孔隙对碳纤维增强环氧树脂复合材料超声衰减系数及压缩性能的影响

史俊伟; 刘松平; 荀国立; 杨刚

doi:10.13801/j.cnki.fhclxb.20191008.001

孔隙对碳纤维增强环氧树脂复合材料超声衰减系数及压缩性能的影响

doi: 10.13801/j.cnki.fhclxb.20191008.001

中航复合材料有限责任公司　检测与评估研究室，北京 101300

基金项目: 国家自然科学基金(61571409；60727001)

详细信息

通讯作者:
刘松平，博士，研究员，博士生导师，研究方向为复合材料及焊接无损检测与评估 E-mail：liusping2014@163.com

中图分类号: TB332
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出版历程
- 收稿日期: 2019-06-20
- 录用日期: 2019-09-02
- 网络出版日期: 2019-10-09
- 刊出日期: 2020-06-15

Effects of voids on ultrasonic attenuation coefficient and compressive properties of carbon fiber/epoxy resin composite

Laboratory of Nondestructive Testing and Evaluation, AVIC Composite Corporation CO. LTD., Beijing 101300, China

摘要

摘要: 孔隙对碳纤维增强环氧树脂（CF/EP）复合材料的力学性能和破坏模式有显著的影响，因此需要建立准确的孔隙率无损检测评估方法，并基于所评估的孔隙率提高CF/EP复合材料压缩性能预测的可靠性。本文主要研究了孔隙对CF/EP复合材料的超声衰减系数和压缩性能的影响，通过降低固化压力至0.7~0.2 MPa和延长预浸料室温贮存时间至30~180天的方法，制备了不同孔隙率的CF/EP复合材料层压板，通过金相验证其孔隙率在0%~3.0%之间，孔隙类型主要为层中孔隙和层间孔隙。通过理论和试验的方法，基于超声反射法建立了孔隙率与超声衰减系数的关系曲线，由孔隙引起超声衰减系数为α_v=1.08P_v²(P_v为孔隙率)，与前人基于超声穿透法所得的超声衰减系数α_v=0.61P_v²较好地符合2倍声程的关系。对不同孔隙率的CF/EP复合材料层压板进行压缩测试实验，特别考虑了贴片和加载方向对测试结果的影响。从细观角度研究了含孔隙的CF/EP复合材料层压板的压缩破坏模式。结果表明：CF/EP复合材料层压板的压缩强度随孔隙率增加而下降，孔隙率增加至2.5%时，压缩强度下降13.7%，孔隙细观特征影响压缩破坏的形式，主要原因是孔隙诱发微裂纹的萌生和扩展，削弱了纤维与树脂间的结合力并引发纤维微屈曲。
- 复合材料 /
- 孔隙率 /
- 超声无损检测 /
- 超声衰减 /
- 压缩强度 /
- 破坏模式
Abstract: Voids produced in the manufacturing process have significant influence on the mechanical properties and failure modes of carbon fiber reinforced epoxy resin (CF/EP) composite, which highly desire an accurate nondestructive evaluation of the void content and a reliable prediction of compressive properties based on the measured void content. In this paper, the effects of voids on ultrasonic attenuation coefficient and compressive properties were investigated extensively. CF/EP composite laminates were fabricated of quasi-isotropic layups and produced by applying various autoclave pressures of 0.7-0.2 MPa and increasing prepreg out storage time of 30-180 days to acquire controlled levels of void content. The void content, ranged from 0% to 3.0%, as well as the microstructure of intra- and inter-laminate voids could be verified and characterized using light microscopy. Ultrasonic reflection test was performed on laminates containing voids and the correlation between ultrasonic attenuation and void content, namely α_v=1.08P_v²(P_vis void content), was established theoretically and experimentally, which gave good duple-path agreement with published experimental results of ultrasonic through-transmission test, namely α_v=0.61P_v². Then, the dependency of the reduced compressive strength on laminates containing voids was investigated experimentally by counter-posing the compressive performances of four types of CF/EP composite laminate specimens, namely 0° un-tabbed, 90° un-tabbed, 0° tabbed and 90° tabbed. The result shows that the compressive strength of CF/EP composite laminates significantly decreases with the increasing of void content, saying a drop of 13.7% with void content up to 2.5%.The microscopic examination of the compressive failure modes shows that the microstructure of voids of CF/EP composite laminates also has a significant effect on the compressive failure mode, which can be attributed to the initiation and propagation of micro cracks, the reduced fiber-matrix bonding as well as the micro-buckling of fibers.
- composites /
- void content /
- ultrasonic nondestructive testing /
- ultrasonic attenuation /
- compressive strength /
- failure mode

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图 1 IMA/M21 CF/EP复合材料单向带预浸料的标准固化曲线

Figure 1. Typical curing curves for IMA/M21 CF/EP composite unidirectional prepreg

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图 2 在水中检测CF/EP复合材料层压板时换能器信号的时域/频域波形

Figure 2. Transducer signal waveform from CF/EP composite laminate immersed in water

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图 3 由超声C-Scan检测结果的灰度分布确定典型孔隙率的取样区域

Figure 3. Sampling areas of typical void content determined by grey distribution of ultrasonic C-Scan results

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图 4 压缩试样取样示意图

Figure 4. Schematic of compressive sample preparation

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图 5 分别取样自延长贮存时间(a)和降低固化压力(b)所制备的CF/EP复合材料层压板孔隙形貌

Figure 5. Metallographic images of CF/EP composite laminates with voids, respectively obtained by increasing prepreg out storage time (a) and reducing autoclave pressure (b)

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图 6 分别取样自延长贮存时间(a)和降低固化压力(b)所制备的CF/EP复合材料层压板的超声成像

Figure 6. Ultrasonic C-Scan images of CF/EP composite laminates with voids, respectively obtained by increasing prepreg out storage time (a) and reducing autoclave pressure (b)

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图 7 超声反射法几何示意图

Figure 7. Geometry and notation for ultrasonic reflection test

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图 8 沿CF/EP复合材料层压板法线方向传播的纵波声速

Figure 8. Velocity of longitudinal waves propagating along the normal direction of CF/EP composite laminate

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图 9 零孔隙率的CF/EP复合材料层压板中衰减与厚度的关系

Figure 9. Attenuation of ultrasound by CF/EP composite laminates stepwise increasing with thickness in zero void content laminates

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图 10 CF/EP复合材料层压板超声反射法中衰减系数α_v与孔隙率P_v的关系

Figure 10. Attenuation of ultrasound plotted against the void content in a regular relationship for CF/EP composite laminates

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图 11 贴片和非贴片条件下CF/EP复合材料层压板孔隙率对压缩强度的影响

Figure 11. Effects of void content on compression strength of CF/EP composite laminates in terms of tabbed and un-tabbed

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图 12 CF/EP复合材料层压板的压缩失效模式对比

Figure 12. Comparison of the compressive failure modes in CF/EP composite laminates

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图 13 CF/EP复合材料层压板的压缩强度与孔隙率的关系

Figure 13. Compression strength as a function of void content in CF/EP composite laminates

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图 14 CF/EP复合材料层压板中孔隙的位置和微观形貌（黑色箭头：层中孔隙；红色箭头：层间孔隙）

Figure 14. Positions and morphologies of voids in CF/EP composite laminates (Blank arrows: Intralaminar voids; Red arrows: Interlaminar voids)

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图 15 CF/EP复合材料层压板中孔隙周边的微裂纹

Figure 15. Microcrack initiation and propagation with a void inclusion in CF/EP composite laminates

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图 16 含孔隙的CF/EP复合材料层压板的纵向压缩破坏模式

Figure 16. Longitudinal compression failure modes in void-containing CF/EP composite laminates

((a) Tension-compression mode fiber bucking; (b) Shear mode fiber bucking; (c) Shear fracture; (d) Interlaminar lateral-cracking delamination; (e) Intralaminar lateral-cracking delamination)

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图 17 含孔隙的CF/EP复合材料层压板中的树脂基体破坏

Figure 17. Resin fracture in void-containing CF/EP composite laminates

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表 1 碳纤维/环氧树脂(CF/EP)单向带预浸料的力学性能

Table 1. Mechanical properties of cured carbon fiber/epoxy resin (CF/EP) unidirectional prepreg

Mechanical properties	Strength/MPa	Modulus/GPa
0°-tension	3 050	178
0°-compression	1 500	146
90°-compression	−	13.7
In-plane shear	94	5.2
Interlaminar shear	97	−
Note: Properties are experimentally characterized at 23℃ by Hexcel.

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表 2 CF/EP复合材料层压板上典型区域的孔隙含量

Table 2. Void content of typical areas in CF/EP composite laminates

Sampling area No.	Determined by ultrasonic test/%	Determined by light microscopy/%
1	0−0.5	0
2^*	0.5−1.0	0.68
3	1.0−1.5	1.05
4	1.0−1.5	1.37
5^*	1.5−2.0	1.52
6	1.5−2.0	1.75
7^*	1.5−2.0	1.75
8	2.0−2.5	2.33
9^*	2.5−3.0	2.76
Note: Equal-grey-distribution areas indicated by superscripts ^* are not large enough for compressive test sampling.

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表 3 水和CF/EP复合材料的声学常数

Table 3. Acoustic constants of water and CF/EP composite

	Densityρ/ (kg·m⁻³)	Velocity c/ (m·s⁻¹)	Acoustic impedance Z/ (10³kg·m⁻²·s⁻¹)
Water^[35]	998	1 483	1 480
CF/EP	1 580	2 940	4 708

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表 4 不同厚度的零孔隙率CF/EP复合材料层压板中所测得的衰减

Table 4. Experimental attenuation of ultrasound by zero void content CF/EP composite laminates with varying thicknesses

Thickness h/mm	Voltage V_f/V	Voltage V_b/V	Mean attenuation A_l/dB	Standard deviation S₁/dB
2.06	160.39	130.58	1.79	0.56
3.90	158.75	70.60	7.04	0.26
5.02	156.33	55.65	8.97	0.06
7.02	159.42	34.22	13.36	0.30
9.02	155.55	25.75	15.62	0.24
12.68	159.88	17.72	19.11	0.20

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表 5 不同孔隙率的CF/EP复合材料层压板所测得的声波衰减

Table 5. Experimental attenuation of ultrasound by CF/EP composite laminates with varying void contents

Area No.	Void content P_v/%	Mean attenuation A_total/dB	Standard deviation S_total/dB	Attenuation coefficient α_v/(dB·mm⁻¹)
1	0	7.04	0.26	−0.23^*
2_a	0.68	10.00	1.13	0.51
2_b	0.68	10.00	1.44	0.51
3_a	1.05	12.50	0.82	1.14
3_b	1.05	12.76	1.72	1.20
4	1.37	15.66	3.25	1.93
5	1.52	17.50	4.23	2.39
6_a	1.75	18.78	0.62	2.71
6_b	1.75	19.13	0.75	2.79
7	1.75	20.95	2.93	3.25
8_a	2.33	31.46	2.00	5.88
8_b	2.33	35.61	1.97	6.91
9_a	2.76	38.01	1.05	7.52
9_b	2.76	41.61	2.16	8.41
Note: If the equal-grey-distribution area is large enough, ultrasonic test will be carried out left and right to the metallographic sampling area, indicated by subscripts a and b respectively.

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表 6 不同孔隙率下CF/EP复合材料层压板的压缩强度

Table 6. Compression strengths of CF/EP composite laminates at various void contents

Sample type	Void content P_v/%	Compression strength $\bar \sigma $/MPa	Standard deviation S_n−1/MPa	Coefficient of variation CV/%
0° un-tabbed	0	677	60	8.9
	1.05	642	54	8.5
	1.37	644	24	3.7
	1.75	632	26	4.1
	2.33	580	31	5.3
90° un-tabbed	0	673	19	2.9
	1.05	632	54	8.6
	1.37	616	45	7.3
	1.75	599	38	6.3
	2.33	505	35	6.9
0° tabbed	0	737	52	7.1
	1.05	699	54	7.8
	1.37	685	34	5.0
	1.75	678	46	6.9
	2.33	636	14	2.2
90° tabbed	0	664	21	3.1
	1.05	656	39	5.9
	1.37	625	64	10.2
	1.75	614	16	2.6
	2.33	573	40	7.0

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