**2. Fatigue tests**

early stages of design is to evaluate components developed in the concept stage or to modify it, for changes in design during its production life act as facelift. **Figure 1** shows the time development reduction when accelerated tests are implemented. Most improvements have to be made prior to mass production and in the early manufacturing process of the tooling [1]. To perform this kind of test, it is necessary to evaluate the critical failures on the component related with the major probability of occurrence. All the load cases are evaluated; however, the target of this kind of a test is to evaluate the component in an easier way, with uniaxial

In other cases, the test is developed depending on the part or the process that has to be evaluated, for example, a new stamped part, or the weld cordon or the sequence of the welding. In those cases, a localized damage is developed with a correlation to its use in normal load conditions, but the important thing is to find the direction and load amplitude generated by use conditions, to get a correlation between the number of load repetitions and how many

The way to develop this kind of test starts with the instrumentation of a car with displacement transducers, accelerometers, force-moment transducers and strain gauges where it is necessary. The instrumented car is measured on different roads, used by different drivers in all the markets and under different weather conditions to acquire loads to measure the changes on

These responses are acquired as signals, which are analyzed to synthetize it in one signal representing all of these driving and use conditions. The new signal used for the durability test is known as spectrum. The reproduction of this spectrum in labs reaches the same damage on the component as in the roads, but the target of this kind of test is to reduce the time of evaluation in a controlled manner to detect the location of failure, the moment of occurrence and its propagation. To accelerate the test, the spectrum is extrapolated and proving grounds are developed. These are faster than duration cars on the roads, but it is possible to reduce the evaluation time developing accelerated test on test benches through extrapolating the loads; to perform this, the component can be mounted as assembly, subassembly or component as itself. Test reduction is reached due to the loads that represent more damage by their ampli-

tude and severity, than that applied on the component in normal-use conditions.

test, where it is possible.

254 Contact and Fracture Mechanics

kilometers or time of use represent it.

**Figure 1.** Development time reduction using accelerated tests.

the responses of the wheels.

It is important to evaluate components in experimental tests because fatigue strength has its inherent scatter due to four main factors: the loading, design, manufacturing and material (**Figure 2**). Experimental results under variable loads differ from analytical predictions owing to the effect of sequence loads [2, 3], Jimenez et al. [4] proposed a modification in Linear Damage Rule to include the effect of sequence in fatigue life prediction.

While manufacturing generally determines the strength and scatter, the geometry can modify the effect of mechanical properties [5, 6] due to the material that has variations on its properties. Loads have the major variability due to the diversity of drivers and factors such as number of passengers, weight on the car and its distribution, weather and its effects on the interchange of the loads between the non-suspended mass and the pave and the loads generated by bumpy ways and maneuvers.

Fatigue strength at the endurance limit is affected by the type of load and the size, reliability and surface roughness of the component [7]. The surface roughness can be improved with

**Figure 2.** Parameters influencing the structural durability of components.

processes such as shot peening, which is important because fatigue cracks usually initiate at the surface in homogeneous materials [8, 9].

the omission of low loads, they depend on the type of material and the application (**Figure 5**).

Production samples and different lots Material from different lots and suppliers processed in different facilities.

> Different users and styles of use, overloads not expected, abuse loads. Responses changed for the environment.

Accelerated Fatigue Test in Mechanical Components http://dx.doi.org/10.5772/intechopen.72640 257

Quality on the specimen Surface. Quality on Surface in critical areas as in notches.

Type of load (CA, VA)\* Loads in service from different users

The main objective is to develop an accelerated spectrum to get a test track.

Accuracy of test equipment Residual fatigue life

**In laboratory In service**

Environment Temperature, humidity in laboratory Temperature, snow, rain.

**Variable Fatigue in components**

Human Skills and expertise of lab staff to

CA-Constant amplitude; VA-Variable Amplitude.

**Table 1.** Main sources of scatter in mechanical fatigue.

perform and evaluate the test.

**Figure 4.** Parameters influencing the structural durability of components.

**Figure 5.** Schematic S-N curve: (a) linear-linear, (b) semi-log, and (c) log–log.

Production and materials.

Loads including environment

\*

In durability tests, the aim is to minimize the likelihood of failure applied for the more aggressive driver using the weakest component. **Figure 3** shows a strength-load interference model [10], which helps to manage the likelihood of failure of a component. As described in **Figure 2**, the component has different sources of scatter and its structural strength is determined on a bell-shaped curve. On the other side is evaluated the scatter for the loads applied to the component. The safety factor is defined by the difference between the central value of applied loads and its difference with the central value of the component's structural strength.

Jimenez et al. [7] reported that the advantage of component testing is that the effects of the material, manufacturing process and geometry are inherently accounted for. Although with controlled process as in test laboratory, fatigue test results have scatter, the main sources of scatter are summarized in **Table 1** [11].

Fatigue evaluation is not simple to predict by analytical methods, and to perform durability assessment and to predict the component´s life, it is necessary to measure the most precise information, and to do this, the loads in service are acquired and analyzed, to reproduce them as shown in **Figure 4**.

To build a track to perform a durability test, it is necessary to get information from the customers through a data acquisition with strain gauges, accelerometers and displacement transducers; then this information is analyzed. The output of this analysis is to get the desired signal that is known as spectrum. The importance of getting the spectrum is to compare the loads with the S-N curve in order to predict the component life through damage accumulated rule. Every step of the development process is evaluated to improve its mechanical response, and after the design is released, tests are performed to monitor the quality of the product to prevent failures in its service life.

To reduce the time required for testing on public roads, accelerated tests are performed on proving grounds. This simulates road damage for different maneuvers, different vertical loads of frames and different longitudinal dynamics for accelerating and braking, lateral dynamics and vibrations [12], combining all the events (normal roads, rough roads, emergency braking, high speed, city and country roads). In addition, the tests can be performed in the laboratory [5]. It is possible to increase the number of repetitions at high or medium loads, avoiding inadmissible stresses that satisfy the test results. Although some proposals [13] have included

**Figure 3.** Failure likelihood in components subjected to cyclic loads.

the omission of low loads, they depend on the type of material and the application (**Figure 5**). The main objective is to develop an accelerated spectrum to get a test track.


**Table 1.** Main sources of scatter in mechanical fatigue.

processes such as shot peening, which is important because fatigue cracks usually initiate at

In durability tests, the aim is to minimize the likelihood of failure applied for the more aggressive driver using the weakest component. **Figure 3** shows a strength-load interference model [10], which helps to manage the likelihood of failure of a component. As described in **Figure 2**, the component has different sources of scatter and its structural strength is determined on a bell-shaped curve. On the other side is evaluated the scatter for the loads applied to the component. The safety factor is defined by the difference between the central value of applied

Jimenez et al. [7] reported that the advantage of component testing is that the effects of the material, manufacturing process and geometry are inherently accounted for. Although with controlled process as in test laboratory, fatigue test results have scatter, the main sources of

Fatigue evaluation is not simple to predict by analytical methods, and to perform durability assessment and to predict the component´s life, it is necessary to measure the most precise information, and to do this, the loads in service are acquired and analyzed, to reproduce them

To build a track to perform a durability test, it is necessary to get information from the customers through a data acquisition with strain gauges, accelerometers and displacement transducers; then this information is analyzed. The output of this analysis is to get the desired signal that is known as spectrum. The importance of getting the spectrum is to compare the loads with the S-N curve in order to predict the component life through damage accumulated rule. Every step of the development process is evaluated to improve its mechanical response, and after the design is released, tests are performed to monitor the quality of the product to prevent failures in its service life.

To reduce the time required for testing on public roads, accelerated tests are performed on proving grounds. This simulates road damage for different maneuvers, different vertical loads of frames and different longitudinal dynamics for accelerating and braking, lateral dynamics and vibrations [12], combining all the events (normal roads, rough roads, emergency braking, high speed, city and country roads). In addition, the tests can be performed in the laboratory [5]. It is possible to increase the number of repetitions at high or medium loads, avoiding inadmissible stresses that satisfy the test results. Although some proposals [13] have included

loads and its difference with the central value of the component's structural strength.

the surface in homogeneous materials [8, 9].

scatter are summarized in **Table 1** [11].

**Figure 3.** Failure likelihood in components subjected to cyclic loads.

as shown in **Figure 4**.

256 Contact and Fracture Mechanics

**Figure 4.** Parameters influencing the structural durability of components.

**Figure 5.** Schematic S-N curve: (a) linear-linear, (b) semi-log, and (c) log–log.
