Experimental investigation on low-velocity-impact and post-impact-tension behaviors of GFRP T-joints after hydrothermal aging

Peiyu You; Cuilong Liu; Xiaobang Yao; Kaixin Xu; Mingjie Li; Ye Wu; Jixiang Luo

doi:10.1515/epoly-2024-0092

Article Open Access

Experimental investigation on low-velocity-impact and post-impact-tension behaviors of GFRP T-joints after hydrothermal aging

Peiyu You , Cuilong Liu , Xiaobang Yao , Kaixin Xu , Mingjie Li , Ye Wu and Jixiang Luo

Published/Copyright: December 19, 2024

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From the journal e-Polymers Volume 24 Issue 1

Abstract

As an integrated structural unit, composite T-joints are used to transfer the load between two vertical planes, such as the wing box of an aircraft. The article aims to investigate the failure mechanism of the glass fiber-reinforced polymer (GFRP) composite T-joints subjected to low-velocity impact on the weak deltoid and post-impact tension after hydrothermal aging. First, the improved vacuum-assisted resin infusion suitable for the fabrication of GFRP T-joints is employed. Second, the hydrothermal aging is conducted at 25°C and 65°C with the same relative humidity of 85% for 1–6 weeks. Finally, the impact resistance and tensile strength are analyzed based on response history and damage morphology. The results show the significant degradation of impact strength and stiffness of GFRP T-joints after hydrothermal aging and with the increase in aging time and temperature. In the failure mode of post-impact tension, the interlaminar cracks in the deltoid propagate in the horizontal and vertical directions, and there is a large gap in horizontal crack length between aged and non-aged T-joints.

Keywords: GFRP T-joints; hydrothermal aging; low-velocity impact; post-impact tension; failure mechanism

1 Introduction

Glass fiber-reinforced polymer (GFRP) composites, gradually from decorative and secondary load bearing to main load bearing, are commonly utilized in aerospace, construction, automobile, etc., due to their exceptional benefits, including high stiffness/strength-to-weight ratio, superior fatigue resistance, and corrosion resistance (1–4). Because of the manufacturing constraints of composite structures, it is essential to link the parts to create complex structures. As a representative component of structural integration, composite T-joint not only reduces the number of fasteners and parts, but also decrease the assembly cost and weight (5–8). It is an important way to realize the high efficiency of composite structure.

As an integrated structural unit, composite T-joints are applied to transfer the load between two vertical planes, such as the wing box of an aircraft (9–11). T-joints facilitate the transmission of gas power to the ribs and stringers of the wing, which mainly endures tensile, shear, lateral bending, and combined loads (12–14). Nevertheless, composite T-joints are typically the weakest components in composite structures since lawful joint design raises the risk of structural failures or catastrophic events. Barzegar et al. (15) conducted a detailed analysis on reinforced composite T-joints, examining adhesives, fiber volume fractions, and geometrical features of composite adherends, as well as the failure behaviors in bending test. Wan et al. (16) developed a sensor using electric resistance, constructed from woven glass fiber covered with multi-walled carbon nanotube, to observe the mechanical performance of composite T-joints during tensile loading.

Furthermore, the mechanical properties of composite T-joints will deteriorate in a complex environment during service. The tensile loading of composite T-joints after laser ablation was conducted by Chuyang et al. (17) to obtain a significant decrease in interlaminar tensile and shear strength in the heating zone. Similarly, transverse impact behaviors of 3D braided composites T-beam at elevated temperatures were investigated by Zhang et al. (18). With the help of thermal coupling technique, Guowei et al. (19) studied the in-plane shear performance and damage mechanism of composite T-joints under initial defects to get a synergistic effect of high temperature and defects that accelerate the damage process. The T-joint with flaws exhibited a progressive softening effect as temperatures rose, accompanied by a steady reduction in the stiffness of the adhesive layer, and the ultimate load of the structure is reduced by nearly a quarter at 150°C. In another service environment, the ultimate strength and fatigue life of composite T-joints are declined obviously after hydrothermal erosion in the ocean, according to Murdy et al. (20). Both the glass-reinforced epoxy and vinylester-epoxy matrix composites exhibited quantifiable static strength reduction (4–36%) following hygrothermal aging. Hydrothermal aging leads to irreversible degradation of fiber and matrix and further affects the mechanical performance of the materials, in which some mesophases were thankfully restored because of the possibility of self-healing to some extent after re-drying. Therefore, this discovery is of great significance for understanding the performance changes of composites in hydrothermal environment and developing of durable materials.

It is unavoidable to be exposed to certain types of low-velocity impact (LVI), such as tools being dropped, large hail, and other incidents. Generally speaking, the damage caused by impact on the materials is not always visible on the surface of the laminate, resulting in unpredictable and unknown property degradation (21–23). The deep negative effect of the LVI on the subsequent tensile performance is found even in a tiny incident energy. In addition, the effect of LVI load on the post-impact behavior depends on a combination of multiple factors, such as original materials, the enhancement mode, the impact position, and so forth (24–26). Jin et al. (27) reported the damage mechanism of GFRP T-joints under LVI at different impact positions and revealed the interlaminar damage evolution of webs and webs/skin via numerical simulation. Besides, Wu et al. (28) studied the failure mechanism of local-strengthening GFRP T-joints under LVI. The results show that the implantation of steel wire mesh is beneficial to the decrease of delamination damage in the web and post-impact tensile performance rather than catastrophic failure. Furthermore, the bolt reinforcement of GFRP T-joints was optimized with the help of the analysis of crack propagation of strategic positions, according to the research of You et al. (29).

For the wing T-joints structure of military aircraft, in addition to the hydrothermal environment of ground parking, it needs to bear aerodynamic heating and potential bird impact. Therefore, it is an urgent problem to determine the influence of LVI after long-term hydrothermal environment on the structural properties of composite T-joints, but unfortunately there is little such research. It is necessary to study the degradation degree after hydrothermal aging of mechanical properties of T-structures via quantitative data, with special attention to the deltoid of crucial parts and the individual interference degree of period, high temperature, and high humidity. Therefore, to investigate the failure mechanism of GFRP T-joints, we carefully conducted the tests of LVI on the weak deltoid and post-impact tension after a variety of hydrothermal aging at different temperatures and times.

2 Experiment

2.1 Fabrication of specimens

To facilitate the fabrication of GFRP T-shape joints, the two angle iron parts are used to the conventional vacuum-assisted resin infusion (VARI) process. Here is the comprehensive information regarding VARI processing: we employed two bolt-tight angle irons as holders, positioned at the bottom of the VARI system. Second, textile diversion nets and glass fiber of 250 g·m⁻² of areal density are sequentially placed on the surface of both angle irons in layers, as seen in Figure 1. The vinyl ester resin, hardening agent (Methyl Ethyl Ketone Peroxide, MEKP), and accelerating agent (Cobalt isooctanoate) were combined in a mass ratio of 100:1:1 and cured at ambient temperature. One side of two angle irons is joined with bolts and the system is sealed using a vacuum bag and sealant tape. The resin was evenly distributed using a vacuum pump during the injection procedure and then left to cure at room temperature for 24 h. Both web and skin laminate with 4 mm in thickness consisted of 12 layers of GFRP with a stacking configuration of [0/90]₆. With the help of the stone cutting machine, the test specimens with uniform sizes of 25 mm in width, 180 mm in length, and 90 mm in height, as shown in Figure 1 were prepared.

Figure 1

Fabrication of T-joints.

2.2 Hydrothermal aging treatment

Specimens were aged in a climatic chamber (SY/GS, Xi’an, CHINA), which provides the temperature and humidity at the same time. The specimens were divided into two groups for aging of water bath at both 25°C room temperature and 65°C high temperature for 1–6 weeks with the same relative humidity of 85%. At each temperature, at least five samples were taken from the chambers for tensile testing.

2.3 LVI and post-impact tensile tests

In Figure 2(a), the LVI tests are conducted on a drop-weight device of INSTRON 9340, referring to the standard of ASTM D7136 (30). Furthermore, to grip the T-shape specimen three steel plates with a 76 mm circle hole are fixed at the rectangular hole of 125 mm × 75 mm for impact test via bolt joints. The impact location is on the deltoid, and the incident energy is 9/17/25 J, i.e., 2.00/2.75/3.33 m·s⁻¹. The history of contact force and displacement of the impactor, of the hemispherical top of 16 mm in diameter and entire mass of 4.5 kg is obtained. Further, to investigate the influence of LVI damage on the latter mechanical-property performance, with the help of the fixture pictured as in Figure 2(b), post-impact tensile tests are carried out on CSS-44100 universal testing machine at a speed of 2 mm·min⁻¹. Both ends of the T-joint are fixed via steel bars and M10 bolts with 10 N·m⁻¹ torque. The specimens are named as X–Y–Z, in which X is process temperature, Y represents the time of hydrothermal aging, and Z represents impact energy. For example, 25–1–9 J is the specimens subject to the impact of 9 J after hydrothermal aging at 25°C for 1 week.

Figure 2

Set up for tests: (a) LVI and (b) post-impact tensile.

3 Results and discussion

3.1 Damage mechanism of LVI

3.1.1 Comparison between GFRP joints with or without hygrothermal aging

The damage area and dent depth of aged specimens are larger than those of non-aged specimens, and the crack length of the deltoid is longer than those of non-aged specimens, as shown in Figures 3 and 4. Under the impact energy of 25 J, compared with the non-aged specimen and specimen in the worst aging conditions (60–6–25 J), their maximum forces are 2,353 and 1,405 N, and their maximum displacements are 14.7 and 23.8 mm. Due to the structural mechanical properties deteriorating via hydrothermal aging, maximum force decreased by 40.03% and peak displacement value increased by 61.9%. The maximum contact force of the unaged specimen is significantly higher than that of the aged specimen, and the maximum displacement is lower than that of the aged specimen, so the maximum contact force of the aged specimen decreases, and the maximum displacement increases under the same impact energy. All the specimens show significant bimodal trends, meaning GFRP T-joints suffered two destructions during the LVI test. First, the deltoid of T-joints crack is created by the impact of hammerhead, the damage area and dent depth gradually increase and generate vertical cracks as the force increases until it reaches the first peak force. Second, as the extent and dent depth are constantly increasing, compression damage occurs in the clamping T-joint. This leads to descending loading capacity, and the impact energy was absorbed by the area of the clamping T-joint and deltoid of T-joints, and the second peak is as shown in Figure 3(a).

Figure 3

Comparison of aged and unaged specimens on (a) force–displacement curves of aging/unaged specimens and (b) damage condition of aging/unaged specimens at the LVI test.

Figure 4

Damage morphology on web and deltoid of all cases after the LVI test at an energy of 9 J.

3.1.2 Influence of aging time on impact resistance

Under the impact energy of 9 J, the typical damage morphology photographs of both web and deltoid areas are shown in Figure 4. As the aging time increases, the sample has low ultimate contact force, and the damage area and dent depth of the sample increase at the same aging temperature and impact energy, as shown in Figures 4, 5, and 6. After 6 weeks of hydrothermal aging, the impact damage of the specimen includes obvious dent and vertical cracks in the deltoid area, because long-term aging will deepen the erosion of the resin matrix, the decline of fiber strength, the degradation of interfacial properties, and the accumulation of micro-defects.

Figure 5

Force–time curves of all cases with varying impact energies and temperatures: (a) at 25°C and 10 J, (b) at 60°C and 9 J, (c) at 25°C and 17 J, (d) at 60°C and 17 J, (e) at 25°C and 25 J, (f) at 60°C and 25 J.

Figure 6

Force–displacement curves of all cases with varying impact energies and temperatures: (a) at 25°C and 10 J, (b) at 60°C and 9 J, (c) at 25°C and 17 J, (d) at 60°C and 17 J, (e) at 25°C and 25 J, (f) at 60°C and 25 J.

As can be seen from Figures 5 and 6, the mechanical properties of 25–6–25 and 60–6–25 J decrease sharply compared to others due to hydrothermal aging. This is due to the emergence of partial damage in the GFRP T-joint during hydrothermal aging. However, with the increase of aging time from 1 week to 6 weeks, the peak contact force of 60–1–25 and 60–6–25 J decreases by 3.61% and 25.5%, and the displacement at the same impact energy increases by 5.06% and 32.37%. Owing to matrix and fiber degradation of GFRP T-joints after hydrothermal aging, the impact resistance of GFRP T-joints decreases with the aging time.

3.1.3 Influence of aging temperature on impact resistance

Figure 7 compares the peak forces and displacements of the specimens in different aging environments. Specifically, the ultimate force decreased by 16.07%, 16.89%, 15.94%, 8.8%, 11.06%, and 8.17%, while the greatest displacement increased by 18.21%, 16.03%, 16.63%, 10.49%, 21.31%, and 6.25%, respectively. According to the comparison between 25°C and 60°C, a lower temperature benefits from mechanical properties. Noticeably, as shown in Figure 7, the ultimate force/displacement of the sample suffered at 25°C hydrothermal aging is 1,530 N/22.4 mm, while the ultimate force/displacement of the sample suffered at 60°C hydrothermal aging is 1,405 N/23.8 mm. High temperatures induce a chemical structural alteration in the resin matrix of GFRP, resulting in the development of micropores and fissures inside the material. For this reason, the temperature of the hydrothermal aging process leads to different impact resistances, and the increase in aging temperature will cause a decrease in impact resistance of samples.

Figure 7

Comparison of maximum contact force and displacement: (a) at 25°C and 10 J, (b) at 60°C and 9 J, (c) at 25°C and 17 J, (d) at 60°C and 17 J, (e) at 25°C and 25 J, (f) at 60°C and 25 J.

3.1.4 Performance of absorbed energy

Figure 8 depicts the energy–time curves for GFRP-T-joints subjected to low-velocity impacts across various hygrothermal environments. At 9 J impact energy, the curves ascend with time, reaching the preset energy value and then experiencing a minor dip. This behavior indicates that the specimens predominantly exhibit elastic deformation at this energy threshold, with small plastic deformation occurring, resulting in a relatively low energy absorption. The subsequent decline in the curves post-peak is attributed to the elastic recovery phase, which releases a portion of the absorbed energy. In contrast, when the impact energy is elevated to 17 and 25 J, the energy–time curves exhibit a distinct pattern. The energy accumulates progressively over time, nearly achieving the maximum preset impact energy, after which it plateaus. This suggests that at these higher energy levels, the specimens are primarily undergoing plastic deformation. The plastic deformation phase consumes a considerable amount of energy, which is reflected in the curves maintaining a steady state after attaining their peaks.

Figure 8

Energy–time curves of all cases with varying impact energies and temperatures: (a) at 25°C and 10 J, (b) at 60°C and 9 J, (c) at 25°C and 17 J, (d) at 60°C and 17 J, (e) at 25°C and 25 J, (f) at 60°C and 25 J.

3.2 Failure behaviors of post-impact tensile loading

3.2.1 Comparison between GFRP joints with or without hygrothermal aging

Figure 9 compares the typical force–displacement curves of aged/unaged specimens. During the tensile test after low-velocity impact event, aging/unaged specimens have the same failure mode. To reveal the damage mechanism, three phases come into question. In the beginning, the T-joint has linear elastic deformation, the surface of the joint has no visible damage, and the load is linearly related to its displacement. As the tensile load increases, the crack length at the web and the tensile force grows up, until it reaches peak load; at this time, delamination and fiber breakage of the bottom plate can be observed. Vertical cracks of T-joints propagate along the web and stop at the deltoid while the horizontal cracks at the skin expand along the base plate of both the sides. When the force of the sample reaches its maximum, the deltoid is totally destroyed by the crack in the skin, resulting in specimen failure.

Figure 9

Typical load–displacement curves.

3.2.2 Effects of different temperatures and aging times on the residual strength of GFRP T-joints

Figures 10 and 11 show the damage topography of the specimen under different damp and thermal aging conditions, respectively. Tables 1 and 2, respectively, count the horizontal crack lengths of the specimens at different temperatures. GFRP T-joints processed under different damp heat aging conditions have the same failure mode. The main failure mode is interface delamination. As the energy increases, the delamination region expands from the point of impact to a fixed boundary. Since the bond between the glass fiber and the resin is more severely damaged by high temperatures, this leads to a deterioration of tensile strength, and the structure is prone to sudden failure. Both the existing research (13,31) and the comparative results of crack length in deltoid area confirm the aforementioned discussion. Due to the increase of aging time, the internal damage of the specimen is more serious. For example, the horizontal crack length of 25–1–9 J is 47.8% shorter than that of 25–6–9 J. Comparing Tables 1 and 2, the length of horizontal cracks in the sample increases with the increase of aging temperature. The aging temperature has more serious damage to the adhesion degree of the sample. Therefore, the increase in temperature and time will cause more serious damage to the specimen.

Figure 10

Tensile damage morphology after hydrothermal aging at 25°C.

Figure 11

Tensile damage morphology after hydrothermal aging at 60°C.

Table 1

Horizontal crack length of GFRP T-joints after hydrothermal aging at 25°C

Aging time (week)	Horizontal crack length (mm)
Aging time (week)	9 J	17 J	25 J
1	3.38	4.01	4.78
2	3.51	4.47	5.90
3	3.61	4.62	6.32
4	4.41	5.57	6.45
5	6.01	6.53	7.31
6	7.07	7.82	7.93

Table 2

Horizontal crack length of GFRP T-joints after hydrothermal aging at 60°C

Aging time (week)	Horizontal crack length (mm)
Aging time (week)	9 J	17 J	25 J
1	4.25	4.51	5.08
2	4.57	4.98	6.07
3	5.19	6.41	6.78
4	6.12	6.88	7.12
5	6.51	7.22	7.49
6	7.31	7.90	8.17

Figure 12 shows the force–displacement curves of different specimens under tensile test. Further, Figure 13 compares the peak forces of different types of specimens when stretched after impact. Under the same impact energy and aging time, the peak tensile load of the T-joint under the condition of 60°C damp heat aging is lower than that of other specimens at 25°C. Compared with the specimens with six aging period at 25°C, the specimens at 60°C and 1/2/3/4/5/6 week decrease by 7.88/26.80/30.33/26.22/25.99/31.14% in maximum force.

Figure 12

Force–displacement curves of all cases: (a) at 25°C and 10 J, (b) at 60°C and 9 J, (c) at 25°C and 17 J, (d) at 60°C and 17 J, (e) at 25°C and 25 J, (f) at 60°C and 25 J.

Figure 13

Comparison of GFRP T-joints under the impact energy of (a) 9 J, (b) 17 J, and (c) 25 J on ultimate force.

3.2.3 Residual strength of GFRP T-joints at different impact energies

The specimen has typical brittle material characteristics, which can be verified by the sharp decline of the force–displacement curve near the limit value as shown in Figure 12. First, the slope of specimens barely changed before reaching the limit, due to the deformation of the specimens being mainly elastic deformation. Then, the contact force sharply drops due to fiber breakage at the deltoid, and damage to the sample includes interlaminar delamination and fiber breakage. At low incidence energy (9 J), failure of the specimens during the tensile test occurs in the deltoid, but with the climb of the processing temperature, destruction extends to the skin. In the tensile test after high-energy impact (17 and 25 J), the degree of damage increased slightly due to the increase of impact energy, but it increased greatly due to the increase of aging time. At the same processing conditions, with the impact energies of 7 J/9 J/25 J, the peak tensile loads after impact are 376.49 N/328.67 N/246.59 N, which are reduced by 12.70% and 34.50%, respectively. This means that in a hydrothermal aging treatment, the mechanical properties and residual strength of the T-joint are significantly reduced.

4 Conclusion

To investigate the mechanical behavior of GFRP joints after hygrothermal aging, multiple groups of specimens based on the combination of aging temperature and time are subjected to low-velocity impact and post-impact tensile loads. According to the experimental results, involving typical response history and critical failure morphologies, the following conclusions can be drawn:

The structural strength of GFRP T-joints reduces greatly after hydrothermal aging since the erosion of internal resin induces a weakened interfacial bonding capability between fiber and matrix, especially in the deltoid area.
The significant degradation of impact strength and stiffness of GFRP T-joints after hydrothermal aging is reflected in a drop in peak force and an increase in failure displacement. Furthermore, the performance of T-joints decreases with the increase of aging time and temperature. Compared with 25–1–25 J in LVI, the peak force of 60–6–25 J decreased by 37.5%, while the failure displacement increased by 56.5%.
The residual strength of T-joints under post-impact tension decreases with the increase of impact energy and aging degree. During the failure process, the interlaminar cracks in the deltoid propagate in the horizontal and vertical directions, and there is a large gap in horizontal crack length between aged and non-aged T-joints. Compared with the T-joints at 25°C for 1 week and at 65°C for 6 weeks subjected to the impact of 25 J, the horizontal crack length of the latter in the deltoid area has increased by 60.8%, from 5.08 to 8.17 mm.

Acknowledgements

This work was sponsored by Foundation of Jiangxi Province of China Educational Committee (GJJ2201503, GJJ201907), and Innovative Projects of NIT (YC2024-S876, YC2024-S886, YC2024-S874).

Funding information: This work was sponsored by Foundation of Jiangxi Province of China Educational Committee (GJJ2201503, GJJ201907), and Innovative Projects of NIT (YC2024-S876, YC2024-S886, YC2024-S874).
Author contributions: Peiyu You: Conceptualization, Methodology, Software Validation, Formal analysis, Investigation, Resources, Data Curation, Writing – Original Draft, Writing – Review & Editing, Visualization, Supervision, Project administration, Funding acquisition. Jixiang Luo: Formal analysis, Investigation, Resources, Data Curation, Writing – Original Draft, Writing – Review & Editing, Visualization, Supervision, Project administration. Cuilong Liu: Conceptualization, Methodology, Investigation, Resources, Data Curation, Writing – Review & Editing, Visualization, Supervision, Project administration, Funding acquisition. Xiaobang Yao, Kaixin Xu, Mingjie Li, and Ye Wu: Data Curation, Writing – Original Draft, Writing – Review & Editing, Visualization.
Conflict of interest: Authors state no conflict of interest.

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Received: 2024-09-10

Revised: 2024-10-23

Accepted: 2024-10-30

Published Online: 2024-12-19

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/epoly-2024-0092

Keywords for this article

GFRP T-joints; hydrothermal aging; low-velocity impact; post-impact tension; failure mechanism

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BY 4.0