Fatigue Loading Frequency Research Articles

A model of corrosion fatigue crack growth in ship and offshore steels

A model describing corrosion fatigue crack growth rate da/dN has been proposed. The crack growth rate is assumed to be proportional to current flowing through the electrolyte within the crack during a loading cycle. The Shoji formula for the crack tip strain rates has been assumed in the model. The obtained formula for the corrosion fatigue crack growth rate is formally similar to the author's empirical formulae established previously. The different effects of AK and the fatigue loading frequency / on da/dN, in region I as compared to region II of the corrosion fatigue crack growth rate characteristics can be described by a change of one parameter only: the crack tip repassivation rate exponent.

Off-axis fatigue behavior of plain weave carbon/epoxy fabric laminates at room and high temperatures and its mechanical modeling

Abstract Off-axis fatigue behavior of a plain-woven carbon/epoxy fabric laminate at room temperature and 100 °C has been studied. Tension–tension fatigue tests are first performed on plain coupon specimens with five kinds of fiber orientations θ =0, 15, 30, 45, 90° at a frequency of 10 Hz in respective temperatures. Test results show that the fatigue behavior of the carbon fabric laminate significantly depends on fiber orientation. The on-axis S – N relationships plotted on logarithmic scales are almost linear over the range of fatigue life up to 10 6 cycle. By contrast, the off-axis S – N relationships are characterized by their S-shape. The log–log plot of the off-axis fatigue data becomes steeper in an intermediate range of fatigue life than before, suggesting increased sensitivity to fatigue, and it subsequently reaches a plateau indicating an apparent fatigue limit. The fiber orientation dependence of the off-axis fatigue strength can substantially be removed by normalization with respect to a modified static strength. These features are common to the fatigue behaviors at room temperature and 100 °C. Additional fatigue tests with a frequency of 2 Hz demonstrate that the shape of the off-axis S – N curve at room temperature is significantly affected by load frequency, as a result of a different specimen heating. Comparison between the high-temperature fatigue data at 2 and 10 Hz reveals that the S-shape of the off-axis S – N curve is caused partly by an intrinsic rate dependence of the carbon fabric laminate. Applicability of a phenomenological fatigue model that considers the effect of fiber orientation and of multiaxial state of stress is also examined for description of the off-axis fatigue behavior at 10 Hz. The results illustrate that the fatigue model can be used for describing the off-axis fatigue behavior of the carbon fabric laminate at room temperature and 100 °C for a given frequency of fatigue loading.

Constitutive Model for Sn–Pb Solder under Fatigue Loading

The paper presents a constitutive model for Sn–Pb solder which captures the response of this complex material subject to a variety of load paths including fatigue loading. Internal state variables are established to characterize grain coarsening and material degradation observed experimentally. A damagecoupled viscoplastic constitutive model is formulated to take into account the effects of temperature and loading rates on mechanical response. The influence of fatigue loading frequency or strain rate, hold time, and temperature on mechanical behavior and fatigue life for 63Sn–37Pb solder alloy is examined. The fatigue failure predictions are compared with those obtained experimentally and found to be satisfactory.

Corrosion Fatigue in 7075-T6 Aluminum: Life Prediction Issues for Carrier Based Operations

Fatigue experiments performed on 7075-T6 Al alloy at various frequencies and stress levels and under alternating wet and dry environments indicate that simultaneous action of corrosion and fatigue substantially accelerates crack initiation and growth rates when compared to pure fatigue (dry air) conditions. In experiments performed under alternating wet and dry conditions, fatigue crack growth rate was observed to increase rapidly in the presence of mildly corrosive (salt solution) and decrease sharply when subjected to noncorrosive dry air. Crack arrest of various durations was observed at transition points between dry and wet cycles. Lowering the frequency of fatigue loading significantly reduced crack initiation and overall life time. S-N curves showed a continuous downward trend without reaching a plateau or threshold. These observations led to the conclusion that aircraft structural integrity can be seriously compromised even under mildly aggressive environments and at subcritical stress levels when cracks are present regardless of changing of environments and/or test frequency.

Effect of strain and strain rate on fatigue-accelerated biodegradation of polyurethane.

A diaphragm-type film specimen was used to study in vitro degradation of poly(etherurethane urea) (PEUU) under conditions of dynamic loading. This geometry allowed both uniaxial and biaxial loading in a single experiment. During testing, the film was exposed to a H(2)O(2)/CoCl(2) solution that simulated in vivo oxidation of PEUU. The combination of dynamic loading and biaxial tensile strain accelerated oxidative degradation. The effects of biaxial strain magnitude and strain rate were examined separately by increasing the frequency of fatigue loading from 0 to 1 Hz with constant maximum biaxial strain and by changing the maximum biaxial strain while maintaining constant strain rate. In the ranges of biaxial strain energy (0.17 to 0.55 MPa) and strain rate (0 to 46% s(-1)) tested, the rate of degradation increased with increasing strain rate whereas strain magnitude had essentially no effect on degradation rate. Although loading conditions affected the rate of oxidative degradation, ATR-FTIR analysis suggested that in all cases the mechanism of degradation did not change. Chemical degradation produced a brittle crosslinked surface layer marked by dimpling and pitting, as observed with scanning electron microscopy. Pits served as stress concentrators and initiated environmental stress cracks under dynamic loading but not under static (creep) loading. Small pits were sufficient to initiate cracks at higher strain rates whereas only large pits initiated cracks at lower strain rates. Consequently, a higher strain rate produced more profuse cracking.

Effects of loading frequency on fatigue crack growth mechanisms in α/β Ti microstructure with large colony size

This paper deals with crack tip/microstructure interactions at 520 °C in lamellar Ti–6Al–2Sn–4Zr–2Mo–0.1Si (Ti6242) alloy under different fatigue loading frequencies. A series of heat treatments were performed in order to produce large colony microstructures that vary in their lamellar and colony size. Fatigue crack growth (FCG) experiments were conducted on these microstructures at loading frequencies of 10 and 0.05 Hz. The lower frequency was explored with and without imposing a 5 min hold-time at the peak stress level during each loading cycle. Results show that the crack growth behavior is sensitive to the loading frequency. For the same microstructure, the crack growth rate is found to be lower at 10 than at 0.05 Hz. The addition of a hold-time, however, did not alter the FCG rate indicating that creep strain during one loading cycle does not contribute significantly in the crack growth process. It is also shown that variations in lamella and colony size have no effects on the FCG rate except for the early stage of crack propagation. Scanning Electron Microscope examinations are performed on the fracture surface in order to identify the relevant crack growth mechanisms with respect to the loading frequency and the microstructure details. Quasi-cleavage of the α/β colonies along strong planar shear bands is shown to be a major mode of failure under all test condition. At a loading frequency of 10 Hz, the crack path proceeds arbitrary along planes either perpendicular or parallel to the long axis of α lamellae, while at 0.05 Hz, parallel-to-lamellae crack paths become favored. Corresponding differences of crack growth behavior are examined in terms of slip emission at the crack tip and interactions with the microstructure details.

Effect of frequency on the fatigue behavior of [�45]4s laminate of carbon fiber reinforced polyetheretherketone (PEEK) composites under tension-tension loading

Fatigue behavior of carbon fiber reinforced poly etheretherketone (PEEK) laminated composite was investigated. The [±45]4s AS-4/PEEK laminated composites under static tensile measurement at various test temperatures were conducted. Three tension-tension fatigue loading frequencies, 1 Hz, 5 Hz and 10 Hz, were selected to study the effect of frequency on the fatigue behavior of [±45]4s AS-4/PEEK laminate. The survival probability of experimental fatigue life data under different stress amplitude tests were estimated and analyzed by the median rank order-statistic cumulative-distribution function and the Weibull distribution function. The S-N curves at different fatigue loading frequencies exhibited a trend of two-segment straight line curves. The increase in surface temperature of specimens was found and the thermal stress history was also investigated by thermo-image techniques during fatigue life testing. The fatigue failure mechanism was investigated by X-ray radiography.

Frictional heating in a unidirectional fibre-reinforced ceramic composite

Ceramic Composites Research Laboratory, Department of Mechanical Engineering and Applied Mechanics, The University of Michigan, 2250 G. G. Brown Laboratory, Ann Arbor, MI 48109-2125, USA Holmes and Shuler [1] recently found that significant internal heating occurs during the cyclic loading of fibre-reinforced ceramics. In their investigation, conducted with cross-ply carbon fibre/SiC matrix composites (hereafter referred to as Cf/SiC), it was observed that the extent of internal heating was strongly influenced by the peak fatigue stress and loading frequency. For example, during ambient- temperature fatigue between fixed stress limits of 250 and 10 MPa, the temperature rise measured at the specimen surface ranged from approximately 0.5 K at a sinusoidal loading frequency of 1 Hz to over 30 K at 85 Hz. For a fixed loading frequency of 75 Hz and minimum stress of 10 MPa, the tem- perature rise ranged from 0.8 K at a peak stress of 50 MPa to 28 K at a peak stress of 250 MPa. It was proposed that the temperature rise observed during cyclic loading was caused by the frictional sliding of fibres within the matrix [1]. Because of the signifi- cant mismatch in thermal expansion coefficients that exists between the fibres and matrix, C~/SiC compo- sites contain extensive processing-related matrix cracking [2]. Fatigue loading of these initially micro- cracked specimens would promote interfacial de- bonding; once debonding occurs, internal heating would occur by the repeated frictional sliding of fibres along the fibre-matrix interface. For a composite that is initially free of matrix cracking, and assuming a mechanism of internal heating involving the frictional slip of fibres within debonded zones, internal heating should begin when a stress level that is sufficient to initiate matrix cracking is reached (at lower stress levels internal heating should be absent). To provide experimental evidence for this mechanism of internal heating, the present investigation utilized a simpler unidirec- tional fibre-reinforced ceramic that was initially free of matrix cracking. As discussed below, a simple experiment which involved monitoring the specimen temperature, while sequentially increasing the peak fatigue stress until matrix cracking initiated, was used to investigate the relationship between matrix cracking and internal heating. A 16-ply unidirectional Nicalon fibre/calcium alu- minosilicate matrix composite (hereafter referred to as Nicalon/CAS-II) was chosen for this investigation (material processed by Corning Glass Works, Corn- ing, New York, USA). The composite, formed by hot-pressing, had a nominal fibre content of 35 vol %. Edge-loaded tensile specimens (see Fig. 1) with a 33 mm gauge length were machined from the 0261-8028/92 $03.00 + .12

Fatigue loading frequency and life of high-strength steel in air and a corrosive environment

Fatigue loading frequency and life of high-strength steel in air and a corrosive environment