ASTM D1002

What the test does

Two flat metal strips are bonded end-to-end so that they overlap by a short defined length, with the adhesive sandwiched in between. The assembled coupon is clamped at each free end in a tensile frame and pulled along its length until the joint ruptures. Only force is recorded. Because the two adherends lie on different planes, the joint rotates slightly as load builds, so the glue line carries peel as well as shear right up to failure.

What it measures, and why it matters

The result is a single figure: peak load divided by bonded area, reported as an apparent shear strength. The word "apparent" is load-bearing. The number moves with overlap length, adherend thickness and stiffness, and surface preparation, so it ranks adhesives and monitors a bonding process rather than supplying a design allowable. The standard is explicit that treating it as a design stress can lead to product failure, and a separate guide, D4896, exists to explain how far single-lap results can be pushed.

Just as informative is the failure surface. Cohesive failure, with adhesive left on both adherends, says the adhesive's own strength set the limit; interfacial failure, where one face comes away clean, points at degreasing, etching or priming. On a production line that distinction is often the reason the test is run.

Specimen

The standard coupon is a strip 25.4 mm wide with a 12.7 mm overlap, giving a bonded area of about 323 mm² in an assembly of about 178 mm overall. Adherend thickness and overlap may be adjusted so that failure occurs in the adhesive rather than by yielding of the metal; that is normal practice, not a deviation. Bonded panels are cut into replicate coupons, since scatter between nominally identical joints is inherent to the geometry; the replicate count is set by the current edition. Conditioning and test take place in a controlled laboratory atmosphere; secondary sources give 23 ± 2 °C and 50 ± 5 % relative humidity, but that clause could not be confirmed against an issuing-body page.

What the machine must be capable of

On the standard bond area, a flexible sealant failing at a few megapascals ruptures near 0.6 kN, while a structural aerospace epoxy at 40 to 45 MPa reaches 13 to 15 kN. A 10 kN frame covers sealants and general-purpose adhesives; 30 kN is the usual choice for structural work and leaves headroom for the mass of the wedge grips.

The method is written as a load-rate test: shear stress applied at roughly 8.3 to 9.7 MPa per minute (1200 to 1400 psi/min). Force accuracy follows the general ASTM practice for verifying testing machines, E4; the referenced-documents list of D1002 itself was not visible on a public page, so confirm against the current edition. No extensometer is needed — there is no gauge length here, only a bonded length.

Gripping decides the result. Self-tightening serrated wedge grips, or pneumatic side-action grips, must be aligned so the line of force passes through the centre of the joint, and shims or tabs at each specimen end bring the bond line onto the load axis. Without them the offset adherends bend the load path and the coupon sees a bending moment the method never intended. Elevated- and sub-zero-temperature lap shear are separate methods, D2295 and D2557, so no chamber belongs on the base test.

What goes wrong in practice

Skipping the alignment shims is the commonest error. The joint already rotates; grip misalignment on top of that drives peel at the overlap ends, and the value drops for reasons unconnected with the adhesive.

Adherend yield is next. If the metal is too thin or too soft for the adhesive under test, the strips yield before the glue line does and the reported stress is a property of the metal. The fix is thicker adherends or a shorter overlap, not a re-run.

Grip damage is the third. Serrated jaws biting hard, thin adherends can notch them at the jaw line and seed a failure there; too little clamping and the coupon walks out under load, leaving a scored tab and a low reading.

Finally, cure state: testing before the adhesive has reached its specified cure shifts results widely while leaving the data looking plausible.

Related and equivalent standards

The nearest counterpart from another body is ISO 4587:2003, covering single-lap shear of rigid bonded assemblies by tension loading. The intent is the same, but specimen dimensions and rate conventions differ, so figures from one should not be pooled with the other without stating which was used.

D905 is the method most often confused with this one. Both shear an adhesive bond, but D905 loads in compression through a block-shear fixture on wood adherends, and there the fixture rather than the frame defines the test. D2295 and D2557 are the elevated- and sub-zero-temperature variants.