Investigation of the load sequence effects in fatigue of damaged composite laminate & characterization of fatigue damage

Carbon fiber reinforced composites are widely used today in various areas and specially in aerospace industry for structural applications. This investigation focuses on the effect of different load sequencing and impact damage on the fatigue behaviour of CFC laminates. The specimens made from plain CFC laminates and low energy impact damaged CFC laminates were subjected to a typical flight block loading sequence and the fatigue strength degradation was monitored through stiffness measurement using load displacement data obtained during block loading. Three different stress/strain levels were used in testing. All the tests were performed using a computer controlled 100 kN servo-hydraulic test machine in load mode at room temperature and in lab air atmosphere on undamaged and low energy impact damaged composite laminates. Fatigue tests were performed with a sinusoidal waveform at 3 Hz. It was observed that lower strain levels did not show any significant effect on the fatigue properties in both the type of loading i.e. low to high and in high to low block loading in case of both the undamaged and impact damaged CFC specimens. Significant.reduction in stiffness was seen at higher strain level i.e. 6500me in both the undamaged and impact damaged CFC specimens. The low energy impact damaged specimens showed early failure at higher strain levels compared to undamaged specimens. The specimens were observed to have delaminated in the high stress fatigue cycling. The observed stiffness reduction due to fatigue cycling and the presence of delamination provide a means of macroscopic identification of fatigue strength degradation in composite materials. The energy plots appear useful tool to assess the damage growth.     

Characterization of the temperature evolution during high-cycle fatigue of the ULTIMET superalloy: Experiment and theoretical modeling

High-speed, high-resolution infrared thermography, as a noncontact, full-field, and nondestructive technique, was used to study the temperature variations of a cobalt-based ULTIMET alloy subjected to high-cycle fatigue. During each fatigue cycle, the temperature oscillations, which were due to the thermal-elastic-plastic effects, were observed and related to stress-strain analyses. A constitutive model was developed for predicting the thermal and mechanical responses of the ULTIMET alloy subjected to cyclic deformation. The model was constructed in light of internal-state variables, which were developed to characterize the inelastic strain of the material during cyclic loading. The predicted stress-strain and temperature responses were found to be in good agreement with the experimental results. In addition, the change of temperature during fatigue was employed to reveal the accumulation of fatigue damage, and the measured temperature was utilized as an index for fatigue-life prediction.     

Crack Arrest and Residual Strength of Composite Panels with Softening Strips

Three materials, E‐glass, Kevlar, and S‐glass, are assessed in terms of their effectiveness as softening strips in the fail‐safe design of composite structures. The softening effect is assessed in terms of its influence on the failure behavior of AS∕3501‐6 graphite‐epoxy laminates consisting of 16 plies in a lay‐up of [±45,O2]2S. Softening strips are created in the basic material by locally replacing 0°‐ply fibers with the candidate softening materials. Initial damage in the panels is introduced by saw‐cutting′ or as a result of an impact load in the geometric center of the specimen. In some cases, the specimens are fatigue‐cycled prior to the introduction of the initial damage; in other cases, the cyclic loads are applied subsequent to the saw‐cutting or impact loading. Moisture effects on strength degradation and the crack‐arrest capability of the softening strips are included. The experimental results indicate that the S‐glass softening material has a greater beneficial effect than Kevlar or E‐glass on the fail‐safe characteristics of the basic material. Finally, analytical correlations of some test results are presented.     

Effects of microstructure on the strength and fatigue behavior of a silicon carbide fiber-reinforced titanium matrix composite and its constituents

The results of a systematic study of the effects of microstructure on the strength and fatigue behavior of a symmetric [0/90]_2s Ti-15Al-3Cr-3Al-3Sn/SiC (SCS-6) composite are presented along with relevant information on failnure mechanisms in the composite constituents, i.e., the interface, fiber, and matrix materials. Damage micromechanisms are elucidated via optical microscopy, scanning electron microscopy (SEM), and nondestructive acoustic emission (AE) and ultrasonic techniques. Composite damage is shown to initiate early under cyclic loading conditions and is dominated by longitudinal and transverse interfacial cracking. Subsequent fatigue damage occurs by matrix slip band formation, matrix and fiber cracking, and crack coalescence, prior to the onset of catastrophic failure. However, the sequence of the damage is different in material annealed above or below the β solvus of the Ti-15-3 matrix material. Mechanistically based micromechanics models are applied to the prediction of the changes in modulus induced by fatigue damage. Idealized fracture mechanics models are also employed in the prediction of the fatigue lives of smooth specimens deformed to failure at room temperature. The article highlights the potential to develop mechanistically based predictive models based on simplified mechanics idealizations of experimental observations.     

不同应力上限和加载速率下的黄砂岩疲劳特性研究

为研究黄砂岩单轴疲劳加载的特性,开展了不同应力上限和加载速率下的单轴疲劳荷载试验。试验结果表明：黄砂岩的疲劳试验曲线受到单轴压缩应力—应变曲线的控制,疲劳极限变形与峰后对应变形一致;砂岩疲劳过程的不可逆变形和耗散能密度均具有三阶段演化规律,依据倒“S”型损伤模型,验证了黄砂岩疲劳损伤三阶段演化规律;分析认为三阶段规律的本质是砂岩的塑性变形和内部孔隙微裂纹生成以及扩展速度的不同所呈现的结果。研究表明应力上限和加载速率对疲劳寿命有显著影响,根据所得应力—寿命公式,可以估计砂岩在一定条件下的疲劳寿命。     

A new device for fretting fatigue testing

Fretting fatigue damage occurs in contacting parts when they are subjected to fluctuating loads and sliding movements at the same time. Fretting fatigue can reduce the fatigue life of materials by half or even more. Fretting fatigue tests are usually performed using universal hydraulic testing devices. The contact pressure is produced by a fixture, typically designed and manufactured by researchers. In this investigation, a new device is introduced in which the fluctuating loading is supplied by a variable crank system (VCSD). The device called VCSD for abbreviation is basically a position control machine in which displacements can be imposed with an accuracy of 0.01 mm. The axial and contact loads are measured by load cells. The friction load is also measured by using foil strain gauges using a Wheatstone bridge configuration. The functionality of the device is examined by making a comparison between fretting fatigue lives of a number of Al7075-T6 specimens tested on a universal testing machine and VCSD. The results show a very close agreement between the functionality of the two testing rigs. The main advantages of VCSD are its higher frequency with respect to universal devices, simplicity, and cheapness. It can be developed further for high and low temperature tests in future.     

An Evaluation of the summation of cyclic plastic strain damage in two high strength aluminum alloys

A damage equation based upon the integration of low cycle fatigue plastic strain ranges was verified experimentally for two high strength aluminum alloys 2024-T4 and 7075-T651. The damage equation which has been used extensively for many fatigue crack propagation theories assumes cyclic damage under increasing plastic strain ranges. In order to verify the damage equation, low cycle fatigue specimens were subjected to a fully reversed strain cycle in which the total strain-range was increased linearly by a constant amount Δ[Δε_d] per cycle. An excellent agreement was obtained between the predicted and observed fatigue lifetimes. The stress-strain response of these alloys was also measured. The experimental results showed that these two alloys cyclically harden substantially and that the single strain increment stress-strain curve is a fair lower bound approximation of the cyclic stress-strain curve.     

Service Life Predictions for Hot Bulk Forming Tools

Hot bulk forming tools are subject to high thermal and mechanical alternating loads which can induce the formation of fatigue cracks in the highly stressed regions of the tool. It this way, premature tool failure occurs with which increased tool costs are associated. It is therefore vitally important to calculate the tool life output during the process design to improve the efficiency. Thermomechanical fatigue tests using the hot‐working tool steel X38CrMoV5‐3 are carried out as the basis for service life predictions in order to characterise the material behaviour subject to cyclic loading. In the tests, the thermal and mechanical loads operating in the tool steel during a forging process are reproduced. In this way, a strain controlled S‐N curve is determined for a specific temperature interval by varying the applied mechanical load. Thus it is possible to consider the damage mechanisms in the material, which operate during the forming process, for computing the service life. Based on the experimentally determined strain controlled S‐N curve, the computation of a fatigue failure is carried out for a practical example with tool fracture. By comparing the material's experimentally determined load carrying capacity with the loading computed by employing the elastic‐plastic material behaviour, the number of forging cycles is ascertained up to incipient cracking. The simulation model introduced here permits an improved prediction of the fatigue crack formation by integrating the cyclic material behaviour subject to similar conditions found in the forging process.     

Retrofitting of Rectangular Columns with Deficient Lap Splices

The cyclic behavior of eight 0.4-scale reinforced concrete column specimens is investigated. The columns incorporated deficient design details to simulate bridge columns built in Washington State prior to 1971. Two columns were tested as reference specimens, five were tested after retrofitting using carbon fiber-reinforced polymer (CFRP), and one was tested after retrofitting using a conventional steel jacket. All the specimens were tested under constant gravity load and incrementally increasing lateral loading cycles. The specimens had rectangular cross sections with aspect ratios of 1.5 and 2.0. The parameters investigated included the amount of CFRP reinforcement, different retrofitting jacket configurations, and different retrofitting materials. For the as-built specimens, two modes of failure occurred, namely low cyclic fatigue of longitudinal reinforcement and lap splice failure. For the retrofitted specimens, no lap splice failure was observed. All the retrofitted specimens failed due to low cyclic fatigue failure of the longitudinal bars. The retrofitting measures improved the displacement ductility, energy dissipation, and equivalent viscous damping. In addition, increasing the amount of CFRP reinforcement improved the performance of the test specimens.     

Localized heat generation during fracture of cyclically loaded steel

A theoretical model describing heat generation in the plastically deforming region near a crack tip under cyclic loading conditions has been developed. It predicts crack-tip temperature as a function of stress-intensity amplitude, cyclic loading frequency, time under load, and material parameters. Crack-tip temperatures are measured in AISI 4135 steel at a loading frequency of 20 Hz by employing a scanning infrared camera system. Measured temperatures compare reasonably well with values predicted by the theoretical model. Temperature changes are small under prevailing testing conditions.     

Biaxial Low Cycle Fatigue Behavior and Martensite Formation of a Metastable Austenitic Cast TRIP Steel Under Proportional Loading

In this paper the biaxial low cycle fatigue behavior under proportional loading of a recently developed metastable austenitic stainless cast steel is presented. Total strain controlled tests were carried out on a 250 kN biaxial servohydraulic tension‐compression testing machine equipped with a biaxial orthogonal extensometer to measure the principal strains in the gauge area of the used cruciform specimens. The principal stresses were determined based on the compliance after the load reversals. The low cycle fatigue behavior under biaxial synchronous loading is compared to the uniaxial behavior. Therefore, biaxial single step tests and a biaxial multiple step load increase test were carried out. The dependence of the stress state on the cyclic deformation curves, cyclic stress‐strain curves and the formation of martensite are described. Finally, the fatigue life relationship according to Basquin and Manson‐Coffin was determined and compared to the Smith, Watson and Topper damage parameter, which provides a satisfactory fatigue life prediction.     

Monotonic and cyclic damage in metallic sheets

This paper aims to provide an account of some interesting features of damage in metallic sheets under monotonic and cyclic loading using the information and understanding developed through a series of experimental investigations conducted on interstitial free steels. The experiments primarily consisted of damage evaluation in un-notched and notched sheets vis-à-vis that in thick specimens under monotonic loading, and that on sheets by interrupted or continuous cyclic loading. Some salient observations indicate that: (i) void nucleation occurs in two different stages, originating from non-metallic inclusions and precipitates. The critical strain for void nucleation at precipitates (?_c) is lower for sheet metals than that in thick specimens. (ii) ?_c is a function of notch length in sheets, and the function assists to estimate the strength of particle/matrix interface, (iii) under cyclic loading, steel sheets exhibit non propagating microcracks below the endurance limit. Above the endurance limit slip bands promote formation of larger fatigue cracks primarily at ferrite grain boundaries. A series of grain boundary cracks link up to form meso-cracks, one of which grows to cause final failure and (iv) the growth of the macro-crack initially occurs in opening mode followed by its propagation in mixed mode through striations, intergranular cracking and through thickness necking prior to failure.     

Strengthening of Reinforced Concrete Bridge Decks Using Carbon Fiber-Reinforced Polymer Composite Materials

This paper presents the results of an investigation of the monotonic and fatigue behavior of one-way and two-way reinforced concrete slabs strengthened with carbon fiber-reinforced polymer (CFRP) materials. The five one-way slab specimens were removed from a decommissioned bridge in South Carolina. Three of the slabs were retrofitted with CFRP strips bonded to their soffits and the other two served as unretrofit, control specimens. Of the five one-way slab specimens, one unretrofit and two retrofit slabs were tested monotonically until failure. The remaining two specimens, one unretrofit and one retrofit, were tested under cyclic (fatigue) loading until failure. In addition, six half-scale, two-way slab specimens were constructed to represent a full-scale prototype of a highway bridge deck designed using the empirical requirements of the AASHTO LRFD Bridge Design Manual. Of the six square slabs, two were unretrofitted and served as the control specimens, two were retrofitted using CFRP strips bonded to their soffits making a grid pattern, and two were retrofitted with a preformed CFRP grid material bonded to their soffit. Three slabs, one unretrofit, one CFRP strip, and one CFRP grid retrofitted, were tested monotonically until failure and the remaining three slabs were tested under cyclic (fatigue) loading until failure.     

Conditioning monitoring by microstructural evaluation of cumulative fatigue damage

The objective of this work is to evaluate the damage induced below and above the fatigue limit (Δσ _t =360 MPa) in pressure vessel steels, such as SA508. Fatigue damage was induced in samples taken from an SA508 steel plate by various loading histories in order to examine the influence of prior cyclic loading below the fatigue limit. Cell-to-cell misorientation differences were measured by the selected area diffraction (SAD) method. Surface cracking was also studied by the replication method. Small cracks were observed after precycling both below and above the fatigue limit. It was, however, found that fatigue test bars had a longer lifetime after precycling below the fatigue limit, while precycling above the fatigue limit caused other specimens to fail even when subsequently cycled below the fatigue limit. Cell-to-cell misorientation usually increases with accumulation of fatigue damage, but it was found that the misorientations measured after precycling below the fatigue limit decreased again at the beginning of the subsequent cycling above the fatigue limit. It should be noted that the misorientation at failure was always about 4 to 5 deg, regardless of loading histories. Misorientation showed good correlation with the fatigue lifetime of the samples.     

Development of short fatigue cracks in aluminum alloy 2524-T3 specimens

The development of short fatigue cracks in a 2524-T3 alloy is studied under cyclic tension conditions. Flat specimens with a stress concentrator in the form of a central hole are analyzed. The replica technique is used to determine the microcrack parameters and to estimate the cyclic damage characteristics of the alloy in the stress concentrator zone. The experimental results are compared to the fatigue lives estimated by a calculation-experimental method using the NASGRO software package. The experimental fatigue life at the stage of short crack initiation is found to be significantly shorter than the calculated fatigue life.     

Fatigue behavior of A356-T6 aluminum cast alloys. Part I. Effect of casting defects

The influence of casting defects on the room temperature fatigue performance of a Sr-modified A356-T6 casting alloy has been studied using un-notched polished cylindrical specimens. The numbers of cycles to failure of materials with various secondary arm spacings (SDAS) were investigated as a function of stress amplitude, stress ratio, and casting defect size. To produce pore-free samples, HIP-ed and Densal™ treatments were applied prior to the T6 heat treatment. It was observed that casting defects have a detrimental effect on fatigue life by shortening not only the crack propagation period, but also the initiation period. Castings with defects show at least an order of magnitude lower fatigue life compared to defect-free ones. The decrease in fatigue life is directly correlated to the increase of defect size. HIP-ed alloys show much longer fatigue lives compared to non-HIP-ed ones. There seems to exist a critical defect size for fatigue crack initiation, below which fatigue crack initiates from other competing initiators such as eutectic particles and slip bands. A fracture mechanics approach has been used to determine the number of cycles necessary to propagate a fatigue crack from a casting defect to final failure. Fatigue life of castings containing defects can be quantitatively predicted using the size of the defects. Moreover, the fatigue fracture behavior of aluminum castings is well described by Weibull statistics. Crack originating from different defects (such as porosity and oxide films) can be readily identified from the Weibull modulus and the characteristic fatigue life. Compared with oxide films, porosity is more detrimental to fatigue life.     

Effect of laminate stacking sequence on the high frequency fatigue behavior of SCS-6 fiber-reinforced Si3N4 matrix composites

In many potential applications, continuous fiber-reinforced ceramic matrix composites (CFCMCs) will encounter cyclic fatigue loadings at high frequencies (25 Hz or higher). While most of the work in the area of fatigue of CFCMCs has concentrated on low frequency behavior, high frequency behavior is equally important. In CFCMCs, stress-strain hysteresis occurs during fatigue and is associated with energy dissipation in the composite. In addition to this, the repeated friction and sliding between fiber and matrix are responsible for a substantial temperature rise at the fiber/matrix interface. In this study, [0/90] and [±45] SCS-6 (silicon carbide)/Si₃N₄ composites made by hot pressing were investigated under high frequency fatigue loadings. The angle-ply laminate showed the same extent of heating as cross-ply laminates, but at much lower stress levels. Frictional heating was caused by sliding at the fiber/matrix interface. Temperature rise due to heat generation in the specimens correlated very well with damage in modulus as a function of fatigue cycles in the composites. Matrix microcracking was more predominant in the angle ply than in the cross-ply composite, due to the much lower stiffness of the angle-ply composite in the longitudinal loading direction.     

Investigating and Understanding the Cyclic Fatigue,Deformation, and Fracture Behavior of a Novel High Strength Alloy Steel: Influence of Orientation

In this paper, the results of a recent study aimed at understanding the influence of orientation on high cycle fatigue properties and final fracture behavior of alloy steel Pyrowear 53 is presented and discussed. This alloy steel has noticeably improved strength, ductility, and toughness properties compared to other competing high strength alloy steels having a near similar chemical composition and processing history. Test specimens of this alloy steel were precision machined and conformed to the specifications detailed in the ASTM standards for tension testing and stress‐controlled cyclic fatigue tests. Test specimens were prepared from both the longitudinal and transverse orientations of the as‐provided alloy steel bar stock. The machined test specimens were deformed in cyclic fatigue over a range of maximum stress and under conditions of fully reversed loading, i.e., at a load ratio of ?1, and the number of cycles‐to‐failure recorded. The specific influence of orientation on cyclic fatigue life of this alloy steel is presented. The fatigue fracture surfaces were examined in a scanning electron microscope to establish the macroscopic fracture mode and to characterize the intrinsic features on the fatigue fracture surfaces. The conjoint influence of microstructure, orientation, nature of loading, and maximum stress on cyclic fatigue life is discussed.     

Problems and feasibilities of testing the cyclic behaviour of thin sheet steels

This report deals with the testing problems of thin sheet steels subjected to cyclic load and introduces at the same time the appropriate techniques to procure the reliable and significant testing results. Practically, a proper judgement of cyclic fatigue behaviours of such conventional deep drawing steels is only possible, if the specimens were to be tested either under strain control mode in elastic-plastic range with sufficient plastic deformations or load controlled in elastic range by using the notched specimens with adequate local strain concentration. Besides the study of specific material properties, as absolutely determined by using the unnotched specimens, the influence of notch geometry can also be investigated by means of the notched specimens. For a promising performance of elastic-plastic cyclic tests on thin sheets the use of a particular specimen shape with uncritical length of high buckling resistance is most essential. Moreover, a special grip with great precision is necessary to ensure an axial and rigid alignment of specimens. In the course of a development an optimal specimen shape and its gripping device has been designed. The cyclic properties of some thin sheet steels, obtained in this way, are also illustrated.     

Crack Propagation in Flexural Fatigue of Concrete

In this paper the behavior of concrete subjected to flexural fatigue loading is studied. Notched concrete beams were tested in a three-point bending configuration. Specimens were subjected to quasi-static cyclic and constant amplitude fatigue loading. The cyclic tests were performed by unloading the specimen at different points in the postpeak part of the quasi-static loading response. Low cycle, high amplitude fatigue tests were performed to failure using four different load ranges. The crack mouth opening displacement was continuously monitored throughout the loading process. Crack propagation caused by quasi-static and fatigue loads is described in terms of fracture mechanics. It is shown that the crack propagation in the postpeak part of the quasi-static load response is predicted using the critical value of the mode I stress intensity factor (KIC). The ultimate deformation of the specimen during the fatigue test is compared with that from the quasi-static test; it is demonstrated that the quasi-static deformation is insufficient as a fatigue failure criterion. It is observed that crack growth owing to constant-amplitude fatigue loading comprises two phases: a deceleration stage when there is a decrease in crack growth rate with increasing crack length, followed by an acceleration stage where the rate of crack growth increases at a steady rate. The crack length where the rate of crack growth changes from deceleration to acceleration is shown to be equal to the crack length at the peak load of the quasi-static response. Analytical expressions for crack growth in the deceleration and acceleration stages are developed, wherein the expressions for crack growth rate in the deceleration stage are developed using the R-curve concept, and the acceleration stage is shown to follow the Paris law. It is observed that the crack length at failure for constant amplitude fatigue loading is comparable to that of the corresponding load in the postpeak part of the quasi-static response. Finally, a fracture-based fatigue failure criterion is proposed.