## 1 Introduction  
In order to improve the reality of a structural response for pre-tensioned steel structures exposed to fire by using numerical simulation, it is very important to capture the full range of the stress-strain relationship at elevated temperature for steel cables, which are always with a complex shape. It is well known that the strain gauge can accurately capture the stress-strain relationship for steel material at ambient temperature, but cannot at an elevated temperature. In previous test, most of the stress-strain relationships of steel material have been tested by extensometers at an elevated temperature. The brittle ceramic arms of an extensometer had to be removed before the ultimate strength in order to prevent being broken due to the limit measuring range. Thus, it can only capture the partial range of the stress-strain curve before the ultimate strength at an elevated temperature [1]. Although the external mechanical device with linear variable displacement transducer can trace the full range of the stress-strain curve at an elevated temperature, the strain resulted from this kind of measurement device is the average strain, which is different from the engineering strain. Furthermore, it is difficult to grip a group of wires in a tensile test due to the complex surface of a steel cable twisted by a branch of wire. The displacement between the front end of an arm and the surface of the specimen always occurred, especially for the test object with a complex geometrical surface such as a steel cable [2]. Recently, a charge-coupled device camera (CCDC) system, which has been improved by the digital image correlation method (DICM), has been used to measure the strain in the test coupon subjected to tension. A computer program based on this correlation method was developed for the calculation of the displacement components and the deformation gradients of an object surface due to deformation [3; 4]. Several experiments were performed to demonstrate the viability and accuracy of the correlation method. To form a gauge length is the primary step to collect test data by CCDC. [PERSON] et al. [5] presented a novel laser-engraving technology to create speckles on the surface of a metallic specimen, which will form gauge length and be kept up till the melting temperature of metallic materials. This kind of speckles was also applied to investigate the elastic modulus of tungsten material up to 1000 \({}^{\circ}\)C. However, it is not economical and convenient to mark gauge length for steel cable specimens compared with a high temperature paint sparkler. In 1996, [PERSON] et al. [4] proposed that the thermal expansion tests of standard inconel specimens up to 650 \({}^{\circ}\)C was conducted by a combined dual element optical fiber lamp and digital camera. The normalized cross interaction function was developed to determine the value of thermal expansion of specimens. In 2010, [PERSON] et al. [6] reported that the blue LED light and band-pass filter imaging system can overcome the decorrelation effect due to high bright light at elevated temperature. This kind of technology was used by CCDC to investigate the thermal expansion of Cr-Ni austenitic stainless steel up to 1200 \({}^{\circ}\)C. In 2012, [PERSON] et al. [7] carried out a series of tensile tests on pre-stressing steel at an elevated temperature by a steady method and transient method, respectively. It indicates that the novel digital image correlation method can measure the strain of tendon at an elevated temperature. In 2015, [PERSON] et al. [8] investigated the mechanical properties of Gh738 material at an elevated temperature using the digital image correlation method. The CCDC is also used to investigate the mechanical response of welded joints, concrete members and structural members at ambient temperature and elevated temperature respectively [9; 10; 11; 12; 13; 14].