## 3 Calibration Tensile Test at Ambient Temperature  
### Uniaxial Tensile Tests on S355 Structural Steel  
Both of the strain gauge and CCDC were used to investigate the stress-strain relationship of S355 structural steel. The dimension of the specimen for a standard uniaxial tensile test is shown as  
Figure 3: Standard tensile test set-up for S355 structural steel at ambient temperature.  
Figure 2: Charge-coupled device camera system.  
Figure 4. The standard uniaxial tensile tests were conducted on a servo-hydraulic testing machine, with a maximum stroke displacement of 75 mm and a capacity of 500 kN, shown in Figure 5.  
There are two groups of specimen measuring by the strain gauge and CCDC system, respectively. For CCDC system measuring, the first step is to sprinkle black paint on the surface of each specimen to reduce the metallic brightness, and then, markers with white spots painting on a black color background were prepared within the middle part of a specimen surface of about 30 mm length. On the other side of the same specimen, a strain gauge with the measuring range of 100,000 \(\upmu\)e was pasted. The specimen was lightly tensioned to about 1 kN to eliminate any mechanical relaxation. Then, the tensile specimen was tested until failure at a straining rate of 0.0005/s, in keeping with the strain rates set out in the EN 6892-1 [17]. As shown in Figure 6, the S355 steel specimen experienced a ductile model with necking phase. The broken cross section in the middle of length formed a cone-shape with cross section area gradually reduced towards the broken surface.  
Figure 4: Dimension of steel specimens for standard tensile test (unit = mm).  
Figure 5: Servo–hydraulic testing machine.  
The stress-strain curves were obtained by a tensile test with strain gauge and CCDC system, respectively, as shown in Figure 7a. The elastic modulus can be determined by the tangent modulus of the initial linear phase of the stress-strain relationship. The effective yield strength is the intersection point of the stress-strain curve and the proportional line offset by 0.15% strain. The ultimate strength is the maximum stress in the stress-strain curve, and the fracture strain is dependent on the specimen broken. The mechanical properties of S355 steel are listed in Table 1.  
Figure 6: View of tensile failure of S335 steel at ambient temperature.  
Figure 7: Comparison of stress-strain curves of S355 steel measured by different methods at ambient temperature. (**a**) Full range of stress-strain curve. (**b**) Partial range of stress-strain curve.  
Test results indicate that the ultimate strength obtained by different measurements was the same. As shown in Figure 7b, the elastic modulus and yield strength, which obtained by strain gauge were smaller than those obtained by CCDC, and the deviation was 1.34% and 1.55%, respectively, as listed in Table 1. The test process displayed that the tensile specimen also experienced a large strain from the ultimate strength till fracture. As the limited of measuring range, the strain gauge can only capture a partial range of the stress-strain curve before ultimate strength. Comparing the strain gauge, the CCDC system can obtain a full range of stress-strain curve until broken as shown in Figure 7a.  
Therefore, it is more efficient to use the CCDC system than strain gauge to measure the specimens with large deformation through the test process.