The morphological characteristics of fracture reflect the quality of materials and play an important role in material quality inspection, fault diagnosis and fracture principle research. It is a factor that can not be ignored when studying the stress state and crack propagation of components. With the development of high-resolution microscope and electron microscope, people can study the mechanism of fatigue damage from the microscopic point of view. For example, scanning electron microscope (SEM), perspective microscope (TEM), atomic force microscope (AFM), scanning tunneling microscope (STM) and other technologies can more clearly observe its microstructure and crack tip structure on the micro scale, and people have a broader understanding of fatigue crack initiation and propagation behavior, It also has a great promoting effect on the grinding of fatigue fracture cracks.
In order to further study the fatigue crack propagation of compact tensile specimens of 42CrMo steel, the macro and micro morphology of the fatigue fracture of the tensile specimens were observed and analyzed on the basis of the crack propagation test.
The compact tensile specimen of 42CrMo steel is damaged as shown in Figure 1.
After the fatigue crack growth test, the compact tensile specimen of 42CrMo steel was destroyed into two parts, one of which was observed for macro morphology and the other for micro morphology.
The micro morphology of fatigue fracture was observed by scios focused ion beam field emission scanning electron microscope. The equipment used is shown in Figure 2.
Compared with optical microscope and transmission electron microscope, scios focused ion beam field emission scanning electron microscope has the following advantages: its sample preparation is simpler, and the maximum size can reach 120mm > < 80mm > < 50mm. The sample can be viewed from multiple angles. The microscope has higher resolution, so it can enlarge the image in a large range, and the image is clear and has a strong three-dimensional sense. It can collect and sort out all kinds of data. The damage to the sample is small and the pollution is small. The operation is simple, and the composition of the sample can also be analyzed.
Scios focused ion beam field emission scanning electron microscope is equipped with beam current measuring device, automatic diaphragm system and automatic axis closing ion beam system; After the sample is sliced, a three-dimensional image is generated under the combined action of ion beam and electron beam; Real time observation of the monitoring function of ion beam processing; Optimum working distance: 1.0nm × 15kV; 1.6nm · lkv, electron beam landing voltage: 20ev-30kv; Beam intensity: LPA － 400na; Magnification: 1 － 1000000x.
Fatigue failure usually originates from the surface of the specimen; Scanning electron microscopy is mainly used to observe the surface morphology of scanning parts. Therefore, when making scanning parts, the following conditions must be met: generally, in order to maintain the original structure, the sample surface, newly cracked fracture and cross section are not treated, so as to maximize the display of the observation part, and the sample is required to have good conductivity With good secondary electron yield, the sample is dry. Therefore, sampling, cleaning, drying and SEM observation are generally carried out during SEM.
The test fracture sample is a rectangle of 50x5mm, and its size is within the observable range of SEM. Further observation can be carried out on the stage. Add absolute ethanol to the vibrator to clean the test piece. Before using SEM, the specimen needs to be dried. Natural drying method is simple, easy to implement and time-consuming. It is usually suitable for samples with hard surface. This sample is treated by the above method, and then the sample is fixed on the stage with conductive adhesive. Finally, the section is observed.