Characterization of mechanical properties of aluminum cast alloy at elevated temperature

The tensile response, the low cycle fatigue(LCF) resistance, and the creep behavior of an aluminum(Al) cast alloy are studied at ambient and elevated temperatures.A non-contact real-time optical extensometer based on the digital image correlation(DIC)is developed to achieve strain measurements without damage to the specimen. The optical extensometer is validated and used to monitor dynamic strains during the mechanical experiments. Results show that Young's modulus of the cast alloy decreases with the increasing temperature, and the percentage elongation to fracture at 100℃ is the lowest over the temperature range evaluated from 25℃ to 300℃. In the LCF test, the fatigue strength coefficient decreases, whereas the fatigue strength exponent increases with the rising temperature. The fatigue ductility coefficient and exponent reach maximum values at 100℃. As expected, the resistance to creep decreases with the increasing temperature and changes from 200℃ to 300℃.     

High-temperature fatigue-life estimation: Extension of a unified theory

A procedure is presented for predicting the fatigue behavior at elevated temperature by extending the unified theory of fatigue damage previously proposed for room temperature. The method predicts the experimental results of high-temperature push-pull tests under isothermal conditions, using the total strain range. The analysis is based on parameters obtained from short-term tensile tests in which the temperature and the strain rate are the same as for the fatigue test. The procedure is applied for fatigue of a stainless steel at 650°C under cyclic axial strain. It has also been applied to published data for three austenitic stainless steels. In general, the present procedure gives estimates closer to experimental results than those obtained from other known methods.     

一种考虑拉伸保持效应的多轴疲劳寿命模型

通过定义考虑拉伸保载效应的CFI因子(creep-fatigue interaction factor)，将拉伸蠕变损伤和疲劳损伤进行非线性耦合. 根据断裂实验的观察，针对拉伸主 导的裂纹萌生、扩展及破坏的多轴疲劳问题，给出了一个基于临界面方法的能量型高温多轴 疲劳寿命预测模型. 所给出的模型可对不同温度、不同载荷特点、不同保载时间的多轴疲劳 寿命进行预测，模型的材料参数不依赖于温度和载荷. 并且此方法可以很方便地推广到其它 因素主导破坏的高温多轴疲劳寿命预测. 通过拟合高温合金Udimet720Li单轴带保持时间的 低循环疲劳(low cycle fatigue, LCF)寿命试验数据，得到了材料常数. 结合黏 塑性有限元分析方法，对高温双轴带保载循环载荷下Cruciform试件的寿命进行了 预测，预测结果基本落在2倍分散带内，达到工程的要求，证明了该模型的有效性.     

Time-dependent ratchetting experiments of SS304 stainless steel

The time-dependent strain cyclic characteristics and ratchetting behaviours of SS304 stainless steel were investigated by uniaxial/multiaxial cyclic loading tests at room and elevated temperatures (350 and 700 °C). The effects of loading rate, peak/valley strain or stress holds, ambient temperature and non-proportional loading path on the cyclic softening/hardening and ratchetting behaviours of the material were discussed. It is shown that: the cyclic deformation of the material presents remarkable time-dependence at room temperature and 700 °C; the cyclic hardening feature and ratchetting strain depend significantly on straining or stressing rate, hold-time, ambient temperature and the non-proportionality of loading path; the time-dependent ratchetting is resulted from the slight opening of hysteresis loop and visco-plasticity together, and the viscosity is a dominating factor at 700 °C; at 350 °C, abnormal rate-dependence and quick shakedown of ratchetting are observed due to the dynamic strain aging of the material at this temperature. Some significant conclusions are obtained, which are useful to construct a constitutive model to describe the time-dependent ratchetting behaviour of the material. It is also stated that the unified visco-plastic constitutive model discussed here cannot provide reasonable simulation to the time-dependent ratchetting at 700 °C, especially to that with certain peak/valley stress hold, since the effect of the high viscosity on time-dependent ratchetting cannot be properly described by using a unified visco-plastic flow rule.     

Experimental study of dynamic plasticity at elevated temperatures

In this paper it is shown that the diffraction-grating technique and the optical-displacement technique used by the writer for the study of plastic wave propagation at room temperature, may both be extended to within 100° F of the melting point of aluminum. In addition to the measurement of stress history at the impact face obtained by the extension of the load-bar technique to elevated temperatures, strain-time, surface angle-time, time of contact, coefficient of restitution, and displacement-time behavior at the free end of the struck specimen may all be determined at elevated temperatures. Typical strain-time behavior is shown at 800, 1000, and 1100° F, for three types of impact situations.     

Influence of temperature on a carbon–fibre epoxy composite subjected to static and fatigue loading under mode-I delamination

In this paper, interlaminar crack initiation and propagation under mode-I with static and fatigue loading of a composite material are experimentally assessed for different test temperatures. The material under study is made of a 3501-6 epoxy matrix reinforced with AS4 unidirectional carbon fibres, with a symmetric laminate configuration [0°]_16/S. In the experimental programme, DCB specimens were tested under static and fatigue loading. Based on the results obtained from static tests, fatigue tests were programmed to analyse the mode-I fatigue behaviour, so the necessary number of cycles was calculated for initiation and propagation of the crack at the different temperatures. G–N curves were determined under fatigue loading, N being the number of cycles at which delamination begins for a given energy release rate. G_ICmax–a, a–N and da/dN–a curves were also determined for different G_cr rates (90%, 85%, 75%, etc.) and different test temperatures: 90 °C, 50 °C, 20 °C, 0 °C, ?30 °C and ?60 °C.     

Elevated-temperature creep and relaxation of torsion-tension members

Based on the assumption that each material satisfies the condition for isotropic hardening for a von Mises material, an incremental solution is developed to predict axial-strain creep curves and maximum shearing-strain creep and relaxation curves for solid circular torsion-tension members subjected to proportionate and nonproportionate stepped loading including creep in tension and relaxation in torsion. Test data are obtained from torsion-tension members made either of annealed OFHC copper at 800°F (427°C) or hot-rolled SAE 1045 steel at 950°F (510°C). The loading histories include either four stepped proportionate load changes, four stepped nonproportionate load changes, or torsion-tension loading in which the axial load remains constant and the torsional load is allowed to relax during two loading periods of the test. Each test duration is about 100 h. Good agreement is indicated between the predicted and measured creep and relaxation curves.     

Experimental investigation of impact of environmental temperature and optimal baseline for thermal attenuation in structural health monitoring based on ultrasonic guided waves

Structural health monitoring (SHM) of any mechanical component is compulsory for its efficient and long-term performance. One of the major challenges to apply SHM technique in real-time inspections is variation in environmental and operating conditions (EOCs). Sometimes the effect of this variation in EOCs is so severe that it influences the SHM system’s response and reduces the accuracy of the inspection process. The goal of current research is to investigate experimentally the impact of environmental temperature on the ultrasonic guided wave signal during damage detection. According to the characteristic of breathing phenomenon of fatigue crack caused by the applied temperature (30 °C–180 °C) under operation condition, behavior of reflection and transmission signal is analyzed in terms of amplitude and group velocity. Based on experiment findings, a wave velocity function has been generated in the Matlab® environment to compute the velocity of acquired signal considering the effect of both temperature and excitation frequency. A corresponding sequence curve is drawn which illustrates that the proposed function is valid when the operating temperature is less than 130 °C because sensor bonding’s characteristics are affected by the further increment in temperature and consequently it would become difficult to illuminate the sole impact of temperature on damage detection results. Impact of temperature on examined material properties and sensor’s bonding strength is also observed in the current study. Analysis of dispersion curves is performed to examine the individual behavior of S0 and A0 wave modes with temperature and to determine the temperature invariant points to reduce the influence of environmental temperature in SHM. Hence current study not only evaluates the impact of temperature on damage detection but also provides an optimal baseline for thermal attenuation in real-time ultrasonic guided wave inspections.     

Cyclic stress-strain data analysis under biaxial tensile stress state

A new device was developed to assess fatigue life under biaxial tensile loading at elevated temperatures. It makes use of an annular disk specimen and can be easily mounted onto a standard push-pull machine so that the axial force is converted into radial forces extending across the disk specimen. Therefore, a positive ratio of the tangential to the radial stress can be imposed at the reduced section of the disk specimen; this ratio depends on the specimen configuration and may be fixed to a value ranging from 0.5 to 0.9 by varying the inner diameter of the disk. The proposed device has performed successfully and was used to study the cyclic behavior of Type-304 stainless steel subjected to various biaxial tensile stress states at room temperature and at 200°C. The data obtained from this experimental procedure have been analyzed to evaluate the effectiveness of some correlations already available for treating the biaxial cyclic stress-strain response in terms of the uniaxial behavior. This analysis shows that a successful correlation should account for all the stress components. The authors discuss the concept used in the modeling of the material cyclic behavior and the formulation of a biaxial fatigue damage parameter necessary for an effective analytical life prediction methodology.     

A Methodology for Obtaining Material Properties of Polymeric Foam at Elevated Temperatures

A new methodology to characterise the elastic properties of polymeric foam core materials at elevated temperatures is proposed. The focus is to determine reliable values of the tensile and compressive moduli and Poisson’s ratio based on strain data obtained using digital image correlation (DIC). In the paper a detailed coverage of the source of uncertainties in the experimental procedure is provided. The uncertainties include those associated with the load introduction, the measurement and the data processing. The design of the specimens and loading jigs are developed and assessed in terms of the introduction of uniform strain. It is shown that due to the mismatch in stiffness between the jig material and the foam the introduction of a uniform strain through the cross section of the specimens is difficult to obtain. A means for correcting for the non-uniform strain across the specimen cross section is developed. To validate the methodology, tests are firstly conducted at room temperature on Divinycell PVC H100 foam. It is shown that the material is highly anisotropic with a stiffness of 50% less in the plane of the foam sheet compared to the through-thickness direction. It is also shown that because of the compliance of the foam, jig misalignment causes large errors in the measurement, and a means for correcting for this is defined. Tests are then conducted in a temperature controlled chamber at elevated temperatures ranging from 20°C to 90°C. A nonlinear reduction in Young’s modulus is obtained with significant degradation occurring after 70°C. The Poisson’s ratio remains fairly stable at different temperatures. A strong theme in the paper is the accuracy and precision of the DIC data and the factors which introduce scatter in the data, along with the uncertainties that this introduces. Particular attention is paid to the affect of the correlation parameters on the derived strain data.     

A unique elevated-temperature tension-torsion fatigue test rig

A unique tension-torsion fatigue test set up is described that allows strain-controlled tests at temperatures exceeding 649°C. The machine uses a large die set as a load frame resulting in lower cost and superior parallel positioning of the crossheads. Disposable weld-on grips were found to be cost effective for elevated-temperature testing. A new extensometer using commercially available capacitance probes was developed which can operate at the elevated temperature without cooling. Capacitance ring probes were utilized in an attempt to measure through-thickness strains. The characteristic behavior of the ring probes is discussed. Design modifications needed to make a successful measurement of through-thickness strains at elevated temperatures are presented.     

Cumulative damage evaluation of steel using infrared thermography

Following the first damage models proposed by Palmgren and Miner, numerous researchers have focused on the problem of predicting the residual life of a material from its load history. Every component dynamically loaded, particularly over the fatigue limit, shows an increase in temperature. The higher the temperature, the higher the load applied. Therefore, in an undamaged material or mechanical component, it is possible to associate each loading stress over the fatigue limit with a temperature value at the hottest point of the surface during the first phase of the test trough a thermo-mechanical characterization (TMC) map. Using the thermoanalysis of steel specimen data, this paper shows that the energetic effect (as a different temperature increments for equal loading uniaxial stress) can be used to evaluate the cumulative damage caused by previous loading. The tests were performed using C40 steel for which traditional fatigue curves in literature are reported.     

Conductive polymers as fatigue-damage indicators

A conducting polymer consisting of graphite particles in on epoxy matrix has been employed as a transducer. When subjected to strain, the conductor undergoes a resistance change which is due to the variation in contact pressure between the particles. Of more importance is the permanent resistance change produced in the conductor as it is cycled. This resistance change is due to a wear mechanism which improves the contact area between adjacent particles. The permanent resistance change is a consistent and reproducible function of the strain range and the number of cycles. As such, the conductor can be employed as a sensor to indicate fatigue damage. Studies conducted with two-level fatigue tests indicate that the sensor can be employed with complex strain histories. The sensor output in two-level fatigue tests of both the high-low and low-high sequence was interaction free but dependent on the magnitude of the strain range. Nevertheless, a simple graphical approach was established to predict fatigue exposure from the output. Stability of the sensor to both time and temperature was examined. Resistance changes associated with time are small and monotonic initially and, after an initial stabilization period of a month, the changes are negligible. Temperature stability represents a more significant problem which will require further work. However it appears that the sensor will perform adequately in the temperature range of 75±15°F.     

Mechanical behavior of Gamma-Met PX under uniaxial loading at elevated temperatures and high strain rates

Gamma titanium aluminides have received considerable attention over the last decade. These alloys are known to have low density, good high temperature strength retention and good oxidation and corrosion resistance. However, poor ductility and low fracture toughness have been the key limiting factors in the full utilization of these alloys. More recently, a new generation of gamma titanium aluminide alloys, commonly referred to as Gamma-Met PX, has been developed by GKSS, Germany. These alloys have been observed to have superior strength and better oxidation resistance at elevated temperatures when compared with conventional gamma titanium aluminides.The present paper discusses results of a study to understand the uniaxial mechanical behavior in both compression and tension of Gamma-Met PX at elevated temperatures and high strain rates. The compression and tensile tests are conducted using a modified Split-Hopkinson Bar apparatus at test temperatures ranging from room temperature to 900 °C and strain rates of up to 3500 s⁻¹. Under uniaxial compression, in the temperature range from room to 600 °C, the flow stress is observed to be nearly independent of test temperature. However, at temperatures higher than 600 °C thermal softening is observed at all strain rates with the rate of thermal softening increasing dramatically between 800 and 900 °C. The room temperature tensile tests show negligible strain-rate dependence on both yield stress and flow stress. With an increase in test temperature from room to 900 °C, the material shows a drop in both yield and flow stress at all levels of plastic strain. However, the measured flow stress is still higher when compared to nickel based super-alloys and other gamma titanium aluminides under similar test conditions. Also, no anomaly in yield stress is observed up to 900 °C.     

Testing the space-shuttle thermal-protection coating

A light-weight insulation material and its protective glassy coating will protect thespace shuttle from temperatures as high as 1250°C (2300°F). The critical performance characteristics of the brittle coating are investigated using testing techniques developed to accommodate these extreme environments and the delicate material. These include an ultimate-strain test-specimen geometry which circumvents problems created by flawed edges, as well as a tension specimen preparation and loading system with which premature failures due to excessive bending moment are avoided. Additionally, an elevated-temperature mechanical strain transducer—useable at more than 870°C (1600°F)—is described. Potential alterations to this sensor are discussed which would make it functional at up to 1600°C (3000°F).     

Finite-strain pressure-tension loading of thick-walled cylinders including temperature effects

Experimental data were obtained from thick-walled cylinders made of hot-rolled SAE 1045 steel at room temperature, annealed OFHC copper at room temperature and at 500°F (260°C), and annealed aluminum alloy 1100 at room temperature and at 305° F (152° C). Experimental pressure-strain curves were compared with curves predicted by two different analytical solutions. One solution is a finite-strain, incremental, compressible analytical solution. The second solution is a finite-strain, total-strain, incompressible analytical solution which was corrected to make it applicable for compressible materials. With both solutions, the material is assumed to be an isotropic-hardening material that obeys the von Mises yield condition. The loading function for the material was obtained from tension specimens tested at the some temperature and loading rate as the thick-walled cylinders. Good agreement was found between each solution and experimental data.     

The Effect of Temperature on Anisotropy Properties of an Aluminium Alloy

The temperature influence on the mechanical behaviour during plastic deformation of an AA5754-O aluminium alloy has been investigated by several experimental tests. First, monotonous tensile tests were carried out from room temperature up to 200°C with a classical tensile machine and with a less conventional testing apparatus involving the heating of the sample by Joule effect. With this second testing apparatus, the strain fields and tensile curves were obtained in function of temperature by means of a non-contacting optical 3D deformation measuring system. Moreover, shear tests were performed in the same temperature range. It is shown that the anisotropy coefficients are rather constant within this temperature range, with a relative variation less than 8%. For both tensile and shear tests, the stress levels are similar at the beginning of straining at room temperature and 150°C, except that the Portevin?CLe Chatelier (PLC) phenomenon disappears at elevated temperature, and then evolves differently. At 200°C, the stress level is clearly below whatever the deformation. In the framework of drawing process, the formability of this alloy at temperatures higher than 150°C seems to be improved.     

Anisotropic Hardening of Sheet Metals at Elevated Temperature: Tension-Compressions Test Development and Validation

New test equipment has been developed to measure the in-plane cyclic behavior of sheet metals at elevated temperatures. The tester has clamping dies with adjustable side force to prevent the sheet specimens from buckling during compressive loading. In addition to the room temperature experiment, cartridge type heaters are inserted in the clamping dies so that the specimen can be heated up to 400 °C during the cyclic tests. For the strain measurement, a non-contact type laser extensometer is used. In order to validate the newly developed test device, the tension-compression (and compression-tension) tests under pre-strains and various temperatures have been performed. As model materials, the aluminum alloy sheet which exhibits a large Bauschinger effect and the magnesium alloy sheet which exhibits different amounts of asymmetry under cyclic loading are used. The developed device can be well-suited to measure the cyclic material behavior, especially the anisotropic and asymmetric hardening of light-weight materials.     

The interaction of creep and fatigue for a rotor steel

Twenty tests were performed on a 1 Cr?1 Mo?1/4 V rotor steel at 1000° F (538°C) to determine the interaction of creep and low-cycle fatigue. These tests involved five different types of strain-controlled cycling: creep at constant tensile stress; linearly varying strain at different frequencies; and hold periods at maximum compressive strain, maximum tensile strain, or both. The experimental data were then used to characterize the interaction of creep and fatigue by the:

1. Frequency-modified strain-range approach of Coffin;
2. Total time to fracture vs. the time of one cycle relation as proposed by Conway and Berling;
3. Total time to fracture vs. the number of cycles to fracture characterization of Ellis and Esztergar;
4. Summation of damage fractions obtained from tests using interspersed creep and fatigue as proposed by the Metal Properties Council;
5. Strain-range-partitioning method of Manson, Halford, and Hirschberg.

In order to properly assess the strain-range-partitioning approach, seven additional tests were performed at the NASA Lewis Research Center. Visual, ultrasonic, and acoustic-emission methods of crackinitiation determination were unsuccessful. An approximate indication of crack initiation was obtained by finding the cycle N_o where the stress-cycle curve first deviated from a constant slope. Predictive methods (based on monotonic tests) for determining the fatigue life in the creep range were examined and found deficient, though they may still be useful for preliminary comparison of materials and temperatures. The extension of the frequency-modified strain-range approach to notched members was developed and the results of notched-bar tests were shown to corroborate this approach, when crack initiation for the plain and notched bars was campared.