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Pin and collar, Adhesion Strength and Scratch Test

There are a number of other mechanical test methods, which are used for quality assurance purposes or are in the process of being developed to measure properties, such as adhesion strength. This section provides a brief description of some of the more commonly used methods that fall into the first category and recent developments in new methods.

Pin-And-Collar


Loading arrangement

Pin-and-collar specimen

Pin-and-collar test

The pin-and-collar test is used to compare shear strengths of adhesives. It is used for quality control purposes only, and not for joint design. The test specimen consists of a cylindrical pin (12.65 to 12.675 mm in diameter) and a slip collar (12.7 to 12.725 mm inner diameter and 11.05 to 11.15 mm wide) with a clearance fit between the pin and collar. The pin and collar can be made of any material, but the most common material is steel. Adhesive is applied to the diameter of the pin and the collar placed onto the pin, ensuring that the inner surface of the collar is fully coated with the adhesive and that there are no air bubbles or voids trapped between the pin and collar. After the adhesive has been cured, the specimen is placed in a test machine with the collar supported by a ring on a compression plate. An upper plate applies a compressive load to the top end of the pin and pushes it through the collar. The load during the test is recorded and the force required to shear the adhesive bond is used to calculate the shear strength of the adhesive. The test method is covered in a number of standards (ISO 10123 and ASTM D 4562 [1, 2]). Pin-and-collar test is usually used in thread-locking applications (e.g. anaerobic adhesives).


Adhesion Strength Tests

There are a large number of techniques that are used for quantifying adhesion strength. Many of these are qualitative, assessing adhesion from the appearance of the fracture surface. Some types of joint test, such as the tensile butt joint or thick adherend shear, can provide good quality engineering data on adhesives, but the stress concentrations near the joint ends make interpretation of local failure stress and strain values a problem. Failure modes often tend to be a mixture of both interfacial and cohesive, and it can be difficult to detect the point of first failure. In this section, three four potential methods will be described. These are listed below [3, 5]:

Pull-off test

Pull-Off Test: This method is used widely to test the adhesion of coatings to substrates and is also used to assess adhesives. The test is attractive as it is quick and simple to perform, requires low cost equipment and produces a quantified measure of the adhesive strength from the maximum force applied to the sample. It is critical in using the test that failure does not occur at the interface between the aluminum stub and the adhesive. The test gives good repeatability and can differentiate between good and bad surface treatments. In the case of good surface preparation, the failure tends to be cohesive in the adhesive with much greater levels of adhesion failure seen with poorer surface treatments [4, 5]. There is often substantial deformation of the adherend in the test that will result in high cleavage stresses. Also, there is little control over specimen alignment.

Stress distribution across hemispherical adherends in a modified butt joint [4]

Profile Butt Joint: The more tightly controlled tensile butt joint produces strength values that are in reasonable agreement with bulk strengths of adhesives. Although specimen preparation and testing is time consuming, the test produces very reliable results. Finite element analysis (FEA) predictions show that stress concentrations exist near the outer rim of the adhesive layer even when the edges are profiled. The modified butt joint specimen with butted adherend ends consisting of interlocking concave and convex hemispheres with 50 mm radii of curvature, producing a bowl shaped adhesive layer appears to offer a method for determining reliable adhesion strengths. The region of highest stress extends from the centre of the specimen over the majority of the adhesive layer. Stress values near the rim are lower. The highest stress predicted is only a few percent greater than the average stress (calculated from the force divided by the bonded area).

Pull-out specimen

Tensile Pull-Out: This method of determining adhesion strength was originally developed for fibre-reinforced plastics in which adhesion is related to the force required to pull embedded fibres from the resin. Analytical routines, based on elastic shear lag analyses, were developed to calculate stresses along the interfaces [5]. The specimen is relatively easy to prepare and testing is straightforward with a simple fixture. The largest stresses are at the interface, biasing the test towards interfacial failure and this point is easily identified from the sharp decrease in load.

The adherend samples must be thin, as a sufficient thickness of adhesive is required around the embedded sample. As the thickness of the adhesive increases, preparation of the adhesive block becomes more difficult. In general, there is a high degree of scatter with inelastic behaviour occurring at the point of interface failure. Also, shear stress at the interface does not seem to correlate with failure. Intense stress concentrations occur near the exit point of the shim (stress near the exit of the adherend is ~3 times the stress along the centre of the adherend [4]. The extensive inelastic deformation of the adherend and the resulting complexity of the interfacial stress distribution lead to problems in the interpretation of results from this test.

Three-point bend test, ISO 14679

Flexural Test Methods: Flexural loading of a laminate introduces interfacial stresses between layers with delamination occurring if the laminate strength is exceeded. This principle can be applied to testing of adhesive joints. The three-point bend test, ISO 14679 [6] has been developed for adhesives. A thick rib of adhesive is applied to the middle of the adherend. The adherends used are rectangular with a length of 50 mm and a width of 10 mm. These adherends are reasonably thin in comparison with the adhesive rib so that the specimen bends easily under load. The adhesive rib (25 mm long, 5 mm wide and ~4 mm thick) acts as a stiffener and is centrally located on the underside of the adherend.

Three-point bend adhesion specimen

As load is applied, the specimen bends and peel stress is generated at the interface between the adhesive and adherend with maximum stress occurring at the ends of the bond. The onset of delamination in these regions leads to a reduction in load. In weakly bonded joints or when very low modulus adhesives are tested the load reduction may be very small and difficult to detect. Test specimens should fail by delamination at the adhesive/adherend interface. This is not always the case. Flexible adhesives tend to undergo cohesive rupture at the base of the rib. Adding a reinforcing strip to the outer edge of the adhesive rib tends to promote failure at the adherend-rib interface. As the adhesive rib is relatively thick, the technique does not readily lend itself to anaerobic adhesives. Furthermore, if the adhesive cures rapidly large temperature rises may occur in such thick adhesive samples, damaging the material.

Specimen manufacture and testing is relatively straightforward. Although data reduction is also straightforward, it has been shown that the analytical predictive analysis considerably underestimates interfacial shear strength as the analysis assumes linear-elastic behaviour and is only applicable to the center of the beam rather than at the ends of the bondline [5].

Scratch Test

The adhesion scratch test is a commonly used method of assessing coating adhesion [7, 8]. The test is a variant on a hardness test and is designed to generate stresses at the interface between the coating and the substrate, which exceed the interfacial bond strength of thin, high adhesion coatings or films. A loaded diamond tipped stylus (10 to 60 N) is drawn across the target surface (or the sample is displaced beneath the stylus) under an increasing load (typically 100 Nmin-1) until some well-defined failure occurs, usually flaking or chipping. The horizontal displacement rate is nominally 10 mm.min-1 with sample size being typically 25 mm x 15 mm.

During the test, the penetration depth and stylus position are recorded. These data can be supplemented with the vertical indenter load, the horizontal force on the indenter and acoustic emission to enable the coefficient of friction and the point of failure to be determined. The scratch can be subsequently analysed with a profilometer, SEM or optical microscopy to ascertain the scratch shape (residual depth, scratch width and pile-up height) to allow the failure mechanism to be identified. Varying the loading rate, the scratch speed and the indenter shape markedly affect the results of the test. Scratch testing equipment is commercially available. Scratches can be single or multi-pass. There has been recent collaborative work involving NPL to provide a scratch test calibration procedure, draft standards and reference materials.

References

Next: Table 1 - Tensile and Peel Test Methods


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