Fracture toughness increase

These effects have been found by Creton et al. [79] who laminated sheets of incompatible polymers, PMMA and PPO, and studied the adhesion using a double cantilever beam test to evaluate fracture toughness Fc. For the original laminate Fc was only 2 J/m, but when interface reinforced with increasing amounts of a symmetrical P.M.M.A.-P.S. diblock copolymer of high degree of polymerisation (A > A e), the fracture toughness increased to around 170 J/m, and then fell to a steady value of 70 J/m (Fig. 9). 

The monomer compositions were copolymerized using several different cure cycles. All of the cure cycles in this series included an additional post cure at 290 °C for one hour. The exact monomer compositions used, their detailed cure cycles and the physical properties of the resulting copolymers are shown in Table 16. As in the previous examples, here too the glass transition temperature went down as the fracture toughness increased. As before the fracture toughness rose as the mole ratio of bisbenzocyclobutene to bismaleimide approached unity. The presence of phenothiazine appeared to increase the fracture toughness in all of the examples although, its effect appeared most pronounced when... 

With increasing zvalue the density of P-SiAlON decreases linearly, similarly lowering Young s modulus, strength, thermal conductivity, hardness and fracture toughness.30 On the other hand, for low zvalues (z< 1) the hardness and fracture toughness increase and when z > 1 they decrease. It has been also depicted that glass-free microstructures could be obtained in P-SiAlON polycrystals with substitutional level z> 2.31... 

A number of articles on the design of ceramic laminates leading to a significant increase of their mechanical properties were published in the past [15-19]. Our work is based on the control of thermal residual stresses by optimization of the layered structure [20, 21], The proposed approach targets the fracture toughness increase of laminate ceramic composites and is based on the preliminary results both from our work [22, 23] and from the work of others [24-26],... 

The compressive residual stress crrl in the outside layers of a laminate shields natural and artificial cracks in the layer. Therefore, the effective (apparent) fracture toughness of such a structure increases. The more compressive residual stress induced, the more shielding occurs. Another important factor that contributes to the apparent fracture toughness increase... 

The fracture toughness increased in line with increases in Young s modulus, E, but not as a consequence of increases in fracture surface energy, y(. 

The temperature dependence of the fracture toughness for the DGEBA/DDS networks is shown in Fig. 12. Crack growth initiation data are symbolized by the open circles, arrest data by the closed circles, and stable data by the half-filled circles. With the exception of the 1007F/DDS network, the initiation fracture toughnesses increase with the test temperature. In contrast, the arrest values exhibit no noticeable temperature sensitivity, and when observed, the stable fracture toughnesses are of the same magnitude as the arrest values. It is... 

Comparing results obtained with POM G30 (Fig. 5b) and POM G15 (Fig.5c) at the same loading rate shows that fracture toughness increases with increasing fibre content, as expected, at 10 mm/min, but fracture toughness decreases with increasing fibre content in impact tests. Also this result can be explained only by a particular combination of two different effects. 

When the layers of SMA were thinner, the measured fracture toughness increased dramatically. Lee and Char argued that the diffusion rate away from the interface becomes more and more important as the initial layer becomes thinner... 

Martinez et al. (2000) examined the chemorheology and phase separation of polysulfone-modified DGEBA-DDM epoxy-resin mixtures. They found a delay in polymerization due to dilution and viscosity effects. The final morphologies are controlled by curing temperature due to the effect of temperature on phase separation. The fracture toughness increased with increasing immiscibility and was at its maximum for a bicontinuous morphology. 

The notch tensile and fatigue crack tests indicated that the two alloys and their welded joints possess sufficiently high brittle fracture resistance. As temperature decreased, the fracture toughness increased. No unstable crack propagation was... 

Figure 8.64 Fracture toughness increases with particle size in a microc-rack-toughened ceramic unless the size exceeds the critical size for spontaneous microcracking.

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