Fatigue Life Diagram Research Articles

A general method for predicting temperature-dependent anisomorphic constant fatigue life diagram for a woven fabric carbon/epoxy laminate

The anisomorphic constant fatigue life (CFL) diagram approach to prediction of fatigue lives of composites, which was developed in an earlier study, is developed further into a more general methodology that can deal with the mean stress sensitivity in fatigue of composites at different temperatures. The temperature dependence of the anisomorphic CFL diagram for a given composite is characterized by the temperature dependence of the static strengths in tension and compression and of the reference S–N relationship for a critical stress ratio. The temperature dependence of the static strengths in tension and compression is first formulated to describe the temperature dependence of the critical stress ratio. To predict the reference S–N relationships at different temperatures, the change in the value of critical stress ratio with temperature as well as the effect of temperature on fatigue should be taken into account. To this end, a new and efficient engineering method is developed which is based on a grand master S–N curve built by means of a modified fatigue strength ratio and a life-temperature parameter of the Larson–Miller type. The generalized anisomorphic CFL diagram approach developed in this study succeeds in efficiently and adequately predicting the CFL diagrams for a woven fabric carbon/epoxy quasi-isotropic laminate at different temperatures and thus the mean stress dependence of the S–N relationships of the laminate at different temperatures.

Anisomorphic constant fatigue life diagrams for a woven fabric carbon/epoxy laminate at different temperatures

The effect of temperature on the constant fatigue life (CFL) diagram for a woven fabric carbon/epoxy quasi-isotropic laminate has been examined. Constant amplitude fatigue tests are first performed at different stress ratios on coupon specimens at room temperature (RT), 100 and 150°C, respectively. The experimental results show that the CFL diagram for the woven CFRP laminate, which is plotted in the plane of mean and alternating stresses, becomes asymmetric about the alternating stress axis, regardless of test temperature, and shrinks as temperature increases. The CFL envelopes for given constant values of life are nonlinear over the range of fatigue cycles, regardless of test temperature, and they take peaks approximately at a particular stress ratio “critical stress ratio” that is given by the ratio of compressive strength to tensile strength. Then, the experimental CFL diagram for each temperature is compared with prediction using the anisomorphic CFL diagram approach that allows constructing the asymmetric and nonlinear CFL diagram for a given composite on the basis of the static strengths in tension and compression and the reference S–N relationship for the critical stress ratio. It is demonstrated that the anisomorphic CFL diagram approach can successfully be employed for predicting the CFL diagram and thus for predicting the S–N relationships for the woven CFRP laminate at any stress ratios, regardless of test temperature.

Fatigue tensile behavior of carbon/epoxy composite reinforced with non-crimp 3D orthogonal woven fabric

An experimental study of the in-plane tension–tension fatigue behavior of the carbon fiber/epoxy matrix composite reinforced with non-crimp 3D orthogonal woven fabric is presented. The results include pre-fatigue quasi-static test data, fatigue life diagrams, fatigue damage progression, and post-fatigue quasi-static test data for the warp- and fill-directional loading cases. It is revealed that the maximum cycle stress corresponding to at least 3 million cycles of fatigue life without failure, is in the range of 412–450 MPa for both loading directions. This stress range is well above the static damage initiation threshold and significantly above the first static damage threshold (determined by the onset of low energy acoustic emission). The second static damage threshold, determined by the onset of high energy acoustic emission and related to the appearance of local debonds and intensive transverse matrix cracking falls within this range. The established correlation between a 3000,000 cycle fatigue stress limit on one side and the second static damage threshold stress on the other is of a high practical importance, because it will significantly reduce the amount of future fatigue tests required for this class of composites. Surprisingly, for equal maximum cycle stress level, the fatigue life under fill-directional loading appears about three times shorter than that under warp-directional loading. The 100,000 cycle, 500,000 cycle and 1000,000 cycle fatigue loading with 450 MPa maximum cycle stress has resulted in so high variations of post-fatigue static modulus, strength and ultimate strain, that no consistent and statistically meaningful trends could have been established; further extensive experimental studies are required to reliably quantify this effect.

Empirical models for matrix cracking in a CFRP cross-ply laminate under static- and cyclic-fatigue loadings

This paper presents empirical models for predicting matrix crack density in a carbon fiber reinforced plastic (CFRP) cross-ply laminate under static-fatigue and cyclic-fatigue loadings. First, a modified slow crack growth (SCG) law, that covers the whole range of stress ratio R of tension-tension fatigue (0 ≤ R ≤ 1), was proposed. The modified SCG law and three conventional SCG laws were then combined with Weibull's probabilistic failure concept for predicting fatigue matrix crack density in a cross-ply laminate. Matrix crack density was expressed as a function of R, the maximum stress in the transverse ply and the number of cycles. Next, fatigue tests were performed for R of 0, 0.2, 0.4, 0.6, and 1 to determine the applicability of these four models. Finally, constant fatigue life (CFL) diagrams were investigated based on the modified model. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers

Construction of constant fatigue life diagram for a carbon fiber composite

Using the constant amplitude fatigue data at various stress ratios, the constant fatigue life (CFL) diagram was constructed for the CFC material which is useful in prediction of fatigue life under variable amplitude fatigue loads. A quasi-isotropic lay-up sequenced carbon fiber-epoxy composite (CFC) laminate was fabricated by resin infusion technique. The tensile and compression tests were carried out to determine the static strength of the material. About 175 mm long, constant rectangular cross-sectioned fatigue test specimens were cut and prepared from the laminate. The stress-controlled, constant-amplitude fatigue tests were conducted in a 100 kN servo-hydraulic test machine, at room temperature and in lab air atmosphere. All the fatigue tests were performed with a sinusoidal waveform and a frequency of 1–3 Hz. The fatigue tests were conducted at four different stress ratio, R = σmin/σmax, Viz., 0.7,0.5 (tension-tension), −1.0 (tension-compression), and 4.0 (compression-compression). Anti-buckling guide was employed during the fatigue tests which involved compressive load cycles. The CFL Diagrams are material specific with predictive capability for any designed levels from selective experimental data.

OS1521 平織CFRP擬似等方積層板の非相似型等寿命線図とその温度依存性のモデル化(OS15-4 複合材料の変形と強度,OS-15 微視構造を有する材料の変形と破壊)

A procedure for constructing the anisomorphic constant fatigue life (CFL) diagram for composites on the basis of the static strengths in tension and compression and the reference S-N relationship for the critical stress ratio, which was developed in an earlier study, is further tested on a woven CFRP quasi-isotropic laminate at room and high temperatures. Constant amplitude fatigue tests were first performed at RT, 100 and 150℃ for different stress ratios to elucidate the effects of R-ratio and temperature on S-N relationship. Then, validity of the anisomorphic CFL diagram approach to prediction of the fatigue lives of the composite for cyclic loading at different stress ratios and test temperatures is evaluated by comparing the predicted and experimental results.

平織ＣＦＲＰ擬似等方積層板の疲労挙動に及ぼす湿熱環境の影響と非相似形等寿命線図の適用性

The effect of water uptake on fatigue lives of a woven fabric CFRP quasi-isotropic laminate under constant amplitude loading at different stress ratios has been examined. First, constant amplitude fatigue tests are performed at room and high temperatures on the specimens immersed in hot water until saturation and on the specimens kept in a dry place. Experimental results show that the fatigue lives of wet specimens are shorter than those of dry specimens, regardless of stress ratio and test temperature. The degree of reduction in fatigue strength due to water uptake is about eleven percent at most, that is independent of stress ratio. Then, the full shapes of the constant fatigue life (CFL) diagrams for the laminate in dry and wet environments, respectively, at different temperatures are identified on the basis of the fatigue test data obtained at different stress ratios. It is demonstrated that they are characterized by asymmetry, nonlinearity and gradual change in shape. Finally, the anisomorphic CFL diagram approach for composites that was developed in the earlier study is tested on the dry and wet CFRP laminates at different temperatures. It is shown that the anisomorphic CFL diagram approach is successfully applied for predicting the CFL diagrams and thus the S-N relationships for the woven CFRP laminate, regardless of moisture content and test temperature.

Fatigue Behavior of Hi‐Nicalon Type‐S™/BN/SiC Ceramic Matrix Composites in a Combustion Environment

The fatigue behavior of Hi‐Nicalon Type‐S™/BN/SiC ceramic matrix composites (CMCs) was investigated in a combustion environment. Two versions of this CMC were tested. The first version was manufactured by a slurry casting process using woven fiber architecture and the matrix was densified by melt‐infiltration (MI) of silicon. The other was a cross‐ply laminate prepared by a traditional lamination process from unidirectional prepreg followed by the MI processing. Several tests were conducted at a frequency of 1 Hz and a stress ratio of 0.05 to develop the fatigue life diagrams for both material systems. The Prepreg MI CMC exhibited higher fatigue strength, especially for cycles‐to‐failure >10,000, in comparison with the slurry cast MI CMC. Micrographic analysis was conducted to document the oxidation and embrittlement, which was observed to be dependent upon the applied stress level and cycles‐to‐failure. Overall, the prepreg MI CMC was less prone to oxidation and had better fatigue properties, demonstrating the potential for long‐term use as a structural high‐temperature material for high‐temperature oxidative service environments, such as those found in gas turbine engines.

A three-segment anisomorphic constant life diagram for the fatigue of symmetric angle-ply carbon/epoxy laminates at room temperature

The anisomorphic constant fatigue life (CFL) diagram approach that was developed in an earlier study is further tested for applicability to the matrix-dominated fatigue failure in symmetric angle-ply carbon/epoxy laminates. An extension of the CFL diagram approach is also attempted to improve the accuracy of fatigue life prediction. The original anisomorphic CFL diagram approach can be used for approximately predicting the CFL diagrams for the [±30] 3S and [±45] 3S laminates, while it fails to accurately predict the CFL diagram for the [±60] 3S laminate due to its significant local distortion. For accommodating the anisomorphic CFL diagram approach to the local distortion in CFL curves due to a significant change in mean stress sensitivity in fatigue, a transitional segment is inserted between the tension–tension and compression–compression dominated segments. It is demonstrated that the three-segment anisomorphic CFL diagram approach allows improved predictions of the CFL diagrams and S– N relationships for the angle-ply laminates.

809 一方向CFRPの室温における主軸等寿命線図の同定(OS9-(3)オーガナイズドセッション《複合材料の変形と破壊およびマルチスケール計算技術》)

The constant fatigue life (CFL) diagrams for a unidirectional CFRP laminate in its in-plane principal directions are identified. First, constant amplitude fatigue tests were performed at various stress ratios in the longitudinal, transverse and a shear dominated off-axis directions, respectively. On the basis of the fatigue test data, the constant fatigue life diagrams for the unidirectional CFRP laminate subjected to longitudinal, transverse and in-plane shear loading are identified. While the shape of the CFL diagram for longitudinal loading is similar to that observed so far for multidirectional CFRP laminates, a significant difference is observed in the shapes of the CFL diagrams for transverse and in-plane shear loading. Finally, a modified CFL diagram is developed to improve the accuracy of prediction of the transverse and in-plane shear CFL diagrams for the unidirectional composite.

108 一方向CFRPの主軸等寿命線図および繊維とマトリックスの疲労関数の同定(OS1-2 微視構造を有する材料の変形と破壊 : 材料特性の評価)

The constant fatigue life (CFL) diagrams for a unidirectional (UD) CFRP laminate in its in-plane principal directions are identified. First, constant amplitude fatigue tests were performed at various stress ratios in the longitudinal, transverse and shear-dominated off-axis directions, respectively. On the basis of the fatigue test data, the constant fatigue life diagrams for the UD CFRP laminate subjected to longitudinal, transverse and in-plane shear loading are identified. While the shape of the CFL diagram for longitudinal loading is similar to that observed so far for multidirectional CFRP laminates, a significant difference is observed in the shapes of the CFL diagrams for transverse and in-plane shear loading. Finally, a modified CFL diagram is developed to improve the accuracy of prediction of the transverse and in-plane shear CFL diagrams for the unidirectional composite.

A fatigue life calculation method for structural elements made of D16CzATW aluminium alloy

A fatigue life calculation method for structural elements made of D16CzATW aluminium alloyIn the calculating of fatigue life of structural elements the methods based on hypotheses of fatigue damage summation are commonly applied. In using the methods, apart from choice of an appropriate hypothesis, it is necessary to know a loading spectrum which usually constitutes a set of sinusoidal cycles of variable parameters (Sa, Sm), as well as fatigue characteristics in the form of Wöhler fatigue diagram, as a rule. Knowledge of the above mentioned data is low during structural design process as designed material objects and possibility of performing measurements are lacking.Hence in this work a calculation method of fatigue life of structural elements, based on the relations between fatigue life diagrams determined in constant-amplitude conditions (Wöhler diagrams) and those obtained under programmed or random loads, is described. The proposed method was experimentally verified by using results of fatigue tests on structural elements made of D16CzATW aluminium alloy.

Cyclic deformation and fatigue properties of very fine-grained metals and alloys

Bulk ultrafine-grained (UFG) metals and alloys prepared by techniques of severe plastic deformation (SPD), in particular by equal channel angular pressing (ECAP), exhibit exceptional mechanical properties. With respect to potential applications of this new class of very fine-grained bulk materials, the cyclic deformation and fatigue behaviour, relative to that of conventional grain size (CG) materials, is of crucial importance. In the present review, the research performed on the fatigue properties of very fine-grained bulk materials during the last decade will be discussed. Mainly bulk UFG materials prepared by ECAP will be considered and, to a smaller extent, also bulk truly nanocrystalline materials. The discussion will focus on simple materials and on more complex alloys and structural materials. Presentation of fatigue performance in total strain fatigue life diagrams has been found to be particularly suitable to compare the strong UFG and the more ductile CG materials in both the high cycle fatigue (HCF) and the low cycle fatigue (LCF) ranges. In general, the fatigue strength of UFG materials is enhanced considerably by grain refinement, in particular in the HCF regime. However, at the same time, the LCF performance is impaired by microstructural instabilities of the strongly hardened but less ductile UFG materials, as manifested in cyclic softening, fatigue-induced grain coarsening (by dynamic recrystallization at rather low homologous temperature!) and massive shear banding. These effects are discussed critically, also with respect to the effects of the route of ECAP-processing employed, the purity of the material – and the mode of fatigue testing. Remedies by mild annealing treatments which have been partially successful in improving the LCF strength by enhancing the ductility at the expense of a moderate loss of strength will be discussed. Examples of modelling of the cyclic deformation and fatigue properties will be presented, and some directions of future research will be outlined.

Ultrasonic detection of fretting fatigue damage at bolt joints of aluminum alloy plates

The fatigue failure evaluation was carried out to determine the effect of the tightening torque of the bolt on the failure mode and the fatigue life of bolted joints of aluminum alloy plates 2024-T3 at different levels of the stress amplitude. These results were shown in the tightening torque and fatigue life diagram. Regardless the stress amplitude, when the tightening torque is relatively lower the mechanical fatigue failure occurred at the edge of the bolt hole, and the fatigue life increased with the tightening torque. When the tightening torque is increased further, the fretting wear failure occurred near the bolt hole due to the gross sliding of the specimen surface, and the fatigue life increased with the increase in tightening torque. At a higher range of tightening torque, the fatigue life decreased with the increasing tightening torque, and the fretting fatigue failure occurred near the bolt hole. A tightening torque boundary between the fretting failure and the mechanical fatigue increased with increasing stress amplitude. The ultrasonic measurement with surface acoustic wave was also carried out during the fatigue testing, and the echo reflected from the fretting fatigue crack was detected at a position 1.8–2.1 mm ahead of the bolt hole in the fastened condition.

205 平織CFRP擬似等方積層板の疲労挙動に及ぼす吸水の影響と応力比の影響(OS6-(2) 複合材料の変形と破壊およびマルチスケール計算技術,オーガナイズドセッション)

The effect of water uptake on the constant amplitude fatigue behavior of a quasi-isotropic plain-woven CFRP laminate at different stress ratios has been examined. First, constant amplitude fatigue tests are performed at room temperature on the specimens immersed in hot water until saturation and on the specimens that are not exposed to water environment. The fatigue strength due to water uptake was about eleven percent which was independent of stress ratio. Then, the full shapes of the constant fatigue life (CFL) diagrams for the laminates in dry and wet environments were identified using the fatigue test data obtained at different stress ratios, which are characterized by asymmetry, nonlinearity and gradual change in shape. Finally, the anisomorphic CFL diagram approach for composites, which was developed in an earlier study, was tested on the dry and wet CFRP laminates. It is demonstrated that the anisomorphic CFL diagram approach succeeds in adequately predicting the CFL diagrams for the quasi-isotropic laminate in wet as dry environment.

S0406-1-3 一方向CFRPのトランスバース疲労強度に及ぼす応力比と板厚の影響(プラスチック基複合材料の加工と評価(1))

Effects of stress ratio and laminate thickness on the transverse fatigue behavior of unidirectional CFRP have been examined. First, constant-amplitude fatigue tests were performed at various stress ratios on two kinds of 90° specimens with different thickness. The transverse fatigue life under tension-tension fatigue loading became shorter with decreasing stress ratio or with increasing specimen thickness. Under compression-compression fatigue loading, on the other hand, both the stress ratio and laminate thickness had no significant influence on the transverse fatigue life. It was found that the fatigue life becomes the shortest at the critical stress ratio (i.e. the ratio of compressive strength to tensile one), regardless of the thickness of specimens. Then, an existing constant fatigue life (CFL) diagram based fatigue life prediction method was tested on the 90° laminates with different thickness. Results showed that it does not allow accurate prediction of the observed transverse fatigue behavior. Finally, a new CFL diagram based mode that consists of three segments has been developed, and evaluated by comparing with experimental results.

203 一方向CFRPの非主軸疲労強度と等寿命線図の同定(OS6-(1) 複合材料の変形と破壊およびマルチスケール計算技術,オーガナイズドセッション)

203 一方向CFRPの非主軸疲労強度と等寿命線図の同定(OS6-(1) 複合材料の変形と破壊およびマルチスケール計算技術,オーガナイズドセッション)

Computational mechanics of Nitinol stent grafts

A finite element analysis of tubular, diamond-shaped stent grafts under representative cyclic loading conditions for abdominal aortic aneurysm (AAA) repair is presented. Commercial software was employed to study the mechanical behavior and fatigue performance of different materials found in commercially available stent–graft systems. Specifically, the effects of crimping, deployment, and cyclic pressure loading on stent–graft fatigue life, radial force, and wall compliances were simulated and analyzed for two types of realistic but different Nitinol materials (NITI-1 and NITI-2) and grafts (expanded polytetrafluoroethylene—ePTFE and polyethylene therephthalate—PET). The results show that NITI-1 stent has a better crimping performance than NITI-2. Under representative cyclic pressure loading, both NITI-1 and NITI-2 sealing stents are located in the safe zone of the fatigue-life diagram; however, the fatigue resistance of an NITI-1 stent is better than that of an NITI-2 stent. It was found that the two types of sealing stents do not damage a healthy neck artery. In the aneurysm section, the NITI-1&ePTFE, NITI-1&PET, and NITI-2&PET combinations were free of fatigue fracture when subjected to conditions of radial stress between 50 and 150 mmHg. In contrast, the safety factor for the NITI-2&ePFTE combination was only 0.67, which is not acceptable for proper AAA stent–graft design. In summary, a Nitinol stent with PET graft may greatly improve fatigue life, while its compliance is much lower than the NITI–ePTFE combination.

Fatigue strength of metals and composites under repeated high-cycle torsion

The limiting stresses are determined and constant fatigue life diagrams for high-cycle torsion with repeated stress cycle are plotted using the limiting-state models obtained based on the hypothesis of a unified constant-life diagram, which is invariant to the number of cycles to failure. The unified constant-life diagram is given by a transcendental power function whose exponent is an additional parameter characterizing the sensitivity of the material to the asymmetry of the stress cycle. The calculated results and experimental data for carbon and alloyed steels and composite materials are in good agreement

A Method for Identifying Asymmetric Dissimilar Constant Fatigue Life Diagrams for CFRP Laminates

The full shape of constant fatigue life (CFL) diagram has been identified for three kinds of carbon fiber-reinforced plastic (CFRP) laminates of [45/90/-45/0]2s, [0/60/-60]2s and [0/90]3s lay-ups, respectively. First, the effects of mean stress on fatigue behavior are observed for the quasi-isotropic [45/90/-45/0]2s CFRP laminate. It is clearly observed that the CFL diagram turns out asymmetric and the peak of the CFL diagram is slightly offset to the right of the alternating stress axis. The CFL plots indicate that the alternating stress component of fatigue load becomes maximal at a critical stress ratio closely equal to the compressive strength to tensile one (i.e. a C/T strength ratio). It is also demonstrated that CFL curves for different constant values of life are linear in the range of a short life, but they turn quadratic in the range of a longer life. Similar features are observed for the other kinds of CFRP laminates. Then, to construct the asymmetric dissimilar shape of CFL diagrams, a new methodology is developed on the basis of the static strengths in tension and compression and the reference S-N relationship for the critical stress ratio. An efficient procedure for determining the reference S-N relationship for the critical stress ratio by using fatigue data for another stress ratio is also developed.