13 research outputs found
Metal Matrix Composites as Potential Armour Materials
To defeat a kinetic energy projectile the armour needs to be extremely hard on the surface, so as to blunt the projectile on initial impact. A ceramic material may be
an ideal choice. However, to absorb and dissipate the complete kinetic energy of the projectile, the subsequent material has to be extremely tough with a very high work of
fracture. It also needs to be a light weight material to keep the overall weight to a minimum possible value. A fibre reinforced polymer matrix composite can meet the
requirement. The innermost laver needs to have a very high ductility so as to avoid any fracture and fragment formation which could be lethal. So it has to be a metallic
material. Thus, the application requires a range of properties starting from that of a ceramic and ending with that of a metal. Metal matrix composites (MMCs) are essentially metals reinforced with ceramic reinforcements, which exhibit a combination of properties of both the constituents and could be tailored to suit the requirements. Discontinuously reinforced MMCs also have the added advantage of being amenable to conventional metal forming operations, which makes it easier to produce them in the required shapes. This paper would provide an introduction to MMCs and review the available literature on their evaluation for possible applications as armour materials.It would also present the results of the preliminary studies on the dynamic hardness measurements of discontinuously reinforced aluminium alloy composites which gives a good indication of their potential use as armour materials
Association of Accelerometry-Measured Physical Activity and Cardiovascular Events in Mobility-Limited Older Adults: The LIFE (Lifestyle Interventions and Independence for Elders) Study.
BACKGROUND:Data are sparse regarding the value of physical activity (PA) surveillance among older adults-particularly among those with mobility limitations. The objective of this study was to examine longitudinal associations between objectively measured daily PA and the incidence of cardiovascular events among older adults in the LIFE (Lifestyle Interventions and Independence for Elders) study. METHODS AND RESULTS:Cardiovascular events were adjudicated based on medical records review, and cardiovascular risk factors were controlled for in the analysis. Home-based activity data were collected by hip-worn accelerometers at baseline and at 6, 12, and 24 months postrandomization to either a physical activity or health education intervention. LIFE study participants (n=1590; age 78.9±5.2 [SD] years; 67.2% women) at baseline had an 11% lower incidence of experiencing a subsequent cardiovascular event per 500 steps taken per day based on activity data (hazard ratio, 0.89; 95% confidence interval, 0.84-0.96; P=0.001). At baseline, every 30 minutes spent performing activities ≥500 counts per minute (hazard ratio, 0.75; confidence interval, 0.65-0.89 [P=0.001]) were also associated with a lower incidence of cardiovascular events. Throughout follow-up (6, 12, and 24 months), both the number of steps per day (per 500 steps; hazard ratio, 0.90, confidence interval, 0.85-0.96 [P=0.001]) and duration of activity ≥500 counts per minute (per 30 minutes; hazard ratio, 0.76; confidence interval, 0.63-0.90 [P=0.002]) were significantly associated with lower cardiovascular event rates. CONCLUSIONS:Objective measurements of physical activity via accelerometry were associated with cardiovascular events among older adults with limited mobility (summary score >10 on the Short Physical Performance Battery) both using baseline and longitudinal data. CLINICAL TRIAL REGISTRATION:URL: http://www.clinicaltrials.gov. Unique identifier: NCT01072500
Recommended from our members
Analysis of brushless doubly-fed, stand-alone generator systems
This thesis introduces a Brushless Doubly-Fed Machine (BDFM), stand-alone
generator system. Development of a BDFM current-forced model, along with the
analysis of air gap power distribution between the two stator windings and the rotor
circuit, pave the way for the characterization and analysis of stand-alone generator
systems.
The main disadvantage of the conventional diesel-driven generators viz., the
frequency regulation through the prime mover system, can be eliminated by using
doubly-fed generators, where the frequency of the ac excitation provides a means of
controlling the generator output frequency electrically. This facilitates the regulation
of both the generator terminal quantities (Voltage and Frequency) through the generator
unit itself, thus making the prime mover governor system cheaper and of less
importance. When compared with converter-based generator systems, the focus of
comparison shifts towards the economic viability since the converter-based system
dynamic responses can be comparable with a BDFM generator system.
For dynamic analysis, this work assumes a diesel engine as the prime mover.
A permanent speed-droop governor is assumed to prove the versatility of a BDFM
stand-alone generator. The stand-alone system is characterized by a seventh order
system, by assuming passive electrical load at the generator terminals.
The steady-state characteristics of the BDFM stand-alone generator system, both
simulation and experimental results, are discussed. A case study is presented in
evaluating the converter rating for a 100kW system and it is shown that BDFM standalone
generator operation about the synchronous speed results in a fractional converter
rating. A brief discussion on system component sizing is presented.
To facilitate the dynamic control and stability studies, linearized models of the
BDFM stand-alone generator are derived. These models can be used even with a prime
mover other than the diesel engine. The results of the studies showing the effect of
generator loading on the system eigenvalues are given.
Simulated dynamic characteristics of the BDFM stand-alone generator system,
under closed-loop control, are presented. The conventional proportional-integral (P1)
as well as the modern fuzzy control methods are used in obtaining the system dynamic
responses. It is observed that the BDFM dynamic characteristics are superior to those
of the conventional stand-alone generator systems.
Finally, experimental results obtained by using a laboratory, prototype BDFM
are included. A shunt connected DC machine drive is used as the prime mover and the
results show satisfactory dynamic responses of the generator terminal quantities, both
under startup and sudden load changes
ZrB2–SiC based composites for thermal protection by reaction sintering of ZrO2+B4C+Si
Abstract Synthesis and sintering of ZrB2–SiC based composites have been carried out in a single-step pressureless reaction sintering (PLRS) of ZrO2, B4C, and Si. Y2O3 and Al2O3 were used as sintering additives. The effect of ratios of ZrO2/B4C, ZrO2/Si, and sintering additives (Y2O3 and Al2O3), was studied by sintering at different temperatures between 1500 and 1680 °C in argon atmosphere. ZrB2, SiC, and YAG phases were identified in the sintered compacts. Density as high as 4.2 g/cm3, micro hardness of 12.7 GPa, and flexural strength of 117.6 MPa were obtained for PLRS composites. Filler material was also prepared by PLRS for tungsten inert gas (TIG) welding of the ZrB2–SiC based composites. The shear strength of the weld was 63.5 MPa. The PLRS ZrB2–SiC composites exhibited: (i) resistance to oxidation and thermal shock upon exposure to plasma flame at 2700 °C for 600 s, (ii) thermal protection for Cf–SiC composites upon exposure to oxy-propane flame at 2300 °C for 600 s
The effect of participate reinforcement on the sliding wear behavior of aluminum matrix composites
The aim of the present investigation is to characterize the friction and wear behavior of aluminum matrix composites reinforced with particulates of SiC, TiC, TiB2, and B4C. Sliding wear tests were conducted at two loads (80 and 160 N) using a pin-on-disc apparatus and under dry conditions. The results of the investigation indicate that the coefficient of friction of the composites is about 30 pct lower than that of pure aluminum, while the wear rates of the com- posites are lower by a factor of about 3 and 100 at loads of 80 and 160 N, respectively. The type and size of the reinforcement have a negligible influence on the wear rate and the coefficient of friction of the composites. However, the volume fraction of the reinforcement has a marginal influence on the wear rate. Though the coefficients of friction and the wear rates of the composites were broadly similar, the Al-TiC composite alone exhibits a somewhat higher wear rate. The above results of the present investigation have been rationalized on the basis of the inverse rule of mixtures and the existing models for friction and wear
Recent development in the fabrication of metal matrix-particulate composites using powder metallurgy techniques
10.1007/BF01154673Journal of Materials Science2981999-2007JMTS