含酸工况下的断裂力学:SENT和SENB试样的一些表征试验

Currently, 对于如何建立酸性使用环境下管道的断裂韧性，目前尚无明确的指导方针. Our Engaged Expert, Rodolfo Amoriello 为了更好地理解SENT测试技术是如何影响酸性环境下的断裂韧性的，我们进行了研究.

There is an increasing requirement for testing fracture toughness in pipelines, especially in sour environment. The presence of sour fluids 和 气体 cases deterioration to pipelines, the effects of which can range from small pinhole leaks to catastrophic failure.

Introduction

众所周知，单边缘缺口张力(SENT)试样通常比致密张力(CT)或单边缘缺口弯曲(SENB)给出的参数更保守 Engineering Critical Assessment (ECA) 和 Fitness For Service (FFS) calculations when used for analysis of the bending of pipes containing flaws.

In general, for carbon steel, H2S浓度的增加导致断裂韧性下降，主要是由于氢脆效应.

As a consequence, 当考虑H2S效应时，最大可容忍缺陷尺寸(由ECA计算预测)较小. Regarding FFS, 这意味着对于相同的缺陷大小，故障评估图上的初始定位更接近(甚至在)临界区域. 在酸性环境中执行SENT测试，预计将为ECA/FFS分析提供更接近实际材料性能的数据, 平衡不太保守的参数，典型的试样几何形状使用更严格的腐蚀环境条件.

设计并测试了一些表征试验，对SENT和SENB样品在空气中的J-R曲线与酸性环境中的J-R曲线进行了比较. In this paper, the results of the SENT tests are discussed.

Test set up

Materials selection, mounting procedure 和 sample preparation before executing the test must guarantee the absence of any contamination 和 the continuous presence of H2S bubbling during all test phases; from pre-loading to the end of the fracture mechanics test. 不对称容器的设计是为了保证正确的溶液浓度，根据标准或客户的规格. Because the test is done with H2S bubbling, 应特别注意确保正确的健康和安全条件以及对操作人员和测量仪器的保护.

Before the fracture mechanics test, 样品在H2S中预先充电，使用特殊的加载框架，能够承受高达250kN的恒定载荷. Load stability is monitored throughout the pre-charging time for crack stability. Tests are stopped when there is crack propagation 和 samples analyzed. Data logs are saved for future investigations 和 checks. 裂缝监测是通过直流电位下降(DCPD)系统实现的，该系统在所有预充帧之间依次和循环地进行电子切换. DCPD electronic amplifiers, 过滤器, 占空比和采样频率的设计在整个测量周期内都具有很高的精度. 该软件是为了控制DCPD系统而开发的，可以实时估计测试过程中的裂纹扩展情况. The pre-charging period is typical of 1 week.

样品从预充液框架中取出，并使用手推车移动到断裂力学测试框架中，以确保H2S的持续存在和测试安排的安全处理.

然后在极低的速度下进行断裂力学测试，最小速度为3.16 10 -3 M√m ⁄s for SENT samples, using the DCPD system. For SENB samples, the minimum speed is 5 10-3M √m /s. The temperature of the solution was in the range 240C ± 20C.

The overall test duration was typically between 8 和 12 hours.

Test apparatus characterization

为了保证数据的准确性，在试验装置上进行了多次对比试验. 下面的图表中报告了使用以下测试条件的一些示例:

– Test Solution: NACE test sol. B, H2S 0.2 bar partial pressure (30%) balanced with N, max O2 < 10 ppb

– Pre-loading: 1wk @ 150MPa

– F.M.Speed: 0.1 √ (only WM);0.4 √ ,4.0 √ [HAZ (Heat Affected Zone) 和 WM (Weld Metal)]

Test results

The test results on SENT samples, X65, BxB 36 mm, surface notch NQ, from cap side, HAZ 和 WM are shown in the J-R curves below. The welding was performed using the same welding team for each sample, using the same equipment 和 on the same material lot. 每组实验点代表10个样本数据的均值和标准差. Another 10 samples have been tested in WM in air for comparison.

酸性环境和不同测试速度的综合影响是明显的:速度越高，数据越接近“在空气中”的曲线。.

Relevant effects associated with pre-loading 和 test in sour conditions are:

– increased dispersion at least twice of data with respect to curves "in air",

-对于所有“在酸性条件下”的样品，在繁殖曲线的第一部分J几乎为0 (Δa从0到≈0).1 mm).

In the case of SENT of large samples, as those tested, 残余应力的影响与样品在相对高负荷下预充一周有关. During the early pre-qualification phase, spontaneous crack propagation was observed after only a few days of pre-loading, @150MPa. 随后的残余应力测量强调了样品制备和预应变在0.4% only mitigated their value.

Conclusion

在H2S溶液中预充一段时间后，在酸性条件下(H2S)进行断裂力学测试，结果更接近实际应用中的材料性能.

设计了专门的测试程序和设备，并对其进行了表征，以保证可用于FFS / ECA的可靠结果.

实验数据表明，酸性条件对预加载和断裂力学均有显著影响. 在H2S存在的情况下，不同的试验速度对j曲线的测定有显著影响.

For more information on SENT, SENB, Fracture Toughness, Fracture Mechanics Tests or to contact Rodolfo Amoriello for further discussion, 请 contact us today.