The following is an excerpt.

Petter Holmström’s fresh PhD thesis brings good news to every industry which moulds fibre-reinforced thermoplastics for withstanding extreme loads.

«I feel sure that my experimental work is sound and will be of value to others. The models I have worked with have potential industrial use», Petter Holmström says. Here he is working on the finishing stage together with his supervisor Professor Arild Holm Clausen. (All photos: Sølvi W. Normannsen)

The short fibres used in reinforced thermoplastics look like tiny little sticks. They measure 0.1 to 1 millimetre in length, and about 15 millionth of a metre in diameter. Control is the key, when moulding them. And the more you control of the behaviour of these sticks, the stronger the plastic you produce – according to PhD candidate Petter Holmström at SFI CASA.

Much of his work aims to increase our understanding of the mechanical behaviour of the material, that is, its ability to withstand extreme loads. «This must be under control when the material is used in load carrying components», the PhD candidate underlines. In addition, his thesis is also about how to represent fibre-reinforced thermoplastics in numerical simulations.

SIMLab’s first in fibre-reinforced thermoplastics

Fibre-reinforced plastic components are used in car bumpers, rotor blades in wind turbines and in sports equipment. The plastic materials are strengthened by adding glass fibres.  And, according to Holmström, you cannot compute how these materials will react to stretching, pressure, impact or crashes unless you know their interior composition.

He is the first, and so far, the only, PhD candidate in the research group SIMLab, that has embarked on the task of evaluating material models for fibre-reinforced thermoplastics. His thesis «An experimental and numerical study of the mechanical behaviour of short glass-fibre reinforced thermoplastics» is close to 400 pages.   

SIMLab’s material model for brittle materials, which is one of the evaluated models, shows a great potential for computing the stiffness and strength for this material class.

Making cars stronger, but not weaker

The use of short glass-fibre reinforced thermoplastics in load carrying components is relatively new. The material replaces metals. Thus, the industry can reduce the weight of cars or other constructions, while they still retain the stiffness and strength.

One of the benefits of moulded thermoplastics is that a single component may replace a part of the structure that were traditionally composed of a number of parts assembled together.  

For industry, not at least the automotive industry, it is vital to reduce weight, time consumption and costs. In addition, increased flexibility in the design process is always an aim.

Different strengths in different directions

When fibres are moulded into a thermoplastic, the material becomes stiffer, stronger and more brittle. The material is injected just like a heated liquid into moulds of any particular shape. Then it cools and hardens to the shape of the mould. Thus, polymers can be used to form highly complex geometric components.

Much of Holmströms work aims to increase our understanding of the mechanical behaviour of the material, that is, its ability to withstand extreme loads.
Petter Holmström has evaluated two material models for glass-fibre reinforced thermoplastics.

According to Holmström, the industry has not paid a lot of attention so far to the anisotropy of thermoplastics. Anisotropy means that a material has different properties in different directions. Tension tests show that glass-fibre reinforced thermoplastics can be twice as stiff and strong in the longitudinal direction as they are laterally. «When we know there is a factor of 2 that depends on the load direction, industry should account for it», he says.

What cannot be simulated cannot be put into use

«When a plastic component is designed to carry loads in interaction with other components, you have to be able to simulate how they react to external loadings», Holmström explains.

The behaviour of the material must be described mathematically in a material model. He states that this is the kind of knowledge that the automotive industry and the rest of the industry wants.

«If you can’t simulate it, you simply cannot use the material – unless we are talking about kitchen utensils».

Like logs in a streaming river

When thermoplastics are heated to a liquid state and are injected to flow into a mould, the orientation of each glass fibre is decided by the flow conditions or the direction of the plastic mass.

It is just like a river for floating logs. The speed of the logs, the strength of the current and the direction of the water are determined by depth, stones or other obstacles.  When there are many obstacles, the logs can easily jam together and be blocked.

In a simple, flat panel that is 3 millimetres thick, there will be few obstacles and the fibres will spread out fairly uniformly. Despite the simple geometry the material will become clearly stratified – which also explains its anisotropic behaviour.

In a complex component, for instance a car bumper, it is different. There will be nooks and corners that hinder the streaming flow of plastic. Thus, just like the logs, the fibres can lump together and be concentrated in certain areas.

A model that computes behaviour  

Petter Holmström’s specimens are made of fibre-reinforced polypropylene and polyamide. The short fibres are 15-20 times stiffer than the polymers they are moulded into. He is the first PhD candidate at SIMLab to have made extensive use of X-ray microscopy to study the interior of the materials.

The X-ray images show myriads of these tiny little sticks spread in what appears to be chaos. Unravelling this chaos, Holmstrøm has compiled statistics on their distribution, their direction and the angles in which the fibres lie in relation to the others.

«This is important knowledge, because the ability of the material to withstand loading is determined by the orientation of the fibres in the moulded component».

Destroy to protect
X-ray computed tomography (X-CT) image of polypropylene with 30 wt.% glass fibres, where the colors correspond to the fibre angles in the plate plane. MFD = Mould Flow Direction.

The PhD candidate has followed the usual SIMLab recipe. That is, to make it simple, to destroy to protect. The different plastic specimens are stretched in different directions, and the results are fully documented.

The load is quasi-static and monotonic, which means you stretch the specimen slowly in one direction until it breaks.

The results of the experiments are used to make a material model that describes the behaviour of the material. Thereafter, the model is used in simulations to see whether they are in accordance with the physical tests.  

Avoid the weak zones

A huge challenge of the industry is that just as the orientation of the fibres depends on the injection process, the mechanical properties of the component also depend on the orientation of the fibres. To optimize the design process, one must first simulate the injection process. Then, the information about the fibre orientations is transferred to strength simulations.

«This is possible to do today, but the industry has not started to use such methods», according to Holmstrøm. As a result, it is difficult to know beforehand whether a product has an unfavourable fibre distribution.

«It is the totality that matters. The weakest zone will collapse. When you design something that should withstand extreme loads. It is all about avoiding the weak zones».

Useful for the automotive industry
Petter Holmström’s specimens are made of fibre-reinforced polypropylene and polyamide.

For the automotive industry it is important to utilize the fact that fibre-reinforced polymers are less stiff and weaker than steel. This means that in a car crash, the energy from the collision will be absorbed by the polymer car bumper instead of the much stiffer metal parts or the body of a pedestrian.

The more the force can spread, the less damage is done to the person who is hit by the vehicle. This demonstrates the advantages of thermoplastics. And all of it can be simulated, as long as we have access to good models that represent the fibre-reinforced component.

This will help industry to save both time and a lot of money.