Tensile test in thickness direction – the Z-test simply explained

When thick sheets are welded or subjected to heavy loads, unfavorable conditions can lead to so-called terrace fractures – brittle cracks across the sheet surface. To minimize this risk, there is a special material test: the tensile test in the thickness direction, also known as the Z-test.

What is a Z-test?

In the Z-test, a tensile test specimen is drawn perpendicular to the rolling direction (i.e. through the thickness of the sheet). The aim is to evaluate the deformation capacity perpendicular to the sheet surface. In contrast to classic tensile tests, the yield strength or tensile strength is not of interest here – the decisive factor is the fracture constriction value Z [%]. This indicates how much the material was able to constrict in the fracture area – the higher the value, the better the plastic behavior in the thickness direction.

What standards are there?

The two most important regulations are

• EN 10164 – for European applications, with classes Z15, Z25 and Z35.

Three individual values from three tests and an average value are shown. One individual value may fall below the required minimum value. Test from a material thickness of 15 mm.

• ASTM A770 – mainly used in international plant and mechanical engineering, with Z classes from Z20 to Z35.

Six individual values are determined. Three from each side of the board (head and foot). The test frequency is by roller panel.

The classes indicate the required minimum value of the constriction at break.

When is a Z-test necessary?

A Z-test is not part of the standard material test, but must be explicitly ordered. It is useful or prescribed in the following cases:

• For welded constructions with thick sheet metal (e.g. beams, pressure vessels)
• Eurocode 3 (EN 1993): Design and construction of steel structures
• For use in nuclear technology or railroad infrastructure (DBS 918 002-02)
• If there is a risk of center segregation (concentration of inclusions in the middle)

What influences the test result?

Good Z-values are favored by high-quality steel production:

• Low sulphur content
• Calcium or magnesium treatment to control inclusions
• Vacuum degassed and fully killed
• Alloying with fine-grain alloying elements
• Low sheet thicknesses – high degree of deformation
• Uniform heat treatment

Conclusion

The Z-test is an important tool for increasing the reliability of steel structures under complex loads. Whether for bridges, crane runways or high-rise buildings – where safety counts, a tested Z-value ensures additional safety in the construction.