Tension-tension Loading Research Articles

Microscale based prediction of matrix crack initiation in UD composite plies subjected to multiaxial fatigue for all stress ratios and load levels

A criterion is developed which predicts matrix fatigue crack initiation for Uni-Directional (UD) composite plies subjected to multiaxial fatigue load at all stress-ratio's: Tension-Tension, Compression-Compression and Tension-Compression. This research is based on physical observations of matrix damage types and their respective driving forces. Existing fatigue prediction formulations, that partly succeed in predicting fatigue life to initiation, are investigated for their key attributes which are subsequently combined in a coherent way. The framework requires 7 material parameters: 4 for Tension-Tension load, 2 for Compression-Compression load and 1 to take into account the accelerated occurrence of damage under Tension-Compression load. The material parameters can be calibrated from only 7 crack initiation S-N curves. 5 experimental datasets, a variety of the two most common reinforcing fibres and matrix (Carbon and Glass fibres combined with an epoxy or PEEK resin), are selected for a thorough verification and validation of the criterion. The agreement of the prediction with experiment is, remarkably, excellent for Glass/Epoxy, good for Carbon/Epoxy and reasonable for Carbon/PEEK. This new framework establishes a solid base for further developments to predict matrix crack initiation and the inclusion of other failure types such as fibre kinking.

Fatigue Hysteresis Behavior of Unidirectional SiC/Si3N4 Composite at Elevated Temperature under Tension-Tension Loading

In this paper, the fatigue hysteresis behavior of unidirectional SiC/Si3N4 ceramic-matrix composite at elevated temperature has been investigated. The hysteresis loops models considering interface friction between fibers and the matrix have been developed to establish the relationships between the fatigue hysteresis loops, fatigue hysteresis dissipated energy and the interface frictional coefficient. Using the experimental fatigue hysteresis dissipated energy, the interface frictional coefficient of SiC/Si3N4 composite at 1000 °C were obtained for different cycle numbers and fatigue peak stresses. The effects of fatigue peak stress, test temperature and cycle number on the evolution of fatigue hysteresis dissipated energy and interface frictional coefficient have been analyzed. It was found that the fatigue hysteresis dissipated energy can be used to monitor the interface debonding and damage evolution inside of the composite.

Fatigue life enhancement of quasi-isotropic symmetric GFRP laminate by doping MWCNTs

Eight layered quasi-isotropic symmetric laminate was fabricated by vacuum assisted method using glass fiber woven rovings. 0.5 wt.% of MWCNTs (Multi Walled Carbon Nano Tubes) was doped in the epoxy as a secondary reinforcement. Glass fiber reinforced polymer (GFRP) composites are strong in static loading but show poor life under cyclic or fatigue loading. Adding a small amount of MWCNT not only improve the static strength of the laminate but also enhanced the fatigue life by several cycles. Added MWCNTs act as crack arrestor and /or crack deflector, which hinders the crack propagation under cyclic loading. Thus, this enhanced the static strength and young’s modulus approximately by 7% and reduced the maximum stress amplitude by 10%. Due to this, the fatigue life in tension-tension loading of the symmetric laminate was enhanced.

The effect of plasma electrolytic oxidation on the mean stress sensitivity of the fatigue life of the 6082 aluminum alloy

In this work the mean stress influence on the high cycle fatigue behavior of the plasma electrolytic oxidized (PEO) 6082 aluminum alloy (AlSi1MgMn) is investigated. The present study is focused on the fatigue life time and the susceptibility of fatigue-induced cracking of the oxide coating and their dependence on the applied mean stress. Systematic work is done comparing conditions with and without PEO treatment, which have been tested using three different load ratios. For the uncoated substrate the cycles to failure show a significant dependence on the mean stress, which is typical for aluminum alloys. With increased load ratio and therefore increased mean stress, the fatigue strength decreases. The investigation confirms the well-known effect of PEO treatment on the fatigue life: The fatigue strength is significantly reduced by the PEO process, compared to the uncoated substrate. However, also the mean stress sensitivity of the fatigue performance is reduced. The fatigue limit is not influenced by an increasing mean stress for the PEO treated conditions. This effect is firstly shown in these findings and no explanation for this effect can be found in literature. Supposedly the internal compressive stresses and the micro-cracks in the oxide film have a direct influence on the crack initiation and growth from the oxide film through the interface and in the substrate. Contrary to these findings, the susceptibility of fatigue-induced cracking of the oxide coating is influenced by the load ratio. At tension-tension loading a large number of cracks, which grow partially just in the aluminum substrate, are present. With decreasing load ratio to alternating tension-compression stresses, the crack number and length increases and shattering of the oxide film is more pronounced due to the additional effective compressive part of the load cycle.

Design of a Flat Plate Specimen Suitable for Biaxial Tensile Tests on Polymer Materials

Since the introduction of fibre-reinforced polymer composites there has been a surge in the use of adhesives for joints and repairs, and polymer resins as the matrix material for fibre-reinforced composites. The failure mechanisms of these materials have been studied by many researchers; however, there is little accurate experimental data under tension-tension loading published. This is due to the lack of a standard specimen design to perform these tests. The authors propose a flat plate specimen design that has been shown to overcome some of the difficulties presented in literature. A series of tests with this specimen configuration were conducted under varying biaxial loading conditions. The results are plotted in the tension-tension quadrant of the materials failure envelope. For the two polymer materials tested; a linear truncation within the first quadrant of the failure envelope was found.

Comparative Assessment for Static Creep and Tension-Tension Cyclic Creep Behaviors of Modified 9Cr-1Mo Steel

Cyclic creep (CC) tests were performed with the magnitudes of the stress range of constant stress ratio (R=0.1) under continuous tension-tension loading cycles at a hold time of 10minute at 600oC for normalized and tempered modified 9Cr-1Mo steel. Cyclic strain-time curves were obtained from the controlled cyclic creep tests, and the cyclic creep behavior was investigated and compared with the static creep (SC) behavior. In the present test conditions, the results of creep rupture time and creep rate showed retardation behaviour when compared with those of the static creep. Creep rupture ductility decreased with a decrease in stress, and the ductility was almost the same for the CC and SC. In the fracture micrographs, the CC showed typical ductile fracture preceded by many dimples which occurred in the equi-axed core. The dimple size and area decreased with a decrease in the stress ranges. The precipitates formed in the CC test specimens were more coarsened than those in CC ones.

Fatigue Failures of Differences Behaviour on CSM/Woven Roving Composite Materials

This paper presents the investigation of composite materials lamination using different materials in the structure of lamination. The main purpose of the study is to evaluate the behaviour of characteristics in composite materials subjected to difference of fatigue loading, leading to understand the criteria that influence the behaviour of composite lamination structure. Therefore, in this research, the orientation of lamination structure used is 00/900and the material selected for the study were chopped strand mat (csm) and woven roving fabric (wr) as a reinforcement and the matrix used were polyester and epoxy resin. The composite lamination structure was produced using hand lay-up technique. The fatigue condition experiment of composite materials in this research was carried under tension-tension loading. With difference in fatigue loading condition, the lifetime of composite structure will be different and the cracking phenomenon in the structure will also be different. It is suggested that, different number of lamination and amount of reinforcement and matrix, produce a variety of materials characteristic with respect to elasticity of material. An implication of the study in this research showed various behaviour of composite materials with different materials used and it showed a difference phenomenon in comparison to metalic materials.

Finite Element Analysis of the Fatigue Crack Tip Parameters in the Glass Fiber Reinforced Aluminum Alloy Laminates under Tension-Compression Loading

Elastic–plastic finite element analyses have been performed to study the compressive stress effect on fatigue crack growth of the glass fiber reinforced aluminum alloy laminates (GLARE) under applied tension–compression loading. The crack opening displacement and plastic zone size were obtained for the aluminum layer in GLARE. The differences of parameters near the crack tip under tension-compression and tension-tension loading were observed. The results have shown that the displacement and reverse plastic zone size continue to change with the applied compressive stress. Based on the finite element analysis results, a fatigue crack propagation model of GLARE has been proposed.

Fatigue and Crack Growth in 7050-T7451 Aluminum Alloy Under Constant- and Variable-Amplitude Loading

The 7050 aluminum alloy is used in many aerospace structural applications. Previous studies have identified that fatigue cracks develop very rough crack-surface profiles, which cause very high crack-closure levels due to a combination of plasticity, roughness and debris. Previously, tests were conducted on compact, C(T), specimens to generate crack-growth-rate data from threshold to near fracture over a wide range in stress ratios (R). New threshold testing methods, based on compression precracking, were used to generate the data in the near-threshold regime. The plasticity-induced crack-closure model, FASTRAN, was used to correlate the data over a wide range in stress ratios and crack-growth rates from threshold to near fracture. To account for the very high crack-closure levels, a very low constraint factor, like plane-stress conditions, had to be used in the model. In addition, the crack-opening loads were measured during these tests using a local strain-gauge method to generate another ΔKeff-rate curve. These two curves differed only in the near-threshold regime. Herein, fatigue-crack-growth tests were conducted on C(T) specimens under spike overloads and simulated aircraft spectrum loading. Fatigue tests were also conducted on single-edge-notch bend (SEN(B)), specimens over a wide range in loading conditions (constant amplitude and three aircraft spectra). All specimens were machined from a single forged block of 7050-T7451. However, no residual stresses were measured in both the SEN(B) and C(T) specimens. Two European standard spectra were used, but modified to have only tension-tension loading. The purpose of this paper was to evaluate the two different effective stress-intensity factor curves for making crack-growth and fatigue-life predictions. Small-crack theory was used to make fatigue-life predictions using inclusion-particle sizes from the literature. Fatigue predictions on the SEN(B) specimens agreed fairly well (±30%) using a 12-micrometer semicircular initial flaw located at the semicircular-edge notch under all loading conditions, except the model was unconservative (factor of three) on one of the severe aircraft spectra (Mini-TWIST+, Level 1). For the C(T) specimens subjected to single-spike overloads, the life-prediction code also produced much more retardation than observed in the tests. However, the predicted crack-length-against-cycles under the Mini-Falstaff+ spectrum were only about 15% longer than the tests. The discrepancy under the single-spike overloads and the severe aircraft spectra was suspected to be caused by the low constraint factor and/or crack paths meandering around overload plastic zones. Ideally, a roughness-induced crack-closure model; in addition to the plasticity model, would be needed to obtain more reasonable results.

OS1539 HPT加工を受けた異なる結晶格子を持つ金属材料の疲労き裂進展挙動

Fatigue crack growth behavior of metallic materials before and after high pressure torsion (HPT) treatment was investigated using RCT-specimen under tension-tension loading. In order to clarify the influence of crystal lattice on fatigue crack propagation process, specimens made of pure Fe (bcc) and 99.9%Ni (fee) were examined. The growth of fatigue crack was observed by optical microscope during fatigue process and the fracture surfaces were observed using scanning electron microscope. From the results, it is found that the relationships between crack growth rates, da / dN, and stress intensity factor range, ΔK, in HPT-processed specimen of the both materials are almost the same at low ΔK region, but in ΔK values is above 30MPam_<1/2>, da / dN of 99.9% Ni increased in comparison with that of Fe-11ppmC. Stripe like pattern was observed in the fracture surface of the both HPT-processed specimens, but not observed in untreated specimen.

Influence of Carbon Nanotubes and Processing on Cyclic Fatigue and Final Fracture Behavior of a Magnesium Alloy

Carbon nanotubes (CNT)-reinforced magnesium alloy (AZ31) was fabricated using the technique of solidification processing followed by hot extrusion. Test specimens of both the composite and the unreinforced alloy were cyclically deformed at two different load ratios spanning tension-tension loading (R = 0.1) and fully-reversed tension-compression (R= -1) loading under total stress amplitude-control. A comparison of the CNT reinforced magnesium alloy with the unreinforced counterpart revealed well over two hundred percent improvement in cyclic fatigue life at load ratio of 0.1 and about two-hundred and fifty percent improvement in the high cycle fatigue life under conditions of fully-reversed loading [R= -1.0]. At all values of maximum stress, the high cycle fatigue response of both the reinforced and unreinforced magnesium alloy was found to degrade at the lower load ratio (-1.0). The synergistic and interactive influences of reinforcement and processing on microstructural development, cyclic fatigue life and kinetics governing fracture behavior are presented and briefly discussed.

Evaluation of tension-compression and tension-tension fatigue life of woven fabric glass/epoxy laminate composites used in railway vehicle

In this study, the fatigue characteristics and life of two types of woven fabric glass /epoxy laminate composites with WR580 glass fiber for carbody structures and GEP224 glass fiber for bogie frames were evaluated and compared with those of conventional metallic materials. Additionally, the laminate composite for carbody structures was reinforced with carbon plies to investigate the improvement of fatigue life. A fatigue test was conducted for tension-tension and tension-compression load with a stress ratio, R of 0.1 and -1, and up to endurance limit of 107 with frequency of 5Hz. In tension-compression fatigue test, the anti-buckling jig was designed to prevent buckling of specimen induced by compressive load. The Goodman diagrams were introduced to evaluate the fatigue life of laminate composites. The test results showed that the fatigue life of the GEP224 woven fabric glass/epoxy laminate composite with the stacking sequence of warp direction had a good performance in comparison with that of SM490A used to existing metal bogie frame. Also, the fatigue performance of WR580 woven fabric glass/epoxy laminate composite with the reinforcement of woven fabric carbon /epoxy ply had shown a significant improvement than that of a bare specimen without reinforcement.

A Case Study Of The Use Of Ultrasonic Peening For Upgrading A Welded Steel Structure

Fatigue tests were carried out to establish the fatigue life improvement that could be obtained from the high-frequency ultrasonic peening technique known as ultrasonic impact treatment (UIT) of longitudinal fillet welded joints in steel viaducts in an urban railway system that was being upgraded for increased capacity after a period of service. The work involved comparative fatigue tests on as-welded and weld toe peened specimens consisting of 30 mm-thick steel plates with longitudinal non-load carrying fillet welded stiffeners. To take account of previous service, the test specimens were fatigue tested for a proportion of their expected lives before the peening was applied, to simulate that service. Two conditions were investigated, peening applied to the unloaded specimen and peening applied while the specimen was held in tension, simulating the dead load stress in the actual structure. In each case, subsequent fatigue testing was carried out under axial tension-tension loading with a constant minimum stress of 120 N/mm2 to represent the dead-load stress. The treated specimens all failed in the fillet weld throat and in an attempt to identify the factors that control the resulting fatigue life for this failure mode the present and relevant published results were re-analysed in terms of the structural stress on the weld throat, as determined by finite element analysis

The Influence of Air Humidity on Fatigue Properties of an AA2524－T34 Aluminum Alloy

Self-aligning four-point bend fatigue tests were conducted to investigate the influence of humid air on the fatigue behavior of an AA2524-T34 Al alloy. Test specimens of the alloy along the rolling direction was cycled at room temperature and frequency of 15 Hz, tension-tension loading(R = 0.1), in 15% relative humidity (RH) and 95% RH up to 107 cycles to failure. The S-N data shows continuous reduction tendency. It was found that with the increase of test relative humidity, the fatigue properties of the alloy is decreased, the fatigue limit in 95% RH was nearly 20% reduction of that in 15% RH, only 67% of its yield strength. Meanwhile, the S-N data is much more scattered than it in lower relative humidity. In 15% and 95% RH, cracks were initiated predominantly at coarse particles but partly at grain boundaries on the surface in 95%RH. SEM results show the characteristics of intergranular fracture in high humidity circumstance.

Statistical Fatigue Property of Multi-Layer Amorphous Metal Composites in Axial Loading

Statistical fatigue property and fracture mechanism of a Fe-based film amorphous metal, Fe78B13Si9, have been examined in the axial loading. Testing specimens including single-layer and multi-layer types were prepared from a long ribbon of Fe78B13Si9 amorphous metal with dimensions of 25µm in thickness and 50mm in width. The high cycle fatigue tests were performed by using electro-hydraulic servo testing machine in tension-tension loading at a frequency of 50 Hz with the stress ratio of 0.1. The testing data showed that fatigue lives of the amorphous metal had a large scatter, especially at low stress levels. By applying common Weibull distribution and mixed-mode Weibull distribution, the statistic fatigue property of the specimens was quantitatively analyzed. The fatigue strength σ at N=107 was in the range of 135-187 MPa and the fatigue ratio to the tensile strength was found to be σ/σB =6-9%. The fracture surface morphology of failed specimens observed by SEM showed evidently three main regions as follows: 1) flat area corresponding to the crack initiation and the early growth stages up to penetrate the thickness of the film specimen. 2) rough area with a lot of chevron patterns corresponding to the stable propagation as the penetrated fatigue crack. 3) relatively plain area with vein patterns peculiar to the static fracture surface of amorphous metals.

A Study on Improvement of Fatigue Life for Woven Glass Fabric/Epoxy Laminate Composite Applied to Railway Vehicle

In this study, the fatigue characteristics and life of woven glass fabric/epoxy laminate composites applied to railway vehicle were evaluated. The fatigue test was conducted by tension-tension load with stress ratio R of 0.1 and frequency of 5Hz. The material used to fatigue test was two types of woven glass fabric/epoxy laminate composite with and without the reinforcement of carbon/epoxy ply. Also, the fatigue life of woven glass fabric/epoxy laminate composite was compared with that of aluminum 6005 used to the car-body and under-frame structures of railway vehicle. The test results showed that the failure strength and life of woven glass fabric/epoxy laminate composite with the reinforcement of three carbon/epoxy plies had a remarkable improvement in comparison with that of bare specimen without reinforcement.

Assessment of Quasi-Static and Fatigue Loaded Notched GRP Laminates Using Digital Image Correlation

This paper presents the results of an experimental study carried out to assess the effects of cumulative damage on the strain response of open-hole (notched) quasi-isotropic [45/0/-45/90]4s E-glass/913 epoxy laminate specimens under quasi-static tension, and constant amplitude tension-tension, compression-compression and tension-compression cyclic loading conditions. In-plane surface strain was measured as a function of applied load and loading cycles for quasi-static and cyclic loading respectively using digital image correlation (DIC) and compared with strain gauge, fibre Bragg grating (FBG) optical sensor and contact extensometer strain measurements. DIC proved successful in monitoring local (near the hole) and global strains, providing critical information on the changes in strain distribution resulting from damage formation and growth incurred.

Towards a Planar Cruciform Specimen for Biaxial Characterisation of Polymer Matrix Composites

This paper details work undertaken towards the development of a standard test method for the biaxial response of planar cruciform specimens manufactured from carbon fibre-reinforced plastic (CFRP) laminates and subject to tension-tension loading. Achieving true biaxial failure in a cruciform specimen without the need for the inclusion of a stress raiser, such as a hole, in the gauge-section, is a subject attracting much research globally and is by no means a trivial exercise. Coupon designs were modelled using finite element analysis (FEA) in order to predict the stress and strain distributions in the central region of the specimen. An Instron biaxial strong-floor test machine was used to test the specimens. Strain gauges were used to measure the strain in the specimen arms and to assess the degree of bending. Digital image correlation (DIC) was used to measure the full-field strain distribution in the central gauge-section of the specimen and this was compared to values measured using strain gauges. The strain readings obtained from strain gauges, DIC and FEA predictions were in good agreement and showed that the strain distribution was uniform in the central gauge-section, but that strain concentrations existed around the tapered thickness zone. These regions of strain concentration resulted in interlaminar failure and delamination of the laminate propagating into the specimen arms.

Thermo-mechanical fatigue behavior of nickel-base powder metallurgy superalloy FGH96 under tension-tension loading

Thermo-mechanical fatigue (TMF) behavior of FGH96, a nickel-base powder metallurgy superalloy, has been studied under tension-tension loading at the temperature range from 550 to 720 ◦C. The results show that TMF fracture mode is intergranular for the in-phase (IP), but transgranular cleavage-like for the out-of-phase (OP) samples. The total content of Al, Ti and Nb in the γ′ phases for the IP or OP samples and the partitioning ratio of γ′/γ in these elements for the IP samples are relatively higher at the lower strain amplitude, which is consistent with the case of the γ′ size that is larger at the lower strain amplitude, the lattice parameter misfit is negative and the absolute value is lower at the lower strain amplitude that is correlative with the change of the γ′ morphology. The deformation at the lower strain amplitude is mainly dominated by the dislocation lines and dislocation pairs in the matrix channels, at the higher strain amplitude dominated by the large numbers of superlattice stacking faults within the γ′ phases.

철도차량용 직물 유리섬유/에폭시 적층 복합재의 피로수명 향상 방안 연구

초록:본연구는철도차량에적용되는직물유리섬유/에폭시적층복합재의피로특성및피로수명에대해평가하였다. 이때, 피로시험은0.1의응력비(R)와5Hz의주파수에서인장-인장하중조건이고려되었으며, 시험편은카본/에폭시플라이가보강된시험편과보강되지않은시험편두가지타입을고려하였다. 또한, 직물유리섬유/에폭시적층복합재의피로수명을평가하기위해철도차량차체와언더프레임에주로사용되는알루미늄6005와비교하였다. 본연구를통하여3장의카본/에폭시플라이가보강된직물유리섬유/에폭시적층복합재시험편이보강되지않은기본시험편보다피로강도및피로수명이크게향상됨을확인하였다.Abstract: In this study, the fatigue characteristics and life of a woven glass fabric/epoxy laminate composite appliedto railway vehicles was evaluated. The fatigue test was conducted using a tension-tension load with a stress ratio Rof 0.1 and frequency of 5 Hz. Two types of woven glass fabric/epoxy laminate composite was used in the fatiguetest: with and without carbon/epoxy ply reinforcement. In addition, the fatigue life of the woven glass fabric/epoxylaminate composite was compared with that of aluminum 6005, which is used in the car body and underframestructures of railway vehicles. The test results showed that the failure strength and life of the woven glassfabric/epoxy laminate composite reinforced with three carbon/epoxy plies had a remarkable improvement comparedwith that of the bare specimen without reinforcement.