Out-of-phase Thermomechanical Fatigue Research Articles

Thermomechanical fatigue behaviour of nickel base superalloy IN738LC Part 1 – Comparison of two unified constitutive models for description of mechanical behaviour

The capabilities of two unified constitutive models to predict the mechanical behaviour of nickel base superalloy IN738LC under uniaxial loading conditions have been investigated over the temperature range 450–850°C. The material parameters of each model have been identified from an experimental investigation and complemented by available data from the literature. Mechanical responses from isothermal tests performed at 450 and 850°C (such as creep, monotonic, and fully reversed isothermal cyclic tests) were used for the identification of model parameters, and model capabilities were examined by comparison with in phase and out of phase thermomechanical fatigue and stress relaxation tests. Both models were found to capture all features of the material responses under uniaxial loading, although with varying degrees of accuracy.

Thermomechanical Fatigue Behavior of a Quasi-lsotropic SCS-6/Ti-15-3 Metal Matrix Composite

The response of a quasi-isotropic laminate of metal matrix composite, SCS-6/Ti-15-3 in a thermomechanical fatigue (TMF) environment was investigated. To achieve this, three sets of fatigue tests were conducted: 1) in-phase TMF (IP-TMF), 2) out-of-phase TMF (OP-TMF), and 3) isothermal fatigue (IF). The fatigue response was dependent on the test condition and the maximum stress level during cycling. The IF, IP-TMF, and OP-TMF conditions yielded shortest fatigue life at higher, intermediate and lower stress levels, respectively. Examination of the failure mode through the variation of strain or modulus during cycling, and post-mortem microscopic evaluation revealed that it was dependent on the fatigue condition and applied stress level. Higher stresses, mostly with IP-TMF and IF conditions, produced a primarily fiber dominated failure. Lower stresses, mostly with the OP-TMF condition, produced a matrix dominated failure. Also, an empirical model based on the observed damage mechanisms was developed to represent the fatigue lives for the three conditions examined here.

Modeling the influence of initial material inhomogeneities on the fatigue life of notched components

Thermomechanical fatigue (TMF) is a low cycle fatigue process in which material life is correlated to the mechanical strain amplitude. However, it is well known that several other factors influence this life. This paper examines several of these parameters and their influence on life using experiments conducted on a second generation directionally-solidified (DS) Ni-base superalloy. The parameters considered include the influence of the temperature extremums (Tmax of either 750 or 950 °C and Tmin of either 100 or 500 °C), strain ratio (R∊), the strain-temperature phasing (in-phase (IP) and out-of-phase (OP)), the influence of dwells at the high temperature end of the cycle resulting in a creep–fatigue (CF) interaction, and material anisotropy associated with the grain growth direction (longitudinal versus transverse). Results suggest that the phasing has a primary role in controlling the mechanism of degradation. IP TMF is dominated by crack formation in volumes surrounding debonded carbides for both continuously cycling (CC) and CF at 950 and 750 °C, while OP TMF is dominated by surface oxidation and repetitive cracking of the oxide that reforms at the crack tip at 950 °C. Decreasing the Tmax to 750 °C the environmental and creep effects are reduced resulting in virtually pure fatigue exposure under OP conditions. With decreasing Tmin from 500 °C to 100 °C was observed an increase in inelastic strain amplitude and corresponding decrease in life. Variations in R∊ were found to have no significant influence on life or stabilized stress behavior. TMF loading in the transverse orientation resulted in life reductions over the longitudinal orientation due to cracks propagating in a transgranular manner. Lastly, only in material exposed to CF with a Tmax of 950 °C rafting of the γ′ precipitates was observed.

Thermo-mechanical fatigue behavior of an angle-ply SCS-6/Ti-15-3 metal matrix composite

The response of the continuous-fiber-reinforced metal-matrix composite SCS-6/Ti-15-3 with orientation [±45] 2 s was investigated under fatigue conditions. Three temperature and load profiles were used to characterize the metal-matrix composite, namely: isothermal fatigue (IF) at 427°C, in-phase thermo-mechanical fatigue (IP-TMF) from 149°C to 427°C, and out-of-phase thermo-mechanical fatigue (OP-TMF) from 149°C to 427°C. At high stress levels (above 400 MPa) the [±45] 2 s laminate displayed in-phase, out-of-phase, and isothermal fatigue lives which were all within 1000 cycles. At these high stress levels the laminate exhibited the most detrimental fatigue response when subjected to the in-phase TMF profile. The IP-TMF and IF curves displayed equivalent fatigue lives at stress levels below 400 MPa. At these lower stress levels (below 400 MPa) the OP-TMF lives were approximately a factor of two lower than the IP-TMF and IF lives. Mechanical strain measurements, Young's modulus calculations, micromechanical analysis, acetate edge replication, metallography, and fractography were all employed to explain the damage mechanisms and their effects on the fatigue life of the laminate. The initial mode of damage for all specimens tested at all load-temperature profiles was fiber-matrix interfacial failures. Matrix plasticity, matrix cracking, fiber rotation, and fiber failures were evident at all applied stress levels in IP-TMF, OP-TMF, and IF. The degree of each of these mechanisms was dependent on both the applied stress level and the temperature profile.

Thermomechanical fatigue damage/failure mechanisms in SCS-6/Timetal 21S [formula omitted] composite

The thermomechanical fatigue (TMF) deformation, damage and life behaviors of SCS-6/Timetal 21S [ 0 90 ] s were investigated under zero-tension conditions. In-phase (IP) and out-of-phase (OP) loadings were investigated with a temperature cycle from 150 to 650°C. An advanced TMF test technique was used to mechanically quantify damage progression. The technique incorporated explicit measurements of the macroscopic (i) isothermal static moduli at the temperature extremes of the TMF cycle, and (ii) coefficient of thermal expansion (CTE) as functions of the TMF cycles. The importance of thermal property degradation and its relevance to accurate post-test data analysis and interpretation is briefly addressed. Extensive fractography and metallography were conducted on specimens from failed and interrupted tests to characterize the extent of damage at the microstructure level. Fatigue-life results indicated trends analogous to those established for similar unidirectional [0] reinforced titanium-matrix composite systems. High-stress IP and mid- to low-stress OP loading conditions were life-limiting in comparison to maximum temperature isothermal conditions. Dominant damage mechanisms changed with cycle type. Damage resulting from IP TMF conditions produced measurable decreases in static moduli but only minimal changes in the CTE. Metallography on interrupted and failed specimens revealed extensive [0] fiber cracking with sparse matrix damage. No surface-initiated matrix cracks were present. Comparable OP TMF conditions initiated environment-enhanced surface cracking and matrix cracking initiated at [90] fiber/matrix (F/M) interfaces. Notable static moduli and CTE degradations were measured. Fractography and metallography revealed that the transverse cracks originating from the surface and [90] F/M interfaces tended to converge and coalesce at the [0] fibers.

Performance of prestressed wire-wound cylindrical die and fatigue test of piano wire: Kim, H., Fukunaga, O. and Yoshikawa, M. J. Japan Soc. Technol. Plasticity July 1990 31, (354), 893–898 (in Japanese)

The response of the continuous-fiber-reinforced metal-matrix composite SCS-6/Ti-15-3 with orientation [±45]2s was investigated under fatigue conditions. Three temperature and load profiles were used to characterize the metal-matrix composite, namely: isothermal fatigue (IF) at 427°C, in-phase thermo-mechanical fatigue (IP-TMF) from 149°C to 427°C, and out-of-phase thermo-mechanical fatigue (OP-TMF) from 149°C to 427°C. At high stress levels (above 400 MPa) the [±45]2s laminate displayed in-phase, out-of-phase, and isothermal fatigue lives which were all within 1000 cycles. At these high stress levels the laminate exhibited the most detrimental fatigue response when subjected to the in-phase TMF profile. The IP-TMF and IF curves displayed equivalent fatigue lives at stress levels below 400 MPa. At these lower stress levels (below 400 MPa) the OP-TMF lives were approximately a factor of two lower than the IP-TMF and IF lives. Mechanical strain measurements, Young's modulus calculations, micromechanical analysis, acetate edge replication, metallography, and fractography were all employed to explain the damage mechanisms and their effects on the fatigue life of the laminate. The initial mode of damage for all specimens tested at all load-temperature profiles was fiber-matrix interfacial failures. Matrix plasticity, matrix cracking, fiber rotation, and fiber failures were evident at all applied stress levels in IP-TMF, OP-TMF, and IF. The degree of each of these mechanisms was dependent on both the applied stress level and the temperature profile.