Experiment

Specimens: The sealant specimens are provided by the National Institute for Standards and Technology (NIST) in Gaithersburg, MD via their consortium of sealant companies who fabricate the specimens [3]. All chemical information (formulations, base chemistry, fillers, etc.) about the samples is hidden from the Forest Products Laboratory and NIST due to the blind nature of this study. Additionally, chemical analysis of the samples is not permitted. The sealants consist of Consortia C and ASTM round robin B (ASTM B). The Consortia C sealant is specially formulated to fail earlier than commercially available seal­ants. Per the limitations of this study, it is not known what formulation attrib­ute was changed in order to achieve this intent. The ASTM B sealant is commercially available with a ±25 % movement rating. The specimens consist of a pair of anodized 6063 aluminum blocks (12.7 x 12.7 x 76.2 mm) bonded to­gether with sealant in the form of a 12.7 x 12.7 x 50.8 mm bond line cured in conformance with ASTM C719-93 [4].

Strain Cycling: The custom built test machine shown in Fig. 1 consists of two parallel aluminum I-beams with up to 18 sealant specimens fixtured between them. The I-beams are driven by two captive stepper linear actuators (size 34, Hayden Kerk, Waterbury, CT) whose position is monitored by two lin­ear variable differential transformers (model HSD 750 250-010, Macro Sensors, Pennsauken, NJ). The programmed displacement follows the temperature pro­file of polyvinylchloride (PVC) based durability engines in operation at NIST. The displacement (A, cm) versus temperature (T, °C) equation for such engines is given in Eq 1. Strain gauges were not applied to individual specimens. Thus, the values reported as strain are based upon the linear variable differential transformer position of the I-beam and are approximate

A = -(T – 4.5)/105

The temperature is recorded from a thermocouple embedded in a piece of PVC pipe exposed to solar irradiation. The hot compression cycling displacement boundaries were set in this experiment such that +25 % strain occurred at —29°C and —25 % strain at 38°C, corresponding to climate norms for the Wis­consin test site. The load response to the applied displacement is independently measured for each specimen by S-type load cells (model SSM-AJ-250, Interface, Scottsdale, AZ).

Motion control, load cell conditioning, and data acquisition during testing was accomplished via a National Instruments (Austin, TX) Compact RIO (cRIO)-9073 integrated 266 MHz real time controller.

Once per week, displacement cycling was stopped and an apparent modulus cycle was run to check for changes in modulus as a result of weather and dis­placement aging; see Fig. 2. The cycle consisted of two peaks of approximately 15 % strain that act to remove the Mullins effects from the sealants followed by a 10 % estimated strain stress relaxation period [5]. The 15 % peaks remove any effects of filler bonds and secondary bonds that contribute to non-reversible stress-strain behavior. Thus, the stress relaxation period occurs at a lower strain than the first two peaks and is free of these effects.

The apparent modulus (Ea) is determined using a stress relaxation test pro­posed by NIST as a new ASTM International sealant test method [2]. The Ea is calculated via Eq 2, where t is time, k is the extension ratio, L is the load, W is the specimen width, and B is the specimen thickness. Here, k is calculated using Eq 3 where A is the displacement and h is the specimen height. This methodol­ogy is taken from the statistical theory of rubber elasticity [6,7]

Ea(t, k) = 3L(t)/(WB(k — k—2)) (2)

FIG. 2—Ea cycle taken on Jul. 6, 2006 consisting of two approximately 15 % strain Mullins peaks followed by a stress relaxation period at approximately 10 % tensile strain.

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k = 1 + A/h (3)

Weather: Solar irradiance, temperature, and relative humidity were recorded during outdoor exposure. Spectral irradiance is recorded via a Smithsonian SERC 18 scanning radiometer model, SR-18. This model records the UVB spec­tral irradiance in mW/ (m2* nm) and the Smithsonian uses a radiative transfer model to calculate the UVA and visible bands. Air temperature and relative hu­midity are recorded via a weather station. The test started in March and con­cluded in August of 2010. A lighting strike in April damaged the equipment resulting in the loss of approximately one month of weather data.