Joint peeling

When a weight was placed on the model bridge, as shown in Fig. 15.3(b), the joints resisted failure until a certain critical load was reached. At this critical load, that is the failure load, the joints peeled apart and the model bridge collapsed. With 10 mm wide strips the failure load was 70 g. Howeva, when impact load was used, the weight required was less, around 40 g. Under wet conditions the load was also Iowa, around 20 g. 

It is evident from this model that the lap joint peels in essentially the same way as the T joint, except for the extra elastic stretching of the material caused by the higher force required. If we assume that the force F depends on the work of adhesion W in the same manner as the peeling joint, but also on the elastic modulus E of the strips and on the size L of the strips, then it is obvious from a simple dimensional analysis that the force must be given by an equation of the kind... 

The correct theory of lap joint peeling can be derived with reference to Fig. 15.9 which shows a long joint which has already aacked substantially. 

Figure 15,9. A long lap joint peeling under the force F. As the crack penetrates further, the shaded elements A and B change their energy state.

A second paradox is the use of the word shear to describe the fracture. It is evident from the calculation used to obtain Equation (15.5) that shear is not mentioned. The joint peels but does not slide or shear (Fig. 15.11). Only tension forces and displacements are needed to explain the failure of the joint. In fact, it would be far more logical to describe this failure as a tension failure, just as the Griffith equation describes tension failure. Of course, shear stresses exist around the crack tip, as in every crack geometry known, but the energy associated with these stresses remains constant as the crack moves and therefore cannot drive the crack. 

As described in Section II of this chapter, there are many types of peel tests available to characterize adhesives. These tests are important because peel stresses arise in the loading of many joint geometries, such as lap joints. Peel tests are severe because they constitute a test of the adhesive in its weakest stress mode. However, the peel test is a comparative test for adhesives and is dependent on many parameters. These parameters, such as peel speed, peel angle, bond thickness, and temperature, must be held constant to obtain valid results. The stress analysis of peeling is highly complicated because of these variable dependencies. 

Fig. 12. The changes in the elements A and B in the lap joint as the joint peels. Element A expands as the force increases from F/2 to F. Element B shrinks as the force is relaxed from F/2 to 0.

Single- and double-lap Joints, single- and double-strap Joints, peel Joints and cylindrical Joints... 

The usual practical situation is that in which two solids are bonded by means of some kind of glue or cement. A relatively complex joint is illustrated in Fig. XII-14. The strength of a joint may be measured in various ways. A common standard method is the peel test in which the normal force to separate the joint... 

Fig. XII-15. Diagram of peel test. A and B, adhesive joint C, double Scotch adhesive tape and D, rigid support. (From Ref. 107. By permission of IBC Business Press, Ltd.)...

A sealant s adhesion is commonly studied by 180 degree peel tests such as ASTM C794 or by tensHe/adhesion joints tests such as ASTM C719. The adhesion test protocol should simulate actual field conditions as closely as possible. Sealants often have good adhesion to dry substrates, but this adhesion may be quickly destroyed by water. Because most sealants are exposed to water over their lifetime, adhesion testing should include exposure to water for some length of time. ASTM C719 is one of the better tests to determine a sealant s adhesion durabHity as it exposes sealants to seven days of water immersion. 

Another distinction to be made is illustrated with the peel test shown in Fig. 1. Application of stress may cause the joint to fail either adhesively or cohesively . Adhesive failure, shown in Fig. la, is thought ideally to correspond to a perfect... 

Fig. 1, Schematic of commonly u.sed methods for testing the strength of adhesive joints, (a) Peel test. Note that the peel angle can be changed depending on the test requirements, (b) Double overlap shear test. In this test, the failure is predominantly mode II. (c) Single overlap shear test. In this test the failure mode is mixture of mode I and mode II. (d) Blister test.

Fig. 25. Evolution of the tack of polychloroprene-aromatic hydrocarbon resin blends as a function of the resin content. Tack was obtained as the immediate T-peel strength of joints produced with 0.6 mm thick styrene-butadiene rubber strips placed in contact without application of pressure. Peeling rate = 10 cm/min.

St. Clair et. al. investigated a series of maleimide and nadimide terminated polyimides and developed LARC-13 [8,9]. Changing the terminal group from maleimide to nadimide, the value of the lap shear strength of a titanium lap shear joint increased from 7 to 19 MPa [9]. They also added an elastomeric component to the adhesive formulation. The introduction of 15 wt% of a rubbery component, ATBN (amine terminated butadiene nitrile polymer) and ADMS (aniline terminated polydimethyl siloxane) enhanced the adhesive properties as follows 19 MPa to 25 MPa (ATBN) titanium T-peel strength 0.2 kN/m to 1.4... 

In addition to epoxy-phenolic adhesives three-part epoxy-phenolic-nitrile rubber systems are used in metal-metal edge joints and honeycomb constructions [208], These add toughness not available in most EP systems and improve peel strengths. When used on honeycomb, the NR-P is normally applied to the aluminum skin and the EP to the honeycomb for assembly. Service temperature limitations are those imposed by the NR-P part. 

Table 1 contains the metal-to-metal engineering property requirements for Boeing Material Specification (BMS) 5-101, a structural film adhesive for metal to metal and honeycomb sandwich use in areas with normal temperature exposure. The requirements are dominated by shear strength tests. Shear strength is the most critical engineering property for structural adhesives, at least for the simplistic joint analysis that is commonly used for metal-to-metal secondary structure on commercial aircraft. Adhesive Joints are purposefully loaded primarily in shear as opposed to tension or peel modes as adhesives are typically stronger in shear than in Mode I (load normal to the plane of the bond) loading. 

In this theory, the adhesion is due to electrostatic forces arising from the transfer of electrons from one material of an adhesive joint to another. Evidence in support of this theory includes the observation that the parts of a broken adhesive joint are sometimes charged [48]. It has been shown that peeling forces are often much greater than can be accounted for by van der Waals forces or chemical bonds. 

The peel strength of soldered lap joint specimens was also not affected by irradiation. For example, the initial peel strength of a lap joint, fabricated from the 43 kg (95 lb) MR-TU, No. 25 tinplate, and the... 

The metallurgical experiments showed that the beta-alpha transition of the tin coating did not occur at irradiation doses of 3-5 Mrad and 6-7.5 Mrad at 5, —30, and —90°C and that the tensile properties, impact ductility, peel strength of soldered lap joints, and microstructure of commercial tinplate and solder were not affected by the irradiation conditions that are used in the sterilization of meat products. 

Patel et al. [70] in a recent publication have explored the adhesive action of the mbber-siUca hybrid nanocomposites on different substrates. The rubber-silica hybrid nanocomposites are synthesized through in situ silica formation from TEOS in strong acidic pH within acryhc copolymer (EA-BA) and terpolymer (EA-BA-AA) matrices. The transparent nanocomposites have been apphed in between the aluminum (Al), wood (W), and biaxially oriented polypropylene (PP) sheets separately and have been tested for peel strength, lap shear strength, and static holding power of the adhesive joints. 

Peel Strength Values of Vulcanized Styrene-Butadiene Rubber (SBR) Rubber/Polyurethane Adhesive/Leather Joints... 

An immersion time less than 1 min, the neutralization with ammonium hydroxide (it extracts the hydrogen from the sulfonic acid and leaves stabilized S03 NH4 ion pair), and the high concentration of the sulfuric acid (95 wt%) are essential to produce adequate effectiveness of the treatment. H2SO4 treatment increases the T-peel strength of treated TR or SBR-polyurethane adhesive joints (Figure 27.2). 

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