Journal of Aerospace Mechanics
Year:2009 | Volume:5 | Issue:2 (16) (STRUCTURES AND MATHERIALS MECHANICAL BEHAVIOR)|Papers Count:7

In this paper, a physical and an analytical model for penetration of the conical projectiles into metallic targets under oblique impact are proposed. The failure by conical projectile is assumed to be asymmetric petalling and the analysis is performed using the energy balance between initial energy and the work done in the penetration. The work done in penetration consists of the required work for plastic deformation (WP), the work for transferring of material to new position (Wd), and the work for bending of the petals (Wb). The results obtained from the analytical model for the penetration of conical projectiles in thin metallic plates were compared with the existing theoretical and empirical results and showed relatively good agreements.

The objective of this study is to develop a general finite element formulation associated with an incremental adaptive procedure established for the calculation of contact pressure distributions in viscoelastic structures. A generalized Maxwell model has been used to model the viscoelastic constitutive equations in which bulk and shear relaxation functions are represented by sum of a series of decaying exponential functions of time. Based on the principle of virtual work, an effective finite element formulation, associated with an incremental relaxation procedure, has been developed. The viscoelastic contact behavior has been studied through an improved augmented Lagrangian approach, based on kinematical conditions of contact bodies. The proper convergence caused by the numerical examples with those obtained from analytical results shows the applicability of presented finite element formulation.

In this article, a non-destructive method for the frequency analysis and multi-crack detection in rotating shafts is presented, using transfer matrix method (TMM) and based on the Timoshenko beam theory. The cracks can be placed in different angles with respect to each other. The presence of cracks in structures change the local flexibility and results in reduction of natural frequency. The TMM is exerted for the frequency analysis of the shafts with discrepant boundary conditions. The resultant frequencies are used for requiring the mode shapes of the shaft corresponding to the two perpendicular planes (x-z and y-z). Subtracting these two mode shapes gives the position of the crack (cracks). Finally, the required frequencies by TMM are compared with some available experimental results. Close adaptation between these results is representing the high accuracy of the novel transfer matrix method.

In this paper, the mean value of strain energy density criterion has been used to predict the critical fracture load in specimens with U-shaped notch under bending loads. An equation for calculation of averaged strain energy density in this zone has been presented. Bending loads under mode I have been considered. Two cases have been studied: 1) the control volume only includes the semicircular arc of the notch root, and 2) the control volume includes rectilinear edge of the notch. The effect of notch root radius and notch depth on the mean value of strain energy density has been studied in two cases above and the critical fracture load rate has been predicted. Finite Element Analysis has been carried out for verification. The studies show that the critical fracture load increases and then decreases by increasing the notch root radius. Also, this parameter decreases exponentially by increasing the notch depth.

Predicting the status of contact between the pipe and the roll through deformation is considered as one of the problems of reshaping process in non-circular pipe production. In the present study, the elastic-plastic analysis is investigated assuming plane strain condition for reshaping process. One of the problems of the mentioned process is the separation of pipe from roll in some zones. The proposed model has the capability of predicting the separation of the pipe from roll, which results in better prediction of the defect. To verify the reliability of the present contact model, the result of the analysis was compared to those of experimental ones and showed good agreements.

In this paper, a method to determine the material parameters of the Lemaitre damage model from the knowledge of the Gurson model parameters is proposed. As for porosity damage, the Gurson model neglects the coupling of elasticity with damage, allowing to use the stress softening effect to make a link between both theories up to a fast identification procedure (based on the expressions of the yield surfaces of the damaged material). An experimental procedure to identify the Gurson damage parameters has been established for A533B alloy steel under stress triaxiality condition. This method is evaluated in the experimental results of A533B steel. The comparisons indicate good agreements between the result obtained by the proposed method and those of experiments.

In this paper, fault isolation of a laboratory scale structural frame, as a multivariate system, has been investigated, using a geometric approach in conjunction with parametric system identification. The proposed geometric approach is based on the assumption that each fault mode may be regarded as a hyper-surface in an appropriate topological space, where the hyper-surfaces are constructed based on the estimated parameters. Finally, by defining proper metric and assuming the unknown mode (of a system) being as a point in space, where the estimated parameters are the coordinates, the fault mode can be identified by minimizing the obtained distances between the point and each of the hyper-surfaces. Based on the number of inputs and measured outputs, the frame was modeled by standard ARX and VARX models in four different forms as Single-Input Single-Output (SISO), Single-Input Multiple-Output (MISO), Single-Input Multiple-Output (SIMO) and Multiple-Input Multiple-Output (MIMO). Also, the performance of the scheme was evaluated in deterministic and probabilistic spaces. The obtained results revealed that the MIMO representation of the frame in the probabilistic space had an acceptable performance which was the highest in comparison with the others and the SISO representation system in the deterministic space had the lowest performance.