Insights into materials testing: Fractography of a tensile sample
7. December 2023
Fig. 1: Image of a shrinkage cavity approximately 3×2 mm in size at 35× magnification.
An example from the work of our testing laboratory and inspection body shows that a closer look is sometimes needed to understand how a material actually behaves. In this case, fractography – the analysis of fracture surfaces – explained an unexpected anomaly in a tensile sample.
An unusual sample in a tensile test
A B10×50 round tensile sample made from spheroidal graphite cast iron (EN-GJS-500) showed a significantly reduced elongation after fracture of A = 2% in the tensile test. Examination under the stereomicroscope and scanning electron microscope (SEM) revealed the cause: a macroscopic shrinkage cavity measuring approximately 3×2 mm in the fracture surface.
Under the microscope, this shrinkage cavity can be recognised by its strikingly smooth, rounded surface structures. These form when the previously molten material solidifies freely.
Interpreting the findings through fractography
In addition to the reduced cross-sectional area, the notch effect of the shrinkage cavity was the main reason why part of the fracture surface failed in a brittle manner. This region can be recognised by its mirror-like, sharp-edged cleavage facets, a typical feature of brittle fracture. The remaining, unfortunately smaller, portion of the fracture surface shows ductile overload fracture, also known as dimple rupture. The name comes from the honeycomb-like structures formed by microscopic plastic deformation of the material.
Graphite nodules in the fracture surface
Both fracture regions also clearly show the black graphite nodules characteristic of spheroidal graphite cast iron. They contain much of the high carbon content of this material. Closer examination reveals that some nodules appear to be missing. The explanation is simple: they are on the corresponding opposite fracture surface.
Conclusion: Fracture surfaces tell a story
This case clearly demonstrates how fractographic examinations can provide valuable clues to a material’s failure behaviour. Even relatively small defects, such as the shrinkage cavity found here, can significantly influence fracture behaviour.
Combining mechanical testing, microscopy and fractography allows such causes to be reliably identified and interpreted.
