Daniel Stefaniak

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.

Residual Stresses in Intrinsic UD-CFRP-Steel-Laminates - Experimental Determination, Identification of Sources, Effects and Modification Approaches

This paper focuses on the reduction of process-related thermal residual stress in fiber metal laminates and its impact on the mechanical properties. Different modifications during fabrication of co-cure bonded steel/carbon epoxy composite hybrid structures were investigated. Specific examinations are conducted on UD-CFRP-Steel specimens, modifying temperature, pressure or using a thermal expansion clamp during manufacturing. The impact of these parameters is then measured on the deflection of asymmetrical specimens or due yield-strength measurements of symmetrical specimens. The tensile strength is recorded to investigate the effect of thermal residual stress on the mechanical properties. Impact tests are performed to determine the influence on resulting damage areas at specific impact energies. The experiments revealed that the investigated modifications during processing of UD-CFRP-Steel specimens can significantly lower the thermal residual stress and thereby improve the tensile strength.

Payload Adapter Made from Fiber-Metal-Laminate Struts

In comparison to other transport systems, launch vehicles are characterized by relatively light but extremely valuable payloads. The launcher’s upper stage structures, e.g. payload adapter and fairing, offer the highest weight saving potential. An effective weight reduction can only be achieved by the combined utilization of high performance materials and adapted construction methods. To improve the structures damage tolerance a new hybrid lay-up has been developed, which combines the properties of both, steel and carbon fiber reinforced plastics (CFRP). This chapter presents a preliminary design of a payload adapter as a framework, which is based on the high performance material properties of unidirectional CFRP-steel-laminates, offering a considerable weight saving potential.

Evaluation of residual stress development in FRP-metal hybrids using fiber Bragg grating sensors

This paper presents experimental measurement methods for the determination and evaluation of process related thermal residual stresses in fiber metal laminates. A cure monitoring system with fiber Bragg grating (FBG) sensors is used to measure the in-plane strains during processing of carbon fiber reinforced plastic (CFRP)-steel laminates. The simultaneous measurement captures the thermal expansion during the heating stages, the cure shrinkage, and the cooling thermal shrinkage. The results enable the characterization of the co-cure bonding process and the stress transfer between the metal and FRP-layers during the creation process. The residual strains, which are used for calculation of the residual stresses, are recorded at room temperature after manufacturing. In addition, an advanced method using FBG-sensors and the deflection of asymmetric hybrid specimens is developed to validate the gained residual stress data. Asymmetrical specimens are created by removing selected layers after cure. Quantitative evaluation is achieved by determination of their curvature and measuring the strain changes with the embedded FBG-sensors. For validation, the methods were successfully demonstrated on two different curing cycles with different resulting residual stress levels. The simultaneous strain measurement enables the investigation of stress development and delivers more in-depth process knowledge for further optimization of the manufacturing process.

Spezifische Herausforderungen für den Einsatz von Faser-Metall-Laminaten

Faser-Metall-Laminate als Kombination metallischer Werkstoffe und Faserkunststoffverbunde sind aktuell Inhalt vieler Forschungsprojekte. Durch eine geschickte Kombination der beiden Werkstoffklassen erhofft man sich, deren inharente Schwachen zu kompensieren. Der Einsatz dieser Materialkombination ist jedoch gleichzeitig mit unterschiedlichen Herausforderungen verbunden, wie Wissenschaftler vom DLR und der TU Braunschweig beschreiben.

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.