Elevated Temperature Digital Image Correlation Using High Magnification Optical Microscopy

Abstract

Full-field thermally-induced strain measurement of small areas or structures is desirable for applications such as microelectronics, but is difficult to achieve at microscopic scales. This data can be used to evaluate product reliability, validate finite element models, and perform failure analysis. Digital image correlation is an ideal technique for these applications, but its feasibility has not yet been demonstrated. Two primary challenges present themselves: thermal expansion of the specimen and hardware push the viewing surface out of the small focal range of the lens, and speckle fields of sufficiently small size are difficult to prepare. In addition, the close proximity of the microscope lens to the specimen can cause the lens to be heated to harmful temperatures. This paper presents a method for successfully performing two-dimensional digital image correlation measurements over a range of temperatures though and throughout a large range of optical magnifications. Test apparatus has been designed to effectively eliminate out-of-plane thermal expansion and heating of the lens. Specimen preparation techniques provide a sufficient high-contrast surface features for strain and displacement calculations. The apparatus design and specimen preparation methods are described, and examples are presented to demonstrate capability. This provides a means to measure full-field two-dimensional strain of microscopic features over a range of temperatures using common laboratory equipment.