**7. Conclusions**

*PR* <sup>=</sup> <sup>100</sup>(<sup>1</sup> <sup>−</sup> \_\_*<sup>L</sup>*

17–33 for preliminary and exploratory tests, 33–50 for research and development tests, 50–75 for design allowable data tests and

*μ* = \_\_1

*<sup>s</sup>* <sup>2</sup> <sup>=</sup> \_\_\_1

**Figure 25.** Test results in steering knuckle analysis.

75–88 for reliability tests.

270 Contact and Fracture Mechanics

**Figure 1** (Eq. (8)).

the tests. The recommended values by Lee et al. [18] are as follows:

This value represents the portion of specimens that may be used in the variability to replicate

Steering knuckle results shown in **Figure 25** have 7 level of loads, and 90 Specimens using Eq. (4) get a percent replication of 92.2. These high values obtained from these results are evaluated to analyze a proposal to estimate an S-N curve. For a component test, recommended samples used depend on the target, for research and development tests 6–12, and for reliability tests 12–24 samples. The minimum samples for two load levels are three specimens for each load level.

The median is the central value of results at each load level, and the tendency is considered at 50% of reliability and is necessary to evaluate it to know the scatter of the factors described in

> *n* ∑ *i*=1 *n*

To evaluate the scatter of the components based on its fatigue results, the standard deviation is evaluated using Eq. (10). To take into consideration, its results have to be between 0.05 and 0.15; for samples without notches, the range is between 0.1 and 0.2, for uniaxial tests the range is between 0.2 and 0.3, while in complex tests, it can reach values between 0.3 and 0.6 [22].

> *<sup>n</sup>* <sup>−</sup> <sup>1</sup> ∑ *i*=1 *n*

(*xi* − *μ*)

*ns*) (8)

*xi* (9)

<sup>2</sup> (10)

Accelerated tests are used to reduce cost and time in the development process. It can also be used to monitor the quality of the components during its manufacturing life. Experimental evaluation is mandatory prior to final release and start of production to analyze the scatter of the manufacturing process and prevent failures in service life. The importance of performing variable amplitude loads tests is because the prediction of fatigue life under the complex spectrum loads is not possible by any damage hypothesis. The spectrum to evaluate the components in the tests is developed with the loads from different customers and markets and use conditions. Experimental results show discrepancies even within the same batch of production, and the statistical value to evaluate the reliability of the lot under test is the standard deviation that shows the influence of the factors described in **Figure 1**. Although the tests are performed under controlled conditions in a laboratory, in specimens with notches, the batch of production is released if the standard deviation of its fatigue results has a maximum value of 0.2. For samples without notches in uniaxial tests, the maximum scatter allowed is 0.3 and 0.6 for complex test [22]. To evaluate the fatigue strength as well as the scatter, it is necessary to perform durability tests, to prevent failures on the service life.
