Failure mode transition

Shih, G.C. and Ebert, L.J. (1986). Interface strength effects on the compressive-flexure/shear failure mode transition of composites subjected to four-point bending. J. Mater. Sci. 21, 3957-3%5. 

B.A. Gailly, H.D. Espinosa Modeling of failure mode transition in ballistic penetration with a continuum model describing microcracking and flow of pulverized media. Int. J. Num. Meth. Eng. 54(3) 365-398 (2002)... 

Two families of transparent polycarbonate-silicone multiblock polymers based on the polycarbonates of bisphenol acetone (BPA) and bisphenol fluorenone (BPF) were synthesized. Incorporation of a 25% silicone block in BPA polycarbonate lowers by 100°C the ductile-brittle transition temperature of notched specimens at all strain rates silicone block incorporation also converts BPF polycarbonate into a ductile plastic. At the ductile-brittle transition two competing failure modes are balanced—shear yielding and craze fracture. The yield stress in each family decreases with silicone content. The ability of rubber to sustain hydrostatic stress appears responsible for the fact that craze resistance is not lowered in proportion to shear resistance. Thus, the shear biasing effects of rubber domains should be a general toughening mechanism applicable to many plastics. 

Several cautions are, however, in order. Polymers are notorious for their time dependent behavior. Slow but persistent relaxation processes can result in glass transition type behavior (under stress) at temperatures well below the commonly quoted dilatometric or DTA glass transition temperature. Under such a condition the polymer is ductile, not brittle. Thus, the question of a brittle-ductile transition arises, a subject which this writer has discussed on occasion. It is then necessary to compare the propensity of a sample to fail by brittle crack propagation versus its tendency to fail (in service) by excessive creep. The use of linear elastic fracture mechanics addresses the first failure mode and not the second. If the brittle-ductile transition is kinetic in origin then at some stress a time always exists at which large strains will develop, provided that brittle failure does not intervene. 

There are various improvements that can be made to the presented model, some improvements could be accomphshed. Foremost among these possible future-work directions is the inclusion of nonisothermal effects. Such effects as ohmic heating could be very important, especially with resistive membranes or under low-humidity conditions. Also, as mentioned, a consensus needs to be reached as to how to model in detail Schroder s paradox and the mode transition region experiments are currently underway to examine this effect. Further detail is also required for understanding the membrane in relation to its properties and role in the catalyst layers. This includes water transport into and out of the membrane, as well as water production and electrochemical reaction. The membrane model can also be adapted to multiple dimensions for use in full 2-D and 3-D models. Finally, the membrane model can be altered to allow for the study of membrane degradation, such as pinhole formation and related failure mechanisms due to membrane mechanical effects, as well as chemical attack due to peroxide formation and gas crossover. 

The check valves 03VH and 05VH are also identical, but they can degrade due to only one degradation mechanism and fail due to a unique failure mode. Three degradation states are considered for this degradation mechanism. The probability density functions of the transition times have the following forms ... 

Finally, the sensor llSP cannot degrade but can fail due to random electronic failures, considered as a single failure mode. The pdf of the transition time related to this failure mode is exponential, with parameter X(5T ) = 10 h, regardless of the configuration of the other components. 

Figure 1 provides a simple example of the visual representations of a Finite State Diagram (a sub-set of Harel Statecharts) and a Petri Net system for a simple repairable failure mode which is revealed or non-revealed. The example is too simple to demonstrate the modeling power of either Harel Statechart or Petri Net models, but it shows the general principles the system is modeled as a time sequence of states and transitions for which transitions rules are employed and pre- and post conditions may apply. The systems are analysed using Discrete Event Simulation methods providing the required results, i.e. the PFD and other interesting quantities. 

Fig. 17.12 Image representing failure modes in impact specimen, (a) A complete break into two pieces for brittle sample, (b) a mixed mode with hinged or a complete break for samples near brittle-ductile transition, and (c) a hinged break for tough samples (Tiwari and Paul 201 Ic)...

The in-plane mechanical, viscoelastic and thermal properties of a satin weave carbon fabric impregnated with an amine cured epoxy resin were studied by Abot and co-workers [74]. The in-plane quasi-static behaviour including the failure modes under tension, compression and shear and all the mechanical properties including elastic moduli and strengths were determined. The viscoelastic properties including the glass transition temperature were also measured as well as the coefficients of thermal expansion. These measured properties for the fabric composites were also compared with their corresponding ones for a unidirectional composite with the same fibre and matrix. 

Under different confining pressures such as 1, 15, 30, 50 MPa, the failure modes of marble specimens are brittle tension and shear failure, shear failure, transitional failure and dilatation shear failure respectively. 

The related add-on challenge is to optimize materials for conjoint failure modes when conjoint, nonlinear and coupled corrosion processes occur, including mechanically induced modes (wear, fretting, fatigue, and creep). Another need is the ability to handle or anticipate changes in solution or processes with time and transitions in corrosion modes. 

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