Deformation field in 316L stainless steel by single shot peening
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摘要: 运用金属材料表面纳米化试验机对单个弹丸撞击316L不锈钢表面进行了撞击实验；采用激光共聚焦显微镜观察了弹坑的三维形貌，测量不同振动频率下弹坑的直径及离面位移；采用云纹干涉法对弹坑周围的面内应变场进行测量，并分析振动频率及撞击方式对弹坑尺寸、塑性应变大小以及塑性应变区范围的影响；采用有限元方法对单个弹丸垂直撞击试件表面的应变场进行数值模拟，与实验结果进行比较，分析了弹坑周围残余应力的分布。结果表明：随振动频率的增加，弹坑直径和离面位移都增加，频率在50~55Hz，弹坑直径有突变，离面位移和振动频率呈线性关系；振动频率越大，塑性应变越大，塑性应变分布范围均大于弹坑直径的2倍；同一振动频率下弹丸垂直撞击比倾斜撞击的塑性应变大，而塑性应变分布范围相差不大；面内残余应变场的数值模拟结果和实验结果吻合较好，最大误差小于10%。Abstract: The experiment of the single shot impacting the 316L stainless steel surface was carried out using a surface nano-crystallization testing machine. Three-dimensional morphology of the dimple was observed with a laser scanning confocal microscope, and the dimple's diameter and off-plane displacement in different vibration frequencies were also measured. The in-plane strain around the dimple was measured by moiré interferometry. The effect of the vibration frequency and the way of impacting on the dimple size, the plastic strain size and the plastic strain zone were also analyzed. In comparison with the experimental result, the strain field was simulated using the finite element method, and the distribution of the residual stress around the dimple was also analyzed. The result showed that the crater diameter and the off-plane displacement increases with the increase of the vibration frequency. When the frequency is from 50 to 55 Hz, the crater diameter experiences mutations. When the shot impacts the surface vertically, the greater the vibration frequency, the greater the plastic strain and plastic strain zone, and the plastic strain zone are two times larger than the crater diameter. The plastic strain by the vertical impact is slightly greater than the plastic strain by the oblique impact, but it has little effect on the plastic strain zone. The experimental U field strain is in fairly good agreement with the numerical simulation result, and the maximum error is less than 10%.
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相对于火箭撬、电炮等加载设备, 利用压缩气体作动力源的气体炮[1-4]具有干净、易实现等优点, 应用越来越广泛。随着各种缓冲材料(如橡胶、泡沫铝等)和缓冲器的开发利用[5-7], 高速负载的缓冲与回收技术也越来越成熟, 使惯性抛射技术的应用越来越广泛, 如各种冲击实验设备、弹丸加载装置等[8-9]。在水冲击实验[10-12]中, 需要发射具有一定速度的规则形状水柱, 作为实验件入水出水冲击过程模拟的实验条件。目前的水流发射装置如水压凿岩机等[13-14], 都通过活塞推射的方式实现水流射出, 速度与冲击力可调节, 但由于活塞作用到水面上的压力不均匀, 水柱形状容易破坏, 不符合水冲击实验的要求。
本文中基于气体炮加载技术和惯性抛射原理, 研制一套水冲击实验装置, 可实现规则形状水柱按照设定速度发射, 为各种水冲击实验提供技术手段。
1. 装置设计
1.1 装置发射原理
该装置在活塞推射技术的基础上, 设计一抛射筒取代活塞, 将水柱装在抛射筒内; 利用气体炮产生的动力加载抛射筒, 达到设定速度时撞击缓冲件得到减速, 筒内水柱在惯性作用下飞出; 根据一级气体炮内弹道理论, 可计算出水柱飞出速度。抛射筒材料采用高强度合金钢, 缓冲件材料采用橡胶, 装置在发射水柱后抛射筒可再次使用。
如图 1所示, 装置工作过程为以下4个步骤:(1)首先关闭阀门, 抛射筒放置到位, 并在气室中充满压缩空气; (2)打开阀门, 气室内压缩空气瞬间释放, 产生的压力p将推动抛射筒(内装待抛物)在炮管内加速; (3)当抛射筒完成加速后, 将获得一定的初速度v, 并撞击缓冲件; (4)抛射筒受缓冲作用减速停下, 最后待抛物在惯性的作用下以速度v0(略小于v)发射出去。
图 1 基于气体炮加载技术的惯性抛射原理示意图Figure 1. Schematic diagram of inertia projecting based on load technology of gas gun根据一级气体炮的内弹道理论[1], 不考虑各种摩擦与损耗时, 抛射筒获得的速度
v=(2ˉpρLf/Dfl/Df)1/2 (1) 式中:p为发射过程中筒后平均压力, ρ、l为抛射筒平均密度、长度, Lf、Df为炮管长度和内径。
基于上述惯性抛射原理来实现规则水柱的发射, 利用流体模拟软件FLUENT进行模拟。如图 2所示, 抛射筒(内装水)以35 m/s的速度向上运动, 某时刻迅速回收抛射筒, 使其在短距离内速度降为零, 水柱因惯性作用飞出, 出筒速度大于30 m/s、水柱形状保持较好。模拟结果表明, 惯性抛射方式可实现规则形状水柱的发射。
图 2 水柱惯性抛射的模拟分析结果Figure 2. Simulation analysis of ejecting water column by inertia effect1.2 装置结构组成
装置总体结构如图 3所示, 主要由气室、活塞、炮管、抛射筒、缓冲件、限位环等组成。气室中充满压缩空气, 活塞打开以后, 气体瞬间释放产生的动力推动抛射筒在炮管内运动, 获得一定速度后撞击缓冲件减速, 抛射筒内水在惯性作用下飞出, 从而实现规则形状水柱的发射。
气室分为前、后2个腔, 前腔用于充压缩气体, 后腔给活塞提供运动空间。
活塞由活塞杆和密封锥2部分组成, 活塞杆在气室后腔运动, 密封锥采用铜材料、可压紧气室前腔放气口。
为了拆装方便, 炮管由炮管口和炮管座2部分组成; 炮管口周壁上开有条形槽, 可及时排放发射过程中释放的空气。
抛射筒由薄壁圆筒和撞击盘组成, 连接部位局部加厚, 可提高强度。
缓冲垫安装于炮管口处, 环向安装间隙大于缓冲垫厚度, 以提供膨胀空间。
限位环采用铜材料, 可在不碰伤抛射筒的前提下起导向作用。
1.3 抛射筒冲击与缓冲分析
设抛射筒尺寸为直径200 mm、长1 m, 装满水后总质量约为100 kg, 经气体炮加载后获得的速度为35 m/s, 根据动能定理, 抛射筒撞击缓冲垫的总能量为61.25 kJ。抛射筒材料采用高强度合金(如30Cr MnSiA), 缓冲垫材料采用丁基橡胶, 通过有限元模拟分析得到, 如图 4所示:抛射筒壁、盘连接根部的应力最大(676 MPa), 在材料的允许范围内(30Cr MnSi A材料的屈服应力可达800 MPa以上), 这说明抛射筒未发生塑性破坏、下次实验可再次使用; 缓冲垫最大压缩量约为62 mm, 属于弹性变形, 压缩后将恢复变形; 炮管的螺栓连接处应力最大(约390MPa), 一般的中等强度材料即可满足; 炮管口和炮管座的连接螺栓应力最大约660MPa, 8.9级强度螺栓可满足连接要求。
图 4 水柱惯性抛射的模拟分析结果Figure 4. Simulation analysis of ejecting water column by inertia effect1.4 抛射筒发射速度测试
采用一种非接触式的光电测速方法, 如图 5所示, 2对光纤传感器(每1对包括1个发射器、1个接收器)相距L, 抛射筒运动过程中依次通过2对传感器并挡住光线, 从传感器信号中读取光线被遮挡的时刻t1、t2, 则抛射筒速度为v=L/(t1-t2)。该测速方法易于控制、测试精度高, 不受水环境影响。
图 5 抛射筒发射速度的非接触式测量方法Figure 5. Uncontacted method for measuring speed of projecting cylinder2. 实验结果
根据上述设计, 研制了一套水冲击实验装置, 主要技术指标为:
(1) 装置重约3.5 t, 高约3 m;
(2) 可发射水柱尺寸为Ø200 mm×1 m;
(3) 水柱发射速度可达40 m/s, 速度测量精度可达0.01 m。
采用该装置开展了实验, 实现了尺寸为Ø200 mm×1 m水柱的发射, 如图 6所示, 水柱出筒时形状较好, 喷出的水柱可以用作各种结构件的水冲击实验, 水柱发射后抛射筒后保持完好。在气室压力为1.0 MPa时, 开展发射实验, 通过非接触式测量方法[15]测定抛射筒获得的速度为30.15 m/s, 而利用式(1)计算得到的速度为32 m/s, 实际值与理论值比较接近, 误差是由于式(1)没有考虑各种摩擦与损耗; 实验时设定不同的气室压力, 即可获得抛射筒不同的加载速度, 从而得到水柱发射的设定速度。
3. 结语
基于气体炮加载技术和惯性抛射原理, 研制了可实现规则形状水柱按照设定速度发射的抛射装置。该装置利用气体炮动力加载装有水的抛射筒, 达到设定速度时撞击缓冲件得到减速, 筒内水柱在惯性作用下飞出; 抛射筒材料采用高强度合金钢, 缓冲件材料采用橡胶, 在水柱发射后抛射筒可再次使用。采用该装置开展了尺寸为Ø200 mm×1 m水柱的发射实验, 水柱速度与形状均满足水冲击实验的要求; 采用非接触的光电测速方法测量了发射速度, 与理论计算结果相符合。
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图 4 弹丸撞击试件的变形示意图
Figure 4. Schematic diagram of deformation after shot impact substrate
图 5 频率为50 Hz时弹坑的三维形貌图
Figure 5. Three-dimensional topography of crater at frequency of 50 Hz
图 10 不同频率时U场应变分布
Figure 10. Strain distribution in U-displacement at different frequencies
图 11 不同频率下U场和V场的应变分布规律
Figure 11. Strain distribution regularities in U-displacement and V-displacement at different frequencies
图 12 弹丸垂直撞击和倾斜撞击时相同频率下U场云纹图
Figure 12. Contour maps in U-displacement at same frequency under vertical or oblique impact
图 13 相同撞击频率不同撞击角度时应变变化规律
Figure 13. Strain regularities at the same impact frequency and the different impact angles
图 14 x方向应变沿y方向变化规律
Figure 14. Change of strain regularity in x direction along the y direction
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