- Email: [email protected]
The influence of adhesive porosity on composite joints
Composites Communications 15 (2019) 87–91
Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
Short Communication
The influence of adhesive porosity on composite joints Ting Zhang
a,b,c,*
d
, Jia Meng , Qing Pan
a,b
, Baozhong Sun
T
c
a
Department of Discipline Engineering, AECC Commercial Aircraft Engine Co, Ltd, Shanghai 200241, China Shanghai Engineering Research Center of Civil Aero Engine, Shanghai 200241, China Key Laboratory of High performance fibers & products, Ministry of Education, Donghua University , Shanghai 201620, China d Shanghai Aircraft Manufacturing Co, Ltd, Shanghai 200436, China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Composite joints Adhesive film porosity Weak bond SLS strength
Polytetrafluoroethylene (PTFE) films with different pore percent were placed at the bonding interface as artificial defects and then a method of fabricating weak bond joints with different bond strength was developed to simulate the adhesive film porosity likely occurring in polymer composite bond joint. It was found that the adhesive interfacial back wave height is approximately linear functions of PTFE pore percent. The joint strength also approximately decreases linearly as the PTFE pore percent decreases. Based on the test and simulation results, functions were proposed to estimate the adhesive porosity and single lap shear (SLS) strength with respect to the A-scan image and single lap shear strength curve in this paper.
1. Introduction Composite bonding structures are widely used, from automotive components to aircraft structures, owing to their advantages such as cost saving and weight reduction [1,2]. One of the limitations of adhesive joints is the difficulty in predicting the joint strength and the failure modes due to the presence of defects in the adhesive [3]. As researched by Bardis and Tomblin in FAA research group [4,5], there are many factors including poor bond thickness control, variations in the adhesive materials, improper mixing of epoxies, pre-bonding adhesive moisture, improper cure parameter, and harsh service environments etc., which will affect the bonding quality and cause adhesion failures or weak bonds. As well, any manufacturing process will inevitably introduce some defects in the final product and it is almost impossible to produce a defect-free joint even under strict controls [6]. Therefore, the prediction of the failure modes and experimental characterization of the performance still remains a challenge, especially for the weak bonds where the bond line has lower mechanical properties than expected [7,8] and which has no possibility to be detected by normal NDI (Non-destructive inspection) procedures [9–11]. However, for a design of the structure, it is much more important to know how defects affect the strength of the bonded component and how to analyze and estimate the strength of a bonding structure with some inevitable defects, especially for some weak bonds caused by the presence of bond defects (such as voids or porosity). In this paper, a method of fabricating weak bonded composite-to-
∗
composite panels was firstly developed to simulate the adhesive film porosity in the bonding interface; then NDI method was used to detect defects in those weak bond panels and the relationship between the NDI sign with the adhesive film porosity was achieved; finally the single lap shear(SLS)testing was used to analyze effects of adhesive film porosity on the mechanical properties of those panels, which could link up the relationship between the NDI signal with the mechanical properties for a bond structure. In addition, bonding structures may include composite-to-composite, composite-to-metal, and metal-to-metal structure. The nature and technical parameters that govern those bonding structures are, in essence, the same or closely related, therefore, this method could be extended to all bonding structure. 2. Experimental detail 2.1. Materials, geometry and manufacture of SLS specimen [12,13]. Two 2 mm-thick carbon fiber reinforced composite panel with the size of 150 mm × 320 mm was manufactured from aerospace grade unidirectional prepreg using manual laying-up and autoclave cure. A modified epoxy adhesive film was applied to the 270 mm × 12 mm overlap region of the bonding joint. Four kinds of PTFE film with different pore percent were embedded over the center of the adhesive film, and a kind of well bonded joint without PTFE film in the overlap region was design as well. Curing of the joints was performed in a heating oven at 180 °C, using a ramp rate of 2 °C/min from ambient, and holding at
Corresponding author. Department of Discipline Engineering, AECC Commercial Aircraft Engine Co, Ltd, Shanghai 200241, China. E-mail address: [email protected] (T. Zhang).
https://doi.org/10.1016/j.coco.2019.06.011 Received 18 May 2019; Received in revised form 21 June 2019; Accepted 22 June 2019 Available online 27 June 2019 2452-2139/ © 2019 Elsevier Ltd. All rights reserved.
Composites Communications 15 (2019) 87–91
T. Zhang, et al.
without deficiencies are list in Table 1. All tests for this test matrix were Room Temperature Dry (RTD) with the moisture content as fabricated.
Table 1 Bond categories in single-lap joints. Category
kissing bond weak bond weak bond weak bond weak bond good bond
Test
SLS SLS SLS SLS SLS SLS
Average PTFE pore percent N (%)
Void percent N' (%)
Thickness (mm)
Label
0 18 32 50 64 100
100 82 68 50 36 0
– 2 2 2 2 2
– SLS-WB-A SLS-WB-B SLS-WB-C SLS-WB-D SLS-GB-N
2.2. Instrumentation and tensile test 2.2.1. Non-destructive inspection A-scan was done with the gain of 22.7 dB, and the scanning probe with diameter of φ 12 mm and an ultrasound frequency of 5 MHz were used (EPOCH650 system). 2.2.2. Test of SLS specimen 2.2.2.1. Test setup. Testing was carried out using displacement control and loaded the specimens at 0.05 in/min. Data for the load and cross head displacement was plotted in real time and saved for each specimen. After specimen failure, the peak load value, lap shear strength and the failure mode were recorded, and the specimen was removed from the clevis arrangement.
180 °C ± 5 °C for 120 min. The constant pressure of 0.7 MPa was uniformly applied. This was in accordance with the adhesive specifications, and sufficient to reach the bond maximum strength. The structure of SLS specimen was advised by ASTM D 3163 [13]. The detailed parameters for five groups of single-lap joints with and
Fig. 1. A-scan images (ae) and curve (f) before mechanics properties testing. 88
Composites Communications 15 (2019) 87–91
T. Zhang, et al.
3. Test results and discussions 3.1. NDI test result Five groups of specimens shown in Table 1 are measured by the ultrasonic A-scanned machine using the parameters given in section 2.2.1before cutting. The PTFE pore percent of 100% represents good bond and the PTFE pore percent of 18%, 32%, 50%, and 64% represent weak bond. The A-scanned images of five groups of specimens are shown in Fig. 1. Note that the A-scan curves represent attenuation of a probe signal through the two composite substrates and the adhesive (bond), so the three back waves in each figure represent surface back wave, bottom back wave and adhesive interfacial back wave respectively. We firstly adjusted the adhesive interfacial back wave height of good bond joint as 18, then we noticed that the height of adhesive interfacial back wave increases with the decreasing of PTFE pore percent as shown in Fig. 1(a). According to Fig. 1(b-e), we calculated that the average adhesive interfacial back wave height of specimen with PTFE pore percent 18%, 32%, 50%, and 64% are 63,49,42 and 32 respectively. The average adhesive interfacial back wave heights of all five groups of specimens were plotted as function of PTFE pore percent as shown in Fig. 1(f). It can be observed that the adhesive interfacial back wave height is generally linear functions of PTFE pore percent. According to Fig. 1(f), for a new specimen, if we can measure its adhesive interfacial back wave height, then we can calculate its PTFE pore percent by deviation calculation method. Owing to the PTFE pore percent N can be converted to adhesive film porosity N' by formula N′=(1-N), so we can cursorily calculate the adhesive film porosity by the adhesive interfacial back wave height.
3.2. SLS test results 3.2.1. Failure load and shear strength The SLS test was used to obtain failure load and evaluate the adhesive joint strength as a function of PTFE pore percent. Consistent data was obtained for each bond category and the Coefficient of Variation (COV) value for each bond group are all less than 4% as shown in Fig. 2(a), which means the design and fabricated method could provide reproducible data. The group A (N = 18%) provided the most reproducible data while the group D (N = 64%) provided the largest spread in failure loads. The failure loads almost linearly increase as the pore percent N increase from 18% to 64%, while when the pore percent equals to 100%, the failure load creeps up to 11.38 KN, which means the inserted PTFE film not only decreases bonding strength, but also affects the structural integrity. The red line in Fig. 2(a) represents the calculated peak load assuming the peak load is the function of bonding area. As seen in Fig. 2(a), the experimental results (dark yellow column) are higher than the calculated result (red line). As the pore percent increases, the experimental result is infinitely close to the calculated result. The flow of the adhesive film during the curing increasing actually bonding areas could explain this phenomenon. The average shear strength σmax of the five specimens in each group was calculated according to the formula in section 2.2.2. Fig. 3(b) shows the relationship between average shear strength with pore percent N. It can be seen that the average shear strength of good bonded specimens (group N) is the largest among all those five groups of specimens. The average shear strength is increased from 13.4MPa (group A) to 35.67MPa (group N) as the PEFT pore percent of the adhesive layer increases from 18% to 100%. Among all weak bonding joint categories, there is an approximately linear decrease in the joint strength as the pore percent decreases. When the pore percent decreases to 18%, the average shear strength reaches to13.4MPa which is about 39% of the average shear strength of good bonded joints.
Fig. 2. Comparison of failure loads (a) and lap shear strength (b) in joints with different PEFT pore percent.
Fig. 3. Microscopic images of the bond joints after failure (a) five groups of specimen ; (b) N = 18% ; (c) N = 64%.
2.2.2.2. Single lap shear strength calculation. The single lap shear strength could be calculated as below formula:
σmax =
Pmax A
(1)
Pmax is the peak load recorded for the test; A is the overlap shear area and σmax is the maximum lap shear strength of the overlap region found using the particular test method. 89
Composites Communications 15 (2019) 87–91
T. Zhang, et al.
Fig. 4. Simulation image(a) pore random distribution (b) pore regular distribution; (c) failure load versus void percent.
0.1 mm in the through-thickness direction. Material for the cohesive element used bilinear mixed mode failure criteria. The constraints were fixed at one end and a displacement was imposed at the other end through multi-point constrained boundary conditions. The influence of PTFE pore distribution (regular distribution and random distribution) was considered when simulated. Fig. 4(c) shows the curve of failure load versus PTFE pore percent, respectively, at the critical location. It can been seen that the failure load increases as PTFE pore percent increases by test result(column with dense pattern) and also simulation (blue column and dark yellow column). As well, the pore distribution has influence on the peak load for SLS specimens and the influence is not significant when the pore percent is high. When the pore percent is higher than 64%, the simulative peak load is much close to the test result, which indicates that the simulation model has good prediction in the failure load for SLS specimens with low porosity (high PTFE pore percent). Also, the simulation model and method will be improved in future for better accuracy and consistency.
3.2.2. Damage morphology of bonding joint The typical damage morphology of bonding joint for five groups of specimen taken by camera is shown in Fig. 3(a). Fig. 3(b) and (c) are the image of detailed failure mode for group A and D with large magnification. Green and black regions represent the adhesive and the surface of the composite respectively. A combination of interface and cohesive failure was observed for the good bond joint (group N); while for the week bond joints, it is obviously that all joints failed at the bond interface of the PTFE films with the upper pre-cured composite substrate. This was evident from the microscopic images in Fig. 3(a) where the adhesive has been observed to bond to the substrate throng the PTFE film pore and peeled from the PTFE film as such. The observed sudden drop in load level occurring at the ultimate failure point is also an indication of the dominant interfacial failure. As the pore size and density increase, the flow of adhesive was getting more serious, which can be proved by the adhesive residual on the PTFE film as shown in Fig. 3(b) and (c). The flow of the adhesive film during curing also could partially explain why the experimental failure load (dark yellow column) is higher than the calculated result according to the bonding area (red line) in Fig. 2(a).
4. Conclusions The current research provided a comparative study of weak adhesively bonded joints. A method of fabricating weak bond joint with different bond strength was developed. The A-scan and SLS test were used to study the influence of adhesive film porosity on the shear strength. It was found that the thin perforated PTFE films embedded in the bonded joint could simulate the adhesive porosity often occurring in polymer composite bond joint. The adhesive interfacial back wave height is approximately linear functions of pore percent. As well, the
3.3. Simulation results A numerical implementation is performed in Abacus/Standard 2D to predict the SLS strength of the specimens with different PTFE pore percent N in the bonding interface as shown in Table 1. As shown in Fig. 4(a-b), the glue elements were modeled with cohesive elements (COH3D8) in standard Abacus. Mesh size is 1.27 *1.27 mm in plane and 90
Composites Communications 15 (2019) 87–91
T. Zhang, et al.
joint strength also approximately linear decreases as the pore percent of PTFE film decreases. Based on above test and simulation results, functions were proposed to estimate the adhesive film porosity and SLS strength with respect to the A-scan curve and lap shear strength curve in this paper.
Struct. 96 (2013) 256–261. [4] J. Bardis, K. Kedward, Effects of Surface Preparation on the Long-Term Durability of Adhesively Bonded Composite Joints, FAA report DOT/FAA/AR-03/53 (January 2004). [5] J.S. Tomblin, C. Yang, P. Harter, Investigation of Thick Bond-Line Adhesive Joints, FAA report DOT/FAA/AR-01/33 (June 2001). [6] K. Smith, Bonded Joints and Structures-Technical Issues and Certification Considerations, FAA Memorandum PS-ACE100-2005-10038 (September 2005). [7] M.D. Bane, L.F.M. da Silva, Adhesively bonded joints in composite materials: an overview, Proc. IMechE Part L: J. Mater. Des. Appl. 223 (2008) 1–18. [8] S. Budhea, M.D. Banea, S. de Barros, L.F.M. da Silva, An updated review of adhesively bonded joints in composite materials, Int. J. Adhesion Adhes. 72 (2017) 30–42. [9] C.C.H. Guyott, P. Cawley, R.D. Adams, The non-destructive testing of adhesively bonded structure: a review, J. Adhes. 20 (2) (1986) 129–159. [10] J.L. Rose, Ultrasonic Nondestructive Valuation Technology for Adhesive Bond and Composite Material Inspection, Adhesive Bonding, Plenum Publishing, Corp, NewYork, NY, 1991, pp. 425–448. [11] M. Michael, J.G. Capetta, A.H. Davidoff, Advanced Ultrasonic Sign Alanalys Is for Bond Strength and Materials Characterization, Nondestructive Characterization for Advanced Technologies, The American Society for Nondestructive Testing, Columbus, OH, 1991, pp. 132–134. [12] ASTM D 1002-10, Standard Test Method for Apparent Shear Strength of Single-LapJoint Adhesively Bonded Metal Specimens by Tension Loading (Metal-To- Metal). [s], America Annual Book of Standards, 2010. [13] ASTM D 3163-01, Standard Test Method for Determining Strength of Adhesively Bonded Rigid Plastic Lap-Shear Joints in Shear by Tension Loading [s], America, Annual Book of Standards, 2014.
Acknowledgements The financial support provided by AECC Commercial Aircraft Engine Co., Ltd., China and the Fundamental Research Funds for the Central Universities in China through Grant No.2232019G-02 to the work presented here is gratefully acknowledged. This work was also supported by the Science and Technology Commission of Shanghai Municipality, China (18DZ2204600). References [1] K.B. Katnam, L.F.M. Da Silva, T.M. Young, Bonded repair of composite aircraft structures: a review of scientific challenges and opportunities, Prog. Aero. Sci. 61 (2013) 26–42. [2] A. Higgins, Adhesive bonding of aircraft structures, Int. J. Adhesion Adhes. 20 (5) (2000) 367–376. [3] G. Mancusi, F. Ascione, Performance at collapse of adhesive bonding, Compos.
91
Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
Short Communication
The influence of adhesive porosity on composite joints Ting Zhang
a,b,c,*
d
, Jia Meng , Qing Pan
a,b
, Baozhong Sun
T
c
a
Department of Discipline Engineering, AECC Commercial Aircraft Engine Co, Ltd, Shanghai 200241, China Shanghai Engineering Research Center of Civil Aero Engine, Shanghai 200241, China Key Laboratory of High performance fibers & products, Ministry of Education, Donghua University , Shanghai 201620, China d Shanghai Aircraft Manufacturing Co, Ltd, Shanghai 200436, China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Composite joints Adhesive film porosity Weak bond SLS strength
Polytetrafluoroethylene (PTFE) films with different pore percent were placed at the bonding interface as artificial defects and then a method of fabricating weak bond joints with different bond strength was developed to simulate the adhesive film porosity likely occurring in polymer composite bond joint. It was found that the adhesive interfacial back wave height is approximately linear functions of PTFE pore percent. The joint strength also approximately decreases linearly as the PTFE pore percent decreases. Based on the test and simulation results, functions were proposed to estimate the adhesive porosity and single lap shear (SLS) strength with respect to the A-scan image and single lap shear strength curve in this paper.
1. Introduction Composite bonding structures are widely used, from automotive components to aircraft structures, owing to their advantages such as cost saving and weight reduction [1,2]. One of the limitations of adhesive joints is the difficulty in predicting the joint strength and the failure modes due to the presence of defects in the adhesive [3]. As researched by Bardis and Tomblin in FAA research group [4,5], there are many factors including poor bond thickness control, variations in the adhesive materials, improper mixing of epoxies, pre-bonding adhesive moisture, improper cure parameter, and harsh service environments etc., which will affect the bonding quality and cause adhesion failures or weak bonds. As well, any manufacturing process will inevitably introduce some defects in the final product and it is almost impossible to produce a defect-free joint even under strict controls [6]. Therefore, the prediction of the failure modes and experimental characterization of the performance still remains a challenge, especially for the weak bonds where the bond line has lower mechanical properties than expected [7,8] and which has no possibility to be detected by normal NDI (Non-destructive inspection) procedures [9–11]. However, for a design of the structure, it is much more important to know how defects affect the strength of the bonded component and how to analyze and estimate the strength of a bonding structure with some inevitable defects, especially for some weak bonds caused by the presence of bond defects (such as voids or porosity). In this paper, a method of fabricating weak bonded composite-to-
∗
composite panels was firstly developed to simulate the adhesive film porosity in the bonding interface; then NDI method was used to detect defects in those weak bond panels and the relationship between the NDI sign with the adhesive film porosity was achieved; finally the single lap shear(SLS)testing was used to analyze effects of adhesive film porosity on the mechanical properties of those panels, which could link up the relationship between the NDI signal with the mechanical properties for a bond structure. In addition, bonding structures may include composite-to-composite, composite-to-metal, and metal-to-metal structure. The nature and technical parameters that govern those bonding structures are, in essence, the same or closely related, therefore, this method could be extended to all bonding structure. 2. Experimental detail 2.1. Materials, geometry and manufacture of SLS specimen [12,13]. Two 2 mm-thick carbon fiber reinforced composite panel with the size of 150 mm × 320 mm was manufactured from aerospace grade unidirectional prepreg using manual laying-up and autoclave cure. A modified epoxy adhesive film was applied to the 270 mm × 12 mm overlap region of the bonding joint. Four kinds of PTFE film with different pore percent were embedded over the center of the adhesive film, and a kind of well bonded joint without PTFE film in the overlap region was design as well. Curing of the joints was performed in a heating oven at 180 °C, using a ramp rate of 2 °C/min from ambient, and holding at
Corresponding author. Department of Discipline Engineering, AECC Commercial Aircraft Engine Co, Ltd, Shanghai 200241, China. E-mail address: [email protected] (T. Zhang).
https://doi.org/10.1016/j.coco.2019.06.011 Received 18 May 2019; Received in revised form 21 June 2019; Accepted 22 June 2019 Available online 27 June 2019 2452-2139/ © 2019 Elsevier Ltd. All rights reserved.
Composites Communications 15 (2019) 87–91
T. Zhang, et al.
without deficiencies are list in Table 1. All tests for this test matrix were Room Temperature Dry (RTD) with the moisture content as fabricated.
Table 1 Bond categories in single-lap joints. Category
kissing bond weak bond weak bond weak bond weak bond good bond
Test
SLS SLS SLS SLS SLS SLS
Average PTFE pore percent N (%)
Void percent N' (%)
Thickness (mm)
Label
0 18 32 50 64 100
100 82 68 50 36 0
– 2 2 2 2 2
– SLS-WB-A SLS-WB-B SLS-WB-C SLS-WB-D SLS-GB-N
2.2. Instrumentation and tensile test 2.2.1. Non-destructive inspection A-scan was done with the gain of 22.7 dB, and the scanning probe with diameter of φ 12 mm and an ultrasound frequency of 5 MHz were used (EPOCH650 system). 2.2.2. Test of SLS specimen 2.2.2.1. Test setup. Testing was carried out using displacement control and loaded the specimens at 0.05 in/min. Data for the load and cross head displacement was plotted in real time and saved for each specimen. After specimen failure, the peak load value, lap shear strength and the failure mode were recorded, and the specimen was removed from the clevis arrangement.
180 °C ± 5 °C for 120 min. The constant pressure of 0.7 MPa was uniformly applied. This was in accordance with the adhesive specifications, and sufficient to reach the bond maximum strength. The structure of SLS specimen was advised by ASTM D 3163 [13]. The detailed parameters for five groups of single-lap joints with and
Fig. 1. A-scan images (ae) and curve (f) before mechanics properties testing. 88
Composites Communications 15 (2019) 87–91
T. Zhang, et al.
3. Test results and discussions 3.1. NDI test result Five groups of specimens shown in Table 1 are measured by the ultrasonic A-scanned machine using the parameters given in section 2.2.1before cutting. The PTFE pore percent of 100% represents good bond and the PTFE pore percent of 18%, 32%, 50%, and 64% represent weak bond. The A-scanned images of five groups of specimens are shown in Fig. 1. Note that the A-scan curves represent attenuation of a probe signal through the two composite substrates and the adhesive (bond), so the three back waves in each figure represent surface back wave, bottom back wave and adhesive interfacial back wave respectively. We firstly adjusted the adhesive interfacial back wave height of good bond joint as 18, then we noticed that the height of adhesive interfacial back wave increases with the decreasing of PTFE pore percent as shown in Fig. 1(a). According to Fig. 1(b-e), we calculated that the average adhesive interfacial back wave height of specimen with PTFE pore percent 18%, 32%, 50%, and 64% are 63,49,42 and 32 respectively. The average adhesive interfacial back wave heights of all five groups of specimens were plotted as function of PTFE pore percent as shown in Fig. 1(f). It can be observed that the adhesive interfacial back wave height is generally linear functions of PTFE pore percent. According to Fig. 1(f), for a new specimen, if we can measure its adhesive interfacial back wave height, then we can calculate its PTFE pore percent by deviation calculation method. Owing to the PTFE pore percent N can be converted to adhesive film porosity N' by formula N′=(1-N), so we can cursorily calculate the adhesive film porosity by the adhesive interfacial back wave height.
3.2. SLS test results 3.2.1. Failure load and shear strength The SLS test was used to obtain failure load and evaluate the adhesive joint strength as a function of PTFE pore percent. Consistent data was obtained for each bond category and the Coefficient of Variation (COV) value for each bond group are all less than 4% as shown in Fig. 2(a), which means the design and fabricated method could provide reproducible data. The group A (N = 18%) provided the most reproducible data while the group D (N = 64%) provided the largest spread in failure loads. The failure loads almost linearly increase as the pore percent N increase from 18% to 64%, while when the pore percent equals to 100%, the failure load creeps up to 11.38 KN, which means the inserted PTFE film not only decreases bonding strength, but also affects the structural integrity. The red line in Fig. 2(a) represents the calculated peak load assuming the peak load is the function of bonding area. As seen in Fig. 2(a), the experimental results (dark yellow column) are higher than the calculated result (red line). As the pore percent increases, the experimental result is infinitely close to the calculated result. The flow of the adhesive film during the curing increasing actually bonding areas could explain this phenomenon. The average shear strength σmax of the five specimens in each group was calculated according to the formula in section 2.2.2. Fig. 3(b) shows the relationship between average shear strength with pore percent N. It can be seen that the average shear strength of good bonded specimens (group N) is the largest among all those five groups of specimens. The average shear strength is increased from 13.4MPa (group A) to 35.67MPa (group N) as the PEFT pore percent of the adhesive layer increases from 18% to 100%. Among all weak bonding joint categories, there is an approximately linear decrease in the joint strength as the pore percent decreases. When the pore percent decreases to 18%, the average shear strength reaches to13.4MPa which is about 39% of the average shear strength of good bonded joints.
Fig. 2. Comparison of failure loads (a) and lap shear strength (b) in joints with different PEFT pore percent.
Fig. 3. Microscopic images of the bond joints after failure (a) five groups of specimen ; (b) N = 18% ; (c) N = 64%.
2.2.2.2. Single lap shear strength calculation. The single lap shear strength could be calculated as below formula:
σmax =
Pmax A
(1)
Pmax is the peak load recorded for the test; A is the overlap shear area and σmax is the maximum lap shear strength of the overlap region found using the particular test method. 89
Composites Communications 15 (2019) 87–91
T. Zhang, et al.
Fig. 4. Simulation image(a) pore random distribution (b) pore regular distribution; (c) failure load versus void percent.
0.1 mm in the through-thickness direction. Material for the cohesive element used bilinear mixed mode failure criteria. The constraints were fixed at one end and a displacement was imposed at the other end through multi-point constrained boundary conditions. The influence of PTFE pore distribution (regular distribution and random distribution) was considered when simulated. Fig. 4(c) shows the curve of failure load versus PTFE pore percent, respectively, at the critical location. It can been seen that the failure load increases as PTFE pore percent increases by test result(column with dense pattern) and also simulation (blue column and dark yellow column). As well, the pore distribution has influence on the peak load for SLS specimens and the influence is not significant when the pore percent is high. When the pore percent is higher than 64%, the simulative peak load is much close to the test result, which indicates that the simulation model has good prediction in the failure load for SLS specimens with low porosity (high PTFE pore percent). Also, the simulation model and method will be improved in future for better accuracy and consistency.
3.2.2. Damage morphology of bonding joint The typical damage morphology of bonding joint for five groups of specimen taken by camera is shown in Fig. 3(a). Fig. 3(b) and (c) are the image of detailed failure mode for group A and D with large magnification. Green and black regions represent the adhesive and the surface of the composite respectively. A combination of interface and cohesive failure was observed for the good bond joint (group N); while for the week bond joints, it is obviously that all joints failed at the bond interface of the PTFE films with the upper pre-cured composite substrate. This was evident from the microscopic images in Fig. 3(a) where the adhesive has been observed to bond to the substrate throng the PTFE film pore and peeled from the PTFE film as such. The observed sudden drop in load level occurring at the ultimate failure point is also an indication of the dominant interfacial failure. As the pore size and density increase, the flow of adhesive was getting more serious, which can be proved by the adhesive residual on the PTFE film as shown in Fig. 3(b) and (c). The flow of the adhesive film during curing also could partially explain why the experimental failure load (dark yellow column) is higher than the calculated result according to the bonding area (red line) in Fig. 2(a).
4. Conclusions The current research provided a comparative study of weak adhesively bonded joints. A method of fabricating weak bond joint with different bond strength was developed. The A-scan and SLS test were used to study the influence of adhesive film porosity on the shear strength. It was found that the thin perforated PTFE films embedded in the bonded joint could simulate the adhesive porosity often occurring in polymer composite bond joint. The adhesive interfacial back wave height is approximately linear functions of pore percent. As well, the
3.3. Simulation results A numerical implementation is performed in Abacus/Standard 2D to predict the SLS strength of the specimens with different PTFE pore percent N in the bonding interface as shown in Table 1. As shown in Fig. 4(a-b), the glue elements were modeled with cohesive elements (COH3D8) in standard Abacus. Mesh size is 1.27 *1.27 mm in plane and 90
Composites Communications 15 (2019) 87–91
T. Zhang, et al.
joint strength also approximately linear decreases as the pore percent of PTFE film decreases. Based on above test and simulation results, functions were proposed to estimate the adhesive film porosity and SLS strength with respect to the A-scan curve and lap shear strength curve in this paper.
Struct. 96 (2013) 256–261. [4] J. Bardis, K. Kedward, Effects of Surface Preparation on the Long-Term Durability of Adhesively Bonded Composite Joints, FAA report DOT/FAA/AR-03/53 (January 2004). [5] J.S. Tomblin, C. Yang, P. Harter, Investigation of Thick Bond-Line Adhesive Joints, FAA report DOT/FAA/AR-01/33 (June 2001). [6] K. Smith, Bonded Joints and Structures-Technical Issues and Certification Considerations, FAA Memorandum PS-ACE100-2005-10038 (September 2005). [7] M.D. Bane, L.F.M. da Silva, Adhesively bonded joints in composite materials: an overview, Proc. IMechE Part L: J. Mater. Des. Appl. 223 (2008) 1–18. [8] S. Budhea, M.D. Banea, S. de Barros, L.F.M. da Silva, An updated review of adhesively bonded joints in composite materials, Int. J. Adhesion Adhes. 72 (2017) 30–42. [9] C.C.H. Guyott, P. Cawley, R.D. Adams, The non-destructive testing of adhesively bonded structure: a review, J. Adhes. 20 (2) (1986) 129–159. [10] J.L. Rose, Ultrasonic Nondestructive Valuation Technology for Adhesive Bond and Composite Material Inspection, Adhesive Bonding, Plenum Publishing, Corp, NewYork, NY, 1991, pp. 425–448. [11] M. Michael, J.G. Capetta, A.H. Davidoff, Advanced Ultrasonic Sign Alanalys Is for Bond Strength and Materials Characterization, Nondestructive Characterization for Advanced Technologies, The American Society for Nondestructive Testing, Columbus, OH, 1991, pp. 132–134. [12] ASTM D 1002-10, Standard Test Method for Apparent Shear Strength of Single-LapJoint Adhesively Bonded Metal Specimens by Tension Loading (Metal-To- Metal). [s], America Annual Book of Standards, 2010. [13] ASTM D 3163-01, Standard Test Method for Determining Strength of Adhesively Bonded Rigid Plastic Lap-Shear Joints in Shear by Tension Loading [s], America, Annual Book of Standards, 2014.
Acknowledgements The financial support provided by AECC Commercial Aircraft Engine Co., Ltd., China and the Fundamental Research Funds for the Central Universities in China through Grant No.2232019G-02 to the work presented here is gratefully acknowledged. This work was also supported by the Science and Technology Commission of Shanghai Municipality, China (18DZ2204600). References [1] K.B. Katnam, L.F.M. Da Silva, T.M. Young, Bonded repair of composite aircraft structures: a review of scientific challenges and opportunities, Prog. Aero. Sci. 61 (2013) 26–42. [2] A. Higgins, Adhesive bonding of aircraft structures, Int. J. Adhesion Adhes. 20 (5) (2000) 367–376. [3] G. Mancusi, F. Ascione, Performance at collapse of adhesive bonding, Compos.
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