Lap Shear Testing of Adhesives

Interview conducted by Stuart MilneMar 22 2017

Rohit Ramnath, Application Engineer at Master Bond, talks to AZoM about lap shear strength and how significant is this property for adhesives.

What are the different strength properties exhibited by epoxy compounds?

Some of the measures of the strength for an epoxy, include compressive, flexural, peel, shear and tensile. They provide a good gauge as to the different forces that an epoxy can withstand. Many Master Bond epoxies are formulated to maintain strength between similar and dissimilar substrates, even when exposed to high and low temperatures, thermal cycling, corrosive environments and more.

What is lap shear strength and how significant is this property for an adhesive?

Lap shear strength testing measures the ability of a material to withstand stresses set in a plane, where the exerted shear force is moving the two substrates in opposite directions. It is one of the most common stresses that a bonded joint can face during service, especially in structural bonding applications. The lap shear strength of an adhesive serves as one of the indicators for its long term sustenance in an application.

How is lap shear testing performed?

Based on the ASTM D1002 test procedure, lap shear strength testing is typically performed for an aluminum to aluminum bond joint. The substrates are usually roughened, cleaned, and dried prior to the application of the adhesive. The parts with the adhesive between them are clamped/fixtured during the curing process. Once the adhesive is fully cured, the substrates would be pulled in opposite directions (in shear) until the substrates separate. The force needed to pull the substrates apart is then used to determine the lap shear strength.

What are the types of substrates that are used in applications, and how important is surface preparation in achieving optimum strength?

Some of the other common substrates that can be used in applications, are polycarbonate, acrylic, PEEK, fiberglass reinforced plastic, Ultem (polyetherimide), polyester, and ABS to name a few. In determining the lap shear strength for such substrates, the plastics would be prepared prior to testing by an effective method such as degreasing, abrasion, physical methods, chemical treatments, or even plasma/corona etching. It is important to note that proper surface preparation plays a pivotal role in achieving the highest possible bond strength.

Can one perform lap shear testing with plastics?

Although the ASTM D10002 testing is typically done with metals like aluminum, adhesives used for bonding plastics can also be tested for their lap shear strength. Such testing is helpful since plastics are widely used across the Aerospace, Electronics, Medical, and Oil & Gas Industries to name a few.

Has Master Bond done any lap shear testing with plastics?

Master Bond tested many different adhesives and plastic substrates over the years. Most recently, we selected a few plastics and tested the following formulations:

• EP21LV
• EP42HT-2
• EP21ARHT
• EP30-2
• EP31ND
• EP29LPSP
• EP30Med

Master Bond engineers tested these epoxies by applying them on a variety of prepared plastic substrates and curing them in lap style joints. The substrates were then pulled in opposite directions (in shear), until the joints were separated. In some cases, the substrate failed before the bond. The plastics used in this testing include:

• Polyester
• Polycarbonate
• Acrylic
• PEEK
• Fiberglass reinforced plastic
• Delrin
• Ultem (polyetherimide)
• ABS

It is important to note that these plastics were prepared prior to testing. The most effective methods for preparing plastic substrates are degreasing, abrasion, physical methods, and chemical treatments. Proper surface preparation plays a pivotal role in achieving high bond strength.

Observations

1. In many cases, there were substrate failures, which indicated that the maximum possible strength was achieved, and that the surface preparation procedures as well as the curing processes were properly followed.
2. Fiber reinforced plastic was the substrate with which epoxies exhibited the highest psi across the board.
3. EP29LPSP seems to have outperformed the others in many cases, especially yielding the highest strength values for acrylic, PEEK and Ultem.

Disclaimer: The findings in this article are not meant to be used for specification purposes.

About Rohit Ramnath

Rohit Ramnath is an Application Engineer for Master Bond Inc., a custom formulated adhesives manufacturer. He analyzes application oriented issues and provides product solutions for companies in the aerospace, electronics, medical, optical and oil/chemical industries. He graduated from Carnegie Mellon University with a Masters Degree in Chemical Engineering, where he wrote his thesis on analyzing drilling fluid emulsions for enhanced oil recovery.