This paper presents a mixed numerical–experimental method for the identification of the four in-plane orthotropic engineering constants of composite plate materials. A biaxial tensile test is ...performed on a cruciform test specimen. The heterogeneous displacement field is observed by a CCD camera and measured by a digital image correlation (DIC) technique. The measured displacement field and the subsequently computed strain field are compared with a finite element simulation of the same experiment. The four independent engineering constants are unknown parameters in the finite element model. Starting from an initial value, these parameters are updated till the computed strain field matches the experimental strain field. Two specimen geometries are used: one with a centered hole to increase the strain heterogeneity and one without a hole. It is found that the non-perforated specimen yields the most accurate results.
To study the behaviour of fibre reinforced composite laminates under static and cyclic in-plane complex stress states, a biaxial loading frame has been developed. A cruciform type specimen is ...biaxially loaded in its plane using four independent servo-hydraulic actuators. An appropriate control unit keeps the centre of the specimen at the same position during the test. For obtaining reliable biaxial failure data the design of the cruciform specimen is of paramount importance. Finite element simulations of the cruciform specimen to study the influence of various geometrical parameters and experiments on nine selected cruciform specimen types using both strain gages and an optical-numerical full field method for strain determination over the entire biaxially loaded test zone have been carried out. In the present paper, the four most important geometries will be discussed. Since stresses in the biaxially loaded zone cannot be calculated from the experimentally applied loads due to the ill definition of the load bearing area, strain values – the only measurable quantity – are used for the comparison of the different geometry types. For the glass fibre reinforced composite material with a (±45/0)
4/±45
T lay-up this led to the proposal of an optimized geometry: a plane cruciform type specimen with a circular central region of reduced thickness and an adapted corner fillet at the intersection of the arms. Using this geometry failure will occur in the biaxially loaded test zone, rather than in the uniaxially loaded arms, at failure strains in the centre of the specimen comparable for the uniaxial load ratios to strains obtained on standard beam specimens. Tests at different load ratios were performed to obtain data as input for failure criteria. This paper focuses on the design of the specimen. In the near future, the biaxial test results will be compared with existing failure criteria.
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