Predictive Maintenance : Best Strategies for Successful Adoption

Nowadays, industries cannot afford to stop producing because of equipment failures that they could have foreseen. These failures can be avoided by carrying out predictive maintenance. Systematic and conditional preventive maintenances are meant to carry out interventions on low-cost pieces, which degradation level can be easily evaluated (oil level control, scheduled spare parts replacement). However, it is quite complicated to detect some degradations that are responsible for a failure, such as microcracks, temperature rise of spinning pieces, and faulty insulation.

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Preventive maintenance is carried out according to extrapolated forecasts resulting from the analysis and the evaluation of significant equipment degradation parameters. Maintenance teams can rely on several analysis techniques called NDT (Non Destructive Testing), that include:

  • Remote visual inspection
  • Dye penetrant inspection
  • Radiographic testing
  • Ultrasonic inspection
  • Thermography
  • Magnetic-particle inspection
  • Leak tightness control
  • Eddy-current testing                                                       

Predictive Maintenance - Remote visual inspection

Remote visual inspection is the first non destructive testing. It provides a cost-efficient primary assessment. Essential information and defaults can be deduced from the external appearance of the piece, such as folds, breaks, cracks and corrosion.

The remote visual inspection has to be carried out in good conditions with a sufficient lighting (350 LUX at least). When the part of the piece to be controlled is not directly accessible, an instrument made of mirrors and lenses called endoscope is used. Hidden defects with external irregularities may indicate a more serious defect inside.

Predictive Maintenance - Dye penetrant inspection

Dye penetrant inspection is used to detect discontinuities (cracks, splits) on metal parts. A visible or fluorescent penetrant liquid is applied to fill the defaults, and then a developer is used.

This technique is used to control:

  • Moulded or forged part
  • Mechanical components after grinding and/or heat treatment
  • Pre/post-weld parts
  • Hot-rolled or stretched parts

Predictive Maintenance - Radiographic inspection

Radiographic testing is a non-destructive testing method that aims at determining the material density of an object using X-rays or gamma rays.

With this technique, it is possible to visualize in 2D the material volume losses of the controlled object.

Radiographic inspection is used in an industrial context to control the internal integrity of a piece and to detect cavities or inclusions inside the piece. It is also commonly used to inspect welds, to detect foundry defects, and to monitor composite structural integrity.

The main advantage of radiographic inspection is that it makes it possible to obtain high-resolution images of the piece’s material uniformity. Contrasted indications can be detected easily; and it is also possible to distinguish different types of indications. It applies to all types of materials (aluminium, steel, copper alloys, titanium, composite material, etc.).

However this method requires expensive installations and consumables, as well as strict guidelines and regulations because ionizing radiations are a threat to health.

Predictive Maintenance - Ultrasonic inspection

Ultrasonic inspection is based on the transmission, reflection and absorption of ultrasonic waves that propagate through the controlled piece. The emitted wave train reflects on the flaws, and then goes back to the transducer – that often acts both as a transmitter and as a receiver. Signal interpretation enables to determine the position and the relative size of the flaw. This method also offers a high spatial resolution and enables to detect volume flaws as well as surface flaws.

Compared to radiographic testing that is also used in volume flaw research, its main advantage is that ultrasonic inspection only requires one access point; also, the radioactive source and ionizing radiations are not harmful, and it is also easier to be more precise on bigger parts. The result is accessible directly, contrary to dye penetrant inspection, which results are generally obtained after a few hours. The main drawbacks are that some metal materials are more difficult to work with (materials with bigger grain size), and that controls can take more time for thinner layers.

Predictive Maintenance - Thermography 

Thermography enables to produce photographs of the thermal scene observed in the infrared range with a thermal camera. With the thermal camera, thermal images of a thermal scene can be obtained.

The camera technology system and the integrated programs enable to make this radiation visible on a reconstituted image. A thermogram is obtained after the heat transcription operation.

Thermal surface effects can be observed thanks to thermal testing. Such a test is nondestructive, quick, easy to carry out, and does not enter in contact with the targeted object (no perturbation). This type of control is mostly used in controlling electrical installations and buildings thermal insulation.

Predictive Maintenance - Magnetic-particle inspection

Magnetic-particle inspection is a non-destructive testing process that aims at creating an intense magnetic flux at the surface of a ferromagnetic material.

When a discontinuity occurs, magnetic field lines undergo a distortion, which generates a “magnetic field leak”, also called “magnetic flux leak”. A product is applied during (simultaneous technique) or after (residual technique) magnetization on the surface to be examined. It can be black and/or fluorescent, and is directly attracted to the flaw by magnetic forces in order to reveal indications.

Indications have to be observed in particular conditions, either in artificial white light or daylight, or under UV radiation (UV-A) or blue actinic light depending on the product used.

Although magnetic-particle inspection is more “restrictive” than dye penetrant inspection, it is more popular when applicable since it is way faster, among other things.

Predictive Maintenance - Leak tightness control

Leak tightness control is a non-destructive testing process that aims at detecting leaks and assessing their importance in terms of flow or flux. The lack of material that makes the leak possible is called leak tightness defect. The leak happens when the leak tightness defect undergoes different pressures. Depending on the fluid (liquid or gas), the leak is characterized as a “flow” or as a “flux”. There are several leak tightness control tests: the most common one is the “bubble test” that aims at immersing the pressurized piece into a container of detection liquid (global control) or spraying it with a surface-active solution (local control).

Bubbles or foam originating from any flaw shall be interpreted as a leakage. The main advantage of this method is that it highlights minimal flaws in the µm range, but its main drawback is that the results are linked to the environment and to the piece preparation; also, it requires chemicals and compressed gas.

Predictive Maintenance - Eddy-current testing

This NDT method consists in creating electrical current induced by a variable electric field within electrical conductor materials using a sensor. These currents called eddy-currents or Foucault currents flow locally within the material. Their distribution and repartition depend on the magnetic field excitation, the geometry, the electrical conductivity characteristics and the magnetic permeability of the piece. If the controlled piece is abnormal, moves are disrupted and lead to visible impedance variations of the sensor that depend on the nature of the anomaly and on its volume dimensions.

Impedance variation analysis provides useful indications to carry out the test. Collected signals are interpreted by comparing signals from the controlled material with those from a test block containing artificial defects simulating the investigated phenomenon.

Eddy-current testing is very popular since it offers different possibilities thanks to its detection sensitivity; it is also very easy to automate. The main advantages of the eddy-current testing are the lack of contact between the probe and the piece to be controlled, the high-speed process, and the easy process integration into the production line.

To conclude, the NDT sector is currently evolving and getting closer to instrumentation: nowadays, detecting flaws is no longer sufficient; it is also necessary to characterize them and to size them. New NDT techniques and processes to detect property irregularities have to be devised, such as metal microstructure variations, texture or roughness variations of the surface, electromagnetic property variations, etc.

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Some of these tests are obviously quite expensive. They are usually used by manufacturing sectors with high production costs, and they are also used on machines with a critical operational reliability (bottleneck machines in production workshops, compressors in leather factories, cupola furnaces in foundries, etc.). The Analytics tool available on the Mobility Work CMMS will enable you to detect which equipment require this type of maintenance.

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