大围长结构化LDPC码线性编码器设计

校验矩阵拥有大围长对于改善LDPC码的性能有着重要的意义.结构化是提高码实用性的关键.提出一种基于列差搜索算法(Column-Difference Search Algorithm)和步进边增长算法(Progressive Edge Growth Algorithm,PEG)的准循环LDPC码构造方法,可以设计任意围长和码率的QC-LDPC码(称为CP-LDPC码).利用该码校验矩阵的近似下三角特性,推导出递推编码方法,使得该码编码复杂度与码长成线性关系.仿真结果表明,CP-LDPC码在BER性能上与随机码以及同属QC-LDPC码的Tanner码和Array码相比有明显提高,优于随机码、Tanner码和Array码1.35dB～3.95dB.     

宜昌长江公路大桥钢箱梁工地焊接施工精度控制

在大跨度钢桥中 ,由于梁段数量多 ,梁段与梁段对接焊缝收缩量难以控制 ,其变化量积累起来并非小数。所以梁段组装时工地焊接对梁段总长的影响较大。重点介绍在宜昌长江公路大桥钢箱梁工地焊接施工中 ,在钢箱梁长度的预控、跟踪修正和精确控制钢箱梁焊接成桥总长度等方面采取的一些关键工艺。     

Crack tip opening angle measurement through a girth weld in an X100 steel pipeline section*

Crack tip opening angle (CTOA) is becoming one of the most accepted methods for characterizing fully plastic fracture. It provides a measure of the resistance to fracture for a material in cases where there is a large degree of stable‐tearing crack extension during the fracture process. Our current pipeline research uses the CTOA test as an alternative, or addition, to the CTOD (crack tip opening displacement) and the fracture energy characterization provided by the J‐integral approach. A test technique was developed for measurement of CTOA that uses a modified double cantilever beam (MDCB) specimen. A digital camera and image analysis software were used to record the progression of the crack tip and to estimate the CTOA. In this article, CTOA data on crack growth orientations perpendicular to pipeline girth welds are presented. The CTOA for X100 high strength bainitic gas pipeline steel is reported. Two different specimen gauge sections, 3 mm and 8 mm, were used and the effect of the specimen thickness on the CTOA is discussed. The results show a change in the CTOA as the crack grows into the heat affected zone (HAZ). A slight improvement in the fracture resistance is measured, and through the weld, a slight decrease in fracture resistance is observed.     

Prediction of welding stresses in WIC test and its application in pipelines

In the present study, the Welding Institute of Canada (WIC) restraint test was used to simulate the restraint conditions of full-scale girth welds on energy pipelines to ascertain the influence of welding process parameters on welding stresses. Finite element models are developed, and validated with neutron diffraction measurements, to evaluate the welding stresses for under-matched, matched and over-matched welds. The effects of heat input, wall thickness and variable restraint lengths of WIC sample are systematically investigated. As a practical outcome, this work can help in selection of the appropriate restraint length for WIC tests to simulate the specified stress conditions in the pipeline, and, ultimately, reduce the risk of Hydrogen Assisted Cold Cracking (HACC) in high strength low alloy.

This paper is part of a Themed Issue on Measurement, modelling and mitigation of residual stress.     

基于BS 7910的管道环焊缝缺陷工程临界评估研究

依据标准BS 7910, 对含缺陷环焊缝进行了工程临界评估。借助管体与焊缝拉伸试验结果确定评定曲线, 选取最低值作为评定断裂韧性值, 采用评估失效图(FAD)方法进行工程临界评估, 在评定计算中考虑了静态应力、残余应力和应力集中等的影响, 并进行了宽板拉伸试验验证。结果表明, 在管道工作载荷下, 根据BS 7910评估方法, 当夏比冲击吸收功CV>60 J时, 环焊缝中的缺陷尺寸均在可接受范围内; 在管道设计极限载荷条件下, 当夏比冲击吸收功CV＞60 J时, 环焊缝在现有条件下评估点均位于FAD曲线内, 结果可接受; 宽板拉伸试验结果进一步验证了环焊缝中各缺陷在评价输入参数条件下是可接受的, 表明含缺陷焊接接头的安全裕度较高。     

基于PEG算法的准循环LDPC码构造研究

为了兼顾LDPC码较高的纠错性能和较简单的硬件实现,提出了一种基于PEG算法的准循环LDPC码校验矩阵的构造方法,该方法首先利用PEG算法构造基矩阵,然后利用提出的移位参数公式来构造循环移位矩阵,再用循环移位矩阵和全零矩阵对基矩阵进行优化扩展,形成的校验矩阵最短环长至少为8环。该方法具有与PEG算法非常接近的纠错性能,尤其是当信噪比高于1.2 dB时要优于PEG直接构造法,而硬件实现比PEG算法简单,且参数选择灵活方便。     

An investigation into the bust girth range of pressure comfort garment based on elastic sports vest

The global man-made-fibre industry has had to endure considerable structural change since the early 1970s. This paper studies both the reasons for the emergence of excess capacity in the industrialised nations and the manner in which producers and governments have dealt with it. It is clear from this study that the pattern and speed of adjustment to different market conditions have varied according to region, and possible reasons are examined.     

不同焊接构造的铸钢节点环形对接焊缝的热点应力分析

铸钢节点与主体构件的环形对接焊缝处存在材料差异、几何差异和焊接残余应力,因此环形对接焊缝是铸钢节点疲劳破坏的重要研究对象。目前,国内对铸钢节点焊缝疲劳问题的研究比较缺乏,传统名义应力方法并不适用于铸钢节点。采用热点应力方法对铸钢节点环形对接焊缝的疲劳问题进行探究,比较4种热点应力外推方法的计算结果。为减小铸钢节点与主体构件不等壁厚对接焊缝产生的应力集中对疲劳性能的影响,分析对比了3种不同的焊接构造,并分别研究在壁厚t、径厚比d/t、壁厚比T/t三种参数影响下的热点应力变化规律。     

基于中国剩余定理和贪婪算法扩展的QC-LDPC码

在缩短阵列码的基础上运用中国剩余定理(CRT)和贪婪算法提出了一种新颖的大围长、码长更加灵活的QC-LDPC构造方法,且所构造的码字的校验矩阵采用楼梯矩阵循环置换而成。与传统CRT构造方法相比,只需已知一个分量码——缩短阵列码,同时新构造QC-LDPC码码长与码率选择比较灵活,围长更大,如果围长一样,则使最短环数量尽可能地少。仿真分析表明:在误码率为10-6时,在相同码率和码长的条件下,利用所提出的构造方法所构造的girth-8(4,k)QC-LDPC码在加性高斯白噪声(AWGN)和瑞利衰落信道中分别与缩短阵列码相比可获得约1.2 d B和2.0 d B的净编码增益,与CRT码相比分别改善了0.3 d B和0.7 d B的净编码增益,且性能与Gallager随机码性能相似但编码复杂度大大降低。