I. Macroscopic Observation: The most direct initial judgment
1. Visually inspect for fracture marks
The screw head is separated from the shank, obviously detached, or only part of the screw remains in the hole; the fracture surface is smooth or exhibits brittle fracture characteristics (e.g., no obvious plastic deformation), often occurring at the first thread below the head or at the beginning of the thread.
2. Check for abnormal connection conditions
The connected parts are loose, misaligned, or cannot be tightened; a sudden "loss of force" during tightening, with a sharp drop in resistance, may indicate internal breakage.
✅ Applicable scenarios: Quick on-site troubleshooting, suitable for complete breaks visible to the naked eye.
II. Functional and tactile verification: Assisting in judging hidden fractures
1. Tightening test
If the screw can rotate freely but cannot be tightened, and there is no clamping force feedback, it may be a mid-section breakage or thread stripping; use a torque tool to set a standard value; if the torque is much lower than normal, it indicates failure and a risk of breakage.
2. Tapping and Listening Method (Applicable to Enclosed Structures)
Lightly tap the area around the screw. Fractured parts often produce a hollow or unusual sound, unlike the dull sound of normal fasteners.
✅ Note: This method requires experience and is for reference only; it cannot replace professional testing.
III. Professional Testing Methods: Precisely Locating the Fracture Type and Cause
When visual inspection is insufficient or the fracture mechanism needs analysis, the following testing methods should be used:
1. Scanning Electron Microscopy (SEM) Analysis of the Fracture Surface
Observe the microscopic morphology of the fracture surface: If it presents "candy-like intergranular fracture" accompanied by "chicken claw marks," it is a typical characteristic of hydrogen embrittlement fracture; differentiate between fatigue fracture (conical striations), overload fracture (cup-cone shape), and corrosion fracture (pitting, rust).
2. Metallographic Examination
Check whether the microstructure is tempered martensite or other hydrogen embrittlement-sensitive structures; analyze whether the carburized layer thickness exceeds the standard (e.g., exceeding 0.25mm), leading to stress concentration.
1. Hydrogen Content Determination
A significantly higher hydrogen content in the fractured component compared to the intact component (e.g., 11.5 ppm vs. 4.2 ppm) can serve as an important indicator of hydrogen embrittlement.
2. Chemical Composition and Mechanical Property Testing
Verifies whether the material meets standards (e.g., SAE 1022 steel); Tests hardness, tensile strength, etc., to check for deviations and rule out material or heat treatment defects.
