Mechanical systems wear out and fail eventually. The ability of a structure to support a load, move though the specified range of motion, or spin degrades with use and time. Even our joints eventually wear out.
Accelerated life testing (ALT) has value as it provides information about a system’s reliability performance in the future. There is plenty of ALT literature concerning the failure mechanisms unique to electronic components and materials but less has been written about mechanical reliability testing.
This is partially due to the limited number of unique electronic components compared to the often custom mechanical designs. Let’s explore an example of mechanical reliability testing (an ALT) to outline a basic approach to ALT design and analysis.
HOT TUB EXTRUDED SIDING ATTACHMENT RELIABILITY
A hot tub manufacture wanted to create durable maintenance-free siding for a line of hot tubs. The solution was to create extruded strips that could simulate different surface finishes, such as wood, metal, or a wide range of custom textures. The advantages of using a polymer included
- color fastness and UV resistance to discoloration,
- no corrosion or rot as with metal or wood siding,
- ease of user maintenance, with only light surface cleaning as needed, and
- ease of construction.
It was the last feature that also created the field reliability issue. The panels were stapled to a wooden frame, which was then attached to the hot tub structure. The staples would come loose over time. The daily temperature changes and the difference in the polymer coefficient of thermal expansion (CTE) between the wood and hot tub frame would eventually work the staples loose. The panels would fall out of alignment or fall out completely.
The desire is to create a mechanical reliability testing procedure that would speed up the daily temperature swings and resulting stresses across the staples or other possible attachment methods. Placing panels or entire hot tubs into thermal chambers was cost prohibitive. Creating test panels on hot tubs and locating them outdoors, even in areas with high daily temperature swings such as a high-altitude desert, was time prohibitive. The current solution did not fail till close to two years after installation.
As with all engineering problems, there are goals and constraints. The goal is to test attachment solutions using the polymer siding and existing frame to find a solution that keep the siding intact with 99% probability over 10 years. The constraints are that there can be no thermal chambers nor outdoor ambient-based-temperature testing and that the test process should take a month or less to replicate 10 years of normal use.
The question then entails selecting the right ALT approach for this situation.
HOT TUB ATTACHMENT PROPOSED SOLUTION
The failure mechanism is the attachments experiencing relative motion owing to the CTE mismatch between the polymer and frame that either deformed the polymer or mechanically loosed the attachment leading to failure. Temperature caused the materials to change dimension.
Temperature is not the only way to replicate the motion and resulting stress across the attachment. One can either measure or calculate the amount of motion the polymer causes, then create a fixture to mechanically cause the same travel (or desire to travel). Leaving a number of panels free of any constraint one can carefully measure the lengths at different temperatures, creating a relationship between length and temperature. Daily outdoor temperatures for the areas with the most complaints for siding failure are available, so those temperatures and the experimentally determined relationship can be used to determine the distance the panel would travel or indirectly the amount of force exerted across the joint.
A fixture was assembled to hold the siding panels on one end as if attached to the hot tub frame. The other end was in a jig that, with a throw of a lever, would move the panel a fixed distance, replicating the expected daily temperature-swing-induced distance. The panels were constrained from bowing or flexing, thus maintaining the strain across the attachment system.
The field data suggested that the current attachment method using staples would fail in approximately 700 to 800 cycles. The first set of samples all failed between 773 and 842 mechanical cycles. These values are close enough for the quick test to suggest that the mechanical reliability testing approach would replicate the field stresses reasonable well.
One question remained: What was the necessary mechanical cycle time dwell under load to replicate the damage caused by the daily temperature change? Although it was possible to replicate the loading pattern minute by minute by slowly moving the mechanical jig’s lever that would take 24 hours of test time to replicate 24 hours of use. Applying the release of the stress too quickly may not allow time for the damage to the attachment to occur. The initial testing did match the field cycles to failure fairly well and the dwell time was approximately a minute; therefore the process was set to include a one minute-dwell time.
In this particular test development, the damage across the staples occurred nearly instantly. Thus there was little need for further exploration of how the failure mechanism behaved under load. Other situations might be very different and might require further experimentation to refine the test cycling process.
Another concern was the mechanically induced heat caused by the loading/unloading cycles. A thermocouple near the staples showed no change from ambient conditions even with fairly rapid 10 load/unload cycles. In addition, visual inspection of the failure sites (fatigued and fractured or loosen staples) did not reveal any indication of polymer melting.
The ALT design should replicate the damage that occurs in normal use as closely as possible. The failures seen during the initial testing closely resembled the failures of fielded systems. Therefore, at this point the mechanical motion jig was deemed ready for ALT use.
RELIABILITY TESTING RESULTS
There were two rounds of experiments. The first was to only explore possible attachment solution candidates. Staples of different lengths, nails, wire stitching, bolts (least preferred because of the cost of assembly), and others all enjoyed a turn on the rack. For each attachment method 10 siding panels were created and tested till the first panel failed. Some failed relatively quickly while a couple did not have any failures for 3,650 cycles. For the first round, the testing was stopped when either there was a failure, in which case the number of cycles to failure was recorded, or the cycle count reached 3,650.
There were two possible solutions (bolts and one other) that survived the initial round. Now, 20 samples (individual siding pieces and attachment to a representative frame) were created; these were divided into two panels of 10 pieces each. Testing was conducted until at least 10 siding elements failed.
A set of samples experienced a year of cycles, then another panel took a turn on the rack. All four sets of samples received one years worth of cycles in rotation. The initial failure occurred with the preferred attachment method with the equivalent of approximately 13 years of daily temperature-induced cycles. The bolts with slots to accommodate the siding motion did not show any failures out to 17 years of cycling, when the preferred solution experienced the tenth failure.
The cycles-to-failure data for the 10 units and 10 right censored units allow a fit to a Weibull distribution and an estimate to be made of the probability of success out to 10 years of thermal cycling. (Recall that one test cycle is being considered equivalent to one cycle in normal use.) The resulting analysis showed that the new attachment method had a 90% confidence of at least a 99.2% reliability at 10 years of thermal cycling.
ASSUMPTIONS AND CONSIDERATIONS
The reliability testing was not perfect. ALT is rarely perfect because it entails using increased stress or cycles of use per day (in this case) and assuming the results will match the experience under normal conditions. The lab does not replicate the full range of environmental stresses. The humidity and temperatures remained nearly constant, along with the lack of UV, water, cleaning agents, and chlorine exposure that may have been a contributing factor to the eventual failures.
In some cases, such as this one, a simple test can provide a means to compare solutions, allowing a robust solution that outperforms other potential solutions to be chosen. The ALT results are only part of the engineering and management decision to select a solution. Cost, ease of assembly, weight, aesthetics, and many other factors also contribute to the final selection of a solution.
The testing in this the case of the hot tub siding most likely helped the team make a good decision with a few weeks of testing. It will take 10 years to know whether it was the right decision.
Bio:
Fred Schenkelberg is an experienced reliability engineering and management consultant with his firm FMS Reliability. His passion is working with teams to create cost-effective reliability programs that solve problems, create durable and reliable products, increase customer satisfaction, and reduce warranty costs. If you enjoyed this articles consider subscribing to the ongoing series at Accendo Reliability.