Changes in mechanical properties in metal-forming processes

This article presents a paradigm for the mechanical properties of a metal that result from the plastic deformation that occurs in forming processes, including multiple-step or cyclic processes. Some of the new concepts included are a new strength designation that includes the sense (tensile or compressive), the direction (longitudinal, transverse, etc.), the specific type of deformation (tensile, compressive, etc.); a concept of an equivalent strain; and the apparent rules of strain strengthening. Examples of strength     

关于焊接残余应力形成机制的探讨

分析了关于焊接残余应力形成过程描述的传统观点的局限性和不足,指出材料力学的“截面法”不能用于分析横向残余应力分量。认为在不考虑材科相变的前提下,焊缝金属冷却时收缩受制也是导致焊接残余应力产生的重要原因。虽然残余压缩应变和残余收缩应变在导致焊接残余应力产生的作用方面是等价的,但其机理却有本质的不同,区别二者有利于研究和开发新的焊接残余应力调控技术。对固有应变理论进行了分析和补充,认为应统一到拉伸塑性应变上。用钢板上堆敷锡钎焊焊道的方法,在母材温升值低于塑性应变所需温差的条件下,由盲孔法测得了板上大范围分布的高值残余应力,且因焊道和母材的线膨胀系数差异较大,其应变测试值与环境温度有关。     

圆柱螺旋弹簧的正断/切断型疲劳断裂模式与提高其疲劳断裂抗力的途径

结合承受扭转切应力和轴向正应力圆柱体的受力分析,讨论了圆柱螺旋弹簧发生的正应力断裂,纵向以及横向切应力断裂等3种疲劳断裂模式。结果指出,喷丸引入的残余正应力(即应力强化机制)只影响正应力而不影响纵/横向切应力断裂模式的疲劳强度(寿命)。但喷丸引起表面形变层内的组织结构改性(即组织结构强化机制)却能提高所有3种断裂模式的疲劳断裂抗力。     

钛合金焊接过程应力应变特点分析

采用有限元分析和试验测试相结合的方法对钛合金薄板钨极氩弧焊的焊接应力应变进行分析.结果表明,在焊接不均匀加热和冷却的综合过程中,熔池金属是在周边已经产生了压缩塑性应变的框架内熔化,并在已经发生了压缩塑性应变的近缝区框架内冷却.焊缝冷却过程中,在"力学熔化"区间,有拉伸塑性应变的卸载过程,但残余状态仍保留了压缩塑性应变.焊缝在冷却、凝固降温过程中,其纵向拉应力的峰值始终低于材料在相应温度下的屈服强度,直至残余状态.     

一种多轴循环本构关系预测方法

该文研究表明,非比例加载下材料多轴循环本构关系中,塑性应变不仅与应力状态有关,还与其应变路径密切相关。根据材料在多轴非比例加载下产生附加强化的机理,基于临界平面法,分析和计算能够表征与应变路径相关性的塑性应变能;在Osgood-Ramberg方程的基础上,结合塑性应变能和利用von-Mises准则合成的等效应变,对多轴循环等效应力应变关系中的循环强度系数和循环应变硬化指数进行修正,并考虑了平均应变的影响;提出一种能反映应变路径相关性的多轴循环本构关系的预测方法。该方法基于材料产生塑性变形和疲劳损伤的机理,考虑了应力状态和应变路径对多轴循环本构关系的影响,并利用45钢和40CrMo钢,在多种典型应变路径下的疲劳试验数据,对该方法的准确性进行验证。结果表明,该方法能同时适用于比例与非比例加载,多轴循环稳态应力响应的预测结果与试验结果吻合良好,方法简单,便于工程应用。     

等截面侧向挤压的力学分析

等截面侧向挤压是实现材料纯剪切变形的有效方法。通过网格实验研究了等截面侧向挤压过程中材料的变形情况 ,结果表明塑性变形区仅位于模具通道拐点连线附近。通过几何分析推导出等截面侧向挤压等效应变计算公式 ,通过上限分析推导出等截面侧向挤压变形力计算公式 ,理论计算结果与实测值进行了对比。     

管材内压液力成形的稳定性分析

管材内压液力成形过程中 ,管坯在内压与轴向压力的共同作用下发生变形。成形过程中常见的缺陷是起皱和破裂 ,这两种缺陷的产生都和管的失稳有关。本文运用薄膜理论分析了管坯在轴向力单独作用下的压缩失稳 ,得出临界轴向压力的计算公式 ;从失稳应变强度入手 ,分析了圆柱管在内压和轴向压力共同作用下的分散性失稳 ,根据失稳应变强度的计算公式 ,分析了应力状态系数 a、厚向异性指数 rh、应变强化指数 n等因素对管的失稳应变强度的影响规律     

钛合金带热沉钨极氩弧焊中热沉作用

采用数值模拟和实验相结合的方法研究了钛合金TC4薄板常规及带热沉的钨极氩弧焊焊接过程中温度及应力应变的分布,考察了热沉对温度场和应力应变场的影响规律,探讨了使用该技术实现应力和变形控制的机理.结果表明:带热沉的钨极氩弧焊焊接过程中,紧随热源之后热沉急冷作用使得试件形成马鞍形温度场,而热沉作用部位温度最低.热沉作用部位的急冷收缩对周围金属产生拉伸作用,使得焊缝及近缝区金属升温过程中产生的压缩塑性应变减小,冷却过程中产生的拉伸塑性应变增大,接头中不协调应变减小,残余应力降低.实验测量与有限元模拟结果吻合良好,证实了采用热沉控制应力与变形的有效性和有限元模型的正确性.     

Strengthening of NiAl nanofilms by introducing internal stresses

Molecular dynamics simulations of notched NiAl nanofilms tension are carried out. The stress–strain curves are calculated for a dislocation-free nanofilm and for nanofilms with periodic arrays of prismatic dislocations introduced in order to create tensile internal stress in the inner part of the nanofilms and compressive stress near their surfaces. It is demonstrated that under uniaxial tensile load the nanofilms with the dislocation loops can show higher strength, strain to failure, and energy to failure compared to the dislocation-free nanofilm. Larger strength of the nanofilms with dislocations is naturally explained by the compressive internal stress at the surfaces which detains crack initiation at the notch under tensile loading. Increase in the strain and energy to failure is due to the particular mechanism of elastic strain observed for the nanofilm. There exists a domain of strain where homogeneous deformation of the nanofilm is thermodynamically unstable. As a result, domains with larger elastic strain appear and elastic deformation of the nanofilm occurs at practically constant stress by growth of the domains with larger elastic strain in expense of the domains with smaller elastic strain. We believe that this mechanism of non-homogeneous elastic deformation is due to competing interaction of atoms of different sorts and thus, it cannot be realized in pure metals but can happen in ordered alloys and intermetallic compounds. Our results demonstrate that strengthening by introducing internal stresses, widely used for macroscopic structures, can also be applied for nanomaterials such as nanofilms and nanowires.     

DEFORM模拟支撑辊的锻造

轧机中的关键零件之一支撑辊在轧制设备运行的过程中承受轧辊传来的交变应力。大型支撑辊由大型钢锭锻造而成,但大型钢锭中存在种种缺陷,给支撑辊的质量带来很多问题。针对此问题,采用WHF法能够锻合锻件内部组织缺陷,可改善锻件质量。采用DEFORM软件,模拟WHF法拔长,研究WHF法在锻造拔长过程中对锻件心部的作用。实验研究表明,WHF法在拔长的第3道次时,锻件心部的应力状态由拉应力完全转变为压应力。同时,到第6道次后,锻件心部等效应变达到1.2。通过锻件心部压应力和等效应变值可知,WHF法能够完全破碎锻件内部的组织缺陷,生产出质量合格的支撑辊锻件。     

高应变速率下钨丝增强锆基块体非晶合金复合材料的变形行为

利用压力浸渗制备出钨丝增强Zr_(41．25)Ti_(13．75)Ni_(10)CU_(12．5)Be_(22．5)块体非晶合金复合材料,采用应变速率为1×10~(-4) s~(-1)的准静态压缩实验及应变速率为2×10~3s~(-1)的动态压缩实验的方法,研究了在动载荷作用下该复合材料的力学性能．结果表明,准静态压缩时,复合材料的强度约为1980 MPa,与单一块体非晶合金相比并无显著提高．而塑性提高约5倍,达到11．5%；动态压缩时,复合材料的最大抗压强度升至约2648 MPa,塑性则在1．8%—7．5%之间,复合材料的应变速率敏感指数为0．022．在准静态压缩下,复合材料的抗压强度受到残余热应力及钨丝失稳弯曲极限压应力的影响；在动态压缩下,除了热应力的影响外,还受到钨丝剪切断裂以及复合材料正弦型弯曲行为的影响．后两者使复合材料的抗压强度在动态加载条件下升高．     

楔横轧二次楔轧制内部孔腔产生原因分析

针对楔横轧二次楔轧制比单次楔轧制更容易产生内部缺陷的事实,通过在H630轧机上实验轧制进行了验证。并利用有限元法比较二次轧制与一次轧制的轧件的应力特点,分析楔横轧轧件的内部应力应变场及其历史演变过程。解释了楔横轧二次楔轧制容易产生内部孔腔缺陷原因是相对于单次楔轧制二次轧制受的横向拉应力、最大剪应力和最大剪应变值较大,交变次数较大;轧件在二次楔轧制时的塑性降低,应力和应变增大。     

热沉影响钛合金薄板焊接残余应力的试验分析

采用切条应力释放法测量了钛合金TC4薄板常规钨极氩弧焊(GTAW)和动态控制低应力无变形GTAW对接试件中的纵向残余应力和纵向残余塑性应变的分布。测量结果表明,钛合金常规GTAW缝中残余拉应力峰值小于其母材屈服强度,焊缝附近存在残余压缩塑性应变;动态控制低应力无变形GTAW焊技术中热沉的冷却作用使得热源与热沉之间的高温金属承受强烈的拉伸作用,产生拉伸塑性变形,部分抵消了焊接过程中已产生的缩短的塑性变形,使得试件中纵向残余塑性应变减小,焊接残余拉应力峰值降低,残余压应力水平降低。切条应力释放法是一种简便有效的薄板焊后残余应力测量方法,能够满足工程应用的精度要求。     

Compressive properties of aluminum foams by gas injection method

The compressive properties of aluminum foams by gas injection method are investigated under both quasi-static and dynamic compressive loads in this paper.The experimental results indicate that the defo...     

Correlation between Poisson ratio and Mohr–Coulomb coefficient in metallic glasses

Yielding in metallic glasses is often described in terms of the Mohr–Coulomb criterion, τ + μσ ≥ τ₀. It suggests that a material yields when a combination of shear (τ) and normal stresses (σ), linked through the friction coefficient (μ), reaches the critical shear stress (τ₀). In this paper, an increase of the friction coefficient for increasing Poisson ratio is foreseen, if the elastic limit observed under uniaxial (tensile or compressive) and shear stresses remain constant. Experimental values of Young and shear moduli and Poisson ratio of metallic glasses have been collected from the literature. Compressive and tensile yield stresses have been also collected and elastic limits have been calculated. The elastic limit observed under compressive stress decreases with increasing Poisson ratio and it appears similar for metallic glasses based on the same metals. The values of the friction coefficient for metallic glasses have been obtained from the fracture angle observed under uniaxial stress (compressive or tensile) and from the ratio between compressive and tensile strength. Experimental data of the friction coefficient appear rather scattered and a clear trend as a function of Poisson ratio cannot be outlined. The increase of toughness of metallic glasses with increasing Poisson ratio has been ascribed to the corresponding reduction of the compressive elastic limit.     

焊接应力场与应变场的计算与讨论

经典的观点认为焊缝存在残余压缩塑性应变,而目前有学者认为焊缝是在冷却的过程中形成的,与加热过程无关,认为焊缝只存在拉伸塑性应变,而不存在压缩塑性应变.针对这一对传统残余塑变理论的质疑,采用有限元方法对薄板熔焊对接接头纵向应力和应变的瞬态变化以及由焊缝中心到母材边缘的纵向应力和应变分布情况进行了计算.结果表明,焊缝及近缝区存在着残余压缩塑性应变,应力状态为拉应力,由焊缝中心到母材边缘其纵向应力由拉应力转变为压应力,纵向压缩塑性变形量逐渐下降,离焊缝较远受温度场影响较小的母材不产生塑性变形.     

关于焊接残余应力消除原理的探讨

介绍了机械拉伸法、温差拉伸法、滚压法和低应力无变形焊接技术消除应和原理的传统观点,认为焊接时焊接时生压缩塑性变形,消除应力的原理在于用拉伸塑性变形抵消、补偿压缩塑性变形。本文提出新的应力应变发展过程,提出焊缝不存在压缩塑性变形,一直受拉伸、熔合线附近处于脆性温度时承受较大的纵向拉伸应变,随着远离熔合线,应变陡降。焊缝消除应力的原理在于用塑性应变减少甚至找消弹性应变,在一般情况下是用拉伸塑性应变减少     

焊接应力变形原理若干问题的探讨(二)

提出焊接残余应力形成和消除原理:焊接残余应力不是压缩塑性应变引起的,而是由于焊缝和近缝区金属在"力学熔点"及以下温度冷却收缩受阻产生的;消除焊接残余应力不是产生拉伸塑性应变以减少、抵消和补偿压缩塑性应变,而是将残余弹性应变转变为塑性应变;消除焊接残余应力并不是必须去除固有应变,部分去除或完全不去除固有应变也能完全消除残余应力.提出随焊后热精确控制应力变形焊接法,既可实现无应力焊接和无应力无变形焊接,也可实现适当压应力无变形焊接和较大压应力微变形焊接;并对传统方法与有限元法进行了分析比较.     

Failure characteristics of three-body model composed of rock and coal with different strength and stiffness

Four different types of three-body model composed of rock and coal with different strength and stiffness were established in order to study the failure characteristics of compound model such as roof-coal-floor. Through stress analysis of the element with variable strength and stiffness extracted from the strong-weak interface, the tri-axial compressive strength of the weak body and strong body near the interface as well as the areas away from the contact surface was found. Then, on the basis of three-dimensional fast Lagrangian method of continua and strain softening constitutive model composed of Coulomb-Mohr shear failure with tensile cut-off, stress and strain relationship of the four three-body combined models were analyzed under different confining pressures by numerical simulation. Finally, the different features of local shear zones and plastic failure areas of the four different models and their development trend with increasing confining pressure were discussed. The results show that additional stresses are derived due to the lateral deformation constraints near the strong-weak interface area, which results in the strength increasing in weak body and strength decreasing in strong body. The weakly consolidated soft rock and coal cementation exhibit significant strain softening behavior and bear compound tension-shear failure under uni-axial compression. With the increase of confining pressure, the tensile failure disappears from the model, and the failure type of composed model changes to local shear failure with different number of shearing bands and plastic failure zones. This work shows important guiding significance for the mechanism study of seismic, rock burst, and coal bump.     

Thermal residual stresses and stress distributions under tensile and compressive loadings of short fiber reinforced metal matrix composites

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