Effect of toughness distribution in the thickness direction on long brittle crack propagation/arrest behaviour of heavy gauge shipbuilding steel

ABSTRACT

Brittle crack arrestability of the heavy gauge steel plates for shipbuilding is now an important issue for the recent mega container ships. In the present work, the brittle crack arrestability of the steel plate with different toughness distributions in thickness is examined in ultra-wide duplex ESSO tests. It is examined whether a running long brittle crack arrests or not in flat temperature condition in ultra-wide duplex ESSO test that are harder mechanical conditions similar to an actual ship hull condition. Test temperatures are selected at which arrest toughness, Kca, evaluated by temperature gradient type standard ESSO test is the same for two test plates. The steel plate with higher toughness in mid-thickness (t/2) than that in quarter thickness (t/4) could arrest a running long brittle crack although the plate with lower toughness in mid-thickness than that in quarter thickness could not arrest it. The typical split-nail shape appeared at the arrested crack front in the plate with higher toughness in mid-thickness than that in quarter thickness. The numerical analyses also demonstrate that the local stress intensity factor at the arrested crack tip is changing sensitively to the crack front shape. It suggests that the higher brittle crack arrestability appears due to the split-nail shape of the arrested crack front enhanced by the inhomogeneous toughness in thickness.     

Application of optical sensor for arc welding

Summary

In CO₂ gas-shielded arc welding, spatter is scattered and adheres to the base metal surface. The main factors affecting any difference in the bonding force remain obscure. This paper examines the bonding force of spatter adhering to the surface of SS400 base metal (rolled steel sheets) under different temperature conditions in CO₂ gas-shielded arc welding using 1.2 mm dia. solid wire. The following four types of base metal surface condition were adopted:

Type 1: As-received surface covered with an oxide film;

Type 2: Ground surface machined with a plain grinder (R_max = 0.6 μm);

Type 3: Surface with fume adhering to the ground surface;

Type 4: Free-ground surface machined with a disc grinder (R_max = 7–16 μm).

The base metal was heated by a 14 mm dia. x 600 mm ceramic heater arranged on the back of the base metal. The bonding force was measured as the shearing force of the spatter. Some 400 spatter particles on each surface were examined. The particles mostly have diameters ranging between 0.4–1.4 mm.

At a base metal temperature below around 450 K, the spatter on the scaled surface is insufficiently hot to melt the base metal and cannot adhere by melting the base metal surface. The shear stress is mostly below around 40 MPa. Despite a rising base metal temperature, however, spatter removal is still possible at less than 300 MPa.

Fume on the surface or the surface roughness do not affect the bonding force as directly as scale on the surface. The temperature, however, affects the bonding force. At a base metal temperature below around 450 K, the bonding force is slightly smaller.

When spatter adheres to the surface, heat causes the microstructure to change with corresponding hardening. The spatter and HAZ of the base metal here show a slightly higher Vickers hardness value (some spatter particles having HV470) than the base metal hardness (HV156–165), although this depends on the surface conditions and temperature.     

Ultrasound inspection of multilayered cellular structures produced by superplastic forming and diffusion bonding

Experience with the application of repair welding, welding-up technology, and semi-automatic welding equipment in repairing structures in mining and metallurgical equipment is discussed.     

Measuring laser weldability of aluminium alloys using controlled restraint weldability test

Abstract

Al–Mg–Si alloys are known to be highly susceptible to solidification cracking. Weldability results of laminated AA6061-T6 plates are presented in this paper when welded in full penetration keyhole mode using a 1030 nm, 10 kW Yb:YAG disk laser welding source with different welding conditions. Making use of the controlled restraint weldability (CRW) cracking test, a boundary has been established between crack and no crack conditions for different preloads. The originality of the CRW test is the cross-shaped test coupon that partitions the pre-stress unequally along the welding path. The CRW is proven capable of ranking the solidification cracking behaviour of weld metals deposited under different welding conditions.     

Effect of energy transfer modes on solidification cracking in pulsed laser welding

Abstract

In this study, solidification cracking in pulsed laser welding of fully austenitic, AISI Type 316 stainless steel has been analysed at different energy transfer modes. The pulse parameters have been selected appropriately to obtain conduction, transition and keyhole mode welds. Conduction and transition mode welds exhibit higher susceptibility to cracking than keyhole mode welds. It is observed that both heat input and energy transfer mode affect the cooling rate and hence influence solidification cracking. Microstructures of the fusion zone have been analysed, and the cooling rate experienced by the weld is estimated from the mean cell size in the weld. It is found that the critical cooling rate below which cracking does not occur is ～10⁴ K s^??1.     

结构裂纹对弹性梁振动特性的影响

研究了裂纹扩展引起的结构柔度变化以及裂纹界面接触对弹性梁振动特性的影响。基于断裂力学和能量原理推导了含横向裂纹弹性梁的局部柔度模型,分析了常见形式裂纹均匀梁的局部柔度,给出了相应的无量纲柔度系数计算公式;结合梁的弯曲振动方程,探讨了含裂纹悬臂梁的振动特性。算例表明：裂纹形式对弹性梁的柔度影响明显,裂纹界面接触会引发参数振动,界面滑动摩擦阻尼对稳定梁的振动响应具有重要作用。     

不锈钢管道对接焊缝焊接热裂纹超声波检测技术

由于安装制造工艺的原因,核电站辅助系统奥氏体不锈钢对接焊缝极易产生焊接热裂纹,此类热裂纹具有长度短、高度小、多集中于焊缝表面和近表面区域等特征,同时又由于奥氏体材料自身的影响,依据标准采用的体积性检测方法不能获得较好的检测效果。笔者针对特定对象,介绍了核辅助系统奥氏体不锈钢对接焊缝焊接热裂纹超声波检测技术及验证结果,为此类焊接热裂纹检测技术提供了参考。     

板裂状围岩下薄矿脉开采稳定性相似模拟试验研究

赣南某钨矿矿体上下盘岩体多为板岩,结构面发育且岩体破碎,在留矿法放矿过程中,上盘岩体暴露后在次生应力的影响下极易发生板裂状滑落或溃曲破坏,使得采幅难以控制。针对此问题,提出了普通留矿法、静态留矿法、散体人工矿柱+锚固留矿法3种回采方法。通过分析上下盘板裂状围岩随开采过程的位移变形情况,最终选择散体人工矿柱+锚固留矿法作为试验采场回采方法。基于正交试验原理,进行了相似材料配比试验,最终确定的材料配比为ω(沙)∶ω(石膏)∶ω(水)=5.0∶1.0∶1.2。相似模拟试验结果表明:采高较小时,锚杆支护对控制围岩的稳定是有效的,然而随着采高的增大,采场出现应力集中,锚杆逐渐失去了作用,因此仅对围岩进行锚固支护并不足以改善地压环境;散体人工矿柱+锚固留矿法利用人工矿柱减小了回采跨度,提高了整个采场的稳定性。对比模拟结果可知,其上下盘及顶板的位移量远小于其他2种回采方法。     

崩落采矿形成冰碛补给型泥石流相似模拟试验

崩落采矿法形成的冰碛补给型泥石流作为发育在高寒山区的特殊地质灾害类型,给矿山的安全高效开采造成了严重影响.为研究崩落空间发展过程与泥石流之间的关系,开展了相似模拟试验,模拟降雨条件下崩落采矿法的放矿过程,研究矿石、冰碛物、水等3种因素对泥石流形成的影响,结果表明:随着放矿的进行,矿石中混有地表冰碛物的量越来越多,放出的...     

基于角点检测的裂纹瓷图像拼接及裂纹重建

对裂纹瓷表面的纹饰进行图像拼接,可更好地应用于纹理等外部特征的分析与瓷器鉴别.图像处理技术可实现快速、无损的陶瓷器检测与分析,弥补了传统检测方法的不足.本文以具有裂纹釉的龙泉青瓷茶杯为研究对象,提取瓷器的多个侧面图像,采用Harris角点检测方法提取图像中的角点,然后进行图像配准和拼接,将多侧面图像拼接为完整图像;通过...