P. Papanikos

Numerical simulation of the laser welding process in butt-joint specimens

Abstract A three-dimensional finite element model has been developed to simulate the laser welding process and predict laser welded panel distortions. The finite element calculations were performed using the SYSWELD FE code, which takes into account thermal, metallurgical and mechanical aspects. The simulation of the laser welding process was performed using a nonlinear heat transfer analysis, based on a keyhole formation model, and a coupled transient thermo-mechanical analysis. The analysis takes into account metallurgical transformations using the temperature dependent material properties and the continuous cooling transformation (CCT) diagram. The geometry examined was a butt-joint specimen that consisted of thick AH36 shipbuilding steel plates, using different welding parameters. Experimental testing was carried out to measure the distortions of the welded specimens and verify the FE approach. The FE results are in good agreement with experimental measurements.

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.

Three-dimensional finite element analysis of cold expansion of adjacent holes

Three-dimensional elasto-plastic finite element analysis was conducted to evaluate the development and growth of the plastic zone and unloading residual stresses resulting from the cold expansion of two adjacent holes. The contact between the mandrel and the hole was modelled using special contact elements. The work examined the effect of the geometry of two adjacent cold-expanded holes upon the resulting three-dimensional residual stress field. Both simultaneous and sequential expansion of the two holes was considered. It was shown that the compressive residual stress varies across the thickness of the specimen and that two-dimensional finite element models are incapable of accurately predicting the residual stresses resulting from the cold expansion process. It was further shown that sequential expansion reduces dramatically the compressive residual stresses. The validity of the developed finite element model was verified with existing experimental results.

Three-dimensional nonlinear finite element analysis of dovetail joints in aeroengine discs

Abstract Three-dimensional nonlinear finite element analysis is made of the dovetail region in aeroengine compressor disc assemblies using contact elements. The study is devoted to examining the effect of the critical geometrical features, such as flank length, flank angle, fillet radii and skew angle upon the resulting stress field. Frictional conditions at the interface between the disc and the blade are also examined. The finite element predictions were validated using three-dimensional photoelastic stress freezing results. Comparisons with the two-dimensional finite element analysis made earlier by Papanikos and Meguid (Fatigue Fract. Eng. Mater. Struct. 17 (5) (1994) 539–550) of the same geometry reveal certain inadequacies. Specifically, the earlier analysis underestimates the maximum equivalent stress along the interface by as much as 40%. This could have serious implications concerning the safety margins of the disc assembly.

Elasto-plastic finite-element analysis of the cold expansion of adjacent fastener holes

Abstract This investigation is devoted to the modelling and simulation of the residual stress field resulting from the cold expansion of two adjacent fastener holes. In this two-dimensional elasto-plastic analysis, the development and growth of the plastic zone and unloading residual stresses were examined using the finite-element method. Attention was devoted to two related issues. The first is concerned with the effect of the separation distance between the two holes, the mechanical properties of workpiece and the expansion level upon the field variables; and the second with the effect of the order of cold working of the two holes upon the residual stress field. The results reveal the important role played by the geometry of the workpiece and the expansion parameters upon the quality of the treatment, as measured by the mechanically-induced residual stresses. The shortcomings of the two-dimensional analysis are highlighted and compared with the results of a three-dimensional model.

Theoretical and experimental studies of structural integrity of dovetail joints in aeroengine discs

Abstract Comprehensive finite element analysis and photoelastic studies using the stress freezing technique are made of the behaviour of aeroengine compressor discs under centrifugal loading. Specifically, the work is concerned with examining the effect of some of the critical geometric features and interface conditions in the dovetail region upon the least damage fail-safe design condition. These features included: inner and outer radii, flank length and flank angle, and coefficient of friction at the blade-disc interface. Two aspects of the work were accordingly examined. The first was concerned with the stressing of a number of discs containing the pertinent geometric features. The second was concerned with the initiation and subsequent propagation of cracks in regions exhibiting maximum stress concentration; the propagating cracks were then tracked using the maximum-minimum principal stress criterion. Finally, the theoretical predictions were compared with photoelastic spin tests. In this case, cracks which were introduced in the photoelastic models by careful sawing were extended incrementally along the theoretically predicted fracture path and isochromatic fringe patterns were recorded for each crack length. These fringe patterns were then used to analyse the stress field in the neighbourhood of the crack-tip.

Analysis of curved cracks emanating from adjacent holes

Abstract This article provides a new method for the comprehensive treatment of cracks of curved trajectory, emanating from adjacent holes under constant amplitude fatigue loading. The theoretical formulations of the newly formed curved crack are based upon the use of a piece-wise subcrack model and the superposition of single dislocation solutions, which reduce the original problem into the solution of singular integral equations in terms of dislocation density functions. The resulting integral equations are then solved using Chebyshev polynomial expansions, yielding the stress intensity factor (SIF). The SIF solutions are used in conjunction with the strain energy release rate criterion to determine the potential incremental fatigue crack growth experienced by a plate containing two adjacent holes. The theoretical predictions are verified with the experimental results of an aluminum 7075 plate containing two arbitrarily oriented adjacent holes subject to constant amplitiude fatigue loading. The results reveal good agreement between the theoretical predictions and the experimental findings.

Finite Element Modeling of the Tensile Behavior of Carbon Nanotubes, Graphene and Their Composites

Due to their extraordinary mechanical properties, carbon nanotubes and graphene serve as ideal reinforcements. However, the effectiveness of reinforcement may be counterbalanced by the presence of defects, which degrades significantly the mechanical properties of nanomaterials, and the negative influence of several material-related and geometrical factors on the effective elastic properties of nano-reinforced composites. This chapter reviews the continuum models that were developed by the authors in order to predict the elastic properties of isolated defect-free carbon nanotubes, to simulate the tensile behavior of defected carbon nanotubes and graphene and to evaluate parametrically the effective elastic properties of nano-reinforced polymers.

Collaborative CAD/CAE as a cloud service

These days manufacturing and design teams continuously collaborate with colleagues across traditional office boundaries and manufacturing floors in an effort to meet with growing demand and competi...