Fatigue strain controlled

Fig. 4. Fatigue data for high temperature alloys (a) As vs cycles to failure for various alloys tested under strain control. (°) = testing at RT in air ...

Cyclic loads were applied to the specimens using a hydraulic feedback load system which could be either load-, stroke-, or strain-controlled. Sinusoidal and linear-ramp waveforms were generally used. Fatigue frequencies were varied from 0.1 to 15 Hz. Unless otherwise stated, mechanical property data were obtained at a fatigue frequency of 10 Hz. 

A typical fracture surface of fibers with a mirror and hackle feature in SiC/SiC ceramics is shown in Fig. 7.24. The cyclic softening observed in SiC/SiC occurs either under stress control or strain control. Since this observation is crack initiation- and propagation-related, it is possible that similar phenomena may be more frequent after softening during fatigue deformation. The Manson-Coffin... 

Low-Cycle Fatigue Properties. Results of low-cycle fatigue experiments under strain control on as-worked W plate material at 815 °C are shown in Fig. 3.1-172. Low-cycle fatigue tests of pure W were performed in the temperature range between 1650 °C and 3300 C [1.184]. A relationship Afaiiure = exp(—aT) was found to be valid up to test temperatures of 2700 °C [1.185]. In all cases the failure mode was intercrystalline. Similar results were also obtained at a test temperature of 1232 °C [ 1.186]. The deformation behavior of Nb and Nb IZr under plastic-strain control at room temperature was investigated and cyclic stress-strain curves published [1.182]. 

An important issue is the influence of an electrochemical environment on the cyclic deformation behavior of metals [74,33-35]. As illustrated by the data in Fig. 1 for a carbon-manganese steel in high-temperature water, environment does not typically affect the relationship between stresses and strains derived from the maximum tensile (or compressive) points of steady-state (saturation) hysteresis loops [36]. Such loops should relate to elastic and plastic deformation prior to substantial CF microcracking. CF data of the sort shown in Fig, 1 are produced by either stress or total strain controlled uniaxial fatigue experiments, identical to the methods... 

Considering smooth specimens, the ranges of applied stress or plastic strain control the fatigue or CF responses of metals for HCF and LCF conditions, respectively. For HCF, smooth specimen CF hfe increases with decreasing elastic stress range, at cycles in excess of the transition fatigue life, Nj-, according to the Basquin equation... 

Experiments to characterize low-cycle CF life according to the Coffin-Manson relationship (Eq 2) foUow from an ASTM standard for LCF of metals in air, and a classic ASTM manual on laboratory methods (see Ref 37 and ASTM E 606, Practice for Strain-Controlled Fatigue Testing). Low-cycle CF... 

Ratcheting is produced by a combination of a sustained extensional load and either a strain-controlled cyclic load or a cyclic temperature distribution that is alternately applied and removed. This produces cycling straining of the material which in turn produces incremental growth (cyclic) leading to what is called an incremental collapse. This can also lead to low cycle fatigue. 

The ASME design fatigue curves are based on strain controlled data in which the best fit curves are constructed by a factor of 2 on stress or a factor of 20 on cycles to account for environment, size effect, and data scatter. 

Fig. 10.6. Formation of slip bands in ALMg 3 in a strain-controlled fatigue experiment Rs = —1, a = 0.5%, grain size 50iim). Optical micrograph (after [148])...

Thermal fatigue is observed mainly in stress-controlled loading because the strain amplitude increases in this case due to the reduction of stiffness with increasing temperature. The heat generated per cycle thus increases with time. If the loading is strain-controlled, thermal fatigue is usually not problematic because the stress decreases in this case. 

Fig. 10.28. Cyclic stress-strain behaviour at the beginning of strain-controlled fatigue experiments (after [130]). The controlled variable is shown on the left, the material answer in the centre...

If a strain-controlled fatigue experiment is performed at a non-zero mean strain, cyclic relaxation may occur in addition to cyclic hardening or softening, with the mean stress decreasing over time (figure 10.31(a)). If, on the other hand, the experiment is stress-controlled at a non-zero mean stress, the hys-... 

Fatigue has been studied to determine the effect of micro-structure on cyclic deformation and LCF initiation (Mah jan and Margolin, Metall. Trans. A, Vol 13, 1982, p 257-268). dmanstatten + grain boundary a and equiaxed structures of different a particle sizes were produced in smooth bcu specimens of a Ti-6246 alloy, heat treated to produce a 0.2% yield stress of about 1100 MPa (159 ksi). Specimens were cycled at room temperature under total strain control. 

Care miist be taken in analyzing this fatigue data and comparing to a given set of conditions as there are so many variables, including specimen geometry, siuface finish, R-ratio, and loading condition factors such as load controlled or strain controlled fi equency, and wave form. 

Wrought Bar Transage 175 Low-cycle strain-controlled axial fatigue at 400 and 425 °C for STA extruded bar... 

The fatigue behavior of titaniiun-nickel alloys is extremely complex and encompasses many different topics. lb summarize, titanium-nickel excels in low-cycle, strain-controlled environments, and does relatively poorly in high-cycle, stress-... 

Figure 10 Histograms of distance between PSB clusters, from the replicate Ni single-ciystal specimens fatigued for 1000 cycles under total strain control Ae. Shear strain = 0.12%, (a) air and (b) +160 mV/SCE in 0.5 N H2SO4 [4].

Fig. 1.7 8a — N fatigue curve of a carbon steel type A 106 B and A 333-6 obtained with cylindrical specimens under strain controlled alternate traction [14, 15]...

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