Focus on adhesives: Strengthening the bond with reliable structural adhesives

When identifying solutions that can offer assurance and longevity, the maintenance and reliability of assets repairs can be challenging. In this white paper, Ian Wade, Technical Services Manager, Belzona Polymerics Ltd, will look into the use of structural adhesives as the first-choice solution

Structural adhesives can be used for affixing metal substrates or components as they provide high modulus and high strength. However, they are not currently internationally recognised like the traditional methods, even though adhesives are already used in a wide range of industries, such as aerospace, rail and construction.

Traditional practice such as welding, riveting, nuts and bolts and mechanical fixing are perceived as the go-to method. However, they all have their inherent inadequacies. Welding can be hazardous to health; riveting, nuts and bolts concentrate the stress locally; while mechanical fasteners can concentrate stress.

This paper introduces a novel, two-component, solvent-free toughened epoxy adhesive material that provides high adhesion to metallic substrates while also being able to withstand high movement or cyclic fatigue in comparison to general epoxy materials.

As well as potential application areas, the article also discusses a number of benefits, including ease of use, load bearing and impact resistance properties.

Introduction

Most industrial maintenance or repair procedures can either involve welding or the use of mechanical fasteners as these can be perceived as easy and quicker, however, these procedures might initially seem to correct the issue but may cause more harm than good. Depending on the repair situation for instance, welding or drilling to connect mechanical fasteners on a storage tank containing flammable liquid is not recommended for obvious reasons. This is where a structural adhesive can really offer a solution for that maintenance repair.

There are many structural fixings used across a whole range of industries that may be part of any maintenance or repair. These include support brackets such as cable trays, antennas, heating coils filter pans or any other internal fixtures in vessels that suffer from corrosion, impact or vibration damage. Within construction there are fire water deluge systems, nozzles connections, facades panels and signs which can require maintenance overtime or adverse weather conditions.

Process equipment or piping can suffer from thinning or the steel or even through wall defects which will need either monitoring or repairing depending on whether the integrity of the equipment has been lost.

Structural fittings are generally for fixing of static members but may be subjected to forces unbeknown at time of installation. This could include thermal cycling of the joints, cyclic loading or vibration due to fatigue of a component.

If there are repairs due to the above, the contractor may be in a situation where a choice of solution can be made, then the strengths and weaknesses need to be identified.

Welding is regularly used for repairs as it is widely available while also being well regulated with high customer confidence and high strength of the repair. It does come with inherent risks in both the use, the material by heat stressing and the user, as welding can cause both acute and chronic health risks.[i] Application of welding repairs onto live piping sections, storage tanks or process systems and equipment should not be undertaken due to the high temperatures involved and, not forgetting, the combustible nature of the process fluid or gas running through or being stored in these components.

Bolted joints are seen as simple and low cost due to the ease of disassembly and reassembly. These can be dissimilar metals, but the use of dissimilar metals will contribute to galvanic corrosion and add weight to the joint, requiring routine inspection and tensioning, while the drilled holes in the support material will mean stress distribution is not uniform and is concentrated at the holes.

Structural adhesives have high bond strength whilst being lightweight. Adhesive is applied to cover the entire joint, resulting in uniform stress distribution and reducing metal distortion under strain.

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Importance of a strong bond

Adhesive bonding is the joining of similar or dissimilar members together while creating permanent high strength bonds, which can transfer structural stress without loss of structural integrity.

Regardless of the joint type used, it is important to understand the different stresses that are imparted onto a bonded assembly. Adhesives perform the best when the stress is two-dimensional to the adhesive, allowing the force to be applied over the entire bond area.

Joints that are well designed for adhesives place most of the stress into compression or shear modes. Adhesives perform the worst when stress is one-dimensional to the adhesive, concentrating the load onto the leading edge of the bond line. Joints placing stress into cleavage or peel concentrate the stress onto the leading edge, which may lead to premature bond failures, especially if subjected to vibration, impact or fatigue

Bonds of high strength are obtained after cleaning of the substrate by removal of any contaminants, followed by the roughening of the substrate generally in the form of grit blasting to international recognised standards[ii]. This is why surface preparation is critical to success, regardless of what type of adhesive is used.

There are three types of bonding that are important to achieve to ensure good adhesion. These are: adhesive, chemical and mechanical.

Adhesive relies on surface energy to generate adhesion to the substrate, while chemical relies on chemical bond formation and electronic bonding to produce adhesion. Mechanical adhesion is due to the creation of an irregular profile that allows a deeper profile to be produced.

The types of structural adhesives available have been summarised in Table 1.

Table 1 – Types of structural adhesives

There are two types of failure mechanisms associated with structural adhesives:

1. Cohesive failure occurs in the bulk layer of the adhesive material. This failure mode is limited by the strength of the adhesive material and can be caused by insufficient curing of the adhesive and applications at a greater thickness than that recommended, among others.
2. Adhesive failure occurs when the mechanical adhesion between the adhesive and the parts being joined is overcome by the loading. This failure mode is associated with inadequate surface preparation, presence of contaminants, or insufficient curing of the adhesive, among others.

Background

Design considerations for Belzona 7311 were based on both technical target requirements and a practicality approach, as summarised in Table 2.

Table 2 – Design considerations

Belzona 7311 was subjected to at least the following tests and evaluation protocols in to ensure that it met the design criteria previously discussed. Where possible, internationally recognised standards were used:

1. Cleavage Adhesion – ASTM D1062[iii]
2. Tensile Shear Adhesion – ASTM – D1002[iv]
3. Tensile Fatigue Resistance – ISO 9664[v]
4. Impact Resistance – ASTM D256[vi]

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Experimental procedure

1. Cleavage Adhesion – ASTM D1062

Cleavage adhesion is used to assess the strength of an adhesive bond between two substrates when exposed to cleavage stress.

Belzona 7311 was applied between two identical grit blasted metallic cleavage test pieces to create a fixed bond area of 125mm² of minimal bondline thickness.

The specimen was allowed to cure then attached to a 25kN tensometer using suitable grips. The tensometer then applies a load at a fixed rate of 1.3mm/min exerting a cleavage force on the specimen until bond failure. This test is repeated five times so an average force can be calculated.

Figure 1 – Cleavage adhesion test

2. Tensile Shear Adhesion – ASTM – D1002

Tensile Shear Adhesion or lap shear adhesion is used to determine the adhesive strength of a material when bonded between two ridged metallic substrates.

Samples are 100 x 25.4 x 2mm and are overlapped lengthwise by approximately 12.7mm and bonded to a minimal bondline thickness with Belzona 7311.

The specimen was allowed to cure then attached to a 25kN tensometer using suitable grips. The tensometer then applies a load at a fixed rate of 1.3mm/min exerting a cleavage force on the specimen until bond failure.

Figure 2- Tensile force

3. Fatigue Resistance – ISO 9664

Fatigue resistance is the highest stress that a material can withstand for a given number of cycles without breaking.

A standard static Tensile shear adhesion test was conducted to determine the mean breaking stress – 24.17 MPa following this 35% of the mean breaking stress value is used as the mean stress in fatigue testing – 35% mean shear stress = 8.461 MPa (24.17 MPa x 35%)

Figure 3- Tensile shear adhesion test

At four different alternating stresses, fatigue testing was conducted at 30Hz until failure:

1. 80% = 6.8 MPa (8. 461 MPa x 80%) Stress amplitude cycles between
2. 60% = 5.1 MPa (8. 461 MPa x 60%) Stress amplitude cycles between
3. 5% = 4.9 MPa (8. 461 MPa x 57.5%) Stress amplitude cycles between
4. 55% = 4.7 MPa (8.461 MPa x 55%) Stress amplitude cycles between

4. Impact Resistance – ASTM D256

Impact tests can be used to assess the toughness of a material, a material’s toughness is a factor of its ability to absorb energy during plastic deformation. Brittle adhesives have low toughness as a result of the small amount of plastic deformation that they can endure. Tougher materials on the other hand can absorb greater energy during fracture and thus, have improved impact resistance.

The Izod impact test allows for samples to be tested in two forms: either ‘notched’ or ‘un-notched’. In our case, the testing will be notched which has a V-shaped notch of approx. 2.5mm in depth with a total defect angle of 45° in the centre of a specimen sample with dimensions of 12.7 x12.7 x 65mm. The notch concentrates stress and allows measurement of crack propagation.

Non-standard testing:

5. 3-Point Load Test

This comparative technique is used to assess the relative flexibility of adhesives when applied to a metallic substrate. In this test a mild steel panel of dissimilar dimensions

Plate 1 550 x 50 x 10 mm thick

Plate 2 225 x 50 x 10 mm thick

are stressed to the point the adhesive fails. The panel is held in position at two points, one at either end of the sample and is gradually stressed at a single point in the centre of the specimen via a hydraulic press, as seen in Figure 3. The greater the displacement i.e., the further the press travels until failure the more flexible the adhesive. The thickness of the adhesive will influence the degree of flexibility so analysis should be duplicated for repeatability purposes. In the case of this testing, at the manufacturing stage the specimens were compressed by hand pressure only, to try and replicate ‘in field’ applications of achieving below the maximum bondline thickness of 2mm.

Figure 5 – 3-Point load testing

Testing results and discussion

1 – Cleavage Adhesion – ASTM D1062

2 – Tensile Shear Adhesion – ASTM – D1002

3 – Tensile Fatigue Resistance – ISO 9664

Figure 6 – Belzona 7311 SN curve

From a mean breaking stress of 35% (8.461 Mpa), Belzona 7311 will survive 10⁶ cycles at 56.6%, with an alternating stress amplitude of ± 4.791 Mpa = 13mpa to 3.67 Mpa

4 – Impact Resistance – ASTM D256

Table 6 – Impact Resistance Results – ASTM D256

5 – 3-Point Load Test

Table 7 – 3-Point Load Test Results

Figure 7 – 3-point load Specimen Testing

Conclusions

Several conclusions can be drawn from the use of Belzona 7311 as a solution for the repair or maintenance of assets:

1. Belzona 7311 offers high resistance to structures that are subjected to forces such as peel, cleavage, vibration or cyclic loading. These include, but are not limited to, support brackets for fire deluge systems, internal and external fixtures on process equipment, wear pads and wind girders on storage tanks.
2. As Belzona 7311 offers an array of additional practical features including ease of application, ability to hold its own structure when placed in vertical applications and superior adhesion to metallic substrates, the toughened epoxy can be used on structural support reinforcements, load transfer supports and metallic staircases and ladders.
3. Plate bonding to repair thinning or through wall defects on areas such as pipe/piping, process equipment, storage tank floating roofs and platform decks can utilise Belzona 7311, as it offers high impact resistance and flexural properties.
4. Belzona 7311 has been optimised for metal-to-metal adhesion and exhibits an extensive data list with more than 20 tests solely based on adhesion. The performance data can be used for Finite Element Analysis (FEA) or simulations to aid in bond designing or qualification of the adhesive in areas that would normally be seen as high risk for standard epoxies such as handrails and walkways.

Figure 8 – Structural adhesive, Belzona 7311, used to bond bracket

References

[i] https://www.hse.gov.uk/welding/health-risks-welding.htm

[ii] ISO 17212:2012 Structural adhesives — Guidelines for the surface preparation of metals and plastics prior to adhesive bonding

[iii] ASTM D1062-08(2015) Standard Test Method for Cleavage Strength of Metal-to-Metal Adhesive Bonds

[iv] ASTM D1002-10(2019) Standard Test Method for Apparent Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-Metal)

[v] ISO 9664:1993 Adhesives — Test methods for fatigue properties of structural adhesives in tensile shear

[vi] ASTM D256-10(2018) Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics

Ian Wade, Technical Services Manager, Belzona Polymerics Ltd. Ian Wade is a AMPP CIP L2 Qualified Technical Services Manager for Belzona Polymerics Ltd based in Harrogate UK. Ian has been with the company since 2010 and is currently studying towards Bachelor of Engineering (Honours) at the Open University. To compliment his vast experience in the engineered composite repair sector, Ian is also a participating member with the British Standards Institute (BSI) who is responsible for the UK input to ISO/TC 67/SC 6 – Processing equipment and systems for petroleum and natural gas industries.