Load Vs. Deflection stiffness testing of rubber parts and components is needed to establish the quality and performance parameters of these manufactured items. Two (2) main methods of evaluating the stiffness properties are by static testing and dynamic testing.
Static testing of materials as per ASTM D412, ASTM D638, ASTM D624 etc can be categorized as slow speed tests or static tests. The difference between a static test and dynamic test is not only simply based on the speed of the test but also on other test variables employed like forcing functions, displacement amplitudes, and strain cycles. The difference is also in the nature of the information we back out from the tests. When related to polymers and elastomers, the information from a conventional test is usually related to quality control aspect of the material or the product, while from dynamic tests we back out data regarding the functional performance of the material and the product.
Dynamic material characterization is a technique that measures stress as a function of strain, or force as a function of displacement and time. It also involves the application of one or more forces at various frequencies, as a means of determining how material changes in a dynamic environment where the material comes under the effect of multiple frequencies. Dynamic characterization testing of engineering components normally includes components like tires, springs, dampers, biomedical implants and vibration isolation components from the automotive, aerospace and biomedical industries. These components perform under time and frequency varying conditions during their entire lifecycle making them ideal candidates for dynamic study both for product development and failure analysis.
The importance of dynamic testing comes from the fact that performance of elastomers and elastomeric products such as engine mounts, suspension bumpers, tire materials etc., cannot be fully predicted by using only traditional methods of static testing. Elastomer tests like hardness, tensile, compression-set, low temperature brittleness, tear resistance tests, ozone resistance etc., are all essentially quality control tests and do not help us understand the performance or the durability of the material under field service conditions.
Figure 1 above shows different test samples tested on the AdvanSES servo hydraulic tester used in material testing at high frequencies the servo hydraulic tester is capable of going up to 100 hertz under sine wave definition hydraulic actuator is the primary source of frequency generation in the instrument and the servo valve in the actuator controls the flow of hydraulic fluid into the actuator so as to apply a controlled displacement at a controlled frequency. The load cell in the instrument measures the loads generated in the sample under the dynamic frequencies. The servo hydraulic tester is primary primarily used to study static and dynamic stiffness, loss and storage modulus and Tan-delta. Fatigue crack growth propagation of rubber samples can also be tested using a high fps camera integrated with the tester. Elevated temperature testing is also available with the use of a temperature chamber with automatic PID control.
- Gent, N. A., Engineering with Rubber, How to Design Rubber Components, Hanser Publishers, 2016.
- Kartik Srinvas, Dynamic Properties of Polymer Materials and Their Measurements.