Painting your digital image correlation samples for accurate measurement
Digital image correlation (DIC) is becoming a common process to measure material displacement during tensile and dome tests.This visual method uses cameras and software to follow the movement of williamhill彩票points defined by a speckle pattern painted on the material surface.
通过跟踪变形过程中的位移矢量，you can measure such events as local deformations,cracking,plastic instabilities,and phase transformations.It is generally accepted that the precision of the DIC displacement measurement is limited by the quality of the image and is reliable down to 0.1 pixel.And the accuracy of your test results depends on your awareness and management of the test procedures:
- Paint a contrasting speckle pattern (usually black) on top of the white paint.
- Mount the sample and begin your deformation operation.
- Compare the final image (at the failure event) to your reference image to determine displacement length and vectors.
你对材料和成形过程做出正确决定的能力取决于你的测试程序及其对DIC成像的影响。As your geometry and forming operations approach the limits of your material,测试的准确性对于做出正确的决定变得更加关键。
While cameras and software can provide highly precise measurements,the accuracy of your test depends on how closely the movement of the paint follows the movement of the substrate.Paint is formulated to withstand the elasticity of the underlying surface during normal operation;it isn't generally formulated to withstand the high levels of deformation during a tensile or dome test.
研究人员发现，油漆粘附力是精确测量位移的一个限制因素。如果材料表面没有适当的油漆，your test risks showing incorrect results.Inadequate adhesion can result in your test equipment measuring the elasticity of the paint rather than the material you are testing.
The accuracy of your results depends on how well you prepare your sample surface and apply the paint.The two important elements of paint performance are cohesion—the strength of the molecular bonds of the paint—and adhesion—the strength of the paint's bond to the substrate.
Cohesive failure can change the geometry of your speckle pattern and the local elasticity of the paint.粘合失效是指涂料从基底表面剥离。当附着失败时,the speckle pattern cannot be relied on to follow the movement of the substrate.Both types of failure can give you inaccurate results.
Paint adhesion remains somewhat poorly understood,but it is generally assumed to have three components:
Reliable adhesion requires proper"wetting,"当油漆流到粗糙表面上所有的山谷时就会发生这种情况。This provides maximum surface area contact for the three components of paint adhesion.
Finding the right combination of paint and application for each type of material requires much trial and error.要考虑的最重要的变量包括：
- 打扫-正确涂漆时，样品表面的清洁度是最重要的考虑因素。Remove all contaminants and particles from the sample before applying the paint.Throughout the process of cleaning,applying,以及固化涂料，小心处理样品，防止将污垢和车身油引入油漆和基底表面。
- Material Properties-黑色金属，aluminum,不锈钢材料具有不同的表面性质和化学性质。您的油漆选择将影响吸附强度和化学键。试着选择适合你的样品材料的油漆。
- Application Method—Select an appropriate spray tip to ensure adequate coverage and cohesion across the test surface.您的环境也应该是干净的，在适当的温度和湿度水平。Experiment with curing times to find the best elasticity and viscosity for your paint.
As with any process,您不能想当然地认为影响结果的变量。Your best approach is to take a close look at all the components of your test and question their reliability.A disciplined and informed process will help you to attain the precision and accuracy you need from your test results,最终提高生产力。