On the determination of the post-irradiation time from the glow curve of TLD-100

The ratio of peak 3 to the sum of peaks 4 + 5 in TLD-100 was measured for various pre-irradiation and post-irradiation time periods, under conditions characteristic of routine personal dosimetry. It was confirmed that the value of this ratio depends only on the elapsed time between the prior readout and the present one, independent of the moment when the irradiation took place during the total time interval (storage time). This effect indicates that fading of peak 3 seems to be due mainly to changes in the unoccupied traps, and not to decay of trapped charges, being almost independent of the presence of electrons or holes in the traps. This observation leads to the conclusion that the suggestions in the past to use the decay of peak 3 in TLD-100 for the measurement of the elapsed time between irradiation and readout may have been wrong. On the other hand, the decay of peak 2 can be used to measure the elapsed time from irradiation, since the rate of decay is different when related to pre-irradiation and post-irradiation times, indicating a much higher decay rate of the trapped charges (Randall-Wilkins decay). However, because of the fast decay rate of peak 2, its use for determination of the elapsed time since irradiation is of little practical significance.     

Cyclic deformation behaviour of austenitic steels at ambient and elevated temperatures

The aim of the present investigation is to characterise cyclic deformation behaviour and plasticity-induced martensite formation of metastable austenitic stainless steels at ambient and elevated temperatures, taking into account the influence of the alloying elements titanium and niobium. Titanium and niobium are ferrite-stabilising elements which influence the ferrite crystallisation. Furthermore, They form carbides and/or carbonitrides and thus limit the austenite-stabilising effect of carbon and nitrogen. Several specimen batches of titanium and niobium alloyed austenite and of a pure Cr-Ni-steel for comparison were tested under stress and total strain control at a frequency of 5 Hz and triangular load-time waveforms. Stress-strain-hysteresis and temperature measurements were used at ambient temperature to characterise cyclic deformation behaviour. Plasticity-induced martensite content was detected with non-destructive magnetic measuring techniques. The experiments yield characteristic cyclic deformation curves and corresponding magnetic signals according to the actual fatigue state and the amount of martensite. Fatigue behaviour of X6CrNiTil810 (AISI 321), X10CrNiCb189 (AISI 348) and X5CrNi1810 (AISI 304) is characterised by cyclic hardening and softening effects which are strongly influenced by specific loading conditions. Martensite formation varies with the composition, loading conditions, temperature and number of cycles.     

A study of road traffic accidents in Saudi Arabia

In Saudi Arabia, with its rapid expansion of road construction and increase in the number of vehicles, road traffic accidents are becoming a serious public health problem. The object of this paper is to discuss the magnitude of this problem and to compare the situation with that of other rich, developing countries which have also had rapid expansion of road construction and increase in the number of vehicles. It appears that Saudi Arabia has lower accident rates but higher casualty and fatality rates than Kuwait.     

Prediction of notched specimen behaviour at ambient and high temperatures in Ti6246

Life prediction methods are essential in the selection of materials for high performance applications. These design criteria allow safe lives to be predicted for areas of geometrical discontinuity where localized increases in stress lead to the early initiation and propagation of fatigue cracks under cyclic loading. This paper explains two methods for predicting the initiation life of a double edged notch specimen (K_t = 1.9), and applies them to the α + β titanium alloy Ti6246 over a range of temperatures. The Coffin–Manson equation is effective for fully reversed cyclic behaviour. However a Walker strain-based parameter was found to be more appropriate when mean stresses are introduced. The analysis encompasses traditional analytical approaches which are limited in relation to the determination of the stresses and strains at the notch root and a finite element analysis based on ABAQUS^®. The FEA is used to characterize loop generation at the notches and to optimize the stress and strain conditions in the critical root positions.The main focus of the paper, however, is high temperature behaviour where creep and environmental damage impact on fatigue crack development. The limitations of the modelling approach under these conditions are discussed.     

Modelling the dose response of peaks 4, 5 and 5b, in TLD-100, as a function of recombination temperature

This investigation focuses on whether the experimentally observed changes, in supralinearity f(D) as a function of recombination temperature, can be, successfully, predicted by the Unified Interaction Model (UNIM). To resolve this question, the measurement of f(D) has been carried out for glow peaks 4, 5 and 5b in LiF:Mg,Ti (TLD-100) at two different heating rates and UNIM analysis is used in an attempt to arrive at a consistent/physically reasonable explanation of all the f(D)/heating rate data for the three glow peaks. It is demonstrated that in addition to the variation in the luminescence and competitive centre capture cross section with temperature changes in the ks (the geminate recombination coefficient) parameter as a function of temperature is also required in order to predict the changes in f(D).     

Advanced multistage deconvolution applied to composite glow peak 5 in LiF:Mg,Ti (TLD-100)

The effects of 'slow-cooling' on the structure of composite peak 5 following low-ionisation density beta/gamma irradiation are described and analysed in both 'slow-cooled' and 'normally-cooled' samples. Computerised glow curve deconvolution is employed with constrained 'peak-shape' parameters deduced from anciliary studies using 4 eV and 5 eV optical excitation.     

Investigation of the ionisation density dependence of the glow curve characteristics of LIF:MG,TI (TLD-100)

The dependence of the shape of the glow curve of LiF:Mg,Ti (TLD-100) on ionisation density was investigated using irradiation with (90)Sr/(90)Y beta rays, 60 and 250 kVp X rays, various heavy-charged particles and 0.2 and 14 MeV neutrons. Special attention is focused on the properties of high-temperature thermoluminescence; specifically, the behaviour of the high-temperature ratio (HTR) of Peaks 7 and 8 as a function of batch and annealing protocol. The correlation of Peaks 7 and 8 with average linear-energy-transfer (LET) is also investigated. The HTR of Peak 7 is found to be independent of LET for values of LET approximately >30 keV microm(-1). The behaviour of the HTR of Peak 8 with LET is observed to be erratic, which suggests that applications using the HTR should separate the contributions of Peaks 7 and 8 using computerised glow curve deconvolution. The behaviour of the HTR following neutron irradiation is complex and not fully understood. The shape of composite Peak 5 is observed to be broader following high ionisation alpha particle irradiation, suggesting that the combined use of the HTR and the shape of Peak 5 could lead to improved ionisation density discrimination for particles of high LET.     

A comparative study on the susceptibility of LiF:Mg,Ti (TLD-100) and LiF:Mg,Cu,P (TLD-100H) to spurious signals in thermoluminescence dosimetry

It is well known that spurious signals can occur in thermoluminescence dosimetry (TLD) whenever contaminants (i.e. dirt, oil, dust) are present on the surface of the TLD card or crystal during the read-out process. For TLD cards, the Teflon material can also contribute to the background noise and this contribution has been found to depend on the material's light absorption. These non-radiation-induced signals contribute to the total light output during TLD read-out and can lead to incorrect dosimetry especially for low-dose measurements such as personal dosimetry. However, these spurious signals are generally in the low-temperature channels and are mostly accompanied by abnormal glow curves. Most of the published reports dealing with this type of spurious TL signal are on the LiF:Mg,Ti (TLD-100) material. The relatively new TLD material, LiF:Mg,Cu,P, is more sensitive and has higher signal-to-noise ratio than the traditional LiF:Mg,Ti. In this study, the effects of disturbing signals to the LiF:Mg,Cu,P (TLD-100H) cards used in personal dosimetry are investigated and compared with those of LiF:Mg,Ti (TLD-100).     

A perspective on road fatalities in Jeddah, Saudi Arabia.

The aim of this paper is to study an epidemiological aspect of fatalities from motor vehicle accidents in Jeddah, Saudi Arabia. All fatal accidents that occur in Jeddah between 1 January and 31 December 1987 are analyzed. The death rates per hundred million vehicle kilometers of travel, per 100,000 registered motor vehicles, and per 100,000 resident population in 1987 were about 1.87, 19.7, and 26.5, respectively. An attempt is made to use international death rates to compare Jeddah with other countries. The figures reveal higher death rates per vehicle travel and per resident population but lower rate per registered vehicle than in some industrialized countries. An additional attempt was made to estimate the cost of road traffic fatalities in Jeddah, which is an extremely difficult task due to lack of reliable data. This study shows that the cost of 1987 road fatalities in Jeddah is estimated to be 648.7 million Saudi Riyals (US$172.5 million).     

Measurements of indoor gamma radiation dose in At-Taif city, Saudi Arabia using CaSO4:Dy (TLD-900)

This paper describes measurements of indoor gamma radiation dose in At-Taif city, Kingdom of Saudi Arabia. CaSO(4):Dy thermoluminescence dosemeters were used for the detection of gamma radiation in 250 houses within the city. The values of indoor gamma radiation dose rate measured ranged between 90 and 221 nGy h(-1) for the time interval from September 2002 to September 2003. The measured dose varied with the type of house and season of the year. The highest average value 192 +/- 7 nGy h(-1) was measured inside apartments made of cement and brick (157-221 nGy h(-1)), and the lowest average value 92 +/- 6 nGy h(-1) in mud houses (58-117 nGy h(-1)). Intermediate values 154 +/- 5 nGy h(-1) (128-177 nGy h(-1)) and 167 +/- 9 nGy h(-1) (144-185 nGy h(-1)) were observed in large halls and villas, respectively. The average indoor gamma radiation dose rate received by the population of At-Taif city is 138 nGy h(-1) and its corresponding annual dose is 1211 muGy y(-1). The calculated mean effective dose to At-Taif city population, assuming an indoor occupancy factor of 80%, is 970 muSv y(-1).     

Phenomenological study of the ionisation density-dependence of TLD-100 peak 5a

Horowitz and collaborators have reported evidence on the structure of TLD-100 peak 5. A satellite peak, called 5a, has been singled out as arising from localised electron-hole recombination in a trap/luminescent centre, its emission mechanism would be geminate recombination and, therefore, its population would depend on incident radiation ionisation density. We report a phenomenological study of peak 4, 5a and 5 strengths for glow curves previously measured at UNAM for gammas, electrons and low-energy ions. The deconvolution procedure has followed strict rules to assure that the glow curve, where the presence of peak 5a is not visually noticeable, is decomposed in a consistent fashion, maintaining fixed widths and relative temperature difference between all the peaks. We find no improvement in the quality of the fit after inclusion of peak 5a. The relative contribution of peak 5a with respect to peak 5 does not seem to correlate with the radiation linear energy transfer.     

Some effects of yield strength on the stress corrosion cracking behaviour of low alloy steels in aqueous environments at ambient temperatures

The present paper describes the results of a literature review on the effects of material yield strength on the threshold stress corrosion cracking (SCC) condition known as K_1SCC, of a series of low alloy steels in various aqueous environments at ambient temperatures. It has been shown that an increase in yield strength reduced the value of K_1SCC and that this effect was more significant over the lower yield strength range 600–1200 MPa; at higher yield strength levels the effects of yield strength were much less evident. A review of the results indicated that SCC transgranular fracture was seen at yield strength values below around 1200 MPa while above this value the fracture path was exclusively intergranular in nature. Note that the transition point between transgranular and intergranular SCC coincided with the point at which yield strength effects were much reduced. A series of models reported in the literature, which attempted to explain the effects of yield strength on K_1SCC in terms of a fracture mechanics framework were examined with the introduction of a critical distance concept. Finally, the predictions of a grain size effect on the point at which the fracture path changed from transgranular to intergranular were taken from one model and used to show that the large data scatter in the region where transgranular fracture was operative, viz., the lower yield strength region of 600–1200 MPa, could be the result of grain size variations between the different reported studies.     

Seasonal performance of a central air conditioner in Dhahran, Saudi Arabia. A computer simulation study

Seasonal performance of four standard, residential-sized, central air conditioners in a typical hot and humid climate of Gulf environment was measured. A computer model was developed to stimulate the residential building cooling loads and the resulting seasonal energy consumption. For an average 100 m² residence located in Dhahran, equipped with a 17.6 kW air conditioner, simulations show a consumption of ≈22200 kWh of electricity for space cooling over 2600 operation hours, this being within 13% of the measured value. The seasonal performance of three lower capacity (10.5, 12.3 and 14.0 kW) air conditioners for the same duty was also predicted. With the 14.0 kW capacity unit the electricity consumption is reduced by ≈ kWh and can satisfy the load for 96.6% of the time. The smallest (10.5kW) of the four systems can supply the house cooling load for only ≈78% of the time with a seasonal energy consumption of 17 350 kWh.     

Computer aided evaluation of concrete strength data at the water tower project in Al Kharj,Saudi Arabia—Case study

Compressive strength data representing concrete used to slipform the water tower shaft were evaluated. The evaluation was performed utilizing the computer model, the complete quality control report (CQC Report), which was set up on the basis of the project specification, and statistical methods and criteria given in ACI 214-77. A continuing record of the strength data was maintained and analysed for individual strength tests, cumulative average, moving average, average required strength, overall standard deviation, coefficient of variation, ranges of individual tests, moving range, within-test coefficient of variation, and the adequacy of control over concrete and testing operations. The test data were used for three main purposes: (a) to assist the batch plant in the production of uniform, high quality concrete for slipforming; (b) to determine the adequacy of testing procedures, and (c) towards establishing and refining the statistical relationships for future similar projects. An early indication of the quality of concrete, which is vital for continuous slipforming, was obtained by estimating the predicted 28-day strengths using 1-, 3- and 7-day data and the strength-maturity relationship developed in accordance with ASTM C 918. Based on the total weighted percentage average errors and the integral absolute error it appears that all three sets of early strentth data were approximately equal in their validity as prediction bases. Analyses of test results show that the same prediction equation which was used to determine the potential 28-day strength can also be used, in the reverse form, to specify the early strength criterion for quality control purposes, provided that the data are normally or near normally distributed. Using CQC Report it was possible to process and analyse the vast amount of data in a very short time, and to provide the slipform contractor with key information regarding the probable quality of the concrete as placed. This allowed the contractor to note immediately a descending trend in quality of the concrete, to make corrective adjustments in the manufacturing process and to continue concreting with a high degree of confidence.     

Kinetic modelling of the optically stimulated conversion of peaks 5a and 5 to peak 4 in LiF:Mg,Ti (TLD-100)

The TC/LC conversion model for peaks 4, 5a and 5 in LiF:Mg,Ti (TLD-100) has been studied by solution of the coupled differential equations describing the charge carrier traffic following optical stimulation. Aspects of the model investigated were (i) the two-component exponential decay of the composite peak 5 TL intensity following the bleach, (ii) the role of retrapping during bleaching, (iii) the hole nature of peak 4 and (iv) the conversion of peak 5a traps to peak 4 traps. The high conversion efficiency is naturally explained due to the absence of conduction band competitive mechanisms in the optical ionisation of the electron in the e-h occupied structure corresponding to peak 5a and thereby leading to the hole-only occupied TC/LC leading to peak 4.     

Localised and delocalised optically induced conversion of composite glow peak 5 in LiF:Mg,Ti (TLD-100) to glow peak 4 as a function of postirradiation annealing temperature

The composite structure of glow peak 5 in LiF:Mg,Ti (TLD-100) has been investigated using optical bleaching by 310 nm (4 eV) light. The glow peak conversion efficiency of peak 5a (Tm = 187 degrees C) to peak 4 traps is very high at a value of 3+/-0.5 (1 SD) whereas the glow peak conversion efficiency of peak 5 (Tm = 205 degrees C) to peak 4 traps is 0.0026+/-0.0012 (1 SD). The high conversion efficiency of peak 5a to peak 4 arises from direct optical ionisation of the electron in the electron-hole pair. leaving behind a singly-trapped hole (peak 4), a direct mechanism, relatively free of competitive mechanisms. Optical ionisation of the 'singly-trapped' electron (peak 5), however, can lead to peak 4 only via multi-stage mechanisms involving charge carrier transport in the valence and conduction bands, a mechanism subject to competitive processes. The conduction/valence band competitive processes lead to the factor of one thousand decrease in the conversion efficiency of peak 5 compared to peak 5a.     

Dry sliding wear behaviour at ambient and elevated temperatures of plasma transferred arc deposited aluminium composite coatings

Abstract

Plasma transferred arc (PTA) surfacing is a surface engineering process in which a coating is deposited on the substrate by the injection of metal powders and/or ceramic particles into the weld pool created by the formation of a plasma plume. The present work involved the tribological evaluation of metal matrix composite (MMC) coatings deposited onto an aluminium alloy using the PTA technique. Coatings were fabricated by the deposition of an Al–Ni powder containing either Al₂O₃ or SiC particles. Dry sliding wear behaviour of the coatings was evaluated at ambient and elevated temperatures. Under sliding conditions of low applied stress and ambient temperature, reinforcement properties such as interfacial structure and fracture toughness have a significant influence on wear resistance. The SiC particles, which exhibit high interfacial bonding and toughness, support the matrix by acting as load bearing elements, thereby delaying the transition in wear mechanism as applied stress increases. As applied stresses exceed the fracture strength of the SiC and Al₂O₃ particles, these particles suffer fragmentation and/or debonding and no longer support the matrix. At higher stresses and elevated temperature, matrix properties such as flow stress and the tribolayer formation play more important roles in determining wear resistance.     

Supralinearity of peaks 5a, 5 and 5b in TLD-100 following 6.8 MeV and 2.6 MeV He ion irradiation: the extended track interaction model

The extended track interaction model (ETIM) is applied to the TL fluence response of peaks 5, 5b and 5a following 6.8 MeV and 2.6 MeV He ion irradiation of LiF:Mg,Ti (TLD-100).     

Characterizing the bond strength of geopolymers at ambient and elevated temperatures

This paper presents characterization of bond strength of geopolymers at ambient and elevated temperatures. The bond strength of 18 different formulations of metakaolin (MK)/fly ash (FA) based geopolymers is evaluated through double shear tests in 20–300 °C temperature range. The test parameters include fly ash content, SiO₂/K₂O ratio, solid-to-liquid ratio and Si/Al ratio. In addition the effect of additives, namely short carbon fibers, basalt fibers and styrene–acrylate emulsion in MK/FA precursor, on bond strength is studied. Data from the tests show that geopolymers exhibit slightly lower bond strength than that of epoxy resin at room temperature, however geopolymers retain much higher bond strength in 100–300 °C range. Addition of small quantity of short carbon fibers in MK/FA precursor does not significantly influence bond strength of geopolymers at ambient temperature, but greatly improve bond strength retention in 100–300 °C through crack control mechanism.