Stress Concentration in 3D Hybrid Joints by Narasimha n

8/12/2019 Stress Concentration in 3D Hybrid Joints by Narasimha n

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Stress concentrations in a three dimensional hybrid

composite-steel joint model

S. Narasimhan, R.A. Shenoi*, J.I.R. Blake and S.W. Boyd

Fluid Structure Interactions Research Group

School of Engineering Sciences

University of Southampton

Southampton SO17 1BJ, UK

Fax: (44)-2380-59-3299

Phone: (44)-2380-59-2356

[email protected]

ABSTRACT

This paper is concerned with the critical stress concentrations that occur in an

adhesively bonded hybrid metal-composite joint. The hybrid joint considered in the

study is based on the design for hanger to weather deck connections currently in

service in the French navy on the La Fayette class frigates. The joint consists of the

materials like E-glass woven roving mat, vinylester resin, balsa wood core and 6mm

thick mild steel. A three dimensional finite element analysis is performed for this

hybrid joint given that its width is considerable. Interesting normal and shear stress

patterns are seen across the width of the joint thus providing an insight to identify the

location(s) of high stress concentration in the hybrid joint. It is highlighted that a two

dimensional analysis will not be adequate where edge-effects are prominent under the

in-plane static load.

Keywords: Hybrid composite-steel joints, three dimensional FE model, Stress

concentrations

* Corresponding Author



INTRODUCTION

The application of hybrid composite-metal joint structures has been gaining

momentum over the past few years in various disciplines of aerospace, land transport

and in marine industry [1-5]. The prime objective in using the hybrid structures is to

reduce weight. The hybrid joint considered here is based on the design for hanger to

weather deck connections currently in service in the French navy on the La Fayette

class frigates. The joint is manufactured from a 3x1 twill weave 780g/m2 E-glass

woven roving (Chomarat 800S4), vinylester (Dow Derakane 411-C50), 150kg/m3

Balsa wood core (Baltek AL600-10 Contourcore) and 6mm thick mild steel (D55).

The joint specimen was prepared by vacuum resin infusion moulding process where 8

plies of glass reinforcement is laid over the top and bottom side of steel and balsa

wood. This joint is asymmetric in geometry with balsa tapering on one side and flat

on other side, as the requirement for a flush outer surface in superstructure to deck

connections (figure 1).

The objective of the work is to identify the initiation and propagation of the crack that

leads to failure of the hybrid joint under a compressive loading. Although various

experimental and modelling studies on hybrid joints have been undertaken earlier [2-

5], not much information is available on stress variations across the width of the

hybrid joint. Hence it is felt that a three dimensional finite element (FE) analysis of

such hybrid joint can be very useful in locating the critical stress concentration.

FINITE ELEMENT MODELLING AND ANALYSIS

Three dimensional analysis is done using a nonlinear FE program code developed

using a 20-noded isoparametric finite element [6]. Initial analysis have shown that the

joint would produce a lateral deflection due to the eccentricity in the compressive

load. To prevent this, a system of anti-bending guides were positioned at steel/balsa

interface. The geometry and FE model of the hybrid joint is shown in figure 1. A three

dimensional FE mesh for the hybrid joint is generated using a 20-noded three

dimensional isoparametric element with 3x3x3 Gaussian integration scheme. The FE

model had 548 elements and 3347 nodes. Load and boundary conditions were applied

similar to that of experimental set up including the presence of anti-bending guides to



arrest lateral deflection. An uniform compressive load of 120kN which is found to be

the ultimate load carrying capacity of the joint from the experiments [5] was applied

in 12 increments of 10kN each. Figure 2 shows good correlation between

experimental and numerical stiffness in the linear region thus validating the FE

program developed with 20-noded 3D finite element.

Considering the asymmetrical geometry and the presence of three different materials

in the hybrid joint, there are different interface layers to observe the stress

distributions. Of those layers, the critical stress values are seen along the FRP/Core

and FRP/Steel interface layer along the flat side of the joint and at the vertical plane

where the tapering ends. Normal and shear stress distributions shown in figure 3 and 4

were obtained from such interface layers. These stress contours show that the stress

values are not uniform across the width of the joint either along the length or through

the thickness of the hybrid joint. Normal stress increases steeply towards the mid-line

of the joint from the edges at the critical location of FRP-Core-Steel interface layer. It

is to be observed that although the maximum stress occurs along the mid-line the

stress variation at the beginning (X=60.0mm) across the width shows maximum stress

value at the ends rather at the mid-line. Shear stress plot in figure 3 also shows a

similar behaviour of maximum stress at the edges than at the mid-line even though the

stress magnitude difference is not much. This non-uniform stress variation across the

width indicates a typical three dimensional behaviour that is seen in typical single lap

joint which is termed as ‘anticlastic’ effect [6]. This arises due to different elongation

between the adherends owing to their differing material properties and the bending of

the hybrid joint because of asymmetry in shape. Figure 4 shows normal stress

variation across the width along the vertical plane at FRP-Core-Steel interface layer.

Here also the stress variation is so significant and highlights the three dimensional

behaviour in third direction as well. Stress variation in the lateral direction for the

hybrid joints had not been reported in earlier works and from these plots, it can be

very well stated that three dimensional FE analysis of joints with considerable width

will be informative in identifying the occurrence of stress concentration. Given that

stresses are maximum along mid-line at one place and maximum at edges at some

other sections, a structural joint of this configuration should be analysed in a three

dimensional domain rather opting for a plane stress/strain model.



From the maximum value of the normal and shear stress components (26 and 82Mpa

respectively), it is more likely that shear component stress is dominant in initiating the

failure and occurs at the FRP-Core-Steel interface. However a steep normal steep

gradient plays a significant role in rapid up the failure initiation and further

propagation as shown in figure 5 for the test specimen.

CONCLUSION

A three dimensional FE model of a hybrid composite-steel joint is generated and

analysed for static compression loading. Numerical and experimental stiffness were to

be found in good correlation up to linear region. Normal and shear stress variations

showed significant three dimensional behaviour and the relevance of analyzing the

joints of such configurations in a three dimensional domain instead of a 2D plane

stress/strain analysis has been demonstrated. Failure of the joint initiates at FRP-Core-

Steel interface layers on the flat side of the joint specimen and propagates further on

either directions.

REFERENCES

[1] Mouritz AP, Gellert E, Burchill P, Challis K. Review of advanced composite

structures for naval ships and submarines. Composite structures 2001;53:21-41.

[2] Wright PNH, Wu Y, Gibson AG. Fibre reinforced composite-steel connections for

transverse ship bulkheads. Plastics, Rubeer and Composites 2000;29(10):549-557.

[3] Clifford SM, Manger CIC, Clyne TW. Characterisation of a glass-fibre reinforced

vinylester to steel joint for use between a naval GRP superstructure and a steel hull.

Composite Structures 2002;57:59-66.

[4] Cao J, Grenestedt JL. Test of a redesigned glass-fibre reinforced vinylester to steel

for use between a naval GRP superstructure and a steel hull. Composite Strctures

2003;60:439-445.

[5] Boyd SW, Blake JIR, Shenoi RA, Mawella J. Fatigue life characterization of

hybrid composite-steel joints. Proceedings of the International Conference on

Advanced Marine Materials: Technology & Application, London, 9-10 October 2003.

p. 79-87.



[6] Narasimhan S, Shenoi RA, Jeong HK. Three-dimensional stresses in adhesively

bonded lap joints with non-identical adherends. Part L: Journal of Materials: Design

and Applications, Proc. of IMechE 2004;218:283-298.

Figure 1: Geometry and FE model of the hybrid joint

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

U_xx Displacement (mm)

A p p l i e d l o a d ( k N )

Expt. result

FE result

Figure 2: Load-Deflection curve



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Stress Concentration in 3D Hybrid Joints by Narasimha n