Threshold stress effect

Two examples of path-dependent micromechanical effects are models of Swegle and Grady [13] for thermal trapping in shear bands and Follansbee and Kocks [14] for path-dependent evolution of the mechanical threshold stress in copper. 

The above-mentioned models differ in the relation that is derived between the rate of pull-out of the individual chain and the crack velocity. These models also differ in their interpretation of the threshold stress and the threshold toughness (Go). Also, V is expected to be dependent on the configuration of the connector chain at the interface. The value of v when connector chain crosses the interface just once is higher than the value when the chain forms multiple stitches, even though Go is not altered. When the chain forms multiple stitches, the block and tackle effect ensures that the viscous processes dominate even at lower velocities, and V is reduced by a factor of N from the value obtained from the single crossing case. These models are discussed by Brown and coworkers [45,46]. 

It appears that the observed breakdown must be explained in terms of the transient behavior of stress-induced defects even though the stresses are well within the nominal elastic range. In lithium niobate [77G06] and aluminum oxide [68G05] the extent of the breakdown appears to be strongly influenced by residual strains. In the vicinity of the threshold stress, dielectric relaxation associated with defects may have a significant effect on current observed in the short interval preceding breakdown. 

Fig. 8.13 Effect of carbon content of annealed mild steels upon threshold stress for cracking...

It may be felt that the initiation of a stress-corrosion test involves no more than bringing the environment into contact with the specimen in which a stress is generated, but the order in which these steps are carried out may influence the results obtained, as may certain other actions at the start of the test. Thus, in outdoor exposure tests the time of the year at which the test is initiated can have a marked effect upon the time to failure as can the orientation of the specimen, i.e. according to whether the tension surface in bend specimens is horizontal upwards or downwards or at some other angle. But even in laboratory tests, the time at which the stress is applied in relation to the time at which the specimen is exposed to the environment may influence results. Figure 8.100 shows the effects of exposure for 3 h at the applied stress before the solution was introduced to the cell, upon the failure of a magnesium alloy immersed in a chromate-chloride solution. Clearly such prior creep extends the lifetime of specimens and raises the threshold stress very considerably and since other metals are known to be strain-rate sensitive in their cracking response, it is likely that the type of result apparent in Fig. 8.100 is more widely applicable. 

Figure 1. Schematic representation of the power-law creep (e a") in conjunction with the effects of the threshold stress Oo and load transfer.

R. John, K.V. Jata and K. Sadananda, Residual Stress Effects on Near Threshold Fatigue Crack Growth Rates in Aerospace Alloys, In Press, International Journal of Fatigue, 2003. 

Fig. 7.93 Effect of artificial aging at 320 °F on the strength and smooth-specimen SCC threshold stress of 7075-T651 and 71 78-T651 aluminum alloys. Source Ref 97...

Effect of molybdenum content on stress-corrosion threshold stress intensity of austenitic stainless steels. Source Ref 166... 

Effect of yield strength on the threshold stress intensity factor (Kjh) for crack propagation in hydrogen gas (a) low-alloy steels [5] and (b) austenitic steels [17]. 

Effect of gas pressure on threshold stress-intensity factor for crack propagation in hydrogen gas Kj ) or fracture toughness in hydrogen gas (XJh) 15, 30, 32]. The data are for low-alloy steels (open symbols), while the data are for carbon steel (filled symbols). 

Bias stability. Additionally and perhaps of a greater concern for RFID applications, most organic devices to date show substantial bias stress effect, where their threshold voltage shifts during use. The mechanisms for this are currently being debated however, the consequence is that organic devices show a history-dependent performance, which is problematic from a circuit design perspective, for obvious reasons. 

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