Early Stage Fatigue Research Articles

Healing of Early Stage Fatigue Damage in Ionomer/Fe₃O₄ Nanoparticle Composites.

This work reports on the healing of early stage fatigue damage in ionomer/nano-particulate composites. A series of poly(ethylene-co-methacrylic acid) zinc ionomer/Fe3O4 nanoparticle composites with varying amounts of ionic clusters were developed and subjected to different levels of fatigue loading. The initiated damage was healed upon localized inductive heating of the embedded nanoparticles by exposure of the particulate composite to an alternating magnetic field. It is here demonstrated that healing of this early stage damage in ionomer particulate composites occurs in two different steps. First, the deformation is restored by the free-shrinkage of the polymer at temperatures below the melt temperature. At these temperatures, the polymer network is recovered thereby resetting the fatigue induced strain hardening. Then, at temperatures above the melting point of the polymer phase, fatigue-induced microcracks are sealed, hereby preventing crack propagation upon further loading. It is shown that the thermally induced free-shrinkage of these polymers does not depend on the presence of ionic clusters, but that the ability to heal cracks by localized melting while maintaining sufficient mechanical integrity is reserved for ionomers that contain a sufficient amount of ionic clusters guaranteeing an acceptable level of mechanical stability during healing.

Early stage fatigue damage occurs in bovine tendon fascicles in the absence of changes in mechanics at either the gross or micro-structural level

Many tendon injuries are believed to result from repetitive motion or overuse, leading to the accumulation of micro-damage over time. In vitro fatigue loading can be used to characterise damage during repeated use and investigate how this may relate to the aetiology of tendinopathy.This study considered the effect of fatigue loading on fascicles from two functionally distinct bovine tendons: the digital extensor and deep digital flexor. Micro-scale extension mechanisms were investigated in fascicles before or after a period of cyclic creep loading, comparing two different measurement techniques – the displacement of a photo-bleached grid and the use of nuclei as fiducial markers.Whilst visual damage was clearly identified after only 300 cycles of creep loading, these visual changes did not affect either gross fascicle mechanics or fascicle microstructural extension mechanisms over the 900 fatigue cycles investigated. However, significantly greater fibre sliding was measured when observing grid deformation rather than the analysis of nuclei movement. Measurement of microstructural extension with both techniques was localised and this may explain the absence of change in microstructural deformation in response to fatigue loading. Alternatively, the data may demonstrate that fascicles can withstand a degree of matrix disruption with no impact on mechanics. Whilst use of a photo-bleached grid to directly measure the collagen is the best indicator of matrix deformation, nuclei tracking may provide a better measure of the strain perceived directly by the cells.

Recent developments in assessing microstructure-sensitive early stage fatigue of polycrystals

Fatigue failure is a leading concern for many applications involving structures for transportation, manufacturing, medical devices, and electronic components. Recent advances in modeling and simulation, coupled with in situ experimental techniques, have enhanced the understanding required to distinguish and characterize mechanisms of fatigue crack formation and early growth at scales of underlying microstructure. In particular, microstructure substantially influences high cycle fatigue resistance and contributes to variability of the fatigue response. This paper reviews the confluence of recent experimental and modeling advances aimed at understanding and modeling of the formation and early growth of fatigue cracks with size on the order of dominant microstructure attributes.

Three-dimensional magnetic microscopy of early stage fatigue in WMZ of low carbon steel plates (JIS-SS400)

Cyclic stresses around welding joint part affect the strength of mechanical components. In order to understand the fatigue phenomena caused by the cyclic stresses, non-destructive methods that can be related to number of stress cycles are necessary. In the present work, we used a newly developed scanning Hall probe microscope (SHPM) equipped with a GaAs film sensor and observed three-dimensional magnetic fields of the specimen before and after four-point fatigue testing at room temperature in air. Low carbon steel plates (JIS SS400) were used in the experiments. It was found that the intensity of the magnet field in a direction perpendicular to the specimen surface was strongly affected by the fatigue cycles. This result means that we can evaluate the fatigue in welding joint area using quantitative magnetic field measurements. Furthermore, it was discovered that the key factor to evaluate the fatigue is the range between ‘S’ and ‘N’ (‘peak to bottom’ values) of the magnetic fields. The three-dimensional magnetic fields decrease before the dislocation does not accumulate enough to generate a small crack at the early stage of the fatigue.

Fatigue in Early Stage among Jordanian Patients with Cancer Receiving Chemotherapy

Fatigue in Early Stage among Jordanian Patients with Cancer Receiving Chemotherapy

Sensors for monitoring early stage fatigue cracking

Three sensor systems were evaluated for their ability to detect early stage fatigue cracking in open holes. The sensor systems were (1) a Meandering Winding Magnetometer Array sensor system that induces eddy currents to monitor conductivity changes; (2) a through-transmission ultrasonic technique that monitors energy loss, and (3) the Electrochemical Fatigue Sensor which detects fatigue-induced changes in a metal surface through their effects on the electrochemical double layer. The sensors were mounted individually or in tandem in samples of 7075-T651 aluminum alloy which were then fatigued using a spectrum loading sequence. The samples were also examined at various stages during fatigue cycling using optical or scanning electron microscopy. Detection thresholds of approximately 100 μm were observed, and calibration curves for crack size in terms of the sensor outputs were obtained.

MWM eddy-current arrays for crack initiation and growth monitoring

Permanently mountable and scanning MWM ® (Meandering Winding Magnetometer) eddy-current arrays provide an effective means of monitoring fatigue in laboratory tests of specimens and test articles as well as in service on structural components, e.g., aircraft high-strength components, heavy machinery components, and bridge members. The MWM-Arrays can detect and monitor early stage fatigue damage and cracks by measuring electrical conductivity or, in the case of ferromagnetic materials, magnetic permeability. Preliminary results of fatigue tests with linear MWM-Arrays, mounted on a shot peened surface or on a shot peened and cadmium plated surface in the semi-cylindrical gage region of custom-designed high-strength fatigue specimen, are presented and discussed. The results indicate that both permanently mounted MWM-Arrays and scanning MWM-Arrays can detect fatigue damage in steels well before formation of cracks as verified by post-test scanning electron microscopy.

The early stage of fatigue crack propagation

It is observed that an early stage fatigue crack propagates along the maximum shear direction rather than perpendicular to the maximum tensile stress direction. In the present paper, an analytical consideration is given concerning the effects of localized plastic deformation on the early stage of fatigue crack propagation. It is seen that the localized plastic deformation shields the local mode II stress intensity factor and leads the micro-crack to propagate along the maximum shear stress direction. In this analytical modeling, the localized plastic zone is modeled as an elliptical inclusion with eigenstrains where the Eshelby inclusion solution [ Proc. R. Soc. A241, 376–396 (1957)] is employed.

The early stage of fatigue crack growth in martensitic steel

In low ductility and high strength steels, the early stage fatigue behavior associated with non-metallic inclusions is a highly localized phenomenon near the inclusions. However, the nature of the fatigue crack initiation process is not clear. In this paper, a special emphasis is placed on the possible differences in the mechanism of initiation and the early growth of fatigue cracks between a martensitic steel and ordinary ductile materials.

Fractographic Study on the Early Stage Fatigue Cracks of Iron in Vacuum

The effect of atmospheric-corrosion on the fatigue crack nucleation and fracture surface morphology in the early stage of fatigue has been investigated by tests in air and vacuum on pure iron. The experiments were conducted under push-pull loading, and both external and fractured surfaces were observed by means of optical and scanning electron microscopy.It was found that the fatigue lives of iron in vacuum were 2.1 times those in air and the fatigue limit was also 1kg/mm2 higher in vacuum than in air. The density of slip lines in the specimens tested in vacuum was much higher than that in air and the cyclic numbers up to fatigue crack nucleation in vacuum were found to be 2.6 times those in air. The morphology of initial transcrystalline fracture surfaces in air was classified in two types and the number of intercrystalline fracture surfaces was decreased as stress amplitude increased. The morphology of initial transcrystalline surfaces in vacuum was similar to that in air. No intercrystalline fracture surfaces were observed in the specimens tested in vacuum. Clear striations were found on the fracture surfaces of the specimens tested in vacuum, though most of the surfaces have a flattened and smeared appearance.