Fatigue strength is a sub-field of materials science which deals with the distortion and failure behaviour of materials under cyclic stress. Components are essentially subjected to cyclic stress. The fatigue strength of the components must be tested because the loading they can endure under cyclic stress conditions is much lower. A key diagram is the Woehler (or S-N) curve. The amplitude of a stress which can be endured is plotted as a function of the number of cyclical repeats.
Fracture failure is examined using
- Static and dynamic fracture mechanics to determine the fracture toughness, the threshold value, and parameters for the crack progression curve
- Resonance testing machines with crack propagation measurement
Experimental stress analysis
- Experimental determination of component stress when in operation
- Stationary and mobile measuring systems for strain gauges, force, deformation, temperature, pressure and acceleration
- Residual stress measurement (hole drilling method)
The Laboratory for Materials and Joining Technology (LFW) is the competence centre for joining technology at Esslingen University of Applied Sciences. The close relationship it has built up with industry has resulted in the LFW laboratory being equipped with the latest processing and testing technology, which is used for research as well as for teaching. The process technology ranges from simple hand tools through to stationary and robot-guided systems. It has a TruLaser Cell 3000 laser-beam machining centre, and equipment for arc welding (MIG, MAG, TIG), resistance spot welding, arc stud welding, soldering and bonding, as well as the mechanical joining processes known as clinching and high-speed tack joining (ImpAcT, Rivtac ®).
Thanks to the metrology equipment available it is possible to examine the process flow of the methods in detail (video analysis of up to 125,000 images per second, thermography, thermal, mechanical and electrical process parameters). In addition, it is also possible to undertake surface analysis by measuring contact resistances in the laboratory.
In student training, scientific research work and technology transfer, the existing methods of mechanical engineering testing, non-destructive testing methods (NDT) and numerical simulations (FEM) are used in conjunction with the process data obtained to assess the suitability of the joint for method development and for quality assurance in production.
Non-destructive testing NDT
Assessing components by means of non-destructive testing has one crucial advantage. It is non-destructive! This means that components can continue to be used after a test has been carried out or 100% testing during the production process is made possible in the first place. The foundations are taught in laboratory experiments and supplemented by research topics relating to active thermography. The strengths and weaknesses of various methods are examined and their possible uses are investigated further. The students are afterwards able to differentiate between the different modes of action and to choose the method which is most suitable. The laboratory has penetrative, magnetic, inductive, electric, potential, thermal and sonic methods.
Experimental and computational verification of fatigue strength
In the “Experimental and Computational Verification of Fatigue Strength” Laboratory, which accompanies the lectures on “Component safety and reliability”, you will perform several measurement trips with a racing bicycle equipped with instruments (strain gauges on fork, handlebars, saddle). The stresses occurring when the bicycle is being ridden are recorded and subsequently analysed with the measurement software. The findings gained then enable you to make a statement about the life or the service life of the bicycle.
Computational Life Time Assessment
In the modern world of engineering fatigue is a crucial factor, and therefore also the ability of its prediction. Although the experimental approach is commonly used, the more cost efficient approach of computational assessment is often used as part of a holistic solution.
Using the software “winLIFE” and the theories of “Nominal Stress Concept” and “Local Strain Concept” the life time of a steering link subjected to a dynamic load will be assessed. Different geometries of the link, as well as, different loading spectrums will be considered and optimized.
Component flow curve
A flow curve characterizes the mechanical behavior of a material. That is the relation between the applied load and the resultant deformation. This characterization defines the properties of a material both in the elastic and plastic areas, as well as, under static and dynamic loads.
In the “Component flow curve” lab we will produce the experimental flow curves of specimens with different geometries, and the relevant theories will be put to the test. For this aim we will apply strain gauges and load the specimens in a tensile machine, both statically and dynamically. The results will be used to determine the relevant properties of the material and to estimate the life-time of the tested component.
The large numbers of students taking the course has resulted in the laboratory experiments putting the focus on teaching the foundations of materials science and process technology so that the first-semester students all have the same level of knowledge. The contents of the experiments serve to provide them with a fundamental understanding of production technology.
Joining technology teaches the theory behind the joining of at least two objects which are to be joined together. The focus in the Materials and Joining Technology Laboratory (LFW) is on the joining of metal components. The laboratory exercise deals with the fundamental principles of positive-fit, frictional connection and bonding processes. A selection of spot and seam welded joints are used to introduce the students to some applications in automotive engineering and mechanical engineering, and also in precision engineering and electrical engineering, which go beyond the fundamentals, and their technical possibilities are illustrated.
Micro-structural transformation in arc welding
The heat input during welding leads to a localised heat-affected zone (HAZ) in the vicinity of the welding spot, which in turn results in a micro-structural transformation (heat treatment of the components). These changes to the micro-structure are usually undesirable, since they permanently change the mechanical and engineering characteristics of a component, among other things. The laboratory experiment shows the effects on the component on the one hand, and also points out suitable method parameters to improve the quality of the joint. Suitable calculation methods are presented to estimate the cooling times (t8/5 concept) for two and three-dimensional heat conduction and the micro-structural transformation resulting therefrom.
Non-destructive testing NDT
The task of NDT is to test components for damage and irregularities without inflicting more damage. NDT ranges from the simple visual inspection through to technically complex tests such as those using X-Rays. These are used in production to test the dimensional accuracy of components, for example, i. e. does the M10 bolt really have a diameter of 10 mm, or for failure analysis. Failure analysis investigates what has caused a component to fail.
In the laboratory exercise, the students themselves perform one method of radiographic examination, one method of magnetic flux testing and one method of visual inspection. The students learn which possible mistakes can be made when using them and what the possibilities and limits of the methods are. After the laboratory exercise, the task is then to attribute the defects detected to a possible cause of the defect.
Research project: Active thermography for the in-line detection of near-surface defects
The objective of zero-defect manufacture of semi-finished products has made great progress in recent decades, but there are still deficits in the in-line detection of near-surface defects. It has not yet proved possible to transfer the results achieved in the laboratory into the production process, and it is this that forms the basis for process control and the avoidance of near-surface defects, too. This is the reason that Esslingen University of Applied Sciences, whose core competence is in applied research, is looking for solutions. The focus is on its utilisation in production.
Research project: Effect of edge machining on the dynamic strength behaviour of high-stability thin sheet steel
Experimental determination of the effect of the edge condition on the strength behaviour under dynamic load of thin sheet steel as a function of the edge machining, the material, the type of loading and the strain gradient. The edges are produced by milling or milling and polishing, laser beam cutting and shear cutting with different process parameters. To determine the effect of the material, experiments are carried out on extreme-strength steel grades DP1000 and CP1000. In addition, experiments under loadings which create a mean stress effect are carried out for purposes of transferability.
For the ultra-high-strength press-hardened steel 22MnB5, the process sequence for laser cutting is to be optimised with regard to the impact on the dynamic strength behaviour.
Research project: In-situ polymerisable single component matrix systems to increase productivity in hybrid lightweight construction (“FAST matrix”)
Composites with a thermoplastic matrix have high levels of toughness and a good crash characteristic when combined with textile reinforcements. The investigation will focus on newly developed single-component matrix systems made of in-situ polymerisable precursors. These polymer precursors have the advantage that they can be introduced with very low viscosity into the fibre material (carbon fibres) (e. g. via RTM methods) and can then be cured in a very short time by thermal induction to the thermoplastic matrix polymer (e. g. polyamide 6). This opens up new possibilities for process and cost optimisation, especially for hybrid lightweight design. The aim of the project is to produce carbon-fibre reinforced plastic model bodies using the new matrix polymers and test them for their mechanical properties.
Research project: RTM CAE/CAx setup of a complete CAE/CAx chain for the RTM method against the backdrop of the manufacture of high-performance composites
High-performance composites are becoming increasingly important as lightweight construction materials given the trend towards lightweight construction. The overriding objective of the research project is to create a complete numerical model of the whole process chain from fibre production through to the finished component. Esslingen University of Applied Sciences is tasked with determining material characteristics under quasi-static and under temporally changing load for the material models in the simulation environment, and with clarifying the damage mechanisms and the damage development.
Research project: High-speed bolt driving (nailing) in electrical engineering - investigation of the effect of component stiffness on the process flow and the quality of the joint
High-speed bolt driving is a new technology which is currently used to join steel and aluminium sheets. To this end, a nail-like joining element is accelerated and driven into sheets. The objective of the German Federal Ministry of Education and Research (BMBF) collaborative project is to investigate the possible applications for the material copper. The purpose is to show whether, for sufficiently large local degrees of forming, not only compression joints (positive-fit and friction joints) but also bonded cold pressure welded joints can be expected with copper materials.