Tension-tension Fatigue Loading Research Articles

Analysis of Acoustic Emissions from a Steel Bridge Hanger

A steel bridge hanger with three fatigue cracks was monitored for acoustic emission (AE) using combined source location, strain gauge monitoring, and waveform analysis. AE activities from all three cracks were clearly identified and classified as crack growth or noise signals using location, strain magnitude, position on strain cycle, and uniqueness of waveforms as the primary criteria. A vast majority of AE from the cracks was found to be due to crack face rubbing and the crushing of corrosion products between the crack faces while limited crack growth emissions were detected. Results from laboratory tests on A588 compact tension specimens under variable-amplitude tension-tension fatigue loading were used to aid in interpreting AE data from the hanger. Crack growth AEs from these tests were detected only on overload cycles mostly above 92% of the maximum load while AE due to crack face rubbing occurred throughout the load cycle.

Analysis of Acoustic Emissions from a Steel Bridge Hanger

A steel bridge hanger with three fatigue cracks was monitored for acoustic emission (AE) using combined source location, strain gauge monitoring, and waveform analysis. AE activities from all three cracks were clearly identified and classified as crack growth or noise signals using location, strain magnitude, position on strain cycle, and uniqueness of waveforms as the primary criteria. A vast majority of AE from the cracks was found to be due to crack face rubbing and the crushing of corrosion products between the crack faces while limited crack growth emissions were detected. Results from laboratory tests on A588 compact tension specimens under variable-amplitude tension-tension fatigue loading were used to aid in interpreting AE data from the hanger. Crack growth AEs from these tests were detected only on overload cycles mostly above 92% of the maximum load while AE due to crack face rubbing occurred throughout the load cycle.

Effect of thermal residual stresses on fatigue crack petting and propagation behaviour in an Al/SlC metal matrix compost : Fitzpatrick M.E., Hutchings, M.F., King, J.E., Knowles, D.M. and Withers, P.J. Metall. Mater. Trans. A (Dec. 1995) 26A (12), 3191–3198

An investigation was undertaken to understand the fatigue process in unidirectional metal matrix composites (MMCs) with center holes. Currently, there is considerable confusion as to the true nature of initiation and growth of cracks from notches in MMCs. A key concern is whether surface cracks are through cracks. Furthermore, an explanation is required for the preferential initiation and growth of four major symmetric cracks at 65–72 degrees from the pole (loading axis). Clarification of these issues is critical for developing rational analysis procedures for damage tolerance.In this investigation, unidirectional eight-ply Ti 15-3/SCS-6 composites with a center hole were subjected to monotonic and tension-tension fatigue loading at room temperature. Optical and scanning electron microscopy were used to evaluate the initiation and growth of the cracks and to determine their angular orientation around the hole in the specimen. Under fatigue loading, it was found that the major cracks were continuous cracks through the depth of the specimen, and they were located at 65–72 degrees from the pole. The Hencky-Von Mises failure criterion, an octahedral shear stress criterion, was found to be successful in predicting the preferred initiation sites for the major fatigue cracks. Fatigue crack initiation appeared to be controlled primarily by shear stress induced reversed inelastic deformation of the matrix in the hole region. In contrast, under monotonic loading, fracture occurred at 90 degrees from the pole, similar to what is observed in homogeneous metals. Crack initiation in this case appeared to be controlled by fiber fracture. Thus, whereas failure was matrix dominated in fatigue, it was fiber dominated under monotonic loading.

Effect of Cooling Rate and Stacking Sequence on the Fatigue Behavior of Notched Quasi-Isotropic AS4/PEEK Laminates

The effect of cooling rate and stacking sequence on fatigue behavior was analyzed for notched quasi-isotropic AS4/PEEK laminates. The fatigue behavior of fast (475°F/min) and slow (1°F/min) cooled specimens of the following two lay-ups was studied: Layup A of (−45/0/45/90)s and Layup B of (45/90/−45/0)s. All specimens were subjected to a load-controlled, tension-tension fatigue loading with a stress ratio R = 0.1 at a frequency of 5 Hz. Damage was monitored by the reduction in dynamic stiffness and penetrant enhanced X-radiography. Post-failure analysis was carried out by scanning electron microscopy (SEM). Influence of cooling rate on the fatigue properties was different in the two stacking sequences investigated. The fatigue behavior changed drastically with the variation in the stacking sequence. A model, based on a constant strain-to-failure, was developed for life prediction and the predicted lives are in good agreement with the experimental values. To help understand the fatigue behavior of the two stacking sequences studied, the interlaminar stress distributions were calculated with a quasi-3D finite element analysis (FEA) for the regions around the hole. Specimens of Lay-up A were predicted to be more prone to delamination as compared to those of Lay-up B and this is consistent with experimental observations.

Acoustic emission during fatigue crack propagation in SiC particle reinforced Al matrix composites

The acoustic emission (AE) behavior during fatigue propagation in aluminum 6061 and aluminum 6061 matrix composites containing 5, 10, and 20 wt pct SiC particle reinforcement was investigated under tension-tension fatigue loading. The purpose of this investigation was to monitor fatigue crack propagation by the AE technique and to identify the source(s) of AE. Most of the AEs detected were observed at the top of the load cycles. The cumulative number of AE events was found to correspond closely to the fatigue crack growth and to increase with increasing SiC content. Fractographic studies revealed an increasing number of fractured particles and to a lesser extent decohered particles on the fatigue fracture surface as the crack propagation rate(e.g., †K) or the SiC content was increased.

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.

Fatigue Damage and Fracture of Carbon Fabric/Epoxy Composites under Tension-Tension Loading

The degradation of a plain woven carbon fiber/epoxy composite under tension-tension fatigue loading was studied in this work. An almost flat S-N diagram was obtained for this composite. As high as 85% fatigue strength of the static strength at 106 cycles was recorded. A new specimen configuration was developed and used. That is, the specimen has two narrow sections. After the specimen fails at one section under static or cyclic loading, the residual strength of the other section will be statically measured. The residual strength for specimens failing under static and cyclic loadings is higher than the static strength for the virgin specimen. A typical S-shape stiffness reduction against logarithmic cycle number is found for high-cycle-fatigue specimens. However, sudden fatigue death of the specimens occurs with no indications in both S-S curves and AE events. Parameters of the parabolic function which fits the convex S-S curve are more sensitive to the stiffness reduction than the observed stiffness. Microscopic examination of the fracture surface was conducted. From those observations, a fatigue damage process model is proposed.

The hygrothermal effect on the fatigue behavior of carbon fiber/ peek laminate composites (I)

The hygrothermal effects on the fatigue behavior of the Carbon/PEEK laminated composites before and after impact damage were examined in this study. The [0/45/90/-45]2s AS-4/PEEK laminated composites were immersed in 80°C hot water for 45, 90 and 200 days,and subjected to falling weight impact with an energy of 8.58 J and then immersed in 80°C hot water for 45 days. It was found that the tensile strength of AS-4/PEEK laminated composites decreased with the increase of exposure period. The injured AS-4/PEEK composites were subjected to a static load and a tensiontension fatigue load at various levels of stress amplitudes. The effect of stress amplitude on the fatigue life was studied. The experimental fatigue life under different stress amplitude tests were estimated by the median rank order statistic cumulative distribution function. Then,the fitting curves for estimated data were analyzed by the Weibull distribution function. The S-N curves for a series of cyclic loads at various survival probabilities were presented. The damage behaviors of composites after fatigue load test were also investigated by scanning electron microscope(SEM). Results indicated the fatigue lives of immersed specimens were shorter than those without hygrothermal effect, the impact damage affects the fatigue life of composite significantly.

Crack Shielding in Ce‐TZP/Al2O3 Composites: Comparison of Fatigue and Sustained Load Crack Growth Specimens

Crack shielding stress intensities in in situ loaded compact tension specimens of two types of ceria‐partially‐stabilized zirconia/alumina (Ce‐TZP/Al2O3) composites with prior histories of subcritical crack growth in sustained and tension‐tension fatigue loading were directly assessed using laser Raman spectroscopy. Crack‐tip stress fields within the transformation zones were measured by measuring a stress‐induced frequency shift of a peak corresponding to the tetragonal phase. The peak shift as a function of the applied stress was separately calibrated using a ball‐on‐ring flexure test. Total crack shielding stress intensity was estimated from the far‐field applied stress intensity and the local crack‐tip stress intensity assessed from the measured near‐crack‐tip stresses. The shielding stress intensities were consistently lower in the fatigue specimens than in the sustained load crack growth specimens. The reduced crack shielding developed in the fatigue specimens was independently confirmed by measurements of larger crack‐opening displacement under far‐field applied load as compared to the sustained load crack growth specimens. Thus, diminished crack shielding was a major factor contributing to the higher subcritical crack growth rates exhibited by the Ce‐TZP/Al2O3 composites in tension–tension cyclic fatigue. Calculations of zone shielding considering only the dilatational strains in the transformation zones accounted for 81% and 86% of the measured values in the sustained load crack growth specimens, but significantly overestimated the shielding in the fatigue specimens. Possible reasons for the diminished crack shielding in the fatigue specimens are discussed.

Fatique and damage-tolerance analysis of composite laminates: Stiffness loss, damage-modelling, and life prediction

The prediction of fatigue life and evaluation of progressive damage for general composite laminates are studied analytically. From theories of damage tolerance, residual-modulus degradation, and residual-strength d degradation, a simple and approximate approach is proposed for the prediction of progressive stiffness loss, matrix-crack density, and delamination area in terms of tension-tension fatigue load and number of cycles for general laminates containing 0° plies. The proposed approach provides four choices for predicting tension-tension allowable fatigue life and for assessing fail-safety for structures made of composite laminates, namely, a residual-modulus criterion, a matrix-cracking criterion, a delamination-size criterion, and a residual-strength criterion. The approximate analytical relationship between S-N curves for general laminates containing 0° plies and for a unidirectional 0°-ply laminate is proposed. Three glass/epoxy laminates of [0/90] s, [±45/0/90] s , and [0/±45] s lay-ups, and one [0/90±45] s T300/5208 graphite/epoxy laminate under tension-tension fatigue are used to illustrate and verify the proposed approach. The analytical results are in good agreement with experimental data.

Composite-to-metal tubular lap joints: strength and fatigue resistance

The axial strength and fatigue resistance of thick-walled, adhesively bonded E-glass composite-to-aluminum tubular lap joints have been measured for tensile and compressive loadings. The joint specimen bonds a 63 mm OD aluminium tube within each end of a 300 mm long, 6 mm thick E-glass/epoxy tube. Untapered, 12.5 mm thick aluminium adherends were used in all but four of the joint specimens. The aluminum adherends in the remaining four specimens were tapered to a thickness of 1 mm at the inner bond end (the bond end where the aluminum adherend terminates). For all loadings, joint failure initiates at the inner bond end as a crack grows in the adhesive adjacent to the interface. Test results for a tension-tension fatigue loading indicate that fatigue can severely degrade joint performance. Interestingly, measured tensile strength and fatigue resistance for joints with untapered adherends is substantially greater than compressive strength and fatigue resistance. The joint specimen has been analyzed in two different ways: one approach models the adhesive as an uncracked, elastic-perfectly plastic material, while the other approach uses a linear elastic fracture mechanics methodology. Results for the uncracked, elastic-plastic adhesive model indicate that observed bond failure occurs in the region of highest calculated stresses, extensive bond yielding occurs at load levels well below that required to fail the joint, and a tensile peel stress is generated by a compressive joint loading when the aluminum adherends are untapered. This latter result is consistent with the observed joint tensile-compressive strength differential. Results of the linear elastic fracture mechanics analysis of a joint with untapered aluminum adherends are also consistent with the observed differential strength effect since a mode I crack loading is predicted for a compressive joint loading. Calculations and a limited number of tests suggest that it may be possible to selectively control the differential strength effect by tapering the aluminum adherends. The effect of adherend material and thickness on fracture mechanics parameters is also investigated. The paper concludes by examining the applicability of linear elastic fracture mechanics to the joints tested.

Fatigue Cracking of Fiber-Reinforced Titanium Matrix Composites

The fatigue crack growth behavior of a unidirectional SCS-6-fiber-reinforced Ti-6Al-2Sn-4Zr-2Mo composite has been characterized. A fatigue cracking map for several unidirectional SCS-6-fiber-reinforced titanium and titanium aluminide composites under tension-tension fatigue loading has been developed. The map is constructed based upon the fiber strength to interfacial shear strength ratio versus the applied maximum stress intensity factor. Also, the effect of fiber bridging on the effective crack driving force in the matrix is analyzed using the shear-lag model and finite element analysis.

Fatigue Damage and Fracture of Carbon Fabric/Epoxy Composite under Tension-Tension Loading.

The degradation of a plain woven carbon fiber/epoxy composite under tension-tension fatigue loading was studied in this paper. An almost flat S-N diagram was obtained for this composite. As high as 85% fatigue strength of the static strength at 106 cycles was recorded. The residual strength after both static and cyclic loadings is higher than the static strength. A typical S-shape stiffness reduction against logarithmic cycle number is found for the high-cycle-fatigue specimens. However, sudden fatigue death of the specimens occurs with no indications in both S-S curves and AE events. Parameters of the parabolic function which fits the convex S-S curve represent the stiffness reduction better than the observed stiffness. Microscopic examination of the fracture surface was conducted. From those observations, a fatigue damage process mode is proposed.

Observations of fatigue crack initiation and damage growth in notched titanium matrix composites: Naik, R.A. and Johnson, W.S. Proc. Conf. Composite Materials: Fatigue and Fracture, Lake Buena Vista, Florida, USA, 6–7 Nov. 1989 vol 3, 753–771

Damage progression in unidirectional (UD) composites in tension-tension fatigue is vital to understanding fatigue in more complex laminates and loading conditions. We therefore developed the UD fatigue model that considers fiber breaks, fiber-matrix debond growth, fiber fatigue as well as interactions of these damage modes. The contribution of each mode of damage in fatigue is assessed and those that dominate fatigue damage are identified. The model accounts for Weibull statistics of fiber strength and stress redistribution around fiber breaks. The predictions for fatigue loading indicate a significantly higher density of fiber breaks and clusters compared to quasi-static loading at the same stress level. The model predicts S–N curves in pure uniaxial tension-tension fatigue loading and does not consider stress concentrations caused near the grips; the predictions therefore tend to overpredict fatigue life compared to experimental S–N curves available in the literature. The model has potential to be developed for different loading conditions.

Crack initiation around holes in a unidirectional MMC under fatigue loading

An investigation was undertaken to understand the fatigue process in unidirectional metal matrix composites (MMCs) with center holes. Currently, there is considerable confusion as to the true nature of initiation and growth of cracks from notches in MMCs. A key concern is whether surface cracks are through cracks. Furthermore, an explanation is required for the preferential initiation and growth of four major symmetric cracks at 65–72 degrees from the pole (loading axis). Clarification of these issues is critical for developing rational analysis procedures for damage tolerance. In this investigation, unidirectional eight-ply Ti 15-3/SCS-6 composites with a center hole were subjected to monotonic and tension-tension fatigue loading at room temperature. Optical and scanning electron microscopy were used to evaluate the initiation and growth of the cracks and to determine their angular orientation around the hole in the specimen. Under fatigue loading, it was found that the major cracks were continuous cracks through the depth of the specimen, and they were located at 65–72 degrees from the pole. The Hencky-Von Mises failure criterion, an octahedral shear stress criterion, was found to be successful in predicting the preferred initiation sites for the major fatigue cracks. Fatigue crack initiation appeared to be controlled primarily by shear stress induced reversed inelastic deformation of the matrix in the hole region. In contrast, under monotonic loading, fracture occurred at 90 degrees from the pole, similar to what is observed in homogeneous metals. Crack initiation in this case appeared to be controlled by fiber fracture. Thus, whereas failure was matrix dominated in fatigue, it was fiber dominated under monotonic loading.

Fatigue behaviour of carbon fibre-reinforced aluminium laminates

A new hybrid composite (CARALL), consisting of thin layers of carbon fibre/ epoxy prepreg sandwiched between aluminium sheets, has been developed. It is shown that this class of materials offers higher modulus, higher tensile strength and lower density than 2024-T3 alloy in the longitudinal direction. Under tension-tension fatigue loading, the hybrid laminates showed superior fatigue crack propagation resistance in the longitudinal direction, which may be attributed to the bridging effect imposed by the intact fibres in the crack wake. It has also been shown that the effectiveness of fatigue crack growth reduction increases with the thickness of the carbon fibre/epoxy layer. The resistance to fatigue crack propagation can be further improved by introducing compressive residual stresses in the aluminium layer by postcure stretching the laminate in the plastic region of the aluminium alloy.

Residual strength in notched composite materials

The main objective was to understand the stress concentration factor of fatigue damage for centrally notched composite laminates of various lay-ups in the proportion of 45, −45, 90 and 0 deg plies. The application of fatigue stress concentration factor was used to predict the residual strength. The Whitney-Nuismer (WN) extended point stress fracture criterion and the extended average stress fracture criterion were adopted. The characteristic length is not necessarily a constant; it depends on the shape of the damage zone, the number of applied cycles, the hole radius, etc. The tests were performed at low-frequency tension-tension (T-T) fatigue loading with constant stress amplitude at room temperature. In simple terms, after thousands of cycles, because of the geometric change of a circular hole to the deformed laminate, the stress concentration factor decreases with the increasing number of applied cycles, which is the reason for the increase of residual strength of the notched laminate. The correlation of the residual strength as predicted by the numerical methods and the fatigue stress concentration analysis is found to be satisfactory, in comparison with the experimental data.

Mechanics and micromechanisms of fatigue crack growth in brittle solids

This paper is concerned with the mechanics and micromechanisms of stable mode I crack growth in brittle solids subjected to compression-compression fatigue and tension-tension fatigue loads. Constitutive models, results of finite element analyses, and experimental observations are described for monolithic ceramics and ceramic-matrix composites, plain concrete, and a transformation-toughened ceramic in an attempt to deduce a general theory on the origin of mode I fracture in notched plates under uniaxial cyclic compression at room temperature. An analysis of the residual stress field which develops at elevated temperatures in response to power law creep and far-field compressive cyclic loads is also presented. The principal “driving force” for mode I fracture in cyclic compression is the generation of a near-tip zone of residual tension, when the deformation at the notch-tip leaves permanent strains upon unloading from the far-field compressive stress. The results indicated that materials with very different microscopic deformation mechanisms, i.e., microcracking, dislocation plasticity, martensitic transformation, interfacial debonding/slip, or creep, exhibit a macroscopically similar, stable fracture under far-field cyclic compression because the zone of residual tension is embedded in material which is elastically strained in compression. It is shown that cyclic compression loading offers a unique method for fatigue precracking notched specimens of brittle solids prior to tensile fracture testing, whereby an unambiguous interpretation of the critical stress intensity factors for crack initiation and growth can be achieved. Fatigue crack growth characteristics of a transformation-toughened ceramic and a creeping ceramic composite under tension-tension fatigue loads are also discussed.

朱子織ＧＦＲＰの疲労試験におけるＡＥ特性

Acoustic Emission behavior has been studied during fatigue tests of GFRP. Notched specimens were prepared from the GFRP sheet used for landing gear door of Boeing 747 jet plane. Specimens were subjected to uniaxial tension-tension fatigue loading at loading rate of 1 Hz. Relationship between the AE behavior and the loading phase was carefully investigated to evaluate the damage accumulation of the specimen. Four stages in AE activity curves were observed. AE activity of each stage exhibited different characteristics in terms of amplitude and energy, and the AE sources could be distinguished by the observation of the AE parameters such as amplitude, energy and duration. Three different types of the fracture surfaces were observed by scanning electron microscope. Different AE characteristic at each fatigue stage corresponded to these different fracture types. It was suggested that AE measurement could be effective for the assessment of damage and the characterization of crack growth process in GFRP under fatigue loading.

The role of microdamage in tensile failure of graphite/epoxy laminates

Damage development during quasistatic tensile loading of several laminates of graphite/epoxy material is examined and compared to damage development in laminates of a similar graphite/epoxy material subjected to tension-tension fatigue loading. Emphasis is placed upon following damage development at the microstructural level. Evidence of the important role of off-axis ply cracks in localizing and controlling fiber fracture in adjacent load-bearing plies for both loading modes is resented. The relationship between fiber fracture density and static load level is presented for tensile loading of unidirectional and cross-ply laminates by direct observation of fiber fracture in situ. The frequencies of occurrence of multiple adjacent fiber fractures are also reported. The cross-ply laminate results are compared with those from fatigue testing. Significant differences are described and discussed.