Strengthening the Bond with Reliable Structural Adhesives

By Ian Wade, Technical Services Manager, Belzona Polymerics Ltd.

(P&GJ) — Most industrial maintenance or repair procedures involve welding or the use of mechanical fasteners as these can be perceived as easy and quick. However, while 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 fixes that may be part of maintenance or repair, including support brackets, which include cable trays, antennas, heating coils or filter pans or internal fixtures in vessels that suffer from impact or vibration damage.

Process equipment or piping can suffer from thinning of the steel or wall defects which, will need either monitoring or repairs, depending on the integrity level.

Structural fittings are usually used to repair static members but may be caused by other forces 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 of this nature, the contractor may need to choose a solution based on a variety of factors.

Welding is regularly used for repairs as it is widely available while being well regulated with high customer confidence and high strength of the repair it does come with its inherent risks 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 and 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.

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

Strong Bond

Adhesive bonding is the joining of similar or dissimilar members together, while creating permanent high-strength bonds that 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 used after cleaning of the substrate by removal of any contaminants followed by the roughening of the substrate through grit blasting to internationally recognized standardsⁱⁱ. 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 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.

Types of Structural Adhesives (Table 1).

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

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.
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 (Table 2).

Table 2: Design Considerations

The adhesive was subjected to the following tests and evaluation protocols in to ensure that it met the design criteria previously discussed. Where possible, internationally recognized standards were used.

Cleavage Adhesion – ASTM D1062ⁱⁱⁱ

Tensile Shear Adhesion – ASTM – D1002^iv

Tensile Fatigue Resistance – ISO 9664^v
Impact Resistance – ASTM D256^vi

Experimental Procedure

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/mi, 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 (TSA) – ASTM – D1002

TSA, 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 2 mm and are overlapped lengthwise by 12.7 mm and bonded to a 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.

Figure 2: Tensile force

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:

80% = 6.8 MPa (8. 461 MPa x 80%) Stress amplitude cycles between
60% = 5.1 MPa (8. 461 MPa x 60%) Stress amplitude cycles between
57.5% = 4.9 MPa (8. 461 MPa x 57.5%) Stress amplitude cycles between
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 because 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 2.5 mm in depth with a total defect angle of 45 degrees in the center of a specimen sample with dimensions of 12.7 x 12.7 x 65 mm. The notch concentrates stress and allows measurement of crack propagation.

Figure 4: ISO 9664 Fatigue stress cycle

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

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 center of the specimen via a hydraulic press (Figure 3).

Plate 1: 550 x 50 x 10 mm thick

Plate 2: 225 x 50 x 10 mm thick

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 replicate in-field applications of achieving below the maximum bondline thickness of 2 mm.