Incomplete Mixing Procedures

In addition to the varying mixing ratios, an adequate physical mixing procedure itself was also of concern in view of the mechanical fitness of the adhesive and

FIG. 4—Averaged shear test results for varying mixing ratios.

Copyright by ASTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014 Downloaded/printed by

FIG. 5—Tensile tests strength for varying mixing ratios.

its impact on durability. The test sequence presented in the section titled “Varying Mixing Ratios" was also performed for specimens with low mixing quality, which are described in this section and are shown in Table 2. Again, all tests were performed at room temperature and at displacement rates of 50 mm/ min for the tensile tests and 5 mm/min for the shear tests. Please note that a quantification of the mixing quality is quite difficult and the assessment of the low mixing quality for the test campaign presented in this section is based on an

FIG. 6—Shear test strength for varying mixing ratios.

Copyright by ASTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014 Downloaded/printed by

TABLE 2—Strength properties for varying mixing ratios—incomplete mixing.

Incomplete Mixing: Ratio by volume

8:1

9:1

10:1

11:1

12:1

Tensile strengtha

39.5N

39.0N

44.2 N

33.2N

45.4N

1.65 MPa

1.63 MPa

1.84 MPa

1.38 MPa

1.89 MPa

Tensile deviationb

7.1 N

4.2N

5.1 N

9.3 N

2.2 N

0.30 MPa

0.18 MPa

0.21 MPa

0.39 MPa

0.09 MPa

Shear strengthc

369.2N

389.4 N

394.4 N

441.7N

404.2 N

0.62 MPa

0.66 MPa

0.66 MPa

0.74 MPa

0.67 MPa

Shear deviationd

12.4N

10.1 N

26.3 N

26.6N

16.6N

0.02 MPa

0.02 MPa

0.04 MPa

0.04 MPa

0.03 MPa

aDog-bone specimen: 25 mm nominal gauge length, 6 mm x 4 mm nominal cross section. bStatistics based on eight to eleven specimens.

cStrength related to one test specimen with adhesive dimensions of 50 mm in length, 12 mm in height and 12 mm in width. dStatistics based on five samples.

optical assessment of the specimens showing inhomogeneity, i. e., white or whit­ish streaks within the otherwise black (properly mixed) material; see Fig. 7. Fig­ure 8 presents the loads versus the deflection curve for tensile tests on the inhomogeneously mixed test specimens comparable to Fig. 1. It is obvious that the scatter of the individual specimens is significantly higher for Fig. 8 than for Fig. 1, which is expected to some extent, due to the irregularities introduced by the incomplete mixing conditions in the adhesive material. An average of these curves is shown in Fig. 9. Compared to Fig. 2, trends are noted towards lower stiffness for incomplete mixing and towards larger differences in the mechani­cal properties between different mixing ratios. In addition, the sequence of stiff­ness versus mixing ratio differs when compared to the homogenous test

FIG. 7—Specimens featuring incomplete mixing.

Copyright by ASTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014 Downloaded/printed by

FIG. 8—Tensile test results for incomplete mixing cases.

specimen described in the section titled “Varying Mixing Ratios.". Nevertheless, the drawing of final conclusions is difficult due to the absence of metrics for incomplete mixing.

The increased scatter in the mechanical characteristics of the specimens is also visible in the strength values obtained in tensile and shear tests. Compared to Figs. 5 and 6, it is obvious that in Fig. 10 and Fig. 11, the span between maxi­mum and minimum values for tensile and shear values is, in general, larger. Furthermore, the mean values are in general lower with the exception for the nominal mixing ratio test specimens subjected to tensile loading.

We assume that the exception is an outline due to the low number of speci­mens. In case of more specimens we would expect that the behavior is in line.