Testing standard
ASTM D695
Standard Test Method for Compressive Properties of Rigid Plastics
At a glance
- Test type
- Compression — the specimen is squeezed
- Published by
- ASTM
- Edition
- D695-26
- Material
- Plastics, polymers & films
- Runs on
- Series 7200 and Series 9000
What the test does
A short prism or cylinder of rigid plastic is stood on a flat hardened platen, unclamped and unbonded. The crosshead drives a second platen down onto its opposite face at a constant slow speed, squeezing the specimen along its long axis. Force and shortening are recorded continuously until the specimen fractures, flattens completely, or reaches a preset load or reduction in length.
What it measures, and why it matters
The method reports compressive strength, compressive yield and offset yield, modulus of elasticity from the straight initial portion of the curve, and deformation at a stated load. Compressive strength governs housings, bushes, standoffs and any moulding that carries a bearing load, and it is the figure a designer needs when tensile data alone would be misleading. Modulus feeds stiffness and buckling calculations directly. Strength at a fixed offset is the usual lot-release criterion for structural mouldings, since it is repeatable where ultimate strength of a ductile grade is not.
Specimen
The standard specimen is a right prism or cylinder — most commonly a 12.7 mm square section — proportioned so its slenderness ratio falls between 11:1 and 16:1, which keeps the specimen short enough to crush rather than buckle. The preferred modulus specimen is 50.8 mm long. Specimens are machined or moulded with ends flat within 0.025 mm and parallel to one another in a plane normal to the loading axis; out-of-square ends are the most common reason to reject one before it is ever loaded. Material thinner than 3.2 mm needs the bolt-on supporting jig. Conditioning follows Practice D618 Procedure A — 23 ± 2 °C and 50 ± 10 % RH for at least 40 h — and the test is run at the same temperature and humidity.
What the machine must be capable of
Unreinforced rigid plastics on the standard 12.7 mm square section usually fail between roughly 1 and 20 kN, while reinforced and highly orthotropic laminates on the same section can run past 50 kN, so a 50–100 kN frame covers the range. Force indication must be verified to ASTM E4. Speed is fixed at 1.3 ± 0.3 mm/min; after the yield point it may be raised to 5–6 mm/min for relatively ductile materials, and only where the weighing system responds fast enough to follow the load drop. Modulus and offset yield need a compressometer classified to ASTM E83 Class B-2 or better, reading shortening between two fixed points — crosshead travel includes frame and platen compliance, so it flatters the modulus badly. The load must be applied axially within 1:1000 through a compression tool of the subpress type; without that alignment the prism loads one edge first and the recorded strength drops. Specimens under 3.2 mm thick need the bolt-on supporting jig, its screws finger-tight only, with the compressometer clipped to the specimen edges. The compression tool may be dispensed with for low-modulus material, roughly 700 to 3500 MPa, provided the platen faces stay smooth, flat and parallel. Ambient laboratory conditions apply.
What goes wrong in practice
Euler buckling is the headline failure: a specimen machined too long for its section bows and fails at a fraction of its true strength, and the curve looks plausible. End crushing is the opposite error — the loaded faces mushroom into the platens and the recorded stress is diluted by a shortening that is not uniform. Off-axis loading from an unaligned tool loads one corner first and depresses strength. Laminates delaminate under end load rather than crushing, so the number describes the interface, not the material.
Related and equivalent standards
ISO 604 is the nearest counterpart and covers the same physical test, but specimen proportions and modulus evaluation differ, so results do not transfer without qualification. Practice D618 supplies conditioning, ASTM E4 the force verification, and ASTM E83 the extensometer classification. For continuous-fibre composites the end-loaded D695 geometry is often modified with a supported gauge section, and dedicated composite compression methods are preferred where the failure mode is delamination rather than crushing.
Running ASTM D695 on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Most unreinforced rigid plastics fail somewhere between about 1 and 20 kN on the standard 12.7 mm square prism, while reinforced and highly orthotropic laminates on the same section can run past 50 kN, so a 50–100 kN frame is the usual choice. | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Strain measurement | An extensometer to ASTM E83 Class B-2, gauge length not specified — the method fixes specimen slenderness ratio (11:1 to 16:1) rather than an extensometer gauge length; the preferred modulus specimen is 50.8 mm long | Certified to ASTM E83 and ISO 9513 Class 1 — non-contact video, clip-on and high-elongation |
| Gripping | Flat hardened compression platens or a compression tool (subpress), plus a bolt-on support jig for specimens thinner than 3.2 mm | Our compression anvils, built to the specimen |
| Environment | Ambient laboratory conditions — 23 °C and 50 % RH per D618 Procedure A, tested at the conditioning conditions | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.