Beyond surface hardness measurements

This article was originally published in the May–June 2026 issue of Fastener + Fixing Magazine. Authored by Ali Çopuroğlu on behalf of Ticemlabs, the article examines the capabilities and limitations of surface hardness testing and discusses why hardness measurements alone may not always provide a reliable indication of the overall mechanical performance of metallic components, particularly in safety-critical applications such as anchors and fasteners.

Metal hardness testing comprises a set of standardised methods used to quantify a material’s resistance to deformation, indentation or scratching. Common techniques include the Brinell, Rockwell, Vickers and Knoop tests, each selected based on material type, thickness and required precision. Among these methods, those that are simpler to apply are generally preferred in practice, as they enable rapid execution, require minimal surface preparation, and provide direct readout, making them particularly suitable for production environments and routine quality control.

In practice, surface hardness tests play a key role in quality assurance, material verification and process monitoring. They provide fast and cost-effective insight into resistance to wear, indentation, as well as localised deformation. In addition, they are frequently used to evaluate the effectiveness of heat treatment processes, detect surface hardening, and ensure consistency, across production batches. For these reasons, testing hardness remains a fundamental tool in industrial applications.

However, while hardness is a valuable indicator, relying solely on hardness values for engineering decisions requires careful consideration. Surface treatments such as galvanisation or case hardening can significantly modify the hardness profile of a material without affecting its core mechanical properties. As a result, discrepancies may arise between measured surface hardness and the actual bulk behaviour, including strength, ductility and fracture properties. This distinction becomes particularly critical in safety related applications, such as anchors or fasteners.

Experimental studies further highlight this limitation. A study conducted at Ticemlabs demonstrated that two materials with nearly identical surface hardness values can exhibit significantly different mechanical performances. Detailed microstructural analysis using Scanning Electron Microscopy revealed that the weaker material possessed a heterogeneous cross-section, where a hardened surface layer coexisted with a core of different properties. This gradient structure led to misleading hardness readings when interpreted without considering the internal material composition.

This issue becomes even more pronounced in anchor applications. When mechanical performance or its consistency is assessed solely through surface hardness, unexpected results in pull-out tests, particularly in anchor failure modes and deviations in shear performance, are highly likely. Since surface hardness reflects only near surface characteristics, it cannot fully represent the overall load-bearing behaviour of the anchor, potentially leading to inaccurate performance assessments.

In metallic systems, processes such as hot dip galvanisation and surface hardening are intentionally applied to modify surface properties. Hot dip galvanisation introduces a zinc coating that enhances corrosion resistance and may influence surface hardness, while leaving the core material largely unaffected. Similarly, surface hardening techniques increase surface hardness while preserving the toughness and ductility of the core. These approaches are widely used in engineering design to achieve a combination of a hard, wear resistant surface and a mechanically robust interior.

In conclusion, surface hardness testing is an indispensable tool for evaluating surface related performance and supporting engineering decisions in areas such as wear resistance and process quality. Nevertheless, when hardness values are used to infer overall mechanical properties, they must be interpreted with caution. Since hardness measurements primarily reflect localised surface conditions, they should be complemented with comprehensive mechanical testing, and an understanding of material processing history, to ensure accurate and reliable engineering evaluations.