"Working Backward," Feature Article, June 2005

working backward

Reverse engineering gets a role in detecting and preserving.

by Jean Thilmany, Associate Editor

many mechanical engineers can recount a youthful story about taking apart a clock, a small engine, or some other mechanism or electrical gadget to figure out how it works and to try to put it back together again—maybe even better than before.

Thus, the future engineer's introduction to reverse engineering.

Reverse engineering can be defined as taking something apart—or breaking it down to its simplest pieces—to determine how it was built or how it works.

Technology vendors now sell hardware that traces over existing parts and assemblies—whether small or large—and inputs the shape into a computer-aided design system so engineers can adjust and rebuild the products. This is the use mechanical engineers perhaps most often associate with reverse engineering: that a company studies a product's basic structure in hopes of developing a competing or interoperable product.

Or the company may simply need a digital file for one of its own products. Even in this technological age, not all product specifications are stored in the computer.

Aside from this common use, engineers across many disciplines find reverse engineering an invaluable tool to discover and learn about a product's structure and design.

Take Colin Gagg, who is an associate lecturer in forensic engineering at the Open University in Milton Keynes, England.

"Understand how something works and you'll be in a much better position to understand how it failed," Gagg said.

He describes forensic engineering as the science of identifying the root cause of a component or system failure in order to prevent future products from failing in the same way.

The complex products that dominate so much of everyday life, coupled with a litigious society in general, means that when a failed part causes property damage, personal injury, or death, forensic engineers are often called in to determine where fault may lie, Gagg said.

In the United Kingdom, more than 4,000 people die in accidents in and around the home each year, he added. Nearly three million people turn up at accident and emergency wards. Intrinsic in those statistics are the numbers of injuries or deaths directly attributable to poor product design or poor manufacture.

Gagg makes his livelihood by understanding how a product failed—failure analysis—and by passing on his skill to students. For this, he uses an arsenal of both engineering and detective tools. He deems reverse engineering to be of paramount importance.

After all, what is reverse engineering, but reconstruction—run backward. Reconstructing the part to find the failure will uncover any inherent defect in product design, in manufacturing, or in incorrect installation or maintenance, Gagg said.

"To recognize how a component or system failed, the engineer must understand how it worked and was manufactured in the first place," Gagg said. "By using reverse engineering, the forensic engineer attempts to produce a complete description of the product that will allow him to work out how it functions.

"By stepping back through the transformation stages, he'll be in a better position to determine most probable or expected points of failure within a component or system."

This is termed the weakest-link principle: the attempt to find the weakest link in the system, the link that failed the entire assembly.

Gagg offers an example from his forensic engineer's casebook.

cracking the container case

Nowadays, it's unusual for components with serious
faults to be released, say Gagg and his co-authors, Peter Rhys Lewis and Ken Reynolds, in their book, Forensic
Materials Engineering: Case Studies (CRC Press, 2004). Sometimes, however, if a component is faulty, it could fail under heavy load.

In one instance, engineers traced back a simple manufacturing mistake that caused freight containers to split and that cost an insurance company a lot of money. The containers were made of aluminum alloy, commonly used to make containers that transport goods by ship, rail, or road.

This forensic case began when a dock worker noticed a split in the end panel of a loaded 33-foot-long container being lifted from a ship. The container showed no obvious sign of external damage and the piece of machinery it held was still anchored inside.

Shortly after that first split was found, workers at other ports noticed similar failures in the same type of freight container. Inquiries showed that the containers had all been made at the same factory during a two-month period, the authors write.

Investigators found that the riveted seam between the two end panels in the side of the container had split open from bottom to top. All other containers had split in the same position; all had been carrying heavy machinery rather than bulky loads distributed throughout their length.

Costs escalated fast. Machines had to be loaded into other containers, empty containers needed to be shipped to those ports, and the damaged containers had to be either disposed of or repaired.

A mechanical engineer found nothing wrong with the original design or with construction of the containers. Something must have gone wrong in manufacturing or with the material to cause structural weakness.

Next came reverse engineering, used in this case to study the manufacturing process. Investigators wanted to find out exactly how the container was manufactured and whether it could have weakened during any particular step in the process.

By tracking each part and how it had been manufactured, the investigating engineers discovered that the container's side panels were sheets of aluminum alloy that were riveted to each other and to the frame along vertical lap joints. All the failures involved the unzipping of the vertical lap joint between the first and second sheets from the end of the container.

In other words, by taking apart the container and focusing on where each piece could have failed at manufacture, investigators were able to pinpoint and focus on the rivets. Although microscopic examination found no internal fault, wear, or corrosion, investigators found that if one rivet near the end of a seam had failed, it would have thrown an extra load onto its neighbors, which might have overstressed the part, causing all the rivets to unzip.

Now the forensic engineers needed to find the specification for the rivets on the engineering drawing. A hardness test they performed on the failed rivets found them well below the strength indicated on the drawing.

The 3-D laser scanner used to compile an architectural plan for the Statue of Liberty measured some of the statue's many architectural details.

Meanwhile, a mechanical engineer addressed the complementary question of what level of stress the rivets were subjected to when they unzipped. The formal report concluded that a batch of containers was produced with sidewall strength below what was intended, because the rivets had been set without being solution treated, as specified on the drawing.

As it turned out, for about a month, an employee at the container factory had omitted the solution heat treatment. Case solved.

Of course, that final report came after much analysis of individual parts, but the investigating engineers narrowed their focus to the rivets because they had broken down the container to find the cause of failure.

Investigating engineers used analysis technology and other engineering technology in this case. They're as necessary to the forensic engineer as a magnifying glass was to Sherlock Holmes. But, as with the magnifying glass, the user has to know how to look, and what to look for.

"I'm of the opinion it's not necessary to have an in-depth knowledge of the state-of-the art analytical methods or numerical modeling," Gagg said. "Yet the expert must have an implicit understanding of the range of tools available, the data they provide, limitations of the derived data, and of any compromise that is made during the course of testing.

"It's my view that a good forensic engineer will glean relevant information through meticulous investigation and by taking a reverse-engineering approach," he said.

The same remains true of any engineer charged with creating a part. After all, if the machine runs perfectly, if the part performs as it should, there will be no need for an investigation down the line.

"I'm suggesting that any engineer should be aiming to train himself to become a failure detective," Gagg said.

liberty plans on hand

Now, let's examine quite a different use for reverse-engineering process. Although this story is about architects, many mechanical engineers will recognize a common reverse-engineering use: the need to have plans on hand for a part created originally without computers or drawings.

In this unique case, the part—or statue, rather—is huge.

After the World Trade Center attacks, the National Park Service realized that although countless photos of the
Statue of Liberty exist—there's probably one in your own home—they couldn't locate detailed architectural drawings that would enable an exact replica to be constructed. Should the statue suffer harm, it would be very difficult to rebuild it without such drawings on hand.

Texas Tech University, the National Park Service, and the Historic American Buildings Survey are now creating digital architectural drawings to detail the 120-year-old statue's every curve, cranny, and dimension. They're doing this through reverse engineering.

The university is capturing the statue's unique architecture with three-dimensional laser scanning technology tied to geometry processing software, which automatically generates an accurate digital model from the scan data.

Reverse engineering—even tracing the Statue of Liberty to get her measurements—can be an old-fashioned detective game.

Even under the best of circumstances, scanning an object that stands 305 feet tall and weighs 225 tons wouldn't be easy. Beyond mere size, the Statue of Liberty presented special challenges, said Glenn Hill, director of Texas Tech's College of Architecture environmental visualization program. Those who scanned the statue had only a small space to move around in while collecting measurements.

"In addition to that, there were always crowds of tourists present, so there were security and safety concerns at all times," Hill said.

The team collected the data using a Cyrax 2500 3-D laser scanner from Leica Geosystems of San Ramon, Calif. Researchers came back with 13 scans, each of a part of Lady Liberty. Then they had to process the immense point-cloud data.

They would need to put the 13 scans together, like a jigsaw puzzle, to create a whole—easier said than done.

"We tried to register these 13 scans together using targets placed in the scene, but we had a lot of difficulty getting appropriate alignment," he said.

Even though the team used a scanner made especially for large objects, it could scan only what was in its line of sight. Thus, any overlap or undercut on the statue that the scanner couldn't see was not captured.

The team had to eliminate all the holes and imperfections in the model before creating a surface model to take into a CAD program for completion.

To help align the scans and to fix the holes, the team turned to technology that creates surface models from scanned data. The software is Geomagic Studio, from Raindrop Geomagic of Research Triangle Park, N.C.

Hill's team is still working to create a surface model of Liberty Enlightening the World.

Normally, holes and imperfections can be fixed automatically in Geomagic Studio to allow quick import to CAD. But, because this is a historical record, John White, director of historic preservation in the College of Architecture, and Elizabeth Louden, associate dean of research, have to document each hole. Paul Dolinsky, chief at Historic American Buildings Survey, has to review the hole fills as well.

The fills must also be recorded as part of the permanent documentation.

The end goal of the project is still months away, but the completion of the project will mark an important chapter in the statue's history.

"When this project is finished, you will be able to take any portion of the statue and replicate it," Hill says. "This is significant because it means the statue as a symbol of freedom will always be safe for future generations."