ASTM D790

What the test does

A rectangular bar of plastic is laid flatwise across two parallel cylindrical supports; nothing is gripped or clamped. A loading nose descends onto the midpoint of the span and bends the bar at a constant rate while force and midspan deflection are recorded. The run ends at rupture or at 5.0 % outer-fibre strain, whichever comes first, so a ductile grade may finish the test unbroken.

What it measures, and why it matters

The method reports flexural strength, flexural stress at any chosen point on the curve, maximum outer-fibre strain and flexural modulus, taken from the initial straight portion of the curve. Modulus is the stiffness figure that feeds wall-thickness calculations and finite-element modelling, and it separates unfilled grades from filled ones. Flexural strength serves lot release on moulding compounds and electrical insulating materials. Because the outer fibre carries the peak stress, the result is sensitive to moulded skin quality, which makes it useful in moulding investigations.

Specimen

The reference bar is 3.2 mm deep and tested flatwise, with the support span set at sixteen times the depth — 51.2 mm for that bar. Other thicknesses move the span with them. Bars are moulded, or machined from sheet with flat, parallel faces and edges free of tool marks. At least five specimens are tested, with a set in each principal direction for anisotropic materials. A bar carrying a visible void, sink mark or machining mark on the face that will be in tension is discarded and replaced, since the flaw rather than the material then decides where the crack starts. Conditioning follows Practice D618: 23 ±2 °C and 50 ±10 % RH, 40 h for specimens 7 mm thick or less, with testing in the same atmosphere.

What the machine must be capable of

Force demand is low. Most moulded bars fail between roughly 20 N and 1 kN, so a frame of around 2 kN carries almost all D790 work, provided the load cell resolves cleanly down to about 10 N rather than merely reaching the top of its range. Thick laminated thermoset bars run at 32:1 or 40:1 can call for several kilonewtons. Force indication must be verified to ASTM E4.

Rate is defined as a strain rate and converted to crosshead speed by R = ZL²/6d. Procedure A runs at 0.01 mm/mm/min and Procedure B ten times faster at 0.10 mm/mm/min, giving about 1.4 and 13.7 mm/min for the 3.2 mm bar on its 51.2 mm span. Supports must be span-adjustable: 16:1 as standard, 32:1 or 40:1 for high-strength reinforced composites that would otherwise shear, and up to 60:1 where modulus is the object.

Modulus requires a Type II deflectometer classified to ASTM E83 Class B-2; Class C is accepted for the other reported values, and Type I crosshead-position measurement is also permitted. The fixture is a three-point rig — two cylindrical supports and a central loading nose, 5.0 ±0.1 mm radii unless otherwise specified. For bars 3.2 mm deep or more, the support radius may be raised to 1.6 times the depth and the nose radius to 4 times, so the sides of the nose do not foul the specimen. Supports and nose must be parallel, with the bar centred between them. Testing takes place in the D618 atmosphere; other temperatures apply only where the material specification calls for them.

What goes wrong in practice

Four failures recur. A loading nose of small radius indents a soft or thin bar, so local crush is counted as deflection and modulus reads low. A reinforced laminate run at 16:1 fails in shear between the plies instead of in tension, and the number recorded is not a flexural strength at all. Ductile grades reach the 5.0 % strain cut-off intact, leaving no strength value to report. And a span left at a previous setting, or taken from nominal rather than measured depth, biases every result in the batch.

Related and equivalent standards

ISO 178 is the nearest counterpart and uses the same three-point geometry, but specimen dimensions, span derivation, rate definition and modulus evaluation all differ, so results do not transfer and must not be pooled. Support radius is the detail most often missed when a D790 rig is reused for ISO work. ASTM D6272 covers the same materials in four-point bending; ASTM D7264/D7264M is the route for polymer-matrix composite laminates. ASTM D618, E4 and E83 supply conditioning, force verification and extensometer classification.