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Fracture toughness and material relationship between speed

Source:   Date:2014-10-28 15:46:34   Views:

Although unrelated to the fracture toughness of materials and loading conditions, sample type, sample size and other conditions, but it and the temperature and loading speed test is a direct relationship. Under static load fracture toughness and fracture toughness under dynamic loading is completely different, fracture toughness at low dynamic loading material than a lot of static loads, ie materials tend to brittleness. When one percent fracture toughness relative impact velocity is large, the material is static loads, even a million. For example, the speed generated by the explosion is very large, the fracture toughness of the material at this time with respect to the static load on a small lot.
Not all materials have a speed sensitive, in general, the sensitivity, the low-strength steel for greater speed, and the speed of high-strength material sensitivity is much smaller, i.e., the fracture toughness with high strength materials changes in the amplitude of the speed change is small.
In impact loading, the material begins to crack formation and further expanded with the expansion of the crack, if the crack propagation resistance force is less than the extended (ie materials, fracture toughness), the crack will stop the expansion, which is the only crack. Some project members, the forces of each part is different, not the same fracture toughness of the material. When the crack generated from low toughness element to the elements or high toughness toughness extended from low to high toughness part of part time due to increased resistance, but also may cause cracks to stop expansion. Also because of the structural rigidity of some members, when the crack, the local stress drop, spreading force fall, less resistance and thus crack arrest.
When energy is consumed crack crack growth resistance (fracture toughness). Located crack unit length (or unit area) is needed to overcome the resistance R, so that power comes from the crack of the wall of the external force acting dW growth and in vivo release of elastic strain energy a dE, crack unit length (or unit area ) energy required for G, then:
In general, as the crack propagation resistance R will gradually increase. To simplify the problem we assume that the amount of R and crack - nothing to do. Extension force G is always related to the length of the crack, if under static conditions:
When the sealer shear conditions (ie, the diameter of the packaging and loading force) OK, sealing and cutting machine shearing forces which can be determined. : You can think is OK, then the 'proportional to a, it is a straight line through the origin (a 20).
With the expansion of the crack, if the power is greater than or equal to the growth rate of AG resistance rate AR, it will be because of instability and crack extension. Therefore, the critical point of unstable crack growth for
That is the critical point is the cut-off point 'curve and the R curve. When the resistance (fracture toughness) is constant, the performance of a horizontal line on the image, G is a straight line through the origin. The critical point is the intersection of the two point A. In the critical point are:
Two G R = G
We can see that over the critical point A, along with the further expansion of the crack, crack driving force G> R = G. This suggests that, after the critical point A, the power 'to overcome the resistance so that the stable crack growth, there are residual energy (G a R). This part of kinetic energy of the crack becomes, so that the accelerated expansion of the crack. Resistance so that the crack advance. From this, the material fracture toughness (ie crack propagation resistance R) and speed is also closely related. K} is material when static loads of fracture toughness, but in the actual processing of the experimental results of fracture toughness at high speed than the fracture toughness Kd when K. static load is smaller. Kd value is difficult to determine the theoretical method, under normal circumstances it fli determined by experiment, but due to closure of all high-speed load time is relatively short, measuring squid to experiment more difficult, so the accuracy of Kd values ??are also poor. Some experimental data of low carbon steel. From the graph we can see that the dynamic fracture toughness of the material varies with increasing velocity decreases, but only slightly. When the speed reaches a certain value, the dynamic fracture toughness of the material close to the minimum. Then, when the speed is further increased, the fracture toughness but gradually increased, and the rate of increase rapidly. Based on these data, sealing and cutting machine at a shear rate of about 5m / s, the fracture toughness value of the material a minimum, if the sealing and cutting machine shearing at this speed, the brittle fracture to increase the degree of bars, plastic fracture reduction, quality section should be better.