ASTM D3039

What the test does

A straight-sided rectangular coupon, cut from a cured laminated panel and usually carrying bonded end tabs, is clamped at both ends in wedge grips. The crosshead pulls it along its axis at a constant rate until it breaks. Force, crosshead displacement and strain from an extensometer are recorded throughout, so the whole stress–strain response is captured rather than the breaking load alone.

What it measures, and why it matters

The method reports ultimate tensile strength and strain, chord modulus of elasticity, Poisson's ratio, and transition strain where the response is bilinear. Strength measured along the 0° direction releases a prepreg lot and qualifies a cured panel; transverse and off-axis strengths are matrix- and interface-dominated, so they expose undercure and poor fibre wet-out that a 0° coupon hides. Chord modulus, taken between 1000 and 3000 microstrain, and Poisson's ratio go straight into laminate theory and finite-element models as ply-level design inputs.

Specimen

Coupons are straight-sided rectangles, not dumbbells — a reduced section would simply fail at the radius in a fibre-dominated laminate. Width and thickness follow the lay-up: roughly 15 mm by 1 mm for 0° unidirectional carbon, 25 mm by 2 to 2.5 mm for transverse and balanced-symmetric laminates. Bonded end tabs about 1.5 mm thick and 25 or 56 mm long protect the gripped ends. Cut edges must be machined square and clean, since edge damage seeds early failure. Conditioning is the standard laboratory atmosphere of 23 °C and 50 % relative humidity, with moisture conditioning to ASTM D5229/D5229M where the test plan calls for it. A coupon that fails at or inside the tab is a grip-induced failure, and is replaced rather than averaged in.

What the machine must be capable of

Force demand swings by more than an order of magnitude with lay-up, so the frame is sized to the laminate, not to the method. A 90° transverse coupon fails at around 2 to 3 kN; a 0° unidirectional carbon coupon at roughly 30 to 40 kN; a thick balanced-symmetric carbon laminate well past 60 kN. Frames offered for this work run from about 50 kN for glass-reinforced laminates to 250 kN for carbon, with 100 kN the common all-round choice. Force indication must be verified to ASTM E4.

The rate is 2 mm/min (0.05 in/min) of constant head displacement, or a constant strain rate of 0.01 min⁻¹, chosen so the coupon fails in roughly 1 to 10 minutes — polymer matrices are rate-sensitive, so the rate used belongs in the report.

Strain measurement needs an extensometer classified to ASTM E83 Class B-1 over a 10 to 50 mm gauge length. Total range beyond about 5 % is wasted, since carbon laminates fail below 2 % and glass between 3 and 5 %; resolution at 1000 to 3000 microstrain is what actually governs the choice, because chord modulus is taken across that window.

Grips are body-over-wedge, mechanical or hydraulic, so holding force rises with load. Side-loading hydraulic wedges clamp perpendicular to the coupon rather than dragging it during closure, and grip force must be adjustable: too little slips, too much crushes the laminate. Faces are surface-grit for untabbed coupons, serrated or diamond-coated for tabbed ones. Load-train alignment is a requirement in its own right. Non-ambient testing brings an environmental chamber and all-temperature grips into scope.

What goes wrong in practice

Tab debond ends the test before the gauge section does: the adhesive bond line releases in shear and the coupon records a low strength that must be discarded, not averaged. Grip-induced failure is the same loss from the other side — clamping too hard crushes the laminate at the tab end. Off-axis loading from a misaligned load train bends the coupon, scattering strength and reading modulus low. Longitudinal splitting and delamination in unidirectional coupons produce a ragged curve and an ambiguous failure load.

Related and equivalent standards

ISO 527-4 is the nearest counterpart and covers the same in-plane tensile properties, but coupon geometry, test speed and modulus evaluation differ, so results should not be assumed to transfer between the two. ASTM D638 handles unreinforced and short-fibre plastics with dumbbell specimens, which fail at the radius in a continuous-fibre laminate. ASTM D5229/D5229M supplies moisture conditioning, ASTM E4 force verification and ASTM E83 extensometer classification.