Root Cause Analysis of Core Rod Fracture During Crimping or Tensile Testing

I. Failure Characteristics

The fracture typically occurs inside the ball-end or socket-end metal fitting. The primary symptom is a tensile failure load that is significantly lower than the rated design value.

II. Root Cause Analysis

1. Excessive Crimping Force: The pressure applied during the crimping process exceeds the radial compressive strength of the core rod, leading to internal structural damage.
2. Improper Crimping Position: The crimping dies are not centered; the compression zone should ideally be located in the middle of the fitting’s connection area.
3. Poor Machining Precision of Fittings: Issues such as eccentricity of the inner hole, insufficient cylindricity, or internal tapers can cause uneven stress distribution.
4. Material Inconsistency: Non-uniform material quality or incorrect heat treatment (hardness issues) in the same batch of fittings (especially sockets).
5. Substandard Core Rod Grinding: Inadequate grinding of the core rod ends, resulting in excessive taper or poor cylindricity, which prevents a uniform fit.
6. Equipment Instability: Poor synchronization of the crimping machine’s 6-jaw or 8-jaw system, leading to asymmetrical pressure.
7. High Elastic Modulus: If the core rod’s heat resistance is over-optimized, it may become overly brittle (high modulus, low toughness), making it prone to “brittle fracture” during compression.
8. Assembly Misalignment: The ball/socket fitting and the core rod are not perfectly aligned on the same axis during assembly, introducing harmful shear stresses.

III. Solutions and Strategic Recommendations

1. Supplier Quality Management: Source metal fittings from reputable large-scale manufacturers to ensure material consistency. Upon arrival, strictly inspect the hardness, internal hole dimensions, and tolerances against technical specifications.
2. Advanced Grinding Technology: Utilize specialized grinding machines and trained technicians to ensure the surface finish and geometric accuracy of the core rod ends.
3. Tolerance Control: Strictly monitor and control the assembly clearance between the fitting and the core rod to ensure an optimal fit before crimping.
4. Optimization of Crimping Parameters: Whenever a new batch of fittings or core rods arrives, conduct destructive tests on 5–8 samples to determine the optimal crimping pressure. For high-tonnage products, consider multi-stage (segmented) crimping.
5. Maintenance of Crimping Equipment: Ensure the stability of the crimping machine. Regularly replace hydraulic oil and implement cooling measures for the equipment during high-temperature summer shifts to prevent pressure fluctuations.

Expert Insight: The reliability of the crimped connection is the “lifeblood” of a composite insulator. By balancing the core rod’s toughness with the fitting’s hardness and using precise crimping parameters, manufacturers can ensure that the interface remains the strongest part of the assembly.