Failure Theories Experimental Evidence

Experiments made always by Gough et al. [6] on cast iron and wrought high carbon steel have shown that the ellipse quadrant Eq. (9.26) could not well represents the results that, on the contrary, could be described in terms of 

as shown in Fig. 9.10. An operative definition of ductile and brittle materials can be done on the base of the ratio between the fatigue limit in traction and that in torsion. For ductile materials it is 

An equation similar to (9.28) has been derived by Papadopulos et al. [17] for brittle materials 

At variance with Eqs. (9.28), (9.31) is judged to be equally vahd also for out-of-phase loads. It is common opinion that as far as brittle materials are concerned the equivalent stress based on the maximum principal stress criterion is effective. 

---

The formation of water and carbon in rapid hydrocarbon combustion, as in explosions, represents the best experimental evidence in support of this mechanism."1 The failure of Bone to detect methanol among the oxidation products in his experiments weakens this theory considerably, however. The only intermediate compound that could be identified was an aldehyde which together with water usually appeared as the early product. This is true of the higher paraffins as well as of methane. 

The Zhurkov theory relating temperature and applied stress to time-to-fail represents a neat and concise picture of mechanical failure. However, a great deal of the experimental evidence gathered to date to support this theory is suspect. Additional studies are needed to either confirm or to deny this simple mechanistic picture of mechanical failure. 

The function of a structural adhesive joint is to transmit an external load to the structural member. If the joint fails to function as it is intended, it will undergo damage or failure. The damage could be actual fracture of the structure, excessive elastic deformation, or excessive inelastic flow. The criteria for what constitutes structural failure depend on the performance requirements of the joint. The fundamental problem in the mechanics of adhesives and joints is to obtain some relationship between the loads applied to the joint and a parameter that will adequately describe the criteria for strucmral failure. The most common criterion for such failure of lap-type joints is actual fracmre of the joint. For a given combination of adherend and adhesive, the stress analyst must decide what the mode or theory of failure would be if the applied loads become large enough to cause failure. The decision as to which theory would realistically determine the mode of failure is usually based on past experience, or upon some form of experimental evidence. ... 

Fig. 3. Vibrational population distributions of N2 formed in associative desorption of N-atoms from ruthenium, (a) Predictions of a classical trajectory based theory adhering to the Born-Oppenheimer approximation, (b) Predictions of a molecular dynamics with electron friction theory taking into account interactions of the reacting molecule with the electron bath, (c) Born—Oppenheimer potential energy surface, (d) Experimentally-observed distribution. The qualitative failure of the electronically adiabatic approach provides some of the best available evidence that chemical reactions at metal surfaces are subject to strong electronically nonadiabatic influences. (See Refs. 44 and 45.)...

The classical description is based on the assumption that Gm is an analytic function of the mole fraction x and the temperature T for a fixed pressure p at and near the critical point of the mixture. In spite of rather good qualitative agreement with experimental results there is more and more evidence that the classical theory seems to be quantitatively inadequate at critical solution points, analogous with the failure of the classical theory at the critical point of a single component system. ... 

Prior to Harwood s work, the existence of a Bootstrap effect in copolymerization was considered but rejected after the failure of efforts to correlate polymer-solvent interaction parameters with observed solvent effects. Kamachi, for instance, estimated the interaction between polymer and solvent by calculating the difference between their solubility parameters. He found that while there was some correlation between polymer-solvent interaction parameters and observed solvent effects for methyl methacrylate, for vinyl acetate there was none. However, it should be noted that evidence for radical-solvent complexes in vinyl acetate systems is fairly strong (see Section 3), so a rejection of a generalized Bootstrap model on the basis of evidence from vinyl acetate polymerization is perhaps unwise. Kratochvil et al." investigated the possible influence of preferential solvation in copolymerizations and concluded that, for systems with weak non-specific interactions, such as STY-MMA, the effect of preferential solvation on kinetics was probably comparable to the experimental error in determining the rate of polymerization ( 5%). Later, Maxwell et al." also concluded that the origin of the Bootstrap effect was not likely to be bulk monomer-polymer thermodynamics since, for a variety of monomers, Flory-Huggins theory predicts that the monomer ratios in the monomer-polymer phase would be equal to that in the bulk phase. 

---