THE INFLUENCE OF LOAD MISALIGNMENT DURING UNIAXIAL LOW-CYCLE FATIGUE TESTING—I: MODELLING

Abstract— A quantitative model has been proposed which predicts the extent of lifetime scatter in low-cycle fatigue due to the influence of bending caused by load misalignment. The main components of the model are the mechanism of bending, the type of extensometer used to control strain and the fatigue characteristics of the material being assessed. Three mechanisms of bending have been studied and it is argued that the most damaging one is a consequence of a lateral offset in the centre-lines of the load-train with respect to either a machine's frame or ram. Scatter in lifetime is a maximum when strain is controlled by a single extensometer (which is generally the case) and when fatigue behaviour is dictated by crack initiation at the largest surface defect. Two types of scatter have been examined, (i) repeatability scatter due to testing practice within a single laboratory, and (ii) reproducibility scatter between laboratories. An example of the magnitude of reproducibility scatter due to bending has been given by using an equation based on the universal slopes method due to Manson.     

Precision of method for determining resistance of bituminous mixtures to de-icing fluids

This paper presents the results of a Round-Robin test to estimate the precision of European method EN-12697-41 “Test methods for hot mix asphalt-Resistance to de-icing fluids”. The purpose of the project was to determine precision data according to ISO 5725, ASTM E691 and ASTM C802. The examined test method is intended for use in requirements specifications for airfield de-icing chemicals and/or as a tool for development of such products. Precision statistics, repeatability and reproducibility standard deviations, are based on observed values from six laboratories and six levels, each level comprising four samples. From a general statistical analysis, which was conducted in addition to precision determination, it could be concluded that the most damaging de-icing agents (treatments) were identified by all participating laboratories both in terms of absolute values and by ranks.     

The Influence of Finite Geometry and Material Properties on Mixed-Mode I/II Fracture of Aluminium

Numerical calculations have been carried out to assess the influence of both finite geometry effects as well as material properties on mixed mode fracture of aluminium. These effects have been studied in close connection to experimental data for two aluminium alloys found in the literature. Interactions between the crack tip and the outer boundary have, for one of these alloys, been quantified in two ways. Firstly, by evaluating a number of non-singular stress on mixed mode fracture have been examined within the framework of a recently suggested effective plastic strain criterion. The other alloy was addressed in order to furnish a limited investigation concerning the sensitivity of this criterion with respect to material properties. The main conclusions arrived at in this paper are: (i) Boundary induced constraints may relocate the transition between different operative fracture modes and hence be responsible for scatter of experimental achieved under different testing conditions. (ii) The two alloys under consideration were predicted to behave very differently due to variations in the flow behaviour. Different behaviour was also confirmed by the experimental results. This revised version was published online in August 2006 with corrections to the Cover Date.     

两种方法评价管材力学性能的研究

通过挤压一个缺口环或闭口环样品来评价不同管材的弹性模量和弯曲强度。在弹性范围内, 弹性模量可由载荷—位移关系和样品尺寸获到; 而弯曲强度则由断裂临界载荷决定。在本研究中, 四种管材被分为两组来研究这两种方法的适用范围。结果表明: 缺口环适合评价刚度较大、极限应变较小的材料, 而闭口环则更倾向于评价低刚度的材料。此外, 三点弯曲的数据也验证了这两种方法的有效性、便捷性及其各自的适用范围。     

Fracture assessment of U-notches under three point bending by means of local energy density

The main purpose of the paper is twofold. First, to provide a new set of experimental results on fracture of U-notched samples, made of two different materials; second, to apply a fracture criterion based on the strain energy density (SED) averaged over a control volume to assess the fracture load of blunt-notched components under three point bending. Two different materials are considered in the tests: a composite material (Al–15%SiC) tested at room temperature and a steel with a ferritic–pearlitic structure tested at −40 °C. All samples are weakened by U-notches characterized by different values of notch root radius and notch depth. The theoretical loads to failure as determined according to the SED criterion are compared with the experimental data from more than 40 static tests and with a SED-based scatter band recently reported in the literature for a number of materials exhibiting a brittle behaviour under static loads.     

Dynamic crack initiation toughness of 4340 steel at constant loading rates

Determination of fracture toughness for metals under quasi-static loading conditions can follow well-established procedures and ASTM standards. The use of metallic materials in impact related applications requires the determination of dynamic crack initiation toughness for these materials. There are two main challenges in experiment design that must be overcome before valid dynamic data can be obtained. Dynamic equilibrium over the entire specimen needs to be approximately achieved to relate the crack tip loading state to the far-field loading conditions, and the loading rate at the crack tip should be maintained near constant during an experiment to delineate rate effects on the values of dynamic crack initiation toughness. A recently developed experimental technique for determining dynamic crack initiation toughness of brittle materials has been adapted to measure the dynamic crack initiation toughness of high-strength steel alloys. A Kolsky pressure bar is used to apply the dynamic loading. A pulse shaper is used to achieve constant loading rate at the crack tip and dynamic equilibrium across the specimen. A four-point bending configuration is used at the gage section of the setup. Results are presented which show a monotonically increasing rate dependence of crack initiation toughness for 4340 high-strength steel.     

The effect of frequency and environment on the fatigue-crack growth behaviour of ASTM A533 Grade B Class 1 weldment material

The effect of frequency and distilled water environment on the fatigue-crack growth characteristics of ASTM A533 Grade B Class 1 weldment material was studied with major emphasis placed on the crack growth along the weld centreline as well as along the heat affected zone (HAZ). A single deterministic fatigue-crack growth model based on the Four Parameter Weibull Survivorship Function is used to describe the dependence of crack growth per cycle, da/dN, on the alternating stress intensity, ΔK, over the entire range of ΔK. The probability distribution function of da/dN is determined to be log-normal by analysing the residuals of the linear regression model that is derived from the Weibull curve-fitting model. No significant effect of frequency is observed in the weld and HAZ materials in laboratory air. A decrease in frequency from 10 Hz to 0.5 Hz in distilled water tends to increase the crack growth rates over the baseline data. Distilled water environment tends to produce serious data scatter in both weld and HAZ materials. As a result, replication of fatigue tests is recommended in order to increase the amount of data which is essential for a better estimate of the median curve. The observed scatter could have some effect in fatigue-crack growth damage tolerance estimates.     

Severe plastic deformation (SPD) and nanostructured materials by machining

Large plastic strains between 1 and 15 can be imposed in chips formed by plane-strain (2-D) machining of metals and alloys. This approach has been used to examine microstructure changes induced by large strain deformation in model systems—copper and its alloys, precipitation-hardenable aluminum alloys, high-strength materials such as titanium, Inconel 718 and 52100 steel, and an amorphous alloy. It is shown that materials with average grain sizes in the range of 60 nm–1 μm can be created by varying the parameters of machining, which in turn affects the deformation processes. Furthermore, a switch-over from an elongated subgrain microstructure to an equi-axed nanocrystalline microstructure, with a preponderance of large-angle grain boundaries, has been demonstrated at the higher levels of strain in several of these materials. This switch-over can be readily controlled by varying the deformation conditions. Dynamic recrystallization has been demonstrated in select material systems under particular conditions of strain and temperature. This study may be seen as providing an important bridge between furthering the understanding of microstructural refinement by large strain deformation and the practical utilization of nanostructured materials in structural and mechanical applications. Conventional plane-strain machining has been shown to be a viable SPD method for examining the underlying processes of very large strain deformation.     

引伸计误差对Rp0.2测量结果的影响

就引伸计标距误差和引伸计系统误差对RP0.2测量结果的影响进行了分析。对几种材料的试验误差分析表明,应变误差造成的RP0.2的测量误差依赖于材料自身的特性。对于普通材料,其应变误差对拉伸性能RP0.2造成的误差可以假设小于1%;但是对于某些合金材料,引伸计误差对拉伸性能RP0.2造成的误差大于1%,在测量这些材料时需要采用比规定级别更高的引伸计。     

Optimum hot forming temperature of AZ61 magnesium alloy

A new concept of stability of materials is introduced by defining the optimum hot forming temperature for any given strain rate. This temperature is obtained through forming maps that are based on Lyapunov concepts and the introduction of a Garofalo equation in the Lyapunov criterion. This new approach is applied to a magnesium alloy AZ61. Torsion tests were carried out in the temperature range 574–734?K and strain rate range 0.7–8.7?s^?1 and the microstructures were determined using optical microscopy. Using the peak stress, optimum workability at 630?K is obtained at 12?s^?1. The results and the maps are compared with data and maps of other authors for AZ61 alloys in various states.     

Nanostructured bulk copper fabricated by accumulative roll bonding

In this study, we tried to fabricate the nanostructured bulk copper alloys by a severe plastic deformation process. The sheets of copper alloys (OFC, PMC90, and DLP) were heavily deformed to an equivalent strain of 6.4 by the accumulative roll-bonding (ARB) process. The microstructure and the mechanical property of the fabricated specimens were systematically investigated. The microstructure was finely subdivided with increasing the equivalent strain by the ARB process. The severely deformed copper alloys exhibited the ultrafine lamellar boundary structure where the mean lamella spacing was about 200 nm. The strength significantly increased with decreasing the lamella spacing in the ARB processed copper alloys. Especially, the tensile strength of the DLP alloys ARB processed by 8 cycles (the equivalent strain of 6.4) reached to 520 MPa, which was about three times higher than that of same materials with conventional grain size of 10-100 microm. On the other hand, the total elongation greatly dropped only by 1 ARB cycle corresponding to an equivalent strain of 0.8, which was around 3%. However, the total elongation increased again with increasing the number of the ARB cycle, and it reached to 10% after 8 cycles. The recovery of the total elongation could be recognized in all studied copper alloys. The obtained stress-strain curves showed that the improvement of the total elongation was caused by the increase in the post-uniform elongation. It can be concluded that the nanostructured copper alloys sheets having high strength without a large loss of ductility could be fabricated by the ARB process.     

Ultrasonic extraction and field-portable anodic stripping voltammetric measurement of lead in dust wipe samples

Dust wipe samples were subjected to ultrasonic extraction (UE) in diluted nitric acid, and then analyzed for lead content using field-portable anodic stripping voltammetry (ASV). Recoveries of lead were determined from wipe materials which were spiked with certified reference materials (CRMs) containing known quantities of lead. Four different wipe materials and four different CRMs were tested, with and without filtration of aliquots of sample extract through 0.45 microm hydrophilic polytetrafluoroethylene filters. The CRMs consisted of paint, soil, particulate, and dust matrices. Wipe materials were chosen from those which have been found to meet the performance aspects of an ASTM standard specification. UE/ASV experiments were carried out in accordance with newly published ASTM procedures for on-site extraction and electroanalysis. Recoveries were found to vary for different wipe materials and CRMs. For several CRMs, quantitative (80--120%) recoveries for UE/ASV were observed for one wipe material whether filtration was used or not, while other wipe materials required filtration for quantitative recovery. In the case of one wipe material which contained detergents, quantitative recoveries could not be achieved whether filtration was used or not. The total analysis time for a sample set of 6--12 samples was 60--90 min, including extraction time and sample manipulation. The results of this work have provided information on the choice of wipe materials that can be used for quantitative lead measurements by UE/ASV in materials that are representative of sources of lead in surface dust.     

HIGH TEMPERATURE FATIGUE DAMAGE IN THREE AUSTENITIC ALLOYS

Abstract— A series of cyclic strain controlled tests have been carried out at 600°C on three high temperature austenitic iron-based alloys. These alloys were AISI type 316 stainless steel, Alloy 800 H and Sandvik 253 MA. The tests were carried out under constant total strain control using a constant strain rate of 0.005 s⁻'. Damage mechanics was applied to the results in order to follow the accumulation of damage. By consideiing the changes in modulus throughout the life of each specimen it was found that damage evolution could be successfully predicted as a function of plastic strain range despite the fact that each alloy had been chosen because of a different stress response at 600°C, namely cyclic saturation, hardening, and softening followed by hardening for the AISI 316, 253 MA and Alloy 800 H respectively. Although each alloy accumulated fatigue damage in a similar manner the longer lives of Sandvik 253 MA and Alloy 800 H at a given total strain range were due to a smaller plastic strain component and a reduced stage I crack propagation rate. In the 253 MA alloy. slip was predominantly planar with some cells occasionally forming at high strain ranges. Slip was localized in Alloy 800 H due to the shearing of small γ precipitates. In the AISI 316 stainless steel, dislocation cells formed at all strain ranges. It is concluded that all these alloys accumulate damage similarly, independent of their deformation behaviour.     

Analytical approach to the strain rate effect on the dynamic tensile strength of brittle materials

An explicit mathematical expression for the dynamic load-carrying capacity of brittle materials under dynamic tensile loads is derived based on a kind of structural-temporal failure criterion [1] and the one-dimensional longitudinal plane wave propagation model. It is shown that the dependence of the dynamic load-carrying capacity on the strain rate can be determined only by the static material parameters such as tensile strength, density, incubation time, critical failure length and constitutive constants, which verifies that the well known strain rate effect on material strength can be considered as an structural rather than material behavior, as pointed out by Cotsovos and Pavlovi? [2] recently. Moreover, it is found that, under constant strain rate, the dynamic load-carrying capacity depends also on the amplitudes of imposed boundary loads, which explains, to a significant extent, the scatter that characterizes the available experimental data. Furthermore, the derived expression can also be used as a foundation of theoretical analyses on other problems involving the strain rate effect such as dynamic size effect, dynamic failure of quasi-brittle materials and composites.     

Determination of stress and strain concentrations in the elastic-plastic materials under bending and torsion

The analysis of stress and strain concentrations of constructional materials subjected to bending and torsion is presented. The known methods of stress determination for any elastic-plastic material assume the linear strain concentration under bending and torsion. The determination of stress and strain concentrations and their presentation as graphs were the main aims of this paper. Five materials were tested. They had different cyclic strain curves and were loaded by torsional and bending moments. The results proved the linear character of the strain distribution for any material in the entire applied loading range.     

Improvement of ASTM compliance offset method for precise determination of crack opening load

There are two serious drawbacks to the ASTM offset method for determination of crack opening load, discontinuity and inconsistency problems. The former is the lack of opening load value in the lower 5% load range due to the procedure that the compliance offset is plotted against the mean load of the segment span of 10% of the cyclic load range. The latter is that the ASTM method underestimates, estimates accurately, or overestimates the opening load, depending on the maximum compliance offset value and the stress ratio. To overcome the two drawbacks, the so-called normalized-extended ASTM method is proposed and evaluated quantitatively in detail using the data of 7475-T7351 and 2024-T351 aluminum alloys. The normalized-extended ASTM method improves the standard ASTM method significantly and provides very good correlation of crack growth rate stress ratio effects. The normalized-extended ASTM method is strongly recommended for crack opening load determination.     

Fatigue life of 2017(A) aluminum alloy under proportional constant-amplitude bending with torsion in the energy approach

We present the results obtained for 2017(A) aluminum alloy under the conditions of pure bending with constant amplitude, pure torsion, and two combinations of proportional bending with torsion. All results can be described by a single criterion based on the parameter of strain-energy density in the critical plane. The critical plane is defined as the plane where the parameter attains its maximum value. The fatigue life is affected by the sum of the densities of normal and shear strain energies in the accepted critical plane. The results are presented in a scatter band with coefficient equal to three for the case of pure bending. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 4, pp. 68–74, July–August, 2008.     

Image analysis measurements of duplex grain structures

Several types of duplex grain size distributions in five different alloys were evaluated using image analysis. Most of the grain structures contained annealing twins. Those with straight interfaces could be recognized and deleted from the image, leaving only grain boundaries. One specimen exhibited curved twin boundaries, caused by deformation, and they could not be discriminated by the system as currently programmed. Grain areas were measured and grouped according to their relationship to the ASTM grain size scale. An area-weighted histogram was shown to be excellent for revealing the nature of the distribution, while a numerical-frequency histogram was insensitive. The intersection of these two curves separated only one of the four bimodal distributions. A deconvolution approach, using the area-weighted curve only, should be evaluated. An arithmetic grain area classification approach using 25 classes based on the data range, to split the two grain area populations based upon the intersection of the number percent and area percent curves, worked well for two of the four specimens. Image analysis detection of grains results in a small portion of the image (about 6–12%) assigned to the grain boundaries. In manual measurement methods, the area occupied by the grain boundaries is not considered, and it does not influence measurements. Thus, compared to manual methods, image analysis undersizes grains slightly producing a relatively small positive bias in the grain size number, which could be ignored, but can be eliminated or reduced.     

Concurrent ratcheting–fatigue damage analysis of uniaxially loaded A‐516 Gr.70 and 42CrMo Steels

This study intends to investigate the concurrent interaction of fatigue damage and ratcheting strain in two commonly used steel alloys of (American Society for Testing and Materials) ASTM A‐516 Gr.70 and 42CrMo, respectively for pressure vessels and high grade machinery parts over uniaxial stress cycles. Ratcheting extension and fatigue damage progress were both characterized cycle‐by‐cycle over life cycles of tested materials. The interaction of ratcheting and fatigue damage was defined based on mechanistic parameters involving the effects of mean stress, stress amplitude and cyclic softening/hardening response of materials. The extent of ratcheting effect was defined by product of average ratcheting strain per cycle, and maximum stress value during a cycle, while fatigue damage was analysed based on earlier developed energy‐based models of Xia–Ellyin, and Smith–Watson–Topper. Overall damage due to ratcheting and fatigue was calibrated through a weighting factor at various mean/ cyclic amplitude stresses. An algorithm was developed to evaluate overall damage due to ratcheting and fatigue stress cycles of materials subjected to various mean and amplitude stresses. The estimated lives at different mean stresses and stress amplitudes for ASTM A‐516 Gr.70 and 42CrMo samples showed good agreements as compared with those of reported experimental data.     

Observations and analyses of secondary bending for riveted lap joints

The phenomenon of secondary bending in riveted lap joints of the configuration representative of connections of the aircraft fuselage sheets in the longitudinal direction is investigated experimentally and analytically. The experiments involved strain gauge measurements of secondary bending stresses carried out in close proximity to the fatigue critical section of the riveted lap joint and fatigue tests performed to study the effect of secondary bending on the riveted joint fatigue life. The strain gauge measurement results allowed validation of a simple analytical model proposed by Schijve to estimate secondary bending moments induced in mechanically fastened joints with eccentricities. Variables considered in the fatigue tests were several joint geometry related parameters known to influence the magnitude of secondary bending in the fatigue critical location. It was shown that the fatigue test data for joints of various geometries, which were considerably scattered if the fatigue lives were presented against the applied stress amplitude, could become consolidated within common scatter bands when the lives were plotted in terms of the combined tensile stress amplitude including the bending stress computed from the model by Schijve.