Behaviour of adhesive-bonded assemblies of galvanized steel sheets under shear loading

Behaviour of adhesive-bonded assemblies of galvanized steel sheets under shear loading E. Ziane*, G. Beranger*, C. Coddet* and J.C. Charbonnier t (*Universit6 de Technologie de Compi~gne/tlnstitut de Recherche de la Sid6rurgie, France)

The mechanical properties of bonded galvanized steel structures may be improved by using different surface treatments. Results indicate that degreasing with trichloroethane leads to the highest failure stress, followed by a 2 M sodium hydroxide etch, an 0.5 M sodium hydroxide etch and treatment with Parcodine; a 1 N sulphuric acid etch is the/east effective. In addition, the examination of fracture surfaces reveals four distinct zones which correspond to the following four delamination steps: a zone of purely adhesive failure accompanied by cracking and partial debonding of the zinc layer; a zone of mixed failure (adhesive and cohesive); a zone of purely cohesive failure; and a zone of mixed failure which is the source of final fracture.

Key words: adhesive-bonded joints; adhesive strength; lap shear testing; surface treatments; galvanized steel

Shear testing of simple lap joint specimens (ASTM D1002) is widely used in order to assess the behaviour of bonded joints, largely due to the simplicity of the test and ease of specimen preparation. Unfortunately, the stress distribution in the joint is complex, which makes the determination of the causes of failure for a given load difficult. For this reason a considerable amount of work has been carried out to establish models describing the development of the stress along the joint and predicting the type of failure from adhesive and susbstrate properties. Volkersen 1 was the first to propose a model describing the stress variation along the joint as a function of the width and thickness of the bond and of the mechanical characteristics of the adhesive and substrate. However, Volkersen did not take into account strains resulting from longitudinal loading of the sheets nor strains due to shear loading of the adhesive. This model was improved by Demarkles2 who included the strains due to transverse loading of the sheets. However, neither of these authors took account of rotation of the sheets caused by eccentric loading. This phenomenon introduces a bending moment into the normal section, producing a normal stress which results in a Mode I opening rather than pure shear (Mode II). Goland and Reissner3 reworked these models and introduced a factor to account for the bending moment

caused by eccentric loading, but ignoring bond thickness. Other authors such as Renton and Vinson4"~ and AUman6 have proposed more detailed analyses, taking account of the bending, stretching and shear of the sheets as well as the shear and tearing of the adhesive. However, all these authors have considered the adhesive as a material behaving perfectly elastically. Hart-Smith 7 has developed the model further in taking account of the viscoelastic behaviour of the adhesive but this work was limited to strains produced by shear stresses. His approach was otherwise similar to that of Volkersen 1. With the development of data processing techniques, the analysis of stresses in bonded joints using numerical methods has now become very attractive. Wooley and Carvers were the first to introduce the finite element method, which produced similar results to those found by Goland and Reissner3, the adhesive having been treated as a perfectly elastic material. Harris and Adams 9 have applied the same method but also taking account of the viscoelastic behaviour of the adhesive, in order to predict failure and the associated fracture mode. However, the elastic-plastic behaviour of the sheets has not yet been accounted for. All these studies to date have been directed towards an analysis of the mechanical behaviour of the system. For this reason, the aim of this work is to study the correlation between deformation mechanisms and

0143-7496/86/020067-05 $03.00 © 1986 Butterworth Et Co (Publishers) Ltd INT.J.ADHESION AND ADHESIVES VOL.6 NO.2 APRIL 1986

67

fracture surfaces, which may allow an improved understanding of the behaviour of the joint during shear loading.

investigations after sectioning of the joint were performed in a Cambridge scanning electron microscope (SEM).

Experimental details

Results

Single lap specimens were prepared according to ASTM D1002 from 1.5 m m thick steel sheets which had been continuously galvanized by the Sendizmir process. Half-specimens were machined to the dimensions 170 × 25 mm, and subjected to one of the following surface treatments: • Degreasing with trichloroethane - - the degreasing

was performed by rubbing the surface with a cotton swab soaked in trichloroethane. • Treatment with sodium hydroxide - - two treatments with sodium hydroxide (NaOH), 0.5 and 2 M, were carried out at 20°C for 5 minutes and at 60°C for I0 seconds, respectively, in order to remove the aluminium-rich film present on the surface of the sheets. • Chemical etch in sulphuric acid - - an acid etch, (H2 SO4, 1 N) was performed at 20°C for 13 minutes in order to dissolve surface layers of specimens. • Treatment with Parcodine - - a treatment with Parcodine !20, a product based on phosphoric acid which emulsifies, degreases and also promotes an etch of the metal (from St6 Continental Parker), diluted to 20% in water was carried out at 20°C for 13 minutes. After the appropriate bonding treatment, a line of adhesive was placed 6 m m from the end of one of the half-specimens. The adhesive was a two-component epoxy resin from Ciba Geigy (reference AV138 + HV998). The half-specimens were assembled in a pressing rig which allowed a reproducible surface covering of 300 m m 2 to be obtained and a constant pressure of 33 kPa to be applied. The adhesive was polymerized by placing the pressing rig in an oven at 100°C for 20 minutes. The average thickness of the film (0.2 mm) was measured on cross-sections in a scanning electron microscope. Tests were carried on a tensile machine (Instron 1115) with test speed of 2 m m min -1 and a 10 kN load cell. The crack growth was followed using two travelling microscopes (30× magnification), placed in front of and behind the specimen. General inspection of fracture surfaces was first carried out using an optical microscope. Detailed Table 1. Specimen

Measurement of failure load in shear The test results are presented in Table l. In order to assess the significance of the results, taking account of scatter, it is prefereable to use a statistical approach based on variance analysis 1°. Thus ifA is the "surface treatment' factor, the n u m b e r of modalities for this factor is c -- 5 and the number of repetitions is n = 5. In addition if: = (n - 1) (,4) = SS(A)

MS(A)

=

MS(E) MS(T)

Fc

= = =

Fa

=

the sum of squares for factor A; t h e n u m b e r of degrees of freedom of factor A; the variance of factor A, MS(A) = SS(A)/(n - 1); the residual variance; the total variance; the calculated Fisher value, Fc(A ) = MS(A)/MtR(E): the Fisher value extracted from standard tables (Fisher distribution) for a confidence level of 1 - a:

then the influence of factor A is regarded as significant ifFc > F a . The parameters of the variance analysis are presented in Table 2. For the present case, Table 2 verifies that the surface treatment has a significant effect on the joint behaviour since F c > F a. Therefore it may be concluded that the surface treatment plays a significant role in the behaviour of the assembly and that a relative ranking, in decreasing order of effectiveness, of the treatments studied is: trichloroethane, N a O H 2 M, N a O H 0.5 M, Parcodine and H2SO 4 1 N. It should be stressed that this order only applies to the system studied here (hot dip galvanized steel) and for a joint loaded in shear. Observations during the test During the test the behaviour of the specimen can be divided into three steps (see Fig. 1). As loading begins the specimen starts to undergo bending, as noted previously by Goland and Reissner 3 and Lukowiak et al II, which leads to rotation of the joint. The second step is the opening of the joint at the edges, in

Test results (in N) Trichloroethane

NaOH, 0.5 M

NaOH, 2 M

Parcodine

H2SO 4, 1 N

1 2 3 4 5

620 660 590 590 600

565 580 500 620 550

652 680 548 632 480

585 420 435 680 555

320 485 500 435 480

Mean

612

563

598.4

535

444

number

Standard deviation

68

26.3

39.2

INT.J.ADHESION AND ADHESIVES APRIL 1986

73.7

97.1

65.5

agreement with the work of Volkersen I and Harris and Admires9. The third step is propagation of the cracks (delamination) to failure.

+ii

• Observation of fracture surfaces

Seven zones can be distinguished on each half of the specimen (see Fig. 2): • Zone 1 (Fig. 3) shows purely adhesive fracture accompanied by cracking and partial removal of the zinc layer. This phenomenon is probably due to the bending load on the specimen, which results in one face undergoing compression and the other (the bonded face) undergoing tension. • Zone 2 (Fig. 4) reveals mixed fracture (adhesive and cohesive). The zones of adhesive fracture are identical to those seen in zone 1 and show similar damage of the galvanized layer. • Zone 3 (Fig. 5) corresponds to cohesive fracture showing regular serrated features. • Zone 4 (Fig. 6) again shows mixed fracture. • Zones 5, 6 and 7 are complementary to zones l, 2 and 3.

J

Fig. 1

Successive configurations of the assembly during loading

Table 2. Effect

A E T

Summary of the different parameters used in variance analysis

SS

161099 89077 250176

Degrees of

Estimator

freedom

MS

4 20 24

40274.8 4453.8 10424

,ca

Fc

9.04 -

a=0.05

a=0.01

3.92 -

4.95 -

P r o p a g a t i o n direction C'

f

b,

at

"' I + I + 1 + 1 : 1 121 121+1;151+1+1

l/

CI m Fig. 2

Aspect of the fracture surfaces

oI

bI

f

Propagation

, I

direction

I N T J . A D H E S I O N A N D A D H E S I V E S APRIL 1 9 8 6

69

the galvanized steel surfaces with trichloroethane or N a O H appears to be more effective than pickling in sulphuric acid. Taking account of the specimen geometry, shear loading of the assembly produces a mixed-mode failure: opening and shear. The fracture surfaces reveal four successive zones (see Fig. 8). An initial zone of adhesive fracture with cracking and partial removal of the galvanized layer is followed by a second zone of mixed fracture (adhesive and cohesive). A third zone is characteristic of a shear fracture and finally a fourth zone is associated with final failure of the specimen.

Acknowledgement Fig. 3 Scanning electron micrograph of zone 1 in Fig. 2, showing adhesive fracture and cracking of the galvanized layer

The authors wish to thank the CECA (Communaut~ Europ6enne du charbon et de l'acier) for partial financial support of this work.

Fig. 4 Scanning electron micrograph of zone 2 in Fig. 2, showing adhesive and cohesive fracture

Fig. 6 Scanning electron micrograph of zone 4 in Fig. 2, showing mixed fracture in the middle section of the joint

f

¢'

d,~/f

=" "

i

J

b'

<

-

0

Fig. 5 Scanning electron rnicrograph of zone 3 in Fig. 2, showing cohesive fracture

The various steps and processes which intervene during the test may thus be summarized as follows. From the application of the load, plastic deformation occurs due to the presence of a bending moment, which causes plasticization of the adhesive. The crack is nucleated at the principal initiation sites a', b', c' and d' shown on Fig. 7. Propagation then takes place in the direction of the arrows until the two cracks join at the line ef.

Conclusions In conclusion, it may be noted that the surface treatment plays a relatively large role in determining the behaviour of this system. Thus simple cleaning of

70

INTJ.ADHESION AND ADHESIVES APRIL 1986

Fig. 7 Schematic of the specimen showing the main fracture initiation sites and the propagation path

5 Renton, W.J. and Vinson, J.R. "l'he efficient design of adhesive ~ : ~ l ~ d e d joints' JAdhesion 7 (1975) pp 175-193 6

AIl~ilni D:J. 'A theory for the elastic stresses in adhesive bonded lap joints' Quart J ofMech andApp/Maths XXX 11 No 4 (1977) pp 415-436

7

Hart-Smith, L.J. 'Stress analysis; a continuum mechanics approach' in 'Deve/opments in Adhesives 2" (Applied Science publishers, 1981 ) pp 1-44

8

Wooley, G.R. and Carver, D.R. 'Stress concentration factors for bonded lap joint' J Aircraft 10 No 8 ( 1971 ) pp 817-820

9

Harris, J.A. and Adams, R.D. 'Strength prediction of bonded single lap joints by non-linesr finite element methods' /nt J Adhesion and Adhesives 4 No 2 (April 1984) pp 65-78

Substrate - ~

Adhes,ve-'~ /

I 4 I 3 12 I I I f.

SubstroteJ

Fig. 8 Schematic transverse section of the joint showing the four successive delamination zones

10

Vigier, M. "Methodes d'assurance qualit#-fiabilit~ d'exp#rimentation' (Maloine S.A., Paris, France, 1981 )

11

Lukowiak, W., Pavan, A. and Collot, C. 'Etude de la rupture d'assemblages m~talliques ~ double recouvrement colles par I'interm6diaire d'une r6sine epoxy solicit6e en traction' Mat~riaux et Techniques Nos 6-7 (1984) pp 30-35

et

References 1

Volkersen, O. "Recherche sur la th6orie des assemblages coil,s' Construction m~tallique 4 (1965) pp 3-13

2

Demarkles, L.R. 'Investigation of the use of a rubber analogue in the study of the stress distribution in riveted and cemented joints' NASA TN3413 (1955)

3

Goland, M, and Reisaner, E. "Stresses in cemented joints' Trans ASME, J App/Mech 66 (1944) pp A17-A27

4

Renton, WJ. and Vinson, J.R. "Analysis of adhesively bonded joints between panels of composite materials' Trans ASME, J App/ Mech (March 1977) pp 101-106

Authors

E. Ziane is with the University Mohamed I., Oujda, Morroco. G. Beranger and C. Coddet are with the Department of Mechanical Engineering of the University of Compiegne, BP 233, 60206 Compiegne Cedex, France. J.C. Charbonnier is with IRSID, BP 129, 78105 Saint-Germain en Laye Cedex, France. Inquiries should be addressed to Mr Coddet in the first instance.

INT.J.ADHESION AND ADHESIVES APRIL 1986

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