圆棒试样单向拉伸试验颈缩阶段试样形状的数学模型

在分析圆棒试样单向拉伸试验颈缩阶段试样形状特征的基础上建立了描述试样形状的数学模型,即试样自由表面上任一点所在垂直于中心轴线截面的半径r关于该截面与颈缩底部最小截面间的距离z(z≥0)的分布函数为r=${r_n} + \frac{{{r_c} - {r_n}}}{{1 + {{(\frac{z}{{{z_1}}})}^{{p_1}}} + {{(\frac{z}{{{z_2}}})}^{{p_2}}}}}$,在以颈缩底部最小截面中心为原点、中心轴线为z轴的直角坐标系中,试样自由表面的曲面函数为√x2+y2=${r_n} + \frac{{{r_c} - {r_n}}}{{1 + {{(\frac{z}{{{z_1}}})}^{{p_1}}} + {{(\frac{z}{{{z_2}}})}^{{p_2}}}}}$;在该数学模型中,试样形状可以通过6个特征参数(rc、rn、z1、z2、p1、p2)来表征。一种低合金钢圆棒试样单向拉伸试验颈缩形状实测数据和3种不同的各向同性均质弹塑性材料圆棒试样单向拉伸试验数值模拟结果验证了上述数学模型的有效性。     

DP600双相钢单向拉伸的交叉颈缩现象与有限元分析

目的探究DP600双相钢单向拉伸时交叉颈缩现象的产生原因。方法在单轴拉伸实验的基础上,利用ABAQUS有限元软件对拉伸过程进行模拟,对交叉颈缩区域进行应力、应变分析。结果即将发生断裂时刻,试样标距内中心位置应力达到最大值,临近中心位置两侧由于"局部卸载"而出现应力大幅减小的情况。结论试样标距内两侧距离中心越远的位置,越接近于单轴应力状态。DP600板材裂纹萌生时,与断裂发生部位越接近,其应力状态与平面应变状态越相似,沿着颈缩线方向基本没有应变产生,材料主要由颈缩线区域的板材厚向减薄颈缩线加宽补偿垂直于颈缩线方向上的拉伸。     

PA6拉伸过程中颈缩力学性能及红外辐射分析

通过万能试验机对尼龙6(PA6)试件进行单轴拉伸实验,同时用红外热像仪测试试件在拉伸变形过程中表面的温度场变化。在10mm/min、20mm/min、50mm/min三种加载速率下分析PA6颈缩形成阶段与发展阶段的应力与应变,最大红外辐射温度与时间和塑性功之间的关系。结果表明:加载速率越大,颈缩形成阶段红外辐射温度增幅越高,颈缩发展阶段的温度变化越大,塑性功转化为红外辐射能的速率越快。     

基于I1断裂准则的圆棒试样单轴拉伸断裂过程分析

基于I₁断裂准则开展了两种材料模型圆棒试样单轴拉伸断裂过程的有限元模拟,结果表明：颈缩变形阶段,最小截面中心位置最先形成裂纹,在裂纹形成瞬间失稳扩展成一定尺寸的圆形裂纹后停止扩展,在后续持续增大的位移载荷作用下,裂纹步进式扩展;裂纹的形成与失稳扩展伴随着轴向作用力的急剧降低,利用该特征可测定材料的断裂强度I_b。     

各向同性均质弹塑性材料圆棒试样单向拉伸试验变形特性与应力应变本构关系的相关性分析

针对各向同性均质弹塑性材料圆棒试样单向拉伸试验,分别从理论上以及采用有限元法分析了变形特性与应力应变本构关系的相关性,探讨了形成颈缩的必要条件及实际颈缩形貌的形成过程。结果表明,等效塑性应变ε_p达到一定临界值后,流变应力σ₀不再随等效塑性应变ε_p的增大而增大的材料的应力应变本构特性,以及试样为非理想圆柱体是产生颈缩的必要条件,颈缩后期内部形成断裂是导致颈缩区最小截面位置弧形轮廓线曲率半径较小的原因。     

材料模型对1Cr18Ni9Ti管材拉伸有限元仿真的影响

为研究材料模型对有限元模拟1Cr18Ni9Ti管拉伸的影响,将管材单向拉伸试验获取的真实应力应变曲线分别拟合成线性硬化和指数硬化材料模型,并用于有限元模拟。经对比分析认为,采用真实应力应变模型的分析结果与实验结果吻合良好,并能正确显示出颈缩发生时刻和颈缩形状;采用指数硬化模型的有限元模拟结果接近真实应力应变模型,颈缩区应力应变分布略显分散;采用线性硬化模型的有限元模拟结果未能显示实际管拉伸后期的局部颈缩形状。     

金属拉伸试验中控制应力速率的途径

通过对金属拉伸试验中拉伸力速率与应力速率之间的关系的分析,得出了用控制拉伸力速率或拉伸时间的途径来控制应力速率,能适应国标GB228-87的状况。     

颈缩现象的ANSYS模拟

在ANSYS10.0平台上,选用solid5单元(耦合单元),采用有限元方法对1/2的三维工字形平板试件直接进行热力耦合分析,并选取不同的拉伸位移来模拟出效果较好的颈缩现象。     

获取金属材料颈缩后真实本构关系的数值迭代方法

为获取金属材料颈缩后的真实本构关系,提出了一种新型的试验-数值耦合方法,即NE(Numerical and experimental)法。该方法的基本原理是将单轴拉伸试验的荷载-位移曲线作为目标,通过有限元迭代运算,不断修正输入的应力-应变曲线,直至荷载-位移曲线的模拟结果与试验结果完全吻合或达到误差要求时,即得到真实应力-应变曲线。基于该原理,开发了一套数值模拟程序CONST以实现测试技术的自动化并节省操作时间。为验证NE法的有效性,将传统理论计算方法与NE法获得的材料真实本构关系进行对比,并对圆棒和矩形试件分别进行拉伸模拟,结果表明NE法对两种试件均适用,且NE法获得的荷载-位移曲线、颈缩变形特征等较理论计算方法更为精确。所提方法解决了金属材料单轴拉伸颈缩至断裂期间获取真实应力-应变曲线存在的困难,对材料性能评价及结构变形分析具有重要的理论及工程应用价值。     

Cohesive Fracture Model Based on Necking

A linear hardening model together with a linear elastic background material is first used to discuss some aspects of the mathematical and physical limitations and constraints on cohesive laws. Using an integral equation approach together with the cohesive crack assumption, it is found that in order to remove the stress singularity at the tip of the cohesive zone, the cohesive law must have a nonzero traction at the initial zero opening displacement. A cohesive zone model for ductile metals is then derived based on necking in thin cracked sheets. With this model, the cohesive behavior including peak cohesive traction, cohesive energy density and shape of the cohesive traction–separation curve is discussed. The peak cohesive traction is found to vary from 1.15 times the yield stress for perfectly plastic materials to about 2.5 times the yield stress for modest hardening materials (power hardening exponent of 0.2). The cohesive energy density depends on the critical relative plate thickness reduction at the root of the neck at crack initiation, which needs to be determined by experiments. Finally, an elastic background medium with a center crack is employed to re-examine the shape effect of cohesive traction–separation curve, and the relation between the linear elastic fracture mechanics (LEFM) and cohesive zone models by considering the cohesive zone development and crack growth in the infinite elastic medium. It is shown that the shape of the cohesive curve does affect the cohesive zone size and the apparent energy release rate of LEFM for the crack growth in the elastic background material. The apparent energy release rate of LEFM approaches the cohesive energy density when the crack extends significantly longer than the characteristic length of the cohesive zone.     

上拉杆螺管缩口工艺及模具设计

通过对上拉杆螺管加工工艺进行分析,提出了采用两端一次缩口工艺,给出了坯料尺寸及缩口力的计算,介绍了缩口模的设计,并对生产中出现的问题,提出了解决措施,从而保证了该零件缩口的顺利进行。     

A Closer Look at the Diffuse and Localised Necking of A Metallic Thin Sheet: Evolution of the Two Bands Pattern

Plastic strain localisation in a sheet specimen was monitored by electronic speckle pattern interferometry during uniaxial tensile tests. The experiments were carried on in the diffuse and localised necking stages until fracture. A kinematic model, which is independent of material characteristics, was used to describe the whole strain rate field with two crossing localisation bands inclined with respect to the tensile direction. Then, the physical features of localisation, such as the width of the two bands, their inclination angles and their maximum strain rates are identified by least‐square from the displacements fields and their evolutions are followed from the onset of diffuse necking up to the failure. In particular, the effect of the average strain rate is considered and bandwidth evolution is analysed in detail. It was found that:

• The band structure appears early, as soon as diffuse necking starts;
• The separation, in terms of strain rate or bandwidth, of the two bands corresponds to the transition between diffuse and localised necking. The localised necking stage can be divided into two sub‐stages: in the first one, the two bands continue to evolve but at different rates, and in the second one, one of the bands stabilises. The transition between the two sub‐stages is influenced by the crossbeam velocity;
• The inclination of the band leading to fracture remains quite stable, while the other rotates towards a situation perpendicular to the tensile direction;
• The band width decreases exponentially versus the maximum local strain. The two bands follow the same evolution path, but one of them progressively lags behind the other until it stops deforming.
• Although the average strain rate was only varied by a factor two, it was found that, when the strain rate increases, the two bands stay together longer and thus that the onset of localised necking is delayed.

     

样品取向对轧制钢板拉伸性能的影响

分别对不同厚度、不同取向(α=0°,45°,90°)的SPCC冷轧钢板、SPHC热轧钢板、SUS304冷轧不锈钢板和SUS304热轧不锈钢板进行了拉伸试验,研究了取向因素对轧制钢板拉伸性能的影响,并分别探讨了钢板厚度、轧制状态和钢板材料等因素引起的拉伸性能各向异性程度的差异。结果表明:样品取向对轧制钢板的拉伸性能有明显影响,轧制方向的抗拉强度和断后伸长率较高,而屈服强度的变化规律则较为复杂;钢板厚度、轧制状态、钢板材料等因素对轧制钢板拉伸性能的各向异性都有不同程度的影响。     

试样加工对金属材料拉伸性能的影响

通过对大量拉伸试验数据进行分析比较,从样坯切取和试样制备两大方面分析了试样加工对金属材料拉伸性能的影响。结果表明:取样方向、取样位置、取样方法、试样形状、试样尺寸以及试样制备方法等都不同程度地影响着金属材料的拉伸性能,在实际检测工作中应正确认识这些因素对拉伸试验结果的影响,拉伸试验前应选择正确的取样部位和取样方向,加工成规定横截面和形状的试样,并避免试样在加工过程中受到热影响和加工硬化影响,同时还应尽可能提高试样的加工精度。     

Tensile testing of silicon thin films

The tensile strength of silicon thin films was investigated using a specimen chucking system dedicated for microscale specimens. The system uses electrostatic force to fix and hold the free end of the cantilever‐shaped specimens. The thin film tensile tester was built using this system. The accuracy and reliability of this method were assured by comparing it with other tensile‐testing methods using single‐crystal silicon specimens. The result shows good agreement between the testing methods. The strength properties of polysilicon thin films, such as the effect of the testing environment and the specimen size and the film fabrication conditions, were investigated.     

焊管焊接接头拉伸试验相关标准的讨论

焊管常用标准对焊缝接头拉伸试样是否去除焊缝余高要求不一。本文从焊接接头拉伸的实际意义出发,探讨了是否去除焊缝余高及断裂位置对试验结果的影响。不去除焊缝余高的情况,更偏重于焊缝结构的整体强度,而不是严格意义上的焊接接头的抗拉强度。为避免分歧及便于对比,西气东输二线管道工程技术条件中统一规定了焊接接头拉伸试验必须去除焊缝余高,且报告断裂位置。     

GB/T228.1-2010中的拉伸试验速率及其控制

阐述了在拉伸试验中,试验速率对材料屈服性能表现出规律性的影响。我国历次拉伸试验标准(GB/T 228)有关试验速率的理论规定都是以应变速率为标准,但在实际操作中又都是以等效的应力速率为标准;而在新标准GB/T 228.1-2010中,则从理论规定到实际操作均实现了以应变速率为标准的方法。另外还讨论了拉伸试验速率的控制方法,以及试验速率控制与试验设备的关系。