Early Fatigue Damage Research Articles

Heat dissipation measurements in low stress cyclic loading of metallic materials: From internal friction to micro-plasticity

The present paper deals with a procedure to measure the very small quantities of heat generated during the very first cycles of mechanical loading on metallic specimens. A dedicated procedure is presented to reach the required sensitivity in terms of heat sources with some original features with respect to the present state of the art, assuming that the heat sources are uniformly distributed within the specimen as such low strain levels. Then steel (cold rolled and annealed) and aluminium specimens were tested at two different aspect ratios ( R σ = 0.1 and R σ = - 1 ). It was shown that for small stress levels, the heat generated was more or less constant with cycle number. This energy was associated to visco-elastic behaviour (internal friction) and it was shown that the same energy could be measured with the two stress ratios when plotted against strain rate (stress amplitude at constant frequency, here 15 Hz). Then, after a certain stress level, an initial outburst of energy was evidenced over about 10–15 cycles before a constant level was reached again. This was associated to micro-plastic adaptation. This procedure provides an advanced tool to tackle the problem of very early fatigue damage detection and is aimed at providing some physical justification to the procedures of rapid fatigue limit detection by self-heating tests.

In-situ Fatigue Damage Investigations in Welded Metallic Components by Infrared Techniques

Abstract The paper reviews the state of the art of thermal damage assessment in metals and proposes a specialized form of lock-in thermography as an efficient tool for the quantification of early fatigue damage phenomena in welded steel structures in the regime of mid- to high-cycle fatigue. It can be proved that advanced thermographic techniques are sensitive to early damage phenomena in the weld toe. They provide a new experimental mean for in-situ investigation of early inhomogeneous fatigue damage phenomena as mesoplasticity and fatigue cracks. The results demonstrate that non-contacting full field temperature measurements using highly sensitive infrared technology can be applied successfully to detect localized fatigue damage at around 10 % to 20 % of the total fatigue lifetime of the tested specimens. The applied method is based on the separation of thermoelastic and thermoplastic temperature effects which cause characteristic local temperature signatures. In contrast to other well-known approaches—as, e.g., measuring the rise of mean temperature during fatigue loading—the applied method offers a very high spatial resolution and resolves extremely localized phenomena. The methodology can be applied to monitor the fatigue process during conventional fatigue testing. The additional effort for the specimen preparation and the installation of the testing setup is minimal.

Characterization of Early Fatigue Damage in Welded Structures by Thermal Imaging

The paper presents background information and experimental results regarding the assessment of fatigue damage in welded steel structures by thermographic investigations of thermomechanical coupling effects. The results confirm the high potential of specialized thermographic methods for the experimental characterization of all stages of fatigue damage in welded and un-welded components. The technique provides a new experimental mean to investigate early inhomogeneous fatigue damage as mesoplasticity and cracks in the weld toe. The method has been successfully applied during fatigue testing of welded components and allows detecting localized damage as early as 10% to 20% of the total fatigue lifetime of the tested specimens.

Mechanisms and kinetics of the early fatigue damage in crystalline materials

The localization of the cyclic plastic strain and its effect on the hysteresis loop, on the surface relief evolution and on the internal dislocation structure of austenitic and ferritic stainless steels are presented. Extrusion and intrusion formation is studied using atomic-force microscopy. The experimental findings concerning the surface relief formation and crack evolution are confronted with existing models of fatigue-crack initiation. Short crack growth kinetics is documented and the short crack growth law and its parameters are compared with the Coffin–Manson law.

Mechanisms of the Early Fatigue Damage in Metallic Materials

The early stages of the fatigue damage in f.c.c. and b.c.c. metals are studied using high resolution techniques. The localization of the cyclic plastic strain results in formation persistent slip bands (PSBs) with specific dislocation structure. Characteristic surface relief is formed at locations where PSBs emerge on the surface. It consists of extrusions and intrusions separated by the original flat surface. Atomic force microscopy is used to study the details of the surface relief. The experimental findings are discussed in terms of the point defect model of fatigue crack initiation.

Volumetric and Surface Positron Annihilation Studies of Fatigue Damage Accumulation in a Steel Alloy

Abstract Fatigue crack nucleation comprises a significant portion of the fatigue life for many structures. The ability to inspect and quantify damage before significant cracks appear could improve safety while allowing components to be retired only when they have reached the end of their useful life. Positron annihilation is one technique that allows for observation of damage accumulation on a much smaller scale than traditional nondestructive evaluation techniques. As-polished and shot-peened 300M steel specimens were fatigued under constant amplitude. Tests were interrupted at several fatigue life ratios between the as-received and failed conditions, and damage was measured with positron annihilation techniques. Positron annihilation was sensitive to both induced residual stresses and fatigue damage build up, but measurement uncertainty was high and sensitivity was low. Positron annihilation presents a potentially revolutionary nondestructive evaluation technique that could give a quantitative measure of early fatigue damage.

EBSD-AFM Hybrid Analysis on Early Fatigue Damage in Austenitic Stainless Steel under Cyclic Torsional Loading

Both the electron backscatter diffraction (EBSD) and atomic force microscopy (AFM) methods were used to investigate the active slip systems and crack initiation behavior in an austenitic stainless steel, SUS316NG, fatigued to one-fourth of the fatigue life under cyclic torsional loading. Most of the active slip systems under torsional loading took place on the primary planes with the largest Schmid factor. Fatigue cracks observed on the surface were classified into four types : GB (grain boundary), TB (twin boundary), SB (slip band) and TC (transcrystalline) cracks. The density distribution of TC cracks was maximum, followed by those of GB, SB, and TB cracks. The development of the surface topography due to fatigue was studied by AFM examination of replicated surfaces of the specimen. On the basis of EBSD and AFM observations, the following four parameters were measured : h, the depth of intrusion vertical to the surface ; S, the slip displacement ; SA, the component of slip displacement parallel to the surface ; SB, the component of the slip displacement perpendicular to the surface trace. Both h and SB values showed a sharp increase at the onset of crack initiation. The critical values of h and SB at crack initiation were 160nm and 170nm, respectively.

Atomic Force Microscopy Study of the Early Fatigue Damage

The possibilities of atomic force microscopy in studying surface features and early fatigue damage in materials are reviewed. Examples of a true relief arising on the surface of cyclically strained materials as recorded by atomic force microscopy are given. Characteristic features of the surface relief are well-defined persistent slip markings consisting of extrusions and intrusions. The shape of extrusions and intrusions can be obtained by observation of metal surface and replicas and in combination with high resolution scanning electron microscopy. The growth of extrusions during fatigue life is reported for austenitic and ferritic stainless steels. The experimental data are compared with models of the localized cyclic plastic straining and predictions of the fatigue crack nucleation models.

Dynamical systems approach to fatigue damage identification

In this paper, we present a dynamical systems approach to material damage identification. This methodology does not depend on knowledge of the particular damage physics of material fatigue. Instead, it provides experimental means to determine what are practically observable and observed facts of damage accumulation, thus making it possible to develop or experimentally verify appropriate damage evolution laws. A concept of phase space warping is used to develop new damage tracking feature vectors from measured time series through phase space reconstruction. These feature vectors provide critical information about the dimensionality of a damage process that was missing from an old scalar metric described in previous work. Damage identification is achieved by applying either proper orthogonal decomposition or smooth orthogonal decomposition to these vectors. The method is experimentally validated using an elasto-magnetic system, in which a harmonically forced cantilever beam in a non-linear magnetic field accumulates fatigue damage. Both damage identification methods yield a single active damage mode that shows power-law-type monotonic trends, which is also consistent with a result obtained using the old scalar metric. These results are shown to be in good agreement with the Paris fatigue crack growth model during the time period of macroscopic crack growth. Using this simple model, accurate time-to-failure predictions are shown well ahead of actual beam fracture. In addition, this identification scheme provides a much-needed insight into the dimensionality of incipient or early fatigue damage accumulation process, which is shown to be describable by only one scalar damage variable for this specific experiment.

ねじり・軸力複合荷重下での疲労における初期微視的損傷に関する研究(疲労)

ねじり・軸力複合荷重下での疲労における初期微視的損傷に関する研究(疲労)

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.

Thermo-acoustic fatigue characterization

The nondestructive detection of early fatigue damage states is of high importance for safety in aircraft, automobiles, railways, nuclear energy industries and chemical industries. Titanium alloys commonly used in aerospace for structures and engine components are subject to fatigue damage during service. In the current study fatigue damage progression in a titanium alloy (Ti–6Al–4V) was investigated using thermographic detection of the heat dissipated during short-term mechanical loading. The initial rate of temperature increase induced by the short-term mechanical loading was used to indicate the current microstructural state and presence of prior fatigue damage. Two methods for thermal excitation were investigated (a) high amplitude mechanical loading and (b) small amplitude ultrasonic loading. A formula that describes the temperature enhancement due to heat generation during one loading cycle is derived from high amplitude loading data. A correlation between the temperature increase during short-term ultrasonic loading and accumulated fatigue cycles is used to suggest a methodology for in-field assessment of fatigue condition.

Early fatigue damage and residual stresses in polycrystalline copper: a scanning probe microscopy study

Early fatigue damage and residual stresses in polycrystalline copper: a scanning probe microscopy study

Estimation of Early Fatigue Damage in Heat Treated En-8 Grade Steel

Generally, the failure of major machinery parts is due to fatigue damage. Because of the structural inhomogeneity of metals, fatigue damage may sometimes occur significantly below the yield strength of the material due to microplastic deformation at low stress levels. Commercial En-8 grade steel (widely used for making secondary metalworking products) was used to estimate the fatigue damage response during cyclic loading nearer to the fatigue endurance limit. Estimation of fatigue damage was carried out with the aid of a nondestructive testing (NDT) method, that is, Elastosonic measurement of fatigue damping coefficient and slope of fatigue damping curves. Results indicate that fatigue damage increases in annealed En-8 steel with an increase in peak stress and with an increase in the number of cycles. However, for hardened and tempered En-8 steel, experimental results may not provide a true indication of fatigue damage during fatigue loading nearer to the endurance limit, most likely due to the more homogeneous structure. Generally, fatigue failure occurs in this grade of steel due to microcrack generation in the cementite of the pearlite phase of annealed steel.

Early fatigue damage in carbon-fibre composites observed by electrical resistance measurement

Early fatigue damage during the first tenth (or less) of the fatigue life was observed in carbon fibre composites by d.c. electrical resistance measurement. The damage was most severe in the first loading cycle and the incremental damage in each subsequent cycle diminished cycle by cycle. For the continuous carbon fibre carbon-matrix composite, the resistance increased irreversibly during early fatigue due to matrix damage and possibly fibre fracture as well. For the short carbon fibre polymer-matrix and cement-matrix composities, the resistance decreased irreversibly during early fatigue due to matrix damage near the junction of adjacent fibres and the resulting increase in the chance that adjacent fibres touched one another.

Effect of Some Parameters on the Plastic Fatigue Behavior with Micromechanical Approach

The fatigue life and the micro-damage heterogeneity for FCC polycrystalline metals are qualitatively evaluated employing a coupled phenomenological micro-mechanical model of the early plastic fatigue damage initiation. This model is based on the slip theory and on the localization homogenization technique associated with the self-consistent scheme. For representing the micro-damage on each slip system, an internal damage variable dS is introduced. The influence of the main parameters of this model (aggregate composition, damage interaction matrix Drs, and loading path) on the related plastic fatigue phenomena as the fatigue lives and the micro-damage heterogeneity is studied. Hence, various fatigue simulations are performed using several types of aggregates and different loading paths (simple and complex). It is shown that such parameters have great influences on the fatigue behavior. Moreover, the distributions of the local damage heterogeneity are studied under the effect of each determined parameter.

Micromechanical modeling of low cycle fatigue under complex loadings — Part I. Theoretical formulation

Abstract A micromechanical model of the early fatigue damage initiation is proposed based on the slip theory. For each slip system, a local micro-damage variable is introduced to describe globally all phenomena related to the level lower than the crystallographic slip system, such as dislocations, atoms, molecules, lattice defects, etc., of FCC polycrystalline materials. This transgranular damage variable is fully coupled with micro inelastic constitutive equations. It is supposed that the local damage appears when the dislocation density reaches some critical values. The obtained model is devoted to describing the cyclic behavior of metallic materials under proportional and non-proportional loading paths neglecting the quasi-unilateral effect as well as the localization of the fatigue damage on the free surface of the specimen.

Use of embedded optical fibre for early fatigue damage detection in composite materials

The use of embedded optical fibre to detect fatigue damage in GRP three-point bend specimens is described. A simple signal attenuation measurement system was used to indicate damage development. The stresses in the optical fibre were calculated using an existing ‘fibre in matrix’ model. The optical fibre fatigue strength was predicted using constants calculated from optical fibre embedded into the composite material. Early fatigue damage was successfully detected in GRP specimens.

Nondestructive detection of fatigue damage

This paper will discuss the various nondestructive techniques which have been used or which are potentially useful for detection of fatigue damage. The ideal nondestructive testing technique would permit very early detection of fatigue damage so that proper assessment of the severity and rate of severity increase of the structural damage leading to failure can be made. Thus, the most sensitive systems would be capable of detecting motion, pileup, and breakaway of dislocations; the next most sensitive systems would be capable of detecting microcracks; the least sensitive systems would only be capable of detecting macrocracks. It is practically expedient to have nondestructive techniques which can successfully detect fatigue damage in each of these regimes since some components can tolerate larger amounts of fatigue damage or larger crack sizes than others without serious concern for the structural integrity of the component. It will be shown that the best nondestructive technique for detecting and sizing of macrocracks is ultrasonics; the best nondestructive techniques for detection of microcrack formation and possible pre‐microcrack fatigue damage are ultrasonic attenuation and acoustic emission; a large number of nondestructive techniques for residual stress (strain) measurements are candidates for extremely early fatigue damage detection, with current interest primarily directed at ultrasonic wave velocity measurements.

Early Fatigue Damage Detection in Composite Materials

Detection of early fatigue damage in composite materials by nondestructive inspection (NDI) techniques has been demonstrated for ±45° Glass/Epoxy, and ±45°/0° Graphite/Glass/Epoxy. Dynamic axial modulus and temperature were monitored continuously with a correlation between temperature rise and modulus decrease observed. The modulus decrease and temperature rise are indicative of irreversible damage in these materials. Torsional modulus measurements and coin tap tests were performed at 0, 106, 5 × 106, and 107 cycles, on all fatigue specimens. Other NDI procedures including holographic interferometry, ultrasonics, penetrant, and X-Ray radiography were performed on two specimens of each material to evaluate their effectiveness in detecting fatigue damage. Ultrasonics and holography proved to be effective; however, at this time, no clear quantitative correlation beween structural properties and NDI measurements has been determined.