ESIS's blog

The Curie brothers, Pierre and Jacques, discovered the piezoelectric effect in 1880. The phenomenon has been exploited in many useful applications, such as for the pickup on the gramophone that registers the sound when it follows the winding groove in the vinyl record surface. It is also used in lighters that ignite a gas when the voltage gap between different locations exceeds the limit to produce a spark. When things are made smaller and smaller, passing mm's, microns down to nano scales, the piezoelectric effect is surpassed by the flexoelectric effect.

Fracture mechanics suddenly provides a step forward to stop climate change. The blogger has often pictured us humans walking on earth, asking ourselves how to get sufficient energy without burning fossil oil and destroying forests. Earth has a crust that is 30 to 50 km thick. Below it, the temperature is 1 to 6 thousand degrees Celsius. We are on the outside of a thin shell and inside is mostly melted rock, forming a sphere with a diameter that is close to 13000 km. For someone seeing this from another solar system, our behaviour must seem strange and pathetic.

The present EFM paper selected for discussion applies artificial intelligence (AI) to fatigue crack growth. The subject is on the outskirts of my competence. To say the least, I am on thin ice when it comes to AI, machine learning, neural networks and similar. Still, I get the feeling that the selected paper describes an interesting step forward. I am sure that it will, sooner or later, be a reliable tool for predicting closing and opening loads at fatigue crack growth. 

Shifting from macroscopic to microscopic plasticity helps us understand mechanisms that can help us develop high-strength metallic materials. Things that prevent dislocation dynamics or generation, such as other dislocations and grain boundaries in polycrystalline materials, lead to higher strength.

The interesting and well-written paper

Around 20 years ago, I gave a fracture mechanics lecture and talked about crack initiation that happens in the plane with the largest tensile stress. True, at least if the material has isotropic properties. The students already knew where an isotropic material would give the largest stress at bending and torsion. I planned to make a desktop experiment with an icicle and a carrot. This was during the autumn with an abundance of icicles everywhere. The carrot, I found at home.

Dynamic fracture is a never-ending story. In 1951, EH Yoffe obtained an analytical solution for a crack of constant length travelling at constant speed along a plane. She used a Galilean transformation to get a solution for arbitrary speeds. The situation seems strange with a crack tip where the material breaks and a lagging tip where the material heals. However, there are applications. One that I encountered was several mode II cracks that travel in the contact plane between a brake pad and a brake disc. The moving cracks were blamed for the causing squeaking noise when braking.

The Nobel laureate Andre Geim made graphene by playing with pencil leads and Scotch tape and coauthored a paper on how to get the Nobel prize the fun way. Before that, he co-authored with his hamster, Ter Tisha, a paper on diamagnetic levitation and demonstrated it on a frog. He was honoured with the Ig Nobel prize for the paper and later became the only person so far who got both the Harvard Ig version and the real Alfred version of the Nobel prize. Geim is one of my favourite scientists, which led me to read the paper 

The advantage of simplicity is that mechanics and physics can be understood and predicted just by using pen and paper. In the end, numerics may have to be used but then you should already have a pretty good idea of what happens. The other way around, starting with numerics and a limited toolbox of models will seldom lead to anything new. 

The fracture of concrete and other semi-brittle materials offers some simplifications that simplify the analytical analysis. The simple check that reveals if something broken requires an elastic or an elastic-plastic fracture mechanical analysis by just trying to fit the pieces together sometimes fails. The suggestion is that if they do not fit together, we have an elastic-plastic fracture and if they do we have an elastic fracture. We may jump to the false conclusion that linear elastic fracture mechanics can be applied.

The subject of this blog is a well-written and technically detailed study of thermal crack initiation where an adhesive joint between two dissimilar materials meets a free surface. The method that is used goes under the group designation finite fracture mechanics. The paper is:

"Predicting thermally induced edge-crack initiation using finite fracture mechanics" by S. Dölling, S. Bremm, A. Kohlstetter, J. Felger, and W. Becker. in Engineering Fracture Mechanics 252 (2021) 107808.