Physical testing remains vital for product development

The use of software and virtual tools within engineering have become increasingly commonplace over the last 10 years. Now, everything from concepts to materials to analysis is done virtually, via a computer.

However, this approach is not without its limitations. Let's not forget that ultimately it is how a product behaves in the real world, in its operating environment, that will be the marker of its success or failure. It is therefore no surprise that there is still high demand for physical testing of materials and products.

Pulling material samples apart and seeing how they have failed, physically, tells a story. And it is this aspect of 'real' testing that can only be interpreted by experienced people, which makes it such an essential tool for material analysis and product development.

The work done by testing laboratories generally falls in to two main categories. The first is testing a material for compliance, making sure something meets quality standards, conforms to regulations, and is made to the standards it needs to be. The other is that of material analysis. This can be used to influence and improve products from design through to manufacture and use.

Richard White, head of testing at Stafford based experts in materials testing and analysis Ceram, says: "We tend to be involved when there are issues around materials, analysing to particular standards as well as improving products. We might find an area for improvement is in the way the materials are used, the way they are incorporated in to a design, their intended use, or the way they are being processed."

Every material has inherent advantages and disadvantages. The knack is to maximise the potential of a material by exploiting its inherent advantages whilst minimising the disadvantages. Subtle nuances in material types or production methodology can make a big difference to real world performance that can go unpredicted if not properly assessed. Thorough and representative environmental testing nearly always shows up something surprising or unexpected.

"A big area we work within is surface analysis," says White. "That capability enables us to look at the outer layers of atoms on a surface. If something has broken or split, it is the surface that is the issue, not necessarily the bulk material. We can focus in and get that very specific information to help us understand the problems."

US aircraft manufacturer Goodrich was looking to develop a chromium coating for use on aircraft turbine engines to provide superior corrosion resistance. The oxidation of chromium in such coatings is functionally important, and for such safety critical use it must be administered correctly.

Oxidation is obtained by carefully controlling the curing of the coating. This also helps to improve the bond strength to the substrate. These aspects are vital in ensuring superior corrosion resistance.

Goodrich approached Ceram with a request to measure the concentration of oxidation in a chromium test coating in support of its development efforts. The challenge was to establish a surface sampling technique which could assess the chromium, apply a sufficiently sensitive technique to detect the anticipated levels of chromium in potential oxidation states, and ensure the technique quantitatively distinguished between the oxidation states present.

Ceram and its team of experts used X-ray Photoelectron Spectroscopy (XPS) to carry out the measurement. XPS is a surface specific technique that samples the coating to a depth of up to 10nm to generate quantitative elemental composition including oxidation state differentiation.

By using the technique in its highest resolution, Ceram was able to supply analytical results on the levels of each chromium oxidation state to an accuracy of 0.1%. This result was achieved in the presence of a wide range of other metallic and non-metallic materials.

The tests saved Goodrich's investment in a potentially non-fruitful area of research and development. In addition, Goodrich received further technical proposals from Ceram on the possibility of carrying out similar work on the sub-surface layers of the coating to establish oxidation state depth profiles within the coating itself.

Equipment
Testing is also a critical issue for plastic compound and masterbatch producers, which are increasingly being asked to test thermoplastics. Since thermoplastics do not have a definite melting point that precisely marks the transition from solid to fluid, analysis of the slow softening as temperature increases, is an important element in determining quality and performance.

"The trend toward stronger materials means serious consideration regarding this aspect of the equipment specification needs to be covered," says Alan Thomas, marketing manager at Zwick. "It is therefore important to ensure that customers purchase a testing machine with adequate force capacity to accommodate both their current and future testing requirements."

Two key tests for thermoplastics are measuring the Vicat softening temperature and heat deflection temperature (HDT). The Vicat softening temperature describes the point at which a 1mm² circular indenter or cylindrical punch is able to penetrate 1mm into a sample at loads of 10N and 50N.

HDT is the temperature at which a standard test bar deflects a specified distance under load. It is used to determine heat resistance and distinguishes between materials that can sustain 'light' loads at high temperature against those that lose rigidity over a narrow temperature range.

"Experts agree that automation is becoming increasingly important in plastic materials testing," says Thomas. "Automated features, such as sample handling and data collection improve laboratory efficiency, increase accuracy and reduce test variability."

Zwick has been keen to enable this function and has produced its Aflow extrusion plastometer designed for high throughput. It is used to determine the melt mass flow rate (MFR) and melt volume flow rate (MVR). To improve speed and reliability it uses parameter control and force control that automatically adjuststest loads up to 50kg. Automatic parameter control is particularly useful for polymers with an unknown MFR, where operator influence can lead to reductions in measurement accuracy.

Author
Justin Cunningham

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