Product category:
FEA and structural analysis software
News Release from: Algor | Subject: FEA software
Edited by the Engineeringtalk Editorial
Team on 14 July 2004
FEA software aids forensic
investigations
Forensic analysis specialists at Herrera, Stafford and Associates of El Paso, Texas, use FEA tools to determine why products fail.
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
This article was originally published on Engineeringtalk on 15 Mar 2000 at 8.00am (UK)
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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 (HS and 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.
HS and A was hired to research the accident's cause and determine whether the cover was flawed or user error had played a part.
HS and A principal Juan Herrera, PhD gathered specific information indicating that the compressor had been used for many years prior to the accident.
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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 and A laboratory using a similar impact wrench revealed that the recommended torque (80ft-lb) was obtained with only 20lb/in2 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 aluminium seal on the bottom of the cover deformed unevenly around the base.
With this information in hand, Anselmo Najera, Design and Analysis Engineer at HS and 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 mould 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 aluminium seal, Najera modelled 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 aluminium 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 1024 to 15,000lb.
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, 600lb/in2, 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 analysed.
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".
In the laboratory, Najera tested each of the 18 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".
As specialists in failure analysis, HS and 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 analyse 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".
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