Ratio Of Minimum Stress Research Articles

A New Approach to Machine Learning Model Development for Prediction of Concrete Fatigue Life under Uniaxial Compression

The goal of this work is to show how machine learning models, such as the random forest, neural network, gradient boosting, and AdaBoost models, can be used to forecast the fatigue life (N) of plain concrete under uniaxial compression. Here, we developed our final machine learning model by generating the following three data files from the original data used in the work of Zhang et al.: (a) grouped data with the same input variable value and different output variable logN value, (b) data excluding outliers selected by three or more outlier detection methods; (c) average data excluding outliers, created by averaging the grouped data after excluding outliers from among the grouped data. Excluding the sustained strength of the concrete variable, originally treated as the seventh input variable in the work of Zhang et al., resulted in improving the determination coefficient (R2) values. Moreover, the gradient boosting model showed a high R2 value at 0.753, indicating a high accuracy in predicting outcomes. Further analysis using data excluding outliers shows that the R2 value increased to 0.803. Moreover, the average data excluding outliers provided the best R2 value at 0.915. Finally, a permutation feature importance (PFI) analysis was carried out to determine the strength of the relationship between the feature and the target value for the gradient boosting model. The analysis results showed that the maximum stress level (Smax) and loading frequency (f) were the most significant input variables, followed by compressive strength (f′c) and maximum to minimum stress ratio (R). Shape and height to width ratio (h/w) were the features with a non-significant influence on the model. This trend was previously confirmed by a Pearson and Spearman correlation analysis.

Experimental research on the mechanical properties of steel fiber recycled aggregate concrete subjected to true triaxial compression

This paper presents an experimental research on the triaxial behavior of 270 steel fiber recycled aggregate concrete (SFRAC) specimens under true triaxial compression (σ1≠σ2≠σ3). By using a servo-hydraulic closed loop testing system, triaxial loading experiments were conducted considering three variables - the volume fraction of steel fiber (Vf), the replacement ratio of recycled coarse aggregate (RAR), and the stress ratio (α). The influences of these parameters on the failure modes, the stress–strain curves and the peak triaxial strength are analyzed. The experimental results indicated: (i) the stress ratio had the most significant effect on the failure mode of SFRAC specimens, where four modes were observed; (ii) the peak triaxial strength of specimens was sensitive to Vf, and a value between 0.6% and 1.2% was found to be optimum; (iii) the minimum stress ratio σ1/σ3 and the second stress ratio σ2/σ3 exhibited opposite effects on the peak triaxial strength of specimens, while RAR showed the least effect. Finally, a three-parameter failure criterion for SFRAC under true triaxial compression was proposed. The experimental results will provide SFRAC triaxial mechanical behavior for future engineering design and applications.

Tension-tension fatigue behaviour of 3D braided SiCf/SiC composite with film cooling holes at 1350 °C in air

The tension-tension fatigue behaviour of SiCf/SiC composites with film cooling holes (FCHs) was investigated at 1350 °C in air. The number of drilled holes were 1, 5, 10 (rectangular arrangement) and 11 (triangular arrangement), and the average diameter was 0.5 mm. The fatigue stresses were 80 MPa and 120 MPa at a frequency of 3.0 Hz, and the R ratio of minimum stress to maximum stress was 0.1. The stress-strain hysteresis curve, strain accumulation and modulus decrease were investigated, and characterization of the fracture morphology and microstructure was performed to reveal the initiation and evolution of damage during fatigue cycles. The results indicated that the FCHs exhibit little effect on the ultimate tensile strength of the composites. The fatigue life decreases with an increasing number of FCHs, and the effect of the FCHs on the fatigue life gradually decreases as stress levels increase. Specimens with triangular arrangement 11-H exhibit the worst fatigue life, and the damage mechanism is significant relative to large-area of fibre oxidation on the fracture surface. The weakening effect of FCHs on the fatigue performance of the composites is mainly due to the oxidation and embrittlement of fibres around the hole, and it is significant relative to initial crack nucleation and propagation in the coating and matrix.

Fatigue of a 3D Orthogonal Non-crimp Woven Polymer Matrix Composite at Elevated Temperature

Tension-tension fatigue behavior of two polymer matrix composites (PMCs) was studied at elevated temperature. The two PMCs consist of the NRPE polyimide matrix reinforced with carbon fibers, but have different fiber architectures: the 3D PMC is a singly-ply non-crimp 3D orthogonal weave composite and the 2D PMC, a laminated composite reinforced with 15 plies of an eight harness satin weave (8HSW) fabric. In order to assess the performance and suitability of the two composites for use in aerospace components designed to contain high-temperature environments, mechanical tests were performed under temperature conditions simulating the actual operating conditions. In all elevated temperature tests performed in this work, one side of the test specimen was at 329 °C while the other side was open to ambient laboratory air. The tensile stress-strain behavior of the two composites was investigated and the tensile properties measured for both on-axis (0/90) and off-axis (±45) fiber orientations. Elevated temperature had little effect on the on-axis tensile properties of the two composites. The off-axis tensile strength of both PMCs decreased slightly at elevated temperature. Tension-tension fatigue tests were conducted at elevated temperature at a frequency of 1.0 Hz with a ratio of minimum stress to maximum stress of R = 0.05. Fatigue run-out was defined as 2 × 105 cycles. Both strain accumulation and modulus evolution during cycling were analyzed for each fatigue test. The laminated 2D PMC exhibited better fatigue resistance than the 3D composite. Specimens that achieved fatigue run-out were subjected to tensile tests to failure to characterize the retained tensile properties. Post-test examination under optical microscope revealed severe delamination in the laminated 2D PMC. The non-crimp 3D orthogonal weave composite offered improved delamination resistance.

Fatigue stress concentration sensitivity and stress ratio effect of a 40CrNi2Si2MoVA steel

The axial fatigue tests at room temperature for smooth specimens with theoretical stress concentration factor, Kt=1 and notched specimens Kt=2, 3 and 5 were performed in a high-strength 40CrNi2Si2MoVA steel for different stress ratio, R, that is the ratio of minimum stress to maximum stress, of −1, 0.06 and 0.5, respectively. Fatigue strength/limit for 1×107 cycles were determined according to a stair-case method. The stress concentration sensitivity and mean stress sensitivity are characterized by q factor and M factor, respectively. Moreover, a new model for evaluate the mean stress effects of high-strength steels was proposed, which is different from Goodman relation and Gerber parabola.

Tension-tension fatigue behavior of a PIP SiC/SiC composite at elevated temperature in air

The tension–tension fatigue behavior of a PIP Silicon Carbide/Silicon Carbide (SiC/SiC) composite was investigated at 1300°C in air. The composite consisted of a new precursor polymer (LPVCS) pyrolyzed SiC matrix reinforced with pyrolytic carbon (PyC) coated 3D braided KD-I SiC fibers. Environmental barrier coatings (EBC) were employed to enhance the durability and air oxidation resistance of the composite at high temperature. Tension–tension fatigue tests were conducted at 1300°C in laboratory air at frequency of 1Hz. The ratio of minimum stress to maximum stress was 0.1, with maximum stresses ranging from 40 to 120MPa. Fatigue run-out was defined as 105 cycles. Strain accumulation with cycles and modulus evolution with cycles were analyzed for each fatigue test. Fatigue limit of SiC/SiC composite with EBC was 60MPa which was slightly higher than that without EBC. The microstructure as well as damage and failure mechanisms of the composites were investigated.

Instability of dilative sand

A type of pre-failure instability that occurs for medium loose to dense sand under fully drained conditions is studied in this paper. It is observed experimentally that when a specimen is sheared along a drained stress path involving a decrease in the effective mean stress, it becomes unstable after the stress path crosses an instability line (IL). The IL specifies a minimum stress ratio under which instability may occur. It is not unique but changes with the void ratio of the soil and the applied effective stresses. The instability occurring under drained conditions is different from that under undrained conditions. The differences and similarities between the two types of instability are pointed out. Practical implications of the study in analysing the failure mechanisms of granular soil slopes under various drainage conditions are discussed.

Cyclic bending experiments on free-standing Cu micron lines observed by electron backscatter diffraction

Polycrystalline Cu samples 20×20μm2 in size were cyclically bent inside a scanning electron microscope until fracture occurred. The microstructural changes were investigated by secondary electron imaging and electron backscatter diffraction. The in situ experiments revealed that, for the coarse-grained samples, it is not the external stress that dominates the cyclic deformation, but the local internal strength. This is in strong contrast to macroscopic bending samples, where deformation always happens near the fixed end of the bending beam and decreases constantly with increasing distance from the fixation. For micron-sized polycrystalline samples, the grain dimensions, dislocation density evolution and grain orientation (Taylor factor) can define the location of failure if the grain size and sample diameter become similar in size. A comparison with cyclic in situ tension–tension experiments (ratio of minimum stress to maximum stress R≈0) reveals that cyclic bending experiments (R≈−1) undergo bulk-like fatigue deformation with extrusions/intrusions, in contrast to the experiments with R≈0. Both the cyclic tension–tension and bending experiments can be described by a Basquin equation, although different mechanisms lead to failure of the samples.

Research on Monotonic and Fatigue Properties of Functionally Graded Composite Beams Crack-Controlled by UHTCC in Bending

This research focuses on mechanical performances of UHTCC-FGC beams under monotonic and fatigue loading. Three UHTCC-FGC beams are tested, including two monotonic beams and one fatigue beam. The stress level of fatigue beam is set as 0.85. The constant ratio of minimum stress to maximum stress is set to 0.2 to avoid any impact and slip between the loading machine and the specimens during testing. The specimen is subjected to sinusoidal cyclic loading at 1 frequency. During the experiments, the fatigue life, deformability and loading capacity of UHTCC-FGC beams are measured and recorded. Meanwhile，the development of cracks and the failure pattern of UHTCC-FGC beams during the test are observed at the same time.

Determination of the autofrettage pressure and estimation of material failures of a Type III hydrogen pressure vessel by using finite element analysis

The autofrettage process of a Type III hydrogen pressure vessel for fuel cell vehicles with preset winding pattern was simulated by finite element analysis (FEA). For a precise finite element analysis, the ply based modeling technique was used for the composite layers; a contour function was derived for the fibers at the dome part to determine the exact winding angle; and the exact composite thickness was also considered. In order to determine the most appropriate autofrettage pressure, stress analysis of the pressure vessel according to its internal pressure was carried out with consideration of the international regulations about pressure vessel design. The minimum stress ratio, the permanent volumetric expansion and the generated residual stress were investigated, and the failure of the pressure vessel under minimum burst pressure was predicted by application of various failure criteria of anisotropic composites.

J191054 Ti-6AI-4V ELI合金鍛造材の低温高サイクル疲労特性に及ぼす応力比の影響([J19105]構造・材料の高度化に向けた宇宙工学と材料力学の展開(5))

The effect of mean stress or the stress ratio, R, (ratio of minimum stress to maximum stress) on the high-cycle fatigue properties of Ti-6A1-4V extra-low interstitial (ELI) forging was investigated at 293 K and 77 K. Fatigue tests were carried out for up to 107 cycles at stress ratios of R=-1, 0.01 and 0.5, and also under the condition where the maximum stress was fixed at 0.2% proof stress ( Crmax = cr02 test). The high-cycle fatigue strength at 107 cycles decreased at each temperature as the stress ratio increased, when the fatigue strength was evaluated in terms of stress amplitude (aa). In the R=0.01 and 0.5 tests, however, the high-cycle fatigue strength at 107 cycles exhibited deviations of aa below the modified Goodman line at both temperatures, indicating that this alloy exhibited an anomalous mean stress dependency. In particular, the amounts of negative deviations from the modified Goodman line at 77 K were larger than those at 293 K. Thus, it is concluded that the anomalous mean stress dependency of this alloy becomes remarkable at low temperatures.

The use of crack layer theory to predict the lifetime of the fatigue crack growth of high density polyethylene

AbstractIn the case of high density polyethylene (HDPE), the fatigue crack propagates in a discontinuous manner, which can be observed by distinct striations. In this article, fatigue crack growth (FCG) experiments were conducted on two grades of HDPE pipe with compact‐tension (CT) and cracked round bar (CRB) specimens. The effects of the stress ratio (R‐ratio) which is defined as the ratio of minimum stress and maximum stress of fatigue loadings and the frequency on FCG behavior were experimentally studied. Although FCG rates showed a great dependence on the R‐ratio in terms of the range of the stress‐intensity factor, the effect of the frequency may be considered to be significant in the low crack growth region. In addition, these experimental data were employed for predicting the lifetime on the basis of the crack layer (CL) theory. Only a few steps of FCG are needed to determine all necessary parameters for CL theory, and the FCG behavior can be reconstructed based on a computer program that has been developed for the application of CL theory. The predictions from this program accord with experimental data. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Fatigue properties of Cu–Cr in situ composite

The new electrical conductive material, Cu–15 mass%Cr in situ composite, has been developed for the application to lead frame, high field magnet and trolley wire. The Cu–Cr composite was fabricated by casting method in vacuum and forged, solution-treated and cold drawn. The value of the cold drawing strains, η, for the composite were 4.66 and 6.94. η is defined by the value of ln( A 0/ A), where A 0 and A are virgin area and final area of material, respectively. The cold drawn Cu–Cr composite was tested using a rotating bending fatigue testing machine and axial loading fatigue testing machine ( R = 0.1, where R is stress ratio of minimum stress to maximum stress) at room temperature in comparison to the data for pure copper. The fatigue stresses decreased with increasing number of cycles to failure. The double-knees, which have been found in other face centered cubic materials, such as aluminum alloys, were not seen on the S– N curves for both alloys. The fatigue strength of Cu–Cr composite was twice higher than that for pure copper. The fatigue strength of Cu–Cr composite cold drawn up to η = 6.94 was higher than that drawn up to η = 4.66 when tested under an axial loading. In the fatigue fracture surface, the fatigue crack initiated from the un-dissolved chromium particle situated beneath the specimen surface and the crack direction was changed along the chromium fiber. It was suggested that, in order to improve the fatigue strength of Cu–Cr composite, it is very important to reduce the size and amount of un-dissolved chromium particles.

Application of a threshold curve model to high-cycle fatigue behavior of small cracks induced by foreign-object damage in Ti–6Al–4V

A threshold curve method previously developed was applied to analyze the high-cycle fatigue behavior of small cracks induced by foreign-object damage (FOD) in a Ti–6Al–4V alloy for which experimental results can be obtained from recent publications. Foreign object damage represents the impact by small hard objects such as stones or pieces of metal and is a major cause of fatigue failure in gas turbine engines. Experimental results found in the literature showed that the small cracks induced by FOD in Ti–6Al–4V alloys could propagate at an applied Δ K as small as 1 MPa m 1/2, i.e. well below the ‘worst-case’ Δ K thR threshold of 1.9 MPa m 1/2 for long cracks in this alloy (load ratio R=0.95). According to the results obtained in the present work, the threshold for propagation of small cracks (≧20 μm) can be as small as 0.8 MPa m 1/2 considering a stress ratio R=0.8, for which the threshold for long cracks is about 2.3 MPa m 1/2. Further, the minimum stress ratio R at which the analyzed cracks can propagate seems to be about 0.6, for which the threshold for large cracks is about 2.7 MPa m 1/2. These results show that for microstructurally small cracks, the absence of crack closure effect is not the only reason for the observed faster propagation rates and lower propagation thresholds when compared with long cracks.

Fatigue Fracture Resistance Analysis of Polymer Composites Based on the Energy Expended on Damage Formation

Analysis of the fatigue crack propagation behavior of filled polytetrafluoroethylene (PTFE) composites using the energy expended on damage formation was performed. A 25% chopped glass fiber filled PTFE with a commercial name of Rulon LR and a 25% polymer particle filled PTFE (Rulon J) were chosen as candidate materials for the study. Fatigue crack propagation (FCP) tests were performed using both unnotched and single edge notched (SEN) specimens under tension-tension load control condition. The notched specimens were prepared with the same notch length to sample width ratio (a/w) of 0.09. The applied frequency was 3 Hz. The maximum stress was 6 MPa, and the ratio of minimum stress to maximum stress was kept at 0.10. FCP data such as the crack length and the number of cycles were used to calculate the crack speed. The hysteresis energies for both notched and unnotched specimens during cyclic loading were obtained. The energy release rate was determined as the first derivative of the relationship between the potential energy and the crack length. The potential energy was calculated as the area above the unloading curve from the hysteresis loops at various crack lengths. The cyclic rate of energy dissipation on damage formation was calculated based on the hysteresis energies for both notched and unnotched specimens. It was found that the fatigue kinetics are related to the cyclic rate of energy dissipation into the active zone evolution. The specific energy of damage (γ) a material parameter characteristic of the fatigue fracture resistance was evaluated using the Modified Crack Layer (MCL) theory. It was found that the value of γ for the particle filled PTFE is 3200 kJ/m3, which is about three times higher than that for the fiber filled PTFE (750 kJ/m3). This indicates that the particle filled PTFE has higher fracture resistance than the fiber filled PTFE. The fracture surface of the second region for both materials was examined using scanning electron microscopy (SEM). Interfacial debonding, drawn ligaments and large voids were associated with the fatigue fracture of the fiber filled PTFE. The particle filled PTFE displayed more fibrillation than that of the fiber filled PTFE, which indicates a higher energy consuming process related to the fatigue crack growth in the particle filled PTFE.

Fatigue Crack Growth Analysis of Polymonochlorotrifluoroethylene (CTFE)

Fatigue crack growth behavior of a polymonochlorotrifluoroethylene (CTFE) was investigated. Tension-tension fatigue propagation tests on geometrically identical specimens were performed at different frequencies. Specimens with different notch to width ratios ( a/w) were also tested. The applied stress level in all tests was kept the same. The maximum stress was 16.0 MPa and the ratio of minimum stress to maximum stress was kept at 25%. Fatigue test data such as the number of loading cycles, the crack length, and the hysteresis loops were recorded. The Modified Crack Layer (MCL) theory was employed to analyze the FCP behavior of this CTFE material. Also the results were analyzed using the Paris equation for comparison. It has been found that the fatigue crack growth behavior of this material cannot be described by the Paris power law due to the three distinct stages of the FCP kinetics. The MCL model was successfully used for extracting material parameters; γ', the specific energy of damage, and β', the coefficient of energy dissipation. The specific energy of damage for this CTFE was 855 kJ/m3 and the value of the coefficient of energy dissipation was 6.2 x 10-6. It was found that the values of γ' and β' are independent of a/w and the frequency ( f), though the FCP kinetics and life-time were dependent on a/w and f The variation in γ' and β' when aiw changed from 0.04 to 0.1 were only ±0.9% and +6.8%, respectively. For different frequencies (from 0.5 Hz to 5 Hz), the variation of these two parameters were ± 0.6% and ± 1.4%, respectively. In situ and post failure analysis of the fatigue fractured samples revealed a clear damage zone adjacent to the main crack. The larger the volume of the damage zone, the longer the fatigue life-time. It was also found that a higher micro-crack density associated with a larger damage zone. A larger damage zone with higher micro-crack density consumes more energy and thus reduces the portion of energy required to fuel the main crack growth.

Optimum material design for functionally gradient material plate

Steady thermal stresses in a plate made of a functionally gradient material (FGM) are analyzed theoretically and calculated numerically. An FGM plate composed of PSZ and Ti-6Al-4V is examined, and the temperature dependence of the material properties is considered. A local safety factor is used for evaluation of the FGM's strength. It is assumed that top and bottom surfaces of the plate are heated and kept at constant thermal boundary conditions. The pairs of the surface temperatures, for which the minimum local safety factor can be of more than one, are obtained as available temperature regions. The temperature dependence of the material properties diminishes, available temperature region as compared with that for an FGM plate without it. The available temperature region of the FGM plate is wider than that of the two-layered plate, especially for the surface temperatures which are high at the ceramic surface and low at the metal side. The influence of different mechanical boundary conditions is examined, and available temperature regions are found to be different, depending on the mechanical boundary conditions. The influence of the intermediate composition on the thermal stress reduction is also investigated in detail for the surface temperatures which are kept at 1300 K at the ceramic surface and 300K at the metal side. Appropriate intermediate composition of the FGM plate can yield the local safety factor of one or more for the four mechanical boundary conditions at once. For the two-layered plate there does not exist, however, any appropriate pair of metal and ceramic thicknesses which would yield the local safety factor of one or more for the four mechanical boundary conditions at once. The influence of the intermediate composition on the maximization of the minimum stress ratio depends on the mechanical boundary conditions. Finally, the optimal FGM plates are determined.

Fatigue strength of vibration welded thermoplastic butt joints

AbstractFatigue data are presented for the strengths of 120‐Hz vibration‐welded butt joints of four resins: the three amorphous polymers polycarbonate (PC), polyetherimide (PEI), and modified polyphenylene oxide; and the semicrystalline polymer poly(butylene terephthalate). Data are also presented for the fatigue strength of 250‐Hz vibration welds of the high‐temperature polymer PEI. For all the welds, fatigue strength was evaluated through 10‐Hz, tension‐tension, load‐controlled tests at an R value (ratio of minimum stress) of 0.1. Surprisingly, for all the stress levels studied, none of the PC test specimens failed at the welds, indicating that the fatigue strength of PC welds equals that of the base resin. This is not true of the other three resins, except at relatively low stress levels. For each of the four resins, macrographs are used to highlight the differences between the failure surfaces of monolithic specimens and specimens that failed at the welds.

Steady Thermal Stresses in a Functionally Gradient Material Plate. Influence of Mechanical Boundary Conditions.

A functionally gradient material which decreases thermal stresses has been developed for structural components and/or mechanical elements in fields such as nuclear, aircraft and space engineering. Steady-state thermal stresses in a plate made of the functionally gradient material are discussed. The main theme of this subject is to determine the available temperature region where the minimum factor of safety is one or more, and how to distribute the components of the material in the functionally gradient material to decrease thermal stresses, for different mechanical boundary conditions. We discuss a two-layer laminated plate and functionally gradient material plate whose components are ZrO2 and Ti-6Al-4V. The optimal functionally gradient material plate, which makes the minimum stress ratio largest, is determined for each mechanical boundary condition.

Effect of stress ratio on fatigue crack growth in a titanium aluminide alloy

Recent studies on fatigue crack growth of an alpha 2 titanium aluminide,Ti-24A1-11Nb intermetallic at room temperature [1-3] indicate a strong dependence of fatigue crack growth rate on the stress ratio, R (ratio of minimum stress to maximum stress) . Figure 1 shows the variation of da/dN with stress intensity factor range, AK for different stress ratios, both in SE(T) and C(T) specimens [1] . It can be seen that the threshold SIF-range decreases considerably with increase in stress ratio . Dependence of crack growth rate on effective stress intensity range, AK Cf K118 -K , should consolidate data of this type into a single curve, where K is the stress intensity corresponding to the load at which the crack opens fully during the loading part of a cycle . It is assumed here, for simplicity, that the opening and closing loads are the same. It was suggested earlier by the authors [1] that an argument can be made for the existence of a mean stress effect on the crack growth rate in this material, based upon the closure levels required to produce equivalent driving force, AK A,, at different values of R. From the data of Fig. 1 at R=0.1 and R=0.7, crack closure and AKtf arguments would require a K, value of approximately 5 MPaVm for the R=0.1 data to consolidate with R=0 .7 data, assuming the latter had no closure. However, load-displacement measurements made near the crack tip using a precise laser interferometry system [4] have revealed relatively small closure load levels (K,/K. not exceeding 0.4 for R=0.1) for constant amplitude fatigue in this material. Thus, the substantial difference in the rate of crack growth at any given AK for R=0.5 and R=0.7 cannot be explained solely on the basis of crack closure . It is therefore considered necessary to include the effect of mean stress, in addition to crack closure, in expressing the fatigue crack growth rate as :