Forensic Analysis Specialists Discover Root Causes of Product Failures Using ALGOR FEA

	Herrera, Stafford and Associates, LLC of El Paso, TX studied this compressor head cover using FEA and laboratory tests to discover the root cause of its failure. The dome of the cover was unrecoverable from the accident site.
	Anselmo Najera, Design and Analysis Engineer at Herrera, Stafford and Associates, LLC of El Paso, TX, conducted failure analyses of the compressor head cover.
	Najera studied eighteen variations of the compressor head cover, which differed in geometry, pressure loads and bolt forces. The two results above compare a case where the bolt force was the same for all bolts and within the manufacturer's recommendations (top) with a case where the bolts were unevenly torqued and the forces exceeded the manufacturer's recommendations (bottom).

Engineers use a wide range of tools and techniques to ensure that the designs they create are safe. However, accidents sometimes happen and when they do, companies need to know if a product failed because the design was inadequate or if some other cause, such as user error, was to blame. Whether a manufacturer incurs the cost of damages, recalls, replacements, not to mention potential legal liability for injuries, often depends on the cause of the accident.

Forensic analysis specialists at Herrera, Stafford and Associates, LLC (HS&A) of El Paso, Texas, are often called upon to determine why products fail. Recently, they used ALGOR FEA to evaluate the reasons for the failure of a compressor head cover at a Texas oil well.

An Unfortunate Accident

HS&A was hired to research the accident's cause and determine whether the cover was flawed or user error had played a part. HS&A principal Juan Herrera, Ph.D. gathered specific information indicating that the compressor had been used for many years prior to the accident. However, the cover failed after the compressor was restarted following repairs and maintenance operations, releasing natural gas which found an ignition source and resulted in an explosion and fire that seriously injured several workers and caused millions of dollars of property damage.

It was determined that a pneumatic impact wrench had been used to tighten the bolts on the head cover instead of the recommended torque wrench. Tests conducted in the HS&A laboratory using a similar impact wrench revealed that the recommended torque (80 ft-lbs) was obtained with only 20 psi in the air line supply. After his research and studying fragments from the failed cover, Herrera concluded that several errors had been committed in assembling the compressor head cover: the bolt/nut assembly had not been lubricated; the torque values were not equal in all eight bolts; the bolts were not equally screwed in as each had different screw depths; the recommended torque value was exceeded and the recommended torquing sequence had not been followed. As a result, the aluminum seal on the bottom of the cover deformed unevenly around the base.

Simulating Failure

With this information in hand, Anselmo Najera, Design and Analysis Engineer at HS&A, turned to the task of conducting a series of FEA analyses to study how different geometric features and loading conditions affected the stress results.

Najera wanted to study design variations with and without the groove that is present on the top surface of the cover to discover whether that design feature led to high stress concentrations. In addition, there were concerns that manufacturing variations might be causing high stresses. When a cast iron part such as the cover is produced, the mold often twists a degree or two. This results in a slight asymmetry in the thickness of the cover and location of the dome's apex. To study whether the asymmetry would affect stress results, Najera planned to create several variations of the model to represent the dome as designed, as manufactured with a shift in the dome apex and as manufactured with both a shift in the dome apex and asymmetry of the thickness.

Najera created structured-mesh models using ALGOR's finite element model drawing tool, Superdraw. "I chose to create a structured mesh because I wanted to specify the exact location of nodes for the loads and constraints," said Najera. "The asymmetry of the dome was created by incrementally modifying the angle while extruding the mesh. Once I had a quarter of the model, it was mirrored to complete the geometry."

To simulate friction between the cover and its aluminum seal, Najera modeled a thin layer of elements and gave it weak material properties so it would easily deform. "The layer of elements between the cover and the seal functions in much the same way that contact elements do."

For all models, the aluminum seal was completely restrained and material properties were applied based on information from the manufacturer. There were two sets of loading conditions that needed to be considered: the force of the bolts holding the cover in place and the pressure within the compressor. A variety of bolt force loads were considered ranging from 1,024 to 15,000 lbs. Najera also considered a scenario in which the bolt forces were not the same for all of the bolts. The maximum pressure of the compressor, 600 psi, was applied on the inside surface of the dome for most of the models and omitted for several analyses to isolate the effect of the bolt loads. In all, 18 different variations of the model were analyzed.

For each linear static stress analysis, Najera looked at the maximum principal stress and compared it to the yield stress of the material. He also looked for areas of stress concentrations. "We found that neither the groove nor the asymmetry of the dome was significant enough to cause a failure," said Najera. "The most significant factor was the force used to tighten the bolts."

Results and Conditions Involved in FEA of Compressor Head Cover
Case	Force/Bolt (lb)	Pressure (lb/in2)	Max. Principal Stress (lb/in2)	Characteristics
1	1,024	0	3,030.25	Groove as Designed Dome centered
2	1,024	0	3,026.72	Groove as Manufactured Dome centered
3	1,024	0	3,070.81	Groove as Manufactured Dome shifted to one side
4	1,024	0	3,073.28	Groove as Manufactured Dome shifted to one side and asymmetry of wall thickness
5	3,000	0	9,003.76	Groove as Manufactured Dome shifted to one side and asymmetry of wall thickness
6	3,000	600	7,908.91	Groove as Manufactured Dome shifted to one side and asymmetry of wall thickness
7	3,000	0	9,047.03	No Groove, Dome outside radius blends with flat area Dome centered
8	3,000	600	7,913.59	No Groove, Dome outside radius blends with flat area Dome centered
9	6,000	0	17,754.07	Groove as Designed Dome centered
10	6,000	600	14,791.90	Groove as Designed  Dome centered
11	6,000	0	18,094.10	No Groove, Dome outside radius blends with flat area Dome centered
12	6,000	600	15,168.20	No Groove, Dome outside radius blends with flat area Dome centered
13	6,000	0	18,007	Groove as Manufactured Dome shifted to one side and asymmetry of wall thickness
14	6,000	600	15,682	Groove as Manufactured Dome shifted to one side and asymmetry of wall thickness
15	9,000	600	24,172.40	Groove as Manufactured Dome shifted to one side and asymmetry of wall thickness
16	12,000	600	32,895.4	Groove as Manufactured Dome shifted to one side and asymmetry of wall thickness
17	15,000	600	41,826.13	Groove as Manufactured Dome shifted to one side and asymmetry of wall thickness
18	12,000 x 6 15,000 x 2	600	39,024.59	Groove as Manufactured Dome shifted to one side and asymmetry of wall thickness

In the laboratory, Najera tested each of the eighteen FEA cases to verify the simulation results. "We were pleased to find good correlation between our FEA and laboratory results," said Najera. "Our client was relieved that our tests proved that user error, not design or manufacturing flaws, was the cause of the accident."

FEA Applications for Failure Analysis

As specialists in failure analysis, HS&A engineers have many opportunities to use FEA. For example, they have used ALGOR to solve design problems, such as why one company's light bulb filaments were easily breaking. "We are also often involved in automobile accident reconstruction," commented Najera, "and are required to analyze automobile components to find out why they failed."

Built-in tools for sharing FEA results are vitally important in product failure investigation, particularly when communicating with non-engineers. "FEMPRO's user-friendly Superview results evaluation and presentation capabilities help us to present our findings," said Najera. "Superview's ease-of-use is constantly improving, with more capabilities accessible through toolbars and right-click options. These capabilities for creating images and animations of results help us to quickly prepare reports to present to our clients, many of whom do not have a technical background."

In conclusion, Najera said, "We've found that ALGOR is an affordable solution for accurately proving the causes of product failures and then explaining those causes to our clients."