Al-4%Cu合金中锯齿形屈服剪切带的变形测量

通过高速CCD（1000 fps）连续记录Al—4％Cu合金材料在拉伸实验中出现的B型锯齿形屈服（PLC）剪切带发生前后一系列非相干光散斑图，结合数字散斑相关法图像分析技术，定量地给出了PLC剪切带及周围区域的位移和应变的空间分布，再现了PLC剪切带随时间演化发展的过程．实验结果清楚地证实了B型PLC剪切带的一种演化机制，即先在试样一侧成核，然后与拉伸轴成一定角度横向贯穿整个试样，最终形成非均匀塑性变形带．实验结果表明，在PLC剪切带形成瞬间变形带外存在弹性收缩变形．     

基于三维数字图像相关法的5456铝镁合金锯齿形屈服研究

利用三维数字图像相关技术研究了常温下不同厚度5456铝镁合金板状试件在加载过程中的锯齿形屈服现象及其变形分布.统计分析锯齿跌落幅值和周期发现,2者均随应变增大而增大,且锯齿周期随厚度增大而增大,而锯齿幅值与厚度相关性较弱.在拉伸方向应变场和离面位移场中观察到了Portevin-Le Chatelier(PLC)带,PLC带宽不随应力跌落幅值变化而变化,随试件厚度增大而增大;PLC带倾角与应力跌落幅值、试件厚度无关,约为60°.将锯齿幅值和变形信息相联系,获得了锯齿幅值与带内最大应变成正比关系这一规律.通过变形时域分析发现,PLC带引起局部拉伸方向应变突增的同时,也引起相同区域离面位移的突变.     

Al-Cu合金中PLC效应时域和空域行为的实验研究

在不同拉伸应变速率下,得到了Al-Cu合金中PLC(Portevin-Le Chatelier)效应在时域上的三类“锯齿形”应力一时间曲线,比较并研究了三种曲线的异同及其产生的定性解释．与之相对应,观察并分析了PLC效应空域上三种类型PLC变形带的空间传播特性．针对3mm厚的试件,探讨了在不同拉伸应变率下,类型A和B的PLC变形带的传播速率随变形过程的演化;得到了在相同应变下,变形带的传播速率和加载应变速率之间的幂律关系．同时,还将时域上的“锯齿形”应力一时间曲线与空域上的带的传播位置时间曲线相对应,得到了PLC效应发生时宏观上时域和空域之间的关系、另外,还讨论了试件厚度对PLC变形带的空间相关性的影响．     

Ni-1C二元合金宏观不均匀塑性变形与断裂行为研究

研究了Ni-1C二元合金宏观不均匀塑性变形与断裂行为。通过对拉伸应力-时间和位移-时间曲线测量,分析了Ni-1C合金波特文-勒夏特利埃(Portevin-Le-Chatelier,PLC)效应锯齿波形和应力跌幅分布特征,利用三维数字图像相关法研究PLC变形带的形核增殖、时空演化以及断裂过程中裂纹的形成和扩展。结果表明,在3mm/min拉伸速率下,Ni-1C合金PLC效应表现为A型锯齿波,应力跌幅呈现幂率分布;PLC变形带在试样的夹头或标距段形核,向试样一侧连续增殖传播;裂纹萌生于试样标距段,逐渐扩展,导致韧性断裂,同时裂纹的萌生及扩展与PLC变形带之间没有关联。     

块状非晶剪切带和微裂纹形核扩展的SEM原位研究

在扫描电镜(SEM)中对Zr57Cul5.4Nil2．6Al10Nb5块状非晶单边缺口试样进行了原位拉伸；用原子力显微镜(AFM)研究了剪切带的三维形貌．研究表明，拉伸时缺口前方产生剪切带，它们逐步发展、长大．尽管剪切带由剪应力产生，正应力在其形成和扩展过程中起重要作用．两剪切带相交时会形成割阶，其长度随相交剪切带中应变量升高而增大．当主剪切带中应变集中足够大后，剪切微裂纹沿主剪切带和基体的交界线形核扩展，并沿剪切面向试样内部扩展几十微米；在正应力作用下，该剪切(Ⅱ型)裂纹张开成I型，并快速贯穿试样厚度，然后沿横向快速扩展导致断裂．     

弯扭复合变形整体壁板喷丸成形工艺研究

为了获得实用的机翼整体壁板喷丸成形工艺数据,建立了喷丸成形板件的弯曲半径、弦向型面波浪度和展向母线直线度与成形工艺参数之间的关系,分析了弹丸速度、喷丸条带间距、试件厚度、施加的预应力等主要工艺参数对具有弯扭复合变形的整体壁板零件喷丸成形的影响规律;针对喷丸成形的球面变形现象,介绍了几种克服这一问题的技术措施,如条带喷丸、预应力喷丸等.研究表明,改变喷丸条带的间距及毛坯板料的厚度对试件弦向型面波浪度及展向母线直线度的影响效果相反;提高弹丸速度将增加喷丸成形的曲率,但在较高转速下提高叶轮转速对增加试件的变形程度影响不大;预应力喷丸成形方式是增加壁板弦向弯曲曲率及改善展向母线直线度的有效措施.     

高碳硅锰钢的动态力学性能

通过扫描电镜、霍普金森压杆和拉伸试验机等仪器研究了不同温度淬火高碳硅锰钢的组织、变形程度和动态力学性能。研究表明,随淬火温度提高,试验钢中残留碳化物数量减少,在880 ℃时,试验钢的残留碳化物全部溶解。动态力学试验后,880 ℃淬火试样因残留碳化物固溶,动态强度增幅较小,对应变速率不敏感,由均匀变形转为破碎。绝热剪切带内和远离绝热剪切带的区域均存在孔洞,远离剪切带区域的孔洞无序分布,且伴随有均匀分布的碳化物。靠近绝热剪切带区域的碳化物存在分布不均匀的情况,且绝热剪切带内部的孔洞沿着热量传播方向扩展,形成沿剪切带分布的裂纹。     

NiTi-NbTi原位复合材料的Lüders带型变形和载荷转移行为

采用电弧熔炼、锻造和拔丝方法原位合成了一种高Nb含量的NiTi-NbTi记忆合金复合材料.TEM显微分析显示,在材料内部纳米尺度的NbTi和NiTi纤维沿丝材轴向交替分布,NbTi纤维体积分数高达约70％.通过同步辐射高能X射线原位拉伸实验研究了复合材料的变形机制.结果显示,虽然NiTi体积分数仅约30％,但复合材料的变形仍受NiTi的应力诱发相变控制.在加载初期,复合材料先发生均匀变形,并且在拉伸曲线出现屈服平台之前,NiTi己发生均匀相变.当平台出现之后,NiTi转而发生Lüders带型相变,进而诱发NbTi也随之发生Lüders带型变形,使整个复合材料都展现Lüders带型变形.Lüders带前沿存在载荷转移现象,载荷由正在发生相变的B2-NiTi同时转移到NbTi相及之前在均匀相变过程中形成的B19'-TiNi马氏体相.     

铝合金薄壁柱壳电磁胀形塑性失稳实验研究

利用高速相机记录电磁驱动6063-T6铝合金薄壁柱壳膨胀变形过程,结合二维数字图像相关法分析试样表面应变场演化,对金属柱壳电磁膨胀塑性失稳过程开展研究,探讨金属高速成形极限及影响因素。实验结果表明:6063-T6铝合金薄壁柱壳电磁高速膨胀成形开始为均匀变形,随变形发展试样表面应变场出现局域化集中并沿45°或135°方向失稳扩展,裂纹沿局域化集中带发展导致碎裂;电磁膨胀局域化过程中分散性失稳时的平均应变与准静态拉伸失稳应变相比并没有明显变化,而局域化带形成时的应变随加载率提高显著增大,分析认为薄壁板电磁高速胀形成形极限提高的"增塑性"主要与惯性效应与结构效应相关。     

共析珠光体钢在冷轧过程中的组织变化

用SEM、TEM及XRD等方法研究了共析钢中珠光体经40％-90％的冷轧变形后所产生的显微组织变化．接渗碳体形态特点,变形珠光体组织可分为以下3种类型：(1)不规则弯曲片层型,即变形后的渗碳体与轧制面呈大角度偏离且不规则弯曲的珠光体．(2)带有剪切带的粗大片层型,即被渗碳体剪切带分开且变形轻微的珠光体．(3)精细片层型,即与轧制方向平行排列、片间距细小且渗碳体严重变形的珠光体．精细片层区域的比例随着轧制压下率的提高而增大．此外,重度冷轧变形还引起渗碳体严重塑性变形和部分溶解．     

聚能射流穿甲后超高强度钢靶板的损伤特征及其机理

超高强度钢靶板在聚能射流穿甲后损伤特征的研究表明,在穿甲过程中,弹孔周围白层的温度超过了相变点,形成了晶粒尺度在20nm左右的马氏体 残余奥氏体混合组织.绝热剪切带（ASB）中的剪切仅发生在相当窄的层面上,其宽度约为2μm,平均切应变量为110,应变速率≥2.24×106s-1;光学显微镜下显示的剪切带宽度是集中剪切变形区（LSDZ）及其热影响区（HAZ）的总宽度.聚能射流穿甲过程中,靶板材料破坏包含了如下几个相互重叠的过程:与射流接触的靶板周围局部区域材料沿射流方向上的整体协调变形;射流与靶板之间的剪切断裂和弹孔表面部分靶材的熔化;集中在极薄层面卜高度局域化的剪切变形;应力波在靶表面反射造成的正向开裂.     

Out-of-Plane Constraint Effect on the Fracture Toughness of Single Edge Notch Tension SpecimensEI北大核心CSCD

The crack-tip stress and strain fields of single edge notch tension (SENT) specimen are similar to those of the full-scale pipe containing surface cracks under longitudinal tension and/or internal pressure. It is well known that material's fracture toughness is not constant, and the specimen size has a significant influence on fracture toughness. It is thus essential to consider the transferability from fracture specimens in laboratory testing to practical structures, i.e., size effects or constraint effects. However, the specimen dimensions for SENT specimens recommended by current design procedures have not validated the out-of-plane constraint effect on the fracture toughness. In this work, the effect of specimen thickness on the crack tip opening displacement (CTOD) of SENT specimen was investigated using an API X90 grade steel. Full-field deformation measurement by digital image correlation (DIC) technique and stretching zone width (SZW) examination were performed to analyze the size effects on fracture toughness. The results show that the critical crack initiation toughness is highly sensitive to specimen thickness, and decreases significantly as specimen thickness increases until the thickness-to-width ratio (B/W) equals to 4, beyond which the effect of specimen thickness becomes relatively weak. As the specimen thickness increases, the maximum longitudinal strain and stretching zone width decrease sharply, and the location of high-strain zones changes significantly; when B/W >= 3, strain is initiated from the area oppo-site the cracked side rather than from the crack tip, indicating a strong loss of plasticity for thicker specimens. A dimension size is recommended for the fracture toughness testing to take the out-of-plane constraint into account for SENT specimen.     

车用铝合金在剪切应力状态下变形及损伤机理研究

采用Iosipescu和双缺口拉伸实验,对车用铝合金(5052)在剪切和准剪切应力条件下的大变形和损伤机制进行研究。断口分析和有限元三向应力度计算表明纯剪切条件下材料主要以剪切滑移带变形并萌生微裂纹,基本不产生孔洞损伤。而准剪切条件下材料损伤存在有微孔洞和剪切带混合并发机制。高三向应力度区首先出现微孔洞裂纹起始,但随着损伤向剪切应力区过渡,三向应力度不断降低,孔洞的长大聚合受到抑止,转而出现剪切滑移带中的平行微裂纹,材料最终主要以剪切滑移失效。     

RESEARCH ON THE FORMATION MECHANISM OF INTERNAL CRACK IN THE CONTINUOUS CASTING SLAB

In view of the periodic bending deformation of solid-liquid interface in the solidification process for continuous casting slab, the variation of temperature gradient and dendritic spacing in the front edge of the solid-liquid interface, and the nucleation and propagation process of crack were studied. It is shown that the bending deformation of the interface results in the temperature field change in the front edge of solid-liquid interface, and the occurrence of temperature gradient along drawing direction results in the growth of secondary dendrites. The initial crack formed during the middle and final stage of solidification may extend to the surface of the casting slab and become an internal crack. The results of the theoretical analysis are basically in agreement with that of the experiment.     

高应变速率下45钢绝热剪切变形特征

基于退火、正火、调质三种不同热处理状态的45钢的帽型试样强迫剪切实验,在Hopkinson压杆的不同速率加载条件下,利用金相显微镜和扫描电镜测试方法研究了材料绝热剪切的变形特征和敏感性,分析了组织因素对绝热剪切的影响规律。结果表明,材料的组织不同,其绝热剪切变形的特征也不同。45钢正火态、调质态中均出现转变带,其余为形变带。随着加载速率和材料强度的提高,绝热剪切带由形变带变为转变带,剪切带宽度越窄,绝热剪切越敏感。     

Ti5Mo5V2Cr3Al合金中绝热剪切带的微观结构演化

利用分离式Hopkinson压杆,对Ti5Mo5V2Cr3Al(TB10)合金帽形试样进行强迫剪切试验,通过光学显微镜和透射电镜技术观测其绝热剪切带(ASB)内的微观结构。结果表明,ASB的过渡区由具有高位错密度的沿着剪切方向的宽度为20～50nm的拉长组织构成;剪切带中心由大量低位错密度(相对ASB的过渡区)的直径为50～100nm的晶粒组成,具有典型的再结晶组织特征。在绝热剪切变形过程中ASB内的平均绝热温升约为784℃。ASB内发生了动态再结晶,晶粒尺寸为50～100nm。     

Surface and Subsurface Deformation of Wear-Resistant Steels Exposed to Impact Wear

The impact wear resistance of four different wear-resistant steel grades was investigated using different impact bodies. Post-test evaluation of the impact tested samples was performed by different techniques including 3D surface profilometry, microhardness indentation, optical and scanning electron microscopy, and energy dispersive x-ray spectroscopy. The tribological response of the steel plates during the impact is strongly dependent on the properties of the impacting body. The subsurface deformation was found to increase with increasing impact energy and/or impact velocity and decreasing steel hardness. On a microscopic scale, a number of interesting mechanisms were revealed within the deformed impact sites. Besides an overall plastic deformation, localized deformation resulting in narrow adiabatic shear bands with an ultra-fine microstructure was observed. Within these shear bands, showing intense shearing strain, nucleation of microvoids was frequently observed. Growth and linkage of these voids lead in crack formation along the shear bands and eventually flake-like wear fragments are detached when these cracks reach the surface.     

纳米晶体镍片剪切带演化的实验研究

以高纯度、高致密度的纳晶金属镍片为研究对象,利用数字图像相关方法研究了其在单轴准静态拉伸载荷下的剪切带化变形行为,系统分析了纳晶金属镍片中剪切带的萌生及演化规律,定量分析了剪切带形核、扩展和失效的重要节点,讨论了纳晶镍剪切带的物理特征。结果显示:剪切带的演化发展是纳晶镍试样塑性变形的主要形式,是纳晶镍试样具有较低延性的原因。     

Crack shape and rust distribution in corrosion-induced cracking concrete

This study investigated a reinforced concrete specimen that had deteriorated in an artificial environment for 2 years. The crack width and the rust distribution were observed by digital microscopy. The variation of the total circumferential crack width along the radial direction is presented using a linear function. Observation reveals that rust does not penetrate into the corrosion-induced cracks before concrete surface cracking. After concrete surface cracking, rust fills the cracks, lining the edges of the cracks due to the circulation of the outer solution. A schematic diagram is proposed to describe crack propagation and rust development.     

TC4合金绝热剪切动态演变过程数值模拟研究

对TC4合金强迫剪切过程进行了数值模拟,获得了绝热剪切过程中变形局域化区域内的von-Mises应力、有效塑性应变以及温度的分布规律。通过分析发现：大量应变集中于帽形试样变形局域化区域内,且在试样两拐角处最大;变形局域化区域内温度明显高于基体温度,且在试样两拐角处最高。结果表明,绝热剪切带内温度达到了TC4合金的再结晶温度,这为TC4合金绝热剪切带内微观组织变形机制的研究提供了依据