Li Xing Huo

Study on Fracture Properties of High-Density Polyethylene (HDPE) Pipe

Butt-fusion welding is the main technology to join high-density polyethylene (HDPE) plastic pipes, which are widely used in transport the water, gas and corrosive liquid. Investigation shows that one of the failure modes of HDPE pipe is the crack slowly grows across the thick direction and leads to failure at last, so that it is very important to study the resistance to crack initiation of HDPE pipe and its butt-fusion welded joint. In this study, the elastic-plastic fracture mechanics parameter, crack opening displacement (COD) is used to describe the fracture initiation behaviors for the HDPE materials and its butt-fusion welded joints. The resistance to initiation fracture of HDPE pipe materials and butt-fusion welded joints were investigated at different temperature by using multiple specimen resistance curve method and silicon-rubber replica method. The results show that saturation initial crack COD- δis of HDPE pipe materials and butt-fusion welded joints decreases with the decreasing temperature. The δis of butt-fusion welded joints is lower than that of HDPE pipe materials. Investigation also proved that the silicon-rubber replica method is more suitable for HDPE engineering material than the multiple specimen method. At the same time the statistic distribution of the δis of HDPE butt-fusion welded joint was conducted. The results show that the value of the δis has the statistic variance inherently. The optimum fitting distribution of COD is Weibull distribution with three parameters.

Reliability Calculation of Welded Pressure Pipe with Circumferential Crack Based on 3-D Elastic-Plastic SFEM

In this paper, the reliability of welded pressure pipe with circumferential surface crack was calculated by using three dimensional stochastic finite element method. This method has overcome the shortcomings of conservative results in safety assessment with deterministic fracture mechanics method. The calculation of reliability was based on three dimensional elastic-plastic stochastic finite element program which was developed by ourselves. The effects of variables such as fracture toughness, bending moment and the depth of the circumferential surface crack on the structure reliability were also discussed. The calculation results indicate that the crack depth has great effect on the reliability of the welded pipe. When the mean value of the crack depth is changed from 3mm to 7mm, the failure probability of the welded pipe will change from 10-8 to 10-2. The bending moment also has great effect on the reliability of the welded pipe. When the mean value of moment is changed from10000 N.m to 15000 N.m, the failure probability of the welded pipe increases dramatically for the same circumferential crack depth. Irrespective of the changing of moment, the pipe has higher reliability if the crack depth is less than 5mm(a/t<0.5, t is the thickness of the pipe). The method has put forward a new way for safety assessment of welded pipe with circumferential surface crack.

Effects of External Loads on the Reliability of Welded Pipe with Circumferential Crack

The reliability of welded pressure pipe with circumferential surface crack was calculated by using 3-D stochastic finite element method. This method has overcome the shortcomings of conservative results in safety assessment with deterministic fracture mechanics method. The calculation of reliability was based on 3-D elastic-plastic stochastic finite element program which was developed by ourselves. The effects of variables such as bending moment, the inner pressure on the structure reliability were discussed. The calculation results indicate that the bending moment has great effect on the reliability of the welded pipe, and the inner pressure has little effect on the reliability of the welded pipe if the inner pressure is less than 10MPa. If the mean value of the inner pressure changed from 0.5MPa to 30MPa, the failure probability will changes from 10-6 to 10-2. The bending moment also has great effect on the reliability. When the mean value of moment is changed from10000 Nm to 15000 Nm, the failure probability of the welded pipe increases dramatically for the same inner pressure. Irrespective of the changing of moment, the pipe has higher reliability if the inner pressure is less than 6 MPa. The method has put forward a new way for safety assessment of welded pipe with circumferential surface crack.

Analyses for Statistic Distribution of the Welded Joint Fracture Toughness about X65 Pressure Pipe Steel

It is important for the design engineer to realize that scatter and variability of materials properties are inevitable and must be dealt with appropriately. On occasion, data must be subjected to statistical treatments and probabilities determined. The fracture toughness of pipe steel has great effect on the safety of welded pressure pipe. The fracture toughness test of circumferential welded joint was carried out in this paper for X65 pressure pipe steel. The fracture toughness experimental results were analyzed by using computer program developed by ourselves. The distribution of weld fracture toughness of X65 pressure pipe steel which may accord with normal distribution, lognormal distribution or weibull distribution can be determined by calculating the results only one time. The program, which plays an important role in structure reliability analysis, can be used to determine the probability statistic distribution of the mechanical properties. The calculated results indicate that the weld fracture toughness of the X65 pressure pipe steel can either accord with normal distribution, lognormal distribution or weibull distribution, but the optimum fitting distribution of the fracture toughness is weibull distribution.

Effects of External Loads on the Reliability of A131 Welded Pipe with Circumferential Crack

In this paper, the reliability of welded pressure pipe with circumferential surface crack was calculated by using three dimensional stochastic finite element method for A131 pipe steel. This method has overcome the shortcomings of conservative results in safety assessment with deterministic fracture mechanics method. The calculation of reliability was based on three dimensional elastic-plastic stochastic finite element programs which were developed by ourselves. The calculation results indicate that the external load has great effect on the reliability of the welded pipe. When the mean value of bending moment changes from 10000N.m to 20000N.m, the failure probability of the welded pipe will change from 3.3750×10-9 to 1.6794×10-1. When the mean value of moment is changed from10000N.m to 20000N.m, the failure probability of the welded pipe increases dramatically for the same circumferential crack opening angle, inner pressure and the crack depth. The method has put forward a new way for safety assessment of welded pipe with circumferential surface crack.

Study on the Mechanical Bend Fatigue of Micro-Joining Soldered Joint with Lead-Free Solder

In recent years, several electronics manufacturers have been working toward introducing lead-free solder and halogen-free print circuit boards (PCBs) into their products. The key drivers for the change in materials have been the impending environmental legislations, particularly in Europe and Japan as well as the market appeal of ‘green’ products. The reliability of the new materials is an important determinant of the pace of adoption. Fairly extensive mechanical fatigue reliability data is also available for micro-joining soldered joint such as Ball Grid Array (BGA) with tin-lead solder. However, similar data is not available for BGAs assembled with lead-free solder. Mechanical reliability is a critical indicator for phone and BGA survival during repeated keypress, and to some extent during drop. In this paper, the mechanical bend fatigue of BGAs with tin-lead and lead-free solders on halogen-free substrates are examined respectively. A tin-silver-copper alloy was used as lead-free solder due to its increasing acceptance, and the results were compared to those from samples assembled with Sn63Pb37 solder. The reliability was examined at both low cycle and high cycle fatigue. Results show that the mechanical bend fatigue reliability of BGA assemblies with lead-free solder is higher than that of BGA assembly with tin-lead solder. Cross section and failure analysis indicated two distinct failure modes - solder joint and PCB failure. A 3-D parametric finite element model was developed to correlate the local PCB strains and solder joint plastic strains with the fatigue life of the assembly. The intermetallic compoumd (IMC) of micro-joining joint interface was analysised in the future in order to study on the effect of IMC on the reliability.

Hot Spot Stress Fatigue Evaluation to Duplex Stainless Steel Welded Joints

Fatigue strength of duplex stainless steel welded joints with longitudinal fillet welded guess and with longitudinal flat side guess welded on plate was evaluated by hot spot stress, hot spot stress S-N curves were produced and compared with normal stress S-N curves. The results show that the hot spot stress fatigue strength of the two kinds welded joints should be expressed by only single S-N curve while nominal stress S-N curves can only be regressed respectively. The test data are greatly above the S-N curves with slope m=3, recommended by IIW, which indicates that the slope m=3(IIW) has great safety factor in practical engineering application. But there is no appreciable difference in fatigue strength between duplex stainless steel and common structural steel with similar welded joints when fixed the test data to slope m=3.

An Improved FOSM Method for Calculating the Failure Probability of Welded Pipes with Cracks

To increase the accuracy of R-F method, it is necessary to solve the problems of the linear expansion of failure function and non-normal variables. In this paper, the improved FOSM method was applied to calculate the failure probability of welded pipes with cracks. The examples show that this method is simple, efficient and accurate for reliability safety assessment of welded pipes with cracks. It can save more time than the Monte Carlo method does, so that the improved FOSM method is recommended for general engineering reliability safety assessment of welded pipes with cracks.