Denis Najjar

Estimating the parameters of a generalized lambda distribution

The method of moments is a popular technique for estimating the parameters of a generalized lambda distribution (GLD), but published results suggest that the percentile method gives superior results. However, the percentile method cannot be implemented in an automatic fashion, and automatic methods, like the starship method, can lead to prohibitive execution time with large sample sizes. A new estimation method is proposed that is automatic (it does not require the use of special tables or graphs), and it reduces the computational time. Based partly on the usual percentile method, this new method also requires choosing which quantile u to use when fitting a GLD to data. The choice for u is studied and it is found that the best choice depends on the final goal of the modeling process. The sampling distribution of the new estimator is studied and compared to the sampling distribution of estimators that have been proposed. Naturally, all estimators are biased and here it is found that the bias becomes negligible with sample sizes n>=2x10^3. The .025 and .975 quantiles of the sampling distribution are investigated, and the difference between these quantiles is found to decrease proportionally to 1/n. The same results hold for the moment and percentile estimates. Finally, the influence of the sample size is studied when a normal distribution is modeled by a GLD. Both bounded and unbounded GLDs are used and the bounded GLD turns out to be the most accurate. Indeed it is shown that, up to n=10^6, bounded GLD modeling cannot be rejected by usual goodness-of-fit tests.

Modelling of the Vickers hardness of paint coatings deposited on metallic substrates

Abstract Hardness measurements of paint coating deposited on galvanised steels used for domestic appliance housing were performed by different techniques depending on the applied load: Vickers ultra-microhardness (0.4–100 mN), Vickers microhardness (0.05–20 N) and Vickers macrohardness (50–1200 N). The Jonsson and Hogmark model based on the area law of mixture used to characterise the composite (substrate+film) response hardness is not consistent with the experimental results. The authors propose modifications, taking into consideration the very strong plastic deformation of the film under the applied load. These modifications are verified by metallographic observations of the deformed coating near the indentation print and introduced in two other models. The hardness variation with the indentation depth is studied and therefore a guideline is proposed to improve the mar resistance of organic coatings.

Multiscale analysis of abrasion damage on stainless steel

Abstract Topography is a surface characteristic which is as important as the chemical composition with regard to its functional properties. Topography can be mainly described by three kinds of roughness parameters: amplitude, frequency and hybrid parameters. The present study aims to perform a multiscale morphological analysis of 316L austenitic stainless steel ground surfaces with 11 different paper grades. It is shown that the values of the most commonly used roughness parameters Ra (average arithmetic roughness) and Rt (peak to valley) depend on the observation scale with values distributed into three characteristic stages. At small observation scales, less than ∼5 μm, the abraded surfaces are not characterised by any particular scaling law. At large observation scales, more than ∼60 μm, the evolution of the roughness parameters indicates that the abrasion mechanism follows an homothetic scaling law for the different paper grades. These two extreme stages are separated by a transition stage that can be characterised by an index, which exhibits an increasing linear evolution with the size of the abrasive grains.

Influence of Hot Molding Parameters on Tribological and Wear Properties of a Friction Material

The manufacturing process of organic friction material used for braking applications is of importance with regard to their properties and performance. This article deals with a friction material manufactured according to two different hot molding conditions. An original analysis of polymerization of the phenolic resin matrix led to the choice of temperature and duration of the hot molding process. Two materials were developed, changing the hot molding process according to these two dependent parameters, and their friction and wear behaviors were investigated for various thermal severities of sliding conditions. The thermal conductivity was also analyzed and found to be higher when the hot molding temperature was low and the hot molding duration was long. Wear test results showed that friction was not significantly affected by the process change. On the contrary, wear appeared to be sensitive to hot molding temperature and duration. Hot molding at elevated temperature for a short duration led to higher wear resistance because the rubbing conditions were not severe. On the contrary, hot molding at low temperature for a long duration was in favor of the wear resistance for higher severities of the rubbing conditions.

Relationships between the heterogeneous microstructure, the mechanical properties and the braking behaviour of an organic brake lining material

Industrial brake lining materials are composite with complex formulations consisting of multiple constituents. Resulting from the fabrication process, the morphology and distribution of the constituents have significant influences on the future properties and braking performance. In this study, an in-depth analysis ranging from the microscale to the macroscale was performed to assess the relationships between the microstructure, the mechanical properties and the braking performance of an industrial brake lining material formulated for heavy vehicles. It was observed that the manufacturing process had different effects on the morphology and size of constituents and on their distribution in the phenolic binder. The morphologies of large organic particles such as rubber and graphite were affected by the mixing procedure, contrary to those of fibres and mineral particles. A transverse anisotropy consistent with fibre orientation due to cold preforming and hot moulding was observed. The microstructure displayed a strong local heterogeneity right up to the mesoscopic scale at which friction and wear mechanisms typically occur. The mechanical properties were analysed with regard to the heterogeneity of the microstructure to determine the scale at which these properties could be considered to be associated with a homogenised behaviour. The rubbing surface after braking showed that load-bearing localisation depends on the nature, morphology and orientation of constituents but that this heterogeneity can be of interest with regard to the braking ability.

Comments on the mixture detection rule used in SPC control charts

When calculating independently the false alarm rate of the eight usual runs rules used in SPC control chart, it appears that the proposed rule designed to detect mixture patterns corresponds to a Type-I error strongly lower than the seven other rules. This discrepancy is underlined and the mixture rule is showed to be useless both for in-control and out-of-control processes. Thus a modification of the mixture detection rule is proposed and the impact of this new mixture rule is then illustrated and discussed using Monte Carlo calculations.

Surface Cracking and Degradation of Dense Hydroxyapatite through Vickers Microindentation Testing

Surface degradation and cracking of dense hydroxyapatite were evaluated through Vickers micro indentation using indentation loads ranged from 25 gf to 2000 gf. Crack lengths, imprint diameters and the number of lateral cracks and chips were measured using SEM. The crack length-indentation load data were analyzed with regard to the specific relations of Palmqvist and fully developed radial cracks. Crack type transition load from Palmqvist to median crack was experimentally assessed through serial sectioning technique. The analytical estimated transition load, based on the theoretical relation of the indentation load and crack lengths showed a good agreement with one obtained from experimental itinerary. Palmqvist and median cracks were identified in low and medium indentation loads, respectively. High indentation load could also lead to the formation of lateral cracks and chips. The tendency for lateral cracking was evaluated taking into account the number of lateral cracks and chips. The chips were found to be appeared just after test in higher indentation load, whereas in medium loads they could be detectable only after several weeks.

Mechanical Behavior Study of Plasma Sprayed Hydroxyapatite Coatings onto Ti6Al4V Substrates Using Scratch Test

Mechanical behavior and fracture mechanisms of plasma sprayed hydroxyapatite coatings on Ti-6Al-4V substrate were assessed taking into consideration two variables: the coating thickness and the substrate roughness. The results show that the specimens having a substrate arithmetic average roughness parameter Ra = 2.29 µm is favorable with respect to Ra = 1.23 µm. For coating thickness above 105 µm, cracks can be observed in the coating/substrate interface and the higher critical load Pc2 (used generally in comparative evaluation of adherence) decreases. A 90 µm coating thickness sprayed on a substrate having an arithmetic average roughness parameter Ra equal to 2.29 µm seems to be the best compromise between microstructure, mechanical resistance (high critical loads and fairly good contact quality) and long term stability in the physiological medium (low dissolution rate) for an orthopedic application.

Hardness Evaluation of Porous Hydroxyapatite Coating

The extensive use of appropriate coatings to improve wear resistance, friction coefficient, electrical properties, corrosion resistance and biomedical application has stimulated a growing interest in their mechanical properties and especially hardness testing that is routinely used for coating evaluation. In this study Jonsson and Hogmark model is applied for the porous hydroxyapatite produced by plasma spraying on Ti6A14V substrate. Firstly, the effect of indentation load on hardness values of coating and substrate are studied. The modified Jonsson and Hogmark model is used to explain the composite hardness behavior and the effect of coating porosity.

Common Features on Damage Mechanisms Identified on Various Metal and Ceramic on Polyethylene Articulating Surfaces of Total Hip Prostheses

This paper presents a methodology for assessing the in-vivo degradation mechanisms of articular components of total hip replacement (THR) prostheses of Charnley type. The experimental procedure revealed that common features can be observed even if the clinical cases under investigation were quite different with regard to the demographic data. It particularly emphasises the detrimental effects of foreign bodies on the damage of the articulating surfaces. These foreign bodies can migrate into the joint space before embedding definitely into the Ultra High Molecular Weight Polyethylene (UHMWPE) acetabular cup surface where they further participate to a third body wear mechanism accelerated by a progressive increase of the femoral head roughness. Our experimental results underline, from a practical point of view, the need for careful manufacturing and clinical handling of porous surfaces, advocate for a systematic assessment of retrieved components, particularly when changed because of unexplained wear, and make questionable the clinical use of multifilament trochanteric cables.