We present measurements of the stress response of packings formed from a wide range of particle shapes. Besides spheres these include convex shapes such as the Platonic solids, truncated tetrahedra, and triangular bipyramids, as well as more complex, non-convex geometries such as hexapods with various arm lengths, dolos, and tetrahedral frames. All particles were 3D-printed in hard resin. Well-defined initial packing states were established through preconditioning by cyclic loading under given confinement pressure. Starting from such initial states, stress-strain relationships for axial compression were obtained at four different confining pressures for each particle type. While confining pressure has the largest overall effect on the mechanical response, we find that particle shape controls the details of the stress-strain curves and can be used to tune packing stiffness and yielding. By correlating the experimentally measured values for the effective Young's modulus under compression, yield stress and energy loss during cyclic loading, we identify trends among the various shapes that allow for designing a packing's aggregate behavior.

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Several recent studies have attributed a large part of asset price volatility to self-fulfilling expectations. Such an explanation is unattractive to many since it allows allocations that need bear no particular relation to those implied by the economist's standard kit of market fundamentals. We examine the evidence presented in some of these studies and find (i) that all of the bubble evidence can equally well be interpreted as evidence of model misspecification and (ii) that a slight extension of standard econometric methods points very strongly toward model misspecification as the actual reason for the failure of simple models of market fundamentals to explain asset price volatility.

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