Adhesive joint specimens

Fig. 1. The end-loaded split (ELS) test fixture and adhesive joint specimen.

Figure 14 Photographs showing the loci of crack growth for different double cantilever beam adhesive joint specimens pulled to failure, (a) The ERR calculated by FEM for this specimen was largest down the center of the adhesive, (b) the ERR calculated by FEM for this specimen was largest near the lower edge of the adhesive, and (c) the ERR calculated by FEM for this specimen was essentially independent of position across the adhesive thickness.

Fracture Tests on Adhesive-Joint Specimens 20.3.1 The Mode I Double Cantilever Beam (DCB) Test... 

Mode I double cantilever beam (DCB) adhesive-joint specimen... 

Deck-to-hull structural adhesive joints specimen geometry (left) and test jig right). Parts made of GFRP... 

Figure 4. Compression Rig for double cantilever beam adhesive joint specimen adherend.

The principal type of shear test specimen used in the industry, the lap shear specimen, is 2.54 cm wide and has a 3.23-cm overlap bonded by the adhesive. Adherends are chosen according to the industry aluminum for aerospace, steel for automotive, and wood for constmction appHcations. Adhesive joints made in this fashion are tested to failure in a tensile testing machine. The temperature of test, as weU as the rate of extension, are specified. Results are presented in units of pressure, where the area of the adhesive bond is considered to be the area over which the force is appHed. Although the 3.23-cm ... 

Eqs. 1-5 hold whether failure is interfacial or cohesive within the adhesive. Furthermore, Eq. 5 shows that the reversible work of adhesion directly controls the fracture energy of an adhesive joint, even if failure occurs far from the interface. This is demonstrated in Table 5, which shows the static toughness of a series of wedge test specimens with a range of adherend surface treatments. All of these samples failed cohesively within the resin, yet show a range of static toughness values of over 600%. 

Great caution must be exercised in exposing any adhesive joint to the simultaneous effects of environment and stress. The stress can act to accelerate the degradation caused by the environment, and vice versa. Joints that will be exposed to both high-humidity environments and high load at the same time are especially vulnerable, and prototype specimens need to be tested. This degradation mechanism and the performance of several epoxy adhesive systems to combined environmental stress conditions are discussed in Chap. 15. 

In addition to tests performed on the incoming materials, test specimens may be made to verify the strength of the adhesive joint. The quality control tests should be those that can quickly and accurately detect deficiencies in the adhesive s physical or chemical properties. ASTM lists various test methods that are commonly used for adhesive acceptance. 

Structural adhesives usually require curing by the application of heat, the addition of a catalyst, the addition of pressure, or a combination of the three. The strength developed in the adhesive joint at various times during the curing process may be determined by lap shear tensile specimens. This test is commonly used to determine when an adhesive or sealant is fully cured or when the system reaches a handling strength so that the assembled product can be moved with moderate care. 

ASTM D 1144 provides a recommended practice for determining the rate of bond strength development for either tensile or lap shear specimens. However, peel and can-teliever tests can also be used effectively. Measured bond strength values of partially cured test specimens are compared with those of a reference (i.e., fully cured adhesive joint) to assess the extent of cure. This method may suit some applications, but it is limited in accuracy because it does not directly measure the degree of cure in the adhesive, and the effect on the joint design and substrates may override the effect of cure development. 

The rotational viscosity method described above to measure working life or pot life is a form of rheological measurement of cure. However, cone and plate rheometry is preferred for accurate measurements because the specimen size and geometry are similar to those that occur in an adhesive joint. 

D 4896 Guide for the Use of Adhesive Bonded Single Lap-Joint Specimen Test... 

Because of the high scattering of experimental results and the great difficulty in reaching the fully cohesive failure of wooden adhesive Joints, a numerical analysis has been made to give a better knowledge of their mechanical behaviour for various parameters (adhesive used. Joint thickness, loading mode, etc...). For the PU resin tested previously in shear, such an analysis has been made on two steps first simulations have been made on bulk adhesive specimen to determine the mechanical behaviour of the resin and the numerical results obtained have been implanted in the FE code CASTEM 2000 [21] for the mTENF bonded specimen loaded by shear. 

It was mentioned above that the simulation method of Termonia [67-72] can be used to calculate the stress-strain curves of many fiber-reinforced or particulate-filled composites up to fracture, including the effects of fiber-matrix adhesion. Such systems are morphologically far more complex than adhesive joints. Many matrix-filler interfaces are dispersed throughout a composite specimen, while an adhesive joint has only the two interfaces (between each of the bottom and top metal plates and the glue layer). If one considers also the fact that there will often he a distribution of filler-matrix interface strengths in a composite, it can be seen that the failure mechanism can become quite complex. It may even involve a complex superposition of adhesive failure at some filler-matrix interfaces and cohesive failure in the bulk of the matrix. 

FIG. 4—Experimental failure load versus bondline thickness for single-lap joint specimens withAraldite 2011 adhesive (from [8]). 

FIG. 8—Schematic of a typical geometry for DCB adhesive joint fracture specimens (shown with adherends of equal thickness) that was analyzed and experimentally tested. The forces (P) were applied by an Instron Universal Testing Machine. 

The dismandabihty of joints bonded with the adhesive was evaluated experimentally. Fig. 34.5 shows the configuration and the dimensions of such joints. Specimens of three sizes (20 mmx20 mm, 50 mmx50 mm and 100 mmx 100 mm, aU with 0.3 mm bond hne thickness) were prepared. The adherends of the specimens were made of aluminum alloy (5052H34, Si <0.25%, Fe <0.40%, Cu <0.10%,... 

Fig. 34.5 Configurations and dimensions of joint specimens bonded with dismantlable adhesive.

Fabric Peel Test Samples were cut from a plain weave cotton fabric (306 g/m ). Samples measuring 2.5 x 8 cm were cut, submerged in phosphate buffer (pH = 7.2), and allowed to air dry for 25 min. MFC measuring 350 (J-L (used as a model for cyanoacrylate-based tissue adhesives) was spread over a 2.5 X 6 cm area of one fabric sample, and a mating sample was placed on top. A 1200-g weight was set on top of the specimen for 1 min. The specimen was then allowed to cure for 1 h. The unglued portion was gripped into an MTS MiniBionix (Model 858) and the force required to separate the films at a displacement rate of 5.0 cm/min was measured. The maximum load after the initial peak was used to calculate the peel force of the adhesive joint. ... 

There are, of course, an infinite number of m values that can be chosen to satisfy Equation 13.46. The determination of m is governed by a basic assumption that the entire energy supplied to the specimen is concentrated on the crack line for crack extension. Thus the choice of m is such that the bending stresses on the adherend are minimized. This, in turn, depends on the modulus of the adherends relative to the adhesive. This necessitates an empirical determination of the appropriate m value through a procedure known as compliance calibration. Figure 13.37 is a schematic of the specimen geometries of TDCB for the determination of Gj, for bulk adhesives and adhesive joints. 

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