Stefan Kreling

Laser Surface Pre-Treatment of CFRP for Adhesive Bonding in Consideration of the Absorption Behaviour

The use of carbon fibre-reinforced plastics (CFRP) is growing significantly in all areas of lightweight construction. Thermosets like epoxy still build a major part of deployed matrix material. A suitable joining technology for CFRP is adhesive bonding. However, the bonding performance is well reduced because of contaminations. To achieve full bond-strength, adhesion of the adhesive to the components is essential. One innovative method for surface pre-treatment is laser radiation. Former investigations show that an important parameter is the absorption. As a result of that, a UV laser (wavelength of 355 nm) and a CO2 laser (wavelength of 10600 nm) were used to pre-treat specimens manufactured from 120°C curing epoxy. The focus is to achieve a selective removal of resin without impairing the fibres. Lap-shear specimens have been pre-treated, bonded with an one component epoxy film adhesive and tested. The laser pre-treated specimens achieve the same bond strength as references prepared by manual abrading. Furthermore, the mechanisms of interaction between laser radiation and matrix material as well as fibres are discussed considering the different heat deposit and absorption behaviour of the emitted radiations.

Using excimer lasers to clean CFRP prior to adhesive bonding

Dr. Fabian Fischer and Stefan Kreling of the Technical University of Braunschweig, and Frank Gabler and Dr. Ralph Delmdahl of Coherent compare the various laser technologies that have been considered for cleaning carbon fibre reinforced plastic (CFRP) and discuss the implementation of the excimer laser technique in particular.

Surface treatment with small laser spots: an approach for the comparison of process parameters

Pulsed lasers with spot diameters in the μm range are widely used for the laminar treatment of surfaces. The laminar treatment is achieved by a two-dimensional overlap of the small laser pulses going from lines to a surface. This publication states the numerical description of the average amount of laser pulses per surface increment and its spread considering processing parameters such as line feed, pulse repetition rate and hatch distance. Based on the numerical results an empirical formula was derived and the basic approach of calculating an accumulated energy as a benchmark for comparing pre-treatment parameters was investigated for different laser systems.

The Production-Related Influence of Iron Oxides on Steel Surfaces on the Adhesion of Fusion-Bonded Hybrid Structures

In view of e-mobility and the still challenging weight of lithium-ion battery packages as well as the demand for a reduction of exhaust emissions, in particular carbon dioxide, the development of innovative lightweight solutions is required. Therefore the manufacture of tailored hybrid structures fabricated within a continuous production line represents a promising approach for the automotive industry. Roll forming of tailored metal sheets is state of the art and already implemented in the automotive production of roof headers, rocker panels, lower radiator support beams or rear seat back beams. The continuous roll forming process of hybrid structures, consisting of steel sheets locally reinforced by carbon fiber reinforced thermoplastic tapes, offers a high degree of automation as well as decreased weight compared to a roll-formed part of pure metal. With the focus on minimizing the process steps in the present work the steel sheets and the thermoplastic tape are joined by fusion bonding whereby the thermoplastic matrix is melted on the steel surface. With the objective of improving the joint strength of the hybrid structure both materials are heated separately by different heating technologies and joined in a subsequent process step. As a consequence the formation of iron oxides on the steel surface, depending on the heating temperature, can be observed. Depending on various heating temperatures, the steel surface is analyzed physico-chemically by Drop-Shape Analysis as well as chemically by using X-ray Spectroscopy. In addition, the evaluation is carried out by mechanical testing of lap shear specimens. The chemical examinations show the formation of iron oxides on the steel surface, based on prior tempering, resulting in improved adhesion to the carbon fiber reinforced thermoplastic tape.

Adhesive bonding techniques for advanced high-strength steels (AHSS)

The application of advanced high-strength steel in auto body engineering enables the reduction of the weight of the body-in-white while fulfilling the high demands of customers, safety regulators, and the automotive industry regarding safety, especially under crash conditions. This high potential can only be used applying adequate joining techniques. Adhesive bonding and adhesive bonding in combination with spot-welding, so-called weldbonding, offer high potential for joining high-strength materials. This chapter deals with the challenges and influences on adhesive bonding and weldbonding of advanced high-strength steel, in particular the influences of joint geometry and surface layers on achievable bond strength.

Sauber und prozesssicher

Der Einsatz von Faserverbundkunststoffen (FVK) im Automobil ist spätestens seit der Bekanntmachung, dass der BMW i3 ein Life-Modul aus kohlenstofffaserverstärktem Kunststoff (CFK) besitzen wird [1], nicht mehr nur auf die Kleinstserie im Sektor hochpreisiger Sportwagen beschränkt. Es bestehen jedoch noch erhebliche Herausforderungen, diese Materialien zu fügen, da gebräuchliche Verfahren — wie etwa das Punktschweißen — nicht eingesetzt werden können und Techniken, bei denen mechanische Verbindungselemente verwendet werden, zu einer lokalen Zerstörung der Fasern und erheblichen Spannungsspitzen führen. Es empfiehlt sich der Einsatz der Klebtechnik, die allerdings eine Vorbehandlung voraussetzt, um Trennmittelrückstände und andere Kontaminationen von den Oberflächen zu entfernen [2]. Mit dem Einsatz von FVK in größeren Stückzahlen entsteht dabei auch die Notwendigkeit, Verfahren zu entwickeln, mit denen automatisiert und mit hoher Prozesssicherheit und -geschwindigkeit die Klebvorbehandlung erfolgen kann.

Clean and reliable

The use of fiber-reinforced plastics (FRP) in the automotive industry is no longer limited to the smallest-batch series of expensive sports cars, at least not since the announcement that the BMW i3 will have a Life-module made of carbon fiber-reinforced plastics (CFRP) [1]. But there are still considerable challenges as regards the bonding of these materials, as conventional procedures (for instance spot welding) cannot be applied and technologies using mechanical fasteners lead to a local destruction of the fibers and considerable stress peaks. The use of adhesive bonding is recommended which, however, requires a pretreatment to remove release agent residues and other contaminations from the surfaces [2]. With the use of FRP in larger quantities it also becomes necessary to develop procedures which allow automated bonding pretreatment with a high reliability and low process times.

Wirtschaftliches Fügen in Serie und im Reparaturfall

Um dem Nachteil des langsamen Festigkeitsaufbaus struktureller Klebstoffe zu begegnen, werden heute vielfach Hybridfugeverfahren wie das Clinch-Kleben eingesetzt. Uberall dort, wo hybride Verfahren nicht anwendbar sind, bedarf es anderer Wege, die zu einem schnellen Festigkeitsaufbau fuhren. In Rahmen eines dreiteiligen Beitrags werden diese umfassend dargestellt. Der folgende erste Teil beleuchtet den Stand der Technik auf diesem Gebiet und beschreibt ein interessantes Untersuchungskonzept.

A contribution to the qualification process of surface pretreatment methods: Sensitivity of mechanical tests to adhesion and delamination

ABSTRACT Focusing the adhesive bonding of thermoset carbon fiber reinforced plastics (CFRPs) due to release agent residues on a surface pretreatment prior to the bonding process is inevitable to guarantee sufficient adhesion. Based on different material and process-related phenomena, a huge variety of parameters (e.g. treatment time) influence the treatment effectiveness. This often leads to intense parameter studies in which pretreatment methods are qualified by different mechanical tests and which require significant effort in terms of time and resources. Those qualification procedures are sometimes standardized by different and mostly company-specific standards. To increase the efficiency of this qualification process by increasing the general validity, this paper presents a comparison of different mechanical test methods (namely single lap shear test, floating roller peel test, double cantilever beam (DCB) test, and cross-tension test) in terms of their sensitivity to the detection of adhesion or delamination defects, even if they normally test different aspects of a joint. However, it could be found that the most common test – the single lap shear test – can only reveal large defect levels, while different tests, e.g. DCB test, show a good sensitivity for all investigated failures and others show intermediate performance.

The impact of prepreg aging on its processability and the postcure mechanical properties of epoxy-based carbon-fiber reinforced plastics

Due to the intention to increase the product efficiency by weight reduction, the application of carbon fiber reinforced plastics and its processing becomes more and more important for the industrial production planning. The aviation industry mostly uses epoxy-based preimpregnated laminates (prepregs). These prepregs are stored in the refrigerated state and may be processed after defrosting only for a specific shelf-life or so-called out-time. This limitation in processing time reduces the efficiency of the entire production, for example due to the waste of material during production stops. To improve the production effectiveness the shelf-life should be as high as possible, to be able to avoid material waste during production stops. Therefore, the processability and the postcure mechanical properties of two representative prepregs were investigated during this study for different aging stages and correlated with the reactivity of the prepregs.