PhD's

Sebastiaan Haanappel

ctw/pt


University of Twente

Faculty of Engineering Technology

Chair of Production Technology

P.O. Box 217

7500 AE Enschede

The Netherlands

Room: Horst N128

Phone: +3153 489 4346

E-mail: s.p.haanappel[a]utwente.nl


List of publications


PhD Project 2

Complex stamp forming


Start / End:

January 2010 to September 2012


PhD Project 1

Performance of thermoplastic composite joints


Start / End:

September 2008 to January 2010


Funding

These projects are funded by the Thermoplastic Composite Research Centre (TPRC). The support of the Region Twente and the Gelderland & Overijssel team for the TPRC, by means of the GO Programme EFRO 2007-2013, is gratefully acknowledged


Complex stamp forming


Introduction

There is a growing interest in the application of uni-directionally (UD) reinforced thermoplastics in aerospace industry. For example, the Fokker Aerospace Group started the development of processing this material in floor beams since the year 2000. This development was supported by the increasing availability of tooling to process these materials. Since these products were successfully integrated in aircraft designs, it is tempting to process these thermoplastic materials in primary loaded structures. Strengths, stiffnesses, weight savings, and low scrap are points of consideration. These can be achieved by blank designs with locally varying thicknesses and ply orientations. These blanks are termed as tailored blanks.


Fig. 1: Example of a tailored blank on the left. Right, the resulting seat back frame.


Source:

Burkhart A., Cremer D., Feasibility of continuous-fiber reinforced thermoplastic tailored blanks for automotive applications, Proceedings of the 5th Annual SPE Automotive Composites Conference, Troy, Michigan, September (2005).


Objectives


Stamp forming a tailored blank poses a number of questions. For example, which quality is needed for the initially flat tailored blank? How do we assess this quality? How does this quality affect the stamp formed part eventually? A tape placement benchmark will be set-up for this purpose. Different tape placement techniques will be utilised and tape-placed laminates will be inspected.


What accuracy is needed for the tailored blank geometry, such that the use of the matched tooling geometry results in products of high quality? How do locally varying thicknesses and lay-ups affect the forming behaviour? For this, simplified products on an elementary level will be stamp formed and inspected.


Nevertheless, a product design is unique. Forming induced defects like spring back/forward and wrinkling are dependent on the geometry of the tooling. Therefore, prediction tools need to be developed. These aim to simulate the thermoforming process to reduce the number of optimisation cycles during the development phase of a product.



Fig.2: Example of tooling (left) to stamp form a circular blank into a dome (right).



Source:

Haanappel S. P., Thije, ten, R. H. W., Akkerman R., Forming predictions of UD reinforced thermoplastic laminates, Proceedings of the 14th European Conference on Composite Materials, Budapest, Hungary, June (2010).


Approach


In order to simulate the forming process of a tailored blank, at least a model is needed that successfully predicts the forming behaviour of a flat blank. As a first step, forming trials with a doubly curved shape were performed. A typical result is how in Fig. 2. A model was developed in order to simulate this forming experiment. The model needs material property data for different deformation mechanisms, as shown in Fig. 3.



Fig.3: Typical deformation mechanisms that appear in thermoforming processes. From left to right: shear along fibres; shear transverse to fibres, inter-ply slip and, out-of-plane bending.



Source:

Haanappel S. P., Thije, ten, R. H. W., Akkerman R., Constitutive modelling of UD reinforced thermoplastic laminates, Proceedings of the 10th International Conference on Flow Processes in Composite Materials, Centro Stefano Franscini, Monte Verità, Ascona, Switzerland, July (2010).


Constitutive models are needed to account for the deformation mechanisms in Fig.3. These models are implemented numerically. The AniForm finite element package will be utilised for simulating the thermoforming process. This package deals with large deformations of highly anisotropic materials and is based on an implicit formulation. Fig.4 shows a typical simulation result of the forming process of the circular blank in Fig.2. Clearly, the location of wrinkles in Fig. 4 can be predicted accurately.




Fig.4: Results of a forming experiment (left), compared with the simulation (right). A good match in wrinkled regions was found.


Source:

Haanappel S. P., Thije, ten, R. H. W., Akkerman R., Forming predictions of UD reinforced thermoplastic laminates, Proceedings of the 14th European Conference on Composite Materials, Budapest, Hungary, June (2010).


The deformation mechanisms in Fig.3 are described by constitutive models. These involve material parameters that need to be characterised. For this, characterisation methods need to be developed and characterisation set-ups have to be designed and build. Eventually, experiments will be conducted and material parameters can be determined. These are subsequently processed in the constitutive models.



Performance of thermoplastic composite joints


Introduction


Conventional riveting of composite structures leads to fibre damage at the location of load introduction. Additionally, due to local load introduction, stress concentrations around the damaged fibres occur. Obviously, the advantages of continuous fibres in these kinds of bonded structures are minimally exploited.


Composite structures are well suited for adhesive bonding. For this technique, joining needs to be done carefully and proper surface preparation is necessary. Alternative competing joining methods are induction welding and co-consolidation.


Abovementioned joining techniques are mainly used in secondary non-critical structures. Their usage in primary structures is limited. The main reasons are the difficulty in predicting the strength, reliability and durability of these joints. Moreover, the absence of a widely accepted procedure to asses these characteristics of composite joints explains the limited application in primary structures.


In order to improve confidence in these joints, it is aimed to put forward benchmark joint characterisation results. These will ultimately lead to a database containing information that concerns the strength and environmental resistance properties of joints in composite structures, as a function of materials, joint type, geometry and production process (pressure, temperature and time).



Approach


As a start-up for this project, various joining methods will be studied for different elementary joint configurations. The shear strength of lap joints and the peel strength of T-joints will be investigated for different materials. Experiments will be performed to compare the various alternative joining methods, supported by numerical modelling to explain and further exploit the results.


 

Fig.1: Lap joint Fig.2: T-joint


Experiments


Strengths of bonds as they appear in the samples in figure 1 and 2 will be determined. Results of the abovementioned bonding techniques will be compared. Figure 3 shows the result of an optical strain field measurement, performed on the overlap region of a single lap joint. These measurements may help arguing the obtained results.


Fig.3: Shear distribution obtained by an optical strain field measurement



Modelling


Why?

§Explanation of experimental results

§Validation of test samples

§Validation of experimental set-up

§Further exploitation of the results, e.g. improving test set-up/joint design


Fig.4: Shear distribution obtained by numerical modelling


Application examples


Composite joints are advantageous in many composite structures. Composite structures are especially applied in applications in which the strength/stiffness to weight ratio is aimed to be maximised. These appear for example in aerospace and car racing structures.



Fig.5: Joints appear in many types of application. For example, aerospace industry processes composite joints in various structures, like wing sections.