**4. Accelerated tests**

Component life testing is commonly designed to validate fatigue strength of a component based on a target customer usage and is based on loads acting on components. The measurement period is usually not long enough to be used directly in a test. The main target for extrapolated signals is based on time measurement restrictions and problems such as synchronicity, spikes, drifts observed on measurement devices. For this reason, the most representative road or proving ground is determined and that is extrapolated to reach the kilometers of the total life.

The advantages of finite element simulation are mainly in the early stages of design where the prototypes are not yet available, and also to improve its design without physical components. But also in this case, the loads for variable or spectrum as well as constant amplitude are developed to correlate with the accelerated tests. All the factors are evaluated in physical tests, and the results are analyzed through statistical results.

The load measures are extrapolated to the requirement. These spectrums are evaluated to include all the behaviors, as is shown in **Figure 12**. Spectrum test is developed using different

**Figure 12.** Spectrum development target.

**Figure 13.** Schematic spectrum development.

loads from the users and different responses are taken into account. *Ca* is the driver A, the most aggressive driver, *Cb* is the average driver, and *Cc* and *Cd* are the drivers that use less aggressive components but for more time. After having extrapolated the goal use, the spectrum is built considering all these measurements. **Figure 13** shows a schematic spectrum development. For variable amplitude test, the spectrum is reproduced and monitored with statistical analysis.

The repetitions of the cycles are found using the linear damage rule of Miner (Eq. (7)), and damage is evaluated using the ratio of the loads *(n)* with the number of repetitions (*N)* tolerated at *i* load level.

$$\mathbf{D} = \sum\_{i}^{n\_i} \tag{7}$$

The damage could be reached when the summation is 1 and there is an effect of sequence load. Depending on sequence effect loads, the damage can be reached too with values above or below 1 [4]. The failure is observed when there is a physical crack.
