铁素体/马氏体双相钢拉伸变形过程中应力应变不均匀性分析

以铁素体/马氏体双相钢为研究对象,经轧制及热处理后进行变形量为5%的拉伸实验,借助配有EBSD成像系统的场发射扫描电镜对实验钢塑性变形后的微区取向进行分析,并结合晶体塑性模拟DAMASK软件模拟其塑性变形行为。结果表明,经轧制变形获得的铁素体/马氏体双相钢,两相应力应变分配不均匀。实验钢中马氏体积累了更多的位错,KAM值更大。临近马氏体区域的铁素体基体Schmid因子更大,可达到0.49,在塑性变形过程中容易优先产生位错和滑移。晶体塑性模拟结果表明,变形初期,铁素体需承担较大的应变、马氏体承担应力以协调整体的变形。当拉伸变形继续发生时,两相应力应变分布的不均匀性将导致两相交界以及晶界处最先发生断裂。     

铺层方式对CFRP-Al单搭接接头胶接性能的影响

使用热压罐制备[45/-45]_4s、[0/90]_4s和[0/45/-45/90]_2s三种铺层方式的CFRP层合板,然后在室温下与Al胶接制备出单搭接试样。使用电子万能试验机、数字图像相关法(DIC)和扫描电子显微镜(SEM)等手段测量胶接接头的拉伸载荷-位移曲线和应变分布并观察断口形貌。基于试验数据分析不同铺层方式下CFRP-Al单搭接接头的拉伸性能,研究了铺层方式对CFRP-Al单搭接接头胶接性能的影响和铺层方式胶接接头的破坏机制。结果表明,在拉伸过程中[45/-45]_4s试样出现塑性变形阶段其拉伸位移最大,而[0/45/-45/90]_2s和[0/90]_4s试样的拉伸位移较小且发生了脆性断裂。铺层方式从[45/-45]_4s到[0/45/-45/90]_2s再到[0/90]_4s,试样的极限载荷和纤维束断裂数量增加、层间剪切力减小、应变集中程度和分层破坏程度降低。     

E玻璃纤维增强环氧树脂基复合材料轴向拉伸力学性能的应变率效应

采用MTS-810材料试验机、Zwick-HTM5020高速拉伸试验机及分离式Hopkinson拉杆（SHTB）实验装置,并结合数字图像相关性（Digital image correlation,DIC）分析方法,对E玻璃纤维增强环氧树脂基复合材料棒材在10^-3~2 400 s^-1应变率范围内的轴向拉伸力学性能进行了较系统的实验研究,获得了不同应变率下材料的应力-应变曲线,揭示了应变率对材料的拉伸强度和断裂应变的影响规律。通过显微分析拉伸试样的断口形貌,揭示了试样的断裂机制及对应变率的依赖性。实验结果表明：E玻璃纤维增强环氧树脂基复合材料的力学性能具有强烈的应变率效应,归一化拉伸强度随着应变率对数线性增加,而归一化断裂应变则随着对数应变率线性减小;断口显微分析显示：E玻璃纤维增强环氧树脂基复合材料的轴向拉伸断裂模式依赖于应变率,低应变率加载下试样发生沿45°方向的剪切断裂,随着应变率增大,试样断裂模式逐渐过渡到沿轴向的拉伸断裂,特别是在高应变加载下,观察到大量的玻璃纤维丝被拉断,同时环氧树脂基体也发生严重的碎裂现象,这反映了基体材料与玻璃纤维之间相互约束作用在增强。     

DP980高强钢动态拉伸力学行为

对比分析DP980高强钢在应变速率10~(-3)~10~3s~(-1)范围内的动态拉伸实验结果,研究其力学行为以及断裂模式特点。结果表明:应变速率从准静态(10~(-3)s~(-1))增加至10~0s~(-1)过程中,强度基本保持不变,塑性下降了7.5%;应变速率从100s~(-1)增加至103s~(-1)过程中,强度不断增大,而塑性在10~0~10~2s~(-1)范围内上升14%,随后在10~2~103s~(-1)范围内下降了24.7%;应变速率敏感系数m始终随应变速率的增加而升高。变形过程中,位错增殖强化和加速阻力是强度上升的主要原因。塑性变形集中在铁素体中,微孔裂纹主要沿马氏体/铁素体交界扩展。试样沿厚度方向上的宏观断口,在应变速率小于101s~(-1)时呈"V"形杯锥状,在应变速率高于10~1s~(-1)时则是与拉伸方向成约45°的纯剪切型。     

选区激光熔化316L不锈钢高应变率压缩下的塑性变形行为

对选区激光熔化316L不锈钢(SLM-316L)的高应变率(1000、2000、3000 s^-1)压缩力学性能进行测试，用扫描电镜和背散射衍射(EBSD)等手段表征冲击加载前后试样的微观结构，并分析晶体结构的差异以及位错滑移、孪生行为等微观变形机制。结果表明：SLM-316L不锈钢在高应变率载荷作用下有显著的应变率强化效应，其微观组织由截面呈不规则多边形的柱状胞晶密排结构组成，高应变率加载使晶体取向的择优性降低、小角度晶界和孪晶界数量增加，且孪晶界在小角度晶界的交叉缠绕区分布密集，试样的塑性变形过程伴随着位错滑移及孪生行为。     

X90高强管线钢的应变时效行为和敏感温度

用光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)观察应变时效处理前后X90高强管线钢的微观组织和冲击断口形貌,进行拉伸实验和Charpy冲击实验测定其拉伸性能和低温冲击性能,研究了这种钢的应变时效行为。结果表明,X90高强管线钢对应变时效比较敏感,敏感温度点为423.15 K。在高于敏感温度点的温度进行时效处理后材料失去连续屈服和强化特性,拉伸曲线由时效前的“Round House”拱顶型转变成为吕德斯型屈服曲线。对于确定的时效时间(t_ag=5 min),随时效温度T_ag的提高X90钢的屈服强度R_p0.2、抗拉强度R_m和屈强比R_p0.2/R_m均呈现提高的趋势,均匀延伸率UEL、断裂应变ε_f、低温冲击吸收总功A_k、裂纹形成功A_i和裂纹扩展功A_p均呈现减小的趋势。时效处理前后这种钢的显微组织没有明显的差异,均为细小针状铁素体+多边形铁素体+板条贝氏体+M-A组元组成的复相组织。预应变和时效处理是管材发生应变时效的主要诱因,生产中可用柔性校平法取代刚性辊压校平法,多步渐进成型法取代一步螺旋成型法控制预应变量;另外,在保证防腐质量的前提下可降低防腐处理温度(小于423.15 K)以降低温度的影响。     

碳/环氧树脂复合材料应变率效应的实验研究

选择两种铺设方式( _S和 _S)的T300/Epoxy(炭纤维/环氧树脂)层合板, 利用MTS试验机以及Hopkinson拉伸杆分别对其进行了准静态拉伸试验(应变率为10-5~10-4 s-1)、 中应变率拉伸试验(应变率为100 ~101s-1)和高速冲击拉伸试验(应变率为102~104s-1)。静态、 动态实验的试件形状及尺寸均相同。获得了不同应变率加载条件下T300/Epoxy的应力-应变曲线。基于所获得的应力-应变曲线, 讨论了应变率对炭纤维增强复合材料力学性能的影响。研究结果表明: 复合材料T300/Epoxy是应变率相关的材料; 层合板的铺设方向对其应变率效应有着显著的影响; 随着应变率的增加, 材料的强度及弹性模量有较大程度的提高, 但破坏应变有所降低。通过对试验结果的数据拟合, 提出了材料应变率相关的动态本构模型。      

针状铁素体钢的动态应力-应变行为

利用Hopkinson压杆技术对X70管线钢进行了冲击压缩实验,研究了在高应变率变形过程中钢的组织演变和动态应力-应变行为.结果表明:经过适当热处理后X70管线钢具有以针状铁素体为主的显微组织.在103s-1应变率条件下,该钢发生了明显的应变强化与应变率强化,且最大应变也随应变率提高而增加;在铁素体板条内形成的大量位错胞亚晶结构和铁素体组织的显著细化,是该钢高应变率增强增塑的主要机制.     

2D-C/SiC复合材料偏轴拉伸力学行为研究

通过对2D-C/SiC复合材料试件进行不同偏轴角度的拉伸实验,研究了偏轴角度对材料拉伸力学特性的影响。通过应变片分别测得了材料加载方向和纤维束方向上的应力-应变行为,对比分析了偏轴角度对上述应力-应变行为的影响;并结合试件断口扫描电镜照片,阐释了纤维束方向上拉伸和剪切损伤间的相互耦合效应。实验结果表明,材料的拉伸模量和强度随偏轴角度的增大出现明显下降;材料纤维束方向上的拉伸损伤和剪切损伤具有显著的相互促进作用。最后,以材料0°拉伸和45°拉伸实验数据为基础,建立了材料的偏轴拉伸应力-应变行为预测模型,模型预测结果与实验结果吻合较好。     

预应变对Nb微合金化09MnNiDR低温钢高温塑性的影响

利用光学和电子显微镜观察断口组织,并测定断面收缩率来研究预拉伸应变对Nb微合金化低温钢09MnNiDR高温塑性的影响。当试样进行5%变形量预拉伸,800℃时断口组织主要为铁素体,晶界处有网状先共析晶界铁素体,断面收缩率为63%;900℃时断口组织主要为贝氏体和针状铁素体以及大量碳氮化物,断面收缩率降至35%;1000℃时断口组织主要为板条马氏体,断面收缩率升至95%。与未进行预应变试样相比,在800～920℃的温度区间,断面收缩率显著降低,预应变明显恶化了Nb微合金化09MnNiDR钢的高温塑性,这主要与预应变促进了尺寸为十几纳米至几十纳米Nb碳氮化物的沿晶界析出有关。     

A Probabilistic Evaluation of the Least Squares Strain Tensor Derived from a Three-Dimensional Modular Strain Rosette Using the Monte-Carlo Technique

Abstract:  Strain gradients give rise to a number of problems in the field of embedded three-dimensional strain measurement. In order to avoid these problems a modular type three-dimensional strain rosette was embedded into known strain fields and the data from the individual gauges compared with theoretical predictions. Finally, the least squares strain tensor was predicted from experimental data analysed using the Monte-Carlo technique and the theoretical results forecast from finite element data taking into account the mechanical properties of the carrier, plug and prismatic bar. Some of the experimental results were found to correlate well with the theoretical values but some values in the least squares strain tensor, in particular under compression and torsional loading, departed considerably from the theoretical values. It was found that the effect of the measurement errors in the individual gauges combined with the matrix operations in the least squares strain tensor were responsible for biasing the resultant tensor data. However, the modular technique provided a solution to the problem of strain gradients.     

Strain Measurement in a Shape Memory Alloy with Strain Gauges

Abstract:  The recovery of shape due to temperature-induced martensite to austenite phase transformation in shape memory alloys (SMA), such as nickel titanium alloys, has been investigated and utilised in applications for many years. One of the problems in incorporating SMA in host materials, such as composites, is the poor bonding characteristics of the SMA. A closely related problem in experiments is the measurement of strains in SMA specimens; as thin wires have been used in many of the experimental investigations, strains have been deduced from length measurements. In the current work, electrical resistance strain gauges have been bonded on SMA strips. Tensile strains up to 8% during tensile loading as well as compressive strains during strain recovery due to subsequent heating have been measured. Strains determined by other methods, such as extensometer and length measurements, are also reported and compared. The major contribution of this work is the successful bonding of strain gauges on SMA and the measurement of large strains (up to 8%).     

Strain measurement by optical correlation

The application of optical correlation measurements to evaluating elastic strain and plastic strain in metals is discussed in this paper. A selected area of the metal surface is illuminated by coherent light. The optical correlation intensity is then measured by transmitting light scattered from the surface through a holographic filter, in which information about the topography of the surface at an earlier time is stored. Changes in surface topography arising from rigid-body displacement, elastic strain, or plastic strain, respectively, cause corresponding changes in optical correlation intensity. Correlation changes arising from surface translation or rotation can be compensated for. An analysis of the process of holographic reconstruction from an elastically strained surface gives good agreement with the experimental results. The correlation technique is sensitive to elastic strains of the order of 10^–5 and to monotonic plastic strains of the order of 10^–4; the change in correlation intensity is essentially linear with increasing plastic strain, up to a maximum strain of about 10^–3.     

A Novel Strain Sensor for Reinforced Concrete Structures

Abstract:  Reinforced concrete (RC) is the most commonly used structural material in civil engineering applications. RC structures have long-term service lives under normal loading conditions; however, overload caused by misuse or statistically remote events such as earthquakes may create damages that, if not detected in time, may eventually cause failure. Hence, it is important to monitor RC structures to take necessary precautions and save human lives. A long-gauge strain (LGS) sensor has been developed to monitor these structures. While it has been developed mainly with concrete applications in mind, the new sensor can also be used in a variety of applications, including measuring strains in pipelines, steel structures, and the like. The proposed sensor system has a very low cost compared with the commercially available competing systems. Prototypes of the proposed strain sensors have been built and calibrated. Test results prove the accuracy, repeatability and reliability of the proposed strain sensor. When the LGS sensor was incorporated into a concrete beam there was very good agreement between the experimental measurement of strain using the LGS sensor when compared with two strain-gauged parallel steel rebars in the same concrete beam.     

The Performance of Three-Dimensional Strain Rosettes Evaluated when Embedded into a Sphere

Abstract:  Two spheres were manufactured, each containing planes of embedded strain gauges that were arranged so that individual gauge readings could be combined in a variety of ways to calculate the least squares strain tensor. Each sphere was tested by loading along its axis of symmetry. It was found that the particular pattern of rosette chosen produced satisfactory predictions of the orthogonal strains but minor off-axis gauge misalignments combined with the matrix operations in the least squares strain tensor led to misleading forecasts of the shearing strains. The method of manufacture used to embed the gauges was largely responsible for these errors. The investigation confirmed that three-dimensional strain rosettes should be made from square plugs and the direction cosines measured on the plug prior to embedment in the model.     

An Examination of Various Methods of Minimising Strain Gradients in 3D Embedded Strain Gauge Analysis

This paper is a review of different methods used to reduce the effect of strain gradients in experimental models with 3D embedded strain gauged transducers. A detailed analysis of various methods used in relation to research carried out on a prosthetic hip implant is investigated. The methods to reduce these gradient effects detail the importance of the selection of the most suitable 3D transducer pattern and establishing the locations on the model where strain gradients are least problematic. It is also shown that the use of large‐scale models helps minimise the effect a gradient may have on data. Another way of avoiding strain gradients is to use axisymmetric models and embed the gauges at the same radius in the model but at a different angular orientation from the central axis. The strain gradient across the 3D embedded strain gauge transducer can be reduced using the methods described here so that significant errors in the final tensor do not materialise.     

Mechanocombinatorially Screening Sensitivity of Stretchable Strain Sensors

Stretchable strain sensors have aroused great interest for their application in human activity recognition, health monitoring, and soft robotics. For various scenarios involving the application of different strain ranges, specific sensitivities need to be developed, due to a trade‐off between sensor sensitivity and stretchability. Traditional stretchable strain sensors are developed based on conductive sensing materials and still lack the function of customizable sensitivity. A novel strategy of mechanocombinatorics is proposed to screen the sensor sensitivity based on mechanically heterogeneous substrates. Strain redistribution over substrates is optimized by mechanics and structure parameters, which gives rise to customizable sensitivity. As a proof of concept, a local illumination method is used to fabricate heterogeneous substrates with customizable mechanics and structure parameters. A library of mechanocombinatorial strain sensors is created for extracting the specific sensitivity. Thus, not only is an effective strategy for screening of sensor sensitivity demonstrated, but a contribution to the mechanocombinatorial strategy for personalized stretchable electronics is also made.     

Strain Measures for Rigid Crushable Foam in Uniaxial Compression

Abstract:  Progressive collapse deformation mechanisms in Rohacell-51WF foam during uniaxial compression has been studied. Measures of a macroscopic engineering strain are identified. The elastic and plastic parts of a macroscopic engineering strain can be predicted by using the compression failure strain, lock-up strain, and time dependent elastic and plastic parts of lock-up strain, which are material parameters. Identification of strain measures in a uniaxial compression test is essential to get material parameters for an elastoplastic model. The viscoelastic recovery property of Rohacell-51WF foam is also described.     

Surface Strain Redistribution on Structured Microfibers to Enhance Sensitivity of Fiber‐Shaped Stretchable Strain Sensors

Fiber‐shaped stretchable strain sensors with small testing areas can be directly woven into textiles. This paves the way for the design of integrated wearable devices capable of obtaining real‐time mechanical feedback for various applications. However, for a simple fiber that undergoes uniform strain distribution during deformation, it is still a big challenge to obtain high sensitivity. Herein, a new strategy, surface strain redistribution, is reported to significantly enhance the sensitivity of fiber‐shaped stretchable strain sensors. A new method of transient thermal curing is used to achieve the large‐scale fabrication of modified elastic microfibers with intrinsic microbeads. The proposed strategy is independent of the active materials utilized and can be universally applied for various active materials. The strategy used here will shift the vision of the sensitivity enhancement method from the active materials design to the mechanical design of the elastic substrate, and the proposed strategy can also be applied to nonfiber‐shaped stretchable strain sensors.     

激光散斑应变仪应用实验

介绍了激光散斑应变仪的功能及技术特性,还详细描述了如何用激光散斑应变仪作材料变形实验,并与传统方法作比较。文章最后给出了一些实验结果。