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Effects of interlaminar residual stresses on the mechanical properties of thermoplastic hybrid composites

Carbon fibre reinforced plastics (CFRP) with a high weight-specific stiffness have been used for highly loaded structural components in racing cars since the early 1980s. Subsequent exploitation in conventional sports cars (Lamborghini and Ferrari) from the 1990s onwards was followed by the first use of CFRP in BMW's i3 and i8 serial electric vehicles. However, due to the additional costs compared to metallic structures and the poor CO2 balance of carbon fibres, this material technology has so far rarely been used in other mass production vehicles (e.g. BMW G11).

An approach of countering the disadvantages of CFRP is to combine it with more cost-effective, glass-fibre-reinforced plastic (GFRP) in order to mix the expensive but highly stiff carbon fibres in a laminate or component with GFRP in a hybrid, load-specific manner. The major limitations associated with this approach are interlaminar residual stresses resulting from the different thermal expansion behaviour of the reinforcing fibres. The aim of the present work is therefore to quantify the interlaminar residual stresses in UD tape based PA6 hybrid laminates and to investigate their influence on the dynamic 3-point bending properties. In addition, the effect of a heat treatment on the interlaminar residual stresses and the mechanical properties will be determined, thus creating a scientific basis for the industrial application of this material technology. As a target result, a cost reduction of 30 % and an improvement of the CO2 footprint by 50 % compared to pure CFRP are aimed for.

The measurability of the interlaminar residual stresses by means of the analytical hole drilling method is proven in the context of this work, which enables the determination of correlations between the existing stresses and the resulting mechanical properties. The interlaminar residual stresses in the hybrid laminates lead to a premature crack formation in the interface between glass and carbon fibre layers and consequently to failure-initiating delaminations. Due to material fatigue under dynamic load, the maximum stress that hybrid laminates can bear is reduced by 21 % to 25 % compared to the corresponding static bending strength, regardless of the lay-up design. For the carbon fiber monomaterial structure, this decrease is 18 %.

The analysis of the dynamic bending properties at a temperature of +90 °C shows that the influence of the interlaminar residual stresses in the material on the mechanical properties declines with increasing ambient temperature. At this ambient temperature hybrid laminates fail under a dynamic 3-point bending load at stress values exceeding 95 % of the static bending strength. However, while at room temperature a positive hybrid effect of 3 % can be observed for a sandwich hybrid laminate, at +90 °C this characteristic value drops to -28 %. The reason is a premature buckling of the carbon fibres on the pressure side which initiates the resulting catastrophic failure of the sample.

The effect of a heat treatment lasting several hours was also investigated. This allows the interlaminar residual stresses to be eliminated, resulting in an improvement of the dynamic bending strength. At room temperature an alternating hybrid laminate thus achieves over +90 % of the dynamic strength of a CFRP laminate at 35 % lower costs. The dynamic hybrid effect of the heat-treated hybrid laminates is 18 %, compared to -4 % in the untreated state.

Lesen Sie die deutsche Zusammenfassung auf
 Michael Kropka

Michael Kropka
Fakultät für Ingenieurwissenschaften
Universität Bayreuth


Free keywords: Carbon fibre reinforced plastics, CO2 footprint, hybrid laminates, glass fibre reinforced plastics, load-specific, interlaminar residual stresses, polyamide 6, heat treatment, hole drilling method, fatigue properties, dynamic 3-point bending, lay-up design, hybrid effect
Institute / chair: Fakultät für Ingenieurwissenschaften der Universität Bayreuth
Language: German
Technical consultant for expert services: Professor Dr.-Ing. Volker Altstädt, Professor Dr.-Ing. Klaus Drechsler
Publication year: 2020
Provider: Wissenschaftlicher Arbeitskreis Kunststofftechnik (WAK) /

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