Non-contact strain/displacement measurement Digital image correlation method: DIC system

The digital image correlation method (DIC) is to give a random pattern called a speckle pattern to the surface of the measurement object by painting, etc., measure the strain and displacement by analyzing the image before and after deformation, and visualize the strain. This method is also called DIC, which stands for Digital Image Correlation. Unlike strain gauges, it analyzes strain and displacement by image analysis, so it is possible to visualize strain in plane and three-dimensional directions without contact.

Seika Digital Image has a development system for sDIC, which is DIC software, and is good at proposing systems that meet customer needs.

By performing sub-pixel interpolation, it is a highly accurate measurement method with a displacement resolution of 1/10 to 1/100 pixel, and is a technology that has been attracting attention in recent years. By using the sDIC system, it is possible to quantify the strain and deformation of an object in a non-contact, non-destructive manner. Also, by using a high-speed camera, it is possible to analyze high-speed phenomena and vibrations.

"sDIC 2D" uses one camera for two-dimensional (xy) analysis, and "sDIC 3D" uses two cameras for three-dimensional (xyz) analysis including the depth direction. In addition to stand-alone software for those who already have an image capture environment, we also offer a system that includes a camera/lens/fixing jig/lighting/analysis PC, etc.

According to customer needs, such as simultaneous measurement with equipment that we have cultivated for a long time in laser measurement such as PIV, simultaneous measurement with other equipment such as laser vibrometers and SEM images (mechanical characteristics by nanoindenter) handled by our company. We will make a proposal in the form.

A digital image consists of a collection of pixels (or picture elements). In addition, the luminance value (brightness information) of each pixel is quantized and takes discrete values. Such digital images are often created by photographing the subject using an image sensor. The luminance value of the subject is sampled for each pixel of the image sensor and then quantized by the A/D converter. Information is lost in this process between pixels and between quantized luminance values.

The figure schematically shows how the information of one line with the subject changes by sampling and quantization. Object information is as smooth as a line on an analog display, but through sampling and quantization it becomes discrete values ​​and some information is lost. Sampling fineness is determined by the number of pixels, and quantization fineness is determined by bit depth. As it happens in the acquisition of digital images, it is necessary to carefully consider the parameters when making a decision.

In the digital image correlation method, the subject is divided into minute areas called subsets, as shown in the red frame in the figure. Each subset is a measuring point. Deformation and strain are calculated by measuring the displacement of the subset. For strain/displacement detection, set the subset interval called step width and cut the mesh. By detecting the movement amount and direction of each subset, the displacement/strain distribution of the measurement target can be obtained. For strain calculations, the measurement length, called the gauge length, is specified in number of step widths.