G. Labeas

Strength prediction of bolted joints in graphite/epoxy composite laminates

Abstract A parametric finite element analysis was conducted to investigate the effect of failure criteria and material property degradation rules on the tensile behaviour and strength of bolted joints in graphite/epoxy composite laminates. The analysis was based on a three-dimensional progressive damage model (PDM) developed earlier by the authors. The PDM comprises the components of stress analysis, failure analysis and material property degradation. The predicted load–displacement curves and failure loads of a single-lap single-bolt joint were compared with experimental data for different joint geometries and laminate stacking sequences. The stiffness of the joint was predicted with satisfactory accuracy for all configurations. The predicted failure load was significantly influenced by the combination of failure criteria and degradation rules used. A combination of failure criteria and material property degradation rules that leads to accurate strength prediction is proposed. For all the analyses performed, the macroscopic failure mechanism of the joint and the damage progression were also predicted.

Fatigue damage accumulation and residual strength assessment of CFRP laminates

The method of progressive damage modelling has been used to assess fatigue damage accumulation and residual strength of carbon-fibre reinforced plastic (CFRP) laminates under fully reversed cyclic loading (R=σmin/σmax=−1). The accumulation of different damage modes has been assessed, as a function of number of cycles, using a three-dimensional fatigue progressive damage model (FPDM). The residual strength of the CFRP laminates has been assessed through the combined use of the FPDM with a static three-dimensional progressive damage model (PDM). By simulating the experimental procedure, the FPDM has been applied up to certain number of cycles, to estimate the accumulated fatigue damage and then, the static PDM has been applied (quasi-static tensile loading) to predict final tensile failure of the laminates, which corresponds to the residual strength of the laminate, after it has been exposed at the specific cycles. The models comprised the components of stress analysis performed using finite elements, failure analysis performed using polynomial stress-based failure criteria and material property degradation performed using degradation rules. The analysis has been validated experimentally (a) by assuming a laminate free of internal defects, and (b) by considering the initial defects, which were determined experimentally for certain laminates. The analysis has resulted in an accurate simulation of the experimentally determined fatigue damage accumulation and residual strength.

Elastic modulus and Poisson’s ratio determination of micro-lattice cellular structures by analytical, numerical and homogenisation methods

The mechanical properties of micro-lattice structures made of interconnected metallic struts are calculated analytically, in order to derive their elastic modulus and Poisson’s ratio in the three Cartesian directions. The geometry of the investigated unit cell is a body centered cuboid, which is a more generic case of the well-known body centered cubic geometry. The Bernoulli–Euler and Timoshenko beam theories are used for the analytical solution of the unit cell response under complex loading. The derived elastic constants are compared to the relative numerical ones. The influence of geometrical parameters on the stiffness coefficients of the cellular structure is parametrically determined. The derived elasticity matrices are introduced in a homogenised numerical model of the core and its results are compared to a numerical model with explicit representation of the core geometry indicating an excellent accuracy, while the solution time is considerably reduced.

Simulation of the Electroimpulse De-Icing Process of Aircraft Wings

The electroimpulse de-icing system of an aircraft wing leading edge is investigated through the development of a methodology for the numerical simulation of the electroimpulse de-icing process. The principle of electroimpulse de-icing is that the ice is removed due to the leading edge local mechanical vibration, which is induced by an electromagnetic pulse. The numerical simulation methodology is based on a nonlinear transient three-dimensional stress analysis of the ice-covered wing, combined to a de-icing criterion that takes into consideration the tensile and shear stresses at the ice-skin interface. The developed methodology is verified on de-icing experimental tests of an aluminum plate. Afterwards, the methodology is applied to the prediction of de-icing of an aircraft wing leading edge. The dominant process parameters are determined to be the coil number and position, the ice thickness and coverage, the radius of wing curvature, and the electroimpulsive load amplitude. A parametric study is performed to determine the influence of the process parameters on the system effectiveness, defined as the percentage of the de-iced surface over the total leading edge surface. From the results of the parametric study, the possibilities of reducing the weight and energy consumption of the electroimpulse de-icing system can arise.

Thermomechanical Simulation of Infrared Heating Diaphragm Forming Process for Thermoplastic Parts

An innovative methodology for the thermomechanical simulation of the infrared heating diaphragm forming (DF) process is proposed. In the first section of the paper, the heat transfer mechanisms between the infrared (IR) heating lamps and the thermoplastic plate are simulated, and the effect of the various preheating parameters on the heating time and temperature distribution is investigated. In the second section, the mechanical deformation of the thermoplastic component is simulated to enable prediction of heat losses due to the plate contact with the mold. Based on the developed simulation methodology, the main process parameters — e.g., the number, location, and power of IR lamps for optimal preheating; the heat losses during plate deformation; and the minimum required mold temperature throughout the forming phase — are derived for five different thicknesses. The optimization results show that the forming parameters considered influence the heating of the plate in a complex and interactive way; in addition, it is found that with increasing plate thickness, the heating time required to reach the desired temperature also increases.

Bird strike simulation on a novel composite leading edge design

Abstract A methodology for the numerical simulation of bird strike on a novel leading edge (LE) structure of a horizontal tail plane is presented. The innovative LE design is based on the ‘tensor skin’ concept, comprising one or more folded composite sub-laminates that unfold during the bird impact, thus providing high-energy absorption characteristics. The simulation technique is based on a non-linear dynamic finite element analysis and is performed in three steps. The first step deals with the development of suitable material damage models capable of representing the high-strain rate behaviour of the composite systems used in the LE structure. The second step deals with the development of a finite element modelling procedure for simulating the complex failure modes and unfolding mechanisms of quasi-static penetration of simple ‘tensor skin’ strips, which are representative of the complete LE composite structure. The third step deals with the numerical simulation of bird strike experiments on two novel aircraft LE designs. The influence on the numerical results of the critical modelling issues such as the mesh density of the highly impacted areas, the substitute bird flexibility as well as the material damage and contact interfaces parameters are discussed in detail. The numerical results are in good qualitative and quantitative agreement with the results of the experimental tests.

Optimization of Laser Transmission Welding Process for Thermoplastic Composite Parts using Thermo-Mechanical Simulation

An innovative methodology for the thermo-mechanical simulation of the laser transmission welding (LTW) process for thermoplastic components is presented. The work consists of two parts. In the first part, a finite element (FE) thermal model is developed, for the prediction of the transient spatial temperature history developing during the LTW process. Experimental measurements have been used for the calibration of the developed thermal model. Through this thermal model, a parametric study on the main welding parameters is performed, in order to investigate their effect on the maximum temperature. Using the parametric study results, the optimal combination of the welding parameters is derived taking into account the welding cost. In the second part, the optimized set is used in a model developed for the thermo-mechanical simulation of the LTW process and the calculation of the thermal stresses, strains, and distortions of the welded parts. The benefit of the proposed methodology is that it offers the capability of optimizing the LTW process, and also provides a reliable estimation of the developed temperature, as well as the thermal stress and strain fields reducing the experimental effort.

Adaptative Progressive Damage Modeling for Large-scale Composite Structures

Progressive damage modeling (PDM) is a well-established methodology for the prediction of damage initiation and evolution in composite structures. However, as conventional PDM methodology involves a large post-processing procedure, it is impractical for application in large-scale structures due to the high computational cost it requires. In this study, the local character of nonlinearity, due to the scale of the damage topology compared to the size of the entire structure, is exploited to propose proper modifications in the ‘classical’ PDM methodology. Specifically, the sub-modeling technique principles are combined and integrated in the PDM methodology and the appropriate modifications required are discussed. Furthermore, two damage prediction indices, which are related to the criticality of damage state at specific sub-areas (material layers) of the structure are introduced, in order to achieve significant reductions of the required computational time. Both the improvements make the application of PDM in large-scale composite structures practically feasible; this is demonstrated in the case of a generic composite shear joint whose numerical model comprises over a million degrees of freedom.

Tensile Behavior and Formability Evaluation of Titanium-40 Material Based on the Forming Limit Diagram Approach

The formability of Titanium alloy sheet material Ti-40 has been experimentally assessed in the present investigation. The investigation is divided into two parts: In the first part, the effect of the strain rate applied during testing, as well as the effect of material axes’ orientation, on the tensile behavior is evaluated via standard tensile tests. In addition, the hardening characteristics as well as the anisotropy parameters (plastic strain ratio) have also been extracted. In the second part, the formability limits of Ti-40 material are experimentally derived using Nakajima tests and the corresponding forming limit diagrams are compared against other commercially available titanium sheet alloys.

Optimisation of laser welding process for thermoplastic composite materials with regard to component quality and cost

Abstract The laser transmission welding (LTW) process is optimised with regard to quality and cost for welded thermoplastic stiffeners on aircrafts fuselage skin. A generic optimisation concept developed in previous authors work is applied. Quality and cost sensitivity analyses were performed to derive material dependent quality function and process dependent cost estimation relationships. Quality function and cost estimation relationships are exploited to derive iteratively the optimal welding parameters. The derivation of the important heating process parameters arises from the numerical simulation of the process thermal cycle by means of finite element method. To optimise the LTW process with respect to quality and cost, a software tool, namely the LTSM-OPT tool, is extended to the LTW process. The optimal process parameters of the LTW system along with the optimal heating cycle for welding thermoplastic lap joints are obtained, in the form of a reference welding temperature along with an allowable process window, which meets the minimum quality requirements. The results of the study were successfully exploited by an aeronautic industry to weld stiffeners on aircrafts fuselage panel.