What is digital image correlation?
Digital image correlation (DIC) is a non-contact optical method used to measure full-field displacements and strains in materials and structures. It is commonly employed in experimental mechanics and materials science to analyze the deformation and motion of objects under various loading conditions.
- The Digital Image Correlation (DIC) technique is a non-contact, full-field measurement approach.
- DIC enables the assessment of the deformation and motion of objects.
- DIC involves comparing digital images of the object before and after deformation.
- DIC is used to track surface patterns on an object.
- DIC captures images before and after deformation to gather data on displacement and strain.
The Basic
Displacement in image correlation
Key Advantages of Digital Image Correlation (DIC)
- Full-field Measurements: DIC provides full-field measurements of displacement and strain across the entire surface of an object. This allows for a comprehensive understanding of the deformation behavior, capturing localized effects and identifying areas of interest that may be missed by point-based measurements.
- Non-Contact Method: This means it doesn’t require physical contact with the object being measured. This is particularly advantageous when dealing with delicate or sensitive materials where direct contact could alter the behavior or introduce errors in the measurements.
- High Spatial Resolution: DIC can achieve high spatial resolution by using high-resolution cameras and advanced image processing algorithms. This enables the detection and measurement of small displacements and strains, providing detailed information about the material’s response.
- Flexible and Versatile: DIC is a versatile technique that can be applied to various materials and structures, regardless of their size, shape, or complexity. It can be used for both static and dynamic measurements, making it suitable for a wide range of applications.
- Real-Time Monitoring: DIC can provide real-time measurements, allowing for the monitoring of deformation and motion during experiments or tests. This real-time feedback can be valuable for controlling and adjusting loading conditions or experimental parameters.
- Post-Processing Capabilities: DIC generates a large amount of data, which can be further analyzed and processed to extract additional information beyond displacement and strain. For example, it can be used to visualize deformation modes, identify stress concentrations, or validate numerical models through comparison with simulation results.
How does Digital Image Correlation work?
DIC works by tracking the movement of patterns or features on the surface of an object captured by a pair of digital images. The process involves the following steps:
- Image Acquisition:
Two images of the object are captured using a digital camera. These images were taken before and after deformation or motion occurred.
- Image Preprocessing:
The acquired images are typically preprocessed to enhance contrast, remove noise, and improve the visibility of the patterns or features on the surface.
- Image Correlation:
The preprocessed images are divided into small regions or subsets, typically square or rectangular. The subsets in the reference image are then compared to the corresponding subsets in the deformed or subsequent image.
- Pattern Matching:
The subsets in the reference image are compared to the subsets in the deformed image using various correlation algorithms to determine the displacement or strain between the two images. The correlation algorithms estimate the best match between subsets by calculating the similarity, or correlation coefficient.
- Displacement Calculation:
The displacement field is obtained by correlating all subsets in the reference and deformed images. The correlation results provide information about the full-field displacement of the object’s surface.
- Strain Calculation:
From the displacement data, strain fields can be calculated by differentiating the displacement field. Strain represents the deformation of the material and can provide insights into the mechanical behavior of the object.
DIC finds applications in various fields, including materials testing, structural analysis, biomechanics, geomechanics, and manufacturing quality control. It enables researchers and engineers to study the mechanical behavior of materials, validate numerical simulations, and assess the performance of structures under different loading conditions.
Measurement Principle
DIC is an image analysis technique that utilizes grayscale digital images to analyze the entire surface of an object under load. It can accurately determine the object’s contour and measure its displacements in three dimensions.
The EduDIC is a cost-effective and user-friendly Digital Image Correlation (DIC) measurement system specifically designed for academic training in experimental solid mechanics and materials testing. It serves as an entry-level tool that allows instructors and students to perform optical DIC measurements for various research, workshops, and laboratory exercises.
The system offers flexibility and adjustability, allowing users to freely align and move the cameras on the mounting bar. This feature enables the utilization of different stereo-angles and baseline distances, thereby enhancing the accuracy of in-plane or out-of-plane measurements.
The FlexDIC system
The FlexDIC system is an optical measurement device that offers complete non-contact, three-dimensional measurement capabilities for analyzing the shape, displacements, and strains of components and structures made from a wide range of materials.
This system is utilized to determine the three-dimensional material properties during various types of tests, such as tensile, torsion, bending, or combined tests. Additionally, it enables deformation and strain analysis in applications including fatigue tests, fracture mechanics, validation of finite element analysis (FEA), and many other areas of study.
The measurement results obtained through DIC are utilized for evaluating the thermal mismatch (coefficient of thermal expansion, CTE) within components. Furthermore, DIC enables warpage analysis, which can be employed for validating finite element analysis (FEA) models. One of the advantages of DIC is its ability to process information in real-time, allowing for quick data acquisition, evaluation, and visualization during the measurement process.
The ThermechDIC system consists of the following:.
- Stereo-rig DIC system with a hot/cold stage
- Temperature controller
- Liquid N2 pump & controller
- Dewar/tank
To facilitate measurements, the DIC stereo-rig can be mounted on a motorized mounting frame or utilized alongside a stereoscopic microscope (µDIC).
The videoextensometer RTSS system:
