The following is an excerpt.

According to PhD candidate Jianbin Xu, a bumpy bike ride is comparable with understanding the Portevin-Le Chatelier effect in some metallic materials.

Young man in a mechanical laboratory
PhD Candidate Jianbin Xu at SFI CASA studies the Portevin-Le Chatelier effect. He defends his thesis on 14 June 2021.

Modelling the Portevin-Le Chatelier effect. Jianbin Xu has spent 4 years at SFI CASA, investigating the effect deformation behaviour of aluminium alloys. More precisely, he has studied the mechanisms related to a phenomenon well-known for almost 200 years: the Portevin-Le Chatelier (PLC) effect. 

COMPARABLE TO A BUMPY BIKE-RIDE

The PLC effect is related to a strengthening mechanism by alloying elements in a solid solution. Still, it has a catch: The additional alloying element also causes the alloy to deform in an unstable manner. This, in turn, can lead to poor ductility and, thus, premature failure. Ductility is a measure of a material’s ability to bend or stretch before it cracks or fails. Jianbin Xu compares the aluminium deformation with riding a bicycle on a dirt road.
«If you ride the bicycle super-fast like a Ferrari, you will not feel the bumps at all», he says. The same thing happens if you cycle as slowly as possible and avoid the bumps by keeping full attention to the road.

Young man in a technical laboraty


The forming process of alloys often varies in temperature. The strain rate also changes with time and from one location to another in a forming operation. «Therefore, simulation of forming processes and related tests needs constitutive descriptions that account for variations in strain rate and temperature. I have tried to understand and simulate the details of the PLC effect».

«WE WANT TO UNDERSTAND THIS PROCESS»

These two options also go for the deformation of aluminium:
«If the metal deforms at a really high strain rate or a low strain rate, the PLC effect will disappear», Xu explains. The third option is to ride at normal speed, feel all the bumps and face a high risk of crashing your bicycle. To continue the comparison: Strong instabilities might occur more often for many of the most commonly used aluminium alloys. «We want to understand this process. And maybe one day we will be able to avoid the phenomenon – and the problem – of the PLC effect », says Jianbin Xu.

A FUTURE GUIDE FOR THE DESIGN OF ALLOYS 

Scientists talk about the PLC effect as «a serrated stress-strain curve that exhibits inhomogeneous deformation in the material». When illustrated, it looks like the profile of a saw blade. For producers of alloys containing magnesium, it is a constant headache. The PLC effect is known to induce blue brittleness in steel, and the loss of elasticity may cause rough surfaces to develop during deformation. Thus, the material renders useless for car-body- components, sheet-forming applications or other casting parts.
«The stuff I have done may serve as a guide for the design of alloys or for quality control in the automotive industry», Jianbin Xu says.

READ MORE: Better Tools to Predict Ductile Failure in Aluminium Alloys
READ ALSO: The Hunt for the Accurate Point of Fracture

HIGH-SPEED CAMERAS AND DIGITAL IMAGE CORRELATION
He holds up a dog bone-shaped aluminium sample as he prepares for another tensile test in the lab at the metallurgy building at NTNU. The specimen comes from a 5xxx series aluminium alloy (i.e. an AlMg alloy), extensively used in beverage packaging and the automotive industry. 

Tensile testing

FIGURE OF TEST SET UP

«Undesirable surface finish and discontinuous thinning during sheet metal forming have caused challenges for these applications», he explains as he mounts the specimen to the grips of the test machine.
He has done unaccountable numbers of experiments at different strain rates and temperatures. The tests are recorded with high-speed cameras, and his work also includes extensive use of the use of a digital image correlation (DIC) system developed in SimLab. Besides, several transmission electron microscopes (TEM) observations are done with the help of Emil Christiansen, a former PhD at SFI CASA.

READ MORE: Rocking Around at The Nanoscale

EXTENSIVE AND DETAILED EXPERIMENTAL WORK
Jianbin Xu is a part of the Lower Scale programme in CASA. According to his supervisor, Professor Knut H. Marthinsen, at the Department of Materials Science and Engineering, Xu has performed extensive and detailed experimental work. Tensile testing at different strain rates and temperatures includes digital image correlation to monitor the macroscopic localizations that form where the effect occurs. He has implemented and analyzed several existing constitutive models to verify their accuracy when describing the test results. Also, Xu has applied one of those models in the Finite Element model (FEM) Abaqus. 
«Which, in principle, makes it possible to simulate the behaviour that is possible to see -through the use of DIC, namely the nucleation and propagation of PLC bands that happen during a virtual tensile test», Marthinsen explains.

MAKES IT EASIER TO PREDICT THE MATERIAL BEHAVIOR
According to Xu, the models make it easier to predict material behaviour. «Actually, I am quite satisfied with the good agreement between the experiments and the simulations. The idea of understanding the deformation mechanism at a deep level triggers me», says Xu.
The PhD candidate comes from Yancheng, a small city in eastern China. He jokes that he is still surprised by the choice he made several years ago about coming the whole way to Norway. As an undergraduate, he studied magnesium alloys, which is another light metal. When opportunity knocked at NTNU in Trondheim, he decided to go. It could not have been that bad here, as he plans to look for other opportunities in Norway after he defends his thesis on 14 June 2021. 

I just add some bricks to the wall that has already been built.

But I am dreaming about solving the problem every day.

CONNECTING LOWER- AND CONTINUUM SCALE
Jianbin Xu seems quite content with the fact that the three objectives that were listed for his work now are mainly fulfilled:  First, he has studied the influence of temperature and strain rate on the PLC band nucleation and propagation. This is done by both experiment and material modelling. Second, implementation and validation of a physics-based model are applied to capture the mechanical behaviour of the material. According to Xu, this work shows an excellent agreement with the experimental results and has solved the long-standing unsolved jerky flow scenario issue brought by the variation of temperatures and strain rates.

4 YEARS OF CHALLENGING AND ENJOYABLE WORK
The third task was the Finite element modelling of the band formation, propagation and band morphology.  «A finite element model was implemented to accurately simulate the subtle features of PLC bands. Simulations and experiments agree quite well», the PhD candidate explains.The soon-to-be doctor says that it has been 4 years of challenging and enjoyable work at SFI CASA. In particular for him, who does not have a mechanics background.


«THERE IS STILL A WAY TO GO»
«The PLC effect is an age-old question and was first discovered almost two centuries ago. It has been extensively studied, and massive amounts of literature can be found. Ground-breaking research hasn’t appeared for quite a long time. There is still a way to go to accurately predict the behaviour of the material. I just add some bricks to the wall that has already been built. But I am dreaming about solving the problem every day». 

The doctoral work has been carried out at the Department of Materials Science and Engineering. Professor Knut Marthinsen has been Xu’s supervisor. Co-supervisors: Professor Bjørn Holmedal at the Department of Materials Science and Engineering and Professor Odd Sture Hopperstad at Department of Structural Engineering.