3 SHEAR TESTS

Lap shear tests were undertaken in October 1997 in conjunction with the design of a treatment for a fourth-century B.C. Pentelic marble lion in the collection of the Nelson-Atkins Museum. Several large sections of the sculpture, weighing approximately 4,000-5,300 Newtons (900-1,200 lbs.) and a mass of 400-540 kg each, were to be adhered to others as part of the reassembly process. Pursuing the desire that these joints be more easily reversible, the conservation staff began to evaluate the use of B-72 barriers and B-72 as an adhesive compared to more commonly used structural adhesives such as Akemi polyester and Araldite AY 103 epoxy. To more directly mirror the bonds that would be made in the project, the test coupons were made of Pentelic marble (Marble Enterprises 1999), identical to the type used to carve the lion sculpture in ancient times.

Fig. 4.

A shear test coupon

3.1 ADHESIVES EVALUATED

Five samples of each of the following adhesive groups were prepared:

1. Araldite AY 103 with HY 991 hardener (100 resin to 40 hardener, w/w), cured for one week.
2. Akemi Marmokitt 1000 transparent knife grade polyester with benzoyl peroxide–based hardener, cured for one week.
3. 22% B-72 in acetone w/v applied as a double coat to both joint substrates and then bonded with epoxy. A 22% solution was chosen because of good working properties as a coating. One coat was applied by brush, left to dry for several hours, followed by an application of a second coat, which was left to dry for one month prior to the application of the Araldite epoxy (AY 103 with HY 991 hardener), which in turn was allowed to cure for one week.
4. 22% B-72 in acetone w/v applied as a double coat to both joint substrates and then bonded with Akemi polyester. One coat was applied by brush, left to dry for several hours, followed by an application of a second coat, which was left to dry for one month prior to the application of the Akemi polyester, which in turn was allowed to cure for one week.
5. Araldite AY 103 with HY 991 hardener with the addition of fumed silica to make a thick paste (100 parts resin to 40 parts hardener to 30 parts fumed silica by weight). These samples were applied using a shim to ensure that the five samples had a consistent adhesive line of 3 mm (0.125 in.) in thickness. In this evaluation the fumed silica was added to increase the viscosity of the resin so that it might be evaluated as a gap filler as well as an adhesive. Improvements of the cohesion and adhesion were not the intention in these specific samples, though the results do suggest that both were increased.

3.2 SAMPLE PREPARATION

Each sample was made of two small slabs of Pentelic marble (shear strength of 4,212 kPa or 611 psi) nominally 19 mm (0.75 in.) wide x 31.75 mm (1.25 in.) long x 9.52 mm (0.375 in.) thick, adhered with an overlap of 12.7 mm (0.5 in.) as described in ASTM D1002 (Skeist 1977; Pocius 1997) and illustrated in figure 4. Since there was a small variation from the nominal dimensions in each of the samples, the actual measured width and overlap length to the nearest 0.025 mm (0.001 in.) were used to calculate the shear area for each specimen. Even small variations in the width and lap length cause substantial changes in the calculated shear area and stress, so the accurate dimensions of each sample were used in the individual calculations. The samples were washed in a dilute Orvus (sodium alkyl sulfate) anionic detergent solution using stencil brushes, rinsed in distilled water, and then air-dried for three weeks. The samples were degreased with acetone, allowed to dry, then adhered with “finger” pressure. Excess adhesive was wiped off with acetone on a dry swab or mechanically removed after full cure.