# An Analysis of Mechanical Properties for Ultrasonically Welded Multiple C1220-Al1050 Layers
## 3 Results of the Experiment
### Evaluation of Mechanical Joint Performance
#### 3.1.3 Strength Curve  
The type and sequence of fractures in the weld zone were mentioned above. This was confirmed by the shear tensile displacement-strength curves obtained from the actual shear tensile test. As was stated previously, when the ultrasonic multi-layer welding specimens are simultaneously tensioned, they have five types of displacement-strength curves according to the fracture type. Figure 10 shows the five typical types of displacement-strength curves obtained from this shear strength test. The E-curve in Figure 10 is the displacement-strength curve of the test specimen of the aluminum indentation fracture of weld zone 1 and the aluminum base material fracture between weld zones 2 and 3. In this case, the strength of the indentation fracture is insignificant compared to the strength of the aluminum base material, so it is impossible to identify the strength of the indentation through the curve. However, the C-curve represents specimen 7, exhibiting the interface fracture type of all weld zones. In this case, the strength values of each weld zone can be identified through the graph. The maximum weld zone tensile strength of C was very small compared to A. In case C, it is possible to identify the strength of each weld from the graph because the strength of each weld was similar. The individual displacement-strength curves of the welds are shown in Figure 11. As mentioned above, weld zone 1 shows three fracture types and weld zone 2 shows two fracture types. Weld zone 3 shows the interface fracture type, and Figure 11 shows the characteristics of the displacement-strength curve according to each fracture type. The strength of each weld was compared by measuring the individual weld strength. Figure 12 compares the strength of each weld using the average of the strength values measured in the 25 welding conditions. The strength of the weld zone was the highest in weld zone 1. Weld zone 1 was where the horn came into direct contact with the aluminum surface, and received an indentation fracture, an aluminum base-material fracture, and an interface fracture. In weld zone 3, the strength was the lowest, at an average of 521.96 N. The average strength of weld zone 1 was 1,355 N, and the strength distribution was more than 1200 N in most parts of weld zone 1 because the dispersion width was very small. However, in weld zone 2, the average strength was 796.84 N and the distribution of the maximum and minimum value was very wide. In addition, the distribution range of strength was also very wide in weld zone 3. The influence of a change in welding conditions on strength was very small in weld zone 1, whereas weld zones 2 and 3 were very sensitive to the welding conditions. Although the melt area was not measured directly, the melt area could be predicted because the magnitude of the tensile load was relatively proportional to the size of melt area, as shown in Figure 12.  
Figure 10: Five kinds of displacement–strength curves represented by multi-layer ultrasonic welding in a one-step shear strength test. Fractures can be broadly classified into three general types, according to their location: aluminum base material fracture, aluminum indentation fracture, and interface fracture. Strength according to each fracture type is compared in Figure 13. The aluminum base material fracture shows the strength of the aluminum material (1200 N or more), and the indentation fracture shows similar fracture strength to aluminum. In the case of an interface fracture, weld zone 1 shows a similar fracture strength to aluminum, but weld zones 2 and 3 show less than 1/2 the strength of weld zone 1. In terms of the frequency of the weld fracture, weld zone 1 showed an aluminum base material fracture type in 19 out of 25 experiments, and showed indentation and interfacial fracture types three times, respectively. Weld zone 2 showed an interface fracture type about 88% of the time. However, weld zone 3 exhibited interface fracture type in all of the 25 experiments.