Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
D5656-10 – Standard Test Method Technical Guide
📐 Specimen Geometry and Preparation
The Thick-Adherend Metal Lap-Shear Test (D5656) is a critical method for characterizing the intrinsic shear behavior of adhesives. The specimen consists of two thick, rigid plates (adherends) adhesively bonded together. Typical adherend materials are high-strength aluminum (e.g., 7075-T6) or steel, with a standard thickness of 6.35 mm (0.250 in). The overlap length is 12.7 mm (0.50 in), and the width is 25.4 mm (1.00 in). The bondline thickness is carefully controlled, typically between 0.5 mm and 1.0 mm (0.020 to 0.040 in), using precision shims. Surface preparation is conducted according to Guide D2651.
| 🟦 Parameter | 📏 SI Dimension | 📐 Inch-Pound Dimension |
|---|---|---|
| Adherend Thickness | 6.35 mm | 0.250 in |
| Overlap Length | 12.7 mm | 0.50 in |
| Specimen Width | 25.4 mm | 1.00 in |
| Bondline Thickness | 0.5 – 1.0 mm | 0.020 – 0.040 in |
⚙️ Test Procedure and Speed Selection
Testing is performed on a universal testing machine verified under ASTM E4. A tensile load is applied, placing the adhesive layer in shear. Because the adherends are thick and rigid, peel forces are significantly minimized compared to standard single-lap shear tests (D1002 / D4896). A slow, quasi-static crosshead speed, typically between 0.5 and 1.0 mm/min (0.02 – 0.04 in/min), is used to capture the full stress-strain response. Shear strain is measured directly across the bondline using averaging extensometers or digital image correlation (DIC), as the location along the overlap can affect the displacement measurement.
⚠️ Important Consideration: The user is cautioned that the stress is not totally uniform across the overlap. Factors such as adhesive stiffness, adherend stiffness, and the location of shear strain measurement can affect the experimentally obtained load-displacement curve.
📊 Key Measured Properties and Analysis
The load-displacement data is converted to an engineering shear stress-strain curve. Shear stress (τ) is calculated from the load and overlap area. Shear strain (γ) is calculated from the extensometer displacement divided by the bondline thickness. Critical parameters derived from this curve include the shear modulus (G), the shear stress at the knee (yield), and the ultimate shear strength and strain.
| 🎯 Property | ⚡ Typical Value Range |
|---|---|
| Shear Modulus, G | 250 – 400 MPa |
| Shear Yield Stress, τy (Knee) | 20 – 35 MPa |
| Ultimate Shear Strength, τult | 35 – 50 MPa |
| Shear Strain at Failure, γf | 0.10 – 0.50 |
💡 Identifying the Knee: As defined in Terminology D907 and D5656, the “knee” is the inflection point on the curve where plastic yielding begins to dominate the deformation. A standardized offset method (e.g., 0.2% strain offset) is often used to consistently determine this value for analysis and design.
❓ Frequently Asked Questions
🔍 What is the main objective of the D5656 test method?
The principal objective is the determination of the complete shear stress-strain behavior of an adhesive, minimizing peel stresses through the use of thick adherends to provide accurate material data for stress analysis and FEA.
💡 How does D5656 differ from a standard single-lap shear test (D4896)?
Unlike the standard single-lap shear specimen which has thin adherends that bend, inducing large peel stresses, the thick-adherend specimen in D5656 is stiff enough to minimize bending. This allows for the direct determination of the adhesive’s shear stress-strain properties.
⚡ What equipment is essential for measuring shear strain in D5656?
An averaging extensometer attached to the sides of the specimen, spanning the bondline, is the traditional method. Modern techniques like Digital Image Correlation (DIC) are also acceptable and recommended for observing the strain field across the entire overlap.
📌 What is the significance of the “knee” in the test results?
Defined in Section 3.2.1, the knee represents the point at which plastic yielding of the adhesive begins to dominate the deformation response. It is a critical design value indicating the transition from elastic to plastic behavior in the adhesive bond.