Continuous stiffness measurement

Li, X., and B. Bhushan, A review of nanoindentation continuous stiffness measurement technique and its applications. Mater. Char., 48, 11-36 (2002). 

A significant improvement in nanoindentation testing is continuous stiffness measurement (CSM). ... 

Crosslinking, on the other hand, is seen to decrease the indentation behavior of UHMWPE. Studies by Gilbert [3], Roy [19], and Zhou [20] each show that mechanical properties measured by indentation are decreased with crosslinking. Park et al. [5], on the other hand, using continuous stiffness measurements, showed increases in modulus for crosslinked materials, which is contrary to the consensus view. This difference could be due to variance in the methods used. 

Both reference samples fused completely out of the sample clamps at tempertures between 160°C and 170°C. The dimensions of the UHMW-PP samples did change considerably (length not changed, width -23 % and thickness +40 %) but the shape of the samples was still rectangular, which permitted us to continue the measurements up to 250°C. The consideable difference in dynamic stiffness (and hence in strength) between the standard... 

Restrictions which may exist for the choice of a commercial reactor need not be imposed at the development stage. In some cases, a reactor of one type may be best for acquiring data in model characterisation, whereas a reactor of another type might be more suitable for full-scale production. (The cautions expressed in Sect. 4 must be taken into account.) Continuous flow back-mixed reactors can be very useful for kinetic studies because the absence of concentration gradients can reduce uncertainties in concentration measurements. When these reactors have attained a steady state, many of the problems associated with stiffness (see above) can be avoided. 

Measuring Vulcanization. The formation of a three-dimensional structure during vulcanization increases the stiffness (modulus) of the compound. Therefore, following the modulus increase versus cure time provides a continuous picture of the vulcanization process. Oscillating disk rheometers provide a useful method to do this (17). In this test, a preweighed sample of uncured mbber is placed into a preheated cavity containing a conical rotor. The cavity is closed and the rotor is set to oscillate within the mbber sample. As vulcanization proceeds, the compound s resistance to rotor movement increases and this resistance is followed as a function of time, thereby generating a continuous profile of cure behavior. These cure curves,... 

In all the above methods, it is necessary to cure specimens of test samples for each of a series of curing times and then perform the desired test on the vulcanizate. However, in the test for continuous measurement of vulcanization complete information could be obtained with saving in time. The mooney viscometer test approaches this objective. However a weakness of the mooney viscometer test is that the test is completed before a measurable modulus value after the scorch point has been obtained. This is because the test sample is destroyed after the induction period is passed due to tearing by continuous rotation of the rotor whether small or large. To overcome this deficiency and to provide a total cure curve for the entire vulcanization cycle, a series of instruments called cure meters was developed. In each of these instruments the stiffness or modulus of the compound was chosen as parameters for vulcanization continuously. The Vulkameter developed by Bayers, Germany was the first of the cure meters developed. 

In addition, some rubbers, for example natural rubber and polychloroprene, stiffen at low temperatures by partial crystallisation. This is a gradual process continuing over many days or weeks and is most rapid at a particular temperature characteristic of each polymer, for example -25°C for natural rubber. Hence, tests intended to measure the effect of crystallisation must detect changes in stiffness or recovery after periods of ageing at a low temperature. 

There was previously a separate ISO standard for adhesion in shear but this was withdrawn in favour of extending the standard for shear modulus to allow the test to be continued to the failure point, i.e. the two methods have been combined. The composite method is contained in ISO 182715 and uses the same quadruple element test piece as did the separate adhesion standard. The double sandwich construction is intended to provide a very stiff test piece which will remain in alignment under high stresses. The present standard quadruple test piece uses rubber elements 4 1 mm thick and 20 5 mm long and these tolerances are much less tight than previously. The measured adhesion strength in shear is less affected by the test piece shape factor then tension tests8 and the wider tolerances should be perfectly satisfactory. The test piece is strained at a rate of 50 mm/min, in line with the speed for most other adhesion to metal tests, and the result expressed as the maximum force divided by the total bonded area of one of the double sandwiches. The British equivalent BS 903 Part A 1416 is identical. 

Parallel Plate. Plashchina et al. (73) studied the creep of HM pectin gels placed between two corrugated parallel plates. Creep compliance curves were obtained for 0.5 to 2.5% pectin at temperatures from 25 to 55 C. Reversible and irreversible strain components were separated. Pectin macromolecules were characterized as being very stiff and only a slight decrease in entropy was required to form a pectin gel (7 ). Mitchell and Blanshard (50) used an automated parallel plate vlscoelastometer to study the creep compliance on low methoxyl pectins. The value of these experiments was that a continuous response was obtained from the gel rather than a single point measurement. 

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