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Shear Tests

A quick perusal of the ASTM and ISO standards (Appendix 1) will quickly reveal a multitude of shear test geometries, each with the objective of producing a state of pure uniform shear in bulk adhesive or within an adhesive layer. This section critically evaluates the following test configurations listed below (see also Tables 3 and 4):

Lap Joints

Various lap joint configurations (with increasing joint improvement from top to bottom)

Single-Lap Joint: Despite all its obvious weaknesses, the lap-shear test is the most widely used method for producing in-situ shear strength data of an adhesively bonded joint. The test consists essentially of two rectangular sections, typically 25 mm wide, 100 mm long and 1.6 mm thick, bonded together, with an overlap length of 25 mm [23]. Variations of this test method are included in both national and international standards [23–25] (see Appendix 1). The single-lap specimen is easy to prepare and test. A fixture is used to ensure correct overlap and accurate alignment of the adherend. This may include control of the fillet. Testing can be conducted using standard tension/compression mechanical test equipment.

Side-view of single-lap joint with end tabs to reduce eccentric loading

The lap-shear strength is given by:

where P is the maximum load, b is the joint width and L is the joint length.

Relative deformation of a single-lap joint for different substrate materials

Typical single-lap joint load-displacement response for different substrate materials

The analysis assumes the adherends are rigid, and that the adhesive only deforms in shear. In fact, the resultant stress distribution, across and along the bond length is very complex. The eccentricity of the load path causes out-of-plane bending moments, resulting in high peel stresses and non-uniform shear stresses in the adhesive layer. This effectively reduces the structural efficiency of the joint [6].

Non-uniform shear distribution along bondline

Increasing adhesive thickness results in a more compliant joint to shear stress. The extra adhesive thickness distributes the shear strain over a larger dimension, lowering the strain per unit length and the stress concentration at the ends of the bondline. Alternatively, using an adhesive with a lower modulus will have a similar effect. An increase in joint width results in an increase in joint strength. Failure load increases in the same proportion as the joint width increases (i.e. doubling the width will double the failure load). This is achieved without affecting the shear stress distribution within the adhesive joint.

Table 5: Failure Load Per Unit Width for AV119 Epoxy Adhesive Joints

Adherend
Thickness/Overlap Length
Load/Width
(N/mm)
CR1 Mild Cold Rolled Steel
1.5 mm thick/12.5 mm overlap
2.5 mm thick
  • 12.5 mm overlap
  • 25.0 mm overlap
  • 50.0 mm overlap

334 ± 11


354 ± 10
428 ± 38
633 ± 63
5251 Aluminium Alloy
1.6 mm thick/12.5 mm overlap
3.0 mm thick/12.5 mm overlap

191 ± 14
325 ± 28
6Al-4V-Titanium Alloy
2.0 mm thick/12.5 mm overlap

457 ± 52
Unidirectional T300/924 Carbon/Epoxy
2.0 mm thick/12.5 mm overlap

369 ± 41
Plain Woven Fabric (Tufnol 10G/40)
2.5 mm thick
  • 12.5 mm overlap
  • 25.0 mm overlap
  • 50.0 mm overlap
5.1 mm thick/12.5 mm overlap



275 ± 28
454 ± 27
511 ± 32

327 ± 27

Note:    The “apparent” shear strength measured using lap joints is given in terms of load per unit width (N/mm) rather than load per unit area (i.e. stress).

Failure load does not increase proportionally with increasing bond length (Table 5). Although increasing the overlap length reduces the average shear stress, the increase is not in proportion to the increase in bond length (NB. The shear stress distribution is non-uniform with the ends of the joint resisting a greater amount of stress than the middle of the bond). In order to increase the load capacity of the joint, it is better to increase bond width rather than bond length. As the lap joint length increases, the mean shear stress decreases, and thus the shear stress concentration at the end of the joint increases. For practical design purposes, the optimumlength-to-thickness L/t ratio is 30. Beyond this limit, any additional increase in the value of L/t is ineffective in reducing peak adhesive shear and peel stresses. This limit is known as the ineffective length.

Efforts to reduce stress concentrations formed at the bondline ends have included the use of tapered or bevelled external scarf and radius fillets at the bondline ends (NB. The use of absolutely rigid adherends will not prevent the formation of stress concentrations at the bondline ends). Tapering the ends can prevent premature failure, but the shear strength of the joint remains unaltered.

Significant increases in the “apparent” shear strength of single-lap joint, compared with square-ended bondlines, can be achieved through the formation of a spew fillet at the overlap ends [26]. Further increases in strength may be achieved by rounding the ends of the adherend.

The main problem with the single-lap shear test is that the average shear strength determined using this method does not correspond to a unique material property of the adhesive and therefore cannot be used as a design parameter (NB. Strength is strongly dependent on the joint geometry).

The rapid and continuous changes that occur to the shear and normal stress distributions within a single-lap shear specimen with increasing monotonic loading raises doubts about tests involving creep or fatigue. Exposure to elevated temperatures or hostile environments can only compound this uncertainty. The test, however, is widely employed to evaluate problems associated with service conditions, providing the user with comparative data on long-term performance of adhesive joints. Both ISO 9664 [27] and ASTM D 3166 [28] specify a method, based on the single-lap joint, for the measurement of fatigue strength. Environmental tests often involve a train of standard or miniaturised joints with or without the application of external loading. The factors that influence the tolerance of the single-lap shear joint to either static or cyclic loading include on lap length (ideally 30 times the joint thickness) adhesive and adherend elastic moduli and thickness and bondline thickness [19].

Double-Lap Joint: Attempts to eliminate eccentric loading, responsible for bending of the adherends and rotation of the bonded region, have resulted in the development of a symmetric variant of the single-lap shear test, known as the double-lap joint. However, bending of the outer adherends is unavoidable, since the load is applied to the outer adherends via the adhesive, away from the neutral axis. The bending moment introduces tensile stresses across the adhesive layer at the free end of the overlap and compressive stresses at the other end. The centre adherend is free from the net bending moment. Double-lap joints are twice as strong as their single-lap counterparts. The joint efficiency of double-lap joints is considered good. Various constructions of this test configuration are included in both national and international standards [25, 27, 29] (see Appendix 1). Adherend dimensions are identical to those employed for single-lap shear.

Advantages Disadvantages

Yields “apparent” shear strength

Compatible with metals, plastics and PMCs

Straightforward/economic

  • Specimen fabrication

  • Testing

  • Data reduction

    BS EN 1465/BS 5350: Part C5/ASTM D 1002/ASTM D 3166

    Suitable for cyclic/environmental testing (QA only)
  • Geometry dependent

    Limited to rigid adherends

    Not suitable for generating design data

    Elevated shear and peel stresses at bondline ends

    Moderate to high bending moments

    Failure attributed to peel stresses

    Special bonding fixture required

    Large uncertainties in measurements

    Modified Lap Joints: A number of design modifications have been recommended to reduce/remove peel stresses associated with single-lap and double-lap joints, and thus increase the strength of the bonded joint. An example is the double-butt strap lap joint with and without tapering (internal and external). Tapering the adherends, minimises peel stresses, increases bond efficiency (i.e. strength) and alters the failure mode from peel to shear. The peel stresses virtually disappear when using external tapers with 30° fillets [6, 19]. Both configurations (i.e. 30° and 90° fillets) are suitable for determining shear behaviour under fatigue, creep and environmental assessment of shear behaviour, with butt separation monitored using extensometers. The cost and time involved in fabricating specimens with internal and external tapers are well in excess of the double-strap joint.

    Tapered (bottom) and bevelled (top) strap joints

    In practice, a large number of bonded systems may need to be evaluated in relatively short time spans. The test specimen needs to be simple and quick to prepare and sensitive to environmental effects. For these reasons a perforated short diffusion path configuration of the single-lap joint geometry has been adopted by a number of industries [30]. Specimens are typically 20 mm wide and 120 mm long with either three 3 mm or 4 mm diameter holes drilled through the bonded section of the specimen. Using the smaller diameter holes reduces the possibility of the joints failing by yielding and fracture of the material between the drilled holes.

    Perforated single-lap joint configuration

    Next: V-Notched Beam Test


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