2017年6月14日 周三

Optical glass like BK7 has been widely used in defferent optical system due to its supreme optical charateristics. But the hard and brittle nature of optical glass has brought great difficulty in machining. Thus studying the ductile-brittle behavior under different circumstances is the fundemental of the grinding, cutting and machining microstructure on the optical glass.

In order to thoroughly study the ductile and brittle behavior of optical glass, many experiments have been conducted in the past few decades, such as single grit scratch tests, multi-grit scratch tests and indentational tests. Among them, the single grit scratch test can provide insight to the ductile-brittle transition mechanism. Many single grit scratch tests are designed with linearly increasing depth or load, in order to obtain the result of the ductile-brittle tansition critical condition. For example…

Though the scratch test can provide observable outcomes of the behavior of brittle material, it’s hardly enough to explain the accumulative nature of grinding. When the surface is ground, multiple abrasive grains scratch the surface simultaneusly or successively. And the scratch path of different grains are probably interferential or even overlapped. xx and xx has combine the experiment method and SPH numerical simulation to study the MRR(material removal rate) in multi-grain scratch test when compared with single grit scratch test. The changing of MRR are mainly contributed by the interference of multiple cracks along the scratch.

Apart from the brittle interference effect of close range multiple grains scratch, the interferencing effect of the stress and the residual stress during scratch tests has hardly been investigated before. According to the Rankine theory, failure like cracks will initiate when the maximum principal stress at any point execeed a critical value. This theory also works on brittle materials and it illustrates the relationship between the ductile-brittle transition and the actual stress distribution. When two grains scratch through the surface at the same time or simultaneously in very close range, the stress they generate can have great influence on the scratch profile of each other especially when the cutting depth is near the brittle-ductile transition critical condition. This effect will greatly change the critical condition widely used in the machining of brittle materials. Moreover, the ascertaining of this effect can also contribute to the machining of micro and sub-micro structures in the ductile mode, as it can help design the machining tool and the processing parameters.

 

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