Authors: Li Hong Han, Derek O. Northwood, Henry Hu
Abstract: Mg-Al-Ca alloys with 1wt.% and 2 wt.% Ca additions (AC51 and AC52) were cast by
the Permanent Mold technique. The microstructures of the as-cast Mg-Al-Ca alloys were observed
by SEM with EDS analysis. The secondary phases were mainly precipitated along the grain
boundaries and exhibited a continuous network microstructure for the AC52 alloy and a divorced
microstructure for the AC51 alloy. EDS microanalysis showed that the solute (Ca) content in the
grains of the AC52 alloy is higher than that in the AC51. A three-sided pyramidal (Berkovich)
diamond indenter was used to characterize the local nano-creep behavior at room temperature
within the α-Mg in grains. The nano-creep results showed that the AC52 alloy has better creep
resistance than the AC51 alloy at all loads at room temperature. The creep exponent n, obtained
from the indentation creep data, changes from 6.3 to 3.0 for AC51 alloy and from 6.6 to 3.2 for
AC52 alloy at a critical stress (132 MPa for the AC51 and 145 MPa for the AC52). The transition in
creep behavior at higher stresses is associated with a change in the deformation mechanisms.
605
Authors: Wen Bo Luo, Xin Tang, Rong Guo Zhao, Jiang Hua Tan, Yoshihiro Tomita
Abstract: In this work, the physical aging and its effect on nonlinear creep behavior of poly(methyl
methacrylate) are presented. After annealing above Tg to release the previous thermal and stress
history, the samples were quenched to 60oC, aged for various times, and were then tested at three
different stress levels (22MPa, 26MPa and 30MPa) at room temperature of 27oC. At each stress level,
the creep strain was converted to compliance and measured as a function of test time and aging time.
The test results show that higher stress accelerates creep rate of the material while physical aging
plays a reverse role. The time-aging time superposition is applicable to build a master creep
compliance curve at each stress level, and it is demonstrated that the shift rate deceases with
increasing stress. Moreover, based on the time-stress superposition principle, a unified master curve
was constructed by further shifting the sub-master curves at 30MPa and 26 MPa to a reference stress
level of 22MPa.
2041
Authors: Wen Bo Luo, Chu Hong Wang, Xiu Liu, Qiang Shen
Abstract: The influence of physical aging on mechanical properties of glassy polymers was investigated in this paper. After annealing above Tg to release the previous thermal and stress history, the polymethyl methacrylate (PMMA) samples were quenched to 27°C, aged for various times (ta), and were then stretched at the same temperature by two ways: (1) step stresses with four different magnitudes varying from 15MPa to 30MPa; (2) constant rate stretch up to fracture. The physical aging effect was monitored by measuring the initial instantaneous elastic modulus (E) and the fracture strength (σf) from the stress-strain curves as a function of ta up to 1368h. It is shown that both E and σf of the material increase with aging time and approach to their asymptotic values, which satisfy the KWW rule, while the isochronous creep compliance decreases with log ta in a linear manner within the aging time range considered in this paper.
914
Authors: Jürgen Malzbender, G. Pećanac, Stefan Baumann
Abstract: Ba0.5Sr0.5Co0.8Fe0.2O3 – δ is a mixed ion-electron conductor with high application potential as high-temperature gas separation membrane. However, in practical use the integrity of this brittle perovskite is challenged by the mechanical boundary conditions of transient temperature exposure. Moreover, long term failure mechanisms such as static fatigue at room temperature and creep rupture at operation temperature might occur. The relevance of both effects for BSCF has been investigated. The slow crack growth at room temperature has been determined using bi-axial bending under different loading rates. The creep rupture at elevated temperature has been analyzed from three-point bending tests. The results indicate favourable behaviour of BSCF in both cases. A low risk of failure due to slow crack growth exists and the strain to failure in combined tensile - compressive mode reaches up to 40 %.
303
Authors: Kong Yue, Wei Qing Liu, Xiao Ning Lu, Wei Dong Lu
Abstract: Three material properties (density, modulus of elasticity, bending creep deflection) of poplar wood modified with low molecular resin was examined and evaluated by an experimental approach. Poplar specimens were treated to achieve full penetration using a vacuum schedule with different low molecular resins. These resins were urea-formaldehyde (UF) and phenol-formaldehyde (PF) prepolymer solution. Creep tests were conducted under a varying climate with 16 °C ~ 31 °C and 40 % ~ 80 % relative humidity. The specimens were loaded in bending for approximately 140 days. Then the curves of the creep tests were analyzed to derive the development of bending creep performance. The results show modification leads to significant changes on material properties. Modification increases both the density and elastic modulus, and also tends to deflect creep deformation obviously less than untreated specimens.
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