Gérard Bernhart

Fibre Reinforced Refractory Castables :An Alternative Solution for SPF Die Manufacturing

Today heat resistant cast steels are the nominal solution for Ti-SPF forming die manufacturing. Nevertheless, this materials present some drawbacks related to delivery time and cost. A fibre reinforced refractory castable (FRRC) is proposed as a new solution for prototype SPF die manufacturing. Due to the general brittleness of refractory castables, a short fibre reinforcement has been investigated in order to avoid catastrophic failure during the forming process. General macroscopic behavior of such materials is very complex and presents large evolutions with the testing temperature. The paper addresses the important benefits of the reinforcement for refractory castable in the case of loading on a complex structure. The capability of the material to support several cracks is shown in the case of a technological sample with a complex shape.

Design of SPF Dies Based on Advanced Material Behaviour Models

Numerical simulation was performed on an axi-symmetric metallic die during a whole fife cycle. It was shown that most important stresses are generated by the thermal gradients during heating up and repeated die removal from the press furnace and also by the clamping pressure during forming. The forming itself causes non dimensioning stresses. Creep relaxation during forming induces plastic deformation and distortion of mould after cooling down. To increase simulation relevance, new behaviour models are investigated for three classes of materials candidate for SPF dies: heat resistant cast steels, fibre reinforced refractory concretes and low cost monolithic ceramics.

Unified isothermal and non-isothermal modelling of neat PEEK crystallization

A differential generalized Avrami’s law is used to model crystallization kinetic of PEEK in considering that PEEK crystallization results from the contribution of two distinct mechanisms. The form of this equation allows to predict with good accuracy both isothermal and non-isothermal crystallization kinetics. Nevertheless, isothermal model parameters are not entirely satisfactory for predicting non-isothermal crystallization and the identification of kinetic parameters is needed for both isothermal and non-isothermal cases. The results show that the Avrami exponents and Arrhenius activation energies remain constant for both conditions and therefore suggest that these parameters are only material dependent. On the other hand, the other kinetic parameters depend on the crystallization condition and vary with temperature and/or cooling rate.

Melt state behaviour of PEEK and processing window interpretation for fast compression moulding process

Fast mould heating is nowadays possible by using induction technology for example with the Cage System® developed by RocTool. It allows heating and cooling kinetics of about 100° C per minute and new perspectives are thus possible to optimize the compression moulding process of long fibre reinforced thermoplastic composites. Indeed, a high forming temperature may favour polymer creep and so on composite consolidation. Nevertheless, the processing time of PEEK composite above melt temperature must be reduced to a few minutes due to the fast thermal degradation of the matrix. On the other hand, high cooling rates may have negative effect on matrix crystallinity. The proposed procedure consist in performing a few minutes isotherm around 300° C during the fast cooling. It would favour a high degree of crystallinity of PEEK without extending the cycle time.

Damages Induced by Thermomechanical Cycles in Superplastic Forming Tools

In this paper, damages accumulated in superplastic forming (SPF) tools during the forming cycles were investigated in various aspects. Finite element simulations were performed on an axisymmetrie mould of heat resistant cast steel to evaluate thermomechanical stresses and strains during the forming cycles, which cause accumulated residual stress and deformation damage in the mould. The dimensional changes due to microstructure evolution of the tool material were studied on specimens for typical thermal cycles. And high temperature corrosion destroying the surfaces of the tool was investigated too.

Charpy test investigation of the influence of fabric weave and fibre nature on impact properties of PEEK-reinforced composites

The aim of the work is to use Charpy impact test for quick evaluations of different Polyether-ether-ketone (PEEK)-reinforced composites to be used for impact protection. In the first part, the influence of weave pattern was first analysed by comparing the impact behaviour of three PEEK composites reinforced with plies of unidirectional (UD) tapes, 5H satin fabrics and 2 × 2 twill fabrics made of high-strength carbon fibres. In the second part, the influence of fibre nature was investigated for the same weave pattern. The impact behaviour of five 2 × 2 twill fabrics made from inorganic fibre (carbon, glass and basalt) and organic fibre (aramid and poly(p-phenylene-2,6-benzobisoxazole) (PBO)) has been compared. Two main types of failure modes were identified: a brittle behaviour mode with high failure strength and a highly deformable behaviour mode in which energy absorption is more important. The balance between brittle behaviour and highly deformable behaviour results from competition between the yarn crimp, weave pattern and fibre properties of the composite. Slight yarn crimp and small ply thickness increase the stiffness of the composite and induce brittle behaviour characterized by fibre failure in tension and a steep peak on the loading curves. This behaviour is observed in UD and 5H satin carbon-reinforced composites or 2 × 2 twill glass and basalt fabric-reinforced composites. In contrast, aramid and PBO 2 × 2 twill fabric composites exhibit high shear strength. The highly deformable behaviour of the specimens during the Charpy impact led, in the case of organic fibres, to a non-breakage of the fibres and consequently to a high level of energy absorption. This behaviour is necessarily interesting in armour applications.

Mechanical Characterization of Carbon Fibres Recycled by Steam Thermolysis: A Statistical Approach

The recent development of technologies for recycling carbon fibre reinforced plastics (CFRPs) leads to the need to evaluate the mechanical response of recycled carbon fibres. As these fibres are likely to be degraded during the recycling treatment, it is very important to determine their tensile residual properties so as to evaluate their ability as reinforcement for new composite materials. Carbon fibres reclaimed by a steam-thermal treatment applied to degrade the epoxy resin matrix of a CFRP are here analysed. Two conditions were chosen so as to reach two degradation efficiency levels of the steam thermolysis. Several carbon fibre samples were selected for mechanical testing carried out either on single filaments using single fibre tensile tests or on fibre tows using bundle tensile tests. It is shown that the single fibre tensile test leads to a wide variability of statistical parameters derived from the analysis. Bundle tensile tests results were able to indicate that fibre strength of recycled carbon fibre is similar to corresponding as-received carbon fibres thanks to a statistically relevant database. Wide number of tested filaments enabled indeed to obtain low scatters.

Méthodes de détermination de la résistance en cisaillement à l’interface. Application au PA66 10%FC

Cet article utilise et compare deux methodes pour acceder a la resistance en cisaillement a l’interface des composites fibres courtes injectes. Les methodes sont appliquees a un composite PA66 (polyamide) fibre de carbone. Les resultats obtenus sont proches. Les methodes presentent la particularite d’utiliser les donnees obtenues par un essai de traction sur une eprouvette haltere injectee. Cela rend ces methodes tres pratiques. A l’inverse, ces methodes presentent une sensibilite a de nombreux parametres (module de la fibre, rayon de la fibre, taux de fibre, module de la matrice et module du composite). Notamment, une attention particuliere doit etre portee au module d’Young du composite considere.

Innovative Superplastic Forming Based on In Situ Infra-Red Sheet Heating

This paper describes the research work that was performed in order to propose an innovative and low cost process route for superplastic forming of aluminum and titanium alloy sheets. The driving idea was to heat only the metallic sheet using heating elements included in the pressure chamber. Several heating configuration have been tested and equipment designs investigated. Based on experimental results and numerical thermal analysis, it was found that halogen heating lamps with a high reflective thermal insulation was the best for the upper pressure chamber, whereas low effusivity Refractory Castables materials seem optimal as low cost forming die. Energy consumption evaluation shows more than 80% energy saving in nominal titanium alloy forming. A pilot forming equipment was developed and first aluminum sheet forming trials give interesting results.

Rapid Tooling for Injection Moulding Process

CROMeP (Research Centre on Tools, Materials and Processes) experience in rapid tooling development is related. Focus is placed on two processes, namely the Direct Metal Laser Sintering/DMLSⓇ and STRATOCONCEPTⓇ currently under consideration for the manufacturing of thermoplastic injection moulds. The principle of the two processing routes as well as the benefits attainable by using those processes will be briefly reviewed. Recent results on the microstructure and the mechanical performances of sample tools built up using both processes are presented.