Crack-growth of medical-grade silicone using pure shear tests

Silicone elastomers are commonly used in the manufacture of single-piece joint replacement implants for the finger joints. However, the survivorship of these implants can be poor, with failure typically occurring from fracture of the stems. The aim of this paper was to investigate the crack growth of medical-grade silicone using pure shear tests. Two medical-grade silicones (C6-180 and Med82-5010-80) were tested. Each sample had a 20 mm crack introduced and was subjected to a sinusoidally varying tensile strain, with a minimum of 0 per cent and a maximum in the range 10 to 77 per cent. Testing was undertaken at a frequency of 10 Hz. At various times during testing, the testing machine was stopped, the number of cycles completed was noted, and the crack length measured. Graphs of crack length against number of cycles were plotted, as well as the crack growth rate against tearing energy. The results show that Med82-5010-80 is more crack resistant than C6-180. Graphs of crack growth rate against tearing energy can be used to predict the failure of these medical-grade elastomers.

Development of areolae and growth of the peripheral prothallus in the crustose lichen Rhizocarpon geographicum:an image analysis study

Areolae of the crustose lichen Rhizocarpon geographicum (L.) DC., are present on the peripheral prothallus (marginal areolae) and also aggregate to form confluent masses in the centre of the thallus (central areolae). To determine the relationships between these areolae and whether growth of the peripheral prothallus is dependent on the marginal areolae, the density, morphology, and size frequency distributions of marginal areolae were measured in 23 thalli of R. geographicum in north Wales, UK using image analysis (Image J). Size and morphology of central areolae were also studied across the thallus. Marginal areolae were small, punctate, and occurred in clusters scattered over the peripheral prothallus while central areolae were larger and had a lobed structure. The size-class frequency distributions of the marginal and central areolae were fitted by power-law and log-normal models respectively. In 16 out of 23 thalli, central areolae close to the outer edge were larger and had a more complex lobed morphology than those towards the thallus centre. Neither mean width nor radial growth rate (RaGR) of the peripheral prothallus were correlated with density, diameter, or area fraction of marginal areolae. The data suggest central areolae may develop from marginal areolae as follows: (1) marginal areolae develop in clusters at the periphery and fuse to form central areolae, (2) central areolae grow exponentially, and (3) crowding of central areolae results in constriction and fragmentation. In addition, growth of the peripheral prothallus may be unrelated to the marginal areolae. © 2013 Springer Science+Business Media Dordrecht.

The influence of environment on foliose lichen growth and its ecological significance

This chapter considers various aspects of the influence of the environment on the growth of foliose lichens and its significance in determining the ecology of individual species. Radial growth (RaG) and growth in mass of foliose lichens is influenced by climate and microclimate and also by substratum factors such as rock and bark texture, substrate chemistry, and nutrient enrichment. Seasonal fluctuations in growth, as measured by radial growth rate (RaGR) per month, often correlate best with average or total rainfall, the number of rain days, or rainfall in a specific season. Temperature has also been identified to be an important climatic factor influencing growth in some studies. Interactions between microclimatic factors and especially light intensity, temperature, and moisture status are important in determining differences in growth in relation to aspect and slope of the substratum. The physical and chemical nature of the substratum has a profound influence on the growth of foliose lichens. Hence, the effects of texture, porosity, rate of drying, and the physical changes of the substratum on growth are likely to influence lichen distributions. Bird droppings may influence growth and survival by smothering the thalli, altering the pH, or adding inhibitory and stimulatory compounds. Nitrogen and phosphate availability may also influence growth. Chemical factors also have an important influence on lichens of maritime rocks, the effect of salinity and calcium ions being of particular importance. Effects of environmental factors on growth influence the competitive ability of a lichen and ultimately its ecology and distribution.

Uses of frontier efficiency methodologies and multi-criteria decision making for performance measurement in the energy sector

Guest editorial Ali Emrouznejad is a Senior Lecturer at the Aston Business School in Birmingham, UK. His areas of research interest include performance measurement and management, efficiency and productivity analysis as well as data mining. He has published widely in various international journals. He is an Associate Editor of IMA Journal of Management Mathematics and Guest Editor to several special issues of journals including Journal of Operational Research Society, Annals of Operations Research, Journal of Medical Systems, and International Journal of Energy Management Sector. He is in the editorial board of several international journals and co-founder of Performance Improvement Management Software. William Ho is a Senior Lecturer at the Aston University Business School. Before joining Aston in 2005, he had worked as a Research Associate in the Department of Industrial and Systems Engineering at the Hong Kong Polytechnic University. His research interests include supply chain management, production and operations management, and operations research. He has published extensively in various international journals like Computers & Operations Research, Engineering Applications of Artificial Intelligence, European Journal of Operational Research, Expert Systems with Applications, International Journal of Production Economics, International Journal of Production Research, Supply Chain Management: An International Journal, and so on. His first authored book was published in 2006. He is an Editorial Board member of the International Journal of Advanced Manufacturing Technology and an Associate Editor of the OR Insight Journal. Currently, he is a Scholar of the Advanced Institute of Management Research. Uses of frontier efficiency methodologies and multi-criteria decision making for performance measurement in the energy sector This special issue aims to focus on holistic, applied research on performance measurement in energy sector management and for publication of relevant applied research to bridge the gap between industry and academia. After a rigorous refereeing process, seven papers were included in this special issue. The volume opens with five data envelopment analysis (DEA)-based papers. Wu et al. apply the DEA-based Malmquist index to evaluate the changes in relative efficiency and the total factor productivity of coal-fired electricity generation of 30 Chinese administrative regions from 1999 to 2007. Factors considered in the model include fuel consumption, labor, capital, sulphur dioxide emissions, and electricity generated. The authors reveal that the east provinces were relatively and technically more efficient, whereas the west provinces had the highest growth rate in the period studied. Ioannis E. Tsolas applies the DEA approach to assess the performance of Greek fossil fuel-fired power stations taking undesirable outputs into consideration, such as carbon dioxide and sulphur dioxide emissions. In addition, the bootstrapping approach is deployed to address the uncertainty surrounding DEA point estimates, and provide bias-corrected estimations and confidence intervals for the point estimates. The author revealed from the sample that the non-lignite-fired stations are on an average more efficient than the lignite-fired stations. Maethee Mekaroonreung and Andrew L. Johnson compare the relative performance of three DEA-based measures, which estimate production frontiers and evaluate the relative efficiency of 113 US petroleum refineries while considering undesirable outputs. Three inputs (capital, energy consumption, and crude oil consumption), two desirable outputs (gasoline and distillate generation), and an undesirable output (toxic release) are considered in the DEA models. The authors discover that refineries in the Rocky Mountain region performed the best, and about 60 percent of oil refineries in the sample could improve their efficiencies further. H. Omrani, A. Azadeh, S. F. Ghaderi, and S. Abdollahzadeh presented an integrated approach, combining DEA, corrected ordinary least squares (COLS), and principal component analysis (PCA) methods, to calculate the relative efficiency scores of 26 Iranian electricity distribution units from 2003 to 2006. Specifically, both DEA and COLS are used to check three internal consistency conditions, whereas PCA is used to verify and validate the final ranking results of either DEA (consistency) or DEA-COLS (non-consistency). Three inputs (network length, transformer capacity, and number of employees) and two outputs (number of customers and total electricity sales) are considered in the model. Virendra Ajodhia applied three DEA-based models to evaluate the relative performance of 20 electricity distribution firms from the UK and the Netherlands. The first model is a traditional DEA model for analyzing cost-only efficiency. The second model includes (inverse) quality by modelling total customer minutes lost as an input data. The third model is based on the idea of using total social costs, including the firm’s private costs and the interruption costs incurred by consumers, as an input. Both energy-delivered and number of consumers are treated as the outputs in the models. After five DEA papers, Stelios Grafakos, Alexandros Flamos, Vlasis Oikonomou, and D. Zevgolis presented a multiple criteria analysis weighting approach to evaluate the energy and climate policy. The proposed approach is akin to the analytic hierarchy process, which consists of pairwise comparisons, consistency verification, and criteria prioritization. In the approach, stakeholders and experts in the energy policy field are incorporated in the evaluation process by providing an interactive mean with verbal, numerical, and visual representation of their preferences. A total of 14 evaluation criteria were considered and classified into four objectives, such as climate change mitigation, energy effectiveness, socioeconomic, and competitiveness and technology. Finally, Borge Hess applied the stochastic frontier analysis approach to analyze the impact of various business strategies, including acquisition, holding structures, and joint ventures, on a firm’s efficiency within a sample of 47 natural gas transmission pipelines in the USA from 1996 to 2005. The author finds that there were no significant changes in the firm’s efficiency by an acquisition, and there is a weak evidence for efficiency improvements caused by the new shareholder. Besides, the author discovers that parent companies appear not to influence a subsidiary’s efficiency positively. In addition, the analysis shows a negative impact of a joint venture on technical efficiency of the pipeline company. To conclude, we are grateful to all the authors for their contribution, and all the reviewers for their constructive comments, which made this special issue possible. We hope that this issue would contribute significantly to performance improvement of the energy sector.

The influence of environment on foliose lichen growth and its biological significance

Radial growth and growth in mass of lichens is influenced by climatic and microclimatic factors and also by substratum factors such as rock and bark texture, chemistry, and nutrient enrichment. Seasonal fluctuations in growth, as measured by radial growth rate (RaGR) per month, often correlate best with average or total rainfall, the number of rain days, or rainfall in a specific season. Temperature is also considered to be an important climatic factor in some studies. Interactions between microclimatic factors and especially light intensity, temperature, and moisture are the most important in determining local annual growth rates. The physical and chemical nature of the substratum has a profound influence on the growth of foliose lichens. Hence, the effects of texture, porosity, rate of drying, and the physical changes of the substratum on growth are likely to influence lichen distributions. Bird droppings may influence growth and survival by smothering the thalli, altering the pH, or adding inhibitory and stimulatory compounds. Nitrogen and phosphate availability may also influence growth. Chemical factors may also have an important influence on lichens of maritime rocks, the effect of salinity and calcium ions being of particular importance. Zinc, copper, and mercury may also be important in lichen growth as they have been shown to affect the chlorophyll content of lichen algae. Effects of environmental factors on growth influence the competitive ability of lichens thus influencing their ecology and distribution.

Within-site variation in lichen growth rates and its implications for direct lichenometry

Variation in lichen growth rates poses a significant challenge for the application of direct lichenometry, i.e. the construction of lichen dating curves from direct measurement of growth rates. To examine the magnitude and possible causes of within-site growth variation, radial growth rates (RaGRs) of thalli of the fast-growing foliose lichen Melanelia fuliginosa ssp. fuliginosa (Fr. ex Duby) Essl. and the slow-growing crustose lichen Rhizocarpon geographicum (L.) DC. were studied on two S-facing slate rock surfaces in north Wales, UK using digital photography and an image analysis system (Image-J). RaGRs of M. fuliginosa ssp. fuliginosa varied from 0.44 to 2.63 mmyr-1 and R. geographicum from 0.10 to 1.50 mmyr-1.5. Analysis of variance suggested no significant variation in RaGRs with vertical or horizontal location on the rock, thallus diameter, aspect, slope, light intensity, rock porosity, rock surface texture, distance to nearest lichen neighbour or distance to vegetation on the rock surface. The frequency distribution of RaGR did not deviate from a normal distribution. It was concluded that despite considerable growth rate variation in both species studied, growth curves could be constructed with sufficient precision to be useful for direct lichenometry. © 2014 Swedish Society for Anthropology and Geography.

Influence of macroscopic residual stress fields on fatigue crack growth measurement in SiC particulate reinforced 8090 aluminium alloy

The effect of residual stresses, induced by cold water quenching, on the morphology of fatigue crack fronts has been investigated in a powder metallurgy 8090 aluminium alloy, with and without reinforcement in the form of 20 wt-%SiC particles. Residual stress measurements reveal that the surface compressive stresses developed in these materials are significantly greater than in conventional metallurgy ingot 8090, because surface yielding occurs on quenching. The yield stresses of the powder route materials are greater than those of ingot produced 8090 and hence greater surface stresses can be maintained. In fatigue, severe crack front bowing is observed in the powder formed materials as a result of the reduction of the R ratio (minimum load/maximum load) by the compressive residual stresses at the sides of the specimen, causing premature crack closure and hence reducing the local driving force for fatigue crack growth ΔKeff. This distortion of the crack fronts introduces large errors into measurements of crack growth rate and threshold values of ΔK.

The influence of ageing on fatigue crack growth in SiC-particulate reinforced 8090

A study of the influence of SiC-particulate reinforcement on ageing and subsequent fatigue crack growth resistance in a powder metallurgy 8090 aluminium alloy-SiC composite has been made. Macroscopic hardness measurements revealed that ageing at 170°C in the composite is accelerated with respect to the unreinforced alloy, though TEM studies indicate that this is not due to the enhanced precipitation of S′. Fatigue crack growth rates in the naturally aged condition of the composite and unreinforced matrix are similar at low to medium values of ΔK, but diverge above ≈ 8 MPa√m owing to the lower fracture toughness of the composite. As a result of the presence of the reinforcement, planar slip in the composite is suppressed and facetted crack growth is not observed. Ageing at or above 170°C has a deleterious effect on fatigue crack growth. Increased ageing time decreases the roughness of the fracture path at higher growth rates. These effect are though to be due to microstructural changes occurring at or near to the SiC/matrix interfaces, providing sites for static mode failure mechanisms to operate. This suggestion is supported by the observation that as ΔK increases, crack growth rates become Kmax dependent, implying the crack growth rate is strongly influenced by static modes.

Hydrogen embrittlement contributions to fatigue crack growth in a structural steel

The detrimental effects of a hydrogen atmosphere on the fatigue resistance of BS 4360 steel have been assessed by a comparison of crack growth rates in air and hydrogen at a low cycling frequency (0.1Hz), and at a number of temperature (25, 50 and 80 °C). The crack propagation rates in air are almost independent of temperature over this range, but those measured in hydrogen differ by more than an order of magnitude between 25 and 80 °C. The greatest enhancement is seen at 25 °C and at high values of ΔK, the maximum occurring between 40–45 MPa √m at each temperature. There is little hydrogen contribution to crack growth at values of ΔK below 20 MPa √m for R = 0.1. The enhancement of crack growth rates is reflected by the presence of ‘quasi-cleavage’ facets on the fatigue fracture surfaces of specimens tested in hydrogen. These are most apparent where the greatest increases in growth rate are recorded. The facets show linear markings, which run both parallel and perpendicular to the direction of crack growth. The former are analogous to the ‘river’ lines noted on brittle cleavage facets, and reflect the propagation direction. The latter are more unusual, and indicate that facet formation by hydrogen embrittlement during fatigue is a step-wise process.

Short fatigue crack growth behaviour:data analysis effects

Consideration of the influence of test technique and data analysis method is important for data comparison and design purposes. The paper highlights the effects of replication interval, crack growth rate averaging and curve-fitting procedures on crack growth rate results for a Ni-base alloy. It is shown that an upper bound crack growth rate line is not appropriate for use in fatigue design, and that the derivative of a quadratic fit to the a vs N data looks promising. However, this type of averaging, or curve fitting, is not useful in developing an understanding of microstructure/crack tip interactions. For this purpose, simple replica-to-replica growth rate calculations are preferable. © 1988.

Short and long fatigue crack growth in a SiC reinforced aluminium alloy

Fatigue crack growth behaviour in a 15 wt% SiC particulate reinforced 6061 aluminium alloy has been examined using pre-cracked specimens. Crack initiation and early growth of fatigue cracks in smooth specimens has also been investigated using the technique of periodic replication. The composite contained a bimodal distribution of SiC particle sizes, and detailed attention was paid to interactions between the SiC particles and the growing fatigue-crack tip. At low stress intensity levels, the proportion of coarse SiC particles on the fatigue surfaces was much smaller than that on the metallographic sections, indicating that the fatigue crack tends to run through the matrix avoiding SiC particles. As the stress intensity level increases, the SiC particles ahead of the growing fatigue crack tip are fractured and the fatigue crack then links the fractured particles. The contribution of this monotonic fracture mode resulted in a higher growth rate for the composite than for the unreinforced alloy. An increase in the proportion of cracked, coarse SiC particles on the fatigue surface was observed for specimens tested at a higher stress ratio.

Surface damage and near-threshold fatigue crack growth in a Ni-base superalloy in vacuum

Fatigue crack growth rate tests have been performed on Nimonic AP1, a powder formed Ni-base superalloy, in air and vacuum at room temperature. These show that threshold values are higher, and near-threshold (faceted) crack growth rates are lower, in vacuum than in air, although at high growth rates, in the “structure-insensitive” regime, R-ratio and a dilute environment have little effect. Changing the R-ratio from 0.1 to 0.5 in vacuum does not alter near-threshold crack growth rates very much, despite more extensive secondary cracking being noticeable at R= 0.5. In vacuum, rewelding occurs at contact points across the crack as ΔK falls. This leads to the production of extensive fracture surface damage and bulky fretting debris, and is thought to be a significant contributory factor to the observed increase in threshold values.

Effects of grain size and microstructure on threshold values and near threshold crack growth in powder-formed Ni-base superalloy

Threshold stress intensity values, ranging from ∼6 to 16 MN m −3/2 can be obtained in powder-formed Nimonic AP1 by changing the microstructure. The threshold and low crack growth rate behaviour at room temperature of a number of widely differing API microstructures, with both ‘necklace’ and fully recrystallized grain structures of various sizes and uniform and bimodal γ′-distributions, have been investigated. The results indicate that grain size is an important microstructural parameter which can control threshold behaviour, with the value of threshold stress intensity increasing with increasing grain size, but that the γ′-distribution is also important. In this Ni-base alloy, as in many others, near threshold fatigue crack growth occurs in a crystallographic manner along {111} planes. This is due to the development of a dislocation structure involving persistent slip bands on {111} planes in the plastic zone, caused by the presence of ordered shearable precipitates in the microstructure. However, as the stress intensity range is increased, a striated growth mode takes over. The results presented show that this transition from faceted to striated growth is associated with a sudden increase in crack propagation rate and occurs when the size of the reverse plastic zone at the crack tip becomes equal to the grain size, independent of any other microstructural variables.

Lichen growth and lichenometry

Lichenometry is one of the most widely used methods available for dating the surface age of various substrata including rock surfaces, boulders, walls, and archaeological remains. It depends on the assumption that if the lag time before colonisation of a substratum by a lichen is known and lichen age can be estimated, then a minimum date can be obtained by measuring the diameter (or another property related to size) of the largest lichen at the site. Lichen age can be determined by variety of methods including calibrating lichen size against surfaces of known age (‘indirect lichenometry’), by constructing a growth rate-size curve from direct measurement of lichen growth (‘direct lichenometry’), using radio-carbon (RC) dating, and from lichen ‘growth rings’. This chapter describes: (1) lichen growth rates and longevity, (2) methods of estimating lichen age, (3) the methodology of lichenometry and (4) applications of lichenometry. Despite its limitations, lichenometry is likely to continue to play an important role in dating a variety of surfaces and also in providing data that contribute to the debate regarding global warming and climate change.

'Growth rings' in crustose lichens:comparison with directly measured growth rates and implications for lichenometry

Some species of crustose lichens, such as Ochrolechia parella (L.) Massal., exhibit concentric marginal rings, which may represent an alternative technique of measuring growth rates and potentially, a new lichenometric dating method. To examine this hypothesis, the agreement and correlation between ring widths and directly measured annual radial growth rates (RaGR, mm a^-1) were studied in 24 thalli of O. parella in north Wales, UK, using digital photography and image analysis. Variation in ring width was observed at different locations around a thallus, between thalli, and from year to year. The best agreement and correlation between ring width and lichen growth rates was between mean width of the outer two rings (measured in 2011) and mean RaGR (in 2009/10). The O. parella data suggest that mean width of the youngest two growth rings, averaged over a sample of thalli, is a predictor of recent growth rates and therefore could be used in lichenometry. Potential applications include as a convenient method of comparing lichen growth rates on surfaces in different environmental settings; and as an alternative method of constructing lichen growth-rate curves, without having to revisit the same lichen thalli over many years. However, care is needed when using growth rings to estimate growth rates as: growth ring widths may not be stable; ring widths exhibit spatial and temporal variation; rings may not represent 1-year's growth in all thalli; and adjacent rings may not always represent successive year's growth.