research Institute, Chicago, IL USA, January 1969; Report No AFML-TR-68-391, US Air Force Materials Laboratory Contract)

5 Lehman, G.M, and Hawley, A.V. 'Investigation of joints in advanced fibrous composites for aircraft structures, etc' AD 861165 (Douglas Aircraft Co, Long Beach, CA. USA. January 1969: Report No AFFDL-TR-69-43, US Air Force Flight Dynamics Laboratory Contract)

6 Soni, S.R. 'Failure analysis of composite laminates with a fastener hole' in 'Joining of Composite Materials', ASTM STP 749 edited by K,T. Kedward (American Society for Testing and Materials, 1981) pp 145-164

7 Wilson, D.W. and Pipes, ILB. 'Analysis of the shearout failure mode in composite bolted joints' in "Composite Structures' edited by I.H. Marshall pp 34-49 (Applied Science Publishers, 1981)

8 Collings, T.A. 'On the bearing strengths of CFRP laminates' Composites 13 No 3 (July 1982) pp 241-252

AUTHORS

The authors are with the D e p a r t m e n t of Aeronaut ics , Imper ia l College of Science and Technology, Pr ince Consor t Road, L o n d o n SW7 2BY, UK. Inquir ies should be addressed to M r Mat thews in the first instance.

1 0 2 COMPOSITES . APRIL 1 9 8 5