Controlled-Stress Cyclic Deformation

The ASTM E1886 test method evaluates the performance of entire fenestration designs and impact protective systems after missile impact by subjecting the unit to static pressure differentials to determine its ability to remain un­breached during a windstorm. Eight loading sequences are used to generate 4500 positive and 4500 negative air pressure cycles, where the duration of each cycle shall not be less than 1 s and not more than 5 s. The applied air pressure is anywhere from 20 to 100 % of the design pressure under service load condi­tions.

Rheometry can be utilized as a screening tool to characterize the perfor­mance of any sealant or adhesive component in a fenestration design or an impact protection system that is subjected to a controlled-load cyclic deforma­tion. Therefore, a test protocol was constructed using the following test param­eters after the sealant test specimen was cured between the parallel-plate fix­tures and subsequently equilibrated at 25 ° C: (1) a stress amplitude of 0.138 MPa (20 psi) equivalent to the specified design pressures of silicone seal­ants; (2) an oscillatory frequency of 0.5 Hz that simulated the fastest oscillation cycle specified in ASTM E1886; and (3) a total of 4500 cycles (as drawn in Fig.

1, one oscillation cycle corresponds to a test specimen experiencing a maxi­mum deformation amplitude once each in the positive and negative direction).

Figure 9 plots the measured deformation profiles of the controlled-stress experiments at 25 ° C in terms of movement following Eq 6. After an equivalent of 2.5 h of oscillatory stress cycling at 0.138 MPa (round 1), each sealant was allowed to recover for a period of 24 h before undergoing a second round under the same set of test conditions. In general, the results from the first round (unfilled symbols) were consistent with the sealant modulus listed in Table 1 (lower modulus~more movement). The deviation from a 1:1 correlation—particularly with respect to Sealants C and D—infers that the seal­ants have distinct stress-strain profiles, which may be a function of multiple variables including formulation and cross-link density.

The ultimate movement observed for each sealant was significantly less than its rated capability (Table 1 ) as well as the magnitude used for the controlled-strain cyclic testing. In these controlled-stress experiments, there ap­peared to be minimal to no distortion in the output sinusoidal waveform as visually observed from the waveform display feature of the AR550 Advantage Instrument Control software.

Overall, the more rigid the sealant, the faster was its response to a steady – state strain. After a 24-hour recovery period, the initial response to a second round of oscillatory stress (dotted symbols) revealed that complete recovery from the initial round was not achieved. Nevertheless, the ultimate movement did not vary significantly to that from the initial round and, more importantly, no obvious signs of fatigue were observed.

Summary

The potential of rheology test methods as a screening tool to isolate and evalu­ate the mechanical durability of the elastomeric silicone sealant component in building assemblies undergoing cyclic deformation was demonstrated. In ab­sence of other artificial degradation pulses, the stress reduction observed in sealants undergoing controlled strain sinusoidal deformation at five cycles per minute was attributed to the Mullins effect. The stress-softening phenomenon occurred within the first 24 hours of cycling; however, three of the four sealants subsequently exhibited signs of recovery during the remainder of the testing period lasting at least four more days.

Under controlled-load cyclic testing at its design load of 0.138 MPa, the sealants exhibited an ultimate deformation within the 2.5-h test cycle well below its rated movement capability with no apparent signs of fatigue. There­fore, these sealant materials tested should be acceptable in an impact-resistant assembly if the frame remains rigid and the stresses induced from the design wind pressures are transferred to the fully cured and adhered sealant joints.

A next step to further characterize the sealants is to ascertain that the cyclic strains or stresses imposed upon the sealants in real systems are quantified properly so that the rheology test methods presented can better assess the per­formance and durability of an individual material.

Time, h

0.0 0.5 1.0 1.5 2.0 2.5

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Number of cycles @ 0.5 Hz