Joint Testing

(Courtesy FEICA)

The choice of test method depends on the properties of the bonded joint in question. Bond strengths are determined using the lap shear test, peeling resistance using the floating roller peel test or T-peel test and adhesion behavior using for example the wedge test. In order to obtain information about the long-term stability of bonds, the samples are equilibrated under the likely conditions of the actual surroundings (equilibration under hot humid conditions, sweating test, salt spray test, cataplasma test, stability test under changing climatic conditions) and are then subjected to mechanical tests.

In the mechanical tests the bonded joints are loaded to the point of fracture. The nature of the fracture (adhesive fracture, cohesive fracture, substrate fracture or a combination of these) provides information about the quality of the bond and about any potential production errors. Regarding tests on samples equilibrated under controlled conditions, a cohesive fracture indicates that the adhesion remained stable despite the influences acting on the bond (e.g. moisture). In contrast, an adhesive fracture indicates that the bond has failed at its most sensitive point, the adhesion. This may indicate incompatibility between the substrate material and adhesive, inadequate surface pretreatment and/or processing/application errors.

Possible processing/application errors are:

  • Pot life/skinning time exceeded
  • Surface too cold
  • Adhesive too cold
  • Adhesive stored for too long
  • Mixing errors

Lap Shear Test in Accordance with DIN EN 1465

The lap shear test is the most commonly used standard test for determining the strength of medium-strength and high-strength bonds. The bond strength of bonded single lap joints on subjecting the substrates to loads is determined by lap shear forces in the direction of the bonded joint. In accordance with the standard, the overlap corresponds to a width of 25 mm and a length of 12.5 mm. Each sample must be measured individually. To do this, a sample is clamped in the self-aligning jaws of the test unit so that the force acts in the centre of the bonded layer, as illustrated below

In accordance with the standard, the test is carried out in such a way that the period of time for separation of the substrates is 65 ± 20 s, with the speed of movement of the jaws being constant. The highest force Fmax that acts can then be read from the force gauge and recorded. The bond strength in N/mm2 (MPa) is then calculated using the following equation:

                                    Fmax        Fmax
Bond Strength  =           -------   =   ------
                                      A            LÜ·b  

where:

Fmax = highest force in N
LÜ = overlap length in mm
b = average sample width in mm
A = bonded area in mm2

Wedge Test in Accordance with DIN 65448

The wedge test is one of the few test methods that allows testing of the quality of bonds under the influence of mechanical, chemical and media loads. In order to carry out the wedge test two metal sheets of a prescribed thickness and pretreated under production conditions are bonded to each other. A wedge is driven into the bond and the end of the crack that is produced is marked. Thereafter the prepared sample is equilibrated under hot humid conditions (e.g. 75 minutes at 50°C and 95% humidity) or in water. The bond which is under stress from the wedge is possibly forced further apart. After being equilibrated the progress of the crack is marked and measured, the bonded area is separated and the surface of fracture is evaluated. The advantage of this test compared to the lap shear test is the fact that results are obtained relatively quickly and direct information is obtained about the effect of aging conditions on the adhesive layer when it is exposed to mechanical loads, as illustrated below

Peel Test

Peel tests determine the resistance of bonded joints to peeling forces. They are chiefly used for comparative evaluation of adhesives and surface treatment methods because they are very sensitive at discerning differences in the adhesion and cohesion behavior in the adhesive film. To carry out these tests, samples are subjected to tensile forces on their non-bonded sides until the adhesive film tears and both halves of the sample are apart. The force required to do this is recorded as is the change in distance between the clamp grips. The result of a peel test is a peel diagram. The fracture pattern provides information about the stability of the adhesion, as illustrated below.