Mixed numerical–experimental technique for orthotropic parameter identification using biaxial tensile tests on cruciform specimens

Design of a cruciform specimen for biaxial testing of fibre reinforced composite laminates

A testing apparatus has been developed to study the behavior of sheet metals and composite materials under monotonic and cyclic biaxial loading conditions. This test facility employs cruciform specimens that are loaded in their plane. Problems encountered while developing the test system are discussed. We also discuss the difficulties common to test methods employing cruciform specimens. These relate to the design of a suitable specimen geometry and to the determination of the stresses throughout the specimen. A method for designing an optimal geometry for these specimens is presented. This method is based on the statistical tools of factorial and response surface designs. The statistical method, coupled with a finite-element analysis of the specimen, was successfully applied to optimize the geometry of a cruciform specimen with a circular reduced central region.

Development of an apparatus for biaxial testing using cruciform specimens

F or experimental investigations concerning the mechanical behaviour under biaxial stress states of rolled sheet metals, mostly cruciform flat specimens are used. By means of empirical methods, different specimen geometries have been proposed in the literature. In order to evaluate the suitability of a specimen design, a mathematically well defined criterion is developed, based on the standard deviations of the values of the stresses in the test section. Applied to the finite element method, the criterion is employed to realize the shape optimization of biaxial cruciform specimens for isotropic elastic materials. Furthermore, the performance of the obtained optimized specimen design is investigated in the case of off-axes tests on anisotropic materials. Therefore, for the first time, an original testing device, consisting of hinged fixtures with knife edges at each arm of the specimen, is applied to the biaxial test. The obtained results indicate the decisive superiority of the optimized specimens for the proper performance on isotropic materials, as well as the paramount importance of the proposed off-axes testing technique for biaxial tests on anisotropic materials.

Optimal design of biaxial tensile cruciform specimens

A screw-driven new biaxial testing machine for the realization of experimental investigations on anisotropic sheet materials, such as composite plates or rolled sheet metals, is presented. The described mechanical concept and servocontrol system allow cruciform specimens to be subjected to large strain biaxial tensile and compressive tests without kinematic incompatibilities. Moreover, for the proper implementation of biaxial tensile tests, the specific problems linked to the anisotropic properties of the investigated materials are taken into account; therefore, for the first time, the biaxial machine is supplied with the original ‘off-axes testing device,’ consisting of hinged fixtures with knife-edges at each arm of the cruciform specimen. A recently developed optimization method for the optimal design of flat tensile cruciform specimens is shortly reviewed. Numerical simulations illustrate the decisive superiority of the optimized specimen compared with specimen designs proposed in the literature, as well as the necessity to use the ‘off-axes’ testing technique in biaxial tests on anisotropic materials.

A new direct biaxial testing machine for anisotropic materials

It is well know that high-strain biaxial testing with axial force and pressure on thin-walled tubes can be interesting from several points of view: capability of testing over the whole range of strain or stress ratios, great versatility of testing. and because of the physical system, the measured strains and stresses are principal. Nevertheless, there are few experimental-biaxial-fatique data due to the complexity of the testing equipment involved. In this paper, an experimental testing rig capable of subjecting thin-walled tubes to combined dynamic loadings in tension-compression with torsion and tension-compression with internal-external pressure is presented. The following components are described: the mechanical and hydraulic system, servohydraulic control, specimen configuration, strain-measuring equipment and data-acquisition system. The computation of stresses and strains, as well as examples of data acquisition, are also shown.

A high-strain biaxial-testing rig for thin-walled tubes under axial load and pressure

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A high-strain biaxial-testing rig for thin-walled tubes under axial load and pressure