FEA Analysis Model to investigate weld cracking on ductwork
(ANSYS) Finite Element Analysis of Ductwork

Finite Element Analysis Determined The Cause of Premature Weld Failures

We performed a forensic investigation, evaluating the quality of construction/ welds and the design of failed exhaust ductwork.

Premature weld cracking had occurred in corner weld joints and in stitch welds used to secure the duct to stiffening angles approximately 3 weeks after the ductwork was placed in service. Finite element analysis (FEA) was performed to calculate thermal stresses.

The exhaust ductwork is used to convey high-temperature exhaust from large diesel engines through the roof to the atmosphere during engine testing and qualification. The diesel exhaust temperature is approximately 1100°F when it enters the ductwork. This hot exhaust enters the ductwork vertically. It then immediately turns 90 degrees at the lower elbow, and travels horizontally for approximately 10 feet before passing through a second (upper) elbow which directs the exhaust upward. This upward exhaust travels through two silencers before exiting to the atmosphere. The region of the cracking in the welds is from the exhaust entrance through the two elbows and up to the first expansion joint.

The first step in the investigation involved a major weld inspection. The shop-applied welds were evaluated for size, length and location – and had to meet the specified criteria as well as the AWS 09.1: 2000 Sheet Metal Welding Code. This code is primarily employed for the exhaust ductwork fabrication as specified by the Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA).

The visually-examined welded joints included corner 1/8″ fillet welds which connect stainless steel plates to form the shell of the ductwork and 1/8″ fillet welds which connect the stiffening angles to the ductwork shell. The inspection determined that the welds appeared to be thoroughly fused to both sides of the joints and the weld profiles were acceptable per 09.1 Section 6, Inspection of Arc Welding Work.

A finite element analysis model was then performed to determine the effect of thermal stresses in the exhaust ductwork. The results determined that the cause of failure was the change of ductwork material from carbon steel to stainless steel.

Carbon steel has a thermal expansion coefficient that is approximately 30 percent less than 316L stainless steel. The thermal expansion coefficient is directly proportional to thermal stress. This means that, given the same thermal conditions, the thermal stresses in the stainless steel ductwork would be 30 percent higher than the thermal stresses in identical carbon steel ductwork. 

Also, carbon steel has a thermal conductivity approximately 2.5 times greater than that for stainless steel at temperatures up to approximately 600 F. Consequently, the temperature gradients through the stainless steel material are greater than the thermal gradients through the carbon steel under the same thermal conditions. This resulted in higher thermal stresses in the stainless steel material.


 

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