Mechanical Characterization of Historic and Artistic Materials

Understanding the mechanical properties of cultural heritage materials is a fundamental aspect of establishing the critical environmental conditions which lead to irreversible change. There is already a substantial amount of data from the collections care field that supports a shift to less stringent climate requirements for collections materials. However, most of these data have been obtained by testing artificially aged samples or simplified mock-ups of objects. These laboratory samples may not represent accurately enough the changes that naturally aged objects undergo. In general, cultural heritage objects are more complex than mock-ups as they combine different materials in a specific construction and with specific climatic and conservation history.

This project is undertaking research on naturally aged cultural heritage materials using a wide range of testing techniques. The mechanical testing laboratory combines micro- and macro-mechanical testing apparatus with full microclimatic control. The suite of mechanical testing techniques allows measurements of stresses and resulting strains in a very wide range on samples of different shapes and dimensions (including microsamples) in various climatic conditions. Furthermore, it allows one to execute a combination of static and dynamic testing to characterize time-dependent properties of materials, conditions of glass transitions, and residual stresses in surface layers. Micro- and nanoindentation techniques make possible measurements on historic samples, which is necessary to understand the aging processes of artistic materials. Macro techniques in turn help to validate the results of micro- and nano-tests and aid in defining material damage criteria.

Used to determine the mechanical properties of macro samples subjected to various deformations under different temperature and humidity conditions. As the name suggests, with the help of UTM, various types of tests can be performed, including peel, bend, tensile, compression, friction test, among others. Common test results comprise characterization of stiffness (Young’s modulus), tensile and compressive strength, creep and stress relaxation. It is also possible to use UTM to determine fracture toughness and perform low frequency fatigue tests.

In material science, DMA is routinely used to determine the viscoelastic behavior of materials, perform creep experiments and study changes in damping capacity and modulus as a function of frequency, temperature and humidity. It can be used for finding glass transitions in polymers materials and performing fatigue tests.

Two units are used. The first is designed for testing materials in wide range of temperatures; the second offers precise control of temperature and humidity during the test. Both measure mechanical properties of centimeter size samples: storage modulus (elasticity) and loss modulus (damping) as a function of time, temperature, humidity (TA DMA850), frequency, stress or combinations of these parameters.

The standard indentation test consists of pressing a probe with a well-defined geometry into a sample surface, with the resulting force-penetration depth (or load-displacement) curve serving as the “mechanical fingerprint” of the material. Nanoindentation employs this technique at smaller scales and has become indispensable in the field of material science and for industrial applications, where small sample sizes predominate. Recently, nanoindentation has been successfully applied in the characterization of historic materials, demonstrating its capability of consistent measurements on submillimeter cross-sectional paint samples.