Engaged in the quality inspection of wood-based panels, not only need to be familiar with the content of product standards, but more importantly, we must know more about the theoretical basis for standard setting. Only in this way can we find problems, analyze problems and solve problems in actual work, and thus continuously improve The quality of quality inspection work and the standardization level of the wood-based panel industry. With the rapid development of China's wood-based panel industry, the variety, output and quality of structural panels are increasing. New standards have been introduced, new quality indicators and new testing methods have emerged, and targeted scientific papers and new knowledge and concepts have also been added. Here is a more important concept - creep, talk about some personal views.
1, the concept of creep
Creep is not a new concept in mechanics, but it is still a relatively new problem in the wood-based panel industry and subject areas, and it will inevitably lead to some misunderstandings. What is creep? For polymer materials, creep refers to the phenomenon that the solid polymer material gradually increases with time due to the constant stress for a long time. The appearance of creep is the result of the rearrangement of the thermal motion of the flexible chain polymer under the action of external forces. However, this orientation rearrangement is hindered by the intermolecular interaction force, and cannot be instantaneously completed, and it takes a certain period of time to exhibit a relaxation process. Brittle solid materials, such as ceramics, glass and stone, focus on the process of crack generation, expansion, instability and ultimately material damage under external forces. These brittle materials may have been destroyed long before the microscopic deformation under the external force that people's eyes can feel, so it is urgent to study the creep problem. Many metal materials increase with time under constant stress for a long time, but the theory of metal bonds is various, and its essence may be a multi-center theory of covalent bonds. It is different from the strength theory of polymers, that is, the mechanical response of polymer materials cannot be used to explain the creep of metal materials.
Wood and wood-based panels are quite typical polymer materials. The basic content of polymer chemistry and physics can be referenced and cited in the research of wood and wood-based panels. Cellulose macromolecules are linear macromolecules, hemicellulose macromolecules are linear macromolecules with a small number of branches, and there seems to be no chemical connection between the branches. The lignin macromolecules are like a cluster of shrubs with branches and branches. Although there are chemical linkages, the probability of occurrence is very low. The lignin macromolecules and hemicellulose macromolecules often combine to form the so-called "blended polymer" structure in polymer physics. It can be seen that the configuration and conformation formed by the arrangement of the atoms of the three macromolecules of wood in space and their aggregated structure are completely consistent with the causes of the creep phenomenon we introduced earlier. Of course, the development of creep depends mainly on the magnitude of the applied force, the ratio of the intermolecular interaction energy to the thermal kinetic energy, and is also completely related to time, temperature and water content. These research results are discussed in detail in polymer physics. In a research on the creep of wood-based panels many years ago, we also found that when the relative humidity of the artificial board environment changes, the creep speed will increase, and the relative humidity is small. Whether it grows bigger or smaller, it is like this. This may be the difference between the creep laws of wood and wood-based panels and the creep of most synthetic polymers. The qualitative analysis of this phenomenon does not seem to be difficult, but it is not so easy to establish a relatively complete mathematical model. There must also be some hard work on the ground. More than 20 years ago, creep research on wood-based panels was once popular, but the prospects for developing wood-based panel products with strong creep resistance are unclear.
2, on the issue of creep
Let's talk about a few issues that are easily overlooked and misunderstood in creep research.
One of the problems, creep and plastic deformation is not the same thing. It can be explained clearly by a simple creep experiment. The experimental device is similar to the static bending strength measurement except that a constant load is added in the middle of the test piece. The size of the load is generally one-third of the elastic limit of the test piece. It can be a weight or a mechanical force. The loading head of the testing machine applies a constant load. As time increases, the deflection of the test piece will gradually increase (in extreme cases, it can even be broken, of course, there must be several necessary preconditions). After a period of time, if the load is suddenly removed, the test piece can be found. The deflection will decrease instantaneously, and then the deflection will gradually become smaller as time goes by, until it is reduced to a certain extent, it will not change, but it is impossible to return to the original straight state. This shows that creep involves transient elastic recovery, delayed elastic recovery, and unrecoverable plastic deformation. Therefore, the creep of the test piece, that is, the elastic deformation and the plastic deformation. Moreover, the size of various deformations can be separated by measurement.
The second problem is that creep and stress relaxation are two different concepts. The definition of creep has been said before. The so-called stress relaxation is the phenomenon that the internal stress of the specimen is attenuated with increasing time under constant strain. Today's mechanical testing machines can be easily and accurately measured. When the curved wood component of the furniture is processed, the softened test piece is bent and fixed on the mold, and after a period of shaping treatment, the constraint of the mold is removed, and the bent test piece can be completely kept with the mold. Consistent, there may be some elastic recovery, which is related to the processing conditions, which is the typical stress relaxation, but it can not be said to be creep, because the test piece is completed under constant strain. The appearance of stress relaxation phenomenon is also the result of the rearrangement of the thermal motion orientation of the polymer segment and the macromolecule. The former is reflected in the strain and the latter is reflected in the stress.
The third problem is that it is recommended not to use the “stress release†method. The “stress release†method may be translated from the English literature. However, after reviewing the monographs of polymer physics and polymer mechanics, no use was found. The introduction of stress release. Because when using the term "stress release", people ask, where is the stress release? Is "stress release" instantaneously completed? From the above brief introduction, we know that the so-called "stress release" is only the process of rearrangement of high polymer segments and macromolecular thermal motion orientation, there is no "release" problem, so the use of "stress relaxation" is more To be exact. From the perspective of thermodynamics, it can be interpreted as the change of entropy of the system.
Eyeshadow Case,Mascara Tube,Mascara Bottle
Feiyi Cosmetic Trading Co., Ltd. , http://www.stfycosmetic.com