On the influence of resin pocket area on the failure of tapered sandwich composites

Abstract

Structural defects such as resin pocket area are inevitably created between surface and core of composite structures during the production of wind turbine blades using vacuum infusion process. In this article, four-point bending tests were performed on tapered sandwich composites to investigate the effect of resin pocket area on the mechanical strength, crack growth path, and failure mode. Specimens were in similar shape to wind turbine blade profiles, and a shear-dominant load was applied to the resin pocket area during the experiments. The extended finite-element method was applied in order to predict crack growth path and failure mode. The average static strength of the specimens including the small size of resin pocket area had almost no change in compare with the specimen with no resin pocket area. Moreover, the medium size of resin pocket area decreased the strength for 3.5% while the large size one enhanced it for 1.75%. Thus, it can be deduced that the defect area does not have a significant effect on the flexural strength of the sandwich composite tapered specimens, but it can arrest the crack. Therefore, the crack propagates in the opposite direction at the interface of the face and core. Although the resin pocket area arrests the crack, it was observed that the size of resin pocket area directly affects the crack growth and its path. The smaller resin pocket area leads to slower crack growth, and early collapse occurs for the larger size of defect area. So, the size of resin pocket area has considerable importance during manufacturing of such structures. Finally, numerical results have shown good agreement with experimental ones.

Keywords

crack arrest crack growth extended finite-element method resin pocket area tapered sandwich composites vacuum infusion process

Introduction

Wind energy has been recently used as one of the primary sources of renewable energy. Wind turbine blades play an important role in energy conversion and generally should be light and have a small mass moment of inertia. The blades should be able to tolerate extreme wind speed and also have a lifetime of at least 20 years (Zenkert, 1995). In order to satisfy such requirements, sandwich composites are appropriate to be used as blade materials because of their high strength to weight ratio. Sandwich composites have been extensively used in large parts of wind turbine blades because of their high bending stiffness and strength to weight ratios. However, there are some noticeable weaknesses in the manufacturing of sandwich structures.

Composite sandwich structures are fabricated by attaching two thin but stiff face sheets to a lightweight but thick core. They are widely used in wind turbine blades, aerospace, and marine applications due to their high flexural strength to weight and stiffness to weight ratios (Chen and Kam, 2011). Huang and Alspaugh (1974) applied a constant-thickness sandwich theory by considering stiffness variation in accordance with local thickness in order to investigate on variable-thickness sandwich beams. Lu and Libove (1991) showed that using uniform thickness sandwich composite theory in non-uniform thickness sandwich beams leads to a considerable amount of error because membrane stresses have transverse shear components in the face sheet. They concluded that such transverse shear components change transverse loads and displacements in the core. In continue, this theory was developed for the study of isotropic and orthotropic sandwich plates with non-uniform thickness (Lu, 1994; Paydar and Libove, 1986). Kassapoglou (1996) achieved the stress distribution in sandwich ramp-down structures in bending and shear loading conditions. He observed that the ramp-down angle and thickness of the core have significant effects on the damage of these structures. Thomsen and Vinson (2002) presented a high-order theory for analyzing sandwich beams and plates with non-uniform thickness. Kuczma and Vizzini (1999) studied the load distribution and damage models in sandwich tapered composites under tension, pressure, and bending loading conditions. They observed that damage is initiated at the tapered area, and surface-core delamination occurs as dominant failure mode. Lindberg (2007) performed four-point bending tests on tapered composite beams to study the effect of variations in core thickness ratio (length of tapered area to core thickness) on strength and failure. Vadakke and Carlsson (2004) conducted an experimental investigation to study the failure mechanism in an artificial through-width face/core debond. Steeves and Fleck (2004) carried out numerical and experimental investigation on detecting failure mechanisms of sandwich composite beams under three-point bending test. They constructed a failure mechanism map and showed that the failure mechanism is dependent on the geometry of beam. Gibson (2011) analyzed initial core shear failure and studied the effect of crack size in composite sandwich beam using elementary laminated beam theory and linear elastic fracture mechanics. She found that debonding can lead to failure if there are pre-cracks at the interface of core and surface. It was also observed that shear failure is the dominant failure mode if the crack size is small compared to the core thickness.

Due to the large size of wind turbine blades and the need for high levels of quality and accuracy, manufacturing by vacuum infusion process (VIP) is the most appropriate method. However, sandwich composites that are used in wind turbine blades must have a non-uniform thickness. Because of non-uniformity in thickness and high vacuum pressure during the manufacturing process, a structural defect is created in the tapered region between upper and lower faces, which is named as a resin pocket area. Such a defect influences the strength and crack propagation in sandwich composites. In this article, the effects of resin pocket area on the strength, crack growth, and failure mode are investigated in tapered sandwich composite specimens by experimental and numerical methods. The four-point bending test was performed in order to create a constant shear load on the tapered area and to prepare a shear failure mode in this area. The extended finite-element method (XFEM) was applied to model crack growth in the resin pocket area and to predict the failure mode by considering the virtual crack closure technique (VCCT) and linear elastic fracture mechanics (LEFM). The results of experimental and numerical investigations were also compared for the specimens with no resin pocket area in order to clarify the effect of such area.

Experimental procedure

Materials and methods

Sandwich composite specimens were made of polyvinyl chloride (PVC) foam (Airex C70.75) as core material. This type of foam has 100% closed cells with constant shear strength through the thickness. Face sheet materials were laminated with six layers of tri-axial $[0 / + 45 / - 45]$ S2-glass fibers and 2040 epoxy resin. In order to achieve practical results, specimens were manufactured by the VIP, which is usually used in the production of large wind turbine blades. All specimens were manufactured at the Sun-Air Research Institute (SARI) at Ferdowsi University of Mashhad. The schematic of core and face sheet materials are shown in Figure 1.

Figure 1.

Schematic of foam core and laminated face sheets.

As it is shown in a segment of the wind turbine blade (Figure 2(a)), the resin was trapped between lower and upper faces, and consequently, a resin pocket area was created due to vacuum pressure during the manufacturing process. There are four types of specimens by considering different values for artificially manufactured resin pocket lengths: a= 0, 5, 8, and 11 mm. The geometric dimensions of the specimens were selected according to ASTM C 393-06 standard (2006) (Figure 2(b)). As shown in Figure 2(b), it is evident that there will be no resin pocket area in the specimen if the amount of this area is zero.

Figure 2.

Geometric specifications: (a) resin pocket area in a real wind turbine blade and (b) dimensions of the specimen.

The artificial resin pocket area was created in the specimens by cutting a part of the foam in the tapered area (Figure 3). Therefore, this part is filled with resin after the completion of injection during the VIP method. Figure 4 shows the specimens with different sizes of resin pocket area. After 24 h of curing at room temperature, the specimens were cured at $45^{°} C$ and $70^{°} C$ for 2 and 16 h, respectively. Main procedures of preparing the sandwich composite are shown in Figure 5(a).

Figure 3.

(a) The foam core before cutting and (b) the cut foam core to create a resin pocket area.

Figure 4.

Specimens with different sizes of resin pocket area: (a) no resin pocket area, (b) a= 5 mm, (c) a= 8 mm, and (d) a= 11 mm.

Figure 5.

Experimental testing: (a) preparation of composite specimens and (b) the Zwick/Z250 testing machine and four-point bending fixture.

Mechanical testing

The four-point bending test was performed at room temperature according to ASTM C 393-06 (2006) at 6 mm/min rate of loading by the Zwick/Z250 testing machine (Figure 5(b)). In order to detect the initiation and growth of cracks, tests were recorded by Canon high-speed camera. The span distance between supports was chosen as 312 mm, and the loading span was selected as 202 mm. As shown in Figure 5, narrow rubber pressure pads were placed under the loading points to avoid any face sheet and core crush failure under loading rollers. Moreover, the flat-bottomed steel loading pads (Figure 5) were utilized above the rubber pressure pads to apply uniform pressure to the rubber pressure pad and to prevent it from non-uniform deformation. The resin pocket area tolerates shear forces as a core because of its trapping between upper and lower composite face sheets. For this reason, the distance between support and loading point should be as close as possible at each side of specimen in order to achieve a shear-dominant loading case (Lundell, 2010).

Theoretical overview

Geometrical discontinuities such as holes, notches, cracks, and non-uniformities in a structure could be considered as the causes of damage initiation and failure of structures (Zamani et al., 2019). As it is known, the fracture mechanics approach supposes an existing crack in the structure. Moreover, LEFM considers a negligible plastic region in the vicinity of the crack tip. Therefore, the stress intensity factor controls damage evolution in the vicinity of the crack.

VCCT

The VCCT is a suitable technique to model the onset and growth of crack in quasi-static problems or under high-speed impact loading. Moreover, it is based on LEFM, and accordingly, it is a good choice for case studies with brittle crack growth. The VCCT, proposed by Irwin (1958), assumes that the strain energy released when a crack is extended by a certain amount is the same as the energy required to close it by the same amount (Krueger, 2002; Leski, 2007).

According to the research on the fracture of PVC foams, it was shown that the LEFM assumptions were satisfied for these materials (Noury et al., 1998). Due to the brittle behavior of the foam and manufacturing-induced resin pocket area, the VCCT could be implemented to model the crack growth. Accordingly, the Benzeggagh and Kenane (BK) rule (1996) (equation (1)) was used to determine the mixed-mode behavior

G_{eqc} = G_{IC} + (G_{IIC} - G_{IC}) {(\frac{G_{II} + G_{III}}{G_{I} + G_{II} + G_{III}})}^{η}

(1)

where G_I, G_II, and G_III are fracture energies of the first, second, and third mode, and $η$ is the power of the rule. G_IC and G_IIC represent the critical fracture energies of the first and second modes of fracture. The maximum tangential stress theory was applied to predict the change in the crack growth angle. The maximum tangential stress theory assumes that cracks initially propagate along the direction of the maximum tangential stress at crack front. Then, cracks propagate when the tangential stress exceeds a critical value (Erdogan and Sih, 1963).

Numerical analysis

Numerical simulations were conducted through ABAQUS commercial finite-element software. Crack initiation and propagation were predicted by the XFEM based on the LEFM approach. Due to the brittle behavior of the foam core and the onset of failure from the foam (Poapongsakorn and Carlsson, 2013; Saenz et al., 2011; Viana and Carlsson, 2002), the maximum principal stress damage criterion was selected for predicting crack initiation. Crack propagation was modeled according to the VCCT employing the maximum tangential stress criteria. First, the initiation site of the crack was predicted by the software using the maximum principal stress damage theory. According to this damage theory, the stresses in the finite-element analysis were compared to the introduced value to the software (i.e. 1.6 MPa from data sheet of AIREX C70, 2011). Second, the software propagates the crack using the VCCT which employs maximum tangential stress theory to estimate the crack growth angle. It is worth noting that the fracture energies of modes I, II, and III are required to be introduced to ABAQUS. The Young’s modulus, shear modulus, and Poisson’s ratio of foam core and laminated composite face sheets are presented in Table 1. It should be noted that the fracture energy constants were obtained from the results of the double cantilever beam (DCB), end notched flexure (ENF), and mixed-mode bending (MMB) tests, which were performed by Sun-Air Research Institute at Ferdowsi University of Mashhad. Moreover, the tensile and shear properties of the laminated composite were obtained by performing experimentations according to ASTM D3039/D3039M-17 (2017) and ASTM D4255/D4255M-15a (2015) standards, respectively. Only half of the specimen was modeled because of symmetry about the x-axis (Figure 6(a)).

Table 1.

Mechanical properties used in the FEA.

	Face sheet	Core
Material	Glass/epoxy	Airex C70.75 (2011)
Young’s modulus (GPa)	$E_{1} = 22, E_{2} = 9, E_{3} = 2$	$E = 0.05$
Poisson’s ratio	$υ_{12} = υ_{13} = 0.27, υ_{23} = 0.4$	$υ = 0.32$
Shear modulus (GPa)	$G_{12} = 14, G_{13} = 5, G_{23} = 5$	$G = 0.024$
Fracture energy (J/m²)		$G_{IC} = 15, G_{IIC} = 13, G_{IIIC} = 13$
Tensile strength (MPa)		1.6

Figure 6.

Half model of tapered sandwich composite.

The foam core and face sheet materials were defined as isotropic and orthotropic, respectively. The plane strain elements of CPE4R with reduced integration were considered for both of face sheet and core materials and the total number of 7083 elements was generated. The average element size of the core 0.65 mm and the size of face sheet elements are 0.7 mm.

Results and discussion

Four-point bending test for specimens without resin pocket area

The specimen without resin pocket area experienced a sudden shear failure mode in the core. Figure 7 shows the crack initiation and growth path in tapered sandwich composite with no resin pocket area. At first, the crack initiates under the loading points (Step 1) and propagates with angle of $45^{°}$ toward the middle of the core (Step 2). Such propagation occurs because shear stresses, which are produced due to loading, lead to principal tensile and compressive stresses with the angle of 45° with respect to the axis of the specimen. Crack growth angle alters after it passes the middle of the core and propagates toward the interface between upper and lower face sheets and causes delamination and collapse of this specimen (step 3). The main reason for the reduction of crack angle after passing the middle of the core is bending-shear coupling effect, which is activated in the tapered area (Vel et al., 2005). Activation of this couple leads to secondary shear stresses. These shear stresses change the direction of principal tensile stresses and crack growth angle.

Figure 7.

Crack growth path in the specimen with no resin pocket area.

Four-point bending test for specimens with resin pocket

Specimens with small, medium, and large sizes of resin pocket area were tested in the same manner as specimens with no resin pocket area. The crack initiation and propagation stages for a specimen with a small resin pocket area are shown in Figure 8. It was found that damage initiation and propagation were as the same as specimens with no resin pocket area (Steps 1 and 2 in Figure 8(a)), but the crack arrested as soon as it reached to the resin pocket area. After stopping in a while, it propagated in the opposite direction at the interface of face sheet and core (Step 3 in Figure 8(a)). It was also observed that the crack propagated more slowly in the third step for small and medium sizes of resin pocket area than in specimens with large resin pocket area. It should be mentioned that the crack propagation time from the onset of crack to the failure of sandwich composite was monitored using a high-speed camera. This time span was considered for deciding whether the crack growth was rapid or slow. Figure 8(b) and (c) reveals the crack growth steps for medium and large sizes of resin pocket area.

Figure 8.

Crack growth steps for different resin pocket sizes: (a) small size (a = 5 mm), (b) medium size (a = 8 mm), and (c) large size (a = 11 mm).

The linear behavior of load–displacement curves for the specimens with no resin pocket area (a= 0), small (a= 5 mm), medium (a= 8 mm), and large (a= 11 mm) sizes of resin pocket area show a brittle fracture, and it can also be concluded that the size of resin pocket area has almost no effect on ultimate strength of structure (Figure 9). It is worth noting that the load–displacement behavior of the specimen could be explained in two parts. The first part is related to a nonlinear behavior that could be attributed to the presence of rubber pressure pads at the onset of loading (Figure 5). The second part is pertained to the linear part of the load–displacement curve. It should be stated that the first part of all curves could be neglected due to their nonlinear behavior, and the linear parts were compared together. It can be deduced that the sample with no resin pocket area (sample 1: reference) and sample 2 (a= 5 mm) have similar slope. Furthermore, Samples 3 and 4 have similar slope while the slope of load–displacement curve for the Samples 3 and 4 is smaller than the slope of the Samples 1 and 2. The reason of difference between the slopes related to these specimens is that the sample gets less stiff as the size of resin pocket area is medium and large (a= 8 and 11 mm). With the same inference, the samples with no resin pocket area and small resin pocket area reveal more stiff behavior and they showed larger slope than samples with medium and large resin pocket area. The peak points in the load–displacement curves are the representatives of failure loads. It means that the onset of crack occurs at this load, and the structure experiences a rapid crack propagation and collapses right after the initiation. Besides, micro-scale damages form before reaching to the peak load, and at the instant of peak load, they coalesce into a major crack which lead to failure. The specimen with the small resin pocket area (a= 5 mm) has larger stress concentration with respect to the specimens with the medium and large resin pocket areas (a= 8 and 11 mm), and accordingly, formation of micro-damages and reaching to the peak load would be faster in the case of the specimen with small artificial defects (a= 5 mm). Consequently, the sample with large resin pocket area (Sample 4) has the longest time to failure and the biggest displacement to failure. The other cause of the biggest displacement to failure for Sample 4 is that it has the largest artificial defect (the resin area) and, accordingly, sample 4 has less stiffness than other samples. This leads to more displacement to failure for the samples that have larger artificial defects.

Figure 9.

Load–displacement curves in four-point bending test, reference sample: no resin pocket area, Sample 2: small size resin pocket area, Sample 3: medium size resin pocket area, and Sample 4: large size resin pocket area.

As tapered sandwich composites are used in the fabrication of wind turbine blades, the creation of resin pocket area is always a concern in this industry. Therefore, the angle of the tapered zone (draft-angle) usually is decreased in order to omit the resin pocket area. But it should be noted that decreasing the slope of tapered zone (its angle) lead to smaller foam core area, and this results in declining the strength of the structure. According to these results of the present research, there is no concern about small and medium size resin pocket areas that are produced during manufacturing process of tapered sandwich composites. Hence, there is no need to decrease draft angle. Consequently, larger bending stiffness could be obtained by having larger draft angles.

Finite-element analysis

Crack growth path in the four-point bending test is simulated in ABAQUS using the XFEM and use of the LEFM which is an appropriate approach for brittle fracture analysis. The BK (Benzeggagh and Kenane, 1996) power law is considered as a mixed-mode crack behavior in the VCCT. The maximum tangential stress theory is applied to predict the crack growth angle during the finite-element analysis. A total of 7083 elements of CPE4R type are considered for numerical analysis. Prediction of crack growth path in the specimen with no resin pocket area is shown in Figure 10. According to this figure, the crack propagation can be categorized into four steps. It is evident that the crack initiates from loading points and propagates with angle of 45° (Step 1) until its growth angle decreases up to center of the foam (Step 2). As the crack approaches the tapered region (zone with non-uniform thickness), there is more reduction in growth angle, and its growth direction tends toward the foam core-face sheet interface in tapered area (Step 3). The crack growth with angle of 45° (as mentioned in the first step) could be attributed to shear stresses. The mentioned shear stresses are dominant cause of principal tensile stresses that have direct effect on crack growth angle (Figure 11). As illustrated in Figure 11, the principal tensile stresses are perpendicular to crack edge, and this leads to propagation up to the middle of foam. By approaching to the tapered zone (Step 4), a shear-bending coupling is activated. It should be mentioned that, in the uniform thickness region of the specimens, the presence of shear stresses lead to tensile principal stresses that are perpendicular to the crack (as shown in Figure 11). As the crack grows and reaches to the tapered zone of sandwich composite, in addition to the previous shear stresses, normal stresses were induced due to bending. Consequently, the combination of shear and bending stresses alters the direction of principal tensile stresses, and this leads to the change in crack growth path (Figure 11).

Figure 10.

Crack growth path in foam core in four-point bending test.

Figure 11.

Principal stresses in foam core.

The results of numerical analysis in Figure 10 are in good agreement with the experimental observations (Figure 7). Besides that, the crack growth path in the presence of small resin pocket area is shown in Figure 12, and it arrests when it reaches the resin pocket area.

Figure 12.

Crack growth path in the specimen with medium resin pocket area: (a) numerical simulation and (b) experimental observation.

As shown in Figure 13(a), it is obvious that the finite-element model presents a brittle behavior as it was observed during experimentation. In this figure, the normalized strength was defined as the strength of the specimens containing resin pocket area with respect to the specimen with no resin pocket area. The nonlinear behavior at the beginning of the experimental load–displacement curve is due to rubber pressure pad compression. This nonlinear behavior caused unexpected error between numerical and experimental results because the rubber pressure pad was not considered in the finite-element modeling. The load was transferred from the flat-bottomed steel loading pad after a while, and the nonlinear effect was eliminated. As can be seen in Figure 13(b), the resin pocket area does not have significant effect on the failure strength, and it only alters the crack growth path and how fast it can propagate until failure.

Figure 13.

(a) Load–displacement response for the specimen with no resin pocket area and (b) normalized failure strength with respect to the size of resin pocket area.

Conclusion

In this article, experimental and numerical investigations were performed to study the effect of the resin pocket area on the failure mode and crack growth in the tapered sandwich composite specimens subjected to static four-point bending load. The four-point bending test was conducted on specimens with no resin pocket area and specimens with small, medium, and large sizes of resin pocket area. Numerical analysis of the crack growth path under the static four-point bending was accomplished by applying the XFEM and LEFM approach. Finally, the following conclusions can be inferred according to the results of the present research:

The damage was initiated in foam core, and the presence of resin pocket area did not have a significant effect on the static failure strength of the tapered sandwich composite. Experimental observations illustrated that the presence of a resin pocket area can slow down damage propagation and can delay the collapse of the structure.

Results revealed that the onset of crack and its propagation path in the foam core for the specimens with no resin pocket area were similar to the specimens with small resin pocket area up to the middle of the core. Afterwards, for the specimens with small resin pocket area, the crack changes its path to the opposite direction in the core as it reaches to resin pocket area, while the crack propagates toward the core-face sheet interface in the tapered region and occurs a rapid failure for the specimens with medium and large resin pocket area. Therefore, there is a significant difference in crack growth path and failure modes of the specimens containing small, medium, and large resin pocket area during static loading condition.

Experimental results showed that the small and medium resin pocket area slows down the crack growth and delays its propagation, but the large resin pocket area accelerates damage and leads to early collapse of the structure. According to these results, there is no concern about resin pocket areas that are produced during manufacturing process of tapered sandwich composites. Hence, there is no need to decrease draft angle in order to eliminate resin pocket area because this reduction has negative effect on bending stiffness of the structure.

The finite-element model was presented to simulate crack growth and failure mode in the tapered sandwich composite under static four-point bending load. Finally, the results of numerical analysis showed good agreement with experimental observations.

Footnotes

Acknowledgements

The authors would like to thank the Sun-Air Research Institute at Ferdowsi University of Mashhad for providing necessary materials and equipment for manufacturing of specimens and sharing their data with us during the course of this research.

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) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Pedram Zamani

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