ASTM C961
ASTM C961: Lap Shear Strength of Sealants
Test Method Overview, Fixture Requirements, Specimen Preparation, and Calculations
ASTM C961 is a standardized laboratory test method used to determine the lap shear strength of sealants bonded between rigid substrates. The method also documents the mode of failure—adhesive, cohesive, or mixed—which is critical for understanding sealant performance in structural glazing, curtain wall systems, and other building-envelope applications.
Sealants in service are frequently subjected to shear stresses caused by differential thermal movement, wind loading, and structural displacement. ASTM C961 provides a controlled and repeatable means to compare sealant formulations, surface preparations, and curing or conditioning protocols under shear loading.
1. Purpose and Scope of ASTM C961
ASTM C961 is intended to:
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Measure the maximum lap shear force a sealant bond can sustain
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Calculate maximum shear stress based on a defined bonded area
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Evaluate bond integrity by recording the failure mode (adhesive vs. cohesive)
The test is primarily comparative and is not intended to simulate all service conditions, but it is widely used for material screening, quality control, and product qualification.
2. Test Apparatus and Fixture Requirements
Substrates
The standard configuration uses annealed float glass substrates with the following nominal dimensions:
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25.4 mm × 76.2 mm × 6.35 mm
(1 in × 3 in × 0.25 in)
Alternate substrate materials may be used if agreed upon between the parties involved, but glass is the reference substrate defined in the method.
Sealant Thickness Control
A template or spacer is used to control the sealant bondline thickness. The nominal thickness specified by the standard is:
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3.18 mm (1/8 in)
Maintaining a consistent bondline thickness is essential for repeatable and comparable results.
Universal Testing Machine
Although the result is reported as shear strength, the test is performed using a universal testing machine operating in tension. The lap joint geometry converts the applied tensile force into shear stress within the bonded region.
Machine requirements include:
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Stable low-speed control
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Appropriate load cell capacity for expected sealant strength
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Ability to operate at the specified test speed
Crosshead speed:
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12.7 mm/min (0.5 in/min)
Grips and Holding Fixture
The standard shows the glass substrates clamped directly into the test machine jaws and pulled apart. In practical laboratory use, the gripping method should:
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Secure glass substrates without inducing edge damage
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Maintain coaxial alignment to prevent bending or “cocking”
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Provide consistent positioning for all specimens
Common solutions include rubber-faced grips, cushioned jaw inserts, or dedicated lap-shear fixtures designed specifically for brittle substrates like glass.
3. Test Specimens
Number of Specimens
A minimum of six (6) specimens is prepared and tested for each sealant condition.
Bonded Area
The standard lap joint overlap area is:
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1.0 in²
(6.45 cm² or approximately 645 mm²)
This area is used directly in the shear stress calculation and must be measured or controlled accurately.
Assembly Geometry
Each specimen consists of two rigid substrates bonded together with a controlled overlap. The sealant layer is uniform in thickness and centered within the overlap region.
4. Specimen Preparation and Conditioning
Substrate Cleaning
Proper surface preparation is critical. One commonly referenced cleaning procedure includes:
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Cleaning with soap and water
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Rinsing thoroughly
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Solvent wiping with methyl ethyl ketone (MEK)
Alternate cleaning methods, primers, or surface treatments may be used if specified by the sealant manufacturer or test program.
Sealant Application
The sealant is applied between the substrates using the thickness template. Care should be taken to avoid air entrapment and to ensure full wetting of the bonding surfaces.
Conditioning
After assembly, specimens are conditioned at standard laboratory conditions for a minimum of 24 hours prior to testing. Additional or alternative conditioning (heat, humidity, immersion, etc.) may be performed if agreed upon and documented.
5. Test Procedure
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Mount the specimen in the universal testing machine with one substrate secured in the upper grip and the other in the lower grip.
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Ensure proper alignment to prevent bending or torsional loading.
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Separate the grips at a constant rate of 12.7 mm/min (0.5 in/min).
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Continue loading until bond failure occurs.
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Record:
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Maximum force achieved
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Mode of failure (adhesive, cohesive, or mixed)
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Approximate percentage of each failure mode
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6. Calculations and Data Reduction
Maximum Lap Shear Stress
The maximum shear stress is calculated using the peak force and the bonded overlap area.
SI Units
τmax=FmaxA\tau_{\text{max}} = \frac{F_{\text{max}}}{A}τmax=AFmax
Where:
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τmax\tau_{\text{max}}τmax = maximum shear stress (MPa)
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FmaxF_{\text{max}}Fmax = maximum force (N)
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AAA = bonded area (mm²)
(1 MPa = 1 N/mm²)
US Customary Units
τmax=FmaxA\tau_{\text{max}} = \frac{F_{\text{max}}}{A}τmax=AFmax
Where:
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τmax\tau_{\text{max}}τmax = maximum shear stress (psi)
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FmaxF_{\text{max}}Fmax = maximum force (lbf)
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AAA = bonded area (in²)
Example Calculation (SI)
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Maximum force: 850 N
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Bonded area: 645 mm²
τmax=850645=1.32 MPa\tau_{\text{max}} = \frac{850}{645} = 1.32 \text{ MPa}τmax=645850=1.32 MPa
Example Calculation (US Customary)
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Maximum force: 190 lbf
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Bonded area: 1.0 in²
τmax=190 psi\tau_{\text{max}} = 190 \text{ psi}τmax=190 psi
7. Reporting of Results
A complete ASTM C961 test report should include:
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Individual results for all six specimens
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Average lap shear strength and standard deviation
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Substrate material and surface preparation method
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Sealant identification, batch, and cure history
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Test speed and conditioning details
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Detailed description of failure mode percentages
8. Sources of Variability and Best Practices
Key factors influencing test repeatability include:
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Bondline thickness control
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Accuracy of overlap area
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Surface preparation consistency
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Specimen alignment during testing
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Conditioning time and environment
Careful control and documentation of these variables is essential for meaningful comparison between sealants.
9. Typical Equipment Configuration
A complete ASTM C961 test setup typically includes:
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A calibrated universal testing machine
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Properly sized load cell
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Glass-compatible grips or lap-shear fixture
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Bondline thickness templates
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Clean, consistent substrate supply
When properly executed, ASTM C961 provides a reliable and widely accepted method for evaluating sealant shear performance under controlled laboratory conditions.
