The effect of core materials on flexural behaviour of sandwich panels with basalt FRP facesheets

Abstract

The flexural behaviour of sandwich panels with basalt fiber-reinforced polymer (BFRP) facesheets was investigated. A total of 10 sandwich panels were subjected to six-point flexural tests to simulate uniformly distributed loading conditions. The variable parameters were the core thickness (30, 50, 60 and 100 mm), core category (polyurethane [PU], extruded polystyrene [XPS], rock-mineral wool [RW]), and hybrid core (PU+RW and XPS+RW). Finally, the shear distribution coefficient of core material for sandwich panels with BFRP facesheets was modified depending on the experimental data. The experimental results showed that an increase in the PU core thickness significantly increased the flexural strength and rigidity of the sandwich panels. For different core materials, the maximum loads of the panels were positive with the shear strength of the core materials. Whereas, XPS+RW hybrid core not only can significantly decreases the costs, but also exhibits the highest bearing capacity and stiffness as compared with the PU+RW hybrid and PU cores.

Keywords

Core materials hybrid core flexural behaviour sandwich panel FRP facesheets

Introduction

Composite sandwich panels have been intensively used since they were introduced in the United States of America during World War II (Djama et al., 2019; Vinson, 2005). Generally, sandwich structures offer advantages, such as high mechanical performance and low density, etc. (Mastali et al., 2017; Sharaf and Fam, 2013). Consequently, over the last few decades, the sandwich structures have been applied in different fields, including automobiles, aerospace, wind energy systems, naval engineering and civil engineering (Birman and Kardomateas, 2018). In civil engineering, sandwich panels are widely used as elements in temporary prefabricated houses for natural disaster survivors because it is convenient to tear open outfits (Abdolpour et al., 2016; Arslan and Cosgun, 2008; Bagarić et al., 2020).

The structural performance of sandwich panels primarily depends on the facesheet and core material (Ji et al., 2013). Steel, aluminium and timber are the most widely used materials for producing composite sandwich panels (Hao et al., 2018; Huang et al., 2021a; Latour et al., 2021). However, these materials are easily corroded or spoiled by the external environment when antiseptic treatment is not applied, resulting in a decline in performance. Fiber-reinforced polymers (FRPs), especially for Basalt FRP (BFRP) are eco-friendly materials with high strength, light weight, and good corrosion resistance, and they are suitable for use in civil engineering structures constructed in corrosive environments (Chen et al., 2017; Huang et al., 2021b; Shams et al., 2015; Wang et al., 2016b). Therefore, the use of BFRP facesheets as a replacement for traditional facesheets can effectively improve the durability of sandwich panels in highly corrosive environments.

The core of sandwich structures should provide sufficient strength and stiffness to support the facesheets (Betts et al., 2021; Papadopoulos, 2005). Although several studies on the mechanical properties of foam core sandwich panels have been conducted, most studies focused on sandwich panels with polyurethane (PU) core materials, which exhibit good heat preservation performance (Mohamed et al., 2015; Tuwair et al., 2015; Tuwair et al., 2016). This type of sandwich structure is mostly used in military barracks and other constructions because of the high cost of PU foams (Castillo-Lara et al., 2021; Lv, 2011; Nasirzadeh and Sabet, 2016). In addition, PU cannot satisfy all the requirements for excellent fire resistance and sound insulation, etc. for housing construction as compared with other core materials (Aditya et al., 2017). Moreover, experimental results have shown that the structural performance of the sandwich panels is markedly influenced by the mechanical properties of the core materials (Sharaf et al., 2010; Shawkat et al., 2008). Another study investigated the mechanical properties of hybrid PU foam and polypropylene composite cores with glass-fiber-reinforced polymer (GFRP) skins (Correia et al., 2012). The test results showed that the stiffness and strength of the hybrid core materials significantly influenced the performance of the sandwich panels. The extruded polystyrene (XPS) has been proven that has better mechanical properties compared with PU. Rock-mineral wool (RW) has outstanding flame-retardant and sound insulation properties, which is suitable to be used as core materials in sandwich panels. Therefore, the flexural behaviour of BFRP facesheets sandwich panels produced with different core materials and hybrid cores should be investigated.

In this study, six-point bending tests on sandwich panels were performed to simulate uniformly distributed loading conditions. In addition, the effects of various thicknesses (30, 50, 60, and 100 mm), categories (PU, extruded polystyrene [XPS], RW, and hybrids (PU+RW and XPS+RW) of core materials on the flexural performance of the sandwich panels were investigated. Furthermore, the shear distribution coefficient formula for the core material of sandwich panels with basalt facesheets under uniformly distributed loads was modified.

Experimental programme

Materials

In this study, the polymer used as the matrix for the facesheets was vinyl ester resin with good corrosion resistance. The hardening and accelerating agents were methyl ethyl ketone peroxide (MEKP) and dimethylaniline, respectively. The vinyl ester resin was mixed with MEKP and dimethylaniline at a mass ratio of 1%:1%:0.1%. The reinforced materials were a basalt chopped strand mat and unidirectional fabric cloth. The facesheet was manufactured through vacuum-assisted resin injection and comprised four layers of basalt chopped strand mat and a layer of unidirectional fabric cloth (Lei et al., 2013). Uniaxial tensile tests on the facesheets with the size of 400 mm*25 mm were performed according to ASTM D3039 by LFV-1000 hydraulic servo tester with a load capacity of 900 kN (ASTM, 2017).

The core materials were PU foam, XPS foam, and basalt RW. The thermal conductivity of the PU foam was 0.023 W/(m∙K), and the densities were 38 and 50 kg/m³ provided by the manufacturer. The thickness, thermal conductivity, and density of the XPS foam board provided by the manufacturer were 50 mm, 0.033 W/(m∙K), and 34 kg/m³, respectively. The thickness, thermal conductivity, and density of the basalt RW board provided by the manufacturer were 50 mm, 0.034 W/(m∙K), and 160 kg/m³, respectively. Because the basalt RW was highly anisotropic, the fiber directions parallel and perpendicular to the plate were adopted as the horizontal and vertical RWs, respectively. The RW board was cut to 600 mm × 300 mm × 50 mm specimens, and the core used for the sandwich panels was composed of three pieces of horizontal RW stitched with vinyl ester resin. For the vertical RW core, the RW board was cut to 300 mm × 50 mm × 50 mm specimens and bonded to the upper and lower BFRP facesheets.

Table 1 lists the mechanical properties of the core materials, determined according to ASTM C297/C297M-04 (ASTM, 2010), ASTM C365/C365M-22 (ASTM, 2022), and ASTM C273-00 (ASTM, 2020). The obtained stress–strain data are shown in Figure 1.

Table 1.

Summary of core materials properties.

Material	Density (kg/m³)	Compressive strength (kPa)	Tensile strength (kPa)	Shear strength (kPa)
PU (ρ1)	38.00	175.00 (3.6%)	157.78 (3.7%)	102.90 (8.4%)
PU (ρ2)	50.00	295.00 (4.5%)	354.45 (5.0%)	237.44 (6.5%)
XPS	34.00	320.00 (6.4%)	535.00 (5.6%)	297.44 (3.3%)
Horizontal RW	160.00	68.50 (10.3%)	25.12 (8.5%)	41.58 (9.7%)
Vertical RW	160.00	260.00 (14.5%)	513.12 (12.8%)	192.19 (14.8%)

Figure 1.

Stress-strain curves of core materials.

Test specimens

A mixture of ethoxyline resin and a curing agent was used to bond the facesheets and the core to form sandwich panels. Iron blocks were uniformly pressed onto the upper surface of the sandwich panels (Figure 2) to ensure that the core bonded to the facesheets. Table 2 lists the parameters of the sandwich panels with the same length and width (1700 mm × 300 mm). In Table 2, P50 indicates that the core is a 50 mm thick PU foam with a density of 38.00 kg/m³; P^#50 indicates that the core is a 50 mm thick PU with a density of 50.00 kg/m³; X50, RW50, and RW*50 indicate that the core materials are XPS, horizontal RW, and vertical RW, respectively, each with a 50 mm thickness. P/RW100 and X/RW100 indicate that the core materials are composed of 50 mm thick PU and RW bonding and 50 mm thick XPS and RW bonding, respectively.

Figure 2.

Sandwich panels (a) Sandwich panels pressed by iron blocks (b) Completed sandwich panels.

Table 2.

Parameters of specimen of sandwich panels.

No.	Specimen	Core material	Thickness (mm)	Category	Hybrid
1	P30	PU (ρ1)	30
2	P50	PU (ρ1)	50	PU (ρ1)
3	P^#50	PU (ρ2)		PU (ρ2)
4	X50	XPS		XPS
5	RW50	RW		Horizontal RW
6	RW*50	RW		Vertical RW
7	P60	PU (ρ1)	60
8	P100	PU (ρ1)	100		PU (ρ1)
9	P/RW100	PU+ RW			PU (ρ1)+ RW
10	X/RW100	XPS+RW			XPS+RW

Test setup and measurement system

The tests for sandwich panels were performed according to EN 1994-1-1:2004 (EN, 2004; Fam and Sharaf, 2010). A servo-hydraulic testing machine (Walter+Bai Testing Machines Co. Ltd) with a loading capacity of 500 kN was used. The stroke-controlled mode was selected, and a loading rate of 2 mm/min was applied. A data acquisition instrument manufactured by Donghua Testing was used to record the strains of the BFRP facesheets and the deflections of the sandwich panels. Six-point bending, comprising two supports and four loading points, was adopted to simulate uniform load distribution across the panels, as performed by Aykac and co-workers (Aykac et al., 2013). The applied loads were recorded using the data acquisition instrument, and linear variable differential transformers were used to measure the vertical deflections in the mid-span and end support settlements. Figure 3 shows the locations of the devices. Based on the test data, the safety factors of deformability (K_D) and strength (K_F) (Ye et al., 2009) can be calculated as follows:

K_{D} = \frac{D_{u}}{D_{d}}

(1)

K_{F} = \frac{F_{u}}{F_{d}}

(2)

where K_D is the safety factor of deformability; D_u is the deformability at the failure state; D_d is the deformability at the serviceability limit state; K_F is the safety factor of strength; F_u is the strength at the failure state; and F_d is the strength at the serviceability limit state.

Figure 3.

Six-point bending test setup (a) General test setup (b) Schematic diagram.

Experimental results and discussion

Effect of core thickness on flexural behaviour

Failure modes of sandwich panels

The failure of the sandwich panels was governed by the shearing of the core material on one side of the test machine (Figure 4(a)). Based on the visual inspection of the specimens, the six-point bending tests resulted in the detachment of the facesheets from the core along the shear crack and extended to the middle of the sandwich panels (Figure 4(a)). Additionally, based on the measurements, the distance between the shear crack and the support decreased with increasing PU thickness (Figure 4(b)–Figure 4(e)). This shear cracks were occurred because the insufficient shear resistance of the core materials and the debonding of facesheets from the core materials which made the force transferred to the core (Abdolpour et al., 2016). This tendency could be attributed to the decrease in the shear span ratio because of the increased effective height of the sandwich panels.

Figure 4.

Failure modes of different core thickness samples.

Load–lateral deflection response of sandwich panels

The load–deflection behaviours of all panels were linear up to failure, regardless of the core thickness (Figure 5). This trend confirmed that all failures were caused by the sudden shear cracking of the core. The load and deflection values for different thicknesses are shown in Table 3.

Figure 5.

Load-deflection responses plots for different thickness.

Table 3.

Summary of test results for different thickness.

ID	Failure state		Span/150 limit state
ID	Load (kN)	Deflection (mm)	Load (kN)	K_F	Deflection (mm)	K_D
P30	3.60	62.14	0.86	4.16	10.00	6.21
P50	4.21	36.96	1.32	3.19	10.00	3.70
P60	5.34	38.30	1.57	3.41	10.00	3.83
P100	8.28	38.47	3.13	2.65	10.00	3.85

With an increase in the thickness of the PU foam core, the flexural strength and stiffness increased significantly, but the failure deflections were slightly influenced, except for P30. The deformation of P30 at the failure state was significantly larger than those of other sandwich panels by approximately 70%. The flexural strength of the sandwich panels increased by 16.9%–129.9% relative to that of P30, depending on the core thickness. Figure 5 also shows the deflection control limit (span/150), mainly used in design codes. The permissible service loads based on the deflection control limits were significantly lower than the failure load of the sandwich panels, contributing to the high values of K_D and K_F calculated using equations (1) and (2), respectively.

In addition, the core thickness plays a critical role in variations in the safety factors. Generally, an increase in the thickness decreased the strength and deformity safety factor. The percentage decrease in K_D was 61.3%, whereas it was 57.0% for K_F. The minimum safety factors for deformity and strength were 3.85 and 2.65 for the span/150 limit state, respectively. Moreover, the minimum K_F was significantly higher than that of the reinforced concrete structure. P30 had the highest deformability safety factor, whereas the other specimens had almost equal values of the deformability safety factor.

Load-strain response of facesheets

Figure 6 shows the load–strain behaviour of the facesheets during loading.The strain gauges were attached to the on each L/6 (250 mm) of upper and lower facesheete of sandwich panels to record the strains. The load–strain responses of all facesheets were somewhat linear, and similar research was conducted by Mostafa and co-workers (Mostafa et al., 2014). In addition, the axial symmetry of the strain curves for compressive and tensile facesheets is shown in Figure 6, indicating a neutral axis close to the centroid of the panels. Table 4 lists the stress, strain, and utilisation rate results of facesheets. The maximum longitudinal strain measured for the facesheets in tension and compression was 0.26%. The measured strains of the specimens were significantly lower than the failure value, suggesting a low utilisation rate of the facesheet. It was observed that the utilisation rate of facesheets decreased with an increase in the thickness, and hence, P30 showed the maximum usage rate. However, the maximum utilisation rate of P30 was 12.53% for the compressive sheet and 10.73% for the tensile facesheet because of its low strain. The low utilisation rate suggests that the facesheets produced in this study can satisfy the experimental demands.

Figure 6.

Load-strain behavior of facesheet for different thickness.

Table 4.

Summary of test results for facesheet of different thickness.

ID	Compression facesheet			Tension facesheet
ID	Strain (%)	Stress (MPa)	Utilization rate (%)	Strain (%)	Stress (MPa)	Utilization rate (%)
P30	0.26	31.28	12.53	0.22	26.73	10.73
P50	0.18	22.19	8.82	0.16	18.79	7.52
P60	0.18	21.48	8.61	0.16	18.84	7.50
P100	0.18	21.61	8.62	0.15	18.47	7.41

Effect of core category on flexural behaviour

The price of different core materials in China is 1500 CNY/m³ for PU (ρ1); 1900 CNY/m³ for PU (ρ2); 500 CNY/m³ for XPS; 800 CNY/m³ for RW. The XPS and vertical RW core materials had excellent competitive costs as compared with PU. Moreover, it is necessary to investigate the flexural behaviours of sandwich panels produced with XPS and RW core materials, considering their excellent mechanical performance.

Failure modes of sandwich panels

Figure 7 shows different failure modes of the sandwich panels for various core categories. The experimental results indicated that the core categories influenced the failure of the sandwich panels. For P^#50, the failure mode was similar to that for P50. In addition, a visible 45° shear crack appeared on the core material, and the location of the crack was similar to those of the PU with a 38 kg/m³ density. As shown in Figure 7, X50 exhibited relative plastic deformation after loading, corresponding to the large deformation of XPS determined from the shear tests (Figure 1). The shear plastic deformation of the core materials caused the entire specimen to undergo a more extensive deformation than P50, and the cracking angle of the XPS core material was larger than that of P50. However, the failure of RW materials was somewhat different from that of the foam core materials. For RW50, a crack in the fiber direction was observed at the right side of the specimen and extended to the splicing interface of the core materials. This phenomenon is similar to the failure of the inter-laminar shear of fiber-reinforced polymer (FRP) materials (Wang et al., 2016a). In contrast to RW50, no complete damage to the RW*50 sandwich panel was observed during loading. The RW*50 specimen was significantly deformed after loading, and the most evident shear deformation occurred symmetrically between the supports and the middle section of the L/8 panels. This excessive deformation of RW*50 caused the BFRP facesheet to touch the support, stopping the loading immediately. The considerable plastic deformation of the vertical RW sandwich panel during the test is a significant property, which differed from those of other core composite panels.

Figure 7.

Failure modes of different core categories samples.

Load-lateral deflection response of sandwich panels

The load–deflection responses of the panels are depicted in Figure 9 and the test results for different core categories are listed in Table 6. As shown in Figure 8, the RW*50 sandwich panel shows an obviously ductility behavior after the ultimate load. Because of the maximum tensile and shear modulus, the RW*50 shows the maximum bearing ability at L/150 limit states. The load–deflection curve of the X50 sandwich panel was initially linear and then became nonlinear, which corresponded to the plastic deformation occurring at the later stage. Because of the excellent tensile and shear performance of the XPS, X50 had a maximum failure load of approximately 8.86 kN, and the failure deflection (55.63 mm) was slightly lower than that of RW*50. The strength and deformability safety factors of X50 reached 3.82 and 5.56, respectively (Table 5), which were significantly higher than those of most composite panels.

Figure 8.

Load-deflection responses plots for different core categories.

Table 5.

Summary of test results for different core categories.

ID	Failure state		Span/150 limit state
ID	Load (kN)	Deflection (mm)	Load (kN)	K_F	Deflection (mm)	K_D
P50	4.21	36.96	1.04	3.19	10.00	3.70
P^#50	7.13	30.66	2.38	3.00	10.00	3.07
X50	8.86	55.63	2.32	3.82	10.00	5.56
RW50	2.25	12.50	2.08	1.08	10.00	1.25
RW*50	5.67	85.56	3.09	1.84	10.00	6.00

With an increase in PU density, the mechanical properties of the PU core material became noticeably affected (Figure 1). Therefore, the P^#50 curve was somewhat different from the P50 curve (Figure 8), and an increase in density significantly increased the strength and stiffness of the sandwich panel. This improvement can be explained by the increment of mechanical performance of the core with higher density (Figure 1). This phenomenon is consistent with the results reported by Sharaf and co-workers (Sharaf et al., 2010). The failure load of P^#50 was nearly 1.7 times that of P50 and was 1.73 kN lower than the failure load of the XPS core specimen. The gradient of P^#50 (which represents stiffness) was relatively consistent with X50. An approximately 17.0% decrease in the failure deflection of P^#50 compared with that of P50 was observed. However, an increase in the core density did not affect the strength and deformability safety factors. At a limit condition of span/150, the values of P^#50 were close to those of the low-density PU core sandwich panel.

The load–deflection curve of RW50 was close to that of P^#50 and X50 first and started to decline beyond the maximum load (2.25 kN). The failure load, deflection, and safety factors of the RW50 were the lowest among the five specimens, suggesting that the horizontal RW is unsuitable as the core material for the mechanical sandwich structures. Nevertheless, the vertical direction of the fibers significantly improved the strength and stiffness of the panel. The initial gradient of the RW*50 curve, that is, the stiffness, was higher than that of X50, corresponding to the shear behaviour of the vertical RW and XPS (Figure 1(c)). After attaining the maximum load (5.67 kN), an evident yield platform appeared in the curve and gradually declined. These characteristics yielded a deformability safety factor exceeding 6.0, although the strength safety factor was not high.

Load-strain response of facesheets

The load–longitudinal strain curves of facesheets for different core categories during loading are shown in Figure 9. Considering the edge of the silica gel gasket under the loading plate is only about 1cm away from the strain gauges, so the layout of strain gauges was changed in the subsequent test, and the clearer strain curve were obtained after the strain gauges were arranged for each quarter span. Because the bearing capacity of the RW sandwich panels decreased after reaching the maximum load, only the strain results for the RW specimens before the failure load is presented in this paper. The detailed test data for the stress, strain, and utilisation rates of facesheets are listed in Table 6. The maximum tensile and compressive strains (0.26% and 0.25%, respectively) were obtained for the X50 facesheet. Therefore, the maximum stress of the X50 facesheet reached 31.50 MPa, and the material utilisation rate exceeded 10%, which was influenced by the deformation capacity of the panels and the shear performance of the XPS material. Because of the low bearing capacity of RW50, the lowest material utilisation rate was 2.90%. The vertical RW core material utilised the BFRP facesheets satisfactorily and increased the material utilisation rate by nearly 150% compared with RW50. The use of vertical RWs is a reasonable arrangement mode because of the excellent flame-retardant and sound insulation properties of RW (Mirnik et al., 2016).

Figure 9.

Load-strain behavior of facesheet for different core categories.

Table 6.

Summary of test results for facesheet of different categories.

ID	Compression facesheet			Tension facesheet
ID	Strain (%)	Stress (MPa)	Utilization rate (%)	Strain (%)	Stress (MPa)	Utilization rate (%)
P50	0.16	18.88	7.51	0.16	18.79	7.51
P^#50	0.23	28.11	11.22	0.22	26.30	10.50
X50	0.26	31.50	12.60	0.25	29.82	11.93
RW50	0.07	8.86	3.53	0.06	7.19	2.90
RW*50	0.18	21.73	8.70	0.17	20.05	8.01

Effect of hybrid core on flexural behaviour

As a porous inorganic fiber material, RW exhibits superior flame-retardant and sound insulation properties. The PU+RW hybrid core not only reduces costs but can also satisfy the requirements for multiple actual uses. In addition, the replacement of PU with XPS, which exhibits the best mechanical properties and has the lowest price, can significantly increase the bearing capacity and stiffness and further decrease the cost of sandwich panels.

Failure modes of sandwich panels

The failure modes of the composite sandwich panels produced with different hybrid core materials are shown in Figure 10. For sandwich panels, the shear of the core materials leads to failure (Figure 10). When the P/RW100 sandwich panel reached the failure state, approximately 45° shear failure of the PU core material occurred. Moreover, interfacial de-bonding failure occurred when the cracks extended. A similar phenomenon was observed for X/RW100; the panel failed during the shear cracking of the XPS core. Based on the stress–strain curves of the core materials (Figure 1), shear dominated the failure load of the sandwich panels. Vertical RW exhibited higher strength than PU and XPS during the initial loading phase; therefore, shear damage was the main failure mode of the PU and XPS cores (Figure 1).

Figure 10.

Failure modes of different core hybrid samples.

Load–lateral deflection response of sandwich panels

Figure 11 shows the load–deflection responses of the panels for different hybrid core materials, and the test data are listed in Table 7. The hybrids of the core materials exerted pronounced synergistic effects and mechanical properties (Figure 11). P/RW100 and X/RW100 exhibited better performance than P100 based on the curve gradients and failure loads. The failure load on the hybrid PU and vertical RW was 8.40 kN, showing a slight increase compared with the entire PU core. At L/150 limit states, the maximum loads showed the same tendency as the failure states. After attaining the failure value, the PU failed in the shear mode, corresponding to a sharp decline in the load curve. Because of the high ductility of the RW, the sandwich panel could still function until after the maximum load as RW*50. Therefore, the deformability safety factor increased from 3.85 to 6.45 with the replacement of 50% PU with vertical RW. Because the XPS exhibited excellent shear behaviour, the failure load of X/RW100 was 10.43 kN, showing a 24.2% increase compared with the P/RW100 sandwich panel. Moreover, the XPS and vertical RW hybrid slightly increased the failure deflection as compared with P/RW100. The strength and deformability safety factors were 1.5 and 7.13, respectively, and these factors satisfied the design code requirements (SAC, 2018).

Figure 11.

Load-deflection responses plots for different hybrid.

Table 7.

Summary of test results for different core hybrid.

ID	Failure state		Span/150 limit state
ID	Load (kN)	Deflection (mm)	Load (kN)	K_F	Deflection (mm)	K_D
P100	8.28	38.47	3.13	2.65	10.00	3.85
P/RW100	8.40	17.47	5.06	1.66	10.00	6.45
X/RW100	10.43	18.50	6.94	1.50	10.00	7.13

Table 8 lists the load per unit deflection values for different core sandwich panels. The P100 specimen resisted twice as much load as P50, corresponding to the core thickness (Table 8). For P/RW and X/RW, the hybrid core exhibited outstanding mechanical properties. The load per unit deflection of P/RW was approximately 1.66 times the sum of P50 and RW50 values, whereas the load per unit deflection of X/RW was more than twice the sum of X50 and RW50 values. Hence, a significantly positive hybrid effect was observed. Moreover, because of outstanding flame-retardant and sound insulation for RW, the XPS+RW hybrid is more suitable as core material of sandwich panel for prefab house.

Table 8.

Load of per unit deflection for different sandwich panels.

P50	P100	X50	RW50	P/RW	X/RW
0.11	0.22	0.16	0.18	0.48	0.56

Table 9.

Summary of test results for facesheet of different core hybrid.

ID	Compression facesheet			Tension facesheet
ID	Strain (%)	Stress (MPa)	Utilization rate (%)	Strain (%)	Stress (MPa)	Utilization rate (%)
P100	0.18	21.61	8.61	0.15	18.47	7.41
P/RW100	0.13	15.74	6.30	0.14	16.35	6.50
X/RW100	0.17	20.82	8.32	0.16	19.23	7.72

Load–strain response of facesheets

Figure 12 shows the variations in the strain of the facesheets during the loading process. The curves were almost linear and symmetrical. Table 9 lists the measured data for the stress, strain, and utilisation rate of the facesheets. With a low deformation capacity, the material utilisation rates and stresses of the P/RW100 and X/RW100 facesheets were lower than those of P100. The P/RW100 utilisation rates of the compression and tension facesheets, which were 6.30% and 6.50%, respectively, were the lowest among the three specimens. With the hybridisation of XPS, the utilisation rates of the compression and tension facesheets increased by 32.1 and 18.8%, respectively. However, the utilisation rate of the facesheets was still low. Thus, the failure of the core material is still the primary consideration in the design of sandwich panels with BFRP facesheets. Based on the structural design requirements for high strength, heat preservation, and sound insulation, sandwich panels with a hybrid XPS and vertical RW core may play a critical role in the future.

Figure 12.

Load-strain behavior of facesheet for different hybrid.

Table 10.

Summary of the shear distribution coefficient for different core material.

ID	Bearing capacity (kN/m²)	Shear distribution coefficient
P30	1.92	0.15
P50	2.93	0.53
P60	3.48	0.61
P100	6.95	0.64
P#50	5.28	0.58
X50	5.16	0.95
RW*50	6.86	0.46
P/RW100	11.25	0.88
X/RW100	15.43	0.96

Modification of bearing capacity formula

The GB/T 23932 (SAC, 2009) proposed a formula for determining the bearing capacity of a single-span simply supported plate under uniformly distributed loads, as follows:

p = \frac{f}{5 W L^{4} / 384 E_{f} I_{o} + K β W L^{2} / 8 A_{c} G_{c}}

(3)

where p is the uniform distributed load (MN/m²), f is the deflection of the panels under normal service conditions (mm), W is the width of the sandwich panel (mm), L is the span of the sandwich panel (mm), and E_f is the elastic modulus of the facesheet (MPa). I₀ is the moment of inertia of the upper and lower facesheets to the neutral axis (mm⁴), K is the non-uniform coefficient of shear stress (K = 1.2), β is the shear distribution coefficient of the core material, G_c is the shear modulus of the core material (MPa), and A_c is the cross-sectional area of the core material (mm²).

In equation (3), the shear distribution coefficient, which represents the percentage of shear force on the core accounted for the total shearing force of the sandwich panel, depends on the core material and usage scenario. However, this formula is suitable for sandwich panels with metallic facesheets, and the material behaviour of metals is different from that of FRPs. Moreover, the calculation of the shear distribution coefficient using the code equation is complicated. Therefore, based on the experimental data, the shear distribution coefficient was calculated for sandwich panels with BFRP facesheets in this study. As shown in Table 10, the value of shear distribution coefficient ranges from 0 to 1, and increases with the improvement of the shear performance on the core materials. For example, because of the excellent mechanical performance of XPS compared with PU (ρ1), the value of shear distribution coefficient increases from 0.53 to 0.96.

Conclusion

In this study, the effects of thickness, category, and hybrid of core materials on the flexural performance of sandwich panels with BFRP facesheets were investigated. The following conclusions were drawn from this experimental study.

1. The sandwich panels with PU core materials failed in shear of core for different core material thickness specimens. Moreover, there is an obviously positive correlation between the PU core thickness and the flexural behaviour of the sandwich panels was observed.

2. For the same thickness, the core category influenced the failure mode and the flexural behaviour of the sandwich panels. Sandwich panels with XPS core primarily failed in shear, whereas through-cracks and significant deformations were observed for the horizontal and vertical RW cores, respectively. The maximum loads of the panels were closely related to the shear performance e core materials. Compared to P50, the failure load of the XPS core material was more than three times higher than that of P100.

3. The failure of the sandwich panel with a hybrid core material was controlled by the shear performance of the core material. The load per unit deflection for X/RW was more than twice the sum of X50 and RW50 values, demonstrating an excellent positive hybrid effect. The combination of XPS and RW could also satisfy the functional requirements of sound insulation when used as a structural component of buildings. Moreover, the cost of the hybrid of the XPS and RW was 56.7% lower than that of PU.

4. The shear distribution coefficient of the core material was calculated based on the experiments. The value of shear distribution coefficient was positively correlated with the mechanical performance of core materials.

Footnotes

Acknowledgements

The authors gratefully acknowledge the National Natural Science Foundation of China (No. 51878149).

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (No. 51878149).

Data availability

The raw and processed data required to reproduce this finding cannot be shared at this time as the data forms part of an ongoing study.

ORCID iD

Xin Wang

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