Adhesion fracture

Fracture mechanics (qv) affect adhesion. Fractures can result from imperfections in a coating film which act to concentrate stresses. In some cases, stress concentration results in the propagation of a crack through the film, leading to cohesive failure with less total stress appHcation. Propagating cracks can proceed to the coating/substrate interface, then the coating may peel off the interface, which may require much less force than a normal force pull would require. 

Bascom, W.D. and Cottington, R.L, (1976). Effect of temperature on the adhesive fracture behavior of an elastomer-epoxy resin. J. Adhesion 7, 333-346. 

Texture is a key component of the quality and palatability of potato products. Texture is generally quantified by measuring the resistance of a product to an applied force. A number of different rheological parameters can be used to evaluate a range of tuber characteristics such as firmness, hardness, softness, adhesiveness, fracturability, etc. There is a considerable amount... 

ISO, Standard test method for mode I interlaminar fracture toughness, G/c, of unidirectional fibre-reinforced polymer matrix Composites. ISO 15024 2001. Blackman, B.R.K., H. Hadavinia, A.J. Kinloch, M. Paraschi and J.G, Williams, The calculation of adhesive fracture energies in mode I revisiting the tapered double cantilever beam (TDCB) test. Engineering Fracture Mechanics 2003. 70 p. 233-248. BSI, Determination of the mode I adhesive fracture energy, Gic, of structural adhesives using the double cantilever beam (DCB) and tapered double cantilever beam (TDCB) specimens. 2001. BS 7991. 

The TDCB test configuration was employed to determine the adhesive fracture energy, G. Tests were conducted at different crosshead speeds between 10 and 1 m/s. A schematic of the test specimen is illustrated in Fig. 1(a). Due to symmetry only half of the specimen is considered in the numerical analysis, Fig. 1(b). The profile of the arms is machined such that the rate of compliance increases linearly with the crack length and hence the derivative of the compliance with crack length remains constant. The beams are contoured to the profile described by Eqn. 1 [2], where h is the height of the beam, a is the crack length and m is a constant (m = 2000 m" in the present work). 

The thickness of the TDCB specimens (S = 10 mm) is sufficient to ensure plain strain conditions. It should be noted that during the test the arms remain within their elastic limit. Therefore, from simple beam theory [7], and by the use of linear elastic fracture mechanics, the strain energy release rate of the adhesive can be obtained using Eqn. 2, where P is the load at failure and E, is the substrate modulus. The calculated adhesive fracture energy was employed in the simulation of the TDCB and impact wedge-peel (IWP) tests. 

THE DETERMINATION OF ADHESIVE FRACTURE TOUGHNESS FOR LAMINATES BY THE USE OF DIFFERENT TEST GEOMETRY AND CONSIDERATION OF PLASTIC ENERGY CORRECTION FACTORS... 

Fixed arm peel and T-peel test procedures are used to measure peel strength for flexible laminates. Analysis of the contributions from elastic and plastic deformations of the peel arms during these tests enables the energy contribution from plastic effects to be subtracted from the energy required to peel the laminate. In this way, the adhesive fracture toughness is determined. 

Adhesion, laminates, fracture mechanics, adhesive fracture toughness, polymers, metal substrates, T-peel, fixed arm peel... 

The important features of Figure 4 are the dependence of both G/f" and Ga on peel angle. This suggests that these parameters cannot be objective material (or laminate) properties. However, Ga can be seen to be independent of peel angle and hence this quantity is considered to be an objective measurement of the adhesive fracture toughness. The Ga values for all five laboratories are shown in Figure 5. 

Figure 5 Adhesive fracture toughness as a function of peel angle for 5 laboratories for fixed arm peel tests on PP laminate system.

There is some scatter in these adhesive fracture toughness data. However, there are no reasons for excluding any of the results. The mean value is 206 jW with a standard deviation of 42 jW. With consideration to the overall level of scatter, this gives good agreement with the results for the data at a peel angle of 90" (214 J/m ). 

Analysis of the data follows a similar route to that for the fixed arm procedure. However, for the T-peel there are adhesive fracture toughness values to determine for each peel arm, (Ga) and (Ga). The adhesive fracture toughness for the laminate (Ga) is then the sum of these two values. 

The average of the adhesive fracture toughness values from all of the labortories is as follows ... 

It is concluded that the adhesive fracture toughness is independent of configuration. However, the peel strength and peel angles do depend on configuration. 

Comparison of Adhesive Fracture Toughness by Different Test Geometries... 

Figure 7 Comparison of adhesive fracture toughness by T-peel and fixed arm peel for data from 4 specific laboratories. Each laboratory has a pair of averaged adhesive fracture toughness values for both tests.

INVESTIGATION OF ENERGY CORRECTION FACTORS IN THE DETERMINATION OF ADHESIVE FRACTURE TOUGHNESS. 

The determination of adhesive fracture toughness involves the measurement of a toughness term with elastic deformation corrections and subtracting from this energy term a contribution due to plastic deformation. Therefore, the relative size of the plastic deformation term will govern the accuracy of the adhesive fracture toughness. 

It is observed from the results in Table 2 that again a common value of adhesive fracture toughness is obtained from either geometry, to within experimental error. 

Despite the difficulties associated with the BMI/copper laminates (as just discussed), an understanding of their adhesion characteristics remains important. In particular there is an interest in the relationship between adhesive fracture toughness and temperature. This can be approached by use of either test geometry. The fixed arm peel procedure can be conducted at different test temperatures. The tensile stress-strain properties of the peel arm can also be measured at these temperatures and adhesive fracture toughness calculated in the usual manner and plotted against temperature. This can be a time-consuming process that can be overcome by use of a T-peel procedure operating as a temperature scan. 

The adhesive fracture toughness versus temperature results for the peel scan and those based on isothermal T and fixed arm peel are as shown in Figure 8. The agreement between the two methods is encouragingly good, particularly in consideration of the low values for adhesive fracture toughness, the low quality coefficient for the data (about 0.2)... 

Figure 8 Adhesive fracture toughness versus temperature for BMI/copper laminate by T-peel scan and fixed arm peel procedures...

---