Sp.G. Pantelakis

Numerical and experimental investigation of the laser forming process

In this paper numerical and experimental results from an investigation on the laser forming process of metallic plates are discussed. The laser forming process has been simulated numerically, a three-dimensional finite-element algorithm having been developed. The algorithm includes a non-linear transient coupled thermal–structural analysis; the temperature dependency of the thermal and mechanical properties of the material being accounted for. The proposed algorithm is capable of calculating the time-dependent temperatures, stresses and strains during the process as well as of predicting the final shape of steel-plates and sheets. Numerical simulation has been carried out for bending as well as for the forming of sine-shaped plates. Validation of the numerical simulation was made experimentally. An experimental parametric study was performed on one-edge-clamped plates irradiated by a CO2 laser. Good correlation between the numerical simulation and the experimental results was demonstrated.

An analytical model for the prediction of distortions caused by the laser forming process

Abstract Simulations of laser forming processes of metallic sheets have so far been performed by means of non-linear elastoplastic finite element analysis which is time consuming and difficult. In this paper an analytical model for the prediction of distortions caused by laser forming of metallic sheets is proposed; it is applied for the case of bending. For developing the analytical model the predominant mechanisms which control the laser forming process have been accounted for; they include the non-uniform temperature distribution throughout the thickness of the plate, the development of plastic deformations on a certain area of the material and the subsequent distortions at the end of the process due to the developed plastic strains. The temperature dependency of the material properties has been considered as well. The developed model is parametric and considers the dimensions of the plate and the laser irradiation parameters. The comparison of the calculations using the proposed analytical model with experimental results as well as with results obtained by finite element simulation has shown a very good correlation.

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.

Tensile and energy density properties of 2024, 6013, 8090 and 2091 aircraft aluminum alloy after corrosion exposure

Evaluation was made for the corrosion susceptibility of aircraft structure aluminum alloys 2024 T351, 6013 T6, 8090 T81 and 2091 T84. Tensile and energy density data were obtained. Stereoscopic and metallographic corrosion analysis were made as well. The specimens were pre-corroded using accelerated laboratory corrosion tests or out-door atmospheric conditions before testing. Noticeable decrease of yield and ultimate tensile stress were detected when the specimen surface was corroded. Dramatic volumetric embrittlement was observed even after short exposure times that were not sufficient for the appreciable development of surface corrosion attack. Observed material degradation behavior is attributed to hydrogen penetration and absorption.

Optimization of the diaphragm forming process with regard to product quality and cost

Abstract The diaphragm forming process used for the cost efficient production of thermoplastic composite components has been experimentally investigated. The process has been optimized with regard to product quality and cost. For the process optimization a new generic concept is introduced. The proposed concept involves quality and cost sensitivity analyses by considering the process parameters as variables. For deriving relations between process parameter variation and component quality features, as well as between process parameter variation and component costs, an extensive experimental parametric study was made. For the investigation, the thermoplastic composite APC-2/AS4 was used. The manufactured parts were different, aircraft structure like, simple shapes. Cost estimation relationships were established by using the activity based cost concept. Using the derived empirical relations for the quality and cost sensitivity analyses, the developed process optimization concept could be implemented. The investigation presented the need for the evolution of the diaphragm forming technique into a new ‘cold’ diaphragm forming technique. To evaluate the ‘new’ technique, a testing facility on experimental (laboratory) scale was devised. Simple shape parts from the thermoplastic Borealis PP/E-glass were manufactured with satisfactory quality. The cost for producing components using the cold diaphragm forming technique was estimated and it was found to be significantly lower compared to the cost for producing the same components using the autoclave or conventional diaphragm forming techniques.

Degradation of Mode-I Fracture Toughness of CFRP Bonded Joints Due to Release Agent and Moisture Pre-Bond Contamination

An experimental investigation of the effects of pre-bond contamination on Mode-I fracture toughness of carbon fiber reinforced plastic (CFRP) bonded joints is presented in this paper. Two pre-bond contamination scenarios were considered; namely, the silicon-based release agent and moisture. The two contamination scenarios were realized in one of the composite substrates prior to bonding. The common characteristic of the two contamination scenarios is that they lead in the formation of defects in the form of weak bonds that cannot be detected by conventional non-destructive testing techniques. The contamination effects on Mode-I fracture toughness of the bonded joints were investigated by conducting mechanical tests on double-cantilever beam specimens and comparing the results with relative measurements taken from reference specimens. Prior to mechanical testing, the bonding quality of the specimens was tested using ultrasonic C-scan inspection. Both the release agent and moisture are found to significantly degrade the Mode-I fracture toughness of the joints. For the release agent, the effect was more significant for silicon concentrations over 5 at%; a complete lack of adhesion was observed for silicon concentrations over 7 at%. At low values of relative humidity, there was a small increase in Mode-I critical energy release rate while at larger values there is a decrease which reaches 26% for the higher relative humidity percentage. The results from the Non-Destructive Testing (NDT) tests verify the inability of conventional NDT to detect the defects resulting at the interface between the contaminated adherends surface and the adhesive for both contamination scenarios.

Evaluation of the effects of variations in chemical composition on the quality of Al-Si-Mg, Al-Cu, and Al-Zn-Mg cast aluminum alloys

The effect of slight variations in chemical composition on the quality of cast aluminum alloys from three different major alloy systems was evaluated. For the evaluation of the alloy quality, an index QD adjusted to damage tolerance requirements that are currently involved for the design of advanced lightweight structures is used. The quality index QD accounts for tensile strength and ductility as well as for material failure through yielding or fracture. For this investigation, experimental results obtained for variations in chemical composition of the alloy systems Al-Si-Mg, Al-Cu, and Al-Zn-Mg were exploited. In total, castings from 37 different batches from 10 aluminum alloys, varying in chemical composition, were evaluated. Quality characterization and alloy quality ranking were made by evaluating results of 512 tensile tests using the index QD as well as, for comparison, the quality index Q, which is currently used by the industry. The results obtained involving the index QD seem to be more realistic, from the viewpoint of damage tolerance design requirements.

Strain energy density prediction of fatigue crack growth from hole of aging aircraft structures

Fatigue crack growth rate depends not only on the load amplitude, but also on the morphology of crack path. The strain energy density theory has the ability to analyze crack growth rate. A strain energy density crack growth model is proposed. It can predict the lifetime of fatigue crack growth for mixed mode cracks while an equation for mode I crack is also obtained. The validity of the model is established with two cases: a center-crack panel and cracks emanating from the edge of a hole. The stress intensity factor expression for the former case is analytical while that of the latter is calculated numerically using finite elements. The results are compared with the testing data. Good agreement shows that the proposed model is useful.

Fatigue crack growth retardation assessment of 2024-T3 and 6061-T6 aluminium specimens

Abstract A fatigue crack growth retardation model is developed. It considers a strip plastic zone with material hardening effect which is taken as one of the basic mechanisms controlling fatigue crack growth. Crack growth is treated incrementally and corresponds to the failure of material elements ahead of an existing crack after a certain critical number of low cycle fatigue. Computed curves are correlated to test data obtained from the 2024-T3 and 6061-T6 aluminium specimens. Deviations from test data increase with increasing crack length.

An automated technique for manufacturing thermoplastic stringers in continuous length

Abstract In the present work an automated Continuous Compression Moulding Technique for the manufacture of stringers in continuous length is presented. The method combines pultrusion and hot-pressing. The technique is utilized for the production of L-shape stringers which are widely applied in aerospace constructions. The investigation was carried out on carbon reinforced PEEK (C/PEEK), as well as, for comparison, on the thermoplastic composites carbon reinforced polyethersulfon (C/PES), glass and carbon reinforced polyphenylene-sulfide (G/PPS, C/PPS) and Kevlar reinforced Polyamide 6 (K/PA 6). For the materials investigated the optimized process parameters for manufacturing the L-shape stringers were derived experimentally. To achieve this goal, the quality of the produced parts was controlled by using non-destructive testing techniques. Parts providing satisfactory quality were also tested destructively to measure their mechanical properties. The investigation results have shown the suitability of the technique to produce continuous length stringers.