Halit S. Türkmen

Nonlinear Structural Response of Laminated Composite Plates Subjected to Blast Loading

We are concerned with the theoretical analysis of the laminated composite plates exposed to normal blast shock waves as well as presenting correlation between the theoretical analysis and the experimental results of the strain time histories. The laminated composite plate is clamped at all edges. On the theoretical side of the study, dynamic equations of the plates are derived by the use of the virtual work principle within the framework of the Love theory of plates. The geometric nonlinearity effects are taken into account with von Karman assumptions. Then the governing equations of the laminated plate are solved by the Runge-Kutta-Verner method. A new displacement function is considered for the theoretical solution of the blast-loaded clamped plate. Furthermore, finite element modeling and analysis for the blast-loaded composite plates are presented. On the experimental side of the study, tests have been carried out on the laminated composite plates with clamped edges for two different blast loads. The results of theoretical and finite element methods are compared with the experimental results. Theoretical and finite element analyses results are in a good agreement. There is a qualitative agreement between the analyses and experimental results in the first load case. The predicted peak strains and response frequency are in an agreement with the experimental results for first load case. Thus the theoretical solution may be used for providing material in the preliminary design stage. There is a difference between the analysis and experimental results in the second load case because of the extremely large deflections. In this study the effects of loading conditions, geometrical properties, and material properties are separately examined on the dynamic behavior, as well.

On the mechanical behaviour of AA 7075-T6 during cyclic loading

The mechanical behavior of an aluminum alloy during uniaxial cyclic loading is examined using finite element simulations of aggregates with individually resolved crystals. The aggregates consist of face centered cubic (FCC) crystals with initial orientations assigned by sampling the orientation distribution function (ODF) determined from the measured crystallographic texture. The simulations show that the (elastic) lattice strains within the crystals evolve as the number of cycles increases. This evolution is attributed to the interactions between grains driven by the local plasticity. Under constant amplitude strain cycles, the average (macroscopic) stress decays with increasing number of cycles in concert with the evolution of the lattice strains. Further, the average number of active slip systems also decreases with increasing cycles, eventually reaching zero as the material response becomes totally elastic at the grain level. During much of the cyclic history only a single slip system is activated in most grains. The simulation results are compared to experimental data for the macroscopic stress and for lattice strains in the unloaded state after 1, 30 and 1000 cycles.

The evolution of crystalline stresses of a polycrystalline metal during cyclic loading

Abstract The presence of a positive average applied stress during cyclic uniaxial loading leads to a reduction in fatigue life of metallic parts. The metals are typically polycrystalline, with stresses varying from crystal to crystal due to differences in lattice orientation and slip system strength. Simulations enable us to better understand how polycrystals behave under cyclic loading and how the changing stress over many cycles influences fatigue life. Specifically, uniaxial cyclic simulations of pre-strained HY100 steel were conducted using an elastic viscoplastic continuum slip model employing a Taylor hypothesis. Stress-controlled loading conditions were employed to mimic fatigue tests on cold-bent bar specimens for three different load levels. The macroscopic axial strains and the crystal axial stresses were monitored during the cycles. The stress–strain response for the first cycle was used to determine the load input for the material point simulations. The peak values of crystal axial stress were found to evolve continuously with the number of loading cycles. It was found that the stress change in a crystal is influenced not only by its own orientation but also by the orientations of the other crystals in the aggregate. Furthermore, the distribution of crystal stresses after thousands of cycles at a lower stress amplitude closely resembled the distribution after tens of cycles at a larger stress amplitude.

Prediction of fatigue response of composite structures by monitoring the strain energy release rate with embedded fiber Bragg gratings

Composite materials are becoming increasingly more valuable due to their high specific strength and stiffness. Currently, most components are operated for a number of service cycles and then replaced regardless of their actual condition. Embedded fiber Bragg gratings are under investigation for monitoring these components in real time and estimating their remaining life. This article presents research conducted on a novel technique for prediction of the remaining life of composites under fatigue loading using embedded fiber Bragg grating sensors. A prediction is made of the remaining life at every cycle based on data collected from the sensors and the previous loading history.

The Nonlinear Dynamic Behaviour of Tapered Laminated Plates Subjected to Blast Loading

In this study, the geometrically nonlinear dynamic behaviour of simply supported tapered laminated composite plates subjected to the air blast loading is investigated numerically. In-plane stiffness, inertia and the geometric nonlinearity effects are considered in the formulation of the problem. The equations of motion for the tapered laminated plate are derived by the use of the virtual work principle. Approximate solution functions are assumed for the space domain and substituted into the equations of motion. Then, the Galerkin method is used to obtain the nonlinear algebraic differential equations in the time domain. The resulting equations are solved by using the finite difference approximation over the time. The effects of the taper ratio, the stacking sequence and thefiberorientation angleon thedynamic response areinvestigated. Thedisplacement-timeand strain-time histories are obtained on certain points in the tapered direction. The results obtained by using the present method are compared with the ones obtained by using a commercial finite element software ANSYS. The results are found to be in an agreement. The method presented here is able to determine the nonlinear dynamic response of simply supported tapered laminated plates to the air blast loading accurately.

Influence of modelling variables on the distribution of lattice strains in a deformed polycrystal, with reference to neutron diffraction experiments

The distribution of (elastic) lattice strains following plastic deformation of an aluminium alloy is examined using finite element simulations of aggregates with individually resolved crystals. The aggregates consist of face centred cubic crystals with initial orientations assigned by sampling the measured crystallographic texture. The simulations show that the lattice strains within the crystals are influenced by various microstructural features, material parameters and modelling assumptions. A hierarchy among these various effects is established, based on applications where low probability events are crucial, such as in fatigue or fracture problems. The simulation results are discussed in detail and compared with experimental data for the macroscopic stress, the lattice strains in the unloaded state and the associated uncertainties.

Dynamic Behavior of a Plate Under Air Blast Load Using Differential Quadrature Method

The effect of blast load on the plate and shell structures has an important role on design decision. Blast load experiments are usually difficult and expensive. Therefore, numerical studies have been done on the response of blast loaded structures. However, because of time dependency of the nature of the problem, numerical solutions take long time and need heavy computational effort. The differential quadrature method (DQM) is a numerical solution technique for the rapid solution of linear and non-linear partial differential equations. It has been successfully applied to many engineering problems. The method has especially found application widely in structural analysis such as static and free vibration analysis of beams and plates. The capability of the method to produce highly accurate solutions with minimal computational efforts makes it of current interest. In this paper, the dynamic behavior of isotropic and laminated composite plates under air blast load has been investigated using the differential quadrature method. The results are compared to the numerical and experimental results found in the literature.Copyright

A Slip-Based Model for Strength Evolution During Cyclic Loading

Motivated by the strong dependence of strain-hardening processes on slip system activity. a slip system hardening formulation that explicitly employs accumulated slip system shear strains and net crystal shearing rates is introduced within a polycrystal plasticity modeling formulation for predicting material response during cyclic deformation. The model, which is a slight modification of the Voce hardening model commonly employed for large strain forming simulations, was employed to model the behavior of 304L stainless steel subjected to uniaxial and multiaxial nonproportional multiple-step experiments and multiaxial multiple-phase angle experiments. The model successfully captured the pseudo-saturation response that is common during the multiple-step tests and captured many of the loop-shape and stress-level features of the multiple-phase angle experiments.

The dynamic response of the sandwich panel subjected to the impact load

In this study, the dynamic behavior of the sandwich panels subjected to the impact load are investigated experimentally and numerically. For this purpose, the sandwich panels are manufactured using the honeycomb core material and laminated composite face sheets. The face sheet materials used in this study are manufactured using an aramid fabric and epoxy resin. The wet hand lay-up technique is used to produce the sandwich panel. The curing is achieved by using a heated vacuum table. The panel is fixed at all edges and the impact load is applied on the panel. The dynamic response of the panel is measured using the strain gauges. The panel is also modeled using Ansys finite element software. The analysis of the impact test is achieved. The numerical and experimental results are compared. The results are found in an agreement. The final results are concluded.

AIR BLAST-INDUCED VIBRATION OF A LAMINATED SPHERICAL SHELL

The scope of this study is to investigate the dynamic behavior of a laminated spherical shell subjected to air blast load. The shell structure considered here is a hemisphere in shape and made of a glass/epoxy laminated composite material. The blast experiments are performed on the spherical shell. The strain-time history of the center of the spherical shell panel is obtained experimentally. The blast loaded spherical shell is also modeled and analyzed using ANSYS finite element software. The static analysis is performed to characterize the material. The dynamic response of the spherical shell panel obtained numerically is compared to the experimental results. It is observed that the response frequency corresponds to the higher vibration modes of the panel. The qualitative agreement is found between the numerical and experimental results.