IMAGING INTERFACE STRESS AND STRAIN IN HETEROGENEOUS FILMS
Interface failure is common in structures consisting of dissimilar materials.Interface stresses arise due to the mismatch of mechanical properties during processing or usage which often results in fracture.While much progress has been made in understanding the interface fracture in hard materials,relatively little is known about the fracture mechanics of soft matter.A familiar yet challenging example is the drying of a colloidai film. Drying has been exploited as a unique and simple preparation technique for colloidal coatings, such as paint, for thousands of years. As solvent evaporates the suspension undergo a fluid to solid transition far away from equilibrium. Enormous stresses build up during drying that often leads to the failure of thin films. The heterogeneous nature of drying makes it difficult to understand the mechanics because the composition and properties vary dramatically in space and time. By utilizing and improving traction force microscopy used in cell biology to measure the force generated by crawling cells, we quantify the spatial distribution of all three components of stress at the interface between drying colloidal films and their substrates. Particularly, we apply this approach to image stresses and strain near the tip of interface and in-plane cracks. With this technique, we are able to better understand the evolution of mechanical properties and fracture mechanics. We recently reported the stresses and deformation profile near the tip of an interface crack between the drying colloidal coating and the elastomer substrate and measured the stress intensity factor 1. Both normal and in-plane stresses on the interface were carefully measured using the deformation of elastomer substrate. Furthermore, the local strain of drying colloidal films can also be measured by analyzing the displacements of tracer particles inside the films. Our goal is to correlate the stresses on the interface and the strain of drying films near the interface so that we can establish the constitutive relationships which are left largely unknown for drying colloidal films. Using the same technique, we are also studying drying and cracking films consisting of other colloidal particles, such as corn starch. Drying starch suspension is a nice and simple model system to siudy (he fracture dynamics and mechanics. We coat a ihin layer of starch-water mixture on our elastomer substrate. It gradually solidifies and forms a nice cracking pattern while deforming the elastomer substrate underneath it. Fig. 1 shows one region of cracks in dried corn starch, superimposed with substrate deformation field, from which the distribution of stresses on the interface can be calculated using the solution we presented in 1 . In addition, the relatively larger size of starch particles will enable us to resolve the microstructure of the dried films and understand the effect of particle properties on the drying and cracking behaviors of colloidal films. Our technique is quite general and can be applied in studying other phenomenon involving interface mechanics such as adhesion, and is particularly valuable for materials with spatially and temporally heterogeneous mechanical properties. It can be adapted to measure interface stresses over a wide dynamic range by changing the elastic modulus of the elastomer substrate.
fracture mechanics colloids ceramics
Y.Xu L.A.Wilen E.R.Dufresne
Department of Mechanical Engineering and Materials Science,Yale University,New Haven,CT 06511,USA Unilever Research and Development,Trumbull,CT 06611,USA Department of Mechanical Engineering and Material Science,Chemical Engineering,Physics,and Cell Biol
国际会议
The Third International Conference on Heterogeneous Material Mechanics(第三届国际非均匀材料力学会议)
上海
英文
320-321
2011-05-22（万方平台首次上网日期，不代表论文的发表时间）