Significant Decrease In Fatigue Life Research Articles

Interior cracking behavior of laser powder bed fused TiC/Ti6Al4V composites under vacuum environment

High and very high cycle fatigue tests were carried out to elucidate the interior crack nucleation behavior in a vacuum environment of a TiC-reinforced Ti6Al4V composite fabricated by laser powder bed fusion. To investigate the failure mechanisms, advanced microscopy techniques such as scanning electron microscopy and electron backscatter diffraction are employed. The first failure mode is the cracking of the inhomogeneous hardening zone (IHZ). The cleavage planes observed inside IHZ suggest that the essence of cracking is lamellar α′-grain fracture. The second failure mode is the lack of fusion (LOF) assisted IHZ cracking. The presence of LOF increases stress concentration and reduces the effective bearing capacity of the material, leading to a significant decrease in fatigue life. The knowledge gained can be utilized to optimize the fabrication process and design of composites, ultimately improving their fatigue resistance and extending their service life in applications.

Construction quality impact in asphalt pavements cost: a framework based on air voids, linear viscoelastic and fatigue behaviour

ABSTRACT The loss of performance against fatigue is evident for situations of poor execution of the surface layer, quantified in this research by the parameter in-field air voids. This loss of performance has a direct impact on highways operating costs. To analyse the loss of fatigue performance, eight field-applied asphalt mixes were evaluated, comparing laboratory compaction (5.50% air voids) with as-built in-field core samples (random variable air voids). The impact of increased air voids on LVE behaviour, fatigue life, and costs were investigated. Decrease in stiffness was observed for in-field air voids greater than 7.00%. Through the S-VECD model and FlexPAVETM software, a significant decrease in fatigue life was observed for in-field air voids greater than 8.00%. Costs, defined as costs per traffic request (N) (US$/N) were significantly affected for in-field air voids greater than this value (8.00%). Based on that, a Payment reduction factor (PRF) was proposed. In this way, it is concluded that the framework demonstrated throughout the paper, based on a mechanistic approach and using the FlexPAVETM, allows the evaluation of the construction quality impact on asphalt pavement cost.

Effect of hydrogen on low-cycle fatigue behavior of HRB400 steel under asymmetric cyclic loading

This paper focuses on the study of ductility changes, ratcheting, and low-cycle fatigue behavior of materials after hydrogen charging. Hot-rolled ribbed bar 400 (HRB400) steel was electrochemically charged with H2SO4 solution, and the samples subjected to different hydrogen charging times were subjected to quasi-static tensile and low-cycle fatigue tests under asymmetric cyclic loading. The tests indicated that hydrogen charging changed the initial yield stress of the material, but the ultimate strength remained almost unchanged; hydrogen charging significantly changed the cyclic hysteresis behavior of the material, which increased the elastic range of the stable hysteresis curve; hydrogen charging changed the pattern of ratcheting deformation. The pattern of ratcheting deformation in the later stage was very different from that of the sample without hydrogen charging; hydrogen charging reduced the ductility of the material and resulted in a significant decrease in fatigue life. The fracturing of the hydrogen-charged samples exhibited apparent quasi-cleavage and intergranular fracture characteristics. The non-hydrogen-charged samples had two failure modes (fatigue failure and increased ratcheting deformation resulting in necking instability), whereas the hydrogen-charged samples had only one failure mode of fatigue failure. Based on the experimental analysis, a fatigue life equation that simultaneously considers the effect of hydrogen embrittlement index, mean stress, and stress amplitude is proposed. The equation was used to predict the low-cycle fatigue life of hydrogen-charged specimens under asymmetric cyclic loading, and the error between the results and the measured values was in the two-factor band.

The Effect of Temperature and Phase Shift on the Thermomechanical Fatigue of Nickel-Based Superalloy

In this paper, the minimum temperature and phase shift effects on the thermo–mechanical fatigue (TMF) behavior of Inconel 713LC are investigated. TMF tests were performed under 0° (in-phase-IP) and +180° (out-of-phase-OP) phase shifts between mechanical strain and temperature. Cylindrical specimens were cycled at constant mechanical strain amplitude with a strain ratio of Rε = −1. Tests were performed with temperature ranges of 300–900 °C and 500–900 °C. The heating and cooling rate was 5 °C/s. Fatigue hardening/softening curves and fatigue life data were assessed. Results show that out-of-phase loading was less damaging than in-phase loading. Scanning electron microscopy (SEM) examination of metallographic sections indicated that the life-reducing damage mechanism was intergranular cavitation under in-phase loading. Transmission electron microscopy (TEM) revealed honeycomb structures for IP loading. The plastic strain localization into persistent slip bands was typical for OP loading. For out-of-phase loading, fatigue damage appeared to be dominant. The increase in the temperature range led to a significant decrease in fatigue life. The reduction of fatigue life was far more pronounced for out-of-phase loading. This can be ascribed to the accelerated crack propagation at high tensile stress under out-of-phase loading as well as the amount of accommodated plastic strain deformation. Based on the SEM scrutiny of metallographic sections and TEM observations of dislocation arrangement, the prevailing damage mechanisms were documented and the lifetime behavior was accordingly discussed.

Estimation of residual fatigue life of polymer composites after preliminary low-velocity impact

Composites are replacing traditional metals and alloys in many industries, including aviation, space, building, automotive, petrochemical, due to their lower weight and physical and mechanical characteristics, which allows them to be introduced into critical high-loaded structures and parts. At the same time, in the process of operation, composite structures can be in conditions of complex thermomechanical action. Data on changes in the residual mechanical characteristics, available damages allow increasing the prediction life, survivability and safety of destruction of composite structures.The paper presents the results of studying the processes of accumulation of fatigue damage and destruction of CFRP specimens with layouts [0º/90º]n and concentrators of an operational nature. The study was carried out with the implementation of a preliminary transverse impact by a falling load according to the three-point bending and subsequent cyclic stretching. In accordance with the developed technique, the value of the ultimate strength (σB) of the carbon fiber under study was determined from quasi-static tensile tests. The nominal fatigue life (Nmax) is obtained from tests with the parameters with the maximum stress in the cycle σmax=0.75∙σB, the asymmetry coefficient R=0.1 and the frequency ν=20 Hz. During the cyclic tests, the frequency and amplitude of the loading were selected in such a way that the heating of the sample did not exceed 20°C relative to the temperature of the beginning of the test. From experiments on impact bending, the potential energy (Emax) was determined, at which the samples were destroyed. Further tests for preliminary impact according to the three-point bending were carried out in the range from 0.3 to 0.9 of the destruction energy (e’). Single blows were made across the entire width of the samples. After each impact, a visual inspection of the samples was carried out. Microstructural studies and analysis of fracture patterns of prototypes under complex shock impacts and cyclic loading were performed using a Carl Zeiss SteREO Discovery V12 stereomicroscope.As a result, experimental dependences of the influence of different intensity of impact bending on the residual fatigue life of carbon fiber specimens were obtained. Analysis of the diagram of the change in the residual fatigue life versus the impact bending intensity made it possible to reveal the presence of a shock sensitivity threshold. Impact actions with energies exceeding the threshold value led to a significant decrease in fatigue life, by more than 10% of the nominal value of Nmax, for the studied CFRP composites.

Fatigue Test of 6082 Aluminum Alloy under Random Load with Controlled Kurtosis.

This paper presents the results of experimental tests carried out on an electromagnetic shaker where the excited element was a specimen with additional weight attached to the slip table. The load was random with a different kurtosis parameter value, i.e., it was performed for non-Gaussian loads. The experiment was accompanied by basic fatigue calculations in the frequency domain and their verification with experimental results. A significant decrease in fatigue life was found to take place with an increase in kurtosis and the maintenance of the same standard deviation of the specimen load. The fatigue effect, caused by the deviation from the normal distribution that was described by the kurtosis parameter, on the fatigue life of aluminum alloy 6082 was presented. An analysis revealed the different amplitude probability distributions for the loading signal and the recorded deformation signal. It was concluded that there was a lack of sensitivity of the numerical model to the change in the kurtosis parameter of the distribution of random loads.

Effects of Hygrothermal Ageing on the Interphase, Fatigue, and Mechanical Properties of Glass Fibre Reinforced Epoxy

Reliability and cost-effectiveness represent major challenges for the ongoing success of composites used in maritime applications. The development of large, load-bearing, and cyclically loaded structures, like rotor blades for wind or tidal energy turbines, requires consideration of environmental conditions in operation. In fact, the impact of moisture on composites cannot be neglected. As a result of difficult testing conditions, the knowledge concerning the influence of moisture on the fatigue life is limited. In this study, the impact of salt water on the fatigue behaviour of a glass fibre reinforced polymer (GFRP) has been investigated experimentally. To overcome the problem of invalid failure during fatigue testing, an improved specimen geometry has been developed. The results show a significant decrease in fatigue life for saturated GFRP specimens. In contrast, a water absorption of 50% of the maximum content showed no impact. This is especially remarkable because static material properties immediately decrease with the onset of moisture absorption. To identify the water absorption induced damage progress, light and scanning electron microscopy was used. As a result, the formation of debondings and cracks in the fibre–matrix interphase was detected in long-term conditioned specimens, although no mechanical loading was applied.

Low cycle fatigue behavior and microstructure evolution of a novel 9Cr–3W–3Co tempered martensitic steel at 650 °C

A series of uniaxial strain-controlled fatigue tests were performed at 650 °C to explore the high-temperature low cycle fatigue (LCF) behavior and corresponding microstructure evolution of a novel 9Cr–3W–3Co martensitic G115 steel. The fatigue test results showed that the steel exhibited a cyclic softening behavior and the degree of softening S increased with the total strain amplitude, which was related to the decrease of dislocation density and subgrain development. Further, statistical analysis of the subgrain size after LCF tests at different strain amplitudes showed that the development of subgrain or cell depended on both the plastic strain level and cycles, among which the effect of plastic strain level is dominant. The variation of degree of softening ΔS was used to make comparison between G115 steel and P92 steels to evaluate the effects of the strain amplitudes on the respective S. The G115 steel experienced the minimum relative increment compared with P92 steel. This resulted from the strengthening by fresh and fine Cu-rich precipitates. The number density of Cu-rich particles increased with the total strain amplitude and that weakened the effect of strain amplitude on degree of softening. SEM was employed to investigate the fracture morphology; representative images of the G115 steel after the LCF tests showing three regions: fatigue crack initiation sites, stable crack growth region, and rupture region. Larger the strain amplitude was, more the crack initiation sites formed, which corroborates the significant decrease in fatigue life with increase in the total strain amplitude. In addition, a modified plastic strain energy approach based non-Masing behavior of G115 steel is proposed to predict the fatigue life.

Fatigue Crack Propagation Analysis of Plaque Rupture

Rupture of atheromatous plaque is the major cause of stroke or heart attack. Considering that the cardiovascular system is a classic fatigue environment, plaque rupture was treated as a chronic fatigue crack growth process in this study. Fracture mechanics theory was introduced to describe the stress status at the crack tip and Paris' law was used to calculate the crack growth rate. The effect of anatomical variation of an idealized plaque cross-section model was investigated. The crack initiation was considered to be either at the maximum circumferential stress location or at any other possible locations around the lumen. Although the crack automatically initialized at the maximum circumferential stress location usually propagated faster than others, it was not necessarily the most critical location where the fatigue life reached its minimum. We found that the fatigue life was minimum for cracks initialized in the following three regions: the midcap zone, the shoulder zone, and the backside zone. The anatomical variation has a significant influence on the fatigue life. Either a decrease in cap thickness or an increase in lipid pool size resulted in a significant decrease in fatigue life. Comparing to the previously used stress analysis, this fatigue model provides some possible explanations of plaque rupture at a low stress level in a pulsatile cardiovascular environment, and the method proposed here may be useful for further investigation of the mechanism of plaque rupture based on in vivo patient data.

The effect of higher harmonic forces on fatigue life of marine risers

Higher harmonic responses measured in flexible riser model tests conducted within the Norwegian Deepwater Programme (NDP) are studied to evaluate the effect of higher harmonics of force on the fatigue life of risers. A large third-harmonic contribution and sometimes a fifth-harmonic are observed for the majority of test cases. Since existing methods of fatigue life estimation account for the effect of hydrodynamic forces only at the frequency of vortex shedding, a new methodology is introduced which takes into account the influence of the higher harmonic components: First, a new response reconstruction technique is used to reconstruct the entire span-wise riser motion in both the cross-flow and in-line directions based on measurements at specific points along the length. Then, a force database obtained from forced in-line and cross-flow motions of rigid cylinders is used to estimate the higher harmonic components of the force along the entire riser length. A significant decrease in fatigue life is obtained when the higher harmonic components of the fluid forces are considered.

Mechanical properties of a bulk Cu0.5NiAlCoCrFeSi glassy alloy in 288°C high-purity water

The tensile properties and fatigue lives of a bulk Cu0.5NiAlCoCrFeSi glassy alloy were measured in 288°C water and compared with those obtained in air at ∼25°C. The bulk glassy alloy retained good tensile properties in 288°C water, i.e., tensile fracture strength and tensile fracture elongation were, respectively, 2660MPa and 2.01% in air and a little lower, 2000MPa and 1.49%, in 288°C water. The results of fatigue tests confirmed the significant decrease in fatigue life in high-temperature water. A decrease in strain rate from 0.5to0.001%s−1 decreased fatigue life by a factor of ∼5.

The role of interfacial coatings on the high frequency fatigue behavior of nicalon/C/SiC composites

A significant number of researchers have shown that, under monotonic loading, a weak fiber/matrix interface is desirable in obtaining toughness and graceful failure in continuous fiber reinforced ceramic composites (CFCCs). Such an interface is usually accomplished by the insertion of a thin coating (typically 0.1--1 {micro}m thick) such as C or BN, which is compliant enough to relieve residual clamping stresses from processing, and whose structure consists of several weakly bonded basal planes. Holmes et al. showed that in a unidirectional Nicalon/CAS composite with a weak interface, increasing loading frequency caused a significant decrease in fatigue life. Due to repeated frictional sliding of fibers, a significant amount of heat is generated by the specimen, and a large temperature rise is observed. In fact, Sorensen and Holmes have shown that immersing specimens in oil, in order to create a lubricating layer at the fiber/matrix interface, can dramatically improve the fatigue life of the composite. Following this observation of Sorensen and Holmes, the authors have hypothesized that thicker interfacial coatings may improve the fatigue life of CFCCs. To this end, Nicalon/SiC composites were examined, with a carbon coating of 0.33 and 1.1 {micro}m. The samples were identical in nature, except for themore » interfacial coating thickness.« less