ASTM D695 – Compressive Properties of Rigid Plastics

Overview & scope

ASTM D695 is the primary American standard for determining the compressive properties of unreinforced and reinforced rigid plastics, including high-modulus composites, when loaded in compression at relatively low, uniform rates of straining or loading. It is maintained by ASTM Committee D20 on Plastics and is widely cited in material datasheets, OEM specifications, and NABL/ISO 17025-accredited laboratory scopes across India.

The standard reports compressive strength, compressive yield strength, modulus of elasticity in compression, and deformation beyond the yield point for rigid plastics tested between parallel platens under controlled speed and environment.

Tip: ASTM D695 is intended for rigid plastics. Materials that merely flatten under load without fracturing or yielding do not exhibit a compressive strength by this method — report yield or stress at a stated strain instead.

What does it measure?

Compressive strength (MPa), compressive yield strength (MPa), compressive modulus of elasticity (MPa or GPa), and deformation beyond yield. It does not measure tensile or flexural behaviour — see ASTM D638 and ASTM D790 respectively, which together with D695 complete the core mechanical characterisation of a rigid plastic.

Sample materials

Rigid PVC, polycarbonate (PC), nylon (PA6, PA66), POM/acetal, PEEK, epoxy and phenolic thermosets, glass-fibre reinforced compounds, laminates, and high-modulus polymer-matrix composites — including machined stock shapes used in bearings, bushes, and structural components.

India context & equivalents

The international counterpart is ISO 604, which the Bureau of Indian Standards has adopted identically as IS 13360 (Part 5/Sec 8). Indian laboratories serving both export and domestic specifications routinely run ASTM D695 and ISO 604 / IS 13360 methods on the same universal testing machine with the appropriate fixtures and software templates.

Industries & applications

Specimen geometry & dimensions

The standard specimen is a right cylinder or prism whose length is twice its principal width or diameter. Slender column specimens with a higher slenderness ratio are used when compressive modulus or accurate stress–strain data are required, and thin sheet materials are tested as jig-supported specimens.

Tip: Test at least five specimens for isotropic materials and ten (five normal, five parallel to the principal axis) for anisotropic materials — and always state the specimen form in the report, because results from different geometries are not directly comparable.

Specimen preparation

1. Prepare specimens by moulding to size or by machining from sheet, rod, or finished parts with smooth, defect-free surfaces.
2. Machine the loaded end faces flat, parallel to each other, and perpendicular to the long axis — out-of-square ends are the most common cause of scattered results.
3. Inspect for voids, sink marks, machining chatter, and edge defects; discard any specimen with visible flaws.
4. Condition at 23°C ± 2°C and 50% ± 10% RH for a minimum of 40 hours per ASTM D618 unless the material specification states otherwise.
5. Measure width and thickness (or diameter) at several points with a calibrated micrometer and compute the minimum cross-sectional area for stress calculations.

Platens, subpress & support jig

• Hardened parallel platens — flat, polished compression platens with one self-aligning (spherically seated) face keep loading uniform across the specimen ends.
• Compression subpress / cage — a guided two-platen tool that maintains platen parallelism and axial alignment, recommended for small block and cylinder specimens.
• D695 support jig — for materials under 3.2 mm thick, the standard specifies a slotted support jig that restrains the thin specimen against buckling so it fails in true compression.
• Centre the specimen on the platen axis; off-centre loading introduces bending and depresses both strength and modulus.

Instrumentation & strain measurement

• Compressometer or strain measurement is required for modulus. Raw crosshead displacement includes machine, platen, and seating compliance and overstates deformation at low strain.
• Use a clip-on compressometer/extensometer spanning the specimen gauge region, or a non-contact video extensometer tracking marks on the column faces.
• Select a load cell so peak force falls between 10% and 90% of rated capacity — reinforced grades can exceed 100 kN on full-size specimens, so check capacity before testing.
• Sample at ≥ 20 Hz; brittle thermosets can fail abruptly and need fast acquisition to capture peak load.

Test procedure

1. Record temperature, humidity, material identification, and specimen form for the batch.
2. Verify current load cell and strain-device calibration; zero both channels.
3. Measure and log specimen dimensions; mount the specimen centrally between the platens (or in the subpress / support jig for thin materials).
4. Bring the platens into light contact without pre-loading the specimen.
5. Run at a crosshead speed of 1.3 ± 0.3 mm/min and record the full force–deformation curve.
6. Continue loading until yield or fracture, or to the deformation limit given in the governing specification.
7. Reject results from specimens that buckle, broom at the ends, or fail from visible defects, and repeat on fresh specimens.
8. Export compressive strength, yield, and modulus with the curve for traceability.

Calculations & outputs

Example: a 12.7 × 12.7 mm prism has A₀ = 161.3 mm². At a peak force of 14,500 N, compressive strength = 14500 / 161.3 ≈ 90 MPa.

Crosshead speed guidance

• 1.3 ± 0.3 mm/min — the standard crosshead speed for ASTM D695 compression testing.
• As specified — always follow the governing material or OEM specification and document the actual speed in every report.

Tip: Plastics are strain-rate sensitive in compression just as in tension — a result reported without the test speed and specimen geometry cannot be compared across laboratories.

Reporting requirements

• Material identification, grade, lot/batch number, and moulding or machining method.
• Specimen form, individual dimensions, cross-sectional area, and number tested (with any rejected specimens and reasons).
• Conditioning, test temperature and humidity, crosshead speed, fixture type (platens, subpress, or support jig), and strain-measurement device.
• Results with statistics: compressive strength, yield strength, modulus (mean, SD, CV%), and the failure mode observed.

ASTM D695 vs ISO 604 — key differences

Both standards measure compressive properties of plastics but use different specimen geometries and speeds, so results are not directly numerically comparable. ISO 604 is adopted in India as IS 13360 (Part 5/Sec 8). Select the standard cited by your customer or target market.

Tip: If a specification simply says "compression test," confirm ASTM D695 or ISO 604 before machining specimens — the geometries differ and archived data will not be comparable.

Recommended Testometric setup

Model suggestions for compression testing

Specifications vary by configuration; contact FITCO India for a tuned method and accessories aligned to ASTM D695.

FAQs