Analysis of the cause of cracking of metal bellows

Oct. 18, 2019

Whether using hydroforming or mechanical bulging, the phenomenon of peak cracking of the metal bellows during the forming process has never occurred in the past processing experience. After the peak crack was found, the metal bellows was re-injected with the same batch of material. No similar phenomena appeared after forming.

After preliminary analysis of cracks, fractures and processing of the failed metal corrugated pipe, it is judged that the crack and the fracture are related to the uneven distribution of material composition. Corrugated metal pipe manufacturers today share the reasons for the cracking of metal bellows.

1. Reason for damage

The results of material analysis show that the chemical composition and mechanical properties of the materials are in compliance with the requirements of GB/T3280-2007. The results of metallographic analysis show that the material is normal austenite structure; the existence of band structure is naturally caused by the metal bellows material after cold forming. The crack region is macroscopically concave and convex. The microscopic morphology is characterized by permeability and tissue shedding. There are dense and chain-like slag inclusions or inclusions on the edge of the exfoliated tissue and its shallow surface section. The presence of inclusions disrupts the continuity of the matrix structure. When the inner surface of the bellows is pressed, the outer surface is subjected to tensile stress, causing the defect area to partially fall off.

2. Failure cause analysis

The breakage of the failed metal bellows peak is due to inclusions in the material. The presence of inclusions in the material destroys the continuity of the matrix structure of the metal material, resulting in non-uniformity of the metal structure and reducing the plasticity and toughness of the metal. Under the action of external force, breakage and local tissue shedding occur at this point, which eventually leads to cracking and failure of the metal bellows peak.

The inner and outer surface states of the corrugated metal pipe before the processing are normal. After the metal bellows were formed, tear-like cracks were found at the peaks thereof.

Corrugated metal pipe

The failed metal bellows are formed by mechanical bulging and formed to <60 tons. After the forming was completed, cracks appeared on the peaks of the two bellows. It can be seen from the outside that there is a crack of about 8 mm in the lower part of the corrugated wave of the metal bellows, and a fracture of about 1.5 mm in the center of the crack. There is a crack about 8.5 mm long under the third corrugation peak, and a rectangular protrusion with a width of about 1.0 mm and a height of about 1.0 mm is located at the center of the crack. When viewed from the inside, a depression having a length of 3 mm was observed on the inner surface of the crack of the outer tube blank of the bellows. No traces of hard object contact were observed on the back of the two cracks.

There is no trace of bumping or squeezing at the position corresponding to the outer tube blank crack on the outer surface of the inner tube blank. When disassembled, no foreign matter was present between the metal bellows layers.

The reason for the cracking of the metal bellows wave peak is due to the presence of inclusions in the metal material structure. The presence of inclusions destroys the continuity of the matrix structure and reduces the ductility and toughness of the metal. Under the action of the forming force of the metal bellows, material breakage and local tissue detachment or bulging occur at the material slag, which eventually causes the peak to crack and fail.

The shape of the material protruding at the third corrugation crack is somewhat similar to the material detachment at the corrugated crack. Therefore, no further analysis of the fracture of the third corrugation was performed, and only the corrugated fracture pattern was extracted for analysis. Therefore, the conclusions drawn from the above analysis still have limitations.

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