October 21, 2026 · Indumor
A steel can be strong and brittle at the same time.
It sounds like a contradiction, but it is not. Strength tells you how much load a part carries before it yields or breaks when the load rises slowly. Toughness tells you how much energy it absorbs before breaking when the load arrives suddenly, and above all when there is a notch — a defect, a corner, a weld — where a crack can start. Two steels with the same strength can have very different toughness. And toughness, unlike strength, changes a great deal with temperature.
Common structural steels have a transition: above a certain temperature they break in a ductile way, deforming and absorbing a lot of energy; below it, they break in a brittle way, with almost no deformation, like glass. That transition temperature depends on the chemistry, the thickness, the manufacturing process and the welding. A structure that works happily in summer can become vulnerable in a hard frost, under the same load.
A brittle fracture gives no warning. There is no prior deformation anyone could see, no noise, no time to unload the structure: the crack runs across the part at high speed. That is how, in the 1940s, several of the Second World War's welded ships broke apart, in cold water and with steel that had not been chosen for toughness; the story is told in how electric welding was born. In the 1950s, after those investigations, the Charpy test became a specification requirement. Today it is routine.
The Charpy test measures that toughness. A 10 × 10 × 55 mm specimen with a 2 mm V-notch is brought to the test temperature and struck with a pendulum. The energy it absorbs as it breaks, in joules, is the result. Three specimens are tested and averaged, with an individual minimum. The standards that describe it are ISO 148-1 and ASTM E23.
That is where J2 comes in. In the European standard EN 10025-2, the name of a structural steel states its guaranteed toughness. In S355, the 355 is the minimum yield strength in megapascals, and the letters that follow state the Charpy test:
The result reads as an average with a floor. For a 27 J requirement, the average of the three specimens has to reach 27 J, and one single value may fall below it, but not under 70% of the required value. In thin plate, where a 10 mm specimen does not fit, sub-size specimens of 7.5 or 5 mm are tested, with proportionally reduced requirements. Besides the joules, the lab can report the fracture appearance — how much is fibrous and how much crystalline — and the specimen's lateral expansion, which say the same thing in other words.
For lower temperatures there are normalized or thermomechanically rolled grades with their own letters, such as NL and ML, tested at −50 °C. In ASTM standards, on the other hand, steels like A36 or A572 Grade 50 do not include the Charpy test by default: it is ordered as a supplementary requirement.
When should you specify it? When the structure will work outdoors in cold places — Patagonia, the high mountains, the northern plateaus —; when it takes impact loads or works in fatigue, like a crane runway; when the plates are thick, because thickness raises the risk of brittle fracture; and when there are many welds with little freedom to move. And thickness rules: at the same grade, a thick plate turns brittle at a higher temperature than a thin one, which is why EN 1993-1-10 has a table that, by grade, service temperature and stress level, sets the maximum allowable thickness for each subgrade. A J2 instead of a JR costs a little more, and in a hard frost it saves your bacon. And if the project calls for it, the requirement does not stop at the material: welding procedures are also qualified with Charpy tests, in the weld metal and the heat-affected zone.
To verify that the steel that arrived is the steel that was ordered, the Charpy result has to be on the material certificate, with the test temperature and the values of the three specimens. If the job is for high-altitude mining, it is also worth reading what changes when building for the mountains.
One last piece of advice from the Indumor shop floor: toughness is specified on the drawing, not assumed. A steel without a Charpy letter can be excellent, but nobody guarantees it; and in the cold, what is not guaranteed does not count. Ask for the letter on the purchase order, and check it on the certificate.
Tell us where it is going and we will tell you which steel grade and tests to specify.
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