Providing consultancy services and materials support to the Transport Industry is a major role of ESR Technology’s Forensic Engineering & Materials team. Often the team are called upon to examine the condition of various safety critical parts, in this instance the team were requested to assess polymer based resilient mounts. Resilient mounts are used in a wide range of application such as suspension for diesel engine rafts and gearbox units on trains.
A common request is to provide condition assessments at various service intervals, this can provide information of possible degradation or provide remaining life predictions. This will involve internal condition of the rubber following sectioning. Other techniques used to assess the condition are described below:
Metrology (Dimensional Measurements)
Both used and new mounts can be measured at various locations in order to compare any dimensional differences occurring through in-service use. Comparing height measurements can be important to give an understanding of whether the rubber in the mounts ‘settle’ over time leading to an out of tolerance component.
Compression tests are commonly performed on rubber components to assess their behaviour under similar loading conditions that they would see in service. Compression testing the mounts allows the user to observe how the mounts deform and how much they displace during loading. Most components under test will adhere to certain specifications that specify the maximum permissible level of displacement when fully loaded. The height the mount returns to after the load is removed is equally as important as the displacement. Ideally the mounts should all return to the same height and when plotting the load v displacement graph, the on-load and off-loads plotted curves should all peak at the same point and on reversal should pass through the origin equally. When the compressive cycle is repeated several times the results will show a hysteresis curve which can reveal differences between the tested mounts, such as a difference in the peak displacement level for the same mount during its test cycle can indicate that the mounts displacement is reducing as it loading cycle continues, i.e. the mount is getting stiffer with the more repeated loading it sees.
Hysterisis curve for rubber mount
Hardness can be defined as a materials resistance to permanent indentation. When testing rubbers the method used is the Shore Hardness. Shore hardness utilises a durometer scale, the two most common scales, which use slightly different measurement systems are the A and D scales; the A scales being used for softer polymers/rubbers and the D scale for harder ones. Each scale yields values between 0 and 100. Different scale durometers use different size indenters and apply different indenting forces. Durometer scale A, for example (used for softer polymers and rubbers) applies an indentation force of ~8N, whereas the D scale durometer applies ~44N force. The hardness values are created by measuring the depth of the indentation which is dependent of several factors such as material hardness, viscoelastic properties of the rubber, shape of the indentation foot and duration of indentation. The hardness of a rubber mount is important as it ties in closely with the compression testing. A harder rubber could possible indicate a stiffer more ridged mount, as can happen with age, and a softer rubber can indicate a more pliable and elastic mount yielding more deflection in service.
Example of Shore A scale Durometer
A visual inspection of the internal structure of a rubber mount is of equal importance as compression and hardness testing. Inspecting a resilient mount visually allows the observation and classification of defects present in the central rubber section and outer steel casing. An example of possible defects in the outer steel casing could include corrosion, pitting, gross inclusions and porosity, shrinkage voids and cracking. Possible defects in the rubber section could include gross porosity, tearing, voids and dis-bond between the rubber and steel surface.
Example of common resilent mount defects
If you have requirements regarding polymer testing or resilient mounts, or you would like to discuss some of the experimental work listed above feel free to contact Mr Tim Gould (email@example.com).