Influence of artificial pre-stressing during the curing of VIRALL on its mechanical properties

Composites Science and Technology 53 (1995) 361-364 0 1995 Elsevier Science Limited Printed

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ELSEVIER

INFLUENCE OF ARTIFICIAL PRE-STRESSING CURING OF VIRALL ON ITS MECHANICAL

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DURING THE PROPERTIES

G. X. SC,” G. Yao,h & B. L. Zhou” a International Centre for Materials Physics, ‘State Key Laboratory for Fatigue and Fracture of Materials, Institute of Metal Research, Academia Sinica, Shenyang 110015, People’s Republic of China (Received 21 July 1994; accepted 19 December

pre-stressed concrete reinforced by steel bars. Tuttle4 introduced this method into the mechanical/thermal analysis of fiber/epoxy composite laminates and indicated that fiber pre-stressing could reduce the thermally induced ply stresses in the composite laminate. Great potential benefit is thereby gained in improving the mechanical properties of composite laminates. The research results of Schulte and Marissen” showed that for a cross-ply laminate pre-stressing the 0” plies during the curing process may cause compressive stresses in the 90” plies after cure and therefore the development of transverse cracks under tensile loading may be delayed and shifted to higher strain and load values. Most of the previous work was aimed at modifying and recreating the inner stress states of the laminates. As for the VIRALL laminate, because Vinylon fiber has ‘work-hardening’ stress/strain behavior, applying the fiber pre-stressing method to VIRALL laminate may not only recreate the inner stress states, but could also lead to strengthening. On the basis of such considerations, we investigate in this paper the influence of fiber pre-stressing on the tensile properties of VIRALL laminate.

Abstract A method for improving the mechanical properties of VIRALL has been proposed on the basis of the tensile behavior of Vinylon strands. The influence of artificial pre-stressing during the curing of VIRALL on its tensile properties has been experimentally investigated. It is revealed that fiber pre-stressing may lead to a dramatic increase in the elastic limit, yield strength and failure strength of VIRALL laminates. These results can be referred to the strengthening effect of Vinylon fibers and the modifications and recreations of the ply stress states in VIRALL laminates. Keywords:

pre-stress,

Vinylon,

VIRALL,

1994)

tensile

properties 1 INTRODUCTION VIRALL (Vinylon-reinforced aluminium laminate) is a new kind of composite with low cost and light weight.‘-3 It is an interlaminar superhybrid composite made by laminating Vinylon (PVA, polyvinyl alcohol fiber) fiber/epoxy layers and aluminium sheets alternately. Compared with the corresponding aluminium alloy, VIRALL has higher specific strength and specific stiffness, in addition to higher impact resonance and superior fatigue damage tolerancele3 such that it might be applied as a partial substitute for aluminium alloy. However, the elastic limit and yield strength of VIRALL is not very satisfactory.’ As, for a structural material, the yield strength is usually a more important parameter than the fracture strength, a low yield strength may induce a series of problems, such as low fatigue strength, etc. We have therefore tried to find a way to improve the elastic limit and the yield strength of VIRALL. The first method studied was fiber pre-stressing, which is a process involving holding the tension load on the fiber during curing of the composite and releasing the load after cooling down to ambient temperature. Pre-stressing is a traditional technique originally used in making

2 EXPERIMENTAL

PROCEDURE

2.1 Materials (1) Aluminium (alloy) sheets; (2) high-strength/highmodulus Vinylon fibers (made from PVA); (3) 6101 resin epoxy adhesive. 2.2 Specimen preparation First, Vinylon strands are stretched under a given load, and then bonded to aluminium sheets with epoxy resin adhesive. The preload is held throughout the cure process at about 120°C and released after cooling to ambient temperature. Five types of VIRALL laminates have been made under five different pre-stresses, 0, 100, 160, 200 and 300MPa. These VIRALL laminates are referred to as VO, VlOO, V160, V200 and V300, respectively. VO means 361

362

G. X. Sui, G. Yao, B. L. Zhou

that there was no pre-stress on the fibers and VlOO means 100 MPa pre-stress was applied, etc. 2.3 Tensile tests All the tensile tests were carried out at room temperature on a Schenck-PSA universal testing machine at a cross-head speed of 2-3 mm min-‘. 3 RESULTS AND DISCUSSION 3.1 Tensile properties of Vinylon strands From tensile tests on Vinylon strands, the tensile modulus was found to be 26.4-28.3 GPa, the tensile strength 1.2-1.4 GPa, and the failure strain 0*070.075. A typical tensile stress/strain curve for a Vinylon strand is shown in Fig. 1. At the beginning, the curve behaves linearly, implying that the Vinylon strand initially deforms elastically. With further increase in strain, the curve rises more slowly. Such a behavior in a stress/strain curve indicates that the fibers are weakening and signals the onset of plastic deformation. Soon after this, the curve rises dramatically, the slope increasing with increase in strain until failure of the fiber. This suggests that the Vinylon fibers are stiffened during the deformation. This increase in work-hardening rate with strain for Vinylon fibers is a distinguishing feature which is not found in most metallic materials, carbon fibers or glass fibers. For most metals, the tensile deformation process consists of elastic, yielding and plastic processes, until fracture. For carbon fiber and glass fiber, deformation is almost entirely elastic. This increasing rate of tensile work-hardening is a characteristic feature of Vinylon fibres, and is due to its particular structural features. According to a description of the crystalline polymer structure in the literature,6 we imagine a schematic illustration of the molecular chains in Vinylon fiber. As shown in Fig. 2, Vinylon fiber is composed of crystalline zones

Strain (8)

Fig. 1. Tensile stress/strain

curve of Vinylon strands.

1 Stretched _ chains

Curved ’ chains 1

\ /

Fig. 2. A schematic

illustration Vinylon.

Folded chains

of molecular

chains

in

consisting of stretched chains and non-crystalline zones made up of both folded chains and curved chains. At the beginning of tension loading, the stretched chains respond elastically, so that the stress/strain relationship is linear. With increasing tensile stress, the folded chains slide along the direction of tension. In this stage a slight increase in stress can cause a large increase in strain, so that the fibers behave weakly and the stress/strain curve is lowered. When the strain increases to a certain level, the curved chains may be forced into the axial direction which results in an increase in resistance to the tensile deformation. The larger the deformation, therefore, the more the curved chains align in the drawing direction and the more difficult it is for the fibers to deform. With the increase in the number of stretched chains, the fibers become increasingly stiff. This then, is what is referred to as the tensilestrengthened effect for Vinylon fibers. 3.2 Some features of the stress/strain behavior of VIRALL As indicated in earlier work,le3 the failure strength of VIRALL is higher than that of the corresponding aluminium alloy and the yield strength is lower. From the stress/strain relationship (Fig. 3) it can be seen that above the yield point, the aluminium retains less stiffness and is therefore less useful. The VIRALL laminate, however, owing to the tensile-strengthened effect of the Vinylon fibers, still has a relatively high stiffness above the yield point and the curve shows a clear tensile-strengthened effect. This deformation behavior of VIRALL is simply the response of that of Vinylon fibers, as mentioned above. The strength-

Influence of pre-stressing on properties of VIRALL

ic”3

Viral1

300 t-

()Y

I

0

Fig. 3. Tensile

Normnl stmin (8)

stress/strain curves of aluminium and VIRALL.

ened effect for VIRALL gives it better stability and greater safety in applications than aluminium alloys and other materials. 3.3 Effects of fiber pre-stressing on the tensile behavior and mechanical properties of VIRALL curves of the The typical tensile stress/strain laminates with aluminium alloy and VIRALL different levels of fiber pre-stress are shown in Fig. 4. It can be seen from Fig. 4 that the stress/strain relationship for VIRALL is greatly influenced by the fiber pre-stressing, and this is reflected in some of the principal mechanical properties of VIRALL laminates. As shown in Table 1, with an increase in fiber pre-stress, a clear increase is found in initial modulus, yield strength, elastic limit strength and failure strength, together with a decrease in failure strain. According to the description in Section 3.1, the chains in the fibers may be forced to align in the direction of the tension axis under a tensile preload. The larger this preload, the more the chains in the fiber are stretched, and the greater is the ability of the

material to resist the deformation of the fibers. Since the elastic modulus of the fiber is its ability to resist elastic deformation of the stretched chains, clearly the initial modulus of fiber must increase with increasing fiber pre-stress. Consequently the initial modulus of VIRALL shows an increasing trend with increasing fiber pre-stress. When deformed into the plastic range, both the VIRALL and the fibers are heavily strained and there are more chains stretched straight in the fibers. This causes the stress/strain curve of VIRALL to turn upwards. The larger the pre-stress, the greater the tensile strengthening effect and the more rapidly the stress/strain curve turns upwards. This results in an increase in yield strength and failure strength with increasing fiber pre-stress. Because of the finite limiting strain of the Vinylon fiber, it is evident that the failure strain of the VIRALL will decrease with increasing fiber pre-stress. From Table 1 it can be seen that the elastic limit strains of aluminium and the VO laminate are almost the same. This indicates that the VO laminate begins plastic deformation at a strain which is just that required for plastic deformation of the aluminium. Furthermore, the plastic deformation of the VIRALL laminate is dominated by the aluminium sheet. For a laminate with fiber pre-stress, the inner ply stress states are modified and recreated. There must be an internal compressive stress in the aluminium sheets in equilibrium with the internal tensile stress in the fiber plies. When the VIRALL laminate is subjected to a tensile load, the compressive stress in the aluminium sheets can cause a delay in reaching the elastic limit, therefore shifting the elastic limit of VIRALL to higher strain and stress values. In other words, the elastic limit of VIRALL is enhanced by fiber pre-stressing. The larger the pre-stress the higher the internal compressive stress in the aluminium sheet, and the higher the stress at the elastic limit of the VIRALL. The elastic limit of VIRALL therefore increases with increasing fiber pre-stress, and so do the yield strength and the failure strength. 4 CONCLUSIONS

”0

I

2 Normal strain (%)

3

4

Fig. 4. Stress/strain curves of aluminium alloy and VIRALL laminates with different levels of fiber pre-stress.

363

AND RECOMMENDATIONS

The process of fiber pre-stressing during the curing of VIRALL laminates may lead to two main responses, one of which is to modify and recreate the internal ply stress states and the other is to set up the tensile strengthening effect of the Vinylon fibers. This process in VIRALL causes a dramatic increase in the initial modulus, the elastic limit, the yield strength and the failure strength. The improvements in these mechanical properties are enhanced with increasing levels of fiber pre-stress. The method introduced in this paper may provide an effective way of improving the creep and fatigue properties of VIRALL.

364

G. X. Sui, G. Yao, B. L. Zhou

Table 1. Some mechanical properties of aluminium alloy and VIRALL

Initial modulus (GPa) 0.2% Yield strength (MPa) Elastic limit strength (MPa) Elastic limit strain (%) Failure strength (MPa) Failure strain (%) Aluminium volume fraction Fiber volume fraction

laminates with different levels of fiber pre-stress

Al

vo

VlOO

V160

v200

v300

69.9 158.3 147.6 0.211 300 15 1 0

48.3 128.4 99.1 0.205 357 5-6 0.57 0.32

49.3 132-9 110.1 0.223 368 5.2 0.57 0.32

50.2 142.2 116.0 0.231 385 4.6 0.58 0.31

50.9 165.4 127.8 0.251 394 3.8 0.55 0.33

51.5 176.0 141.8 0.275 410 3.4 0.56 0.33

ACKNOWLEDGEMENT

The authors would like to express their appreciation to the National Natural Science Foundation of China for financial support.

C. X., Composites, 24 (1993) 433-5.

2. Sui, G. X., Zhou, B. L., Zheng, Z. G., Zhou, C. T. & Shi, 3 C. X., J. Mater. Sci. Technol., 9 (1993) 382-4. Sui, G. X., Zhou, B. L., Zhou, C. T. & Shi. C. X., J. ’ Mater. Sci. Lett., 13 (1994) 234-5. 4. Tuttle, M. E., J. Comp. Mater., 22 (1988) 780-92. 5. Schulte, K. & Marissen, R., Comp. Sci. Technol.,

44

(1992) 361-7.

REFERENCES 1. Sui, G. X., Zhou, B. L., Zheng, Z. G., Zhou, C. T. & Shi,

R. I., Structured Polymer Properties-The Identification, Interpretation and Application of Crystalline Polymer Structure, John Wiley & Sons, 1974.

6. Samuels,