Discussion

The three silicone sealants studied all toughened and also appeared to stiffen with increasing movement rates. The toughening of the sealants results from a simultaneous increase in maximum strength and strain, which translates into substantially increased fracture energy, which corresponds to the area under the stress-strain curve. For a well-balanced blast mitigating window design, the

image214

FIG. 10—Individual stress-strain curves measured for Sealant A at the different move­ment rates.

increased toughness results in greater blast capacity, assuming the silicone seal­ant represents the weakest link in the performance chain of the design.

In tension, both tensile strengths and corresponding strains increase by a factor of about 2 to 2.5 for all three sealants with an increase in movement rate from 50 mm/min to 5.0 m/s (see Tables 2 and 4 and Figs. 9 and 12). In the

TABLE 3—Tension tests results for H0 (unweathered) and HW (weathered) specimen con­figurations at a movement rate of 5 m/s.

Sealant

Mean Stress and Strain

H0 (Unweathered)

HW (Weathered)

A

stress o-max [MPa]

3.46

2.89

strain є [%] at F = Fmax

180

126

B

stress o-max [MPa]

2.85

2.46

strain є [%] at F= Fmax

177

168

C

stress o-max [MPa]

2.44

2.36

strain є [%] at F= Fmax

238

217

image215

FIG. 11—Tension tests results for H0 (unweathered) and HW (weathered) specimen configurations at a movement rate of 5 m/s.

TABLE 4—Tension test results for Sealant B for different specimen configurations (H0, HL, and HS) at various movement rates (effect of sealed joint sizes).

Geometry

Mean Stress and Strain

50 mm/min

0.5 m/s

1.0 m/s

2.5 m/s

5.0 m/s

H0

stress nmax [MPa]

1.38

2.16

2.36

2.74

2.85

(12 X 12)

strain є [%] at F= Fmax

91

170

155

193

177

HL

stress nmax [MPa]

1.36

1.91

2.30

2.50

2.95

(24 X 12)

strain є [%] at F= Fmax

69

148

135

159

171

HS

stress o-max [MPa]

1.40

2.12

2.38

(12 X 6)

strain є [%] at F= Fmax

55

64

35

image216

FIG. 12—Tension test results for Sealant B for different specimen configurations {.H0, HL, and HS) at various movement rates {effect of sealed joint sizes).

TABLE 5—Shear test results for Sealant B obtained with specimen configuration M at various movement rates.

Geometry

Mean Stress and Strain

50 mm/min

0.5 m /s

1.0 m /s 2.5 m /s 5.0 m /s

M

stress rmax [MPa]

0.64

1.46

1.74 2.09 2.22

strain у [%] at F = Fmax

111

210

210 230 234

TABLE 6—Shear test results for Sealant B obtained with various movement rates.

specimen

configuration L at

Mean Stress Geometry and Strain

50 mm/min 0.5 m/s

1.0 m /s

2.5 m /s 5.0 m /s

L Force Fmax [N]

Displacement u [mm] at F =F

max

2411 3384 3.7 6.1

3867

7.5

4717 5201 7.3 7.6

TABLE 7—Mean compressive stress a50 % [MPa at strain є = 50 % obtained with specimen configuration H0 at various movement rates (50 mm/min to 1.0 m/s).

Sealant

50 mm/min

0.5 m/s

1.0 m/s

A

4.26

8.04

10.70

B

4.04

4.79

5.12

C

2.50

3.56

3.77

shear experiment on Sealant B, a corresponding increase in shear strength by a factor of 3.5 and strain by a factor of 2.1 is observed (see Table 5). Furthermore, in the combined shear and tension experiment carried out on Sealant B, in­creases in the force and displacement by factors of 2.2 and 2.1, respectively, were measured over the range of movement rates (see Table 6).

Earlier studies on the strain rate sensitivity of silicone sealants were limited to much slower movement rates (0.01 to 125 mm/min) in tension (see, for example, Ref [39] as an early study), but had found similar results.

In compression, the increase in strengths with movement rates showed a stronger dependency on the formulation of the sealants than the corresponding increases in tension, shear, or in combined load. As can be seen from Table 7, the strengths at 50 % compression increased by a factor of 2.5, 1.3, and 1.5 for Sealants A, B, and C, respectively, with an increase in movement rate from 50 mm / min to 5.0 m / s.

As can be seen from Fig. 10, an increase in movement rate typically also appeared to result in an increase in Young’s modulus of the sealant.

The relation between microstructure and constitutive response of rubbers is well understood (see, for example, Ref [40]), and can be summarized as follows. The viscoelastic nature of amorphous rubbers (such as silicones above glass transition and crystallization temperatures) derives from the mobility of the polymer chain on the atomic scale (rotations between molecular units) and on the macroscopic scale (straightening of the chain between cross-links). The strain rate sensitivity reflects the timescale required for these polymer chain reorientations to take place. At low strain rates the polymer chains have suffi­cient time to reorientate themselves and the storage modulus of the rubber is low. At high strain rates, the deformation of the polymer chains is restricted to bending and stretching of the chemical bonds, and the storage modulus of the rubber can increase by up to three orders of magnitude.

Accelerated weathering according to ETAG 002 does have an effect on me­chanical properties and adhesion of the sealants. A decrease in strengths (-3 to -17 %) and strains (-5 to -29 %) at maximum load in relation to nonweath­ered specimens is observed. Furthermore, while specimens of Sealant A all failed cohesively, some specimens of Sealants B and C displayed partial adhe­sive failure.

The geometry of the sealed joint has little effect on the maximum stress levels achieved, however, a thinner glueline (specimen Type HS) results in lower strains at maximum force.

Summary

Bomb blast mitigating window designs are clearly growing in popularity in response to increased terrorist risks. These window designs are complex with many variables affecting their performance. One essential component needed to develop a successful bomb blast mitigating window system is the silicone sealant. Only a silicone sealant can reliably meet the demands of these systems: strength to anchor the laminated or filmed glass in the frame, flexibility, long­term adhesion and proven structural glazing durability. Testing of silicone seal­ant in a manner simulating bomb blast conditions in terms of movement rates shows an increase in tear energy (toughness) and often also in Young’s modu­lus. By providing data on the behavior of silicone sealants at high movement rates, a design professional is armed with information needed to innovate, model, and develop systems, which will perform successfully and hopefully save lives.