Applicability of empirical models for evaluation of stress ratio effect on the durability of fiber-reinforced creep rupture–susceptible composites
Fiber-reinforced polymer–matrix composites are known to exhibit loading rate- and time-dependent mechanical response. Their fatigue strength is determined by a complex interaction of damage processes governed by loading duration and cycle number. Apart from mechanistic approaches, a number of empirical models of various sophistication have been proposed to predict the durability of composites, differing in the amount of experimental data needed for their application. The accuracy of several such models is evaluated by comparing the prediction to the experimentally determined stress ratio effect on fatigue life of glass fiber-reinforced polyester–matrix composite. It is found that the accuracy of prediction generally improves with increasing the amount of test data needed for model calibration. However, the most accurate method of fatigue life estimation, among the selected ones, is by the modified Goodman diagram.
A CRITICAL ASSESSMENT OF KASSAPOGLOU'S STATISTICAL MODEL FOR COMPOSITES FATIGUE
Kassapoglou has recently proposed a model for fatigue of composite materials which seems to suggest that the fatigue SN curve can be fully predicted on the basis of the statistical distribution of static strengths. The original abstract writes expressions for the cycles to failure as a function of R ratio are derived. These expressions do not require any curve fitting and do not involve any experimentally determined parameters. The fatigue predictions do not require any fatigue tests for calibration". These surprisingly ambitious claims and attractive results deserve careful scrutiny. We contend that the result, which originates from the reliability theory where exponential distributions is sometimes used to model distribution of failures when age (or wearout) has no influence on the probability of failure, does not conform to a fatigue testing with the resulting SN curve distribution. Despite Kassapoglou's attempt to use a wearout law which seems to confirm this result even with wearout, we contend that a proper statistical treatment of the fatigue process should not make wear-out constants disappear, and hence the SN curves would depend on them, and not just on scatter of static data. These concerns explain the large discrepancies found by 3 independent studies which have tried to apply Kassapoglou's model to composite fatigue data.
Coexistence of Virulence and Antibiotic Resistance Genes in Pasteurella multocida Isolated from Diseased Rabbits
Pasteurella multocida (P. multocida)is considered a predominant pathogenic bacterial agent causing respiratory manifestations (snuffles) in rabbits with considerable economic losses and unfavorable prognosis in Egypt. A few recent P. multocida strains exhibit resistance to the most commonly used antibiotics in the veterinary field. Therefore, the present study was carried out to investigate the prevalence of both virulence and antibiotic resistance genes among P. multocida isolated from diseased rabbits in Sharkia Governorate, Egypt. Only 10 out of 100 tested rabbits` nasal swabs were finally confirmed positive for P. multocida of serogroup A by polymerase chain reaction (PCR). Antibiotic susceptibility testing of the recovered isolates revealed that they were all multidrug resistant (MDR) with a predominance of resistance to amoxicillin, ampicillin, amoxicillin/clavulanic acid, neomycin and tetracycline (100% each), followed by kanamycin and streptomycin (90% each). All the recovered isolates were further subjected to PCR screening of some common virulence and antibiotic resistance genes of interest . With the exception of toxA gene, the other virulence associated genes (ptfA, Omp87 and nanB) were found among all the examined isolates. Totally, all MDR P. multocida isolates contained at least one antibiotic resistance gene with aphA1 being the most prevalent (100%), followed by ermX gene (40%). Antibiotic resistance genotyping demonstrated the presence of multiple antibiotic resistance genes among majority of the isolates (40%) with only one isolate harboring 4 genes encoding identical resistance phenotypes. Evidentially, all MDR P. multocida isolates possessed at least 3 virulence genes accompanied by the attendance of antibiotic resistance genes. These findings evidenced that rabbits are potential sources of pathogenic P. multocida strains harboring virulence and antibiotic resistance genes. Therefore, it is evident that there is an urgent need for the judicious use of antibiotics in rabbits` treatment systems to successfully mitigate the propagation of drug resistance across P. multocida species.