Failure of screwed joints

Part 1: TYPOLOGY

The failure of a bolted connection (also called "failure") can occur by :

 

1/ Sudden failure

 

2/ Progressive failure (Fatigue)

 

3/ Hydrogen embrittlement

4/ Corrosion

1: SUDDEN STATIC RUPTURE

Excessive tensile stress

 

Stresses are evenly distributed in the section

 

net failure perpendicular to the axis of the screw shank (brittle failure)

 

fracture with initiation zone and final fracture at the periphery with the presence of shear lips (ductile fracture)

 

failure with formation of radial lines due to the presence of hardening taps (semi-fragile failure)

 

Unless there is a metallurgical defect, a screw breaks at the first thread in engagement Example: 10.9 screw broken during tightening (overtightening)

Example: Sudden semi-fragile failure: location of the origin of the failure

 

Excessive torsional stress

 

Stresses are distributed in the section with a maximum at the periphery and a minimum in the core.

Significant plastic deformation can be observed (twisting effect) with the presence of a vortex fracture surface




2: FATIGUE FAILURE

Rotational bending

 

Typically, this type of failure is characterised on the screw section by :

 

  • multiple peripheral initiators with ridge or radial lines

 

  • front lines or stop lines

 

  • two identifiable zones: a zone of fatigue propagation and a zone of sudden final failure

 

Plane Bending and Alternate Plane Bending

 

In the case of plane bending, the initiation is at a single point.

 

In the case of alternating plane bending, the initiation occurs at two diametrically opposed points









Influencing factors

 

Fatigue failure is characterised by a notch effect, with initiation generally occurring on the surface.

 

The geometry of the part is therefore a determining factor and variations in cross-section should be avoided (limit grooves, increase fillets, etc.)

 

The surface of the final fracture zone is representative of the static equilibrium of the assembly, as it is obtained by exceeding the residual mechanical characteristics.

The machining zones are potential initiation zones; a rolled thread is more efficient than a cut thread, from the point of view of fatigue resistance.

3: HYDROGEN EMBRITTLEMENT

Principle and origin

 

Mainly during surface treatment (or during welding) there is absorption of hydrogen on the surface and then diffusion which causes a loss of cohesion of the metal network.

 

During an electrolytic surface treatment, the risk appears in different phases:

 

Pickling

 

Cathodic degreasing

 

Electrolysis

 

Fracture is of the delayed type with a fragile character.

 

The risk increases with the mechanical characteristics. Failure occurs during tightening or a few hours after commissioning, even in the absence of an additional external load.

 

Prevention is achieved by a degassing heat treatment, which must be carried out within four hours after leaving the electrolytic bath. See NF EN ISO 4042, NF EN 12329 and NF EN 12330)

 

1/4 : CORROSION

Corrosion pitting is the cause of this type of failure, and the surface condition determines it. The surface defect becomes a stress concentration zone that can cause failure.

Part 2: MAIN CAUSES

1: POSITION AND TYPES OF BREAK

Fracture in nets often occurs at the first net in engagement as this is an area of maximum stress concentration.

Under-head failure is mostly the result of an insufficient connection radius.

 

Fracture in the shank, which is very rare, mainly concerns screws with a reduced shank or fusible screws.

2: CAUSES OF SUDDEN BREAKAGE

3: CAUSES OF FATIGUE FAILURE

4: ROOT CAUSES AND PREVENTION

Apart from the emblematic phenomenon of hydrogen embrittlement, almost a third of ruptures are due to assembly defects! They can therefore be avoided without redesigning the assembly.

 

Some recommendations:

  • Qualify the quality class of the screw

 

  • Choose a nut compatible in quality class with the screw

 

  • Avoid heterogeneous assemblies (galvanic corrosion effect)

 

  • Ensuring clamping with pre-tension

 

  • Ensuring that the prestressing is maintained: for example, a locknut prevents loosening but does not ensure that the prestressing is maintained

  • Choose the right washer and avoid crushing it

(Source : étude CETIM)