Method for the characterisation of the mechanical behaviour of straw bales

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.