Meaning
Optical measurement technique that tracks surface deformation over time by comparing successive photographs of a speckled pattern on a plastic component. Digital image correlation creep provides a high resolution map of how a polymer part moves and stretches under a constant load. This method is used to study the long term stability of plastic assemblies and predict when they might fail.
The absence of physical contact with the sample prevents the sensor from influencing the result. It is particularly useful for measuring the complex deformation of parts with irregular shapes or thin walls. The technique reaches its limit when the surface of the part becomes too distorted for the software to track the speckle pattern.
Deformation Mapping
A high contrast speckle pattern is applied to the surface of the plastic part before the test begins. As the part is subjected to a constant load, a series of digital images are taken at regular intervals. The software then compares these images, tracking the movement of each individual speckle to calculate the local strain across the entire surface.
This allows engineers to see exactly where the material is stretching the most and where the stress is concentrated. This level of detail is impossible to achieve with traditional testing methods, which only provide a single measurement at a specific point. The mapping reveals how the material flows and redistributes the load over time.
It is an essential tool for understanding the creep behaviour of complex moulded parts.
Stress Relaxation
Over time, polymers under load will undergo a process of molecular rearrangement that leads to a slow increase in deformation. This phenomenon, known as creep, is a major factor in the long term performance of plastic components. Digital image correlation creep allows researchers to observe this process in real time and see how it is affected by temperature and humidity.
By analyzing the rate of deformation, they can predict the useful life of the part and ensure that it will not fail prematurely. This information is critical for designing parts that must maintain their shape and function over many years. The data also helps to validate the material models used in finite element analysis.
Accurate creep data ensures that the final product will be reliable and safe for the intended application.
Measurement Constraint
The accuracy of the digital image correlation technique depends on the quality of the speckle pattern and the stability of the lighting and camera setup. If the speckles are too large or too small, the software will struggle to track them accurately. Any movement of the camera or the sample that is not caused by the load will introduce errors into the results.
To minimize these errors, the test is usually conducted in a controlled environment with a fixed camera mount. The technique also requires a clear line of sight to the surface being measured, which can be difficult for parts with complex internal geometries. Despite these constraints, it remains one of the most powerful tools for studying the mechanical behavior of polymers.
Every test provides a wealth of data that can be used to improve the design and performance of plastic parts.
