According to the fracture mode of parts, it is divided into edge crystals, cleavage, quasi-cleavage, fatigue, toughness, slippage, corrosion, etc., and the stress concentration of fracture sources such as bumps, knife marks, cracks, non-metallic inclusions, etc.

At least 3 - 5 samples of each failure mode were analyzed to ensure representative results.

Use mechanical processing or wire EDM to cut the sample suitable for scanning electron microscope analysis and observation, smooth the bottom of the sample, use a brush to clean the surface to remove surface debris, immerse it in anhydrous ethanol for 10 - 15 minutes ultrasonic cleaning, clean the surface of contaminants, such as oil stains, after taking it out, there should be no foreign matter on the surface of the sample, clean and dry before setting aside.

The fracture should be placed separately before observation to prevent contamination of the sample.

During macroscopic observation, the fault location, crack propagation direction, and the size of the final fault area should be recorded to preliminarily judge the failure mode.

During microscopic analysis, the spring sample should be placed in the sample chamber, fixed on the sample stage with conductive glue, the surface morphology should be positioned at low magnification of the fracture, and the source area, expansion area, final fault area, and valuable area should be observed at gradual magnification, and clear SEM image photos should be taken.

Under normal circumstances, the spring is subjected to the torsional warping force of bending, and its fracture has obvious radial fatigue propagation traces from the source area to the surrounding area under the low fold and microscopic morphology, the fatigue source area is mainly fractured along the crystal, the fatigue expansion area is obvious, and the final fault area is mostly U-shaped, generally ductile fracture, serious oxidation, and the lead attachment is often accompanied by lead adhesion in the spring quenching crack area of the lead bath heating. The morphology of the fracture caused by spring quenching cracks is shown in Figure 2 . The micromorphology of the fatigue fracture along the fracture and the fatigue propagation zone in the fracture fatigue source area are shown in Figure 3 and Figure 4 , and the lead adhesion in the electron microscopy spectroscopy analysis of the spring quenching crack area is shown in Figure 5 .

Folding refers to the interlayer rolling of materials during the manufacturing process to form a kind of “false skin”, which is improperly cold-processed and usually presents continuous or intermittent straight lines, furrows, or fish scale cracks. The fracture in the fatigue source area is severely oxidized, and the two sides are concave and protruding, and the fatigue extension area is obvious. The morphology of the fatigue fracture caused by spring folding is shown in Figure 6 , and the microscopic morphology of the fatigue source area, fatigue propagation zone, and final fault area caused by spring folding are shown in Figures 7-9 .

Pits are mechanical impacts that form small depressions locally. The fracture of the fatigue source area has obvious depressions and severe oxidation, independent or small amounts, mostly caused by mechanical damage, obvious fatigue flaring in the fatigue expansion area, and most of the final fault area is U-shaped, generally ductile fracture. The morphology of the fracture caused by the spring pit is shown in Figure 10 , and the micromorphology of the fatigue glow in the fatigue source area and fatigue expansion area caused by the spring pit is shown in Figure 11 and Figure 12 .

There are obvious corrosion pits and serious oxidation at the fracture in the fatigue source area, and the surrounding or the entire spring surface is densely corroded with pits, which are mostly caused by rust on the surface of the spring wire. The morphology of the fatigue fracture caused by the spring corrosion pit is shown in Figure 13 , and the microscopic morphology of the fatigue source area and fatigue propagation area caused by the spring corrosion pit are shown in Figure 14 and Figure 15 .

The fracture of the fatigue source area has obvious traces of mechanical damage from the source area to the surrounding area under the low magnification and microscopic morphology, the fracture of the fatigue source area has obvious traces of mechanical damage, severe oxidation, the fatigue glow pattern in the fatigue expansion area is obvious, the final fault area is mostly U-shaped, generally the ductile fracture, the morphology of the fatigue fracture caused by spring mechanical damage is shown in Figure 16 , the microscopic morphology of the fatigue source area and fatigue expansion area caused by spring mechanical damage is shown in Figure 17 and Figure 18 , comprehensive force analysis, The characteristics such as fracture morphology can determine that the spring is a bending and torsional composite fatigue fracture under mechanical load.

There are obvious radial fatigue propagation traces from the source area to the surrounding area under the low magnification and microscopic morphology of the spring fracture, and there are obvious long strip scratch marks at the fracture in the fatigue source area, severe oxidation, obvious fatigue glow in the fatigue expansion area, and the final fault area is mostly U-shaped, with generally ductile fractures. The fatigue fracture morphology caused by spring scratches is shown in Figure 19 , and the microscopic morphology of the fatigue source area and fatigue expansion zone caused by spring scratches are shown in Figure 20 and Figure 21 .

There are obvious traces of radial fatigue propagation from the source area to the surrounding area under the low magnification and microscopic morphology of the spring fracture, obvious non-metallic inclusions at the fracture in the fatigue source area, obvious fatigue glow pattern in the fatigue expansion area, and the final fault area is mostly U-shaped, generally ductile fracture, the morphology of the fracture caused by spring inclusions is shown in Figure 22 , and the microscopic morphology of the fatigue source area and expansion area caused by spring inclusions is shown in Figure 23 and Figure 24 . The results of electron microscopy analysis of non-metallic inclusions in the source area of spring fatigue are silicate inclusions, as shown in Figure 25 , and the comprehensive force analysis, fracture morphology characteristics, and micro-electron microscopy spectroscopy can attribute the spring failure to the stress concentration generated by the inclusion edge, and finally, the bending-torsion composite fatigue fracture caused by the spring failure.

The spring is affected by an abnormal shear external force, and its fracture has obvious traces of approximate parallel expansion from the source area under low magnification and microscopic morphology, and the fracture has obvious friction failure traces, and the fracture is a shear ductile fracture. The morphology of the fracture of the spring external force damage is shown in Figure 26 , and the microscopic morphology of the fracture of the spring external force damage is shown in Figure 27 .

Under normal circumstances, the spring is affected by the torsional warping force of bending, and its fracture has obvious radial fatigue propagation traces from the source area to the surrounding area under low magnification and microscopic morphology. There are obvious wear marks at the fracture in the fatigue source area, and the fatigue flaring in the fatigue expansion area is obvious. The microscopic morphology of the fatigue fracture caused by spring wear is shown in Figure 28 , and the microscopic morphology of the fatigue source area and fatigue propagation zone caused by spring wear are shown in Figure 29 and Figure 30 .