Errors during the construction phase with serious consequences

The screw head suddenly breaks off. For no apparent reason. The consequences are incalculable damages to the building.

Falling parts can endanger human lives. The cause is the so-called hydrogen-induced brittle fracture, which often occurs with a time lag after installation. Today, we know that there are three coherent factors: a critical hydrogen charge, a critical material state, and a critical stress state.

Steel profile sheets which are not exposed to the elements, may be fixed in industrial lightweight construction using screws made of case hardened steel. Under certain conditions however, there is the risk of hydrogen-induced brittle fracture, in which the screw head breaks off without a recognisable reason. Through the use of sealing washers, brittle fracture can be effectively avoided.

Industrial and commercial buildings, which are manufactured in lightweight construction, generally consist of steel skeleton constructions that are planked with trapezoidal profiles of different thicknesses. The steel profile plates are screwed directly to the steel girders with self-drilling or thread-tapping screws. Screws made of case-hardened, zinc-plated carbon steel can be used for fastening of the supporting elements and the loadbearing shell. A higher grade material, such as corrosion resistant stainless steel, is not necessary, since the load-bearing shell is usually still provided with an insulating layer and then closed with a cover shell. As a result, the screw joint is permanently protected from weather influences.

Example of an optimal fixing with sealing washer of the load-bearing shell and the cover shell

In the daily routine, many construction sites do not run as originally planned: delivery delays, changes in planning or difficult weather conditions often lead to unintentional delays between the individual construction phases. In detail this could mean, that the steel screws are weather exposed over several days or weeks, even though the material is not optimised for this. Things become dangerous when the screws have been fastened in the steel girder with a high tightening torque or are fixed at an angle. Then the failure of the screw happens without previous visible deformations or corrosion. Therefore, it is not possible to predict the time of fracture. Damages can occur months after completion of the construction work. The often serious consequences might include closed construction sites, high complaints costs and damages to the company image. In addition a danger to human life, by falling components, can not be ruled out.

The collapse of the Berlin Congress Hall is one of the most spectacular damage cases caused by so-called hydrogen embrittlement. In 1980 the 600 ton southern roof brim above the main entrance of the hall collapsed. Many people were injured and a 27-year-old man was killed in the accident. In the analysis, experts discovered a whole series of design errors in the construction.

Connection without sealing washer

Brittle fracture has occurred

Connection with sealing washer

SEM image of the fracture surface of a lowdeformation fracture of a micro screw in the region of the under-head radius.

In the detailed view of the upper picture, gaping grain boundaries, intercrystalline split fractures and zones with linear residual ductility (crow’s feet) are visible on the split fractures

How does hydrogen-induced brittle fracture occur?

Influencing factors and avoidance measures


Prerequisites for hydrogen-induced brittle fractures are
  • the presence of a hydrogen supply
      - during the manufacturing process
      - while using the screw at the construction site
  • the material condition
  • the mechanical loading of the screw

The risk of a hydrogen-induced brittle fracture is only present when all three parameters are critical.

Optimisation in the manufacturing process

(at the EJOT plant)
 

The risk of the hydrogen-induced brittle fracture is reduced or can be detected early, amongst other things, by the material selection, exact process parameters and production-accompanying tests. In the case of galvanically applied zinc layers, however, the risk potential of a hydrogen-induced brittle fracture can never be excluded 100 %, according to the state of technology (for example, DIN EN ISO 4042). In addition, there is usually no conclusion possible about the exact supply of hydrogen in operation.

In simple terms, the release of hydrogen is minimised in the finishing process during surface treatment. The heat treatment ensures optimum core and edge hardness as well as sufficient toughness of the material. In addition, the hydrogen loading is further reduced by hydrogen annealing. Before delivery, the screws are subjected to a load test to minimise the risk.

Optimisation of the processing at the construction site

An efficient measure to reduce the risk of hydrogen-induced damage is the use of sealing washers. EJOT offers versions with pre-assembled sealing washer for all self-drilling and self-tapping screws of the JA, JZ and JT families..

Increased safety during installation

Sealing washers reduce the mechanical load and limit pretension force or tensile stress of the screws. The visible compression of the sealing washer during fastening allows a controlled tightening of the self-drilling screw. Tearing off of the screw head or stripping of the screw thread during installation is effectively avoided.

Increased safety after installation

Surface protection layers of the profiled sheets, which are damaged by the fastening process, can be safely covered with the sealing washers. The vulcanised plastic is characterised by high weather and UV resistance. This way the screw connection point is protected from weather influences until completion of the construction work. Due to the enlarged contact surface of the sealing washer, the values of the pull-through capacity are also increased.


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