A Comparative Study of Delayed Hydride Cracking in Zr-3.5Sn-0.8Nb-0.8Mo and Zr-2.5Nb

A new test procedure for measuring the resistance to delayed hydride cracking was developed. The critical stress intensity factors for delayed hydride cracking and the crack growth velocities of Zr-3.5Sn-0.8Nb-0.8Mo alloy with different heat treatments were evaluated and compared with Zr-2.5Nb. It was found that Delayed Hydride Cracking (DHC) crack growth velocity increases with the alloy strength, and the critical stress intensity factor is independent of heat treatment history or alloy composition.

Subcritical Crack Growth Velocities (v-K Curves) of Dental Bioceramics

In this study the subcritical crack growth (SCG) behaviors of five dental bioceramics were evaluated in order to plot the crack growth velocity versus stress intensity factor (v-K) curves. Disc-shaped samples of two sintered porcelains, two glass-ceramics, and a glass-infiltrated ceramic composite were prepared and tested in artificial saliva using a biaxial flexure jig. The SCG parameters were evaluated by the dynamic fatigue test using five constant stress-rates and in an inert condition. Among the tested materials, the lithium disilicate glass-ceramic showed the highest susceptibility to strength degradation by SCG, whereas the glass infiltrated alumina composite showed the lowest susceptibility. The v-K curves showed that the SCG susceptibility significantly affects the crack growth velocity of the different bioceramics.

Dynamic Fracture Behaviors of Polypropylene/Polyamide-6 Blends

The dynamic fracture properties of polypropylene/nylon-6 (PP/PA6) blends, with different weight fractions and different compatibilizars, are investigated by reflected dynamic caustics experiments. According to the shadow spot patterns around a moving crack tip, which are recorded during the dynamic fracture process, dynamic fracture toughness and crack growth velocity can be estimated to evaluate the fracture properties of polymer blends. Moreover, combined with microscopic observation, the damage mechanism of the polymer blends is discussed.

Acoustic Emission during Fatigue Testing of Pressure Vessels

It is known that the basic mechanism of fatigue crack growth in ductile pressure vessel steels, which is repetitive blunting and re-sharpening of the crack tip, is a weak acoustic emission (AE) source. On the other hand, a large number of AE events occur during cyclic crack growth. Most AE events are caused by repetitive friction at the fracture surfaces, but these AE events show relatively weak correlation with the crack growth velocity. In the paper it is shown, based on examples – fatigue crack growth starting from an artificial weld defect, cycling of a natural crack defect, crack initiation and growth in gas cylinders - which information can be gained by relatively simple analyses of the AE data from cyclic pressure tests.

A four-point bending technique for studying subcritical crack growth in thin films and at interfaces

A technique was developed to obtain the subcritical crack growth velocity in a 4-point bending sample by analyzing the load-displacement curve. This was based on the observation that the compliance of a beam increases as the crack grows. Beam theory was used to analyze the general configuration where two cracks propagated in the opposite directions. A simple equation relating the crack velocity to the load and displacement was established, taking advantage of the fact that the compliance was linearly proportional to the crack lengths; thus the absolute crack length was not important. Two methods of obtaining crack velocity as a function of load were demonstrated. First, by analyzing a load-displacement curve, a corresponding velocity curve was obtained. Second, by changing the displacement rate and measuring the corresponding plateau load, a velocity value was calculated for each plateau load. While the former was capable of obtaining the dependence of crack velocity versus load from a single test, the latter was found to be simpler and more consistent. Applications were made to a CVD SiO2 system. In both cases of crack propagation either inside the SiO2 layer or along its interface with a TiN layer, the crack growth velocity changed with the stress intensity at the crack tip exponentially. As a result, a small crack will grow larger under essentially any tensile stresses typically existing in devices, provided that chemical agents facilitating stress corrosion mechanisms are also present.

Progressive Debonding of Multilayer Interconnect Structures

ABSTRACTProgressive (or time dependent) debonding of interfaces poses serious problems in interconnect structures involving multilayer thin films stacks. The existence of such subcriticai debonding associated with environmentally assisted crack-growth processes is examined for a TiN/SiO2 interface commonly encountered in interconnect structures. The rate of debond extension is found to be sensitive to the mechanical driving force as well as the interface morphology, chemistry, and yielding of adjacent ductile layers. In order to investigate the effect of interconnect structure, particularly the effect of an adjacent ductile Al-Cu layer, on subcriticai debonding along the TiN/SiO2 interface, a set of samples was prepared with Al-Cu layer thicknesses varying from 0.2–4.0 μm. All other processing conditions remained the same over the entire sample run. Results showed that for a given crack growth velocity, the debond driving force scaled with Al-Cu layer thickness. Normalizing the data by the critical adhesion energy allowed a universal subcriticai debond rate curve to be derived.