Abstract: A test assembly structure having a first specimen support, a displacement mechanism joined to the first specimen support and a second specimen support. A loading assembly is joined to the second specimen support and configured so as to engage a specimen held by the second specimen support with a specimen held by the first specimen support. A self-reacting structure is joined to the loading assembly having a flexure substantially rigid in the direction of loading of the loading assembly and substantially compliant in the direction of displacement of the displacement mechanism. A second flexure can be configured to support the second specimen support and/or loading assembly on a base. The second flexure is substantially compliant in the direction of loading of the loading assembly and substantially rigid in the direction of displacement of the displacement mechanism.