Accelerated Life Testing
Accelerated Life Testing
Assembly Magazine July 2021
By Jim Camillo
Like so many things in manufacturing, product testing is not a one-type-fits-all-applications procedure. Among the more well-known types of testing that manufacturers regularly perform are leak, mechanical performance, material (such as tensile strength) and computer simulation modeling.
Accelerated life testing (ALT) is one of the lesser-known procedures, but it is no less important. This type of testing involves capturing product life data under accelerated stress conditions, which cause the unit to fail more quickly.
“Our testing is determined by extensive work with end-users to understand the worst-case accessories, materials, loading and environment,” explains Lee Brendel, resident engineering manager for North America at Stanley Black & Decker Inc. (SBD). “We make test rigs to match these scenarios and effectively perform ALT. Key components of the rig are a power supply, a controller and various types of sensors to capture data. Often times, the testing involves a very complex routine, so specially trained technicians and engineers run our tests.”
Kitchen-cabinet-hardware maker Blum Inc. performs a different type of product testing at its assembly plant in Stanley, NC. In a lab there, pneumatic cylinders are used to open and close cabinet drawers and doors around the clock to test the company’s hinges and slides. The goal is to see how many times the hardware can be cycled until it fails and to help engineers learn the mechanisms of failure.
This type of testing is called “typical life” because the product test is operated under normal conditions. Its main benefit is helping manufacturers make more-accurate predictions about the lifespan of the hardware and to refine their designs.
When companies need to obtain reliability results more quickly, however, they turn to ALT. Performed correctly, accelerated testing can significantly reduce time to market, and lower both product-development and warranty costs.
Faster Today, Better Tomorrow
Manufacturers can perform one or more versions of ALT to obtain the data they need. Qualitative accelerated tests are used primarily to reveal probable failure modes for the product so that engineers can improve the product design. These tests are performed on small samples with the test units subjected to a single severe level of stress. If the specimen survives, it passes the test. Otherwise, appropriate actions must be taken to improve the product’s design to eliminate the causes of failure that were identified during the test.
One type of qualitative accelerated test is highly accelerated life testing or HALT. Gary Delserro, PE, president of Delserro Engineering Solutions, says HALT is a process proven to lower product manufacturing costs, improve customer satisfaction, gain market share and increase profits.
“Companies have reported savings in the millions after using HALT,” claims Delserro. “The test can accelerate a product’s aging process from actual months into test minutes, and it can help you discover weaknesses in your product during the design stage. Combined vibration, temperature and electrical stress variables, as well as internal fluid pressure, are typically used during HALT to induce failures and uncover fault points. By using combinations of loads, we can uncover design or manufacturing process flaws before they reach your customer.”
Another type is the highly accelerated stress test (HAST)—also called the pressure cooker test—which uses temperature and humidity as the environmental parameters to test electronics component reliability. Its purpose is to evaluate a product’s humidity resistance by increasing the test chamber’s water vapor pressure to a much higher level than that inside the product. This process temporarily accelerates moisture infiltration into the product.
Qualitative accelerated life (QLAL) tests only introduce failure modes that will be encountered in real life situations. As a result, it does not provide information that can be used to quantify the life characteristics of a product under normal use conditions.
A quantitative accelerated life (QTAL) test, on the other hand, is designed to quantify the life of the product and to produce the data required for ALT analysis. Data obtained from these tests enable manufacturers to accurately estimate a probability density function for the product under normal use conditions and to calculate product reliability, probability of failure, mean life, failure rate and B10 life, which is the time when 10 percent of the product population will have likely failed.
QTAL tests can employ usage rate acceleration or overstress acceleration to speed up the time-to-failure for the product. With overstress acceleration, one or more environmental factors (temperature, voltage, humidity, etc.) known to cause failure under normal conditions are increased. Usage rate acceleration is appropriate for products that do not operate continuously or at a very high rate under normal conditions.
A washing machine is a good candidate for usage rate acceleration testing. One machine manufacturer assumes that the average machine is used about six hours per week, but wants to quickly and accurately determine its performance level at 28 weeks (just over six months). To achieve this, the company runs the machine continuously for one week (168 hours) to simulate 28 weeks of operation under normal use conditions.
Countering Chrome Corrosion
Within every skid-steer, side loader, forklift and snow plow are multiple lift-cylinder rods made of steel. And on the outside of each of these rods is a chrome plating that prevents corrosion and extends wear life. Geneva, IL-based Industrial Hard Chrome Ltd. (IHC) specializes in applying chrome plating to these rods, as well as tubing and machine-cut blanks.
“The worst thing you can do to an installed rod is to not use it,” notes Bruce Tucker, quality manager at IHC. “Depending on the machine, the rod may sit engulfed in hydraulic oil at times. But, often it’s exposed to the elements. Chrome plating keeps the rod from corroding due to exposure.”
After plating, the rods undergo ALT based on ASTM B117, Standard Practice for Operating Salt Spray (Fog) Apparatus. IHC owns, and uses daily, three specially designed chambers (made by Singleton Corp.) for this type of testing.
Up to 50 16-inch-long rods at a time are placed in each chamber to see how well the chrome plating withstands accelerated exposure to a salt fog and extreme temperatures. Each rod is cut from a 24-foot-long plated steel bar, according to Tucker.
Before testing begins, a stainless steel bubble Humidifying Tower is filled with deionized water and mounted on the outside of the chamber. A premixed 5 percent saline solution is poured into two other beakers with internal funnels. These beakers are attached to different towers, which are placed inside the chamber at designated locations.
Next, the operator sets the air pressure, pH and temperature parameters. Finally, he places two PVC racks into place, puts the rods on top (each at a 15-degree angle with some space in between) and closes the chamber.
When the operator turns the chamber on, the air goes into the Humidifying Tower before picking up the 5 percent saline solution and spraying it through the two internal towers. A salt fog mist is evenly applied to the rods during testing.
“Each chamber is opened every 24 hours, but only for the shortest time,” explains Tucker. “The technician first checks the beakers to see how much saline has been collected. Collecting about 1 to 2 milliliters per hour is standard. Then he rechecks the parameters to make sure they’re correct.”
This cycle is repeated until the rods have been tested for at least 200 hours. Nearly every IHC customer requires a testing time less than 200 hours, but Tucker uses this number as a barometer to make sure the chrome plating is the highest quality possible.
When testing is complete, the technician opens the chamber and removes the beakers. He manually records the amount of saline collected and other related data on a log sheet. Shortly thereafter, he enters this data into a proprietary software program for sorting and analysis.
“The rods are then removed and carefully examined for any signs of corrosion,” says Tucker. “Only when the data verifies that the chrome plating has passed the ALT and each rod has no sign of corrosion does the order get shipped.”
Tool Test Time
As the world’s largest manufacturer of power tools, SBD faces a unique daily challenge: Maintaining and expanding its customer base to stay at the top. The way the company meets this challenge is by producing high-quality tools.
Testing has been an essential part of making such tools for more than a century, notes Brendel, although not all current tool brands made by the company have been around that long. These include Stanley, DEWALT, Black & Decker, Craftsman, Porter Cable, Mac Tools, Proto and FACOM.
“We make many different power tools, in various versions, and each of them undergoes many tests, including ALT,” says Brendel. “To handle this volume, we operate several large test centers globally, where new product development takes place as well. SBD also has a test lab in each of its manufacturing plants throughout North America, South America, Europe and Asia.”
A power drill, for example, may go through up to 20 different tests as part of an ALT program. These tests are done at every stage of development—from prototyping to final production—and can include the tool enduring many thousand cycles of operation.
One common drill test involves applying a simulated load with a braking device to the tool. SBD engineers then determine how much current the motor draws in such a situation, and how much additional load is placed on the motor and transmission.
“In general, every tool and every system has a unique test method we’ve developed based on years of data and refinement,” says Brendel. “All of our products go through ALT during new product development, after any design changes, and on an ongoing basis when in production. End-user research always directs our development of test specs and methods, to make sure we’re designing the products to meet the customer’s needs.”
Brendel says that SBD strives to make its ALT as aggressive as possible without creating unrealistic failure modes. He acknowledges that this is easier to do for established products, like drills, than for new ones.
“For brand-new products, during failure analysis we have to consider the test method as a potential root cause,” says Brendel. “This may require us to slow down the test, or correlate it to other non-accelerated test data. Also, we may have to determine if the product requires a design change, if the test method needs adjustment, or just figure out what the acceleration factor is for that specific failure.”
The Engineered Approach
Founded in 1992, Delserro Engineering Solutions (DES) specializes in ALT, HALT, stress, highly accelerated stress screening (HASS), reliability, durability, vibration, shock and environmental testing. Customers include large and small companies in the electronic, telecommunication, aerospace, transportation, medical, and consumer products industries, as well as manufacturers that serve U.S. government agencies.
“For custom product life cycle testing, we meet with each customer and write up an agreed-upon plan that specifically defines everything before we run any test,” explains Delserro. “This plan covers details like how many cycles to run a product, in how short a time and where to add stress.”
Each rig used for testing is custom made, and varies in size from a small benchtop unit to one that covers an area of up to 6 cubic feet. The rig frame is made using the T-slot aluminum building system from 80/20 Inc. Delserro likes that the system’s aluminum profiles offer the flexibility to build one type of frame, tear it down and build a completely different type of frame.
“Testing five or six of the same product is recommended to obtain a large-enough data sample for statistical analysis using the Weibull++ ALT module made by ReliaSoft,” continues Delserro. “We then provide the customer with a thorough final test report.”
One customer recently asked DES to perform HALT on a new circuit board design that featured potted componentry. This test involved exposing several boards to hot and cold temperatures, followed by rapid temperature ramping, vibration and combined temperature and vibration stresses.
According to Delserro, the hot and cold temperature step exposed one hard failure (design weakness) and multiple soft failures, such as power resets and lack of communication. The boards did function properly during the rapid thermal ramping and vibration steps.
However, the combined temperature and vibration stage revealed the same problem as that shown during the hot-cold test. In addition, the potted componentry cracked on multiple boards during testing and caused communication problems.
DES completed its testing in three days. A relatively short time, objectively speaking, but long enough to show the customer a product design weakness that most likely would have caused several warranty and reliability issues down the road.
Another HALT-related project involved testing two military products using DES’s accepted procedure. Testing was done to compare the results for an existing proven product vs. a new one.
Various stresses were applied to determine each product’s temperature and vibration operation and destruction limits. Recoverable operational problems and hard failures were found during the testing of both products. These results provide the company an opportunity to improve each product’s reliability.
“The general rule is the more safety or service critical a part or product is, the more it needs to undergo ALT,” concludes Delserro. “For example, a key component in a medical device should be tested. But, a three-dollar toy, probably not.”