Peter Mitschang

Continuous monitoring of three-dimensional resin flow through a fibre preform

A basic requirement for an accurate numerical simulation of the resin transfer moulding process is a set of exact permeability coefficients of the applied textile reinforcement. The permeabilities are usually obtained from the measurement of flow front propagation through a stack of planar fibre preforms. While measuring the in-plane flow is considered as rather simple, the detection of a flow front that moves perpendicular to the laminate plane emerges to be difficult. Therefore, the knowledge of the transverse impregnation behaviour is still sparse and transverse permeability values were determined for a very few fabrics only. The knowledge of an exact transverse permeability is important for the three-dimensional simulation of flow through thick sectioned parts and for specific RTM-related processes like resin film infusion or SCRIMP®. This paper addresses the monitoring of flow front propagation utilizing ultrasound transmission. A testing rig was developed for monitoring three-dimensional ellipsoidal impregnation, which is induced by point injection. Two multidirectional non-crimped fabrics were characterised by the three-dimensional measurement. For the determination of the transverse permeability the three-dimensional filling was emulated by a numerical flow simulation software.

Aspects of the stitch formation process on the quality of sewn multi-textile-preforms

Abstract Stitching technologies are considered to be one of the key technologies for automated manufacturing of complex textile preforms which are used for liquid composite moulding of fibre reinforced plastic parts [Proceedings of Sixth International Conference on Automated Composites (1999)]. The sewing or stitching process is applied for different purposes during the production of dry fibrous reinforcements as well as for structural aims in the composite component (through-the-thickness reinforcement), thus, the requirements on the stitch itself are wide spread [Complex Multi-textile Preforms – The Potential of Sewing. 90(4) (2000) 43; Stress Conc Compos Sci Technol, 59 (1999) 2125; Tech Textiles 43 (2000) 120; Compos Part A 31 (2000) 571; D 82 Dissertation RWTH Aaachen (1999)]. A detailed prediction of properties of the stitched reinforcements requires an understanding of the stitch formation process and the interaction between textile reinforcements, the stacking sequence thereof, the stitching process parameters and sewing thread properties.

Induction Heated Joining of Aluminum and Carbon Fiber Reinforced Nylon 66

Joining different material types, like metal and thermoplastic fiber reinforced polymer composites (TP-FRPC), offers a large potential for innovative light weight applications. This kind of bonding depends on mechanical, physical, and chemical interactions and is, therefore influenced by joining partner surface treatments. This study describes adhesion models and the effect of surface treatments of AlMg3-CF/PA66-joints. Joining by means of induction heating is an appropriate joining technology for the bonding of metal/TP-FRPC as it is utilized by a rapid heat generation. The characterization of the bonding mechanisms and the influence of the surface treatments are presented by single-lap joints and by microscopic analyses.

Process and performance evaluation of ultrasonic, induction and resistance welding of advanced thermoplastic composites:

The possibility of assembling through welding is one of the major features of thermoplastic composites and it positively contributes to their cost-effectiveness in manufacturing. This article presents a comparative evaluation of ultrasonic, induction and resistance welding of individual carbon fibre-reinforced polyphenylene sulphide (PPS) thermoplastic composite samples that comprises an analysis of the static and dynamic mechanical behaviour of the joints as well as of the main process variables. The induction welding process as used in this research benefitted from the conductive nature of the reinforcing fibres. Hence, no susceptor was placed at the welding interface. Resistance welding used a fine-woven stainless-steel mesh as the heating element and low welding pressures and times were applied to prevent current leakage. Triangular energy directors moulded on a separate tape of PPS resin were used to concentrate ultrasonic heat at the welding interface. The static single-lap shear strength of the joints was found similar for induction and ultrasonic welding. A 15% drop in the static mechanical properties of the resistance welded joints was attributed to incomplete welded overlaps following current leakage prevention. However, the fatigue performance relative to the static one was similar for the three sorts of joints. A comparative analysis of process variables such as welding time, required power and energy was also carried out.

Approach to net-shape preforming using textile technologies. Part II: holes

Considering cost-effective fibre-reinforced plastic (FRP) parts manufactured by resin transfer moulding (RTM) technologies, net-shape preforms can contribute to mechanical post treatment free manufacturing, hence reducing cycle-time and cost. In addition to this, RTM specific problems, such as fibre pinching when closing the tool can be avoided. The wide range of textile technologies used for FRP parts offers various possibilities to achieve net-shape parts within the preforming process including bores or inserts (see part II). The potential of the presented technologies is evaluated and discussed. Apart from standardised testing methods, a new side-impact testing was specially designed for determining the in-plane damage-tolerance of net-shape manufactured FRP parts. Folding to net-shape shows poor quality but improves mechanical properties. Rim-protections with braided inserts or over-edge stitching technologies which provide exact preform dimensions, improved mechanical performance. Especially side-impact tolerance or in-plane damage tolerance was increased. Ultra-sonic testing was successfully applied to visualise the differences in damage tolerance and propagation between these new rim technologies.

Tracing the Void Content Development and Identification of its Effecting Parameters During in situ Consolidation of Thermoplastic Tape Material

One of the ways to identify the quality of structures made from fibre-reinforced thermoplastic material is by examining its void content percentage. Significant improvement in mechanical properties can be achieved by minimising it. The purpose of this work was to develop a simulation tool from existing available model in literature, to trace out the void development inside the laminate during the manufacturing and identification of major influencing process parameter. The effects of consolidating force, process velocity, hot gas flow in the heating region, and repetitive passes were investigated through simulation. A series of experiments was carried out on several AS4/PEEK laminated plates manufactured by automatic tape placement process. Simulated void distribution through thickness and density were compared with measured values to trace the effecting input parameters. Thickness build-up with successive lay-ups is also monitored online and the average thickness lies in close proximity to the predicted range. Major influencing process parameters were identified.

Parametric study on processing parameters and resulting part quality through thermoplastic tape placement process

Thermoplastic tape placement with in situ consolidation is a unique process, in which prepreg tape is precisely positioned, laid-up and immediately consolidated on the tool surface in a single step. This study addresses some of the remaining issues that restrict the industrialisation of the thermoplastic tape placement. The consolidation quality of the laminates depends on several parameters underlying the consolidation mechanism, for example the tape placement setup, the processing parameters, and the material characteristics. In order to identify the critical parameters of the process, simulations were carried out to study the process sensitivity on individual parameters.

Structure and properties of flax/polylactide/alumina nanocomposites

Composites composed of polylactide (PLA), woven flax fiber textiles (weave style of 2 × 2 twill and 4 × 4 hopsack) and boehmite alumina were produced by hot press. The spraying technique served for the pre-dispersion of the alumina nanoparticles. The aqueous alumina slurry was produced by mixing the water with water dispersible alumina. The dispersion of the flax structures and alumina particles in the composites was studied by scanning electron microscopy. The polylactide composites were subjected to water absorption and instrumented falling weight impact tests. The creep and thermomechanical properties of the composites were determined in short-time creep tests (performed at various temperatures), thermogravimetric analysis and dynamic-mechanical thermal analysis, respectively. It was found that the incorporation of alumina particles reduced the water uptake compared to the polylactide/flax blends. The impact energy and stiffness value of polylactide/flax blends was markedly higher than that of PLA but reflected the effects of composite structures. Incorporation of alumina particles enhanced storage modulus and the creep resistance compared to the polylactide/flax blends but slightly incremented thermal resistance at high temperature. No clear trend in the flax weave style-effect was found in the thermal behavior. The creep master curves were constructed by applying the time–temperature superposition principle. The Findley power law could satisfactorily describe the creep compliance versus time traces for all systems studied.

Thermal Residual Stress Simulation in Thermoplastic Filament Winding Process

The present paper describes an advanced model to simulate the residual stresses of reinforced filament wound thermoplastic composites. The nonlinear characteristics of the problem are reduced by decoupling some of the phenomena and the associated dependent variables into submodels. Four submodels are used here: fiber motion, thermal, kinetic and rheological as well as a stress-strain submodel. The models were tested on a selected GF/PP sample chosen from a sensitivity study which is presented also. The effect of individual process parameters on the residual stresses in filament wound parts is rated by a sensitivity study.

Preform technology: a necessary requirement for quality controlled LCM-processes

A considerable number of process versions of Liquid Composite Moulding (LCM) technologies have been commercialised during the last few years. Only recently there have been attempts to reduce costs and to implement more complex components by the application of preform technology. This article indicates the most appropriate application fields for LCM techniques for the manufacture of fibre reinforced polymers. Different LCM methods are described and summarised. The preforming techniques and particularly the sew -and -cut philosophy for developing a tailored reinforcement is explained. Investigations designed to elucidate the influence of sewing parameters and thread properties have led to an understanding of the requirements and preform characteristics needed to operate the quality controlled LCM process. Some concepts related to the control of the resin injection are described, and a realization of a decision-tree-concept is shown. Finally, a comparison of cost effects for different preforming methods shows a need to define a “critical preforming effort” to manufacture quality components as well as economically optimised parts.