Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation

 

 

Material Properties - Case Study No. 121: Rotary Kiln Lifter Thermal Residual Stress and Strain

 

Residual stresses that are caused by thermal overstrain will camoflage themselves well. In the design phase, assuming that components grow uniformly for the sake of simplifying calculations is dangerous; nothing will be saved. A thermal finite element analysis is worth every penny.

Sample Section of Lifter/Shell

When a fracture sample was cut into three sections for analysis, the crack marked "A" opened up. It had been closed, prior to cutting. The sample was from the end of a lifter that was located inside a rotary kiln. The sample included the lifter and shell, shown here in the picture to the left (the lifter is the vertical piece while the shell is the horizontal piece). The region of the kiln where the sample was taken operated under very high temperature conditions (500° to 900° C). The next two pictures below show a close up view of crack "A". Both pictures reveal the magnitude of strain that was restrained within the section prior to cutting. In other words, crack "A" was restrained and in a closed position at section "2"; once the cut was made the crack "popped" open.

Close Up View of Sprung Crack "A"

Section View of Crack "A"

The picture below shows crack "A" in section 2 at location "C". Note the three large cracks, numbered 1 through 3. Cracks usually line up normal to the direction of the tensile stress. The only reason this section did not open up was the fact that the cracking ended close to "C" and there was still plenty of uncracked material to withstand the tensile stresses in section "2".

The tensile stresses were residual and were the result of the lifter being overheated and expanding in an essentially fixed position. The middle of the lifter arched upward pushing the ends into the shell. The action created high compressive stresses that yielded the material in the crack regions of "A" and "B" (in the top picture). Upon cooling, the strains created from the compressive yield stresses turned into equivalent tensile stresses.

Close Up View of Section No. 2 @ "C"
The phenomenon is sometimes referred to as a material having a "memory." If a material is either heated up or strained past its yield point, there will be residual stresses with an opposite sign. In some cases, as in this one, those residual stresses are added to the operating stresses of the component. As such, a component can fail under "design" conditions. In this example, the unusual thermal growth was not anticipated in the original design. The incorrect assumption here was that the lifter would safely grow outwards in the unrestrained region and that the welded region would grow linearly with the shell at a uniform rate and negate any thermal stresses along the weld. However, a thermal finite element model was created and analyzed after numerous unrelenting lifter failures occurred. The model showed that the lifters grew much faster along the axis of the shell, than the shell itself, and bowed. The bowed lifters caused the ends to compress into the shell. The stress was sufficient to yield the material in compression and create nearly equal residual tensile stresses. Those tensile stresses are what are pulling the lifter away from the shell in the picture shown above (cracks 1 through 3).

 

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