Research progress in evaluation of slow crack growth resistance of polyethylene pipes based on strain hardening modulus

doi:10.19491/j.issn.1001-9278.2022.09.020

Abstract

Abstract:

In this paper， the three failure modes of pipes were summarized and the molecular mechanism and fracture mechanics mechanism of slow crack growth were briefly described. The principle and test process of the strain hardening modulus （SH） method were introduced in detail. The paper also reviewed the research progress in the correlation of material microstructures， the domestic and foreign reliability research of SH method for the evaluation of the slow crack growth resistance of pipes， and the application of SH method for the evaluation of the environmental stress cracking resistance of materials. Finally， the applications of SH method for the control of outlook on product quality was discussed.

Key words: strain hardening modulus, slow crack growth resistance, polyethylene, pipe

CLC Number:

TQ320.77

LI Yan, TANG Xiaoxu, ZHANG Weijie, HUANG Ruipeng, ZHANG Shan. Research progress in evaluation of slow crack growth resistance of polyethylene pipes based on strain hardening modulus[J]. China Plastics, 2022, 36(9): 148-159.

Figures/Tables 21

Fig.1. Failure mode of pipes

Fig.2. Molecular chain shape and structure

Fig.3. Schematic diagram of the relationship between molecular mechanism and slow crack growth resistance

Fig.4. Schematic diagram of fracture mechanics mechanism

Fig.5. Schematic diagram of crack growth

厚度/mm	压塑温度/℃	平均冷却速率（脱模温度<40 ℃）/℃·min^-1	预热条件/min	全压压力/MPa	全压时间/min
0.3或1.0	180	15±2	5~15	5	5±1

Tab.1. Conditions for compression moulding of the sheet

Fig.6. Dimensions of the test specimens

项目	尺寸/mm
夹具之间的初始距离（L）	30.0±0.5
标距长度（l₀）	12.5±0.1
窄平行部分长度（l₁）	16.0±1.0
夹区的平行部位间距（l₂）	46.0±1.0
总长度最小值（l₃）	70
半径（R₁）	10.0±0.5
半径（R₂）	8.0±0.5
窄平行部分宽度（b₁）	4.0±0.1
末端宽度（b₂）	20.0±1.0
厚度（h）	$0 . 3 - 0.03 + 0.05$ 或1.0±0.1

项目	尺寸/mm
夹具之间的初始距离（L）	30.0±0.5
标距长度（l₀）	12.5±0.1
窄平行部分长度（l₁）	16.0±1.0
夹区的平行部位间距（l₂）	46.0±1.0
总长度最小值（l₃）	70
半径（R₁）	10.0±0.5
半径（R₂）	8.0±0.5
窄平行部分宽度（b₁）	4.0±0.1
末端宽度（b₂）	20.0±1.0
厚度（h）	$0 . 3 - 0.03 + 0.05$ 或1.0±0.1

Tab.2. Dimensions of the test specimens

Fig.7. Tensile curve of the strain hardening test

试样	共聚单体类型	共聚单体含量/1 000 C	晶片厚度/nm	M_w/kg·mol^-1	M_w/M_n	应变硬化模量/MPa
PE1	1⁃己烯	3.4	144.64	564.5	27.4	71.6
PE2	1⁃己烯	2.3	177.99	468.2	20.4	63.0
PE3	1⁃丁烯	2.5	180.52	502.6	24.2	55.4
PE4	1⁃丁烯	1.5	187.30	465.3	23.0	52.6
PE5	1⁃丁烯	1.2	252.21	456.8	21.2	45.8

Tab.3. Relationship between microstructural parameters and strain hardening modulus［61］

Fig.8. MWD and SCB distribution determined by GPC and CCD distribution determined by TREF for a Ziegler⁃Natta and Chromium⁃based resins［62］

试样	催化剂	共聚单体类型	密度/kg·m^-3	M_w /kg·mol^-1	Mw/Mn	短支链分布/1000C	应变硬化模量/MPa
PEAD1	齐格勒⁃纳塔	1⁃丁烯	955.3	260	30	2.7	52.0
PEAD2	齐格勒⁃纳塔	1⁃丁烯	957.0	290	26	3.4	53.5
PEAD3	铬系	1⁃己烯	957.4	430	68	2.1	57.6
PEAD4	铬系	1⁃己烯	955.0	462	79	1.9	64.0

Tab.4. Physico⁃chemical characterization of a series of high SCG resistance commercial PE pipe resins［62］

Fig.9. Strain hardening modulus of ethylene/1⁃butene and ethylene/1⁃hexene copolymers as a function of comonomer content normalized weight average molecular weight［63］

Fig.10. Maximum natural drawing ratio of ethylene/1⁃butene and ethylene/1⁃hexene copolymers as a function of comonomer content normalized weight average molecular weight［63］

Fig.11. Relationship between strain hardening modulus and crack growth rate［66］

试样	共聚单体类型	NPT法		SH法
试样	共聚单体类型	破坏时间/h	标准偏差	应变硬化模量/MPa	标准偏差
PE1	1⁃己烯	2 275	12.58	51.4	1.28
PE2	1⁃己烯	2 300	16.5	54.8	1.15
PE3	1⁃丁烯	2 250	10.35	48.6	0.84

Tab.5. Comparison of test results and standard deviation of SH method and NPT method［68］

Fig.12. Failure curves of PE100 and PE100⁃RC materials from different raw materials suppliers with Cyclic CRB Test in direct correlation considering the three material classifications［69］

Fig.13. Correlation of SH modulus <GP> of PE 100 and PE 100⁃RC materials from different raw material suppliers determined with the SH Test［69］

Fig.14. Correlation of SH Test modulus and single point values when the global load is12.5 MPa from Cyclic CRB Test of PE 100 and PE 100⁃RC materials from different raw material suppliers［69］

Fig.15. <Gp> obtained from tensile test at room temperature and 0.5 mm/min vs stepwise isothermal segregation （SIS）ratio［70］

Fig.16. NDR obtained from tensile test at room temperature and 0.5 mm/min vs SIS ratio［70］

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