Ch.V. Katsiropoulos

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.

Effect of Water Absorption on Strength of the Aeronautical Composite Material Fiberdux HTA/6376

The effect of water absorption on tensile strength and fatigue life of the Fiberdux HTA/6376 composite laminated material was experimentally and numerically investigated. For the investigation, a quasi-isotropic and a cross-ply layup were considered. Tensile and fatigue tests were carried out with dry and wet specimens subjected to water absorption up to the water uptake saturation point. Fractographic analyses of fracture surfaces were conducted to detect the type of accumulated damage. To describe the mechanical behavior of the material a FE-based progressive damage model was developed. Material properties used in the model are those of the saturated material evaluated experimentally. Convergent results reveal an appreciable degradation in the ultimate tensile strength and fatigue life of the composite material owing to water absorption.

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.

Process and cost modelling applied to manufacture of complex aerospace composite part

Abstract Novel methods for process simulation and cost analysis have been applied during manufacturing process development of a rotor blade pitch horn. The aim is to reduce costs and lead time on the one hand and to enhance part quality on the other hand. Fabric draping has been simulated using the kinematic draping code PAM-QUIKFORM incorporating new processing strategies. Draping strategies were optimised using a genetic algorithm taking into account manufacturing constraints, which led to a fabric shear reduction by up to 10° with the optimised strategy implemented in manufacturing. A novel material generation of prebindered carbon fibre tows has been used to enhance rigidity and dimensional accuracy of the preform and to minimise processing time. State-of-the-art preforming technology has been incorporated in the process significantly increasing the degree of automation. The process had been analysed based on the activity based costs methodology deriving product costs as sum of costs of all activities involved. Development efforts have been concentrated based on the analysis in order to optimise cycle times with a nearly even duration of the subprocesses. In comparison to a manual prepreg manufacturing process, cost savings with the novel, semiautomated preforming process could be quantified to ∼20%.

Assessing effect of different surface treatments on fracture toughness behaviour of adhesively bonded 8552/AS4 composite joints

Abstract Adhesive bonding of aeronautical components made of carbon–fibre reinforced plastics is a popular alternative to mechanical fastening. The continuing research is focused on the optimisation of the surface treatments so as to improve the mechanical properties. In this work, the effect of two atmospheric pressure plasma (APP) treatments before bonding on the fracture toughness behaviour of adhesively bonded joints was experimentally investigated. The laminates were in contact with different ancillary materials during the manufacturing process, thus leading to eight different treatment alternatives. For the investigation, a quasi-isotropic layup was subjected to modes I and II fracture toughness test. To support the understanding of the mechanical behaviour observed, non-destructive testing evaluation as well as failure mode analysis at macroscopic level was carried out. As a result, APP showed promising performances regarding surface preparation, revealing an appreciable dependence of the fracture toughness behaviour on the selected alternatives.

Assessment of imperfect bonding of adhesively bonded U-joints using ultrasonic inspection

Abstract Ongoing research in the area of adhesive bonding is focused on the optimization of the bonding method used as well as on the use of newly developed high quality adhesives. In this work, the bonding quality of adhesively bonded composite U-joints was experimentally investigated using C-Scan ultrasonic inspection. The reference adhesive used was Epibond, while the newly developed adhesive was the one developed within the ABITAS project. Based on the geometry of the U-joint, different scan areas of the two existing bondlines were defined in order to effectively capture the quality of the entire bonded area. Furthermore, two different bonding methods were evaluated in terms of bonding quality assessed through the ultrasonic inspection tests. In this frame, U-joints including normal cured and spot cured bonded areas have been investigated. The evaluation was carried out on the basis of the total defected area by using the clustering mode of ULTRAWIN image analysis software. This mode enables the quantitative assessment of the defective percentage (%) out of the total scanned area. The results reveal a significant amount of voids in the components investigated. In most cases, spot curing areas are clearly distinguishable. The spot curing process causes a variation of the adhesive layer thickness and the creation of bubbles. Comparison between the conventional and the newly developed adhesive was made; for components using the conventional adhesive, damage is located mostly around the rivets, probably due to the increased local pressure.

An overall assessment of a new adhesive bonding process for composite materials, with regard to quality and cost

An assessment of innovative adhesive bonding process has been performed with regard to quality and cost. In this frame, the effect of two different atmospheric pressure plasma surface treatment conditions on the fracture toughness behaviour of adhesively bonded joints was experimentally investigated. Furthermore, the mechanical performance of a newly developed aerospace structural adhesive has been characterised experimentally in order to assess the quality of the bonded elements. To assess the feasibility of the new process, a complete cost-estimation analysis of the process has been carried out based on the activity-based costing modelling approach, thus serving to the estimation of the total cost/duration of the process. To this end, the newly developed process is assessed with regard to quality and cost. It could be shown that the new process offers tempting alternatives to the existing adhesive bonding and joining processes used in the aeronautic industry.