Using Accelerated Aging to Evaluate Automobile Components

The capability to bring automotive products to market in a shorter time than ever before, at a lower cost, with higher reliability is extremely important for Siemens North American Motor Operations (NAMO) located in London, Ontario, Canada. Siemens NAMO is a worldwide leader in manufacturing electric motor drives/systems and electric motor/fan systems for North American, European, and Asian automobile manufacturers.

In less than two years, Siemens successfully implemented a technology transfer for an accelerated-aging test program developed by Advanced Reliability Engineering Technology (ARET). By using the ARET computer model and procedures program, Siemens has reduced durability test time significantly without the need for expensive, specialized equipment.

Purpose of Accelerated Aging

Accelerated aging, when properly applied, yields the same type of anomalies as would be identified during normal durability testing. By allowing the design engineer to condense a product’s life into just a few days, design problems and process-related issues can be corrected in a timely manner. Accelerated aging can allow several design iterations, each with test validation, to be performed in a few weeks. Products can be robustly validated before they go to market. Defect rates can be held to less than 0.5% without expensive over-design.

In the past, durability, qualification, or reliability testing typically followed normal simulation environments in which systems were exposed to environments within their expected operating limits. Accelerated aging tests products in environments that are above the design limits. It is important to note that they do not encroach on the system’s destruct regions. All the test environments are in the design forgiveness region—the area where a product is stressed but can operate without permanent damage.

All hardware ages as a function of the environment to which it is exposed during its normal life. Test exposures that duplicate the use environment are classed as simulation tests. These are useful but extremely time-consuming.

To validate product reliability in a meaningful manner, equipment must be subjected to environments that expose it to the outer design limits. This accelerated aging ensures that there will be no degradation as the product ages.

For example, consider a car that is warranted for 50,000 miles. At an average speed of 30 miles per hour, running 24 hours a day, the in-warranty life is about 70 days. Long-haul truckers want a vehicle to perform failure-free for 2,000,000 miles. At an average speed of 50 miles per hour, 24 hours a day, this is 4.6 years. Engineers are not allowed 70 days and certainly not 4.6 years to validate their designs. The necessity to validate design forgiveness led Siemens NAMO to adopt its accelerated aging program.

Implementation

Accelerated aging subjects parts or assemblies to test environments that:

    Exceed the design limits. Replicate the failure anomalies normally seen during operation. Do not encroach into the destruct region.

Since all products age as a function of their use environments, they have different aging factors resulting from where they are located within the vehicle and how they will be maintained. Since an automobile has at least five use environments—the engine compartment, under the dash, the passenger compartment, the trunk, and the chassis—the Siemens NAMO accelerated testing needed to establish the aging factor for each product in its use environment. Then, based on the calculated aging factor, a profile is developed exceeding the use environment.

Many components would have a life expectancy of greater than 60 years in a friendly environment were it not for degradation caused by mechanical wear. Some examples are switches, relays, potentiometers, and motor/generators containing brushes and bearings.

Having the capability to calculate the aging factor for each use location allows an engineer to develop accelerated test profiles with test-exposure periods of no more than one-tenth the normal life. This yields calculated aging factors equal to or greater than the use environment.1

Accelerated Testing Examples

Rotating assemblies such as DC motors are designed to meet a minimum life expectancy as dictated by the brush material. One environment that dictates end of life is operational hours at the maximum operating temperature. Brush life is a direct function of its use environment.

Tests have been run on rotating assemblies to compare durability test results (simulation) with accelerated aging test results (stimulation) with a 1:1 correlation. Accelerated aging tests validate that end of life occurs in approximately one-tenth of the time that the same failure would take during durability testing. This correlates with experience by users.

We can apply accelerated aging to components whose life is limited by abrasion when we understand that wear-out of these materials occurs exponentially as a function of heat rise. Since brushes wear as a function of heat with little or no effect from cycling while bearings are affected by both thermal cycling and high temperature, a unique test must be developed for each component family.

Product aging is considered in four operating modes:

    Normal use. Upper and lower design limits. The region beyond the design limits which is the design-forgiveness region. The region where destruction occurs because coefficients of expansion beyond the elastic limits of the materials cause fatigue failures.

Valid aging tests stay away from the region that causes destruction.

For example, for a rotating component in an engine compartment, you first need to develop the climatic conditional environment. For a vehicle carrying a 36,000-mile warranty, Table 1 shows the aging factor to be a minimum of 14 for the conditions shown. The table also displays scenarios for vehicles with up to 150,000-mile warranties and corresponding age factors.

Table 2 depicts three test groups with corresponding accelerated aging factors. The first group shows normal product use with typical environment, durability, and design aging factors. The second group illustrates durability profiles for different warranty considerations, and the third demonstrates development of accelerated aging profiles.

Many automobile products are designed for 36,000-mile to 150,000-mile service, which is a normal three- to 10-year warranty life. The table illustrates how changes in exposure environments alter the related aging factor. There is a correlation between use environments and durability tests because they closely replicate each other.

As warranty periods increase, so does the test time to validate product reliability. This minimizes the time available for the engineer to make design improvements. For this reason, accelerated aging is of the utmost importance.

Developing and Applying the Profile

Manipulating the simulation environments allowed Siemens to develop a stimulation environmental profile. The company applied aging factors for various warranty mileages and was able to reduce automobile component test time. This was done by varying thermal properties:

    The mass rate of change per °C per minute. The number of thermal cycles. The total hours of exposure to the test environment. The span of temperatures in °C.

Using this procedure, test times were reduced to less than two weeks. Some of the applied conditions are beyond the design limits but do not encroach into the destruct region.

Logarithmic aging (10 times or more) starts to occur as a test sample is subjected to environments that exceed the product’s design region. If a test sample is subjected to environments that are near or in the destruct regions of the product’s thermal properties or elastic limits, an aging rate of 100 times or more would occur. Care must be taken to prevent this.

Siemens goals are to deliver failure-free products at a lower cost. To this end, the accelerated-aging program reduces the time to market and assures high reliability. When a mechanical system requiring a 2,000-hour life can be tested in 168 hours, there is sufficient time to make design improvements and validate the design integrity.

Reducing design time on a Siemens product from a nine-month cycle to less than two months results in at least 60% cost savings. Initial accelerated test profiles (less than two weeks)—nowhere near the material destruct region—allow ample time to optimize the final profile.

Siemens developed unique environmental test sequences and subjected the products to conditions that correlated with wear-out and precipitated-failure mechanisms. This became possible because detailed knowledge of the relationship between use, design forgiveness, and destruct limits was developed. The company then could apply aging at rates 10 times or more the normal usage, speeding the time to market for scores of products.

The Bottom Line

There are different approaches to the development of accelerated-aging profiles. Some systems use static climatic changes, and others have a mechanical abrasive wear which must be the driving force in developing the failure mechanism evaluation. Accelerated aging is a technique, the effectiveness of which depends on how well it is applied.

Reference

1. Capitano, J., “Explaining Accelerated Aging,” EE-Evaluation Engineering, May 1998, pp. 46-51.

About the Authors

Dr. Joseph Capitano is president of Advanced Reliability Engineering Technology. During his career, he has been involved in the improvement of reliability for products ranging from underwater, vehicular, test equipment, and aircraft to space shuttles and spacecraft. Dr. Capitano holds undergraduate degrees in electrical technology and management and an M.B.A. and a Ph.D. in management and logistics and is a Registered Professional Engineer in California. Advanced Reliability Engineering Technology, 3203 Shadylawn Dr., Duarte, CA 91010, (626) 358-0933, e-mail: [email protected].

Rob Anderson is manager of engineering services at Siemens NAMO, (519) 680-5481, e-mail: [email protected].

Boris Sverzhinsky is the manager of the accelerated testing program at Siemens NAMO, (519) 680-5547, e-mail: [email protected].

Siemens NAMO, 1020 Adelaide St. S., London, Ontario, Canada N6E 1R6.

TABLE 1

Warranty

Years of Warranty

Hours of Life

Low Temp. (°C)

High Temp. (°C)

Life Cycles

Rate of Change (°C/min)

Aging Factor

36,000

3

1,200

40

107

8,760

5

14

50,000

5

1,667

40

107

14,600

5

22

75,000

7

2,500

40

107

20,440

5

32

100,000

8

3,333

40

107

23,360

5

39

150,000

10

5,000

40

107

29,200

5

53

TABLE 2

 

Group

Test Conditions

Warranty Mileage

Rate of

Change (°C/min)

Life Cycles

Hours of

Life

Temp.

Range (°C)

Days in Test

Aging Factor

1

Product Use

36,000

5

8,760

1,200

60

50

14

1

Durability Test

36,000

10

1

2,000

87

84

14

1

Design Parameters

36,000

10

1

2,000

87

84

14

2

Durability

50,000

10

1

3,000

87

208

25

2

Durability

75,000

10

1

4,000

87

271

29

2

Durability

100,000

10

1

5,500

87

292

34

2

Durability

150,000

10

1

8,000

87

333

46

3

Designed Accelerated Profile

75,000

20

1

168

130

7

25

3

Designed Accelerated Profile

100,000

20

1

280

130

12

32

3

Designed Accelerated Profile

125,000

20

1

336

130

14

35

3

Designed Accelerated Profile

150,000

25

1

336

150

14

57

Copyright 2000 Nelson Publishing Inc.

May 2000

 

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