409 research outputs found
Strain rate behaviour of thermoplastic polymers
Polymers are increasingly used in structures that have to withstand impact
conditions. This thesis describes an investigation of strain rate properties at
room temperature of four engineering polymers; polyethylene (high density,
HDPE and ultra high molecular weight, UHMWPE), nylatron and
polyetheretherketone (PEEK 150g).
A split Hopkinson pressure bar (SHPB) system was used to study the
response of these polymers in compression tests at high strain rates up to
10' S-1. Stress equilibrium in SHPB samples was studied theoretically by
examining multiple reflection effects during the initial elastic loading of the
polymers; this study proved very useful in the analysis of SHPB tests. To cover
a wide range of strain rate, compression studies were also made at low strain
rates (10-3 _10-2 S-1) using a Hounsfield screw machine. Viscoelastic models
have been applied to these results. These models fit quite well with the
experimental results of HDPE, UHMWPE, and nylatron, but not to the PEEK due
to the yield drop in the stress - strain curves, especially at high strain rates.
An exploding wire technique was used as an axial impulsive loading system
for hollow cylindrical samples. An image converter camera at framing intervals
of 21ls or 10 Ils recorded the radial expansion of the cylinder. The expanding
cylinder was used as a driving system for a new technique called the freely
expanding ring method, which was used to obtain the stress - strain behaviour of
polymeric thin rings placed as a sliding fit on the cylinder. This method
produced very high tensile strain rates up to fracture (> 10' S-1).
Comparisons have been made between results obtained from the quasi-static,
SHPB, and expanding ring tests. The freely expanding ring and SHPB results
were in good agreement indicating similar tensile and compressive high strain
rate behaviour.
The mechanical properties of the above polymers are strongly dependent on
strain rate. The Young's modulus and the flow stress increase with increasing
strain rate. Nylatron showed high strain rate strain softening at high strain, this
was due to the high temperature rise during loading, when the transition
temperature (Tg) of the material (50 QC) was exceeded. However, the other
materials showed continuous hardening behaviour. Plots of the flow stress at
5% and 10% strain vs log strain rate showed a linear increase up to a strain rate
of about 103 S-1. Above 103 s-1, the stress rose more rapidly, but then showed
significant drops for nylatron and PEEK. These drops in stress are probably due
to both micro crack initiation in the sample and also high temperatures around
the crack tips
High-speed photographic studies of blast wave impact phenomena
An exploding wire technique (EWT) is described, which provides high
rates of loading by producing cylindrical blast waves of high
reflection pressure with short duration. These waves have been used
for internal impact of hollow polymeric cylindrical specimens. The
EWT overcomes the small size restrictions of other high strain rate
testing methods like the Split-Hopkinson pressure bar technique and
drop weight. [Continues.
Diplomatic Immunity and the Abuse of Domestic Workers: Criminal and Civil Remedies in the United States
Superplastic behaviour of AZ91 magnesium alloy processed by highâ pressure torsion
An investigation has been conducted on the tensile properties of a fineâgrained AZ91 magnesium alloy processed at room temperature by high pressure torsion (HPT). Tensile testing was carried out at 423 K, 473 K and 573 K using strain rates from 1Ă10â1 sâ1 to 1Ă10â4 sâ1 for samples processed in HPT for N = 1, 3, 5 and 10 turns. After testing was completed, the microstructures were investigated by scanning electron microscopy and energy dispersive spectroscopy. The alloy processed at room temperature in HPT exhibited excellent superplastic behaviour with elongations higher than elongations reported previously for fineâgrained AZ91 alloy produced by other severe plastic deformation processes, e.g. HPT, ECAP and EXâECAP. A maximum elongation of 1308 % was achieved at a testing temperature of 573 K using a strain rate of 1Ă10â4 sâ1, which is the highest value of elongation reported to date in this alloy. Excellent highâstrain rate superplasticity (HSRSP) was achieved with maximum elongations of 590 % and 860 % at temperatures of 473 K and 573 K, respectively, using a strain rate of 1Ă10â2 sâ1. The alloy exhibited lowâtemperature superplasticity (LTSP) with maximum elongations of 660 % and 760 % at a temperature of 423 K and using strain rates of 1Ă10â3 sâ1 and 1Ă10â4 sâ1, respectively. Grainâboundary sliding (GBS) was identified as the deformation mechanism during HSRSP, and the glideâdislocation creep accommodated by GBS dominated during LTSP. Grainâboundary sliding accommodated with diffusion creep was the deformation mechanism at high test temperature and slow strain rates. An enhanced thermal stability of the microstructure consisting of fine equiaxed grains during deformation at elevated temperature was attributed to the extremely fine grains produced in HPT at room temperature, a high volume fraction of nano ?âparticles, and the formation of ?âphase filaments
Bioremediation of marine oil spill using beeswax
1227-1231Oil spills can have devastating environmental, economical, and social impacts. The use of bioremediation is a preferred technique for cleaning up oil spills because it facilitates the process of naturally converting oil into non-toxic byproducts without any further local environmental disruption. An experimental study was conducted to degrade marine oil spill using beeswax as a biostimulant for bioremediation and also to study the chemical variation. Beeswax, a nutrient source for proliferation of olephic (oil eating) bacteria, was added in intervals of five days and the corresponding dissolved oxygen (DO) was measured using Azide Winklerâs method. The recuperation of the DO concentration from 3.1 mg/l to 7.4 mg/l is an indication of the reduction in oil content. An oxygen sag curve and a graph showing temporal variation of chemical parameters such as nitrite, nitrate, and ammonium were plotted. During the period of study, the oil content reduced from 10 ml to 5 ml. As the addition of biostimulant resulted in an increase in the rate of bioremediation, the use of beeswax as a biostimulant appears to be an effective tool for the clean-up of oil-contaminated marine waters without adverse environmental impact
The use of polyurethane foams for the extraction of platinum (II) from hydrochloric acid solutions in the presence of tin (II) chloride
A preliminary investigation prior to the study of the extraction of platinum, was conducted on the Co(NCS)âÂČâ» /foam system to, (a) develop a suitable experimental apparatus in order to study the loading of platinum foams and (b) obtain fundamental kinetic and thermodynamic data concerning the sorption process. The sorption of platinum(II) as complexes of [Pt(SnClâ)nClâ-n]ÂČâ» and [Pt(SnClâ)â
]Âłâ» from hydrochloric acid solutions by polyurethane foams was examined in detail. Our studies show that the capacity of the foam to extract platinum depends on the following solution conditions of, temperature, initial platinum concentration, hydrochloric acid and hydrogen ion concentrations, the Sn:Pt ratio and the presence of interfering cations and anions. The rate of platinum sorption is shown to be dependent on temperature, initial platinum concentration, the Sn:Pt ratio and the mass of the foam. The foam's ability to selectively extract platinum from solutions containing platinum and certain base metals is examined. Results obtained show that in all cases almost complete extraction of platinum was achieved. Of all the base metals investigated only Cu(II) co-extracts to a small extent with the noble metal. In this work a procedure has been developed in which we are able to quantitatively recover both platinum and tin from loaded foams, by dissolving the polyurethane in warm nitric acid. A simple distillation allows for the satisfactory separation of platinum from tin
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