Cychc loading

Another important appHcation of LEFM is the rate of growth of a fatigue crack under cycHc loading. This is also controlled by the stress intensity factor through an equation of the following form (110) ... 

Fig. 16. Self-similar crack propagation in an isotropic material. The crack propagates in a direction perpendicular to the cycHc loading axis (mode I loading).

HG Nelson, Hydrogen-induced slow crack growth on a plain carbon pipeline steel under conditions of cychc loading , in Effect of Hydrogen on Behavior of Materials, AW Thompson and IM Bernstein, Eds., The Metallurgical Society of AIME, New... 

Bresler B, Bertero W. Influence of high strain rate and cychc loading of imconfined and confined concrete in compression. Heidebrecht A C, Emery J J, Speirs J W, et al, ed. Proceedings of Second Canadian Conference on Earthquake Engineering. Ontario, Hamilton McMaster University 1975, p. 1 13... 

Strain rate y (frequency of cycHc loading) Increases with strain rate G increase with y G/Gmait probably not affected if G and G are measured at same y Stays constant or may increase with Y... 

Under repeated cychc loading, a material may fail at stresses well below the single-cycle failure stress found in a typical tensile test. This process is called fatigue and is usually depicted by plotting the maximum stress versus the number of cycles to failure. 

In this chapter, we assume a monotonically increasing or static load. The application of fracture mechanics to the case of cychc loads is the topic of section 10.6.1. 

To compare the behaviour of such a ceramic under cychc loads with that under static loads, the accumulated loading time of cyclic loading can be converted to an effective loading time teff, the time that would cause the same damage under static loads. This is discussed in detail in Munz j Fett [104]. 

By definition, ESC is influenced by the level of the appHed multiaxial stress. It is expected that below an assigned value of stress in a specific medium ESC will not occur. Fatigue crack growth experiments at various constant levels of AK can be appHed to study the influence of the crack-tip loading on the ESC behavior in a systematic way. As an example, cast CT specimens of PMMA (Mn = 4.6 X 10 ) were tested in air and IPA at different levels of AK (0.6,0.7, 0.8, 0.9 MPa. v ). The tests were performed as described earlier, with the application of the medium after a certain time of cycHc loading in air. From the plots of the crack length ratio vs. number of cycles, the following observations can be made (Fig. 23) ... 

In addition to monotonic loading conditions, certain repetitive operations could also lead to mechanical failures. A typical example is a PWB used in cell-phone applications. The PWB could be subjected to multiple small deflections each time the ceU-phone keypad is pressed.This can cause an incremental deterioration in the mechanical response of the PWB and lead to solder joint interconnect failures in BG As mounted on the PWB. This sort of failure mechanism is more prevalent in relatively thin PWBs used in cell-phone applications. However, similar failures could also occur if, for example, multiple runs of ICT are performed on PCAs in high-end server/network applications. An industry standard test method has been developed by JEDEC to characterize the response of a PCA to cychc loading conditions (JESD22-B113). The layout of the test board as specified in the test method is similar to that for the monotonic four-point bend test. Some data on the cyclic bend response of Sn AgCu solder joints can be found in Reference 39. 

The explanted bone samples should be tested intact to estabhsh reference mechanical properties [78]. Loads should be appHed in a manner that minimizes focal loading of the specimen, so that accurate, representative material properties are measured. Preconditioning the specimens by cycHc loading, over several complete load-unload cycles, to a peak compressive force of approximately one average body weight (70 kg) is reasonable to do. This technique tends to smooth out variabdity in the data, and increases the likelihood of obtaining reproducible data. 

Garcia, M. and M. M. Knight. 2010. Cychc loading opens hemichannels to release ATP as part of a chondrocyte mechanotransduction pathway. / Orthop Res 28(4) 510-5. 

Better joint behavior can be achieved using steel rods mnning through columns and the ends of beams and epoxied into place. This system takes advantage of the ductile behavior of the steel rod which can yield not only in tension but also in compression as buckling is prevented by the surrounding timber. The cychc load behavior is good, with characteristics similar to reinforced concrete. 

---