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Most engineering structures are affected by complex multi-axial stresses that arise from load inhomogeneity, geometry and material. However, laboratory tests predominantly use different types of loading regimes, which include creep, static and fatigue tests, and simple specimens that are subjected to uniaxial stress states. Due to the difference between uniaxial test design data and the functional behaviour of multi-axial stress components, much research work has been conducted to ensure an adequate understanding of this relationship.