I guess for calculating impact parmaeters like maximuym force, impact time and maximum deflection for slow impacts the stiffness derived from the static youngs modulus applies. But what about semi slow impacts with speeds from say 1-10m/s
E.g. for rubber the dynamic stiffnes may be 1,4-1,5 times higher than the static stiffness. The problem applies to weight drops in gyms. Often the weights are covered with rubber or urethane plastic. Mats may be shredded rubber tiles, EVA, PUR foam under rubber etc.
Hello Mr BUERS,
Following your question I propose these elements :
- Perform experiments on your system to evaluate the frequency distribution of the impact you consider.
- Test with dynamical / Thermal mechanical analysis ther dependence of the stiffness of the material to frequency and temperautre.
- Perform time temperature superposition to extrapolate stiffness properties for high frequencies
- Use stiffness vs frequency table for stiffness definition in your models, when you perform direct analysis, transient response etc.
- Also you can consider the dfependence of stiffness to strain level.
- We can discuss about this.
Sincerely
VIALA Romain
Hello Romain Viala
Thanks for your reply!
I work as an acoustics consultant and we do have limited access to laboratory equipment except sound level meters, acclerometers and impact hammers. However I do have an an ultrasonic sound speed meter which can measure compressional sound speeds and thus E-modules when the density of the material is known. Even if the signal is 40 kHz ish I think this gives the static stiffness.
We do have a durometer for Shore A hardness tests, which may be translated to E-modules through some empirical functions.
Moreover we could do static deflection tests under load or do the dynamic testing straight out using high speed camera 1000 fps during impacts.
I also have tested a similar procedure to measurements of dynamic stiffness for floating floor resilient materials, using a weight manual as the mass and instrument with an acclerometer and an impact hammer.
What surprises me is that no publications on impact and contact mechanics I have come over mentions this problem of static versus dynamic stiffness.
One of the problems is of course that the answers one gets by measuring using different methodes give different answers. So what should I use In the models? It is very easy to adjust the contact parameters (E-module and thickness) until the expected force level or sound level is reached for a given impact energy and contact stiffnesses and areas.
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