Detection process of screw fracture

Update:02-11-2020
Summary:

Composite slot pan head screw, material is AISI 1022A, […]

Composite slot pan head screw, material is AISI 1022A, specification is 10-32×0.5″, surface is silver-white electro-galvanized. The screw fails to reach the specified torque value when it is installed, and it breaks. Take 1 piece of fracture failure and uninstalled pieces in the same batch 3 pieces, 3 pieces of different qualified batches, for analysis.

Test process and results

The fractured parts of the failed parts are the joints between the head and the rod, and there is no obvious necking phenomenon, and the fracture is characterized by torsion fracture morphology

The chemical composition of the broken parts is tested, and the results meet the requirements of AISI 1022A material standard

The hardness of the broken parts was tested and the results were HRC20-21.

The fracture observation shows that the multi-source fracture source originates from the surface of the joint between the head and the rod, and the final fracture area is inside the bolt. The fracture is a typical torsion fracture.

The metallographic structure of the failed parts and the qualified parts are inspected separately. The structures of the failed parts and the qualified parts are the same, and both are uniform granular pearlite structures.

The chemical composition of the broken parts is tested, and the results meet the requirements of AISI 1022A material standard

The hardness of the broken parts was tested and the results were HRC20-21.

 

 

Analysis and discussion

The screw breaks when the specified torque value is not reached during installation. The broken parts of the failed parts are the joints between the head and the rod, and there is no obvious necking phenomenon, and the fracture is characterized by torsion fracture.

This batch of screws has not undergone heat treatment, and its metallographic structure is granular pearlite, which is a raw material organization. The screw hardness value of granular pearlite tissue is lower.

Because the screw hardness value is low, it cannot meet the installation requirements. When the torque value does not reach the specified value under the action of the installation stress, the screw head and rod joint will be twisted and turned off where the stress is concentrated.

Conclusion and inspiration

(1) The screw has a low hardness value and cannot meet the installation requirements, and it breaks under the action of installation stress.

(2) The screw has not been heat treated, resulting in a low hardness value that cannot meet the installation requirements.

(3) This screw can only adapt to small installation stress. If installation stress is required, heat treatment is recommended to improve strength and hardness.

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