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Straw bale construction is a building technique that has excellent thermal performance and limited impact on the environment. Improving knowledge of the mechanical properties of straw bales is important to understand the behaviour of straw bale buildings, especially in case of natural calamities, where straw bales may carry mechanical load and act as a “surviving cell”. The results of extensive tests conducted on bales of different materials are presented. The influence of the material, bale density, bale orientation, baling process and loading rate on the mechanical properties of straw bales was investigated. Force-displacement curves obtained from monotonic compression tests were analysed and relationships between the mechanical properties of straw bales and their geometry and density were determined. Continuous measurements of bales lateral displacement allowed Poisson's ratio to be calculated and, using a simple model, the strain to which bale strings are subjected during loading was estimated. Young's modulus was shown to mainly depend on the square of the density, while no influence of the loading rate and of strings pre-tension was observed. The Poisson's ratio did not remain constant during loading and it exhibited a different trend depending on the orientation of the bales. Moreover, it was observed that for flat bales a rearrangement of the straw fibres during loading occured and the maximum strings strain remained limited. Strings strain reached higher values for on-edge bales instead, and strings bursts occured more frequently. [ABSTRACT FROM AUTHOR]

The use of straw bale construction is strongly on the rise. Despite the need for a deep understanding of the mechanical behaviour of straw bales, there is little research on the testing of single unplastered straw bales and a standard test method does not exist. In this paper, a method able to evaluate the mechanical behaviour of single straw bales is proposed. Force and displacement of the bale in all the three directions was measured in real time without stopping the test; this allowed to best deal with the time-dependent nature of the mechanical behaviour of the bales to be. The test apparatus included a hydraulic press for loading plus digital cameras and a 3D laser scanner for measuring the lateral displacement of the bale. The method was validated by testing six rice bales (three bales laid flat and three on-edge). Results showed that there is no significant difference in the elastic modulus between flat and on-edge orientations. For on-edge bales, string burst was observed, whereas for flat bales no string failure occurred. By using digital image correlation it was observed that straw bales exhibit a typical deformation pattern which is due to the baling process. Measurements also showed that the Poisson's ratio does not remain constant along the longitudinal direction during loading and it is null along the transverse direction. The proposed method can be implemented to evaluate the influence of a variety of parameters and loading conditions on straw bales mechanical response. [ABSTRACT FROM AUTHOR]

The paper discusses the restrictions the positive definite character of the strain energy imposes to the values of the Poisson's ratios in a linear elastic orthotropic constitutive law. By exploiting the reciprocity principle and the conservation of energy, non-trivial constraints arise, leading to a particularly expressive form of the domain of existence, named Tetrahedron-Ellipsoid locus, in the space of three independent Poisson's ratios. The presented analysis establishes several notable restrictions for the elastic behavior of orthotropic composite materials, in all the cases in which there are great differences in the Young's moduli. For instance, near the edge of this locus, some Poisson ratios may belong to an interval not including the zero value; a very interesting peculiarity, perhaps not yet figured out in literature. In order to assess the coerciveness of the formulation, the results are applied to common wood essences, to several Italian growing species of bamboo and Arundo donax , natural composite functionally graded orthotropic materials. In the latter cases, an extensive experimental investigation is carried out to detect several Poisson's ratios, not studied in literature. Poisson ratios are obtained directly by strains measured by means of both physical strain gauges and Digital Image Correlation (DIC). [ABSTRACT FROM AUTHOR]