Destructive Testing of Adhesive Bonds
Did you ever have one of those Monday mornings where you just wanted to throw something at the wall? Rip things apart? Smash things on concrete? Light something on fire? Irradiate something?
At Permabond, we don’t wait for Monday morning – we break things every day!
Destructive testing is not only essential to understanding adhesion properties – it can be a whole lot of fun. However, there are challenges in writing appropriate destructive testing protocols.
- Specimen selection – In order to put the bonded joint into an Instron for shear testing, it must be of certain shape and size. For example, you may purchase a zinc coated component but to find lap shears with the identical zinc coating and same surface finish might be impossible.
- Adhesive application – if your actual assembly has an induced gap, be sure to induce the same gap in the lap shear. It is also important to apply and cure the adhesive in the same manner as would be done in actual use. If you plan to use a room temperature two component epoxy but choose to heat cure that epoxy for testing – you’ve got apples and oranges.
- Thickness of substrates – The thickness of the substrate can affect the bond integrity especially if different coefficients of thermal expansion and contraction are at play.
Technical data sheets have a plethora of information on adhesive strength, temperature tolerance, and chemical exposure, but testing is always recommended to ensure adhesive performance is appropriate for each individual application because each factor is affected by the others.
The stress, environment, temperature, and time or speed between transitions all affect bond strength.
The key to destructive testing appropriately is to understand the types and combination of stresses the joint will experience in actual use. Following are some typical tests.
Shear strength is the measure of how difficult something is to cut (think pruning shears). Shear strength is one of the most common means of destructive testing and can be done in a few ways; the most simple is to bond two lap shears and pull. The reason shear strength is used to represent the strength of two bonded lap shears is that the load is parallel to the surface the load acts on. The shear stress causes one lap shear to displace with respect to the opposite lap shear.
There are ISO standards for the speed at which the laps are pulled and the temperature the test is performed under. Changing the speed, temperature, and environmental parameters to match the expected real world operation of the item being bonded will provide more valuable information. Slow pulling, for example at an inch per minute, is clearly a very different scenario than a fast jerk.
Tensile Strength is a straight pull, it is often used to test the strength of the adhesive itself (not bond strength). Tensile stress will cause the specimen to elongate and eventually break. Care is taken to form adhesive into test bars, then the test bars are pulled until they break. In addition to revealing it’s ultimate tensile strength, this test provides other information about the adhesive. For most adhesives, during the first portion of the tensile test the load and the elongation (change in length) the substrate undergoes is linear – if stress is removed during this elastic phase the material will return to its original state. The information from this line can provide the Modulus of Elasticity.
Stress/Strain= Modulus of Elasticity or Young’s Modulus
The point at which the line is no longer linear is the Yield Strength, where the substrate begins to deform. At this point and any point after it, the material will no longer return to its original state (original length) once the stress is removed. The specimen has begun to deform and with additional strain will ultimately break.
Impact testing determines an objects ability to resist high speed loading. Simply put – hitting the object at high speed. This is helpful in determining stress absorption or failure.
For most adhesive bonding purposes, peel tests vary in the angle of the peel. Roller methods or moving table methods are generally used for laminating applications.
Evaluating how temperature affects bond strength is necessary. Although service temperature ratings are provided for most adhesives, the service temperature may not directly correlate to bond strength at elevated temperature. First, it is important to understand that a classification of an adhesive as a high temperature resistant adhesive is based on the adhesive class. For example a standard cyanoacrylate adhesive resists up to 82C so a cyanoacrylate that resists 160C is considered high temperature resistant, but standard single component epoxy adhesives routinely resist 180C and are not categorized as high temperature resistant even though this is higher than the 160C of a high temperature resistant cyanoacrylate.
Service temperature is provided only as a guide, and testing which takes into account the bond area, speed of temperature change, the type and amount of stress on this joint, the coefficients of thermal expansion between the substrates, and the gap of the adhesive should be performed to ensure success.
Similarly chemical exposure (including water, humidity, etc…) will affect the bond differently at different concentration, length of exposure, dry time between exposure, and the joint design (specifically how much of the bond is exposed to the chemical).
The whole is greater than the sum of its parts. The types of stressors on a bond (temperature, environmental, strain, rate) all interact and collaborate to make the entirety of the abuse on the bond. Independently each has an effect, but to grok the combined effect takes some creativity.
Climb a ladder and drop it ten times. Then bake it and drop it hot another ten times. Then take it out in zero degree weather and drop it ten times. Then boil it and take it out into zero degree weather and throw it.
Bake, Climb, Drop.
Burr, Climb, Drop.
Boil, Burr, Climb, Throw.
Permabond adhesives are thermoplastic – they won’t reflow or melt at high temperature but they do soften at high temperature and can harden at low temperature. This physical change can affect the adhesive’s ability to expand and contract and thus affect the stress absorbing properties of the adhesive during impact.
Similar to the difference between dropping and throwing, the effects of water vary with time, temperature, and pressure.
Typical water absorption tests are performed on adhesives by either soaking for 24 hours at room temperature or by boiling for two hours. The samples are weighed before and after testing to determine the percent absorption. Left to dry, the adhesive will desorb the water and return to its original form. In and of itself this slight increase in size/weight of the adhesive is minimal, however the affects to adhesion may be greater. As the adhesive swells the softening, especially under significant load, can do two things 1. Adversely affect bond strength or 2. Provide more stress absorption.
In addition to soaking and boiling water, humidity testing, salt water spray testing, and steam/pressure testing are all common.
Like other forms of radiation, sunlight can affect the color and function of various plastics. Adhesives are often tested for long term exposure to light. Some adhesives like MS Polymers remain virtually unaffected by light, others, including certain epoxies, can harden and yellow.
Flame tests are common in the transportation industry. There are a variety of tests; criteria for fire resistant and fire retardant tests can be found here.
So yes, in addition to dropping, throwing, baking, boiling, freezing, irradiating, pulling, pushing, smacking, and squashing – we do light things on fire intentionally.
Provided you didn’t burn it, looking at how the bond fails is important.
- Substrate failure = the substrate or specimen failed but the bonded joint remained intact
- Adhesive failure = the adhesive comes of one or both substrates
- Cohesive failure = the adhesive remains on the substrates but is split down the middle
If you are interested in assistance designing the best means of destructive testing for your application contact Permabond.