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Non-Destructive testing

Non-destructive testing is the inspection of a component in a manner that will not make it unsuitable for further service. This testing may be carried out at the time of production or when the component is in service. Each method is suited to detecting particular faults and may be suited to designing of a component or as a quality control measure. Many components used in Personal and Public Transport are subjected to non-destructive testing techniques tram and train tracks, suspension components of busses trains and motor bikes, laminated flooring of busses.

Outcomes

This material addresses aspects of the following syllabus outcomes:

A student:

H1.2 differentiates between the properties of materials and justifies the selection of materials, components and processes in engineering.

H2.1 determines suitable properties, uses and applications of materials in engineering

Source: Board of Studies (1999) Stage 6 Engineering Studies preliminary and HSC courses. Sydney: Board of Studies

Visual inspection

Visual inspection is the most common and simplest non-destructive testing method; it is limited to detection of surface flaws and faults. Railway tracks are often tested this way.

he test involves a close visual inspection of a components surface, the surface may be illuminated or the inspector may use a magnifying glass, mirror or other optical aid in order to gain accurate results.

Visual inspections are simple to carry out at minimal cost. Accuracy depends on the skill, experience and knowledge of the inspector.

If a permanent record is required, a photograph or videotape or inspectors report may be kept. Recent advances in digital photography make this recording an excellent method.

Liquid penetrant testing

he test specimen is first thoroughly cleaned and dried before the test. A liquid penetrant is applied to the surface; spraying, dipping or brushing may do this. Over a period of time the liquid penetrant is drawn into any surface faults by capillary action, any excess liquid is removed. Depending on the process being used, the surface is coated with whiting or a developer. Faults open to the surface will appear as a discoloured line in the whiting. Fluorescent or coloured dyes drawn into the faults are readily seen under ultraviolet light or as a line in the developer.

Liquid penetrant tests are simple, versatile, portable and inexpensive. The results are easy to interpret but only surface faults can be detected. If a permanent record is required a photograph or videotape or inspectors report may be kept. The use of laser scanners and digital control allows this process to be used as a mass production technique.

Magnetic particle testing

agnetic particle tests are suitable only for ferrous metals capable of being magnetised. Almost any size or shaped component can be tested.

he test specimen is first thoroughly cleaned and dried before the test.
When magnetised ferromagnetic specimens have a distorted magnetic field in the region of the fault or defect. This distortion can be seen with the application of magnetic particles as a powder or suspended in a liquid. These particles are often coated in a fluorescent material enabling inspection under ultraviolet light. The flaw can be seen as a disturbance in the flow lines.

ults perpendicular to induced field are easily detected, whereas faults parallel to the induced field may be misinterpreted. To avoid this, inspections and magnetisation, should be carried out from different orientations.

If a permanent record is required a photograph, videotape or inspectors report may be kept. Specialised techniques of recording the defect patterns are also available.

Acoustic monitoring

coustic monitoring is used to detect growing or active faults. When materials are deformed, stressed or undergo some permanent deformation such as a crack propagation, they emit very high frequency sound waves. These acoustic emissions can be detected with the use electronic equipment such as transducers, microprocessors and amplifiers. These acoustic emissions indicate any dynamic change within the material.

This test can be done on any material but is particularly useful in detecting delamination in layered materials and fibre failure in composite materials. It can be used as a monitoring system and with the use of multiple sensors can be used to accurately determine the location of a fault.

X Ray testing

X Rays can be used for detection of internal flaws and faults in most engineering materials. X ray testing is expensive due to the costly equipment, film and processing required. There is also the need for the implementation of specialised safety equipment and procedures.

Complex shapes require examination from two different angles.

X ray or Gamma radiation is passed through the test specimen and then recorded upon a photo-sensitive film. The flaws or defects are indicated as dark areas on the film because faults absorb less radiation than the material itself.

Due to its expense X ray detection is generally used during product development, or in laboratory testing.

Gamma radiation is suited to field or on site applications as less complex equipment is used.

Ultrasonic testing

Ultrasonic testing offers immediate results and a high degree of accuracy for cracks and internal faults such as gas porosity. The test is suitable for metals, plastics, glass, concrete and ceramics. Components that are thin, small, have complex shapes or have rough surfaces are difficult to test.

Ultrasonic testing involves sending high frequency vibrations (100 kHz to 200 kHz) through a material and sensing their reflections. The high frequency vibrations are produced by a transducer, which uses a piezoelectric crystal to convert electrical oscillations into mechanical vibrations. The transducer is placed on the surface of the material to be tested. Vibrations penetrate the material and are refracted and reflected at discontinuities within the material. Another transducer picks up the reflected signal which is displayed on an oscilloscope. The resulting reflection indicates the internal integrity of the test specimen. Flaws are shown as a peak, the size of the peak indicates the size of the fault.

Ultrasonic signals can be recorded if a permanent record is needed. Ultrasonic testing is also used in high-speed automated productions eg, railway tracks.

Digital technology

With the rapid development of computer technology in terms of scanning and imaging techniques, digital photography and computer control may non-destructive testing methods can be incorporated into the manufacturing process. These technological advances have greatly improved quality control and enabled the testing to be carried out at high speed with great accuracy.

Many of these testing techniques can be carried out on site due advances in computer technology that have enabled the miniaturisation and portability of the equipment.

Students may like to investigate other methods of non-destructive testing such Strain Testing, Leak Testing, Thermal Techniques and Eddy Current Testing.

Internet resources

Non Destructive Testing Australia

Past HSC question

Question 15 c

Age-hardened aluminium alloys are used in aircraft construction.

Name a specific alloy and discuss the suitability of the alloy for this application.

Identify and describe a non-destructive test method that would be appropriate in inspecting for cracks.

Sample answer

Duralumin (or 96% Al/4% Cu or 2xxx alloy) is an alloy used for this application.

It has a high strength to weight ratio compared to other materials (eg steels); this saving in structural weight can be allocated to greater load carrying capacity. Components can be easily formed in the soft condition and then strengthen as the age-hardening process occurs over time.

Aircraft are subject systematic visual inspection by both pilot and maintenance workers. The pilot and/or maintenance workers walk around the aircraft looking for any signs of cracks or any other damage particularly in known fault prone areas.

Standards Package CD for Engineering Studies available in all schools. Samples should be read and studied in conjunction with the Glossary of Key Words and Examiners comments .