How can you determine whether the surface of a titanium alloy fastener has microcracks, nicks, or folds?
The simplest and quickest way to determine
For titanium alloy fasteners, the following two methods allow for quick judgment.
1. Visual Inspection + Direct Bright Light
Turn on a bright light (a cell phone flashlight will work) and shine it at a 45° angle onto the threads, head, and flange surface.
If you see continuous bright lines, dark lines, burrs, or steps, these indicate tool marks, folds, or cracks.
The natural color of titanium alloy is a matte silver-gray; any abnormal lines will be very noticeable.
2. Low-magnification loupe (10–20x)
Most commonly used in the workshop; costs just a few dollars.
The root, crest, R-radius, and head corners of the thread are high-risk areas.
Look for:Lines of varying depths → Tool marks
Metal layer misalignment, peeling, or curling → Folding
Fine, forked, or irregular cracks → Microcracks
Mnemonic: Check the thread root for lines, the corners for peeling, and the surface for cracks.
How to Distinguish Between the Three Types of Defects
1. Tool Marks
Appearance: Parallel, uniform, and regular fine lines
Location: Thread flanks, inner bore walls, and flange surfaces
Cause: Cutting marks left by turning tools or taps
Consequences: The deeper the tool marks, the shorter the fatigue life
2. Folding (Metal Folding / Forged Folding)
Characteristics: Overlapping metal layers, curled edges, burrs, irregular patterns
Locations: Corner of the head, flange edges, thread entry points
Causes: Metal is compressed and overlaps during forging, turning, or thread rolling
Hazards: High-risk areas for fatigue failure; must be deemed non-conforming
3. Microcracks
Appearance: Long, slender, hairline-like cracks
Location: Thread roots, stress concentration R-corners
Characteristics: Do not disappear when moved under a light
Hazards: Directly lead to failure during use
Advanced Testing
1. Fluorescent Penetrant Inspection (FPI)
The most standard method for detecting surface cracks in titanium screws.
Apply penetrant → Apply developer powder
Cracks appear as bright yellow-green lines, leaving even the smallest cracks exposed.
Folds and nicks do not show up; only cracks will appear bright.
2. Post-blasting inspection
After sandblasting, the surface has a uniform matte finish:
Scratches appear as light and dark shadow lines
Folds appear as peeling or uneven areas
Cracks appear as distinct black lines
Chengjin Titanium Industry’s Internal Evaluation Criteria
Minor knife marks: Acceptable, but must not be deep
Visible knife marks or continuous knife marks: Rejected
Any creases or peeling: Rejected without exception
Any micro-cracks: Scrapped without exception (Cracks in titanium alloys cannot be repaired)
A Simple Summary
Use strong, angled light to identify lines; examine the base of the tooth with a magnifying glass. Parallel lines indicate knife marks; overlapping raised edges indicate folds; split hair-like strands indicate cracks. Any item with cracks should be discarded.