Bolt stress detection

Bolt stress testing is a key technology for evaluating the internal stress state of bolts under long-term service or specific working conditions. Its core purpose is to ensure connection reliability, verify material properties, and prevent failure risks caused by stress relaxation or concentration.

Bolt stress testing is a key technology for evaluating the internal stress state of bolts under long-term service or specific working conditions. Its core purpose is to ensure connection reliability, verify material properties, and prevent failure risks caused by stress relaxation or concentration. The following is an introduction from five aspects: detection principles, methods, standards, influencing factors, and equipment applications:

1、 Detection principle

Bolt stress detection is based on material mechanics and acoustic properties, and the stress value is inferred by measuring the physical quantity changes caused by stress (such as sound velocity, strain, displacement, etc.). The core principles include:

Acoustic elastic effect: When ultrasonic waves propagate in materials, stress can cause changes in sound velocity, and stress can be calculated by measuring sound velocity or time of flight (TOF). For example, when the axial stress of the bolt increases, the ultrasonic velocity decreases linearly, which can be accurately converted into stress values through calibration curves.

Strain resistance effect: Resistance strain gauges are attached to the surface of bolts, and resistance changes occur as the bolt deforms. The resistance changes are converted into strain values through a Wheatstone bridge, and then combined with the elastic modulus of the material to calculate stress.

Lattice distortion principle: X-ray or neutron diffraction indirectly measures residual stress by analyzing changes in lattice spacing of materials, suitable for surface or deep stress distribution analysis.

2、 Testing method

According to the detection scenario and accuracy requirements, bolt stress detection methods can be divided into two categories: non-destructive detection and destructive detection:

Non destructive testing:

Ultrasonic stress testing:

Principle: By utilizing the acoustic elastic effect, stress is calculated by measuring the propagation time or sound velocity changes of ultrasonic waves in bolts.

Advantages: No need to damage bolts, can monitor service status online, suitable for harsh environments such as high temperature and high pressure.

Application: Key bolt testing in aerospace, nuclear power, wind power and other fields, such as engine blade bolts and nuclear reactor pressure vessel bolts.

Magnetic elastic stress detection:

Principle: Based on the magnetic anisotropy effect of ferromagnetic materials, stress changes cause changes in magnetic permeability, and stress is inferred by measuring magnetic field changes through magnetic sensors.

Advantages: Fast detection speed, suitable for batch screening.

Limitations: Only applicable to ferromagnetic materials such as carbon steel and alloy steel.

X-ray diffraction method:

Principle: When X-rays penetrate materials, changes in lattice spacing cause diffraction angle shift, and stress is calculated by measuring the offset.

Advantages: High precision, capable of analyzing surface or near surface stress distribution.

Limitations: The equipment is expensive, the detection efficiency is low, and professional operation is required.

Destructive testing:

Drilling method:

Principle: Drill holes on the surface of the bolt to release residual stress, and calculate the original stress by measuring the strain changes around the drill hole.

Limitations: Requires bolt damage, only applicable for single inspection or failure analysis.

Slicing method:

Principle: Cut the bolt to obtain a cross-section, and indirectly evaluate the stress state through metallographic analysis or hardness testing.

Limitations: Damaged bolts, only used for laboratory research or root cause analysis of failure.