GB/T 18294.7-2026Technical identification methods for fire investigation - Part 7: X-ray imaging analysis (English PDF)

5.2 Technical Requirements

5.2.1 The spatial resolution of the X-ray imaging system should be no less than 4 lp/mm, and the low contrast resolution should be no greater than 2.3%. Note

1.Spatial resolution, also known as line-pair resolution, refers to the highest spatial frequency in a specified set of test charts that an X-ray imaging system can resolve. It is usually expressed in units of... The number of resolvable line pairs within a distance. Note

2.Low contrast resolution, also known as density resolution, refers to the lowest contrast detail of a specified object that can be distinguished under uniform background conditions.

5.2.2 The maximum tube voltage of the X-ray generator should be no less than 130kV, and the radiation output should be stable.

5.2.3 Under operating conditions, the reference control level of dose equivalent rate outside the protective device of the X-ray imaging system should not exceed 2.5 µSv/h.

6.1 Selection

6.1.1 Examine the submitted samples and select those that can prove the cause of the fire for X-ray image analysis.

6.1.2 When disassembly may lead to changes in the internal structure, X-ray image analysis can be performed directly on the submitted sample.

6.1.3 Ensure that the physicochemical properties of the sample do not change under X-ray irradiation; if the sample undergoes physicochemical changes under X-ray irradiation... If the nature of the change is altered, the change should be predictable and should not have a significant adverse impact on the equipment and related personnel, nor affect subsequent analysis.

6.1.4 Ensure that the size and weight of the sample are within the limits allowed by the equipment.

6.1.5 Ensure that the sample does not crack or break, and prevent some sample from falling into the sample chamber and affecting the normal operation of the equipment.

6.2 Cleaning and Recording

6.2.1 Clean the selected samples and record the external characteristics of the samples before and after cleaning using methods such as photography, for the purpose of recording internal and external characteristics. Comparative analysis.

6.2.2 During the cleaning process, the focus should be on removing high-density deposits such as metal that may obstruct the key areas to be observed, ensuring that the key areas of the sample are clear. The key parts are not damaged.

6.2.3 Record the placement of the sample in the sample chamber.

7.2 Setting up an X-ray imaging system

7.2.1 X-ray intensity Adjust the tube voltage and tube current according to the density and thickness of the sample to determine the appropriate X-ray intensity in order to obtain a clear image.

7.2.2 X-ray detection device Based on the size and type of the sample, as well as the key dimensions to be observed, select and determine the X-ray detection device.

7.2.3 Location of X-ray emitting device and X-ray detection device Adjust the relative positions of the X-ray emitting device, the detection device, and the sample, and adjust the magnification and clarity of the X-ray projected image. Under the condition of meeting the magnification, the position of the X-ray emitting device should be adjusted first to obtain a clear X-ray projection image.

7.2.4 Image Quality Adjusting parameters such as grayscale and contrast of X-ray projection images improves image quality and clearly reveals the internal structural features of the sample.

7.3 Acquiring Projected Images The computer system's control software is used to adjust various parameters, observe the sample, select characteristic locations for photography, and obtain X-ray projection data. image.

8.2 Result Judgment and Expression

8.2.1 Compare and analyze the external feature images and X-ray images of the sample to objectively describe the internal structural features of the sample.

8.2.2 In X-ray projection images, areas with lighter gray levels and higher brightness have lower density, while areas with darker gray levels and lower brightness have higher density.

8.2.3 When analyzing melt marks of copper, aluminum, iron conductors and their alloys, the melting properties should be determined by combining the macroscopic morphological characteristics of the melt marks and the distribution of internal pores. Perform the analysis.

8.2.4 When analyzing electrical equipment and components with complex internal structures, such as batteries and capacitors, distinguish the differences between the sample and the comparison sample, and comprehensively analyze the external characteristics. The characteristics of the part are analyzed to determine the reasons for the changes in its internal structural features.

8.2.5 As needed, compare the X-ray images of the sample, specimen, and control sample to analyze the changes in their internal structure.

8.2.6 When X-ray image analysis is used in conjunction with other methods, the results of multiple methods should be combined for comprehensive analysis.

8.3.1 Characteristics of Circuit Melt Marks

8.3.1.1 Characteristics of wire melting marks refer to the features that indicate the presence of wire melting marks inside the fire residue, including but not limited to.

a) The ends of the conductors exhibit spherical, pitted, nodular, or other irregular projection features; typical features are shown in Figure A.1 of Appendix A.

b) The phenomenon of two or more wires crossing and sticking together, with spherical, nodular or other irregular projection features at the sticking point.

8.3.1.2 According to GB/T 16840.1, it is determined to be a short-circuit melt mark, and there are a large number of large, directional pores inside. If the characteristics are consistent, it can be identified as a secondary short-circuit melt mark, and typical features are shown in Figure A.2.

8.3.2 Structural characteristics of lithium-ion batteries

8.3.2.1 Structural characteristics of lithium-ion batteries refer to the features that reflect the integrity of the battery's internal structure, including but not limited to.

a) If the battery as a whole exhibits swelling and rupture, or if the outer casing shows signs of explosion from the inside out, it can be determined that the battery has undergone a thermal runaway process;

b) The battery casing is intact, but there are cavities inside caused by missing materials, which indicates that it has undergone a thermal runaway process;

c) The current collector inside the battery exhibits significant turbulence, indicating that it has undergone a thermal runaway process. Typical characteristics are shown in Figure A.3b). (Normal) State correspondence diagram A.3a);

d) The presence of localized missing current collectors inside the battery indicates that it has undergone a thermal runaway process. Typical characteristics are shown in Figure A.3d). State correspondence diagram A.3c).

8.3.2.2 When a lithium-ion battery that has undergone thermal runaway has localized damage to its internal current collector, the battery can be disassembled and analyzed according to... GB/T 16840.1, GB/T 16840.4 and GB/T 19267.6 specify the identification of melt marks on the current collector in the defective part.

8.3.3 Structural Features of Electric Heating Appliances Structural features of electric heating appliances refer to the characteristics that reflect the integrity of the heating appliance's casing and heating wire, including but not limited to.

a) The heating wire is damaged, with obvious metal accumulation at the defect location, and the diameter of the metal accumulation area is significantly larger than the heating element. The diameter of the component indicates that the heating wire has melted at high temperature.

b) If the outer casing of the heating wire sheath is intact but the internal heating wire has melted at high temperature, it can be determined as an electrothermal melt.

c) If the outer casing of the heating wire sleeve has few defects, but the internal heating wire exhibits multiple high-temperature melting characteristics, it can be determined as an electrothermal melt.

d) If the outer casing of the heating wire sheath has few defects, but the internal heating wire has many defects and exhibits characteristics of high-temperature melting of the heating wire, it can be determined as an electric... Thermal fracture, typical characteristics are shown in Figure A.4.

8.3.4 Switch On/Off Characteristics Switch on/off characteristics refer to the features that reflect the on/off status of switches such as knife switches, residual current devices, and air circuit breakers, including but not limited to.

a) If the positions of the moving and stationary contacts inside the switch are clearly identified, and good contact between the moving and stationary contacts is confirmed, the switch can be determined to be active. Road status;

b) If the positions of the moving and stationary contacts inside the switch are clearly defined, and it is confirmed that the moving and stationary contacts are not in contact with each other, the switch can be determined to be... The typical characteristics of the open circuit state are shown in Figure A.5.