Flex Endurance Test Basics

While most wire installed in an aircraft is placed in static conditions, there are still several harnesses that must support regular flexing. Applications such as landing gears, wiring attached to panels/doors, and flight control surfaces have regular flexing. What is the best approach to selecting and verifying wires/cables for these applications? It turns out that there are several options.

Basics of Flex Testing

Flex Life and Flex Endurance are cyclical mechanical flexing evaluation techniques. This process involves attaching an insulated wire/cable sample to a fixture capable of rotating +90° and -90° from the vertical position. A total of 360° of rotation constitutes one cycle. Mandrels are used to ensure that the samples are flexed at a consistent bend radius based on the sample diameter or the product specification.

While similar, the EN3475 and the AS4373 have different approaches to the evaluation technique.

EN3475

The EN3475-512 method is used to determine the average number of cycles that a sample can withstand before continuity is lost in the conductor. A minimum of three samples are used to determine this average. The average from testing should exceed the product specification to pass this standard.

AS4373

The AS4373-704 method is used to determine the number of cycles that a sample undergoes before the conductor strands begin to break. This standard involves two parts:

1. Six samples are flexed until the wire is completely separated and the weight drops. The number of cycles is recorded, and the average is calculated.

2. With the average calculated in part one, 50% of that value is flexed on two virgin samples. This is repeated for two samples per 10% increase, up to 90%. These samples are visually inspected after flexing and the remaining number of conductors are counted.

Differences in Methods

The AS4373 test method involves the complete separation of the conductor and insulation of the wire/cable sample being flexed. This baseline is used to determine when the conductors begin to break, which will affect the current carrying capacity of the sample. This test method can be used to extrapolate the longevity of test samples that undergo constant mechanical flex stressing. To evaluate when continuity loss occurs, the EN3475-512 method should be employed.

The EN3475-512 method analyzes when there is complete separation of the conductor due to flexing. This method does not determine when the conductor strands begin to break and should be used when comparing two different samples to determine which is the most durable.

What These Test Methods Are Not

As designed, these test methods are not intended to provide a life prediction of wire/cables in high flex conditions. The parameters of flex life testing far exceed the requirements of most wire and cable samples in use. These parameters include applied weight, bend radius, and flex range of motion.

The weight applied to the samples is far beyond the tension that will be applied to the sample in the application. The standard specified weights for AS4373 are 4lbs for 22AWG wire or 20% of the wire break strength if another wire gauge is being tested. Most live wires should not be used to bear weight as doing so could lead to a decrease in current carrying capacity, which will result in heating of the wire.

Another aspect of flex life testing that is not common to the application is the bend radius of the sample. The AS4373 method calls for mandrels that are 6 times greater than the outer diameter of the wire specimen. In application, the wire/cable should not exceed the defined minimum bend radius of the sample. During flex testing the wire/cable is flexed close to if not beyond the minimum bend radius repeatedly to accelerate the damage to the sample.

Additionally, the sample is flexed to ±90° from the vertical axis. This range of motion is uncommon in most applications and is used to accelerate the deterioration of the samples under test.

Accelerated Aging or Life Prediction

There are two ways to approach mechanical stressing:

• Rapid comparative testing: Comparing two or more similar wire/cable samples under the same test conditions to determine which is the most durable under those conditions or one sample with varying test parameters. This method uses a similar setup to the AS4373 and EN3475 test methods and provides quick results that cannot be extrapolated.
• Long-term life prediction: Uses a configuration similar to how the sample will be routed in the application and will be more accurate to represent the damage to the samples in practice. This method takes longer than rapid comparative testing as custom test rigs have to be engineered and built to fit that suit the wire. Results can be accelerated by changing the parameters of the test to be more severe (temperature, bend radius, flex angle per cycle, etc).

Both options for mechanical stressing can be supplemented with progressive conductor health checks. These health checks can be customized to better fit the profile of the wire under test. Cables intended for high-power applications, can be tested intermittently for changes in conductor and insulation resistance. Signal-carrying lines can be tested for the aforementioned changes as well as changes in attenuation and time delay reflections. The selection of approach to mechanical testing is based on the individual user needs, and in fact, a case can be made where both are viable – such as using a modified version of the rapid comparative testing to down-select potential solutions, then use the long-term life prediction on one or two to estimate use life. For aircraft, this can then inform the life limits of parts and help establish replacement intervals.

Wrap Up

The methods for wire/cable flexing are as varied as there are applications. For basic comparative testing, following the standardized test method is the best means to down-select those few wire/cable types that showed the longest durability. Once this determination has been made, the application specific configuration can be tested if desired. Some flexing of cables is to be expected, if there is regular flexing, then wire/cable solutions with high conductor strand count (with smaller conductor strands) may be a viable solution.

For those looking for wire/cable flex testing, Lectromec’s ISO 17025:2017 accredited lab has the tools and capabilities to address your project needs. Contact us to find out more.