Thin films play a key role in surface and engineering. They are widely used in battery manufacturing, flexible displays, sensory skins and semiconductor logic [1], [2], [3], [4], [5] and have a significant impact on the electronics industry [6], [7]. To avoid the possible delamination or cracking [8] and ensure the reliability and stability of coating-substrate structures, it is crucial to characterize the mechanical properties of thin films [9]. Blister test [10] is one of the few tests that can simultaneously measure multiple mechanical parameters, such as the elastic modulus, residual stress and Poisson’s ratio of thin films. To accurately determine the mechanical properties of thin films by blister tests, the most important step is to measure the deformation of thin films.

Strain gauges are widely used in deformation measurements for mechanical analysis [11], [12], [13]. However, they have some limitations in many practical applications. For example, strain gauges in blister tests easily fall off due to deformation of the films, which may lead to failure or inaccurate deformation measurements. It is also well known that strain gauges can only give the so-called point strains and the full-field deformation of thin films cannot be obtained. Thus, the deformation information may be extremely insufficient to characterize the mechanical properties of thin films.

The digital image correlation (DIC) method [14], [15], also known as the digital speckle correlation method, is a noncontact photomechanics method based on speckle image analysis. DIC extracts full-field deformation information by matching the corresponding locations in images captured before and after the deformation of a specimen. The difference between the coordinates of the center points of each pair of matched subsets serves as the measured displacement. As a powerful and flexible tool for surface deformation measurement, subset-based DIC has been widely accepted and commonly used for deformation measurement of thin films [16], [17]. Yang et al. [16] characterized the strain field of thermal barrier coatings using a DIC technique. He et al. [17] studied the Young’s modulus of thin films based on a dual DIC system. Based on whether the out-of-plane deformation can be obtained, the DIC method can be divided into 2D-DIC and 3D-DIC [18], [19]. Compared with 2D-DIC, 3D-DIC can measure not only the in-plane deformation but also the out-of-plane deformation; thus, it can be employed for deformation measurement in blister tests. By using conventional 3D-DIC technology, the measurement accuracy of a simple out-of-plane deformation can meet the needs of most engineering applications. However, for some special cases, such as serious stress concentrations existing in specimens [20], [21] or inevitable shedding of surface speckles under high temperature [22], conventional subset-based DIC (2D & 3D) may lead to large errors in measurement results. Generally, there are two inherent disadvantages for conventional subset-based DIC. First, this method can only obtain the displacements at the subset centers [23], thereby losing the calculation points near the specimen edges, which may contain very useful information because stress concentration and greater deformation usually exist on edges. Second, owing to the lack of mechanical constraints, conventional subset-based DIC cannot ensure the continuity of the displacement field within the entire measured area [23]. Moreover, when analyzing speckle images by the conventional 3D-DIC method, decorrelation between the reference and the deformed images could occur due to a very large deformation or other reasons, such as speckle falling off or illumination effect [24]. This will cause the missing of deformation information for some areas.

It is worth noting that the displacement distribution of thin films in blister tests is usually complex and nonuniform, and obtaining the whole and accurate displacement field is important for understanding the deformation and failure mechanisms of thin films. To determine multiple mechanical properties simultaneously and quickly, inverse techniques are frequently employed [24], which are highly dependent on the continuity and accuracy of the measured displacement field. This implies that conventional subset-based DIC methods are not suitable for thin film deformation measurements. Therefore, it is of great practical significance to develop an effective noncontact method to obtain the whole and accurate displacement field with spatial continuity. Mesh-based/global DIC methods [25], [26], [27], [28] can achieve this goal, but they usually suffer from the disadvantages of complex operations and extremely low calculation efficiency, making it very difficult to meet the requirements of engineering applications. To the best of the authors’ knowledge, mesh-based/global DIC methods are mainly used for the measurement of in-plane displacement fields, and only a few studies involve out-of-plane displacement field measurements [27], [28].

As an alternative to the conventional 3D-DIC methods, an improved 3D-DIC method considering mechanical constraints (3DMC-DIC) was proposed in this paper for out-of-plane displacement field measurement of thin films. 3DMC-DIC retains the outstanding advantages of conventional methods, such as easy implementation, fast convergence and high efficiency. In addition, it considerably improves the drawbacks of conventional methods, such as the lack of spatial continuity and the loss of boundary displacement information. The effectiveness of the proposed method was verified by simulated experiments, and it was applied to real thin-film deflection distribution measurements in a blister test. The rest of the paper is organized as follows: Section 2 introduces the basic principles of 3DMC-DIC. Section 3 presents the blister test and the detailed procedure of out-of-plane displacement measurement of thin films. Finally, several concluding remarks are provided at the end of the paper.