The material selection platform
Adhesives Ingredients
The material selection platform
Adhesives Ingredients
Impact Strength Test Methods for Adhesives

Test Methods to Measure Impact Strength of Adhesive Joints

Adhesives react differently at high loading rates than they do under more normal static loading conditions. Hence, the study of the behavior of adhesive joints under impact loading is now a very active field of research, driven by significant industrial interest.

Many industries, such as the automotive & aerospace, are employing adhesive joints with high impact strength. These structural bonds are not only important to obtain application-specific mechanical performance, but they also provide adequate safety levels for their occupants during collisions.

Therefore, understanding the behavior of adhesive joints under impact is crucial for designing stronger and safer structures. Explore some of the widely used test methods for assessing impact characteristics and fracture behavior of structural adhesives.


Impact Tests Used in the Adhesives Industry

Impact Tests Used in the Adhesives Industry

The ability of an adhesive joint to withstand a sudden load is of interest in a wide variety of applications. The automotive and the aerospace industries are just two of the major adhesive markets that place a high value on impact resistance and fracture behavior of structural adhesives. In the auto industry, this is called “crash worthiness”.

As a result, a variety of test methods have been devised for applying rapid loads. However, all have shown various limitations, related mainly to the difficulty in setting up the specimen and the need to attach the impact apparatus rigidly to a stiff and heavy base.

Learn here, the important tests that have been used to determine the fracture mechanics properties of adhesives under high loading rates. These are generally laid down by National or International Standards and include:

There is, however, a lack of confidence in their predictive capability under “extreme” conditions, especially impact loading. Certain tests are more appropriate for fundamental property measurement, such as measurement of:

  • Energy release rate (G)
  • Fracture energy (Gc), or
  • Critical stress intensity factor (K)

The less complex test methods are more appropriate to the formulator or end-user in the ranking of adhesive systems, surface treatments, etc. Understand here, the in-depth knowledge about which test method is most appropriate for specific situations.

Let's start with the consequence of high rates of loading and standards for measuring impact resistance of adhesives first...

The Consequence of High Rates of Loading

The Consequence of High Rates of Loading

Adhesives react differently at high loading rates than they do under more normal static loading conditions. The toughness of an adhesive joint may decrease under impact-loading conditions. This is due to the fact that polymeric materials exhibit plastic and viscoelastic deformations, and as a result, the energy absorption characteristics come into play.

An analogy illustrates the chemical structure of polymers, molecular entanglement, and their relation to viscoelasticity. This analogy is of a polymer to a bowl of spaghetti. If one slowly pulls apart spaghetti, the noodles will separate without much resistance.

The noodles will separate from one another even though the original mixture was highly entangled. However, if the spaghetti noodles were separated quickly, the entanglements of the noodles would provide resistance, (there is no time for orderly separation) and fracture of the noodle strands is a likely result. Similarly, polymers are likely to exhibit brittle fracture when strained at high rates, and ductile fracture when strained at lower rates.

Factors Affecting Adhesive Joint's Fracture Behavior

The adhesive joint’s fracture behavior may vary depending on:

  • The rate of loading
  • The joint design, and
  • The test temperature

A significant problem is that the performance of the tested joint under impact loading is often a function of the test method and not necessarily indicative of the actual joint or its service environment.

Although a structural adhesive may have very high static shear strength, the toughness of an adhesive joint may decrease considerably under impact-loading conditions. Toughness is the adhesive’s ability to absorb energy. It is directly related to the area under the stress-strain curve when the adhesive joint is tested.

Tough adhesives generally have both a high degree of elongation as well as a high ultimate strength. Toughness is a characteristic that depends on the viscoelastic or time-dependent nature of the adhesive material.

Bonding Dissimilar Materials-AS SG

Standards for Measuring Impact Resistance of Adhesives

Standards for Measuring Impact Resistance of Adhesives

A number of adhesive tests have been developed to measure impact resistance. Several of the more commonly used tests are described in the table below. Most of these tests such as the drop weight test have been developed to mimic impacts that occur in common applications. As a result, there is no universally accepted impact test, but several that have been developed specifically for certain safety-critical industries or applications.

Test Specimen Characteristics
Block shear
(ASTM D950)
Bonded block hit by a pendulum
  • Impact velocity of 3.3 m/sec.
  • No correlation between measured force and adhesive fracture energy.
  • Considered poor at discriminating between adhesives.
Cantilever beam
(ASTM D3433)
Straight and tapered double cantilever beam
  • Fracture energy can be determined at variable test rates (10-5 to 5 m/sec).
  • Considered to be the easiest and most reliable method of measuring fracture energy.
Three point bend
(Similar to ASTM D790)
Notched, rectangular bar
  • Results are highly dependent on temperature and notch severity.
  • Impact velocities of 2.9 m/sec (Charpy) and 2.44 m /sec (Izod).
Impact wedge
(ASTM 3762)
ASTM D1002 or D3163 lap shear specimen
  • Developed primarily for determining side impact on bonded automotive joints.
  • Uses pendulum similar to ASTM D950.

In these tests, a pendulum or dropped weight is often used and represents the most convenient and inexpensive test apparatus. High-speed servo-hydraulic actuators and associated instrumentation for force measurement and data capture can also be used for producing impact velocities of up to 5 m/sec.

Laboratory joints are generally tested to rate the toughness of adhesives and to develop fracture behavior properties, and then full-scale crash tests (Figure below) are used to verify the crash worthiness of bonded assemblies.

Crash Tests to Measure Impact resistance of Bonded Structures
Full-scale Crash Tests to Measure Impact Resistance of Bonded Structures in the Auto Industry

Another unique feature of adhesive bonded joints in the auto and aerospace industry is the joint geometry. For weight reduction, structures generally require the bonding of relatively thin (e.g., 1 mm) substrates such as steel or aluminum alloys.

Many of the more common impact test methods described in the table above are more appropriate for thicker substrates. As a result of these unique characteristics, the auto and aerospace industries have developed and embraced certain tests methods to measure the performance of structural adhesive under impact loading and these have become National or International Standards.

Block Shear Test

Block Shear Test

The block shear test is defined by ASTM D950 and BS 5350. It does not produce material parameters, but it measures the total energy required to fail a standard test specimen as shown in the figure below:

Block Shear Test Specimen
Block Shear Test Specimen (From ASTM D950)

The test specimen consists of a 25mm square, 10 mm thick adherend block bonded to a larger base. The specimen is mounted in a grip and placed in a standard impact machine. A pendulum is positioned so that the face of the upper adherend is struck as closely as possible to the bond. The adherend block is struck with a pendulum hammer traveling at 5 m/s, and the energy of the impact is reported in pounds per square inch of bonded area.

With this test method, discrimination between different adhesive types is poor. However, discrimination can be achieved with formulation differences within the same type of adhesive. The adherends must be bonded with the surfaces parallel or the impact strength value is severely reduced. The test results are relatively independent of the type of substrate depending on its deformability. There is no correlation between the measured force and the adhesive’s fracture energy.

Cantilever Beam Tests

Cantilever Beam Tests

a. Double Cantilever Beam (DCB) Test

The double cantilever beam (DCB) test is the most commonly used method for measuring the initiation and propagation values of Mode I fracture energy GI under static and cyclic loading conditions (see ASTM D3433, BS 7991 and ISO 25217). This test method is useful in that it can be used to develop design parameters for bonded assemblies. The initiation and propagation values of fracture toughness, Gc, under static and cyclic loading conditions can be measured.

In this test method, a tensile load is applied to a specimen (figure below) with an embedded release film (~25 mm deep) at the specimen mid-plane. The tensile force acts in a direction normal to the crack surface. Specimens are typically 25 mm wide and 356 mm long. The adherend thickness is typically 3-5 mm.

Double Cantilever Beam Test Specimen
Double Cantilever Beam Test Specimen

Crack length is measured using either a traveling microscope, a crack gauge, or a video camera. The use of a crack gauge enables crack measurement to be automated. For static tests, the coefficient of variation in Gc is typically 20% or higher. Both static and fatigue testing under ambient and hostile environmental conditions can be undertaken using this test method.

The critical strain energy release rate or fracture toughness Gc is calculated from the following equation:

Calculating Fracture Toughness in Adhesives

P is the applied load
E is the flexural modulus of adherend in the longitudinal direction
b is the specimen width
a is the crack length, and
h is the adherend thickness

Fabrication and Testing of DCB Specimens

Fabrication and testing of DCB specimens are straightforward and relatively inexpensive. Testing can be conducted using standard mechanical test frames. Specimen fabrication is identical to that employed for wedge cleavage specimens. While the DCB test method is intended for use in metal-to-metal applications, it may be used for measuring fracture properties of adhesives using plastic adherends provided consideration is given to the thickness and rigidity.

In general, systems of similar type toughness can be compared as can the effect of environment on the toughness of a single system. A Gc value represents a lower limiting value of fracture toughness for a given temperature, strain rate, and adhesive condition. This value may be used to estimate the relation between failure stress and defect size for a material in service.

b. Tapered Double Cantilever Beam (TDCB) Method

The tapered double cantilever beam (TDCB) method provides additional information on the fracture mechanics properties of the adhesive over a higher strain rate for determination of Gc, fracture energy. It is used for Mode I testing of adhesives with metallic adherends and is well suited to tests where the crack length is difficult to measure, especially environmental testing. The large size of the adherends (310 mm long, 50 mm wide and 10 mm thick) also ensures that plastic deformation of the adherends is minimized, thus facilitating the use of low yield stress metallic substrates and tough adhesives.

The TDCB method is covered in ASTM D3433, BS 7991 and ISO 25217. It is considered to be the easiest and most reliable method for measuring the adhesive fracture energy. The test specimen (Figure below) has been used to determine the rate of crack growth under various cyclic loading and environmental conditions.

Tapered Double Cantilever Beam Test Specimen
Tapered Double Cantilever Beam Test Specimen

For higher rate tests a high-speed servo-hydraulic test machine can be used, and crack growth data can be measured by high-speed photography. The main disadvantage of the TDCB method is the relatively high cost associated with specimen fabrication.

Three Point Bend Test

Three Point Bend Test

The three point bend or Charpy test is a standard method for measuring the fracture energy of metals and polymers. It has also been used with bulk adhesive and joint specimens as shown in the figure below:

Three Point Bend Test
Three Point Bend Test

The test specimens are 80 mm long with a cross-section of 10x12 mm. The specimens can be notched with the use of a sharp blade or by release material inserted where the crack is to be formed. The specimens can be tested on a pendulum hammer type machine or on a servo-hydraulic machine for high rates of loadings. Preference is given to the servo-machine for higher rates of loading.

The specimen is tested similar to the common three point bending test (ASTM D790) used to determine flexural strength and modulus, except the specimens are tested on a pendulum hammer type machine or on a servo-hydraulic machine for high rates of loading.

The length of the crack is measured. The specimen is then loaded monotonically. A plot of the load versus the crack opening displacement is used to determine the load at which the crack starts growing. This load is substituted into the following formula to find the stress intensity factor, K, or fracture toughness:

Calculating Stress Intensity Factor

P is the applied load
B is the thickness of the specimen
a is the crack length, and
W is the width of the specimen

The three point bend test requires considerable time to prepare the specimens, and problems often occur in inserting the crack into the specimen. The sharpness of the crack leads to different values of fracture energy. Generally, the measured fracture energy values are similar to those with tapered double cantilever beam tests but with greater variation.

Impact Wedge Peel Test

Impact Wedge Peel Test

The impact wedge peel (IWP) test is an International Standard Method (ASTM ISO 11343) that is employed to measure the resistance to cleavage fracture of structural adhesive at a relatively high-test rate of 2 to 3 m/sec and at defined temperatures. The auto industry uses this test method both for the development of new adhesives and as a quality control procedure.

The IWP test specimen is shaped like a tuning fork, and a wedge is forced through the bonded portion of the specimen as indicated in Figure 6. The specimens should be 90mm long and 20mm wide. The joint is made from sheet metal substrates that are between 0.6mm and 1.7mm thickness. The substrates should be bonded over a length of 30mm to give a tuning fork profile.

Impact Peel Test Specimen
Impact Peel Test Specimen

No starter crack or notch is used with these specimens as sometimes employed in other impact test methods. This helps to eliminate any variability due to notch size or severity. The free arms of the specimen are clamped to the fixture with a spacer block between them. The wedge is then driven through the bonded portion of the specimen using a high-speed servo-hydraulic machine. As the wedge is impelled dynamically through the bonded substrates, very high local tension stresses occur in the bond – a critical load case for adhesives.

The wedge velocities recommended by the ISO are 2m/sec for steel and 3m/sec for aluminum alloy substrates. However, tests can be performed at lower velocities and at various test temperatures. Several possible high-speed hydrolytic testing machines and instrumentation packages can be used with the IWP specimen.

An oscilloscope can be used to record the displacement versus time output from the testing machine and the force versus time signal. These traces then can be transferred to a computer analysis package to calculate the results. One possible commercial test set-up utilizes an Instron Dynatup Model 9250HV and impact data acquisition and analysis system.

Determining Stable/Unstable Crack Growth

In the IWP test, racks were found to propagate through the adhesive by either a stable form of crack growth or an unstable form of growth. The figure below illustrates typical traces for both these types of crack growth. Non-stabilized adhesives fail after crack initiation instantaneously and the substrates separate. For the stabilized adhesive the crack front is more or less close to the wedge front. As the two substrates are still partially bonded, the adherend has to deform and more impact energy can be absorbed.

Typical Traces for Stabilized and Unstabilized Crack Growth
Typical Traces for Stabilized and Unstabilized Crack Growth as Determined by the Impact Peel Test3
(Steel substrates bonded with one-part epoxy adhesive)

The ISO Standard specifies that the average cleavage force is calculated from the force versus time data, disregarding the first 25% and the last 10% of the curve. The energy is also calculated, by integration over the same part of the curve.

By changing the speed of test, rate dependencies can be observed and taken into account for material modeling. Also, by varying the test temperature, the time-temperature-superposition principle can be used to estimate the joint characteristics at different impact speeds or at different temperatures.

The figure below provides a summary of different adhesive types for their impact peel strength at various test temperatures. For crash worthiness, general purpose adhesive will not keep the substrates together.

Non-stabilized structural adhesives can withstand low energy impact, but at lower temperatures their resistance capacity is minimal. Structural semi-crash stable adhesives could resist medium impact energy levels adequately. For higher impact levels, however, the crash stable adhesives fulfill the needed performance requirements at all service temperatures.

Impact Peel Strength of Different Adhesives at Room and Low Temperatures
Impact Peel Strength of Different Adhesives at Room and Low Temperatures4

Analyzing the Data

It has been noted that adhesives may exhibit stable crack growth at moderate temperatures (i.e., room temperature), but unstable crack growth at lower temperatures (-40°C). In fact, most structural adhesive failures are stable at room temperatures at test rates of 10-5 to 2m/sec. At -40°C only the toughest adhesives exhibit stable failure.

It has also been noticed that higher impact wedge peel forces are measured for steel than for aluminum substrates. There is a significant amount of plastic deformation of the substrates and, thus, this test method gives an indication of the performance of the adhesive/substrate combination rather than a measure of the bulk material properties of the adhesive such as in the case of Izod or Charpy impact test.

A correlation of cleavage force and the adhesive fracture energy, Gc, can be calculated. This has been achieved by developing a finite element model for the failure of the impact wedge peel specimens. This allows the cleavage force and the fracture energy of the adhesive to be predicted accurately from crack length versus time data, obtained from high-speed photography.


The IPW test is primarily qualitative but is very discriminating in determining variations in adherend surface preparation parameters and adhesive environmental durability. The test has found application in controlling surface preparation operations and in screening surface preparations, primer and adhesive systems for durability.

In addition to determining crack growth rate and assigning a value to it, the adhesive–joint failure is evaluated and reported. For example, adhesion failure; cohesion failure; or adherend failure are noted after opening up the specimen at the conclusion of the test period.

  1. Tomczyk, A.J., Test Methods for Adhesive Fracture Properties – Overall Summary, AEA Technology, Osfordshire, UK, February 1997
  2. Instron Application Report, “Impact Performance of Adhesive Bonds Under Impact – ISO 11343”, www.instron.com, 2007.
  3. Blackman, B.R.K., et. al., “The Impact Wedge Peel Performance of Structural Adhesives”, Journal of Materials Science, Vol. 35, 2000, pp. 1867-1884.
  4. Droste, A., “Crash Stable Adhesives in Application and Simulation”, DYNAmore GmbH, 2006.
  5. https://www.sciencedirect.com/topics/engineering/fatigue-testing

Leave a comment

Want to comment?

No Account yet?

Rate this Content
Be the first to comment on "Test Methods to Measure Impact Strength of Adhesive Joints"

Back to Top