Most engineering structures are influenced by complex multiaxial stresses that arise from loading, geometry and/or material inhomogeneity. However, laboratory testing predominantly uses different kinds of loading regimes, which include creep, quasi-static and fatigue tests – simple specimens that are subjected to uniaxial states of stress. Because of the contrast between design-based uniaxial test data and the functional operation of components with multiaxial stresses, more research work is being conducted to ensure that the relationship between the two is adequately understood.

A specimen lending itself directly to biaxial testing is a cruciform (a cross-shaped plate) loaded in-plane by four orthogonal actuators. We developed testing systems to control the movement of the specimen center, removing unwanted motion and bending on the specimen. In a conventional servohydraulic testing machine, one end of the test specimen is held stationary while the other end is displaced in tension or compression by an actuator. For most tests this is a very satisfactory method of operation, but it does mean that the center of the specimen moves during the test cycle. For some tests it is necessary to keep the center of the specimen stationary - for example, when it is required to study the center of the specimen through a microscope while the test is in progress. For this type of testing, a second actuator is added to the system so that deforming movements can be applied to both ends of the specimen. So, there are two actuators and two variables to be controlled: the center position of the specimen, and the total deformation applied to the ends of the specimen. The control problem arises because there is no natural pairing of actuator and controlled variable - movement of each actuator affects both the center position and the deformation of the test specimen.

Equal deformation of each end of the test specimen causes no movement of the center position; displacement of each actuator in the same direction moves the center position without altering the deformation. Each loop operates independently and there is no interaction between the in-phase and out-of-phase drive signals to the actuators. This principle, as described for two opposed actuators, is also applicable to a cruciform test set up with four actuators. Tests can also be run in strain and load control modes. A recently installed cruciform system is designed for complex TMF tests - where measured total strain signals must be corrected in real-time with respect to thermal expansion and elastic strain content

Our cruciform systems, with 100 kN or 250 kN capacities, provide a highly aligned and stiffened load frame, together with hi-fidelity seal-less, hydrostatic bearing actuators that provide superior control of the specimen test.  This loading system is well matched to the 8800 Series Digital Controller, and our new WaveMatrix™ dynamic testing software that provides an intuitive user interface for complex, multi-axis tests.

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