Friday, November 27
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what is Non destructive testing ( NDT)?

Today, non -destructive testing is most often understood as an analysis of the reliability and other properties and the main working characteristics of the entire object or its individual elements (sections), not related to the removal of this object from work or its dismantling. In other words, we are talking about checking the product without any destruction (nondestructive inspection-NDI).

And this is precisely its main difference from destructive control. A simple example. In order to assess the tensile strength of an object, in any case, it is necessary to apply a load, after which the object will no longer be suitable for operation (car crash tests can be referred to such methods).

As for the basic methods of non-destructive testing, then they are such methods as :

  • magnetic
  • eddy current
  • radio wave
  • optic
  • acoustic (ultrasonic)
  • radiation
  • thermal
  • electric
  • penetrating substances

And here it is worth noting that with all the variety of non-destructive testing methods, the most common use is the magnetic particle and ultrasonic methods. As for, for example, radiation control, it is used much less frequently. But it is radiation monitoring devices that allow you to control large thicknesses of materials, as well as those types of materials whose diagnosis by other methods is difficult (in particular, composites). The use of the acoustic method of non-destructive testing is also due to its particular advantages: this is, above all, the possibility of diagnosing defects of the internal type, the relative simplicity of non-destructive testing devices, and a wide range of materials suitable for examination. In this regard, it compares favorably, for example, with magnetic, vortex, and electrical control methods, which make it possible to diagnose only the surface and subsurface layer of metals.

For more details about the features of each non-destructive testing method, that explained 3 things:

 

  • Non-destructive testing method
  • Defects detected by this non-destructive testing method
  • Disadvantages of the method

1. Visual Measuring

Defects on the surface of an object with a size of 0.1 mm or more

Low ability to detect small surface defects

detect-ability depends on subjective factors.

2. Magnetic powder

Diagnostics of objects made of ferromagnetic-type steels: surface and subsurface (2-3mm) defects with a length of 0.5 mm and an opening width of 2 μm

It can be used for non-destructive testing of non-magnetic coatings (cadmium, chromium, etc.). Coatings with a thickness of up to 20 μm have virtually no effect on the correctness of control and the detection of defects

Magnetic particle testing of elements and structures made of non-ferromagnetic steels is not possible if there is no zone on their surface necessary for the application of indicator materials and magnetization, or products with structural inhomogeneities and / or sudden changes in cross-sectional area with the presence of discontinuities with an opening plane that does not coincide with the direction of the magnetic field , or making an angle of 30 degrees or less with it

3. Capillary

Defects of the surface and through type with an opening of the order of 1 μm

Using the instruments of this non-destructive testing method, it is possible to detect only surface defects or through defects without determining their exact depth.

The complexity of mechanization and automation of the implementation of the method.

The need for thorough surface treatment of the controlled object

4. Ultrasonic

Suitable for non-destructive testing of products from both metals and non-metals

Allows to identify all types of defects in the main material, welds, heat affected areas

High speed, control performance at low cost and human safety

5. Ultrasonic Flaw Detector Mobility

The surface of the object must be prepared for the introduction of ultrasound, and in the case of welded joints, preparation of the roughness directions is also necessary (they should be perpendicular to the seam)

Contact liquids (water, paste, oil) are required. Moreover, in the diagnosis of vertical or strongly inclined surfaces, these contact fluids must have a certain density

The use of “ground” transducers (with a radius of curvature of the sole of R equal to 0.9-1.1 R of the radius of the controlled object), not suitable in this form for the diagnosis of flat surfaces

The devices of this method of non-destructive testing do not allow to answer the question about the size of the detected defect, measuring only its reflectivity in the direction of the receiver (while this value does not correlate for all types of defects)

It does not allow controlling connections in which both elements are forged, cast, or stamped; angled inclined (with a deviation from perpendicularity of more than 10 degrees) welded joints of tubular elements between themselves or with other elements, as well as metals with a coarse-grained structure, products of small size and complex shape.

6. Radiation

Internal defects of welded joints (cracks, imperfections, pores, slag inclusions)

Low detection of surface defects

The method of non-destructive testing does not allow to detect inclusions and pores with a cross-sectional diameter, cracks and lack of penetration with an opening plane that does not coincide with the direction of transmission

When using appropriate devices, it is necessary to ensure radiation safety

7. Thermal

Identification of passages of coolants, leaks, violations of insulation coatings, areas of heating of electrical contacts

Dependence of the correctness of measurements on the environment and weather conditions

8. Leak Detection

Leak detection

Detection of end-to-end defects only

9. Acoustic emission (AE)

It detects surface and internal defects, including – and this is especially important – defects that are still only in the development stage (from tenths of a millimeter). Due to this, it allows classification of defects, including the degree of their danger

A fairly sophisticated technology that requires expensive equipment and instruments.

Acoustic emission signals are usually difficult to distinguish from interference.

The need for subsequent diagnosis of controlled objects by other methods.

10. Vibrodiagnostic

Detection and diagnostics of pulsation of the flow of a technological medium, oscillations of moving parts

Strict additional requirements for the method of mounting the sensor.

The dependence of the vibration level on a number of factors

Difficulties in isolating the vibration signal

11. Eddy Current

Detects surface and subsurface (depth -1 – 4 mm) defects

It is used only for the diagnosis of products from conductive materials.

12. Electric

Allows assessment of insulation integrity

Assumes the need for contact with the object

Strict requirements for the cleanliness of the surface of the object

The complexity of automating the process of non-destructive testing

The dependence of the correctness of the measurement results on the state of the environment

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