Fracture analysis of polymer materials

Fracture analysis of polymer materials
Basic introduction
Polymer materials are a new type of structural material, which is widely used due to its excellent chemical and physical properties. In actual use, cracking or breaking may occur due to unreasonable design or use. According to the classification of fracture mechanism, its fracture mode can be divided into many types, and fatigue cracking/fracture belongs to one of them.

Fracture analysis of polymer materials is one of the important ways to analyze the failure of polymer materials. Fracture analysis is of great significance to the research and development of materials. By analyzing it to find the principle and understand the mechanism, it can improve product quality, improve the process or play a role in judging responsibility. Fracture refers to the phenomenon that the material is randomly broken by external force and separated into two or more parts. The surface or cross section after fracture is called the fracture. By analyzing the morphological characteristics of the fracture, the nature and cause of the failure of polymer materials can be studied. Fracture morphological characteristics analysis is one of the important methods for polymer material research. Under the action of external load, the gradual cracking of polymer materials goes through three stages: crack initiation, stable expansion and rapid expansion.

Fracture analysis of polymer materials

Fracture principle
The plastic deformation of polymer materials is caused by the deep molecular structure. During the fracture process of the material, the mutual influence of the expansion of the cavities and the plastic strain will make the fracture process complicated. Plastic deformation caused by slip in simple polymer grains cannot occur as in metal grains.
Fracture classification
The fracture of polymer materials is divided into brittle fracture and ductile fracture. Brittleness is always associated with the elasticity of the material in essence. The deformation of the sample before fracture is uniform, causing the crack of the sample to quickly penetrate the plane perpendicular to the stress direction. Generally, brittle fracture is caused by the tensile stress component of the applied stress, and ductile fracture is caused by the shear stress component.
Analysis method
1. Macroscopic observation

Use the naked eye or a magnifying glass to observe at low magnification. Generally, only observation can be made, and no photos can be taken. The observed information is usually described in words. Its magnification is less than 50 times, generally about 10 times, which can be used to observe the roughness of the fracture surface, the characteristics of crack initiation, expansion and final fracture zone, and to determine the crack direction, crack source location, load type and level, etc. Usually this method is used to collect information for further optical microscope observation. Low-magnification observation only requires that the fracture surface is clean and free of pollution.

2. Optical microscope observation
The most commonly used optical microscope is a stereo microscope. When it is necessary to further magnify and observe some characteristic details on the macroscopic fracture, an optical microscope can be used to directly magnify and observe. In addition, analysts usually use low-magnification optical microscopes to take photos of the entire fracture to obtain more comprehensive and detailed fracture information, and also provide basic information for the next microscopic observation.

3. Scanning electron microscope observation
Scanning electron microscope is a method of microscopic observation. The magnification can reach thousands or even tens of thousands times. It is usually used in conjunction with an energy dispersive spectrometer (EDS). In fracture analysis, it mainly has two uses: morphology observation and micro-area component analysis. Due to the poor conductivity of polymer materials, a certain thickness of conductive materials such as gold and platinum must be sprayed on the fracture before scanning observation. In addition, in order to prevent burns on the fracture surface, the observation voltage needs to be controlled, generally 5-15Kv is appropriate. When the energy dispersive spectrometer performs component analysis, three forms of point scanning, line scanning and surface scanning can be used. The specific selection can be based on actual needs.

Scanning electron microscope observation

(Scanning electron microscope/SEM)

The above three observation methods are in a progressive relationship in terms of process, first low magnification and then high magnification, first macroscopic and then microscopic. The above analysis techniques are more commonly used. In addition to these analysis techniques, metallographic microscopes, transmission electron microscopes, etc. can also be used for observation. The specific selection is also based on actual needs.

Fracture morphology
The quantitative relationship between the characteristic parameters of the fracture surface morphology of materials and the mechanical properties of materials is widely used in the fields of material fracture research, failure analysis and research and development of new materials. It is generally divided into mirror area, ribbed morphology, arc streak line, and stress whitening area.
(1) Mirror area
The mirror area is formed by the extension and rupture of cracks along one or a few silver streaks. The growth of silver streaks depends on the process of the orientation transformation of the polymer chain on the boundary from random coils to the middle of the silver streak. The smaller loading speed and higher test temperature make the silver streak growth time more sufficient.
(2) Rib-like morphology
The rib-like morphology is composed of alternating rough bands and smooth bands, with the rough band in front and the smooth band in the back; the rough band is composed of many uneven small planes, and silver streaks can usually be observed on the smooth band.
(3) Arc-shaped streaks
Arc-shaped streaks are the morphological features left by crack extension, crack arrest and crack restart when the polymer material breaks. Lowering the test temperature and increasing the loading rate often lead to a decrease in the distance of arc-shaped streaks on the cross section of the polymer material.
(4) Stress whitening zone
The stress whitening zone is the area of ​​plastic deformation of the polymer material. It is the result of silver streaking or shear yielding of the polymer material under the action of external force. Its peripheral fracture surface morphology is often fiber morphology, micro-pits and parabolic patterns.

Fracture analysis of polymer materials Images