The influence of dynamic strain aging on resistance to strain reversal as assessed through the Bauschinger effect

The Bauschinger effect of three commercially produced medium carbon bar steels representing different microstructural classes with similar tensile strengths and substantially different yielding and work-hardening behaviors at low-strain was evaluated at room temperature and in situ at temperatures up to 361 °C. The influence of deformation at dynamic strain aging temperatures as a means to produce a more stable dislocation structure was evaluated by measuring the resistance to strain reversal during in situ Bauschinger effect tests. It was shown that the three medium carbon steels exhibited substantial increases in strength at dynamic strain aging temperatures with the peak in flow stress occurring at a test temperature of 260 °C for an engineering strain rate of 10⁻⁴ s⁻¹. Compressive flow stress data following tensile plastic prestrain levels of 0.01, 0.02 and 0.03 increased with an increase in temperature to a range between 260 °C and 309 °C, the temperature range where dynamic strain aging was shown to be most effective. The increased resistance to flow on strain reversal at elevated temperature was attributed to the generation of more stable dislocation structures during prestrain. It is suggested that Bauschinger effect measurements can be used to assess the potential performance of materials in fatigue loading conditions and to identify temperature ranges for processing in applications that utilize non-uniform plastic deformation (e.g. shot peening, deep rolling, etc.) to induce controlled residual stress fields stabilized by the processing at temperatures where dynamic strain aging is active.     

Apparent strains in unmounted strain gauges at high temperature

A detailed investigation into the source of the unmounted apparent strains in resistance strain gauges over a temperature range of 20°C to 550°C is presented. Different measuring systems give different values of unmounted apparent strain. Formulae for two different measuring systems relating the unmounted apparent strains to resistance values are derived. Values computed from these formulae using measured values of resistance are compared with values of unmounted apparent strains measured directly. In addition, formulae which are essential in order to relate the actual strain to the indicated strain are derived for each of the systems. It is shown using the derived expressions that the magnitude of the unmounted apparent strains can be reduced considerably by connecting a shunt resistor across the active element.     

不锈钢焊接凝固裂纹的三维数值模拟

焊接凝固裂纹的产生与否取决于两个方面的因素,材料凝固裂纹阻力和凝固裂纹驱动力,凝固裂纹阻力可以通过实验方法来获得,而凝固裂纹驱动力主要采用有限元计算方法得到.本文通过对SUS310不锈钢,在裂纹敏感温度区间内,焊缝金属动态应力应变场演变过程三维模拟计算的研究,得到了10 mm厚板的凝固裂纹驱动力曲线.利用动态单元再生技术,解决了焊接凝固过程的三个因素-焊接熔池的变形、初始温度的变化和凝固收缩的影响.并且模拟计算得到的驱动力与实验测量的凝固裂纹阻力曲线进行了比较,预测了焊缝金属的凝固裂纹.     

Demodulation scheme for fiber bragg grating sensors based on active control of the spectral response of a wavelength division multiplexer

We present a closed-loop technique for measuring wavelength shifts associated with fiber Bragg gratings by using a fused biconical wavelength division multiplexer (WDM). The spectral response of the WDM is actively tuned by stretching of the coupling region to maintain a fixed coupling ratio at the reflected Bragg wavelength. The closed-loop operation allows sensitivities usually associated with a highly selective WDM to be obtained without compromising the measurement range. A simple theoretical model is presented together with experimental results for temperature and strain measurements.     

Experimental study on the dynamic tensile behavior of a poly-crystal pure titanium at elevated temperatures

A new technique for measuring dynamic tensile behavior of metallic materials at elevated temperatures was developed. This technique employs a rapid contact heating method to obtain a stable and nearly homogenous high temperature field in the testing gage of the specimen. As an application of this new technique, a commercially pure titanium (CP-Ti) was tested in the strain rate range of 300 s⁻¹–1400 s⁻¹ and in a temperature range of 298 K–973 K. Quasi-static experiments (10⁻³ s⁻¹, 10⁻² s⁻¹) were also performed in the same temperature range for comparison. The testing results indicated that both temperature and strain rate have pronounced influence on the mechanical behavior of CP-Ti.     

温度应变同时测量的光纤光栅系统的研究

为了同时传感温度和应变,在同一根光纤的两个不同地方写入相同的Bragg光栅,设计制成一种特殊机构的传感系统。由于测量结构特殊,两处光纤光栅的反射峰具有成比例的应变响应系数和不同的温度响应,测出温度后,应变也同时算出。实际测量表明,该系统结构简单,测量精度高,能进行实时测量。     

Thermomechanical treatment of 2014 aluminium alloy

Abstract

The effect of thermomechanical treatment on the flow stress, fracture strain, structure, and precipitation behaviour of commercial grade 2014 aluminium alloy has been investigated. Specimens in the supersaturated and aged conditions were plastically deformed in torsion tests in the temperature range 293–493 k and strain rate range 2·8 ×10^?3?2·5 s^?1. It is stated that the starting condition of the alloy acts dominantly on the flow stress, fracture strain, and thermally activated processes, which take place during aging. An increase in temperature results mainly in a reduction of flow stress in the aged alloy and an increase in flow stress in the supersaturated alloy. The supersaturated alloy exhibits negative strain rate sensitivity over the entire range of applied temperature while for the aged alloy it is exhibited only in the temperature range 293–393 K. The effect of temperature and strain rate on the fracture strain of the supersaturated alloy is negligible, but the fracture strain of the aged alloy increases with temperature and decreases with strain rate. In the supersaturated alloy, the process of strain aging is dominant during deformation at room temperature and at higher temperatures precipitation aging and recovery are dominant. In the aged alloy, strain aging is dominant in the temperature range 293–443 K and recovery is dominant only at the highest test temperature (493 K).

MST/616     

Local Measurement of Stress–Strain Behaviour of Ductile Materials at Elevated Temperatures in a Split‐Hopkinson Tension Bar System

A split‐Hopkinson tension bar system is modified to allow for measuring the stress–strain behaviour of ductile materials at large strains, high strain rates and elevated temperatures. The specimen is heated by induction, and a pyrometer provides a laser‐based temperature measurement that controls the testing temperature in a feed‐back loop. A high‐speed digital camera system and an edge detection algorithm are used to obtain local measures of strain after necking of the axisymmetric specimens. Using the local strain measurements and Bridgman's analytical formulas, it is feasible to find the equivalent stress–strain curve to fracture for different levels of strain rate and temperature. Thermal and thermo‐mechanical finite element simulations of the test set‐up are used to evaluate the validity of the proposed experimental method.     

Modern fibre optic sensors

Fibre optic sensors are currently used as transducers for many physical phenomena. Variations on the scheme of measuring a small gap in real time with Fabry-Perot interferometry yield sensors for local strain, extension between points, crack growth, fluid pressure, temperature, surface shear and acoustic emission in solids. Chemical changes can also be monitored with long period gratings using affinity coatings. The embedding of fibre optic elements in structural materials presents a technical challenge in the use of fibre optic sensors. Fibre egress has been addressed by the use of strain relieving sheathing and the introduction of wireless telemetry. The smart footbridge at the University of Missouri — Rolla highlights the application of fibre optic sensors in civil structures. This bridge is equipped with extrinsic Fabry-Perot interferometer sensors measuring flexural strain, shear strain and temperature, providing year round information from the structure.     

DETERMINATION BY THREE METHODS OF YOUNG'S MODULUS OF A 0.2 MM TITANIUM ALLOY STRIP

The value of Young's modulus of a 0–216 mm (0–0085 in) thick sheet of weldable titanium alloy (13% V, 11% Cr, 3% A1) was measured at room temperature by three different methods simultaneously. The results and their analysis give insight into the merits of strain measuring standards at present available at FFA and of the specialised strain measuring methods used at FFA and already described (Ref. 1).     

高温下混凝土SHPB动态力学性能试验研究

采用霍普金森压杆(Split Hopkinson pressure bar,SHPB)试验装置和高温炉进行了高温下普通混凝土的抗冲击性能试验研究,通过比较实测动态强度和应力-应变曲线,揭示了温度和应变率对高温下混凝土动态力学性能的影响规律.试验结果表明：除200℃~300℃范围外,高温下混凝土具有显著的应变率效应,且温度越高动态应变率效应越显著;温度和应变率对混凝土归一化应力-应变关系曲线上升段的形状影响很小,可采用统一的函数形式.200℃~300℃低温区混凝土不仅无明显动态效应,且存在静力强度衰减现象,此特性需引起注意并有待进一步研究.     

温度和应变率对原位合成钛基复合材料动态压缩性能及破坏机制的影响

使用分离式Hopkinson压杆（SHPB）系统,在温度293~973 K、应变率6 000~10 000 s-1下,对原位合成TiC颗粒和TiB晶须混合增强钛基复合材料（TMCs）的动态压缩性能进行了研究。试验结果表明:在373~573 K、673~773 K和873~973 K范围内TMCs流变应力随温度的增加而显著减小;在较低温度（低于373 K）和较低应变率（6 000~8 000 s-1）下,TMCs呈现小幅的应变率硬化特征,而在较高温度（573 K及以上）时各应变率下TMCs均存在应变率软化特征,且温度越高材料应变率软化效应越明显。材料失效/断裂机制分析表明:应变率软化机制主要是绝热软化及其产生的绝热剪切带（ABS）中微裂纹的形成和扩展的综合作用;在较高的应变率和较大应变下ABS中会产生微裂纹,温度较低时TMCs塑性不足以抑制或阻碍微裂纹的扩展,从而导致TMCs在宏观上迅速破坏;材料破坏时以钛合金基体塑性断裂为主,但在局部伴随部分增强相脆性断裂。      

Temperature and strain rate effect on tensile properties of 9Cr–1·8W–0·5Mo–VNb steel

Abstract

Tensile tests have been carried out on 9Cr–1·8W–0·5Mo–VNb steel (grade 92) over wide ranges of temperature (300–923 K) and strain rate (3×10^?3–3×10^?5 s^?1). The tensile strength of the steel decreased slowly with temperature at relatively lower temperature range, whereas rapidly in the higher temperature range with a plateau in the intermediate temperature range. The decrease in strain rate decreased the tensile strength of the steel both at lower and higher temperature ranges. Elongation to fracture and reduction in area increased with increase in temperatures and decrease in strain rate at higher temperature regime with a plateau in the intermediate temperature regime. The ductile mode of tensile failure has been observed in the investigated temperatures and strain rates. The plateau in the variation of tensile strength with temperature, the negative strain rate sensitivity of tensile strength and minimum in ductility of the steel in the intermediate temperature range are considered as a consequence of dynamic strain ageing. The rapid decrease in tensile strengths and increase in ductility at high temperatures have been attributed to the dynamic recovery.     

Verification of the duplex gauge using variable strains and temperatures

Four steel specimens were equipped with "duplex nances", a recently proposed transducer based on two different electrical resistance strain gauges, capable of correcting the effects induced by temperature. The examined innovative transducer, in comparison with traditional strain measuring systems, has the advantage of utilising two strain gauge channels of the same type, which significantly simplifies automatic measuring systems. The specimens were subjected to temperature and strain field variations in the ranges of=30–150°Cand=100–700μm/m respectively The temperature and strain values obtained from the duplex gauges were compared with those measured by means of traditional methodology, i.e. a strain gauge and a thermocouple.     

The time and strain required for development of minimum strain rates in ice

Uniaxial compression tests were carried out on laboratory-made polycrystalline ice samples with randomly oriented grains. Tests covered the temperature range ?5.0°C to ?32.5°C and the octahedral stress range 0.5 to 12 bar.Plots on log-log scales of strain rate against time and against total octahedral strain for the experiments, reveal that within these temperature and stress ranges, a minimum strain rate occurs after a time determined by a straight line locus, and at a total octahedral strain of about 0.6%.Some consequences for further studies of minimum flow rates at lower temperatures and stresses are outlined.     

High-temperature low-cycle fatigue of Alloy 800H

Low-cycle fatigue tests have been carried out on Alloy 800H in the temperature range 22–760 °C with either diametral strain measurement with conversion to axial strain or direct axial strain measurement. The axial strain was cycled between equal positive and negative values in the range of 2–0.3%. The number of cycles to failure as a function of the plastic strain range deviated from the Coffin–Manson Law at the lower strain ranges. At one temperature where tests were interrupted for intermittent SEM examination of the specimen surfaces, cracks were detected very early in the test. Determination of crack propagation rates by measurement of striation widths on the fracture surfaces allowed comparison of these rates with ΔJ values for the stress–strain hysteresis loops.     

THERMAL-MECHANICAL LOW-CYCLE FATIGUE UNDER CREEP-FATIGUE INTERACTION ON TYPE 304 STAINLESS STEELS

Abstract— In this study, in-phase and out-of-phase thermal fatigue tests at the temperature ranges of 473–823 and 573–873 K were carried out on three kinds of 304 stainless steel as well as isothermal low-cycle fatigue tests at 823 and 873 K, in order to investigate the properties of thermal fatigue strength under creep-fatigue interaction. Based on the relation between the fatigue life and the failure mode, the time-dependent effect on the fatigue life was discussed. Also, an attempt was made to apply the strain range partitioning method to the thermal fatigue life prediction. It is difficult to evaluate the thermal fatigue life at high temperatures simply from the isothermal fatigue life under the same strain condition. It was also found that an unbalanced creep strain during tensile loading, which increased the number of intergranular cracks, gave the largest damage to the material. By the strain range partitioning method, it was possible to predict the isothermal fatigue life and the thermal fatigue life at the low temperature range within a factor of 1·5. On the other hand, the thermal fatigue life at the high temperature range could only be predicted within a factor of 3. However, further detailed investigations are required on the technique of partitioning the inelastic strain range and predicting the effects of dynamic strain ageing and recovery during strain holds.     

Substitutional dynamic strain ageing in an iron — 1.1 at. % niobium alloy

The mechanical properties of an Fe-1.1 at. % Nb alloy have been studied in compression over the temperature range 300 to 1100°K. The substitutional niobium atoms were responsible for dynamic strain ageing which resulted in a small peak in the temperature dependence of the flow stress, negative strain rate sensitivity, and serrated stress strain curves.The serrations were preceded by a strain rate and temperature dependent critical strain. These dependencies were analysed using theories that have been successfully applied to substitutional strain ageing in fcc structures. The analysis showed that, unlike in fcc structures, the apparent activation energy for the onset of serrated flow increased at the faster strain rates; this was attributed to the vacancies produced by the plastic deformation rapidly annealing out due to the high temperatures involved at the faster strain rates.     

Microstructure evolution of ZK40 magnesium alloy during high strain rate compression deformation at elevated temperatures

Abstract

Microstructure evolution of the homogenised ZK40 magnesium alloy was investigated during compression in the temperature range of 250–400°C and at the strain rate range of 0·01–50 s^?1. At a higher strain rate (?10 s^?1), dynamic recrystallisation developed extensively at grain boundaries and twins, resulting in a more homogeneous microstructure than the other conditions. The hot deformation characteristics of ZK40 exhibited an abnormal relationship with the strain rate, i.e., the hot workability increased with increasing the strain rate. However, the dynamic recrystallisation grain size was almost the same with increasing the temperature at the strain rate of 10 s^?1, while it increased obviously at the strain rates of 20 and 50 s^?1. Therefore, hot deformation at the strain rate of 10 s^?1 and temperature range of 250–400°C was desirable and feasible for the ZK40 alloy.     

Flexural composite oscillators for the measurement of anelastic and elastic properties of solids at frequencies of 1 to 10 kHz

Longitudinal composite oscillators for measuring internal friction, piezoelectric modulus, and strain modulation effects are usually limited to a frequency range of 30 to 200 kHz. If the same crystals are vibrated in flexure, a longitudinal strain can be introduced with the resonance frequency below 3 kHz while at the same time keeping the inherent high Q of the composite system. This paper develops the theory for the strain amplitude and damping for the flexural composite oscillator made up of two quartz crystals plus specimen and, if appropriate, spacers. This high Q technique of vibrating in flexure has applications for strain modulation and damping experiments.