ISO 178

What the test does

A rectangular plastic bar rests freely on two parallel supports — nothing is gripped. A loading edge 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 when the specimen ruptures or reaches 5 % flexural strain, whichever comes first, so a ductile grade may finish without breaking at all.

What it measures, and why it matters

The method reports flexural stress at any point on the curve, flexural strength, flexural strain, and flexural modulus taken between 0.05 % and 0.25 % strain. Modulus is the stiffness figure that feeds part-wall calculations and finite-element models, and it is the number that classifies a grade as rigid above 700 MPa or semi-rigid between 70 and 700 MPa. Flexural strength serves lot release on moulding compounds and grade-to-grade selection. Because the outer fibre sees the highest stress, the test is also sensitive to skin quality, so it is used in moulding investigations.

Specimen

The preferred specimen is a bar 80 mm long, 10 mm wide and 4 mm thick, moulded or machined with flat, parallel faces and edges free of notches and machining marks. The span is set from the thickness at L = 16h, which puts the supports 64 mm apart for the preferred bar; any other thickness moves the span with it. At least five specimens are tested, and more where higher precision is wanted or the material is anisotropic. Any specimen that breaks outside the central third of the span, or that shows a visible void, sink mark or damaged edge, is discarded and replaced. Conditioning and testing both take place in the ISO 291 atmosphere 23/50 — 23 °C and 50 % RH with class 2 tolerances of ±2 °C and ±10 % RH — unless the material standard directs otherwise.

What the machine must be capable of

Force demand is modest and scales with strength: the preferred bar draws roughly 1.7 N for each megapascal of flexural strength, so a soft semi-rigid grade needs a few tens of newtons, an unfilled engineering thermoplastic several hundred, and a short-glass-filled compound over a kilonewton. A 1–5 kN frame covers the range, but the load cell must resolve the low end cleanly rather than merely reach the high end. Force indication must meet ISO 7500-1 Class 1.

Rate is defined as flexural strain rate, not crosshead speed. Method A runs at 1 %/min throughout; Method B runs 1 %/min for the modulus portion then switches to 5 %/min or 50 %/min according to the material's ductility. The frame must hold the tabulated speeds of 1, 2, 5, 10, 20, 50, 100, 200 and 500 mm/min within ±20 % up to 10 mm/min and ±10 % above, with 1 mm/min reserved for specimens 1 to 3.5 mm thick; 2 mm/min is customary for the preferred bar.

Deflection measurement sets the accuracy of modulus. Type IV tests need a direct deflectometer at ISO 9513 Class 1; type II tests accept Class 2, as do type III tests provided machine compliance is corrected for; type I, strength only, needs no deflection instrument.

The fixture is a three-point rig with an adjustable span, a loading edge of 5.0 ±0.2 mm radius, and supports of 2.0 ±0.2 mm radius for specimens 3 mm thick or less and 5.0 ±0.2 mm above that. Supports and loading edge must be parallel within ±0.2 mm across the width; a rig that is out of parallel twists the bar and reports a low, scattered strength.

What goes wrong in practice

Machine-compliance error is the commonest: crosshead travel includes the frame's own flex, and using it as deflection inflates apparent sag and depresses modulus. Ductile grades often reach conventional deflection — 1.5 times thickness, about 3.5 % strain — without breaking, so no strength value exists and reporting one is wrong. A loading edge of the wrong radius indents soft material, adding local crush to the deflection. Moulded skin is oriented, so specimens tested on opposite faces give different strengths.

Related and equivalent standards

ASTM D790 is the nearest counterpart and runs the same three-point geometry, but its specimen dimensions, span ratio, rate derivation and modulus evaluation differ, so results do not transfer and must not be pooled. The support radius is the detail most often got wrong when a D790 rig is reused for ISO work. ISO 291 supplies the conditioning atmosphere, ISO 7500-1 the force-verification class, and ISO 9513 the extensometer classification.