An analysis of the two concepts the failure and crack propagation in various materials has always been of interest to researchers. Thus, it is of necessity to investigate the failure of construction steel as one of the most widely used materials in the industry. Numerical modeling is always a compliment to the analysis of laboratory samples. One important issue, particularly in failure problems, is to study the behavior of laboratory samples according to their dimensions. In the current research, the effect of sample thickness size on crack tip behavior is numerically examined. A standard CT SPECIMEN is commonly used to evaluate the failure of ductile materials. The crack tip behavior along the thicknesses of the laboratory samples is a combination of plane stress and plane strain behavior. Accordingly, in the present study, the effect of thickness on the numerical samples was investigated via the numerical result validation. The validated results then were used as a complement to the experimental results. Modeling and analysis of the numerical samples of varying thicknesses indicated that, with progression from the sample thickness center towards the free edges, the behavior shifts from plane strain to plane stress. In the case of the standard CT SPECIMEN with 25 mm crack length, the samples with greater than 15 mm thickness have an almost plane strain behavior, and the results are proved to be reliable. Then, with further analysis and taking into account the parameters dependent on sample size, loading value, and stress-strain values perpendicular to the equation plane, an equation was presented which can be used to realize to what extent the behavior in the free edge of the CT SPECIMEN operates in the form of plane stress or plane strain.

Background and purpose: Coronal CT scan of paranasal sinuses with contiguous images is the gold standard technique in diagnosis of sinusitis. Considering the expense and high dose of radiation in conventional CT scan, clinical measurements and simple radiography is not enough sensitive and specific, some workers have suggested limited screening (non contiguous) instead of conventional CT image. This study compared the diagnostic accuracy of coronal CT with limited screening (4 screens) to conventional CT in diagnosis of sinusitis.Materials and methods: On 100 patients suspected of sinusitis referred to Emam Khomeini hospital. Coronal CT scan (with 16 screens) was performed. And on the basis of relevant scout view, four images of limited screenings (non contiguouse images) were selected and printed. Images of the both CT techniques were reported seperately by two radiologists. (Totally, four CT reports for each patients were given) The results of standard and limited screening CT reports given from the first radiologist were compared with the reports of the same, given by the second radiologist.Results: The comparitive results for the standard and limited screening techniques reported by two radiologists referred to as kappa value were 0.95 and 0.97 respectively. Which indicate a good aggreement between two radiologists in giving the reports of the images. Limited screening technique generally had the rate of 94.1% and 93.5% for sensitivity and accuracy respectively. General prevalence of sinusitis in the patients under study was 67%. Maxillar sinus was the most involved one with result of 98.7%.Conclusion: Limited screening CT technique for paranasal sinuses can be a proper technigue as a first evaluative measurement in the patients suspected for sinusitis. For this, can replace simple radiography. Advantage of this technique can be due to use of less dose of radiation, more speed of radiography and less expenses.

Under cyclic loading, the plastic zone becomes complicated during unloading. The energy-absorbing cyclic plastic zone is a stimulant for crack growth and can be a criterion for determining the damage around the fatigue crack tip. Presenting analytical models for determining the shape and size of the plastic zone often makes restrictive assumptions such as elastic-perfectly plastic response. In this research, the effects of non-linear kinematic hardening behavior of pure copper on the cyclic plastic reaction of the crack tip in different conditions were investigated. Chaboche nonlinear material model was used to determine the hardening parameters. According to the numerical results, the cyclic plastic zone around the crack tip was constant in the same load range but load ratio had a slight effect on this zone. Moreover, presence of the kinematic hardening in the cyclic loading caused reverse plastic zone to be predicted smaller than analytical model. According to the results, for materials such as pure copper with kinematic hardening behavior, the cyclic plastic zone increases with increase in the crack length. Therefore, the cyclic plastic zone, as well as other parameters in the fracture mechanics can be a proper criterion for fatigue crack growth studies.

In this paper, the effect of bandwidth and loading level parameters changes of random load histories on the fatigue crack growth is studied by the central limit theory. In this method, the life probabilistic distribution and probability of failure function and reliability function are derived by the central limit theory. The walker equation is used to account the stress ratio effect in the fatigue crack growth rate equations. Then, the theoretical results are verified by the test results in the three random loading conditions with the various bandwidths and loading levels. A good agreement among the theory and test results was observed. The probability of failure and reliability diagrams via number of cycles is presented too.