Determination of the Failure Stresses for Fluid-filled Microcapsules that Rupture Near the Elastic Regime R. Mercadé-Prieto & R. Allen & D. York & J.A. Preece & T.E. Goodwin & Z. Zhang Received: 28 August 2011 / Accepted: 21 February 2012 / Published online: 8 March 2012 # Society for Experimental Mechanics 2012 Abstract The encapsulation of liquids within an external wall or shell is an important technology often utilized in the production of many commercial products. The mechanical characterization of such microcapsules is paramount in or- der to fully understand their performance in their target environment. Some microcapsules, with wall materials such as inorganic based compounds, rupture at small deforma- tions, commonly near the elastic regime. The study herein presents a general methodology that enables calculation of the failure stresses leading to the elastic-like rupture of microcapsules under parallel compression testing. Two sce- narios of failure, brittle and ductile, were considered. Anal- yses of the critical stresses present within the microcapsule wall during different degrees of fractional deformation were obtained using finite element modelling, resulting in similar values for both the brittle and ductile scenarios. The correlations presented were used to determine the failure stresses of tetraethoxyorthosilane-methyltrimethoxysilane (TEOS-MTMS) microcapsules with a model core oil, which are 11–14±10 MPa. The data were further analyzed using Weibull distributions. Keywords Elastic behaviour . Finite element modelling (FEM) . Fracture stress . Micromanipulation . Microcapsule Introduction The microencapsulation of liquids is both a common and useful technology which may be utilized in order to enhance the stability of the encapsulated material, whilst also con- trolling its release. The mechanical testing of such micro- capsules to the point of rupture is often performed in order to characterize the final product, and investigate its suitabil- ity for applications whereby triggered release of the liquid core material by mechanical forces is desired [1]. Develop- ing an understanding of the mechanical properties of micro- capsules is therefore critical in this endeavour. The mechanical testing of microcapsules is commonly per- formed between two parallel plates using a unique apparatus which has been designed specifically for the compression and analysis of microcapsules of 1–1000 μm, the use of which is referred to as a micromanipulation technique [2]. One of the most studied microcapsule shell materials for the encapsulation of liquids is melamine-formaldehyde (MF), particularly in the encapsulation of perfume oils. MF micro- capsules containing liquid oils commonly break (rupture, burst, fail or fracture) at large fractional deformations (~0.4–0.7) ε, defined as the displacement of the microcap- sule divided by its diameter, and it has recently been shown that this occurs after extensive plastic deformation [3–5]. Electronic supplementary material The online version of this article (doi:10.1007/s11340-012-9605-5) contains supplementary material, which is available to authorized users. R. Mercadé-Prieto : R. Allen : Z. Zhang (*) School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK e-mail: Z.Zhang@bham.ac.uk D. York Procter and Gamble Technical Centre, Newcastle upon Tyne NE27 0QW, UK J.A. Preece School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK T.E. Goodwin ENCAPSYS®, Appleton, P. O. Box 359, Appleton, WI 54912-0359, USA Experimental Mechanics (2012) 52:1435–1445 DOI 10.1007/s11340-012-9605-5