After choosing the right glue, preparing the surface, choosing the technique of applying the glue and finally gluing the selected elements, you still wonder if the adhesive joint you choose will meet the requirements set for it? Will it prove to be strong enough, durable and resistant to various factors? In this article, we will suggest how to dispel these doubts and what available methods of testing adhesive joints to use.

The use of adhesive joints in many industries is fully possible thanks to the development of a methodology for testing the quality of these joints. It is based mainly on strength tests. Many methods have been developed that enable testing of adhesive joints in conditions appropriate for their work - the presence of shear, peeling and tensile stresses and in the conditions of cracking of the adhesive itself.

Procedures for testing adhesive joints:

• shear test,
• roller tear test,
• glue strip test,
• aging tests.

SHEAR TEST

Shear is the most common type of load acting on adhesive joints. The shear strength of an adhesive is regarded as its main strength property. The shear test is defined in DIN EN 1465 and is the most commonly practiced. This method is used when the bonded elements are made of specific materials (e.g. steel, wood, glass, composite) and of specific dimensions.

Sample geometry:

- element length 100 mm

- width 25mm

- thickness 1.6 mm

- overlap length 12.5 mm

Pic. 1 - sample before shear test prepared according to DIN 1465

Two samples prepared in this way, glued together (simple lap joint) are stretched on the surface of the adhesive. It is very important in this case to use the right thickness of glue. The shear strength of an adhesive generally decreases with increasing layer thickness. This is mainly due to the change in the state of stress in the weld. In this method, apply force parallel to the glued surface and cause it to increase continuously until it breaks.

After rupture of samples, rupture images should be analyzed. Breakups are divided into:

• adhesive failure - the glue separates from the bonded part (one of the elements has been incorrectly selected or prepared for gluing);
• cohesive failure - destruction of the internal structure of the adhesive, the desired image of the failure (adhesive remains on both elements);
• breaking of the bonded part - desirable in practice, but does not provide information about the strength of the adhesive;
• mixed breakup - mix of different breakup images.

Pic. 2 - Image of the flexible adhesive sample breaking off (aluminum and screen-printed glass)

There are also other methods included in the shear process as methods of testing adhesive joints using a different type of adhesive:

• anaerobic adhesive for mounting coaxial elements (e.g. mounting a bearing on a shaft) - compression/shear test according to DIN 54452, torsion/shear test DIN 54455.
  

• anaerobic thread adhesive - strength test of glued threaded joints DIN EN ISO 10964.
  
• if it is not possible to prepare the sample thickness according to the standard, it is possible to use a shear stress test /thick shear sample DIN 54451.
  
• there is also a long-term strength test by the temporal creep limit DIN EN 15336.
  

ROLL TEARING

The method of testing the adhesive peel strength consists in creating a situation in which one of the glued elements will be peeled off from the other, which will result in the creation of a tensile stress on the adhesive. The roll tear test is defined by DIN EN 1464. The use of this method also depends on the type of material with certain dimensions.

Sample geometry:

rigid part length 300 mm

flexible part length 250 mm

width 25mm

thickness of the rigid part 2.5 mm

thickness of flexible part 0.5 mm

lap length 200mm

Pic. 3 - sample of the tape before the tear test on the roll

The samples prepared in this way are subjected to a force almost perpendicular to the bonding surface, and its continuous increase causes tearing and then further tearing. The difference between a shear test and a peel test is that in shear, the force acts over the entire surface of the bonded sample, and in peel, the force acts linearly on the bonded sample.

The peeling method can be used both in the case of peeling two rigid elements apart and in the case of peeling a flexible element from a rigid substrate. This method is often used when testing different types of tapes. However, at the design stage of the adhesive joint, the possibility of peeling force should be avoided, as adhesives are the least resistant to it. It is strongly recommended to design the adhesive joint in such a way that the forces that will act on it are shear forces.

Other methods for testing peel loads are:

• Test for peel loads DIN EN ISO 11339
  
• DIN 65448 wedge test
  
• Bending/peeling test DIN 54461
  

ADHESIVE STRIP TEST

The adhesive strip test is defined by the DIN 54457 standard and consists in applying an elastomer strip (elastic adhesive) to a specific surface and then tearing the strip off while systematically cutting it. This gives the belt another chance to tear.

Pic. 4 – adhesive strip test prepared according to DIN 54457

AGING TESTS

In order to optimally test the strength of the adhesive joint, mechanical tests are performed first (described above), then aging tests and then mechanical tests are performed again. After such a cycle, we have a picture of how aging affects the adhesive joint.

We define adhesive aging as the changes that occur in the adhesive bond over time. Adhesive joints age due to multiple loads, which contributes, among others, to to reduce their strength and durability. Loads affecting the aging process of adhesive joints are:

1. Temperature

At elevated temperatures, the cohesion of adhesives decreases, chemical reactions and aging processes (e.g. water penetration) accelerate. Low temperature contributes to, for example, the brittleness of the adhesive bond.

2. Moisture/chemicals

If the adhesive joint is exposed to the influence of high humidity or other chemicals, there may be, for example, reduction of adhesion by breaking the physical interaction, reduction of cohesion - softening effect or hydrolysis of polymer chains. Not only the adhesive joint but also the glued elements themselves are exposed to aging (e.g. metal is exposed to corrosion and plastics to softening).

3. Radiation (usually UV)

Exposure of the adhesive joint and glued elements to excessive radiation may result in a reduction or complete loss of adhesion, the appearance of embrittlement or a change in color through discoloration.

4. Mechanical loads

A mechanically treated adhesive joint ages faster, and plastic deformation and microcracks may occur from a certain load level.

5. Fungi/Microorganisms

Certain types of fungus under the right conditions can feed on adhesive components and thus affect the strength of the joint.

6. Roaming modifiers

In the production of plastics, various types of fillers, plasticizers as well as anti-adhesive agents are used, which over time can migrate from the inside of the plastic to the outside, weakening the adhesive joint.

< p>The following aging tests are distinguished:

• constant climate test - samples are stored in constant climatic conditions, e.g. 85⁰C/85% humidity/2000h, one cycle can last from 30 to 90 days;
• variable climate test - samples are stored in periodically changing climatic conditions. This type of test is often used in the automotive industry. Exemplary cycle: from room temperature, the sample is heated to 80⁰C within 2h, then the sample stays at 80⁰C for 4h, and then for next 2h be cooled down to -40⁰ At -40⁰C the sample stays for 4h. The cycle is repeated as needed.
• immersion test - samples are stored (soaked) in the substance at a fixed temperature;
• salt mist test - spraying samples with 5% common salt solution. This test imitates the influence of the Mediterranean climate on the adhesive joint;
• catoplasmic test - a sample wrapped in a wet compress in a sealed plastic bag filled with deionized water is placed in an oven and heated at 70⁰C for 7 or 14 days. Then the sample is cooled down to -20⁰C and left for 15h. The cataplasm test is the most destructive of adhesive bonds and is often chosen when the adhesive requires rigorous testing prior to use.

Pic. 5 - test sample of the adhesive strip prepared according to DIN 54457 after the cataplasm test

Offer of adhesive joint tests

The Melkib company, as an entity operating in the industry of adhesives used in industry for many years, has the ability to test adhesive joints in a certified laboratory in accordance with applicable standards. The most common test is the cataplasm test, combined with the DIN 1465 shear test as well as the DIN 54457 adhesive strip test. which coordinates the process from the selection of the adhesive and its application technology, through tests and research, to the use of the adhesive in a given production process.

What is the cost of research?

It depends on the number of samples and the scope of research, it's hard to provide a quote without this information. If you are interested in conducting such research, please contact. We will prepare an individual quote.