*3.2.1 Aluminum alloy 2025 stress: Strain curve*

The stress–strain curve is obtained by loading objects at a constant speed and measuring the amount of deformation in the tensile test. In this test, a specimen without any crack is installed on the tensile and is pulled at a speed of 1 mm per second. Using the results of this test, the stress–strain curve is plotted and the yield stress and ultimate stress in aluminum alloy 2025 are obtained. After testing and plotting the stress–strain curve, the ultimate stress level was 381.67 MPa and the yield stress was 275 MPa. The ultimate stress level for aluminum alloy 2025-T3 is 400 MPa in Ref. [12]. **Figure 12** shows stress–strain curve of aluminum alloy 2025 specimen that extracted from propeller blade. The difference between the measured stress and the reference stress is due to the life of the specimen used in the test because as the life of aluminum-containing copper alloy increases, this aluminum becomes brittle.


**Table 2.**

*Max. Acoustic emission count and cycle number of start signal changes in each specimen.*

### *Advances in Fatigue and Fracture Testing and Modelling*

#### **Figure 11.**

*Acoustic emission signal amplitude and cumulative count cumulative count vs. standard cycle diagram for all 9 specimens.*

**Figure 12.** *Stress–strain curve of aluminum alloy 2025.*

#### *3.2.2 Acoustic emission count and stress versus time diagram*

After performing the bending fatigue test and recording and analyzing the acoustic emission parameters due to crack initiation, it is necessary to subject the cracked specimens in the bending fatigue test to the tensile load to determine the parameters and characteristics of acoustic emission in fatigue crack growth. After performing the tensile test on 5 of the cracked specimens in the bending fatigue test, it is time to plot the count and stress vs. time diagram. This diagram shows the rate count and stress at each point in time of the test. Because all sources of additional signals and noise are blocked, the received signals are related to the acoustic emission activities inside the specimen. In general, these acoustic emission activities may be the result of plastic deformation or the growth of fatigue cracks created in the specimens. Because the test specimen is aluminum alloy 2025 with long life and brittle material and there is no sign of deformation in the specimen,

## *Determining the Characteristics of Acoustic Emission in the Fatigue Crack Growth of Aluminum… DOI: http://dx.doi.org/10.5772/intechopen.99360*

the signals received by the sensor cannot be the plastic deformation signals so these signals are due to the growth of fatigue cracks.

After examining the count and stress vs. time diagram, it was found that with increasing stress, which is obtained by dividing the tensile force on the cross-section of the specimen, the count also increases. As mentioned in the previous paragraph, this increase in the count is related to the growth of cracks in the aluminum specimen. According to the **Figure 13**, which shows the count and stress vs. time diagram for specimen NO. 1, which is randomly selected from 5 specimens to explain in detail, the count does not increase continuously and the increase in count occurs after increasing the slope of the stress diagram.

**Figure 14** shows the count and stress vs. time diagrams for all 9 specimens. As can be seen from the figure, in all specimens, the count increases sharply at the end of the test time, which indicates the highest crack growth activity during the test or an increase in crack growth rate with increasing force.

The highest increase of count for specimen NO. 1 occurred from 160 seconds to 167 seconds, where the highest rate of crack growth was observed. **Figure 15** shows the condition of the crack in 3 different times. Figure (a) shows the crack condition before the tensile test, when in the bending fatigue test the test is stopped immediately after observing the crack initiation. Figure (b) is after increasing the count

**Figure 13.** *Acoustic emission count, and stress vs. time diagram of specimen NO. 1.*

**Figure 14.** *Acoustic emission count, and stress vs. time diagram for all 9 specimens.*

**Figure 15.** *Crack condition during specimen NO. 1 test (a) before tensile test (b) in 160 second (c) in 167 second.*

at 160 seconds, where the count increases to 106 and the cumulative count to 189. Figure (c) also refers to a time of 167 seconds, where a sharp increase in the count, first at 165 seconds at 530 and then at 166 seconds at 602.

As the crack growth and the count diagram show, as time goes on and the stress and force increase, the count rate increase too, so the internal activity of the material and the crack growth increase, so that the maximum crack growth rate at the end of the test of each specimen.
