Adhesion fracture toughness

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...

It has not been possible to rigorously define the maximum level of correction that can lead to sensible values for adhesive fi-acture toughness, although corrections up to about 70% seem to provide adequately accurate values for adhesive fracture toughness. Examples of laminates where the corrections are above 80% have been shown. Even at such levels, the procedures for determination of adhesive fracture toughness provide probably the best approach currently available. However, further investigation of what is an acceptable level of correction is required. This is under investigation by the authors. 

The main limitation of the peel tests described above is that the adhesive fracture toughness, Ga, is determined from the difference between two numbers, that is, G — Gp. When modem, structural adhesives are employed to bond the substrates these two numbers may be large, and as a consequence, the repeatability of the Ga values determined may be poor. Recent research has investigated the application of various validity criteria for peel tests. One such criterion has been to limit the size of the plastic correction term, Gp, to a fixed percentage of G, for example, a limit of (Gp/ G) < 80% has been utilized. Another criterion proposed has been to limit the maximum strain in the peel arm. Such criteria are still currently under development. 

Kawashita LF, Moore DR et al (2006) Protocols for the measurement of adhesive fracture toughness by peel... 

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