What do lotus flowers have to do with wall paint? More than you might think at first. Lotus leaves have a fascinating feature: tiny wax bumps that repel water by beading off and taking any dirt particles with it. The lotus effect serves as an inspiration for individual developments in the industry for self-cleaning wall paints, glass, and bricks.
THE PROBLEM OF NOTCH STRESS
This example illustrates how bionics work. The term originates from the words ‘biology’ and ‘electronic.’ According to Dr. Lothar Harzheim, a bionic approach is when “principles of nature are deciphered and applied to a technical or structural process.” This is also true for the lightweight construction projects in Opel / Vauxhall’s development center, where Harzheim is responsible for optimization. The physicist and his colleagues apply both bionics and mathematical algorithms to their work on a daily basis.“We want to build parts that are both sturdy and lightweight.”
One of the big challenges is to come up with the ideal shape for the individual chassis components. Driving puts a great deal of strain on the parts, whether steering knuckles or differential cages. The job of the optimizers is to solve one of the biggest problems in applied mechanics: notch stress.
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We asked ourselves: how is it possible for branches to be whipped around by the wind and the snow and not get broken?
TEARS IN THE COMPONENT
Notch stress occurs in sharp grooves or holes in mechanical components. This stress can cause parts to tear and then break. Harzheim’s objective is to reduce local notch stress. And nature shows how to do it. You can see it in the formation of notches on trees and bones, for example. “These elements have grown for millions of years according to the laws
of evolution that allow them to take shape without acquiring dangerous stress points,” says Harzheim. “We asked ourselves, how is it possible for branches to get whipped around by the wind and the snow and not get broken?”
The answer: Notch stress is unheard of in organic weight-bearers such as trees, since they distribute mechanical stress evenly along their surfaces. They grow according to the principle of storing material in overloaded places and removing material in underloaded places. Harzheim analyzed this rule and embedded it in a software application, known as the CAO method (Computer Aided Optimization). CAO can be used to develop new computer components virtually according to the growth rule – incrementally, like the rings inside a tree trunk. “This is how we are able to minimize the stress, reduce the mass of the components, and therefore reduce weight,” says Harzheim.
The researcher’s designs were implemented successfully. “We managed to reduce the notch stress in the spare wheel well strike plate of the latest Astra five-door model by 37 percent,” says Harzheim. The rule of thumb for metal components is that a ten-percent reduction in notch stress will double the lifespan of a component.
The growth rule helps in optimizing the not only the surfaces of components, but also their interiors. SKO software (Soft Kill Option) is used in this case instead of CAO. SKO is suitable for optimizing cast parts, such as engine mounts, that need to be cleared of all unnecessary material inside. The design follows this requirement. The example for this improvement is provided by bones, which not only store material in underloaded places, but also break down material using scavenger cells.
EXAMPLES FROM NATURE
Tree fork in nature
Example of a not optimized part: Red marked the critical section
An example of a computer-assisted optimized component: The tree fork shape results in a homogeneous surface stress.
- EXAMPLE: DIFFERENTIAL CAGE
Overview: Fatigue-critical point
Red marks the critical section: It´s a fatigue-critical point
Result of the growth rule: 28 percent reduction in pressure