964,102 research outputs found
Influence of shear stress applied during flow stoppage and rest period on the mechanical properties of thixotropic suspensions
We study the solid mechanical properties of several thixotropic suspensions
as a function of the shear stress history applied during their flow stoppage
and their aging in their solid state. We show that their elastic modulus and
yield stress depend strongly on the shear stress applied during their
solid-liquid transition (i.e., during flow stoppage) while applying the same
stress only before or only after this transition may induce only second-order
effects: there is negligible dependence of the mechanical properties on the
preshear history and on the shear stress applied at rest. We also found that
the suspensions age with a structuration rate that hardly depends on the stress
history. We propose a physical sketch based on the freezing of a microstructure
whose anisotropy depends on the stress applied during the liquid-solid
transition to explain why the mechanical properties depend strongly on this
stress. This sketch points out the role of the internal forces in the colloidal
suspensions' behavior. We finally discuss briefly the macroscopic consequences
of this phenomenon and show the importance of using a controlled-stress
rheometer
Thermo-mechanical stress of bonded wires used in high power modules with alternating and direct current modes
Today, power electronic reliability is a main subject of interest for many companies and laboratories. The main process leading to the IGBT failure is the cycling thermal stress. Indeed the current ïŹow induce local heating and then mechanical stress. This paper deals with electro thermal stress under steady and transient current states. The main objective is to test bonded wires with active current cycle. Consequently, the thermo mechanical stress is obtained. A numerical 3D ïŹnite element model is presented and some experimental results are given. Indeed an infrared system monitors the temperature dispatching from an experimental test bench under active current cycle. The overall study is a ïŹrst step before a global simulation (electrical thermal-mechanical) in order to optimize some geometric parameters of the packaging
Aluminum alloys with improved strength
Mechanical strength and stress corrosion of new BAR and 7050 alloys that include Zn instead of Cr have been studied and compared with those of 7075 aluminum alloy. Added mechanical strength of new alloys is attributed to finer grain size of 5 to 8 micrometers, however, susceptibility to stress corrosion attack is increased
Origin of slow stress relaxation in the cytoskeleton
Dynamically crosslinked semiflexible biopolymers such as the actin
cytoskeleton govern the mechanical behavior of living cells. Semiflexible
biopolymers nonlinearly stiffen in response to mechanical loads, whereas the
crosslinker dynamics allow for stress relaxation over time. Here we show,
through rheology and theoretical modeling, that the combined nonlinearity in
time and stress leads to an unexpectedly slow stress relaxation, similar to the
dynamics of disordered systems close to the glass transition. Our work suggests
that transient crosslinking combined with internal stress can explain prior
reports of soft glassy rheology of cells, in which the shear modulus increases
weakly with frequency.Comment: 6 pages, 4 figure
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