Investigation into Creep Properties of Kapton Films Under Different Initial Stresses

doi:10.12141/j.issn.1000-565X.230168

Abstract

Abstract:

As a kind of aerospace membrane material, polyimide film’s (Kapton film) creep effect is very important to its structure. In order to study its creep mechanical properties, firstly the study selected Kapton film with a thickness of 25 μm, and carried out the uniaxial creep tensile test under the four stress levels of 35%, 50%, 65% and 80% of the ultimate tensile strength. Secondly, according to the creep mechanics curves obtained, the creep characteristics of the film under different initial stresses were analyzed, and the intrinsic mechanism was discussed by combining the creep elongation. Five creep constitutive models of classical Kelvin, classical Maxwell, four-element Burgers, three-parameter generalized Kelvin and five-parameter generalized Kelvin were used to fit the experimental data, and the fitting effects of each model were compared and analyzed. The results show that Kapton film has obvious viscoelastic properties, which should be considered in the design. The initial stress has a significant effect on the creep properties of Kapton film. The greater the initial stress, the higher the strain at the initial creep stage, the higher the strain retention value at the steady creep stage, and the more obvious the viscoelasticity is. The tensile fracture stress levels in different directions lead to the differences of creep properties, and the total strain of TD (Transverse Direction) is greater than that of MD (Machine Direction) under each stress state. With the increase of the initial stress, the creep elongation of the film first increases and then decreases. This is because the influence of the initial stress on the stress state and dislocation motion inside the film is complex, and there is an equilibrium point. The five-parameter generalized Kelvin model adopted in this paper can well predict the creep properties of Kapton film, and the coefficient of determination of the fitting results is more than 0.99, followed by the Burgers model. The fitting coefficients of classical Kelvin, three-parameter generalized Kelvin and classical Maxwell model are between 0.71 and 0.86, and the fitting results meet the needs of practical engineering.

Key words: Kapton film, viscoelastic mechanics, creep properties, initial stress, creep constitutive model

CLC Number:

TU532.2

LIU Yan, XUE Xinyuan, FAN Lei, et al. Investigation into Creep Properties of Kapton Films Under Different Initial Stresses[J]. Journal of South China University of Technology(Natural Science Edition), 2024, 52(5): 127-138.

Figures/Tables 20

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Table 1

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Table 5

Parameter fitting results of Burgers model"

初始应力水平	方向	E₁	E₂	$η 1 / 1030$	η₂
35% σ_b	MD	4 479.65	101 081.81	-5.5	21 695.07
35% σ_b	TD	3 214.78	75 164.31	-5.5	17 189.11
50% σ_b	MD	2 880.30	77 548.33	-5.5	10 445.77
50% σ_b	TD	1 590.13	24 374.81	-5.5	5 939.93
65% σ_b	MD	1 234.66	19 811.71	-5.5	3 513.48
65% σ_b	TD	402.94	7 603.19	-5.5	897.70
80% σ_b	MD	851.37	19 222.41	-5.5	2 296.37
80% σ_b	TD	669.14	12 340.96	-5.5	2 000.04

Table 5

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序号	弹性模量/MPa	拉伸应变/%	拉伸断裂应力/MPa		序号	弹性模量/MPa	拉伸应变/%	拉伸断裂应力/MPa
MD1	799.046	0.659	184.725	TD1		402.384	0.928	145.348
MD2	833.900	0.610	174.478	TD2		335.288	1.010	141.940
MD3	862.178	0.599	173.737	TD3		380.357	0.977	149.217
MD4	925.843	0.552	168.353	TD4		404.123	0.924	144.461
MD5	806.752	0.670	189.018		TD5	435.310	0.894	144.994
平均值	845.543 7	0.617 8	178.062	平均值		391.492	0.946 4	145.192

初始应力水平	方向	总应变量/mm	加载应变量/mm	绝对蠕变量/mm	蠕变伸长率/%
35% σ_b	MD	5.497	3.506	1.991	56.79
35% σ_b	TD	5.546	3.289	2.257	68.62
50% σ_b	MD	11.196	6.961	4.235	60.84
50% σ_b	TD	14.715	7.683	7.032	91.53
65% σ_b	MD	29.114	14.066	15.048	106.98
65% σ_b	TD	61.471	25.811	35.660	138.16
80% σ_b	MD	58.625	34.613	24.012	69.37
80% σ_b	TD	92.605	49.751	42.854	86.14

初始应力水平	方向	E₀	E₁	η₁
35% σ_b	MD	4 479.92	1.00	7.90
35% σ_b	TD	3 214.96	1.00	7.89
50% σ_b	MD	2 880.48	1.00	7.90
50% σ_b	TD	1 590.31	1.00	7.88
65% σ_b	MD	1 234.82	1.00	9.99
65% σ_b	TD	403.01	1.00	20.00
80% σ_b	MD	851.43	1.00	19.98
80% σ_b	TD	669.20	1.01	99.85

初始应力水平	方向	E₀	E₁	E₂	η₁	η₂
35% σ_b	MD	13 683.81	8 337.76	7 439.01	9 827.04	292 746.47
35% σ_b	TD	8 344.70	6 074.05	5 668.84	9 423.46	260 792.92
50% σ_b	MD	9 235.89	4 908.66	6 054.88	8 999.84	207 836.36
50% σ_b	TD	4 408.88	2 829.81	1 845.91	8 309.39	94 763.48
65% σ_b	MD	10 248.10	1 817.93	1 474.09	3 683.70	51 740.00
65% σ_b	TD	9.996×10⁷	463.98	608.11	2 552.23	41 774.98
80% σ_b	MD	3.978×10⁹	945.39	1 544.48	5 754.27	135 252.36
80% σ_b	TD	9.996×10⁸	987.57	671.79	144 310.41	6 173.63

初始应力水平	可决系数
	经典Kelvin模型		经典Maxwell模型		Burgers模型		三参数Kelvin模型		五参数Kelvin模型
	MD	TD	MD	TD	MD	TD	MD	TD	MD	TD
35% σ_b	0.728	0.714	0.813	0.823	0.967	0.954	0.813	0.823	0.997	0.995
50% σ_b	0.734	0.729	0.831	0.848	0.943	0.971	0.831	0.848	0.993	0.997
65% σ_b	0.807	0.852	0.788	0.718	0.975	0.957	0.788	0.718	0.999	0.997
80% σ_b	0.800	0.829	0.713	0.778	0.943	0.942	0.713	0.778	0.996	0.994