Stress Whitening and Dynamic Design Load

During the characterization of the TSSA film adhesive it was noted that the ma­terial started to whiten when exposed to a certain stress which was significantly

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FIG. 13—Cantilever pull-off test equipment used in outdoor exposure in southern Germany.

lower than the ultimate tensile stress. Stress whitening is a well understood phe­nomenon in thermoplastic materials (see, for instance, literature [41,42]); how­ever, its occurrence in elastomeric materials is rather seldom. In plastic materials stress whitening is generally attributed to a microcracking (crazing). Stress whitening has also been described in elastomer-modified (rubber-tough­ened) plastics [43,44] where the cause of the whitening has been considered to be cavitation (microvoid formation) between the polymeric network and the elastomeric particles induced by the dilatational deformation. Cavitation and crazing are related: when the density of microvoids increases to a critical value, they expand rapidly together to form craze [45]. The stress whitening observed then results from the grouping of quite tiny but highly concentrated crazes. However, cavitation itself may also cause stress whitening by inducing changes in the refractive index of the material. Information on stress whitening occur­ring in elastomers is rather limited. Some layered silicate (clay) filled elastomers were shown to display stress whitening when undergoing deformation [46,47]. In these cases, stress whitening was attributed to microvoid formation at the polymer/filler interface. Furthermore, tensile stresses at the interface of poorly aligned tactoids (stacks of parallel clay platelets at about 1 nm separation) were believed to contribute to void formation that was evidenced via stress whitening [46]. Except for the special cases of platelet-filled elastomers, there appears to be a paucity of information on stress whitening of elastomers.

In order to demonstrate the phenomenon and its reproducibility in the structural silicone film adhesive, some of the information obtained on stainless steel button point fixings on glass exposed to various stresses will be discussed below. However, the phenomenon can be observed in any test specimen (tensile dumbbell, single lap shear, bonded buttons, etc.) and independent of the load­ing state (uniaxial, biaxial, torque, etc.).

Test specimens were prepared by bonding stainless steel buttons of differ­ent diameters with the structural silicone film adhesive (1 mm film thickness) to standard (uncoated) float glass coupons in a typical autoclave process used for the production of laminated glass at BGT Bischoff Glastechnik Bretten

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(Germany) using the same procedure as described in section Cantilever Pull-Off Test and Static Design Load.

The test specimens (using 20 and 50 mm buttons) were then stored at labo­ratory room conditions (approximately 23°C and 50% relative humidity) for 2 weeks prior to testing them to destruction at a rate of 6 mm/min in a tensile – test machine using suitable attachments for tensile and shear loadings (see Fig. 14). Some of the test specimens were stored at 90°C for additional 6 h prior to the testing, placed as quickly as possible into the tensile tester, and then tested to destruction without temperature control. Based on separate measure­ment of cooling rates it is assumed that the average temperature of these speci­mens during the test was about 80°C.

Figure 15 shows the findings in tensile and in shear loading for the buttons with 50 mm diameter. Note that the zero load displacements are probably due to initial specimen slippage in the extensometer or flexibility (lack of stiffness) within the test specimens. The onset of whitening was visually observed and manually recorded. As can be seen, the onset of whitening occurred, quite reproducibly, at a stress of around 2.0 to 2.5 MPa, regardless whether the speci­mens were subjected to tensile or shear forces and irrespectively of the test tem­perature. Excluding the tests were failure of the glass substrate occurred, failure always occurred cohesively within the structural silicone film adhesive at stress levels of >4 MPa.

Figure 16 shows test specimens undergoing tensile testing at the onset of stress whitening and with the whitening fully developed.

The preliminary testing indicates that the stress whitening effect in the structural silicone film adhesive is a response to a consistent stress level, regard­less whether the dilation of the specimen was carried out in tensile, shear, or in torque. The whitening was observed to be reversible; under cyclic loads the whitening disappeared when the specimen was unloaded and reappeared when reloaded. Furthermore, the whitening did not appear to propagate until the ma­terial was loaded to a higher load state.

FIG. 14—Test specimens inserted in tensile tester with suitable attachments for tensile and shear loading.

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FIG. 16—Development of stress whitening during tensile extension test.

Stress whitening is generally considered to be a proven sign of a material’s plastic deformation as it occurs in plastics at the outset of yielding [45]. How­ever, this is certainly not the case for the structural silicone film adhesive, as stress whitening was observed at a much lower value than the maximum stress point. It is hypothesized that the stress whitening in the TSSA is due to cavita­tion at the polymer/filler interface which is fully reversible. Further studies are currently underway with the intent of characterizing the reversibility of the whitening in more detail (hysteresis). Once the whitening phenomenon and the associated stress level is more fully understood, the authors expect that a safety factor of two can be confidently applied to the stress whitening limit state in order to determine the design load (estimated to be around 1.0-1.3 MPa) for conditions where the material must only resist transient loads, such as wind loads.