Erik Kappel

Spring-in and Warpage — Progress in Simulating Manufacturing Aspects

After demolding, plane structures made of fibrous polymer-matrix composites often show a warpage and, owing to spring-in angles, do not reach the designed shape. Considerable effort has been put into the development of methods for simulating the distortions in order to avoid them by countermeasures. Besides the empirical trial-and-error-based procedure, there is a choice between simulation-based and semianalytical approaches. This paper focuses on the latter ones as being fast and sufficiently accurate for an initial design. Spring-in is observed in curved structures. A simple formula relates the amount of spring-in to the difference between their in-plane and transverse properties. Furthermore, it has been shown that an equivalent coefficient of thermal expansion can be determined with the use of L-shaped coupons. With the aid of this coefficient, the spring-in of more complex structures can be calculated with a sufficient accuracy. Experiments reveal a considerable warpage of plain structures, especially of slender plates. This happens in the cases of one-sided molds, obviously depends on the part/tool connectivity, and is caused by the difference in thermal expansion, as well as by the development of cross-linking in the thickness direction. Other parameters of influence may be gradients in the fiber volume fraction or resin-rich layers. A simple formula based on a model of two beams on top of each other was developed. It relates the artificial coefficients of thermal expansion to the curvature of the compound beam. Warpage measurements on simple test specimens allow one to determine these coefficients, which then can be used in detailed FEM analyses of more complex structures in order to anticipate their behavior on demolding.

Fertigungsinduzierte Deformationen von Hochleistungsverbundstrukturen

Faserverbundstrukturen sind aus heutigen Leichtbauapplikationen im Flugzeugund Automobilbau nicht mehr wegzudenken. Dies begründet sich vor allem mit den hervorragenden gewichtsspezifischen mechanischen Eigenschaften des Werkstoffs, aber auch durch die mit ihm verbundene Formgebungsfreiheit hinsichtlich der Bauteilgeometrie, die besonders im Automobilbau zunehmend Anwendung findet. Zusätzlich zum reinen Stoffleichtbau, bei dem die geringe Dichte des kohlenstofffaserverstärkten Kunststoffs von etwa 1,6 kg/dm3 im Vordergrund steht, ermöglicht ein Faserverbund einen hohen Integrationsgrad. Der Integrationsleichtbau ermöglicht durch den Wegfall unnötiger, zumeist kostenintensiver Verbindungsstellen zusätzliche erhebliche Gewichtsund Kostenreduktionen. Das Vereinen von mehreren Karosseriebauteilen zu einem gemeinsamen Seitenteil, wie beim BMW i3, oder die Realisierung einer integral-versteiften Faserverbundflügeloberschale, wie sie seitens des DLR-Instituts für Faserverbundleichtbau und Adaptronik entwickelt wurde, sind Beispiele für den Integrationsleichtbau. Dieser Trend hin zu Großbauteilen mit gesteigerter Komplexität wird durch eine Eigenheit der Schichtverbunde erschwert, die sich

Compliant Aggregation of Functionalities

The aggregation of functionalities offers additional benefits to the customers such as reduced weight, reduced life cycle costs and an increased range of applications. For a compliant aggregation of functionalities according to given requirements clear instructions on how to conduct lightweight design are essential, but often not available today. High performance lightweight structures are made from carbon fiber reinforced plastics increasingly. Due to the specific composite manufacturing process four different levels of function-integration are conceivable. The pre-fabrics or components of the composite can include smart materials with enhanced functionalities. The structure design can better exploit the composite potentials of anisotropic material properties. Passive components integrated into the structure provide additional functionalities as for example de-icing and lighting protection. In adaptive systems active elements significantly improves the ability of the structure to adapt changing environmental conditions. The development of the potentials resulting from the compliant aggregation of functionalities is presented in this chapter.

About the Spring-In Phenomenon: Quantifying the Effects of Thermal Expansion and Chemical Shrinkage

A straightforward approach to predict spring-in deformations of angled composite parts is presented. Therefore, a proposal by Radford is extended in order to calculate the spring-in contribution due to chemical shrinkage. For this, the volumetric shrinkage of neat thermoset resin, which is in the range of 2–7%, is transferred to equivalent strains on ply level assuming no shrinkage in fiber direction. As the fiber volume fraction (FVF) affects mechanical and chemical properties significantly, the spring-in angle is affected as well. Therefore, the numerical investigation accounts for the spring-in angle and its thermal and chemical contributions depending on the FVF. Classical laminate theory (CLT) is utilized to homogenize layup expansion and shrinkage properties. For validation purposes, model predictions are compared with measurement results gained from one manufactured test specimen. Good agreement between analytical and experimental results is found. Furthermore, the chemical contribution of the total spring-in angle ∆φ turned out to be significantly larger than the thermal contribution.