finite element modeling

Nondestructive evaluation of mechanical properties and health monitoring of composite structures is of great importance. The aim of this research was to propose a nondestructive evaluation method based on the propagation of Lamb waves to investigate the matrix cracking damage in laminated cross-ply composites. For this purpose, after developing the finite element model of a glass/epoxy composite, the Lamb wave behavior was studied in two antisymmetric (A0) and symmetric (S0) modes with different excitation frequencies. The dispersion curves of the A0 and S0 modes of the Lamb wave are then obtained by considering different crack densities. It was observed that the effect of matrix crack density and the loss of mechanical properties of the cross-ply composites on the phase velocity of the S0 Lamb wave mode was higher than that of the A0 mode. Furthermore, the detection of matrix cracking was better performed using the Lamb wave velocity at high frequencies close to the cut-off frequency. It was observed that the cut-off frequency decreased by increasing the crack density, and the drop in the Lamb wave velocity increased by increasing the number of 90° layers in the cross-ply composite. It was concluded that the Lamb wave propagation simulation method can be used as a virtual laboratory for nondestructive evaluation of composites and detection of matrix crack density in them.

This study aimed to propose an NDE method based on the guided wave propagation for assessing the thickness loss (corrosion) in the metal pipes coated with composites. First, the finite element model of a steel pipe with the thickness of 4 mm and diameter of 200 mm coated with a layered composite material was developed, in which the composite coating constituted by the chopped strand glass fiber mat and woven roving glass fiber cloth layers. Then, the fundamental antisymmetric guided wave mode with the frequency of 100 kHz was propagated in the longitudinal direction of the structure and the phase velocity of the propagated wave was measured in specimens with different corrosion extent in the steel pipe. In the first step, a uniform corrosion was induced throughout the pipe, and in the next step, it was induced in a part of the pipe with a specific length and circumferential angles of 90, 180 and 360 degrees. It was shown that the reduction in the wave phase velocity in the corroded regions compared with the intact regions was between 9% to 33% for different corrosion extents. Besides, the detection of the corrosion in the steel pipe and its location and extent was properly performed using the guided wave propagation method. It was concluded that the simulated guided wave propagation method can be used as a virtual lab for the development of methods for nondestructive evaluation of pipes coated with composites and detection of the location and extent of corrosion in them.

The nickel-based superalloys high sensitivity to various types of cracks during fusion welding and post welding heat treatment, leads to limitations in the repair welding of damaged parts of this material. Strain aging crack is a type of prevalent crack in the IN738LC superalloys that occurs during the post-weld heat treatment, which was investigated in few studies. In this study, using numerical modeling, the susceptibility to strain aging cracking in IN738LC superalloy welded by TIG method without filler metal was investigated. The results of thermal-mechanical modeling as well as experimental evaluation of cross-section of the autogenous welded samples after post-heat treatment showed that the transverse plastic strain of (rather than longitudinal) plastic is a suitable preliminary criterion for evaluating the susceptibility of superalloy to the strain aging cracking and it seems that if the plastic transverse strain is more than 4.6%, the probability of this crack occurring after post weld heat treatment will be very high.

Nondestructive evaluation (NDE) of the mechanical properties and thickness loss of bilayer structures is of great importance for health monitoring purposes. This study aimed to propose an NDE method based on the Lamb wave propagation for the inspection of bilayer metal-composite structures. The finite element model of a steel substrate coated with a layered composite material was developed, where the composite coating constituted by chopped strand glass fiber mat and woven roving glass fiber cloth layers. The fundamental antisymmetric Lamb wave mode (A0) with different excitation frequencies were generated and propagated on the modeled bilayer specimens. The dispersion curves for the A0 Lamb wave mode were obtained for the simulated specimens, considering different thicknesses and a range of material properties decay for the metal substrate and composite coating. The obtained results showed that the effect of the thickness and decay in the mechanical properties of the substrate on the A0 Lamb wave mode velocity was more than the effect of the thickness and decay in the mechanical properties of the composite coating. Besides, the estimation of coating thickness was performed more accurately at low frequencies, while the decay in the mechanical properties of the coating and substrate was better evaluated at higher frequencies. It was concluded that the simulated Lamb wave propagation method can be used as a virtual lab for the development of methods for nondestructive evaluation of bilayer metal-composite structures with different material properties and thicknesses.

Microlattices often show a higher strength-to-weight ratio and energy absorption than other porous materials, due to its smart geometry. nowadays basic efforts are made to study mechanical behavior of sandwich structures with microlattice core, in most of the studies, the sandwich geometry is flat. Curved sandwich structure with microlattice core, due to its curved geometry and outstanding advantages of microlattice, can provide a structure with high resistance and efficiency. In the present study, flat and curved sandwich microlattices are simulated by Abaqus software and the structures are analyzed under transvers loading. The results of the analysis for a unit cell are compared with the results of previous studies and the efficiency and accuracy of the present modeling are confirmed. The studied models have different curvature and face-sheets thickness. The results show a slight increase in the face-sheets thickness or curvature yields a significant increase in the resistance of these sandwich structures.

The replacement of damaged components is not affordable in important structures such as aircraft, ship or gas pipelines. So, the repair of a defective structure is an acceptable process. Composite patches are used to repair the damaged metal and composite structures in different industries, especially the aerospace industry. Assessment of the repaired structure is a challenging topic in order to ensure the restoration. The thermography technique is one of the most powerful non-destructive testing methods that is used to survey the repaired structures. In the present study, the defects of the de-bonding type between the based aluminum structure and the carbon/epoxy patch made by 4 layers with layup configuration [04] have been investigated by step heating thermography method. Defects locate close to the patch edges because it is more likely that debond onset in a repaired structure at edges in practice. Furthermore, detection of the edge defects is more difficult than the middle defects because of edge effects. The step heating thermography results have been processed by using pulse phase thermography (PPT) approach. Also, the simulation of thermography testing procedure carried out by finite element modeling (FEM). Finally, the results of the experiment and finite element modeling have been compared and good accuracy has been obtained in step heating thermography and PPT algorithm.

The purpose of this study is to demonstrate the advantages of computer simulation and parametric studies in improving the copper electroforming process. For this purpose, the finite element model for a cone geometry was prepared using the Comsol software, and the effect of key parameters including applied current density, solution electrical conductivity, electrode spacing, and anode length, on the thickness uniformity. In order to validate the model, a conical shell was produced in the laboratory by electroforming method, and the thickness distribution was compared with the simulation results. The comparison of the results showed that using the tertiary current distribution method to simulate the electroforming process is a precise and efficient model and can be used for parametric studies. Finally, after parametric study, it was determined that all selected variables had a significant effect on the thickness uniformity. In addition, it was found that the most to least significant variables were applied current density, solution conductivity, anode-cathode spacing, and anode length, respectively.

Ferrite - martensite dual-phase (DP) steels are a subset of advanced high strength steels which can be produced by applying heat treatment on low-carbon steels. The strength and toughness of DP steels are greater than those in ferrite – pearlite steels with the same chemical composition. In this study, mechanical properties and forming limit diagram of ferrite – pearlite and DP steels with the same chemical composition were investigated and compared. For this purpose, inter-critical quenching heat treatment was applied on a low-carbon steel with ferrite – pearlite microstructure to produce ferrite – continuous martensite DP steel. Tensile and hardness tests were used to determine the mechanical properties, and Nakazima test was used to determine the formability of ferrite – pearlite and DP steels. Forming limit diagram of steels was also simulated using finite element method in macro scale, and compared with experimental results. The results of mechanical tests showed that the yield stress, tensile strength and hardness of produced DP steel were increased 65, 91, and 87% respectively, in comparison to the same mechanical properties of ferrite – pearlite steel. Based on Experimental and simulation results of Nakazima test, the formability of DP steel is better than ferrite – pearlite steel. There was good agreement between simulation and experimental results.