Applications of the Brazilian disk test to study mechanical properties of various materials observed in-situ in an Environmental Scanning Electron Microscope (ESEM)

article
Article image: 
Rhiannon Heard1, Kalin Dragnevski1& Gary Edwards2
 
1. Laboratory of In-situ Microscopy and Analysis, Department of Engineering Science, University of Oxford, Parks Road, Oxford, OX1 3PJ, UK
2. Deben UK Limited, Brickfields Business Park, Old Stowmarket Road, Woolpit, Bury St. Edmunds, Suffolk IP30 9QS, UK
 
Biography
Dr Kalin Dragnevski heads up the Laboratory for In-situ Microscopy and Analysis (LIMA), a unique facility comprising state of the art characterisation equipment allowing simultaneous investigation of the structure-property composition relationship in materials systems at different length scales. The Laboratory is part of the Solid Mechanics and Materials Engineering Group in the Department of Engineering Science at the University of Oxford. The Group, which has a long tradition in Oxford, initiated by Hooke’s work on the elasticity of springs, leads internationally recognised research in areas including fretting fatigue, residual stress measurement, sintering and cracking and high-strain rate behaviour of materials.
 
Corresponding author
Dr Kalin Dragnevski, Laboratory of In-situ Microscopy, and Analysis, Department of Engineering, Science, University of Oxford, Parks Road, Oxford, OX1 3PJ, UK
 
INTRODUCTION
 
In-situ tensile testing based on the use of dog bone shaped specimens is a well-established experimental technique. However, for many brittle systems this sort of test is not possible. An alternative means of testing that has been developed for these is the Brazilian disk test1. In this paper, we will demonstrate how we have further explored and developed this alternative experimental technique in an attempt to study the tensile behaviour of a variety of brittle systems in-situ in an Environmental Scanning Electron Microscope (ESEM).
 
THE BRAZILIAN DISK TEST – WHAT IS IT AND HOW DOES IT WORK?
 
The use of the Brazilian disk test in situ was first reported in our paper2 published in the journal, Experimental Techniques. In 2017, the paper was recognised with the prestigious DR Harting Award given by the Society for Experimental Mechanics (SEM) at their Annual Conference held in Indianapolis, USA. The paper plus two others3,4 were the result of a final year research project, which focussed on experimental techniques for structural and mechanical characterisation of materials in situ using ESEM.   
 
The test is based on diametral compression loading of a disk of material, which induces a tensile stress in the centre of the disk perpendicular to the loading direction. Increasing the load causes the tensile stress to increase until a crack initiates in the middle of the disk. Under load the crack would grow until the disk eventually breaks in two. Figure 1 illustrates the loading geometry of the Brazilian disk test. Key advantages over commercial tensile tests are simplicity of specimen preparation, especially important for fragile and brittle materials, which because of their nature in many cases cannot be microscopically characterised after testing. 
 


Figure 1.
 Schematic representation of the Brazilian disk test geometry2.
 
In order to perform this test on micro- and nano-levels a specially designed fixture was manufactured and fitted onto a conventional miniature Deben tensile stage, as shown in Figure 2. The set up was used to investigate the mechanical response and structural changes of a number of industrially relevant materials, including polymer latexes, pharmaceutical excipients and chocolate. 
 


Figure 2.
 3D model of the curved anvil design used to perform the Brazilian disk test2.

COLLOIDAL FAILURE MECHANISMS

The first example that we worked on to demonstrate the Brazilian disk test involved studying the tensile behaviour of brittle colloidal systems. The latex used in this study consisted of 75wt. % polymethyl methacrylate (PMMA) particles with diameters in the region of ~700nm, dispersed in water.
 
For the in situ tests, latex was cast directly onto 12mm diameter SEM stubs and allowed to dry for 24h at room temperature. The brittle disk was then carefully lifted and positioned between two custom designed curved anvils fitted on a conventional Deben miniature tensile stage (Figure 3). The arrangement was then inserted into the chamber of a Carl Zeiss Evo LS15 ESEM. Imaging was carried out under N2 gas (~10Pa), at accelerating voltages in the region of 12.5–13.5kV and working distances in the region of 10-15 mm. For the actual mechanical tests, the loading stage was used in compression with displacement rates varying from 0.01 to 0.5 mm/min. The latex disks were continuously imaged whilst compression took place. During testing, It was also possible to halt compression in order to get close up images of a crack that had not yet spanned the whole diameter of the disk2.
 


Figure 3.
 Photograph of the tensile stage housing the custom-built anvils used for in situ Brazilian tests [2].
 
From the results presented in Figure 4 it is clearly seen that the used configuration was successful in creating the desired perpendicular tensile stresses and yielded diametral cracks across the latex specimens. It was also found that when end crushing was minimal, multiple cracks initiated at several points on the disk and many factors appeared to influence the propagation of cracks through the latex. These factors include: particle packing arrangements, surface defects and the presence of other cracks. Following the onset of fracture, it was found that the cracks were likely to connect and form a single failure route, here described as the path of least resistance. The force-displacement data revealed that the disks failed at loads of ~0.5 N and displacements of ~0.2 mm. The data also revealed that the force in the disk fluctuated. This could be explained by many different mechanisms, including changes in crack growth direction; the crack passing through regions of different particle densities and therefore regions of differing strength and also through multiple crack initiation2.
 






Figure 4.
 Photograph of a failed latex disk (top) and example ESEM images of diametral cracks passing between latex ‘crystals’ (middle) and crack tip passing through a hexagonally close packed region (bottom )2.

COLD CHOCOLATE UNDER STRESS

In the second example, the full results of which are detailed in our recent paper published in inFocus5, we used the Brazilian disk test to study the tensile behaviour of chocolate as a function of temperature. Once better understood this can potentially help with solving problems with transportation and storage, which would ensure that the consumer gets the product not only in the desired state, but also taste. 
 
Tensile testing measurements were made at temperatures ranging from 25°C to -20°C. For these, a specially designed Peltier cooling module was used in conjunction with a standard microtensile stage from Deben (Figure 5). The circular specimens were made from Lindt Chocolate, which contains 85% cocoa mass (cocoa powder and cocoa butter), with the remaining 15% Demerara sugar.  As above, the experiments were carried out in situ using ESEM to facilitate a clearer understanding of the structure-property-composition relationship and thus identify the cause of breakages.
 


Figure 5
. Brazilian disk test set up for chocolate tensile testing5.
 
The results, presented in graphical format in Figure 6, indicate that there is a negative linear correlation between strength and temperature in the region -3°C to 25°C. Below –3°C, crystallisation appears to occur and hence the results do not follow the linear pattern. At 25°C, melting begins and also does not follow a linear relationship.
 


Figure 6.
 Tensile strength vs. Temperature for chocolate5
 
The ESEM images obtained during testing (Figure 7) provide further conclusive evidence of the structural changes taking place within the chocolate specimens; at room temperature the chocolate appearance is polymer-like, with no obvious structure, whereas at -20°C some crystallisation is clearly visible.  
 


Figure 7.
 ESEM images of Chocolate at room temperature (left) showing a typical ‘messy’ polymer structure and -20°C (right) showing evidence for crystallisation5.
 
The results clearly demonstrate that the tensile strength of chocolate increases as the temperature decreases. Therefore, it can be suggested that the ideal transport temperature to prevent breakage must be balanced with the impact on taste and texture (indicated by a glossy surface). A suitable range would be between -3 °C and no lower than -20 °C given there is minimal difference in ESEM images at -3 °C and room temperature (room temperature being the ideal storage temperature) but show a clear difference at -20 °C. Hence, above this temperature, chocolate will likely be able to return to its ideal structure.

MICROMECHANICS OF PHARMACEUTICS

The final example in our paper considers some recent data obtained from another undergraduate research project investigating the micromechics of pharmaceutics. Yield is a vital parameter in all manufacturing processes. Remarkable figures show that the pharmaceutical industry only has a success rate of 70% yield (tablets) while the makers of potato crisps enjoy better than 99.999%6. Our study looked at pharmaceutical excipients, an inactive substance used as a carrier for the Active Pharmaceutical Ingredient (API), which generally make up the bulk of tablets. We carried out in situ examination of tablets on a microscopic level to facilitate a clearer understanding of the structure-property-composition relationship of pharmaceutics and thus identify the cause of breakage.
 
To illustrate the Brazilian Disk test, we report our results for Avicel, microcrystalline cellulose which has excellent compressibility properties and is used in solid dose forms. For the actual tests, tablets with suitable diameters, were made in a specially designed mould from powders compacted at different pressures. The testing configuration generated the classic Brazilian Disk test results, where a sudden crack propagates from the centre of the tablet perpendicular to the loading direction. Use of the ESEM provided evidence on a microscopic level of the tablet crack propagation as shown in Figure 8. As mentioned above, the test was conducted at various compaction pressures, concluding that the tablets had a higher tensile strength at higher compaction pressures (Figure 9). Testing produced repeatable results for both in situ and ex situ conditions, however due to Avicel’s high moisture content, the ex situ measurements were much higher when compared to the in situ ones.
 


Figure 8.
 ESEM image of Brazilian Disk test failure for Avicel.
 


Figure 9.
 Tensile strength for Avicel at various compaction pressures.
 
We have also used digital image correlation (DIC) to look at ATAB samples; another widely used excipient in the Pharmaceutical industry. Here, the Deben stage configured for the Brazilian Disk test was placed under a Point Grey Camera with a 200 mm lens (FLIR). Due to the relative smoothness of the tablet surface, it was coated with a silica-based paint, which produced a speckled pattern. This enabled the tablet's movements to be tracked by the camera mounted directly above the sample. DaVis 8.3.0 software (LaVision) was used to capture the data obtained and generate displacement fields, as well as to highlight the points of maximum strain.
 
During the image analysis it became apparent that the strain data is inaccurate, due to the significant change in height during cracking. For this set up the DIC is only able to map the 2D surface and hence any disruption will lead to the image becoming pixelated and data loss. Despite this, one can see the onset of strain in the x and y-displacement directions. The concentration of stress along the centre of the crack shown in Figure 10 shows the crack propagation clearly develops in the centre in an infinitesimal amount of time highlighting the brittleness of the failure.
 


Figure 10.
 DIC maps of ATAB tablet obtained during Brazilian Disk testing. The central displacements occurred within a couple of frames showing the rapidity of the brittle fracture and emphasising the ceramic nature of the excipient.
 
CONCLUSIONS
 
The results presented and discussed here demonstrate the range of novel applications that have been facilitated through the development of the in situ ESEM Brazilian Disk test. In particular, the Pharmaceutical data shows the ease with which brittle specimens can be prepared for use with this method, where otherwise the tensile strength results would have been unattainable had they required the classic dog-bone shape. Furthermore, the Brazilian disk test is shown to be a very flexible technique, which can be combined with temperature and Digital Image Correlation and used for both in situ and ex situ studies to gather information on strain and temperature dependence of brittle materials under tensile loading. Thus, the production of such a unique technique has opened a plethora of opportunities for investigating brittle samples on a microscale level, a handful of which are detailed here. 
 
REFERENCES
 
1. K. I. Dragnevski, A brief overview of in-situ mechanical testing in the environmental scanning electron microscope, Micro and Nanosystems, Volume 4, Issue 2, June 2012, Pages 92-96; 
2. O. Islam, K. I. Dragnevski & C. R. Siviour, In Situ Environmental Scanning Electron Microscope Observations of Colloidal Failure Mechanisms Using the Brazilian Disk Test, Experimental Techniques, Volume 39, Issue 6, 1 November 2015, Pages 12-18;
3. O. Islam, K. I. Dragnevski & C. R. Siviour, On some aspects of latex film drying – ESEM observations, Progress in Organic Coatings, Volume 75, Issue 4, December 2012, Pages 444-448;
4. O. Islam, K. I. Dragnevski & C. R. Siviour, An ESEM/EDX methodology for the study of additive adsorption at the polymer-air interface, Springer Proceedings in Physics, Volume 164, 2015, Pages 145-151; 
5. R. Heard, Cold Chocolate under stress: An alternative approach to study high-rate behaviour in polymers, In-focus Magazine, Issue 48, December 2017;
6. PricewaterHouseCoopers, The economics of regulatory compliance in pharmaceutical production, Technical report, 2001. 
Website developed by S8080 Digital Media