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Online ISSN : 1347-5320
Print ISSN : 1345-9678
ISSN-L : 1345-9678
Mechanics of Materials
Comparative Analysis of Static and Fatigue Strength of Carbon Fiber and Al 6061-T6 Double Strap Joint
Shahid Iqbal, Adnan Tariq, Wajid Ali Khan, Waseem Shahzad, Muhammad Azeem, Waqas Javid, Haider Ali, Muhammad Yasir, Muhammad Shakeel
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キーワード: double strap joint, mechanical characterization, epoxy YD-128, thermoplastic polyurethane, temperature effect
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2022 年 63 巻 8 号 p. 1120-1126

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Abstract

An experimental investigation of static and fatigue strength of double strap joints, using thermosetting and thermoplastic adhesives, between aluminum alloy 6061-T6 and carbon fiber metal laminate (CFML) has been carried out during this research. The static properties of joints, using thermosetting adhesives, were also determined while varying the temperature values. Two types of specimens were prepared: one, using YD-128 epoxy and the other using thermoplastic polyurethane by applying hand layup method. Results of both tensile and fatigue testing indicated better strength for thermosetting double strap joint both in elongation and fatigue cycles. For experimental validation of joints, finite element modeling has also been used during this research. Furthermore, the thermosetting double strap joint with maximum shear strength has been tested under a series of high temperatures. Temperature dependence of thermosetting epoxy adhesive indicated an increase in tensile strength of joint below the service temperature. However, a 50% drop in joint’s shear strength was observed when temperature was raised from 25°C to 60°C. Similarly, an increase in tensile strength of about 8.11% was observed at temperatures between −20°C to 25°C. The mode of failure for all combinations of thermosetting joints is mixed mode and cohesive failure whereas adhesive failure was observed for thermoplastics joints.

1. Introduction

Composite materials are being used increasingly for a variety of applications such as aviation, aerospace, and automobiles. The importance of composite materials has especially been recognized in aviation and automotive industry due to simplified design and better specific properties.1,2) Composites are replacing traditional metals (Aluminum, Steel alloys) due to their strength to weight ratio, fatigue performance, corrosion resistance, and high specific properties, which are pre-requisites in engineering structures.3–5) Adhesive bonding is an attractive alternative to mechanical fastening and has therefore been having wider applications. For the last few decades, Adhesive bonding has become very popular because it offers many advantages over traditional mechanical fastening techniques. One of the main advantages of using adhesives compared to conventional bonding techniques is higher fatigue resistance and longer service life.6) However, the big challenge in modern-day composites to be used in combination with metals is joining through various adhesive bondings. The idea needs investigation into hundreds of joint configurations, bonding types, and mechanical properties. The introduction of adhesive joints has reduced the aviation structures’ weight by 10–25% with increase in fatigue strength by up to 15%.7) Adhesive bonding is the best replacement of mechanical fastening (riveting, bolting and welding) but presents a challenge as the joint strength is dependent on the matrix component and not reinforcement itself. Therefore, better understanding of the mechanical behavior of adhesively bonded joints, requires a detailed analysis at the interface of the adhesively bonded joints.8–10)

This research experimentally investigates the variation in mechanical properties at different temperatures and recommendations are made to improve these properties accordingly. This type of joint configuration is most common in aviation structures where the metallic ribs and longerons are needed to join with wing and fuselage skins without adding unnecessary mass and without any compromise on joint strengths.

Fiber Metal Laminates (FML) are gaining increasing importance in the field of aviation, where adhesive bonding joints are interlinked with metallic parts.7,11) Budhe et al.5) and Khawaja et al.12) noted that hybrid joints have replaced mechanical joints and simple adhesive joints in the automotive and transportation industry due to higher strength to stiffness ratio resulting in better fuel efficiency. The use of these materials is increasing in daily use and are thus replacing mechanical fastening of metallic joints. Ghoddous13) characterized single lap strap joint and stiffened adhesive joint and investigates the stress distribution using finite element model. Chen et al.14) investigated the failure characteristics of single lap joint and noted that several analytical and mathematical models have been developed to predict the mechanical properties of such joints, but the variability of fatigue strength still needs extensive investigations for various adhesives, metals and composite combinations. Goudarzi and Khedmati15) characterized single lap and double butt lap joints of aluminum (5083-H32) and glass fiber reinforced composites with epoxy adhesive and investigated the load-bearing capacity and failure mechanism of the joint. Mattos et al.16) evaluated the tensile strength of single lap joint of ceramics composites and epoxy for oil and transportation industry.

Bernasconi et al.17) used finite element methods to study two types of samples for fatigue cracks’ growth, one on a flat structural material single lap joint and the other a double cantilever beam to conclude that crack propagation in such joints occurs in an adhesive layer having robust alignments of cracks. They also observed that trails on the single lap joint had relatively better accuracy concerning the double cantilever beam.

Meneghetti et al.18) investigated carbon-epoxy overlays in the single lap joint and concluded that edge geometry, length, and bond type play a key role in determining the strength of such joints using optical and electron microscopy to understand the common failure phenomenon. Researchers19–22) investigated an adhesive joint between aluminum alloy-6061 metal to another metal using epoxy and polyurethane and pointed out that joint strength relied upon adhesive lengths.

The effect of elevated temperature on steel double strap joint for three different types of thermosetting epoxy resins and showed that maximum shear strength of joint decreased near the glass transition temperature (Tg) range.23–25)

Nguyen et al.26) reported the mechanical performance of adhesively bonded CFRP/Steel double strap joints at temperatures of 20°C and 60°C around the glass transition temperature Tg (42°C) of the adhesive. They concluded that the displacement of adhesively bonded steel/CFRP double strap joints increased with temperature whereas the joint stiffness decreased gradually. At elevated temperature the mode of failure of double strap joint was found to be cohesive.

It is evident from the literature that there is plenty of research available to characterize the adhesively bonded joints computationally and experimentally under different operating and processing conditions. However, there is limited literature available on the static and fatigue strength of double strap joint of carbon fiber metal laminates (CFML) and Aluminum Alloy Al 6061-T6 using YD-128 and TPU. Therefore, this research is an effort to characterize double strap joint with thermoset (YD-128) and thermoplastic (TPU) adhesives under different environmental conditions. The effect of these adhesives on the static and fatigue strength of double strap joints is experimentally investigated and compared with simulated results.

2. Materials and Method

Aluminium alloy 6061-T6 and Woven Carbon Fabric TC-33(3K) were purchased from Farmosa Plastic Group Pvt Ltd. and used as constituent materials for double strap joint. Epoxy YD-128 manufactured by Kudko Limited and Thermoplastic Polyurethane (TPU) & Tetrahydrofuran mixture were used as adhesives. Mechanical properties of adherents and adhesives are presented in Table 1.27,28) The flexural strength of YD-128 with DMA hardener is 99.2 MPa and adhesive strength is 6.4 MPa. The tensile strength of TPU is 42 Mpa.29)

Table 1 Mechanical Properties of Different Materials.27,28)

2.1 Test specimen preparation

Test specimens were prepared and tested for their tensile behavior using ASTM standard D 3528-96.30) The stepwise procedure is presented in Fig. 1. Aluminum sheets were used as the base material which was chemically treated using acetone to remove any dirt or impurities present on the surface and degreased by using 180–2000 grit size sandpaper, followed by acetone washing. The sheets were then anodized before joint preparation. Aerospace grade woven carbon fibers TC-33(3K) was cut into required length of 63 mm according to standard.30)

Fig. 1

Various Phases of Sample Preparation.

Two different adhesives were used in this research that included thermoplastic (polyurethane with tetrahydrofuran) and thermosetting adhesive (Epoxy YD-128) used in pristine form. The laminated joints were prepared using hand layup method and curing was carried out under 100 KN compressive load for 48 hours to squeeze out any excessive resin and achieve uniform distribution of the matrix material at room temperature thus resulting in uniform fiber volume fraction in the laminate. Figure 2 shows the sample preparation procedure.

Fig. 2

Various Phases of Sample Preparation using Hand Layup Method.

Eighty (80) samples were machined out by milling machine using carbide cutters from the fabricated sheets as per ASTM dimensions of the samples for testing static and fatigue properties of double lap strap joint of thermosets and thermoplastic adhesives with different reinforcements. The final machined specimen is shown in Fig. 3(a) and 3(b).

Fig. 3

(a) Final Machined out Specimen from the Bonded Sheets. (b) Configuration of Double Strap Joint.

3. Test Procedure

3.1 Tensile and fatigue testing

Tensile testing was carried out by using universal testing Machine (Model: DBSL-SJ-10t) with load capacity of 100 KN whereas, fatigue testing was carried out by using Zwick Fatigue Tester (Model: EQ-25). Tensile testing was performed according to standard30) with the load capacity of 100 KN on universal testing machine for finding the shear strength of adhesively bonded double strap joints. Tests were conducted at crosshead rate of 1.27 mm/min. Twenty (20) specimens, of each material, were tested. The universal testing machine and Zwick fatigue tester with specimen are shown in Fig. 4 and Fig. 5 respectively.

Fig. 4

Universal Testing Machine with Specimen.

Fig. 5

Fatigue Tester with Specimen.

Fatigue testing was performed according to standard ASTM D 3166-9931) at room temperature using Zwick Fatigue tester. The behavior of specimens was observed under the influence of low and high cycle tensile loading using fluctuating stress amplitude. A total of 40 specimens with different compositions were tested. The loading frequency was set constant at a rate of 10 Hz for each type of specimen. The cyclic load range was defined by maximum load and 10% of maximum obtained from tensile testing. The fatigue test was performed at 50%, 70% and 80% static strength of the joint for each combination of the joints. The S-N curve, of each specimen, showed the number of cycles at which specimen failed.

3.2 Simulation

3.2.1 Geometric model and mechanical properties

A double lap strap joint bonded with two different adhesives was studied computationally in this part of the research work. Carbon fibers were bonded with the Aluminum lap joint through two different adhesives. The mechanical properties used for the simulation are listed in Table 2. And also, the flexural strength of YD-128 adhesive is 6.4 MPa.

Table 2 Mechanical Properties of Aluminum 6061-T6, Woven Carbon fiber TC-33k, TPU and Epoxy YD-128 with DMA hardener.

The analysis was carried out using COMSOL Multiphysics. The classic boundary conditions were applied in this double lap joint model. The adhesives were modelled in between the lap joint of two Aluminum plates and at the interface of carbon fiber and Aluminum plates.

3.2.2 Meshing and initial boundary conditions

Two different mesh sizes were used in this analysis. For adhesive bonded joints and carbon fibers at the adhesive-adherent interface, small element size was used, whereas for aluminum plates coarse size elements were used as shown in Fig. 6. It is, generally, essential to model the adhesive layer through mesh elements that are smaller than the adhesive layer thickness so that a smooth transition is effective between the adhesive and adherent layers. Most of the elements were free tetrahedral mesh elements for both adherent and adhesives layers.

Fig. 6

FE Model of the Double Strap Joint.

For tensile testing, classical boundary conditions were used. The mesh independence test was performed on adhesive interface and maximum recorded error was less than 0.1%. The boundary conditions selected here were studied from the research of Banea et al.32) One side of the sample was constrained with fixed support while on the other side a forced constraint was applied. The stress analysis for double lap joint was performed using Von Mises yield criterion to check the stress distributions in the adhesive and adherent layers. The adhesive layer was assumed to be homogenous and isotropic. Both the adhesive and adherent layers were modelled in such a way that there existed some common nodes that accounted for continuity of stress and deformation.

4. Result and Discussion

The tensile test result was investigated while keeping in view the relation of failure load and extension produced. Fatigue results are reported in the form of number of cycles at which joint failed. Fractography results show the mode of joint failure. It can be seen in Figs. 7 and 8, that there is an almost linear relationship between the applied load and corresponding extensions both for Epoxy YD-128 and Thermoplastic TPU based adhesives, therefore, the model can be considered accurate and is able to give a good prediction for the failure load of the specimens. It is also in agreement with the results of Refs. 33–38). From the experimental results, the CFML with YD-128 failed as the load reaches 13.48 KN and 13.53 KN for the simulated results. However, CFML with TPU failed at 4.32 KN as calculated experimentally and 4.25 KN for simulated results (Table 3). The experimental results were verified through simulations as shown in Fig. 9 and 10. Both of the results show a similar trend. Colors in Fig. 9 and 10 show the Von Mises stresses present between Al 6061-T6 and carbon fiber metal laminates (CFML) of adhesively bonded double strap joints.

Fig. 7

Load-Extension Relationship of Joint with Thermoset YD-128 and Thermoplastic TPU.

Fig. 8

Load-Extension Relationship of Experimental and Simulation Results with Thermoset Epoxy YD-128 and Thermoplastic TPU.

Table 3 Tensile Test Comparison among experimental and FEA results.
Fig. 9

Simulation Results Highlighting the Failure Zone of Epoxy YD-128 Joint.

Fig. 10

Simulation Results Highlighting the Failure Zone of Thermoplastic TPU Joint.

Furthermore, the failure zone for both the studies is also the same and, in both cases, the specimen fails due to epoxy failure.

Fatigue test was performed to calculate the life span and specimen’s behavior under cyclic loading. Figure 11 represents the number of failure cycles for double strap joint with Epoxy YD-128 and Thermoplastic TPU.

Fig. 11

Comparison of Number of Cycles of CFML with YD-128 and TPU.

For fatigue testing a relation between average number of failure cycles and average shear stress of CFML with both thermosetting and thermoplastic is presented in Table 4.

Table 4 Experimental Result of Fatigue Testing.

4.1 Temperature effect

Epoxy YD-128 double strap joint was investigated for variation in temperatures ranging from 25°C to 60°C. Tensile strength indicates a linear correlation with increase in-service temperature. Weakening of joint at elevated temperatures is associated with glass transition temperature (Tg) of epoxy bonds which at specifically high temperatures results into reduction in material strength. The Tg for epoxy YD-128 bond is 57°C. The useful range for service temperatures for joint therefore lies in between −20°C to 55°C. The higher temperature ranges, however, have been witnessed to have a reduction in joint strength of more than 50%. The rise in service temperature from 25°C to 40°C produced a reduction in joint strength of 18.4%. Similarly, below freezing temperatures, an increasing trend in the joint’s tensile strength has been observed. For example, a decrease in temperature between −20°C to 25°C produced an increase in strength of about 8.11%. Figure 12 indicates the increasing trend of tensile strength with a reduction in service temperature to −20°C. The decreasing trend in tensile strength of the joint is also shown for elevated service temperatures i.e. from 25°C to 60°C. This has been found in agreement with the previously reported results of Nguyen et al.26)

Fig. 12

Experimental Results of Load vs. Extension at Temperatures above and below than Ambient Temperature.

4.2 Mode of failure of joint

The mode of failure for each combination of thermosetting adhesive joint was cohesive and mixed mode failure as clear detachments of metal strips from bonded sections could be observed (Fig. 13). It indicates that surface preparation technique applied to adherend surfaces was according to the standard and the curing period was proper. While in some areas CFML and adhesive delamination were present due to weak adhesion between aluminum and carbon/epoxy laminates. For the thermoplastic’s adhesive joints, the mode of failure was observed to be adhesive failure. Fractography using optical microscope revealed that air bubbles were trapped within thermoplastic adhesive during fabrication. The presence of these air bubbles decreased the bond strength. Fractography image in Fig. 13 shows encircled areas which confirm the presence of air bubbles in the joints.

Fig. 13

Mix Mode Failure of FML’s after Mechanical Testing.

5. Conclusions

This research has presented a comparison among the tensile and fatigue properties of thermoset and thermoplastic adhesives at room and other service temperatures. From the experimental results, it is concluded that:

  1. (1)    Carbon Fiber joints with epoxy YD-128 showed better tensile and fatigue strength than the thermoplastic TPU adhesive.
  2. (2)    The nature of failure of joints in all cases of epoxy YD-128 joint is observed to be cohesive in nature due to clear detachments of carbon fiber on metal strips from bonded sections, which indicates that surface preparation technique applied to adherend surfaces was sufficient and curing periods were proper.
  3. (3)    Thermoplastic adhesive bonded joint did not produce much of a difference in its mechanical properties in carbon fiber as laminate. Curing of thermosetting adhesive requires specific conditions and temperature and if ensured can result in better fatigue life of bonded joints with carbon fiber as laminating material.
  4. (4)    Epoxy YD-128 joint under a wide range of service temperatures is found to be of variable tensile strengths. The elevation in temperature between 25°C to 60°C witnessed a reduction in joint strength. An overall reduction of 59% in joint strength was observed at elevated temperatures.
  5. (5)    The useful working range of service temperatures, for joints, lies between −20°C to 25°C and resulting in an increase of 8.11% in tensile strength of the joint.

Acknowledgments

The authors are thankful to Mechanical Engineering Department, Wah Engineering College, University of Wah for their kind support.

REFERENCES
 
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