Investigations of Mechanical Properties of Powders by means of a Uniaxial Tester
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The work presented in this thesis has been carried out in cooperation with HiT (Telemark University College) and POSTEC (Powder Science and Technology). The main intention of the work was to do investigations of mechanical properties of powders by means of the uniaxial tester at POSTEC dept of Tel-Tel. The uniaxial tester is a shear tester, originally developed as a simpler alternative to more complicated shear testers. Normally a sample is consolidated to a specified consolidation stress, before relaxation and unloading followed by failure stress measurements. The sample tested in a uniaxial tester does not undergo steady state consolidation, and the uniaxial tester can thereby not be used for silo design. But it is a good tool for quality control and for qualitative comparison of powder flow. Before doing more fundamental research, some optimization of the tester and the test procedures were performed. The stress history of the powder during consolidation was found to influence on the measured failure strength. The main operator dependent part of the testing procedure is the filling of the die. It is therefore found that the filling should be made as operator independent as possible. The rest of the testing is more operator independent, and when operated by a skilled operator, the uniaxial tester showed very good repeatability. Prior to this work, the uniaxial tester has mainly been used for consolidation of powder samples followed by failure stress measurements to compare flowability of different powders. As part of this work, the flowability of L-glutamic acid and an aromatic amine was compared. The effect of chord length distribution and crystal morphology on the flow properties of the two materials was investigated. The two materials behave very differently when it comes to flow properties. The morphology has an influence on the flow properties, but there is no clear relationship between chord length distribution and flow properties for the tested crystals. The effect of cyclic loading under consolidation on the measured failure strengths of different powders has been investigated. The strength of the tested powders seems to increase when cycling to the same level as the consolidation stress level up to a certain level. For higher consolidation stress levels the strength is more and more reduced because of the cycling. A decrease in failure strength after cyclic consolidation was not expected. It may be caused by reduction in binding forces when expanding the sample during cycling. A large part of the PhD work turned out to be an investigation on the elastic behaviour of powders. A preliminary equation for the elastic behaviour of powders has been derived. The equation is based on Hooke’s law of elastic deformation in the contact points between particles as well as other simple assumptions, but seems to describe the elastic unloading of most powders fairly well. The derived equation shows that the stress during unloading is proportional to the square of the deformation, and has been compared with experimental results for a range of powders and has been validated by least mean squares curve fitting. The porosity of the powders seems to be one of the main factors influencing the elastic properties of powders. Also particle shape and size, as well as the state of the powder will influence on the elastic properties. The reloading of powders seems to follow a more linear relationship, in line with Hooke’s law, than the unloading. Much of the work carried out during my PhD study is covered in this thesis. Eight articles, which cover parts of the work more in detail, are also attached.