Cracks bulk material

Times to failure for various stainless steels tested in MgClj have been shown to increase with increasing proportions of martensite present Perhaps the role of martensite under anodic dissolution conditions is comparable to that of ferrite in duplex stainless steels where the enhanced dissolution of one phase prevents crack initiation in the other. There is, of course, another aspect of martensitic transformation that should be mentioned, i.e. the transformation of austenite to martensite either in the bulk material or at a growing crack tip that can give increased susceptibility to... 

Usually, the molecular strands are coiled in the glassy polymer. They become stretched when a crack arrives and starts to build up the deformation zone. Presumably, strain softened polymer molecules from the bulk material are drawn into the deformation zone. This microscopic surface drawing mechanism may be considered to be analogous to that observed in lateral craze growth or in necking of thermoplastics. Chan, Donald and Kramer [87] observed by transmission electron microscopy how polymer chains were drawn into the fibrils at the craze-matrix-interface in PS films [92]. One explanation, the hypothesis of devitrification by Gent and Thomas [89] was set forth as early as 1972. 

The ultrasonic vibration of 20 kHz applies an intermittent force to the material to be cut and generates a crack (cut) at the tip, controlling its propagation or growth thereby minimising the stress on the bulk material. 

There are two main techniques used to measure the fracture toughness of ceramics fracture stress and hardness indentation. The former measures the load to fracture of a pre-cracked specimen using a single edge notched beam (SENB) or a chevron notched beam (CNB) sample. The main drawback of this technique is ensuring that the crack tip is atomically sharp. The second method uses the crack formed at the corners of the indentation produced during a Vickers indentation hardness test. This technique is rapid and relatively inexpensive. However, the toughness values measured are those of the surface, unlike the values obtained by fracture of the pre-cracked beams which are a measure of the bulk material properties. 

Effect on the Bulk Property of the Adhesive. Moisture can alter the properties of the bulk material by changing its glass transition temperature, inducing cracks, or chemically reacting with the polymer—a process called hydrolysis. But before these mechanisms occur, the moisture must first find its way into the bulk polymer. 

Figure 7.5. SEM micrograph of a cross section in a Ni-40 at.% Si/SiC specimen (20 min at 1350°C) showing a smooth interface without any graphite precipitate. Some cracks in bulk materials are seen but not at the interface which is comparatively strong. From (Rado et al. 1999) [23].

Fracture processes are associated with deformations. In glassy thermoplastics craze formation is the most frequent pre-failure deformation process. Just ahead of the crack tip, where the stresses are particularly concentrated, molecular chains of the polymer are drawn out of their amorphous arrangement in the bulk material into fibrils (see Fig. 1.1 and Chapter 1) under the action of the principal tensile stress... 

The question arises whether the strain level and its variation with time reflects a real bulk material behavior. A first estimation of the strain e in the crack tip... 

On the other hand, under subcritical load of long duration, when the crack does not grow, an increasing craze size (see Sect. 3.1) indicates that bulk material is fibrillated without any detectable synchronous fibril failure as shown by the interference optical micrographs in Fig. 3.27b taken in a time interval of 3.4 10 s during... 

The H values measured along the crack zone are lower than the microhardness of the bulk. In this case the indentations were made close to the notch tip. The indenter tip was placed into the crack and the outer part of the indenter pressed on the bulk polymer. Assuming a two-phase system consisting of the crack and the bulk material, a lower H value than that of the bulk must be measured because the microhardness of the cracked portion (Hcrack) is equal to zero (eqs. (3.9) and (3.10)). 

In summary, it can be concluded that using nanoindentation hardness measurements on the crack tip, down to penetration depths of 0.8 pm, it is possible to detect very small craze zones in glassy polymers. The microhardnesses for all investigated samples can be divided into three regions (1) the cracked region, (2) the crazed zone and (3) the bulk material. It was also found that the microhardness of the crazed material is larger than the microhardness of the bulk polymer due to the orientation of the polymer chains within the craze fibrils. 

A reliable mechanical test to measure the adhesion of the interface is required. The standard method to quantify adhesion is to drive a crack at the interface between the two bulk materials and measure the critical energy release rate, (jc, to propagate such a crack. The implicit assumption made in most measurements of Qc is that the external work is dissipated in the plastic deformation of a small volume close to the crack tip. 

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