Published on 15-Feb-2024

Non-Destructive Testing Of Non-Metallic Materials

Non-Destructive Testing Of Non-Metallic Materials

Image Credit @ Element Materials Technology


The use of non-metallic materials dates to 100,000 BC, when Stone was the primary material. This brittle, un-mouldable material, set the precedent for the advent of technological advancement and led to the use of more materials. Experimentation with clay and heat eventually led to the use of ceramics in 6000 BC and by 4000 BC, primitive ceramic manufacturing units were formed as the material gained popularity.

Materials like glass began to be produced after 2000 BC. Glass was created by the heat treatment of a mixture of sodium, quartz, and lime. Glass is also brittle in nature and is formed by hot molding. Fiberglass was not created and popularized until around 1931 AD, during the concurring period of the Iron Age. Bakelite was used as a matrix to create fiberglass during that period.

1950 AD marked the advent of Plastics being manufactured and widely used. Plastics by nature are coldly mouldable, which means they can be molded using lower heat levels. They were easy to machine and cut and are brittle at low temperatures. This period also marked the production and popularization of polymers on a large scale. The matrix for fiberglass was hence replaced which polymers, which widened the application of fiberglass to the field of fiber optics.

Various methods of laying up matrix and reinforcements, and a variety of materials like Epoxy resins, Carbon fibers, and Kevlar fibers opened as potential avenues to be used in composite materials. Epoxy resins are a type of thermosetting polymer that was discovered in 1930, first available in a solid form, but eventually grew to be produced as an adhesive.

Carbon fibers are filaments of carbon that possess high strength and consist of carbon atoms bonded in long chains. Kevlar, discovered in 1965, is a plastic that is manufactured from a compound called poly-para-phenylene-terephthalamide. Kevlar is considered one of the strongest fabrics available and is five times stronger than steel. It has a high heat resistance and possesses a high tensile strength-to-weight ratio.

Silicon carbide (SiC) gained popularity post-1970 AD. SiC is considered one of the hardest materials known to mankind, second only to diamonds. SiC is a compound formed between the hard covalent bonding of two atoms out of the four carbon atoms and one silicon atom present in the material. Silicon carbide has good thermal conductivity, a low coefficient of thermal expansion, and high corrosion and oxidation resistance, making it a viable option for modern machinery and technology.

Superconducting ceramics gained momentum around 1984. Superconducting materials can conduct direct current without any energy losses, provided they are cooled below a certain temperature. High Tg superconducting ceramics can operate at higher temperatures, however, they are brittle in nature.

Non-metallic materials, especially composites are proving to be the materials of the future because of their resistance to corrosion and oxidation, lightweight, and high-strength properties. Failure in non-metallic materials often occurs due to hidden defects and deformities.

As materials such as polymers, ceramics, fiber-reinforced composites, etc. gain momentum in the industries the need for accurate and convenient testing has become vital. 

Non-destructive testing aids in assuring the quality of the materials while under manufacturing, in operation, or post-failure without changing the physical state of the material. This saves resources and provides efficient data on product quality and ensures conformity of product performance to industry standards.


Difference Between Non-Metallic Materials & Metallic Materials

Metallic and non-metallic materials are set apart by the following factors:

Insulation:

  • Non-metallic materials are insulating in nature, whereas metallic materials conduct heat and electricity very well. This makes non-metallic materials more feasible for electrical applications.


Cost:

  • Non-metallic materials are quicker to produce and are cost-effective. They are produced at a higher rate than metallic materials. Due to their lower weight and resistance to friction, they endure less damage over time and cost less to repair and maintain.


Corrosion Resistance:

  • Non-metallic materials are resistant to corrosion, heat, and chemical degradation. Metals however need extra measures like coatings to protect them from damage due to corrosion, heat, and chemicals.


Material finishing processes:

  • Metallic materials require machining post-treatment to upgrade the insulation levels of the material. Plastics, however, do not require any finishing and painting and hence save time and resources.

Also Read, What is Pipeline Isometric Drawings

Types Of Non-Metallic Materials & Their Properties

Non-metallic materials have become an integral part of the energy production industry and manufacturing processes. Components like pipes, non-metallic vessels, material packaging, joints, etc are popularly manufactured using non-metallic materials. Non-metallic materials are also utilized in producing thermal insulation, electrical insulation, and various support elements. 

Applications like Ryertex-based products, Seals, gaskets and pipe seals, and thermal insulation make non-metallic materials versatile and accessible. Popular non-metallic materials include the following: 

Ceramics: 

  • Humanity has been utilizing ceramics for ages, creating it by heating clay. Clay is used for applications where the material is required to have electrical resistivity, and high compressive strength, where low dielectric loss is required, in condensers, high voltage devices, for support, insulation, and fatigue-resistant elements.

Ceramic behavior is determined by the composition of the ceramic, its porosity, density, its grain size, and the types of ceramics mixed.


Polymers:

  • Polymers are versatile materials that are used for the impregnation of materials, such as the matrix in composite materials where fibres are used as reinforcement, as efficient thermal and electrical insulators, for damping forces and to avoid heat dissipation.

When polymers cross the glass transition temperature, the modulus and strength of the material decrease, thermal expansion increases, and mechanical and dielectric damping increases at the glass transition temperature.

Polymers are bound between the two forces of strong covalent bonds within the polymer chains and the weak Van der Waals forces between the polymer chains.


Fiber composites:

  • Fibre composites are composite materials that can be of four varieties, that are metal matrix composites, ceramic matrix composites, carbon/carbon composites, and polymer matrix composites. 

Fiber composites are beneficial for constructions with lower weights, building products with high stiffness and tensile strength, and fatigue-resistant products. Fiber composites are also used in hydrogen tanks to build the cryostat, to provide electrical and thermal insulation, to build compensation elements that have thermal expansion in negative values, and for zero-expansion elements.


Functional Non-metallics:


Superconducting ceramics:

  • Superconductor ceramics are used in nuclear magnetic resonance, mass spectrometers, and particle accelerators to create high amounts of magnetic and electric fields to manipulate beams and accelerate them.


Conducting polymers:

  • Conductive polymers include materials like polyacetylene, polyaniline, polypyrrole, polythiophene, poly-para-phenylene, polyfuran, and poly(phenylenevinylene). These polymers have alternate single and double bonds in the backbone of the chain. 


Conducting carbon fibers:

  • Carbon fiber is a material with the main component as carbon. These materials have high electrical conductivity. Carbon fibers are of three types, mainly Rayon carbon fibers, PAN-based carbon fibers, and isotropic pitch-based carbon fibers. Doping (adding impurities to the material) of carbon fibers with bromine or fluorine increases its conductivity.


NDT Methodologies Used For Testing Non-Metalic Materials

The field of non-destructive testing has adopted multiple advanced techniques, with the escalation of use and varieties of non-metallic materials because of their unique properties and advantages. 

Their differences from standard, metallic materials make these materials ineligible to undergo specific non-destructive processes. However, due to the adaptability of non-destructive testing technologies, multiple testing options have been made available for non-metallic testing. A few of those methods are as follows:

1. Spectrum Evisive Scan:

This non-destructive testing method uses microwave interferometry to analyze objects under test. It can be used to detect cold fusion in HDPE welds, delaminations, misalignments, foreign material inclusions (in PE pipe fusion welds and panel backing materials), and voids caused due to improper mixing, deficiency in glue, lack of contact and the presence of air bubbles). The exposure levels caused due to microwaves are not hazardous and do not need additional precautions and are hence safe for operator handling.

2. Optical Fibre NDT:

The use of optical fibers to assess civil structures and composite materials to detect defects, internal stresses, curing, and cracks has increased in popularity. Devices using this technology can be mounted on devices through operation to give a constant study of their performance and integrity. This process can also be used on automated mechanisms.

3. Infrared Thermography:

This method uses thermal imaging devices to detect emitted infrared radiation. This expensive apparatus uses a thermal camera to detect atypical heat patterns in operating machinery and helps detect defects and deformities, hence determining if the machinery is fit for use.

4. Microwave Test Method:

This method of non-destructive testing helps study the material composition, mechanical energy propagation, and nature of critical defects. Microwaves are induced on the test subject and reflected energy along with the transmitted signal creates an interference pattern. This pattern creates a measurable voltage at each receiver in the apparatus which can be used to form an image to analyze potential defects and deformities.

This method can be used to examine fiber-reinforced resin composites, polyethylene, and reinforced rubber components, hence being an available tool in the process and fuel extraction applications.

5. Differential Scanning Calorimetry:

This method is utilized to detect the composition of test subjects, which are generally non-metallic materials, and provides its melting temperature and glass transition temperature. This is a thermoanalytical tool that examines the uptake of heat energy of the test subject.

6. Electron probe micro analyzer (EPMA):

This method of NDT is used on small volumes of solid materials to examine their chemical composition. 

7. Near-infrared spectroscopy (NIR):

This method provides an in-depth analysis of a test subject, preferably a non-metallic material with the combination of the intensity, width, and peak of the infrared spectrum.

8. X-ray fluorescence spectrometry (XRF):

This method of non-destructive testing gives a thorough study of the chemical composition of the test subject by rapidly determining multiple elements simultaneously.

9. Ultrasonic Testing:

This is one of the most widely used methods of non-destructive testing and uses transducers in housing to induce ultrasonic waves to detect defects and deformities in an object under test. 

Material thickness, voids, porosity, delamination, and cracks are detectable by Ultrasonic Testing. It applies to most materials and can be easily automated.

10. Ultrasonic C-scan​:

The test subjects in this non-destructive testing method are immersed during analysis. It can scan surfaces at different angles and test procedures can be easily programmed and automated. It covers a large test area and can be used in large-scale testing of materials.

Material thickness, voids, porosity, delamination, and cracks are detectable by Ultrasonic C-scan. Like Ultrasonic Testing, it applies to any material, however, it requires immersion of the test subject.

11. Scanning Acoustic Microscopy:

This is another variation of ultrasonic testing methods that scans the test subject under immersion. It can scan minute defects because of its smaller scanning bed. Unlike Ultrasonic C-scan, this method is limited to small-sized test subjects, but its feature of scanning the subject in slices or sections allows for the detection of otherwise hard-to-detect deformities.

Material thickness, voids, porosity, delamination, and cracks are detectable by Scanning Acoustic Microscopy. Like Ultrasonic C-scan, it applies to any material, however, it requires immersion of the test subject and limits the size of the test subject.

12. X-ray computed tomography (XCT) and radiography:

This methodology is used to analyze three-dimensional volumes. Its main application lies in the field of medicine. In this method of non-destructive testing, the scanner rotates around the object under test and captures multiple two-dimensional radiographs, which are combined to create a three-dimensional image.

The thickness of walls, porosity, and voids can be detected using this method.

13. Laser Shearography:

This non-destructive testing method helps test materials under stress for the rate of change of the displacement in its plane. A camera is used to analyze the surface with the help of a laser pattern. This method can be used on a subject under operation or in a laboratory by inducing a load using a vacuum hood or heating lamps.

This method is used to test composite materials and can detect voids and delaminations.

14. Digital image correlation (DIC):

This non-destructive testing method helps determine the strains, Young’s modulus, and Poisson’s ratio for composites and other materials. This method is optical in nature and is no contact, gives precise results, and is non-interferometric.

This process can be highly automated and can be designed to detect deformities in a single run. Digital Image correlation can be used in construction and nuclear engineering and can be used for macro and micro-operations.


Industry Standards For testing Of Non-Metallic Materials

Some of the industry codes and standards that regulate the quality and testing processes of non-metallic metals are as follows:

American Society of Mechanical Engineers- NM.3.1 Part 1 – Thermoplastic Material Specifications

American Society of Mechanical Engineers- NM.3.2 Part 2 – Reinforced Thermoset Plastic Material Specifications

American Society of Mechanical Engineers- NM.3.3 Part 3 – Properties

American Society of Testing and Materials- D792 - Standard Method for Density and Specific Gravity (Relative Density) of Plastics by Displacement

American Society of Testing and Materials- D638 - Standard Test Method for Tensile Properties of Plastics


Conclusion

Accurate determination of the properties of materials is vital to any process or field of engineering. A thorough analysis of material properties before, during, and in between operations helps understand the structural and chemical integrity of the material and helps the operator and organization assess its fitness for operation.

Advancements in materials science and engineering will lead to the formation of more sophisticated and adaptable materials, and non-destructive testing will prove to be the testing method of the future, as it ensures speed, efficiency, and sustainability, and can accommodate automation.




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