Incremental indentation

Figure 2.5 Schematic incremental indentation by a conical indenter. See text for...

Increment. See preferred increment. Indenting. In structural brickwork, the omission of a suitable series of bricks so that recesses are left into which any future work can be bonded. 

An alternative to the measurement of the dimensions of the indentation by means of a microscope is the direct reading method, of which the Rockwell method is an example. The Rockwell hardness is based on indentation into the sample under the action of two consecutively applied loads - a minor load (initial) and a standardised major load (final). In order to eliminate zero error and possible surface effects due to roughness or scale, the initial or minor load is first applied and produce an initial indentation. The Rockwell hardness is based on the increment in the indentation depth produced by the major load over that produced by the minor load. Rockwell hardness scales are divided into a number of groups, each one of these corresponding to a specified penetrator and a specified value of the major load. The different combinations are designated by different subscripts used to express the Rockwell hardness number. Thus, when the test is performed with 150 kg load and a diamond cone indentor, the resulting hardness number is called the Rockwell C (Rc) hardness. If the applied load is 100 kg and the indentor used is a 1.58 mm diameter hardened steel ball, a Rockwell B (RB) hardness number is obtained. The facts that the dial has several scales and that different indentation tools can be filled, enable Rockwell machine to be used equally well for hard and soft materials and for small and thin specimens. Rockwell hardness number is dimensionless. The test is easy to carry out and rapidly accomplished. As a result it is used widely in industrial applications, particularly in quality situations. 

The fiber modulus and matrix shear modulus are also required for the analysis. The fiber s coordinates are recorded directly from the stage controllers to the computer. The operator begins the test from the keyboard. The x and y stages move the fiber end to a position directly under the debonder tip the z stage then moves the sample surface to within 4 yum of the tip. The z-stage approach is slowed down to 0.04 jan/step at a rate of 6 steps/s. The balance readout is monitored, at a load of 2 g the loading is stopped, and the fiber end returned to the field of view of the camera. The location of the indent is noted and corrections are made, if necessary, to center the point of contact. Loading is then continued from 4 g in approximately 1 g increments. Debond is determined to have occurred when an interfacial crack is visible for 90-120° on the fiber perimeter. The load at which this occurs is used to calculate the interfacial shear stress at debond. 

It can be seen that ceramic multilayer structures have been produced with increments of the hardness of up to 60 GPa, increasing the hardness by up to a factor of almost 3. Initial work in this area has developed a number of ideas, such as the effect of modulus mismatch, which in some cases give good agreement with the models suggested but in many others do not. It is suggested that at least some of this discrepancy can be accounted for by differences in the microstructure and residual stress-state of the film, both of which are often poorly characterized. Furthermore there is very little direct evidence about how these structures deform and in particular about how different layers must be strained in order to accommodate the indenter when it is pressed into the sample. Further advances in this area will require the greater use of numerical techniques to analyse the complex stress and strain behaviour under the indentation, coupled with the use of recently developed techniques that allow the localized deformation behaviour to be observed in detail. 

The microhardness of the SiC compact measured under a 19.6 N load increases by incorporating SiC-coated MWCNTs, as shown in Table 10.7. The hardness reaches 30.6 GPa for the content of 5 vol% SiC-coated MWCNTs. This high hardness is considered as an apparent value due to the elastic recovery of the indentation after loading. This interesting phenomenon is discussed later. On the other hand, the increment of hardness by incorporating uncoated MWCNTs is very low compared with the increment by incorporating SiC-coated MWCNTs. This behavior may be due to the poor adhesion with the matrix. 

Next, the authors confirmed the relationship between the immobilization of IgG and the deformation efficiency of the azopolymer. The relative immobilization efficiencies were plotted as a function of indent depth, as shown in Fig. 9.16. Incremental changes in the immobilization efficiency were observed by increasing the depth of the indents. However, the immobilization efficiency of the H-azopolymers was higher than that of the CN-azopolymers across the whole range, and this is also shown in Fig. 9.15. This difference shows that the degree of photoimmobilization is not only affected by the deformation capability but is also a property of the surface of the azopolymer and is related to the chemical structure of the azobenzene that is incorporated in the azopolymer. Whitesides and coworkers have also reported that immobilization is influenced by the properties of the surface (Ostuni et al., 2001). 

Incremental depth of penetration is measured between that caused by a minor load given as 10 kg and a major load using either a 120 diamond cone indenter or 1/16-in, 1/8-in, 1/4-in, or 1/2-in steel ball penetrators. Hardness is read directly on the dial indicator. 

All indentation equipment contains some mechanism for applying the load to the surface. For the commonest technique, the static hardness test, it is inherent in the apparatus that the load be applied incrementally and not instantaneously. These two methods of load application produce fundamentally different values for the hardness of any sample. When the load is applied incrementally, then the indentation is in equilibrium with the load throughout the test. 

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