Determination of actual material properties in existing structures using small specimens

During reassessment of an existing steel structure, the designer is often confronted with the fact that the current design loads exceed the loads considered in the original design. Furthermore, fatigue damage may exist that needs to be accounted for in the assessment of the future design life. A possible approach in such a case is to use the actual values of f_y and f_u, because these commonly exceed the nominal values by a considerable margin. For a fatigue assessment by means of a crack growth analysis, fracture toughness properties are required which are usually not well documented, especially for older structures. To obtain specimens to determine the material properties by standardized material tests, a sizable quantity of material needs to be removed from the structure at the critical location where the design loads exceed the resistance, which is undesirable.

This paper presents material testing methods using small specimens, for which only a small quantity of material is needed. The material is extracted by drilling a M39 bolt hole, which has a limited influence on the static resistance of the structure. Furthermore, a high fatigue detail category can be attained by applying a preloaded bolt. Tensile tests are performed on the removed material which give equivalent results to the current standardized tests. Fracture toughness tests are also performed, again with similar results as commonly used standardized tests. Validation of the test methods is shown through comparison with standardized tests.