Browsing by Subject "shear stress"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Impact Force and Stress Distribution of Drop Impact
    (2021-08) Sun, Ting-Pi
    Drop impact is ubiquitous and relevant to many important natural phenomena and industrial applications. Although the kinematics of drop impact has been extensively studied through simulations and high-speed imaging, the understanding of drop impact is still far from fully understood. The studies of dynamics such as the impact force and the stress distribution of drop impact are still relatively scarce. The impact force and the stress distribution lead to the most important consequence throughout the industrial processes and are crucial factors to erosion on substrates or waterjet cutting. Here, we systematically investigate the impact force and the stress distribution of drop impact through experimental studies. To measure the impact force of drop impact, we synchronize the high-speed camera and the piezoelectric force sensor to obtain the temporal evolution of the impact force and the morphology of drop impact over several orders of magnitudes of Re. We verify the force-time scaling proposed by the self-similar theory at the high $Re$ regime. In the finite Re regime, we consider the effects from the viscosity of liquids and analyze the scaling by using a perturbation method, which matches our experimental results very well. The influence of viscoelasticity is also discussed. To obtain the temporal evolution of the stress distribution, which has not been measured experimentally, we develop a novel technique "high-speed stress microscopy." We confirm the propagation of the self-similar non-central maximum pressure and shear stress predicted by theories, and the shear force is also quantified. Moreover, we discover the impact-induced surface shock waves, which are crucial to the origin of erosion induced by drop impact. Furthermore, we measure the shear stress distribution of drop impact on micropatterned surfaces with high-speed stress microscopy. We investigate the influence of micropillars on substrates to the displacement distribution, the shear stress distribution, and the shear force. We hypothesize that the change of shear stress distribution may result from the formation of vortices. Finally, the results show that on the micropatterned surface, the maximum shear stress is suppressed, which is helpful for mitigating erosion to substrates. Our studies provide the experimental results for understanding the dynamics of drop impact. In addition to the pioneering works of measuring the stress distribution, high-speed stress microscopy can be applied to complicated conditions such as non-Newtonian drop impact and varying the ambient pressure. Besides, it opens the door for experimental exploration of the detailed information inside an impacting drop, including the patterns of the flow and the boundary layer.
  • Thumbnail Image
    Item
    Investigation of Shear Distribution Factors in Prestressed Concrete Girder Bridges
    (Minnesota Department of Transportation, 2016-09) Dymond, Benjamin Z; French, Catherine EW; Shield, Carol K
    As shear requirements for prestressed concrete girders have changed, some structures designed using older specifications do not rate well with current methods. However, signs of shear distress have not been observed in these bridge girders and they are often deemed to be in good condition. The primary objective of this research program was to investigate the accuracy of existing shear distribution factors, which are used to estimate bridge system live load effects on individual girders, and provide recommendations on shear distribution to be used in Minnesota with four components: a full-scale laboratory bridge subjected to elastic and inelastic behavior, field testing of bridges, a numerical parametric study, and integration of results to develop a screening tool to determine which structures benefitted from refined analysis. Laboratory bridge inelastic testing indicated shear force redistribution after cracking and before ultimate failure. Use of elastic distribution factors is conservative for shear distribution at ultimate capacity. Elastic laboratory testing was used to validate the finite element modeling technique and study the behavior of a barrier and end diaphragm, which affected shear distribution; ignoring their effects was conservative. Parametric study results indicated that a ratio of longitudinal stiffness to transverse stiffness could be used as a screening tool. If the stiffness ratio was less than 1.5, shear demand from a simple, conservative grillage analysis may be more accurate than shear demand from AASHTO distribution factor methods. Grillage analysis shear demand results due to permit trucks may also be more accurate, regardless of the screening tool ratio.