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Long endurance fatigue performance of bonded structural joints
Long endurance fatigue performance of bonded structural joints G.C. Mays and G.P. Tilly
Resin bonded joints are being used for a variety of applications in highway bridges. Such uses involve operation under different combinations of dead and live loading plus weathering. This paper is concerned with the fatigue performance of steel-to-steel lap joints for endurances of up to 108 cycles. The effects on fatigue performance of curing temperature and operating temperature have been examined and work in progress to investigate effects of weathering is described. Special attention is given to performances at long endurances and low stresses relevant to traffic loading during service. Key words: adhesive-bonded joints; epoxy resins; fatigue testing; environmental testing; steel lapped joints.
In recent years resins have been used in increasing quantities and for a variety of purposes in highway and structural engineering. The wide range of physical properties of these materials makes them attractive in a number of ways. Initial applications have involved use of epoxy resins in nonstructural situations such as binders for skid-resistant running surfaces and epoxy modified asphalt. In highway bridges uses have been as diverse as coating to protect steel reinforcement from corrosion, nosings of expansion joints, the connections between pre-cast post-tensioned segments of concrete superstructures, hybrid anchorages for hanger cables of stayed bridges, external steel plating to strengthen concrete bridges and there are schemes for composite decks incorporating bonded steel plating. The latter applications differ from the others in that they are subjected to traffic induced stresses so that the question of fatigue performance arises. Much of the enthusiasm for resins in civil engineering arises from the successful and continued development of their usage in aeroframes. Material properties have been thoroughly researched and when coupled with service experience with modern aircraft dating from about 1965 it follows that adequate data for structural integrity under relevant environmental conditions have been obtained. Service performance is rather variable; in many applications the joints require regular inspection and repair. Fatigue testing has been conducted on a 'thick adherend lap-shear specimen' and a 'thick adherend double-cantilevered-beam' specimen under pulsating-tension loading 1. The testing conditions included temperatures of 600C and condensing humidity. Unfortunately the fatigue properties obtained in these tests give no more than an indication of what can be expected of bonded connections in civil engineering because different types of resin are used, environments are different and workmanship under the conditions prevalent
on construction sites leads to standards lower than can be obtained in factories. Even more important, operational lives of aeroframes are about 20 years whereas highway bridges in the UK are assessed for lives of 120 years. Tests to establish the performance of bonded joints for aeroframes have usually been for endurances of up to about 107 cycles and some have been up to 108 cycles. Fatigue in bridges can arise from a variety of causes, eg hanger cables are subject to wind induced vibrations and decks under normal trafficking are subject to repetitions of wheel loads. Depending on the type of highway (whether single or dual carriageway etc) components in decks can experience up to 7 x 108 cycles due to loading by axles of commercial vehicles. A wide range of environments can occur in highway bridges; temperatures can vary between - 2 5 ° C and + 75°C on asphalted decks 2 and very high humidities (up to 100% relative humidity) can occur inside steel box girders. Rates of loading can vary from very slow, associated with congested traffic at crawl speed, to relatively fast and may typically be equivalent to a frequency of 1 Hz 3 Until recently there has been little work on the fatigue of bonded connections using types of resins and loading which are relevant to civil engineering. Work by Mays and Harvey4 involved 4-point bending fatigue of 'open sandwich' beams in which concrete was cast onto 0.6 mm thick steel plates. The materials were bonded together with a 1 mm thick layer of adhesive; four types of adhesive were used. Some of the beams were waterproofed on all but their top surfaces and subjected to two types of wetting with tap water and one type with salt solution. In all cases rusting developed at the steel surface and the fatigue performances were generally lowered. Cusens and Smith s tested steel-to-steel double lap joints using three types of adhesive and four types of surface treatment: (i) standard clean rough surface; (ii) standard
0143-7496/82/020109-05 $03.00 © 1982 eutterworth & Co (Publishers) Ltd
INT.J.ADHESION AND ADHESIVES APRIL 1982 109
surface blasted with grade 50 sand; (iii)standard surface allowed to rust over 15% of area; and (iv) standard surface dipped in salt solution and rusted over 20% of area. Subsequent tests showed that the static strengths were reduced by between 12 and 47% for the three adhesives and treatments (ii), (iii) and (iv). The present paper describes work to determine the fatigue performance of steel-to-steel lap joints using two resins typical of those used in structural engineering. The tests involve the following conditions: 1. Specimens cured at different temperatures and tested at room temperature; 2. Specimens cured at room temperature and tested at temperatures ranging from --25°C to + 55°C; and 3. Specimens weathered prior to testing.
Test specimens All specimens were of the double-lap type, the majority being constructed from bright mild steel flats, 25.4 x 6.35 mm in cross-section. Those to be subjected to temperature cycling were constructed from 6.35 mm mild steel plate and subsequently cut into strips 25.4 mm wide. The overall length of the specimens was 600 mm with a lapped length of 80 mm. The specimens were constructed with a target uniform glue-line thickness of 0.65 mm. Two grades of two-part, cold cure epoxy resin adhesive were used. The majority of specimens were constructed with Adhesive No I which is a white thixotropic paste mixed with approximately 10% of its own weight of black liquid hardener of low viscosity and understood to be an aliphatic polyamine. A small number of specimens were manufactured with Adhesive No 4 which is a white pigmented epoxy resin paste mixed with 55% of its own weight o f liquid hardener of medium viscosity and understood to be an aromatic polyamine. Steel surfaces were initially cleaned of any rust or millscale and degreased, first with paralTm and then with acetone, prior to blasting with metal grit to a finish resembling Swedish Standard A Sa 2½ 6. The ends of the main laps were covered with PVC tape to ensure that, under test, all load was transmitted in shear. Adhesive was applied to both main and side pieces using a spreader tool and the laps assembled in specially constructed jigs illustrated in Fig. 1. Specimens could be cast in batches of up to 16 to ensure that assembly was completed within 40 minutes of first introducing the hardener to the resin.
Fig. 1
Jig for preparation of specimens
one of two environmental cabinets capable of either cooling the specimens down to - 2 5 ° C or heating it up to + 75°C. In the event the maximum temperature was limited to + 55°C. The temperature of the surface of the steel specimen was monitored by an independent thermocouple probe. The cabinets were capoable of maintaining the surface temperature within 1 C of the designated figure. Details of the fatigue testing rig and specimen arrangement within the low temperature cabinet are shown in Fig. 2, The majority of this work was performed with Adhesive No 1 although a small series was conducted with Adhesive No 4 for comparison. Static tests were carried out at temperatures ranging from - 2 5 ° C to + 75°C on a separate batch of lap joints manufactured with Adhesive No 1. These tests were performed at a rate of loading of 3 kN/min within the same environmental cabinets and on the same testing machine as described for the fatigue tests.
Low curing temperature A supplementaroy programme wa s introduced to investigate the effect of a 0 C curing temperature on the subs~uent
Test procedures Specimens for test were initially cured at 25 -+2°C in a converted deep freeze cabinet. Seven days after casting, specimens were removed from the jigs and static control tests performed on 3 samples from each batch. The remaining specimens were stored at 25"C until ready for fatigue testing. They were held in mechanical tensile wedge grips and cycled, using a hydraulic serve-controlled actuator, under load control with a sine wave form at a frequency of 25 Hz. The frequency of load cycling was selected to be as high as possible and was based on initial trials to ensure that neither dynamic nor temperature effects were introduced. Specimens were tested in tension between a peak load, selected to give failure at endurances up to 100 million cycles, and 10% of that peak, ie the stress ratio (R) was 0.1. Test pieces were enclosed for the duration of the test within
110
INT.J.ADHESION AND ADHESIVES APRIL 1982
Fig. 2 Details of fatigue testing rig and specimen arrangement within low temperature cabinet
fatigue performance of the joint at room temperature. The specimens were inserted, immediately after assembly, into a modified deep freeze cabinet which maintained the + O . . . . . temperature at 0 - 2 C. At penodzc intervals single speczmens were removed for static tests at 0°C. When the static strength had stabilized fatigue testing of the remaining specimens was commenced at room temperatures. The majority of this work was performed with Adhesive No 1 although a small series was conducted withAdhesive No 4 for comparison.
2
'Weathered' specimens Work at Transport and Road Research Laboratories, Crowthome, UK (TRRL) 7 has shown that under normal weathering, corrosion can develop at the surface of steel bonded to concrete. This can reduce the shear strength of the bonded connection and, more important, provide potential initiation sites for fatigue. Work has therefore been started to determine the performance of steel-to-steel joints after weathering. Two forms of accelerated weathering are being used to investigate the effect of micro-cracking in the adhesive layer and rust at the steel/adhesive interface on the subsequent performance of joints made with AdhesiveNo 1. In an attempt to introduce micro-cracking, 300 mm wide lap joints were subjected to 1000 cycles of temperature variation from -- 7°C to + 35°C and return. Each complete cycle lasted 4 hofirs. The wide laps were subsequently cut into strips, the edges being discarded, before storing at 25°C in advance of fatigue testing at a range of temperatures. The steel for a further set of joints was initially allowed to rust over approximately 20% of the surface area. After casting and curing in the normal manner these specimens were stored in a chamber at 20°C and 90% relative humidity. At approximately 3-weekly intervals the joints were exposed for 3 days to the natural environment adjacent to the Tay Estuary. Atmospheric chloride levels were monitored using an Ambler candle housed in a louvred box. The arrangement during exposure is shown in Fig. 3. After a total period of about 18 months after casting these specimens are to be tested in fatigue at a range of temperatures. Results and analysis Fatigue tests at various temperatures The fatigue tests on specimens constructed with Adhesive No 1 and cured at 25°C were carried out in two phases: Series I specimens were obtained from 2 batches having a mean control strength of 22.6 N/mm 2 at 20°C and were subsequently tested at 0°C or - 25°C; Series 2 specimens were obtained from 3 batches having a mean control strength of 12.4 N/mm 2 at 20°C and were subsequently tested at temperatures ranging from room temperature (20°C) to 55°C. Values of log stress range against log endurance are plotted in Fig. 4. With the exception of results at 55°C, it is evident that the fatigue data may be grouped into two bands, 0°C to -- 25°C and room temperature to 45°C. Linear regression analyses were made on the assumption that the relationship between stress range (Or) and endurance (iV) can be described by the expression: (ar)mN = K
as used for welded joints in BS 5400 part 10 s. The exponent m is 25 and the value of K representing the lower limit of performance over the temperature range -- 25°C to + 45°C
Fig. 3
Louvred b o x and arrangement for weathering
10 _ - 2 5 to O°C '
5
•
20 to 45-C
3 -~
Regression lines
E
\v__55°C
U 2 b
//Did not fail O-25°C C] 4 5 ° C • O ~7 5 5 O 20 Z~
I
105
v
35 i'
I
106 107 N u m b e r of cycles, N
108
Fig. 4 Fatigue performance o f joints made with Adhesive No 1 cured at 25°C (log o r vs log N); regression lines are f o r mean - - 2 standard deviation; one specimen tested at 0 ° C failed at 11 990 cycles and one specimen tested at 55 ° C failed at 16 150 cycles
is found to be 8.7 x 102a and the value of Or at 107 cycles is 4.76 N/mm 2. The fatigue performance is considerably weaker at 55°C and the stress range at I07 cycles is about 2.2 N/mm 2. All fatigue failures occurred at the steel/adhesive interface. At temperatures of up to 45°C the mode was 100% adhesion, at 55°C the mode involved 'pull out' from the joint leaving a smear of adhesive on the steel surface. The latter is indicative of time dependent behaviour. The fatigue tests on specimens constructed with Adhesive No 4 were carried out at a temperature o f - 2 5 ° C only. The control strength of this batch at room temperature (20°C) after 7 days curing at 25°C was 8.8 N/mm 2. The results are presented in Table 1. Despite the smaller static strength of this adhesive its fatigue performance at - 2 5 ° C appears to be at least as good as that of Adhesive No 1 at the same temperature. The change in static strength with temperature of a further batch of specimens constructed with Adhesive No 1 and cured at 25°C is shown in Fig. 5. The control strength • of this batch at 20 O C was 16.7 N/ram 2 . At lower temperatures the strength tends to fall, reaching 12.7 N/mm 2 at -- 25°C, and failures tend to occur at the adhesive/steel interface. At higher temperatures the strength increases rapidly to a peak at approximately 30°C with failure occurring largely within the adhesive layer. Thereafter strength falls reasonably linearly regaining its room temperature value at 45°C and reaching 8.0 N/mm 2 at 55°C. By 75°C it has further reduced to 2.4 N/mm 2, the fracture
INT.J.ADHESlON AND ADHESIVES APRIL 1982
111
Table 1.
Fatigue performance of Adhesive No 4* at -- 25°C
Stress range (N/ram 2) 9.1 8.3 7.5 7.5 6.8 4.4
Cycles to failure (x 103)
(or)mN =
30
2O
E
<>
v
K
can be used to describe the relatioaship between stress range and endurance. Fatigue strengths are reduced due to the lower curing temperature but less severely than the static strength. For example, with Adhesive No 1 the reduction was about 25%. An investigation of the effect of elevated curing temperatures up to 80°C on the static strength of similar joints constructed with these adhesives has previously been conducted9. The results for both steel and aluminium laps are given in Table 2. They show that with Adhesive No 1 there is a tendency for strength to improve with increasing cure temperature but with Adhesive No 4 there is a small decrease after curing at 80°C. Fatigue tests have been carried out at endurances up to 1.5 x 106 cycles on steel laps cured at 23°C and 80°C 4. With AdhesiveNo 1 fatigue performance is improved by the higher curing temperature but with Adhesive No 4 there is little effect, The fatigue performance of Adhesive No 4 is found to be generally superior to that of Adhesive No 1 despite its lower strength.
15.4 116.7 364.6 4 488.8 > 2 0 000.0 > 20 000.0
*Cured at 25°C
Z
are given in Fig. 6 for both adhesives. Again, an expression of the form:
<>
<
~10-
Comparison with US data related to airframes
Surprisingly little fatigue testing has been done in the acreframe industry on laboratory specimens but effort has been concentrated on component testing. Fatigue is considered to be less of a problem than stability. Consequently a lot of 0
-25
-15
-5
5
15 25 35 Teml0er~ture (*C)
45
55
65
75
Fig. 5 Effect o f temperature on static strength o f joints made with Adhesive No 1
surface of the adhesive exhibiting considerable plastic deformation. There is no apparent correlation between static strength and fatigue performance in the range --25°C to 45"C. However, the slightly superior performance in the band 0°C to - 25"C may be wholly or partly due to the greater static strength of the Series I specimens. The fatigue performance is temperature dependent and it therefore follows that a fatigue assessment must take into account not only the design spectrum of loads but also the variation in fatigue strength with temperature and the times at different temperatures. For example, the temperatures of steel deck plates in highway bridges can exceed 550C for about 10 hours per annum. This means that over 120 years about 0.8 x 106 cycles of stress may be caused by the axles of commercial vehicles when the temperature exceeds 550C.
Table 2.
Curing temperature (°C)
Adhesive No 1
Adhesive No 4
Steel/steel AI/AI (N/mm 2) (N/mm 2)
Steel/steel AI/AI (N/mm2) (N/mm2)
23 40 60
16.8 10.0 13.0
7.5 7.5 9.9
113 12.1 12.3
4.2 3.9 4,3
80
16.7
13.1
11.5
3.4
*Cured at up to 80°C Results from Reference 9
~ 5 o
I~
z
112
INT.J.ADHESION AND ADHESIVES APRIL 1982
4 cure o'c test 20"¢o/
Regroups~ n line
v
The static strengths at 20°C of specimen batches constructed with Adhesives No 1 and No 4 and cured at 0°C for a period of 1 week were 8.7 and 3.5 N/ram2, respectively. After a further period of 7 weeks at 0*C strengths had risen to 10.8 and 5.3 N/mm 2, respectively. Thus the effect of curing the resins at 0*C is to lower the static strength by 50 to 60% and to prolong the required curing period when compared with a 25"C curing temperature. The fracture surface of Adhesive No 4 exhibited a regular, layered pattern with cracks running transverse to the length of the lap, presumably caused by thermal contraction during o curing. Results of fatigue tests on specimens cured at 0 C
'C ~est - 25 to O°C
6
r,
Fatigue tests after curing at different temperatures
Static strength of adhesives*
t.
2 .~Dicl not ~ i l o Adl%,sive 1 o Adhesive 4
I 110s
I
10 e 10 ~ Number of cycles. N
10e
Fig. 6 Fatigue performance o f j o i n t s made with Adhesive No 1 and No 4, cured at 0=C (log o r vs log N) (upper line taken f r o m Fig. 4); regression line f o r mean -- 2 standard ~ a t i o n : one Sl~ecimen joined with Adhesive No 1 failed at 1480 cycles end t w o joined with Adhesive No 4 failed at "low" cycles ( 1 0 1 8 0 and 1 2 6 8 0 )
work has been carried out to determine long term behaviour under static stresses and different environments. Service experience with metal-to-metal joints is very variable, some perform well, others require a lot of maintenance. Fatigue tests were conducted by Marceau et al 1 on axial shear specimens of 2024 T3 aluminium alloy. The adhesive was 0.25 mm thick woven scrim and the joint was cured at 121°C for 1-5 hours under a pressure of 0.34 N/mm 2. Mean static strengths at room temperature and mean fatigue strengths at 24°C and 60°C are compared with the results of the present study in Table 3. Also shown is the effect of a lower frequency on fatigue performance at 60°C. The performances of the more sophisticated aluminium-epoxyscrim-aluminium joints are markedly superior to the steelepoxy-steel joints tested in this investigation. The different performances are almost certainly due to the adhesive. The dramatic reduction in fatigue performance reported at 60°C as a result of reducing the frequency from 30 to 0.0002 Hz suggests that special attention needs to be paid to assessing performance at elevated temperatures over a range of frequencies. From the aerospace experience it is also clear that attention should be given to performance under load in different environments.
hand to study the effect of ageing, through accelerated temperature cycling or natural exposure of pre-rusted joints, on fatigue performance. Aluminium-epoxy-scrirn-aluminiumjoints as used in the aircraft industry have a markedly superior fatigue performance to the steel--epoxy-steel joints representative of civil engineering applications. The effect of loading rate on fatigue performance at elevated temperatures requires special attention. Acknowledgements
The work described in this investigation was carried out within the Wolfson Bridge Research Unit, University of Dundee, under contract to the Transport and Road Research Laboratory. However, the views expressed are those of the authors and are not necessarily those of the Department of the Environment, or of the Department of Transport, or any other Government Department. Crown copyright is reserved on this paper © CROWN COPYRIGHT. Reproduced by permission of the Director TRRL, 1982. References
Conclusions
The fatigue performance of epoxy-resin bonded steel lapjoints is temperature dependent. For the resin used in this study there is little change in performance over the temperature range - 2 5 ° C to 45°C but it is considerably weaker at 55°C. For the range - 2 5 ° C to 45°C, the lower bound can be given by the expression: or2SN = 8.7 × 1023 Any fatigue assessment must take into account the design spectrum of loads, the variation in fatigue strength with temperature, and the times at different temperatures. For optimum performance of bonded joints, curing temperatures as low as 0°C should be avoided. Further work is in
1
Marceau, J.A., McMillan, J.C. and Scardino, W.M. 'Cyclic stress testing of adhesive bonds', Adhesives Age (April 1975) pp 37-91
2
Emerson,M. 'Extreme values of bridge temperatures for design purposes', TRRL Report LR 744 (Department of the Environment, Department of Transport, Crowthorne, 1976)
3
Tilly, G.P. 'Fatigue problems in highway bridges', Transportation Research Record No 664 Washington (1979) pp 93-101
4
Mays, G.C. and Harvey, W.J. 'Fatigue performance of adhesive bonded joints for bridge deck construction', IABSE Colloquium, "Fatigue of steel and concrete structures," Lausanne, March 1982
5
Cusens,A.R. and Smith, D.W. 'A study of epoxy resin joints in shear'. The Structural Engineer 58A No 1 (1980) pp 13-18
6
'Pictorial surface preparation standards for painting steel surfaces', Svensk Standard SIS 055900 - 1967 (Swedish Standards Institute, Stockholm, Sweden)
7
Calder, A.J.J. 'Exposure tests on externally reinforced concrete beams - first two years', TRRL Supplementary Report SR 529 (Department of the Environment, Department of Transport, Crowthorne, 1979)
8
'Steelconcrete and composite bridges', BS 5400 (British Standards Institute, London, 1978)
9
Ching, G. 'Durability of epoxy adhesive joints in concrete, steel and aluminium', MSc Thesis (University of Dundee, April 1979)
Table 3. Comparison with fatigue performance of aluminium joints
Joint type
AI/epoxy-scrim/Al* Steel/epoxy/steel
Mean static strength at room temperature
39.5
Mean stress range to cause failure
Temp (°C)
Or
A t 107 cycles at 30 Hz
24
15
60
11.7
60
3.8
A t 3000 cycles at 0.0002 H z
* Results from Reference 1
22.6 (Series 1) 12.4 (Series 2) 16.7 (Series 3)
(N/ram 2)
Temp (°C)
Or (N/mm 2)
-- 25 4.8 to q- 45 55 2.2 -
-
Authors
Mr Mays is with the Department of Civil Engineering, The University, Dundee, and Dr Tilly, to whom inquiries should be directed in the first instance, is with The Bridge Division, Transport and Road Research Laboratory, (Department of the Environment, Department of Transport), Old Wokingham Road, Crowthome, Berks, RG11 6AU, England.
INT.J.ADHESION A N D A D H E S I V E S A P R I L 1982
113
Resin bonded joints are being used for a variety of applications in highway bridges. Such uses involve operation under different combinations of dead and live loading plus weathering. This paper is concerned with the fatigue performance of steel-to-steel lap joints for endurances of up to 108 cycles. The effects on fatigue performance of curing temperature and operating temperature have been examined and work in progress to investigate effects of weathering is described. Special attention is given to performances at long endurances and low stresses relevant to traffic loading during service. Key words: adhesive-bonded joints; epoxy resins; fatigue testing; environmental testing; steel lapped joints.
In recent years resins have been used in increasing quantities and for a variety of purposes in highway and structural engineering. The wide range of physical properties of these materials makes them attractive in a number of ways. Initial applications have involved use of epoxy resins in nonstructural situations such as binders for skid-resistant running surfaces and epoxy modified asphalt. In highway bridges uses have been as diverse as coating to protect steel reinforcement from corrosion, nosings of expansion joints, the connections between pre-cast post-tensioned segments of concrete superstructures, hybrid anchorages for hanger cables of stayed bridges, external steel plating to strengthen concrete bridges and there are schemes for composite decks incorporating bonded steel plating. The latter applications differ from the others in that they are subjected to traffic induced stresses so that the question of fatigue performance arises. Much of the enthusiasm for resins in civil engineering arises from the successful and continued development of their usage in aeroframes. Material properties have been thoroughly researched and when coupled with service experience with modern aircraft dating from about 1965 it follows that adequate data for structural integrity under relevant environmental conditions have been obtained. Service performance is rather variable; in many applications the joints require regular inspection and repair. Fatigue testing has been conducted on a 'thick adherend lap-shear specimen' and a 'thick adherend double-cantilevered-beam' specimen under pulsating-tension loading 1. The testing conditions included temperatures of 600C and condensing humidity. Unfortunately the fatigue properties obtained in these tests give no more than an indication of what can be expected of bonded connections in civil engineering because different types of resin are used, environments are different and workmanship under the conditions prevalent
on construction sites leads to standards lower than can be obtained in factories. Even more important, operational lives of aeroframes are about 20 years whereas highway bridges in the UK are assessed for lives of 120 years. Tests to establish the performance of bonded joints for aeroframes have usually been for endurances of up to about 107 cycles and some have been up to 108 cycles. Fatigue in bridges can arise from a variety of causes, eg hanger cables are subject to wind induced vibrations and decks under normal trafficking are subject to repetitions of wheel loads. Depending on the type of highway (whether single or dual carriageway etc) components in decks can experience up to 7 x 108 cycles due to loading by axles of commercial vehicles. A wide range of environments can occur in highway bridges; temperatures can vary between - 2 5 ° C and + 75°C on asphalted decks 2 and very high humidities (up to 100% relative humidity) can occur inside steel box girders. Rates of loading can vary from very slow, associated with congested traffic at crawl speed, to relatively fast and may typically be equivalent to a frequency of 1 Hz 3 Until recently there has been little work on the fatigue of bonded connections using types of resins and loading which are relevant to civil engineering. Work by Mays and Harvey4 involved 4-point bending fatigue of 'open sandwich' beams in which concrete was cast onto 0.6 mm thick steel plates. The materials were bonded together with a 1 mm thick layer of adhesive; four types of adhesive were used. Some of the beams were waterproofed on all but their top surfaces and subjected to two types of wetting with tap water and one type with salt solution. In all cases rusting developed at the steel surface and the fatigue performances were generally lowered. Cusens and Smith s tested steel-to-steel double lap joints using three types of adhesive and four types of surface treatment: (i) standard clean rough surface; (ii) standard
0143-7496/82/020109-05 $03.00 © 1982 eutterworth & Co (Publishers) Ltd
INT.J.ADHESION AND ADHESIVES APRIL 1982 109
surface blasted with grade 50 sand; (iii)standard surface allowed to rust over 15% of area; and (iv) standard surface dipped in salt solution and rusted over 20% of area. Subsequent tests showed that the static strengths were reduced by between 12 and 47% for the three adhesives and treatments (ii), (iii) and (iv). The present paper describes work to determine the fatigue performance of steel-to-steel lap joints using two resins typical of those used in structural engineering. The tests involve the following conditions: 1. Specimens cured at different temperatures and tested at room temperature; 2. Specimens cured at room temperature and tested at temperatures ranging from --25°C to + 55°C; and 3. Specimens weathered prior to testing.
Test specimens All specimens were of the double-lap type, the majority being constructed from bright mild steel flats, 25.4 x 6.35 mm in cross-section. Those to be subjected to temperature cycling were constructed from 6.35 mm mild steel plate and subsequently cut into strips 25.4 mm wide. The overall length of the specimens was 600 mm with a lapped length of 80 mm. The specimens were constructed with a target uniform glue-line thickness of 0.65 mm. Two grades of two-part, cold cure epoxy resin adhesive were used. The majority of specimens were constructed with Adhesive No I which is a white thixotropic paste mixed with approximately 10% of its own weight of black liquid hardener of low viscosity and understood to be an aliphatic polyamine. A small number of specimens were manufactured with Adhesive No 4 which is a white pigmented epoxy resin paste mixed with 55% of its own weight o f liquid hardener of medium viscosity and understood to be an aromatic polyamine. Steel surfaces were initially cleaned of any rust or millscale and degreased, first with paralTm and then with acetone, prior to blasting with metal grit to a finish resembling Swedish Standard A Sa 2½ 6. The ends of the main laps were covered with PVC tape to ensure that, under test, all load was transmitted in shear. Adhesive was applied to both main and side pieces using a spreader tool and the laps assembled in specially constructed jigs illustrated in Fig. 1. Specimens could be cast in batches of up to 16 to ensure that assembly was completed within 40 minutes of first introducing the hardener to the resin.
Fig. 1
Jig for preparation of specimens
one of two environmental cabinets capable of either cooling the specimens down to - 2 5 ° C or heating it up to + 75°C. In the event the maximum temperature was limited to + 55°C. The temperature of the surface of the steel specimen was monitored by an independent thermocouple probe. The cabinets were capoable of maintaining the surface temperature within 1 C of the designated figure. Details of the fatigue testing rig and specimen arrangement within the low temperature cabinet are shown in Fig. 2, The majority of this work was performed with Adhesive No 1 although a small series was conducted with Adhesive No 4 for comparison. Static tests were carried out at temperatures ranging from - 2 5 ° C to + 75°C on a separate batch of lap joints manufactured with Adhesive No 1. These tests were performed at a rate of loading of 3 kN/min within the same environmental cabinets and on the same testing machine as described for the fatigue tests.
Low curing temperature A supplementaroy programme wa s introduced to investigate the effect of a 0 C curing temperature on the subs~uent
Test procedures Specimens for test were initially cured at 25 -+2°C in a converted deep freeze cabinet. Seven days after casting, specimens were removed from the jigs and static control tests performed on 3 samples from each batch. The remaining specimens were stored at 25"C until ready for fatigue testing. They were held in mechanical tensile wedge grips and cycled, using a hydraulic serve-controlled actuator, under load control with a sine wave form at a frequency of 25 Hz. The frequency of load cycling was selected to be as high as possible and was based on initial trials to ensure that neither dynamic nor temperature effects were introduced. Specimens were tested in tension between a peak load, selected to give failure at endurances up to 100 million cycles, and 10% of that peak, ie the stress ratio (R) was 0.1. Test pieces were enclosed for the duration of the test within
110
INT.J.ADHESION AND ADHESIVES APRIL 1982
Fig. 2 Details of fatigue testing rig and specimen arrangement within low temperature cabinet
fatigue performance of the joint at room temperature. The specimens were inserted, immediately after assembly, into a modified deep freeze cabinet which maintained the + O . . . . . temperature at 0 - 2 C. At penodzc intervals single speczmens were removed for static tests at 0°C. When the static strength had stabilized fatigue testing of the remaining specimens was commenced at room temperatures. The majority of this work was performed with Adhesive No 1 although a small series was conducted withAdhesive No 4 for comparison.
2
'Weathered' specimens Work at Transport and Road Research Laboratories, Crowthome, UK (TRRL) 7 has shown that under normal weathering, corrosion can develop at the surface of steel bonded to concrete. This can reduce the shear strength of the bonded connection and, more important, provide potential initiation sites for fatigue. Work has therefore been started to determine the performance of steel-to-steel joints after weathering. Two forms of accelerated weathering are being used to investigate the effect of micro-cracking in the adhesive layer and rust at the steel/adhesive interface on the subsequent performance of joints made with AdhesiveNo 1. In an attempt to introduce micro-cracking, 300 mm wide lap joints were subjected to 1000 cycles of temperature variation from -- 7°C to + 35°C and return. Each complete cycle lasted 4 hofirs. The wide laps were subsequently cut into strips, the edges being discarded, before storing at 25°C in advance of fatigue testing at a range of temperatures. The steel for a further set of joints was initially allowed to rust over approximately 20% of the surface area. After casting and curing in the normal manner these specimens were stored in a chamber at 20°C and 90% relative humidity. At approximately 3-weekly intervals the joints were exposed for 3 days to the natural environment adjacent to the Tay Estuary. Atmospheric chloride levels were monitored using an Ambler candle housed in a louvred box. The arrangement during exposure is shown in Fig. 3. After a total period of about 18 months after casting these specimens are to be tested in fatigue at a range of temperatures. Results and analysis Fatigue tests at various temperatures The fatigue tests on specimens constructed with Adhesive No 1 and cured at 25°C were carried out in two phases: Series I specimens were obtained from 2 batches having a mean control strength of 22.6 N/mm 2 at 20°C and were subsequently tested at 0°C or - 25°C; Series 2 specimens were obtained from 3 batches having a mean control strength of 12.4 N/mm 2 at 20°C and were subsequently tested at temperatures ranging from room temperature (20°C) to 55°C. Values of log stress range against log endurance are plotted in Fig. 4. With the exception of results at 55°C, it is evident that the fatigue data may be grouped into two bands, 0°C to -- 25°C and room temperature to 45°C. Linear regression analyses were made on the assumption that the relationship between stress range (Or) and endurance (iV) can be described by the expression: (ar)mN = K
as used for welded joints in BS 5400 part 10 s. The exponent m is 25 and the value of K representing the lower limit of performance over the temperature range -- 25°C to + 45°C
Fig. 3
Louvred b o x and arrangement for weathering
10 _ - 2 5 to O°C '
5
•
20 to 45-C
3 -~
Regression lines
E
\v__55°C
U 2 b
//Did not fail O-25°C C] 4 5 ° C • O ~7 5 5 O 20 Z~
I
105
v
35 i'
I
106 107 N u m b e r of cycles, N
108
Fig. 4 Fatigue performance o f joints made with Adhesive No 1 cured at 25°C (log o r vs log N); regression lines are f o r mean - - 2 standard deviation; one specimen tested at 0 ° C failed at 11 990 cycles and one specimen tested at 55 ° C failed at 16 150 cycles
is found to be 8.7 x 102a and the value of Or at 107 cycles is 4.76 N/mm 2. The fatigue performance is considerably weaker at 55°C and the stress range at I07 cycles is about 2.2 N/mm 2. All fatigue failures occurred at the steel/adhesive interface. At temperatures of up to 45°C the mode was 100% adhesion, at 55°C the mode involved 'pull out' from the joint leaving a smear of adhesive on the steel surface. The latter is indicative of time dependent behaviour. The fatigue tests on specimens constructed with Adhesive No 4 were carried out at a temperature o f - 2 5 ° C only. The control strength of this batch at room temperature (20°C) after 7 days curing at 25°C was 8.8 N/mm 2. The results are presented in Table 1. Despite the smaller static strength of this adhesive its fatigue performance at - 2 5 ° C appears to be at least as good as that of Adhesive No 1 at the same temperature. The change in static strength with temperature of a further batch of specimens constructed with Adhesive No 1 and cured at 25°C is shown in Fig. 5. The control strength • of this batch at 20 O C was 16.7 N/ram 2 . At lower temperatures the strength tends to fall, reaching 12.7 N/mm 2 at -- 25°C, and failures tend to occur at the adhesive/steel interface. At higher temperatures the strength increases rapidly to a peak at approximately 30°C with failure occurring largely within the adhesive layer. Thereafter strength falls reasonably linearly regaining its room temperature value at 45°C and reaching 8.0 N/mm 2 at 55°C. By 75°C it has further reduced to 2.4 N/mm 2, the fracture
INT.J.ADHESlON AND ADHESIVES APRIL 1982
111
Table 1.
Fatigue performance of Adhesive No 4* at -- 25°C
Stress range (N/ram 2) 9.1 8.3 7.5 7.5 6.8 4.4
Cycles to failure (x 103)
(or)mN =
30
2O
E
<>
v
K
can be used to describe the relatioaship between stress range and endurance. Fatigue strengths are reduced due to the lower curing temperature but less severely than the static strength. For example, with Adhesive No 1 the reduction was about 25%. An investigation of the effect of elevated curing temperatures up to 80°C on the static strength of similar joints constructed with these adhesives has previously been conducted9. The results for both steel and aluminium laps are given in Table 2. They show that with Adhesive No 1 there is a tendency for strength to improve with increasing cure temperature but with Adhesive No 4 there is a small decrease after curing at 80°C. Fatigue tests have been carried out at endurances up to 1.5 x 106 cycles on steel laps cured at 23°C and 80°C 4. With AdhesiveNo 1 fatigue performance is improved by the higher curing temperature but with Adhesive No 4 there is little effect, The fatigue performance of Adhesive No 4 is found to be generally superior to that of Adhesive No 1 despite its lower strength.
15.4 116.7 364.6 4 488.8 > 2 0 000.0 > 20 000.0
*Cured at 25°C
Z
are given in Fig. 6 for both adhesives. Again, an expression of the form:
<>
<
~10-
Comparison with US data related to airframes
Surprisingly little fatigue testing has been done in the acreframe industry on laboratory specimens but effort has been concentrated on component testing. Fatigue is considered to be less of a problem than stability. Consequently a lot of 0
-25
-15
-5
5
15 25 35 Teml0er~ture (*C)
45
55
65
75
Fig. 5 Effect o f temperature on static strength o f joints made with Adhesive No 1
surface of the adhesive exhibiting considerable plastic deformation. There is no apparent correlation between static strength and fatigue performance in the range --25°C to 45"C. However, the slightly superior performance in the band 0°C to - 25"C may be wholly or partly due to the greater static strength of the Series I specimens. The fatigue performance is temperature dependent and it therefore follows that a fatigue assessment must take into account not only the design spectrum of loads but also the variation in fatigue strength with temperature and the times at different temperatures. For example, the temperatures of steel deck plates in highway bridges can exceed 550C for about 10 hours per annum. This means that over 120 years about 0.8 x 106 cycles of stress may be caused by the axles of commercial vehicles when the temperature exceeds 550C.
Table 2.
Curing temperature (°C)
Adhesive No 1
Adhesive No 4
Steel/steel AI/AI (N/mm 2) (N/mm 2)
Steel/steel AI/AI (N/mm2) (N/mm2)
23 40 60
16.8 10.0 13.0
7.5 7.5 9.9
113 12.1 12.3
4.2 3.9 4,3
80
16.7
13.1
11.5
3.4
*Cured at up to 80°C Results from Reference 9
~ 5 o
I~
z
112
INT.J.ADHESION AND ADHESIVES APRIL 1982
4 cure o'c test 20"¢o/
Regroups~ n line
v
The static strengths at 20°C of specimen batches constructed with Adhesives No 1 and No 4 and cured at 0°C for a period of 1 week were 8.7 and 3.5 N/ram2, respectively. After a further period of 7 weeks at 0*C strengths had risen to 10.8 and 5.3 N/mm 2, respectively. Thus the effect of curing the resins at 0*C is to lower the static strength by 50 to 60% and to prolong the required curing period when compared with a 25"C curing temperature. The fracture surface of Adhesive No 4 exhibited a regular, layered pattern with cracks running transverse to the length of the lap, presumably caused by thermal contraction during o curing. Results of fatigue tests on specimens cured at 0 C
'C ~est - 25 to O°C
6
r,
Fatigue tests after curing at different temperatures
Static strength of adhesives*
t.
2 .~Dicl not ~ i l o Adl%,sive 1 o Adhesive 4
I 110s
I
10 e 10 ~ Number of cycles. N
10e
Fig. 6 Fatigue performance o f j o i n t s made with Adhesive No 1 and No 4, cured at 0=C (log o r vs log N) (upper line taken f r o m Fig. 4); regression line f o r mean -- 2 standard ~ a t i o n : one Sl~ecimen joined with Adhesive No 1 failed at 1480 cycles end t w o joined with Adhesive No 4 failed at "low" cycles ( 1 0 1 8 0 and 1 2 6 8 0 )
work has been carried out to determine long term behaviour under static stresses and different environments. Service experience with metal-to-metal joints is very variable, some perform well, others require a lot of maintenance. Fatigue tests were conducted by Marceau et al 1 on axial shear specimens of 2024 T3 aluminium alloy. The adhesive was 0.25 mm thick woven scrim and the joint was cured at 121°C for 1-5 hours under a pressure of 0.34 N/mm 2. Mean static strengths at room temperature and mean fatigue strengths at 24°C and 60°C are compared with the results of the present study in Table 3. Also shown is the effect of a lower frequency on fatigue performance at 60°C. The performances of the more sophisticated aluminium-epoxyscrim-aluminium joints are markedly superior to the steelepoxy-steel joints tested in this investigation. The different performances are almost certainly due to the adhesive. The dramatic reduction in fatigue performance reported at 60°C as a result of reducing the frequency from 30 to 0.0002 Hz suggests that special attention needs to be paid to assessing performance at elevated temperatures over a range of frequencies. From the aerospace experience it is also clear that attention should be given to performance under load in different environments.
hand to study the effect of ageing, through accelerated temperature cycling or natural exposure of pre-rusted joints, on fatigue performance. Aluminium-epoxy-scrirn-aluminiumjoints as used in the aircraft industry have a markedly superior fatigue performance to the steel--epoxy-steel joints representative of civil engineering applications. The effect of loading rate on fatigue performance at elevated temperatures requires special attention. Acknowledgements
The work described in this investigation was carried out within the Wolfson Bridge Research Unit, University of Dundee, under contract to the Transport and Road Research Laboratory. However, the views expressed are those of the authors and are not necessarily those of the Department of the Environment, or of the Department of Transport, or any other Government Department. Crown copyright is reserved on this paper © CROWN COPYRIGHT. Reproduced by permission of the Director TRRL, 1982. References
Conclusions
The fatigue performance of epoxy-resin bonded steel lapjoints is temperature dependent. For the resin used in this study there is little change in performance over the temperature range - 2 5 ° C to 45°C but it is considerably weaker at 55°C. For the range - 2 5 ° C to 45°C, the lower bound can be given by the expression: or2SN = 8.7 × 1023 Any fatigue assessment must take into account the design spectrum of loads, the variation in fatigue strength with temperature, and the times at different temperatures. For optimum performance of bonded joints, curing temperatures as low as 0°C should be avoided. Further work is in
1
Marceau, J.A., McMillan, J.C. and Scardino, W.M. 'Cyclic stress testing of adhesive bonds', Adhesives Age (April 1975) pp 37-91
2
Emerson,M. 'Extreme values of bridge temperatures for design purposes', TRRL Report LR 744 (Department of the Environment, Department of Transport, Crowthorne, 1976)
3
Tilly, G.P. 'Fatigue problems in highway bridges', Transportation Research Record No 664 Washington (1979) pp 93-101
4
Mays, G.C. and Harvey, W.J. 'Fatigue performance of adhesive bonded joints for bridge deck construction', IABSE Colloquium, "Fatigue of steel and concrete structures," Lausanne, March 1982
5
Cusens,A.R. and Smith, D.W. 'A study of epoxy resin joints in shear'. The Structural Engineer 58A No 1 (1980) pp 13-18
6
'Pictorial surface preparation standards for painting steel surfaces', Svensk Standard SIS 055900 - 1967 (Swedish Standards Institute, Stockholm, Sweden)
7
Calder, A.J.J. 'Exposure tests on externally reinforced concrete beams - first two years', TRRL Supplementary Report SR 529 (Department of the Environment, Department of Transport, Crowthorne, 1979)
8
'Steelconcrete and composite bridges', BS 5400 (British Standards Institute, London, 1978)
9
Ching, G. 'Durability of epoxy adhesive joints in concrete, steel and aluminium', MSc Thesis (University of Dundee, April 1979)
Table 3. Comparison with fatigue performance of aluminium joints
Joint type
AI/epoxy-scrim/Al* Steel/epoxy/steel
Mean static strength at room temperature
39.5
Mean stress range to cause failure
Temp (°C)
Or
A t 107 cycles at 30 Hz
24
15
60
11.7
60
3.8
A t 3000 cycles at 0.0002 H z
* Results from Reference 1
22.6 (Series 1) 12.4 (Series 2) 16.7 (Series 3)
(N/ram 2)
Temp (°C)
Or (N/mm 2)
-- 25 4.8 to q- 45 55 2.2 -
-
Authors
Mr Mays is with the Department of Civil Engineering, The University, Dundee, and Dr Tilly, to whom inquiries should be directed in the first instance, is with The Bridge Division, Transport and Road Research Laboratory, (Department of the Environment, Department of Transport), Old Wokingham Road, Crowthome, Berks, RG11 6AU, England.
INT.J.ADHESION A N D A D H E S I V E S A P R I L 1982
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