Crack Defect Visual Inspection
Crack defect visual inspection: for cracking, crazing and micro-cracks on the surfaces of composite materials, coatings, glass and metal parts, low-angle light or 3D height information forms a clear image, distinguishing parting lines from real cracks.
defect Overview
For fine cracking defects, low-angle light and height information form the clearest image; the difficulty is distinguishing the parting line
A crack is a linear fracture on the material surface, including micro-cracks, crazing and stress cracking; under low-angle light it becomes visible through edge shadows, or its opening can be exposed using 3D height/normal information. Machine vision judges on this basis; the main risk is misjudging normal linear structures such as parting lines, engraved patterns and seams as cracks, and shallow cracks may be masked by directional lighting, so multi-directional or 3D support is needed.
Cracks and scratches both appear as linear features in the image, but their causes differ: a scratch is usually superficial damage from external abrasion, while a crack is a fracture of the material itself, often accompanied by an opening and depth. A shallow crack also forms a clear image from its bright and dark edges under low-angle light, but a more reliable approach is to obtain height/normal information with 3D line laser or photometric stereo, where the opening shows a clear depression.
Cracks are very easily confused with "intentional linear features". Parting lines on injection-molded and die-cast parts, and seams and weave directions on composite surfaces, are all normal lines. Judgement must use a template or positional priors to exclude these known lines, and then flag linear cracking that appears in unexpected positions as suspect.
Another difficulty is the blind spot of directional illumination: cracks parallel to the light path are darkened. For safety-critical parts (such as structural parts and seals), we recommend multi-directional low-angle light coverage, or direct use of 3D height information, to avoid missing fine cracks that could compromise safety.
Occurrence Causes
Only when the cause is known can you decide which station should check for it
- Material stress: stress cracking from uneven cooling during forming or shrinkage during curing
- Process defects: micro-cracks from improper injection molding / die casting parameters or ejection force
- Fatigue and aging: cracking caused by alternating loads and environmental aging
- Design lines: parting lines, seams, and weave patterns misread as cracks (these are normal)
- Downstream damage: cracking from assembly forces and drop impact
imaging Key Points
Whether it can be detected depends first on whether it can be captured
Low-Angle Light
Grazing light creates bright and dark edges at a crack opening and is well suited to making surface micro-cracks form a clear image
3D line laser / height map
Directly measuring the opening depression is the most reliable way to separate a surface scratch from a real crack
Photometric Stereo
Multi-directional illumination is combined into a normal map to separate cracks from texture
Template Exclusion
Register parting line and seam positions to exclude known normal linear structures
judgement Method
Crack judgement combines two pieces of evidence: "line-like features + height opening". The template first excludes parting lines and seams, then measures the length, maximum opening depth, and contrast of line-like connected components on the saliency map. Line-like features with height depression are judged as cracks first.
Threshold bias: for safety-critical parts, an escaped crack defect is extremely costly, so the judgement should be on the strict side; but false calls on parting lines cause heavy over-rejection, so the template must be accurate. The specific length and opening thresholds are defined quantitatively by the quality department and saved with the recipe.
| Judgement Dimension | Description |
|---|---|
| Height Opening | 3D sink depth distinguishes cracks from shallow scratches; preferred for safety parts |
| Position | Whether it lies outside the parting lines / seams allowed by the template |
| Length | Short micro-cracks and long cracks handled by grade |
| Contrast | Maximum difference between bright and dark edges under low-angle light |
| Multi-Directional Coverage | Multi-directional or 3D imaging eliminates the blind spots of parallel light paths |
Applicable algorithm
3D height / normal analysis
Use a line laser or photometric stereo to capture the depression at the opening and distinguish cracks from scratches
- The most reliable cracking criterion
- Recommended for safety parts
Linear feature extraction
Extract linear structures from the low-angle light saliency map and suppress texture
- Multi-direction fusion avoids blind spots
- Filter by length and depth
Template difference judgement
Registration excludes parting lines/seams to find additional linear cracks
- Avoid false rejection of known lines
- Centering offset must be corrected
Common Materials
Common Industry
Common Question
How Do You Distinguish Cracks from Scratches?
Can Parting Lines Be Misjudged as Cracks?
Will Shallow Cracks Escape Detection?
Why Is 3D Recommended for Crack Detection?
Who Sets the Rejection Threshold for Cracks?
What Is the Thinnest Crack That Can Be Detected?
Is crack inspection on curved parts more difficult?
Submit sample testing
Crack inspection uses low-angle light and 3D height information to form a clear image and uses templates to exclude normal lines such as parting lines.
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Send Us Your Defect Samples and We Will Measure Them and Show You the Results
Whether a defect can be detected depends on whether imaging captures the defect features. Provide OK and NG samples and we will run actual imaging and judgement tests on the equipment.