CAO Haijian()， CHEN Hongxia()， HUANG Xiaomei()

1School of Textile&Clothing，Nantong University，Nantong226019，China2Analysis&Testing Center，Nantong University，Nantong226019，China

Abstract：Three-dimensional(3D)fabric composite is a newly developed sandwich structure，consisting of two identical parallel fabric decks woven integrally and mechanically together by means of vertical woven fabrics.In this paper，six types of 3D fabric sandwich composites were developed in terms of compressive and flexural properties as a function of pile height(10，20 and 30 mm)and pile distance(16，24 and 32 mm)in pile structures.The mechanical characteristics and the damage modes of the 3D fabric sandwich composites under compressive and flexural load conditions were investigated.Besides，the influence of pile height and pile distance on the 3D fabric sandwich composites mechanical properties was analyzed.The results showed that the compressive properties decreased with the increase of the pile height and the pile distance.Flexural properties increased with the increase of pile height，while decreased with the increase of pile distance.

Key words：three-dimensional(3D)fabric sandwich composite；compressive property；flexural property；pile height；pile distance

Introduction

Lightweight is a trend for composites. Three-dimensional(3D) sandwich composites have been widely used in aerospace， automobile， marine， energy and buildings for their excellent properties， such as low weight， high strength， high modulus and thermal insulation[1-3].

Traditional 3D sandwich composites， such as honeycomb and foam sandwich composites， have been commonly used in many fields[2-3]. However， the traditional 3D sandwich composites can be only used in structural parts of non-load-bearing， but not in structural parts of load-bearing. This can be explained that the face-sheet and piles of the traditional 3D sandwich composites are bonded by glue， which always bring out delaminating and damaging under mechanical loads or impact in the humid and hot environment[1-3].

In order to overcome the traditional 3D sandwich composites’ shortage， a new type of 3D sandwich composites， three-dimensional fiber sandwich composites(3D fiber composites)， as shown in Fig. 1， have been developed in recent years and widely used in various fields for their excellent properties， especially good designing and complete molding[4-10]. But recently studies found that the 3D fiber composites were sensitive to compressive loads and low-velocity impact loads because piles were mainly composed of fibers[11-13]. Therefore， it’s necessary to do more work to redesign the 3D fiber composites especially in designing of the pile structures.

In this work， a new structure sandwich composites of 3D fabric composites， consisting of two identical parallel fabric decks woven integrally and mechanically together by means of vertical woven fabrics， were designed and fabricated， as shown in Fig. 2. And also the flat compression and flexural properties of the 3D fabric composites were studied.

Fig. 1 Structural diagram of 3D fiber composite

Fig. 2 Structural diagram of 3D fabric composite

1 Experimental

1.1 Specimen preparation

Six types of the 3D sandwich fabrics were designed in terms of compressive and flexural properties as a function of pile height(10， 20 and 30 mm) and pile distance(16， 24 and 32 mm) in pile structures, as shown in Table 1. The preparation method of the 3D sandwich fabrics can be referred in Refs.[11-12].

Table 1 Structural parameters of the 3D fabrics

The 3D fabric sandwich composites were prepared by a standard hand lay-up technique with a mixture of epoxy resin E51 and polyethenoxyamines H023 in a mass ratio of 3∶1. The mould surface was first cleaned with acetone and coated with a mold release agent. A Teflon release film was then placed on the mold. About 35% the required amount of resin was poured over the film surface and spread with the aid of a plastic squeegee. The 3D sandwich fabrics were then placed on the film and totally rolled to assure that resin could be impregnated completely into the 3D sandwich fabrics . Finally， the remaining 65% resin was then applied on the other side in a similar manner. The specimens were kept at room temperature for about 24 h or 5 h in an oven at 50 ℃. After drying the samples and releasing the films， the specimens were then prepared for the test, as shown in Fig. 3.

Fig. 3 3D fabric sandwich composites with pile height of 10 mm

1.2 Compressive test

All the mechanical tests were performed on universal testing machine Instron 5969( Instron Corporation， USA)， with the maximum load of 150 kN. Five specimens were tested in each test and the final results were obtained from the average value. The environment conditions of the laboratory were controlled at(23±2)℃ and(50±10)% relative humidity.

Compressive tests were carried out according to GB/T 1453—2005 standard[14]. For compressive tests， the sizes of specimens were 60 mm×60 mm and loading speed was set at 2 mm/min. And the compressive strength can be calculated according to Eq. (1).

(1)

whereσis compressive strength(MPa)，Pis failure load(N)， andFis sectional area(mm2)， respectively.

The compressive modulus of the piles can be calculated according to Eq. (2).

(2)

whereEpis compression modulus of the piles(MPa)， ΔPis the increasing value of loads in curve of loadsvs. deformation(N)，his the thickness of the specimen(mm)，tfis the face-sheet thickness(mm) and Δhis the increasing value of compressive deformation corresponding to ΔP(mm).

1.3 Flexural test

Flexural tests were carried out according to GB/T 1456—2005 standard[15]. For flexural tests， the size of specimens was 100 mm×30 mm， the span length of two indenters was 60 mm， and loading speed was set at 2 mm/min. The flexural strength of the face-sheet can be calculated according to Eq. (3).

(3)

whereσfis flexural strength of the face-sheet(MPa)，Pis the failure loads(N)，lis the span length of two lower fixtures(mm)，bis the width of samples(mm)，his the thickness of samples(mm)， andtfis the face-sheet thickness of specimens(mm).

1.4 Damage photographing

The photographs of the damage surfaces and the sectioned surfaces were taken using a digital camera(Sony ILCE-5100L, Japan) with two 80 W light sources. The light passed through the specimen from the bottom. The magnitude and location of the damage was observed on the face-sheet and piles of the specimens.

2 Results and Discussion

2.1 Mechanical characteristics

2.1.1Compressivecharacteristics

The compressive characteristics were shown in Figs.4-5. Taking the 3D fabric sandwich composites with pile height of 10 mm for example， as shown in curve a of Fig. 4， with the increase of strain, the compressive stress was increased linearly in the initial stage. And the 3D fabric composites had no obviously changed. Compressive stress then increased rapidly and nonlinearly till reached the maximum with the increase of strain， and the resin matrix and fibers on the face-sheet were partly fractured. The color of the connection between the face-sheet and piles changed white， and the sound of crack could also be heard. Then compressive stress began to decrease with the increase of compressive strain. At the same time the fabrics of piles began to collapse. Finally， the fabrics of piles completely collapsed and the 3D fabric composites were damaged completely， when the compressive stress increased again.

It could also be seen that the main failure mode was the instable one of the piles when the 3D fabric sandwich composites were suffering compressive loads， as shown in Fig. 5. Thus the compressive properties would be affected more obviously by the pile height and pile distance.

2.1.2Flexuralcharacteristics

The flexural characteristics were shown in Figs. 6-7. Taking the 3D fabric sandwich composites with pile height of 10 mm for example， as shown in curve a of Fig. 6， the flexural stress increased linearly with the increase of strain at first. And the 3D fabric composites had no obviously changed. Flexural stress then increased slowly and nonlinearly till reached maximum with the increase of strain， the fibers on the face-sheet were partly fractured and expanded， and the resin matrix were partly fractured too， and the sound of crack could also be heard. Finally， flexural stress decreased rapidly with the increase of flexural strain， at the same time， the upper face-sheet fractured completely and the 3D fabric sandwich composites were damaged completely.

Fig. 4 Curve of compressive stress vs. strain

Fig. 5 Damage photographs of 3D fabric composites suffering compressive loads with pile height of 10 mm

Fig. 6 Curve of flexural stress vs. strain

The main failure mode was the brittle fracture of the face-sheet when the 3D fabric sandwich composites were suffering flexural loads， as shown in Fig. 7.

2.2 Influence of pile structures on mechanical properties

2.2.1Influenceofpileheightoncompressiveproperties

Figures 8-9 represent the typical compressive response of the 3D fabric sandwich composites with the pile height of 10， 20 and 30 mm， respectively. It is clearly revealed that compressive properties decreased with the increase of pile height[3， 16]. The compressive strength decreased from 1.65 MPa to 0.65 MPa， and compressive modulus decreased from 11.03 MPa to 5.65 MPa as pile height increased from 10 mm to 30 mm.

The reason can be explained that the pile could be considered as a compressive rod， thus according to the Euler formula about critical load， the maximum forcePis expressed as

(4)

whereEpis the Young’s modulus for the pile(MPa)，Iminis the cross sectional moment of inertia of the pile(m4)，μis a coefficient depending on the pile constraint conditions， andhis the height of the pile(mm). From Eqs. (1)，(2) and(4)， we can find that the 3D fabric sandwich composites with shorter piles should have more excellent compressive properties， which is consistent with experimental results. Especially in the pile height range of 10， 20 and 30 mm, a significant decrease was observed in Figs.8-9.

Fig. 8 Influence of pile height on compressive strength

Fig. 9 Influence of pile height on compressive modulus

2.2.2Influenceofpileheightonflexuralproperties

The influence of pile height on flexural properties was also examined in the work. Figures 10-11 represent the typical flexural response of the 3D fabric sandwich composites with the pile height of 10， 20 and 30 mm， respectively. It is clearly revealed that flexural properties increased with the increase of pile height[3， 16]. The flexural strength increases from 2.00 MPa to 3.01 MPa， and flexural modulus increases from 16.97 MPa to 24.99 MPa as pile height increases from 10 mm to 30 mm.

The reason can be explained according to the Euler formula， and the flexural stiffnessDis expressed as Eq. (5).

(5)

whereEfis flexural modulus of the face-sheet (MPa). Equation (5) shows that the 3D fabric sandwich composites with higher piles should have more excellent flexural stiffness and flexural properties， which is consistent with experimental results. Especially in the pile height range of 10， 20 and 30 mm, a significant increase was observed in Figs.10-11.

Fig. 10 Influence of pile height on flexural strength

2.2.3Influenceofpiledistanceoncompressiveproperties

The influence of pile distance on compressive properties of the 3D fabric composites was shown in Figs.12-13. It can be seen that compressive properties decreased with the increase of the pile distance[17-18]. The compressive strength decreased from 2.20 MPa to 0.61 MPa， and compressive modulus decreased from 7.94 MPa to 6.52 MPa as pile distance increased from 16 mm to 32 mm.

The reason can be explained that the pile could be considered as a compressive rod， and thus the maximum stressσMcan be given by

(6)

whereNis the numbers of the piles, EMis modulus of the pile(MPa). Equations (6) and(2)show that increasing the pile distance， namely reducing the numbers of the piles， will result in a considerable decrease of the compressive stress and modulus， as shown in Figs. 12-13.

Fig. 11 Influence of pile height on flexural modulus

Fig. 12 Influence of pile distance on compressive strength

Fig. 13 Influence of pile distance on compressive modulus

2.2.4Influenceofpiledistanceonflexuralproperties

The influence of pile distance on flexural properties of the 3D fabric composites was also examined in the work. Figures 14-15 showed that flexural decreased with the increase of the pile distance[17-18]. The flexural strength decreased from 2.92 MPa to 2.06 MPa， and flexural modulus decreased from 24.5 MPa to 17.05 MPa as pile distance increased from 16 mm to 32 mm.

The reason can be explained that decreasing the pile distance will result in a considerable increase of the flexural strength and modulus， which is caused by the adjacent piles overlapped and cooperated with each other to bear higher load.

Fig. 14 Influence of pile distance on flexural strength

Fig. 15 Influence of pile distance on flexural modulus

3 Conclusions

In this work， a new type of sandwich structure， the 3D fabric sandwich composites consisting of two identical parallel fabric decks woven integrally and mechanically together by means of vertical woven fabrics， was developed. The compressive and flexural behavior of the 3D fabric sandwich composites were investigated too. The study revealed that the main compressive failure mode of the 3D fabric sandwich composites was the instability of the piles， and the compressive properties were affected more obviously by the pile height and pile distance. The compressive properties decreased with the increase of pile height and pile distance. The study also revealed that the main flexural mode of the 3D fabric sandwich composites was brittle fracture of the face-sheet. At the same time the flexural properties were affected by the pile structures. The flexural properties increased with the increase of pile height， and decreased with the increase of pile distance. The work will lay a foundation for the optimization of design and mechanical properties of the 3D fabric sandwich composites.