1.What are the advantages and disadvantages of X-ray diffraction?
Principle: Based on Bragg's law. When X-rays irradiate a material's crystal lattice, diffraction occurs. Residual stress causes elastic distortion of the lattice spacing, thus altering the diffraction angle. By precisely measuring the displacement of the diffraction angle, the magnitude and direction of the stress can be calculated.
Advantages:
Absolute quantitative measurement: Stress values (MPa) and direction can be directly obtained.
High spatial resolution: Capable of measuring minute areas (point stress) and stress gradients (e.g., distribution along layer depth).
Mature theory and well-established standards (e.g., ASTM E915, GB/T 7704).
Disadvantages:
Expensive equipment, complex operation, requires specialized personnel and a specific environment.
Typically used for laboratory sampling analysis; difficult for rapid online detection.
Shallow measurement depth (typically 10-30 micrometers), reflecting surface stress.
Applications: Primarily used for precise laboratory analysis, new process development, quality arbitration, and calibration of other methods.
2.What are the advantages and disadvantages of ultrasonic testing?
Principle: Residual stress alters the elastic constants within a material, thus affecting the propagation speed of ultrasonic waves (acoustoelastic effect). By precisely measuring the propagation time or frequency changes of specific wave types (longitudinal waves, transverse waves, surface waves), the stress value can be deduced.
Advantages:
Fast and portable: Enables large-area scanning and online monitoring.
Measures average stress within a certain depth (depending on the ultrasonic frequency and wave type).
Low requirements on the object being measured, requiring only good acoustic coupling.
Disadvantages:
Measurement accuracy is significantly affected by material texture and grain size, requiring calibration for specific materials.
The equipment algorithm is complex, and the absolute measurement accuracy is typically lower than that of X-ray methods.
Applications: Suitable for rapid screening on production lines, detecting longitudinal stress distribution trends in strips and plates, and on-site evaluation of large components.
3.What are the advantages and disadvantages of magnetic detection?
Principle:
Barkhausen noise: Residual stress affects the irreversible movement of magnetic domain walls in a material, thus altering the intensity of the Barkhausen (BN) signal. This noise signal is received by a sensor, allowing assessment of the stress state.
Magnetic anisotropy: Stress causes anisotropy in the magnetic permeability of a material. Stress can be calculated by measuring the magnetoresistance in different directions.
Advantages:
Extremely fast detection speed, ideal for continuous scanning and online applications.
Sensitive to changes in surface stress.
Relatively portable equipment.
Disadvantages:
Strongly dependent on calibration and greatly affected by the material's microstructure (e.g., grain orientation, hardness).
Typically used for qualitative comparisons or relative stress measurements of known materials; relatively weak absolute quantitative capability.
4.What are the advantages and disadvantages of borehole drilling for inspection?
Principle: A shallow blind hole (typically 1-2 mm deep) is drilled at the test point. This releases localized stress, causing minute deformation around the hole. A pre-attached strain gauge measures this strain, and the magnitude and direction of the original stress at the drilled point are calculated using theoretical formulas.
Advantages:
Mature technology, high reliability, and quantitative results.
Equipment cost is relatively lower than X-ray methods.
Can measure stress at a certain depth below the surface.
Disadvantages:
It is a micro-destructive testing method, leaving small holes on the product surface.
The operation involves multiple steps (surface treatment, mounting, drilling, measurement), resulting in slower speed.
For thin plates (such as cold-rolled coils), the drilling may penetrate the surface, causing failure.
5.What are the advantages and disadvantages of the cutting/strip method for detection?
Principle: A sample is cut from a steel strip (e.g., along the transverse or longitudinal direction). Due to the release of residual stress, the strip will exhibit significant bending (warping). By measuring the radius of curvature of the warp, the original average stress distribution can be deduced.
Advantages:
Simple, intuitive, and extremely low cost.
It can reflect the stress equilibrium state along the entire length of the sample.
Disadvantages:
Highly destructive; the sample is completely unusable.
It can only obtain the overall average stress or stress distribution trend, not local point stresses.
Quantitative calculations require simplifying assumptions, resulting in limited accuracy.