Compressive Strength Testing of Foam Plastics: Standards, Methods and Applications

Compressive strength testing of foam plastics is a fundamental mechanical evaluation used to verify the load-bearing capability of cellular plastic materials such as EPS, XPS, PUR/PIR, EPE and EPP in building insulation, protective packaging and structural applications. Testing is performed to recognized international standards including ISO 844, ASTM D1621, EN 826 and GB/T 8813, and is commonly carried out in accredited third-party laboratories. This article explains the test principle, sample requirements, procedures, calculation methods and typical applications of foam plastic compression testing.

Technical Overview

Foam plastics, also known as cellular plastics, are polymeric materials with a gas-filled cellular structure that combine low weight with thermal insulation, energy absorption and load-bearing functions. Typical rigid foams include expanded polystyrene (EPS), extruded polystyrene (XPS), rigid polyurethane (PUR), polyisocyanurate (PIR) and phenolic foam, while common flexible and semi-rigid foams include expanded polyethylene (EPE), expanded polypropylene (EPP) and flexible polyurethane.

Compressive strength testing quantifies the ability of a foam material to resist crushing loads. According to ISO 844 and GB/T 8813, if the specimen yields at a strain below 10%, the compressive strength is the maximum stress carried up to the yield point; if no yield occurs before 10% strain, the compressive stress at 10% deformation is reported instead. The test principle is straightforward: a conditioned specimen is compressed uniaxially between two parallel rigid platens at a constant crosshead rate while the force–displacement curve is recorded and converted into a stress–strain curve.

• ISO 844 – Cellular plastics, determination of compression properties
• ASTM D1621 – Standard Test Method for Compressive Properties of Rigid Cellular Plastics
• GB/T 8813 – Determination of compression properties of rigid cellular plastics (technically aligned with ISO 844)
• EN 826 – Determination of compression behaviour of thermal insulating products for building applications
• Product standards such as EN 13163 (EPS), EN 13164 (XPS), EN 13165 (PUR), GB/T 10801.1 and GB/T 10801.2, which specify declared compressive stress levels

Testing Samples

Compression testing can be performed on virtually all rigid, semi-rigid and flexible foamed plastic products, from flat insulation boards to molded packaging components. Specimen selection should reflect the loading direction experienced in actual service.

Rigid Insulation Boards (EPS, XPS, PUR/PIR)

Specimens are cut from finished boards with facings removed, and are tested perpendicular to the board face because this is the direction in which floor, roof and wall systems are loaded. Depending on the product grade, declared compressive stress levels for load-bearing insulation typically range from 100 kPa to 700 kPa. For polyurethane foams, which are anisotropic, the rise direction (perpendicular or parallel to the foaming direction) must be identified and reported.

Molded and Shaped Foam Products

Molded EPS packaging corners, end caps, boxes, container linings and the cores of structural sandwich panels are sampled from the thickest functional zone of the part. Where the product is manufactured with integral skins, the test may be performed either with or without the skin, provided the condition is clearly documented.

Flexible and Semi-Rigid Foams

EPE, EPP, crosslinked PE and flexible polyurethane foams used for cushioning and seating are evaluated for compressive stress at specified deformation. For applications where recovery after loading is critical, compression testing is frequently supplemented with compression set testing in accordance with ISO 1856.

Specimen Preparation and Conditioning

ISO 844 specifies a preferred specimen of 50 mm × 50 mm × 50 mm; for products thinner than 20 mm, layers may be stacked without adhesive to achieve adequate test thickness, and the lateral dimensions shall not be smaller than the thickness. ASTM D1621 uses a standard specimen of 50 mm × 50 mm × 25 mm. At least five specimens should be tested for each condition. Specimens are conditioned for a minimum of 16 hours at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291 or ASTM D618. Because compressive performance correlates strongly with density, apparent density is usually determined in parallel per ISO 845.

Testing Items

The core deliverables of a compression test program on foam plastics typically include:

• Compressive strength (yield-based), σm, for materials that yield below 10% strain
• Compressive stress at 10% deformation, σ10, for materials that do not yield before 10% strain
• Compressive yield stress and yield strain
• Compressive modulus determined from the initial linear region of the stress–strain curve
• Complete force–displacement and stress–strain curves for design and analysis purposes
• Related test items: compressive creep under sustained load (EN 1606 / ISO 7616), compression set (ISO 1856) and apparent density (ISO 845)

Testing Methods

Apparatus and Calibration

A universal testing machine fitted with flat, rigid and parallel compression platens is used, ideally with a self-aligning (spherical seat) platen to compensate for slight specimen non-parallelism. The force measuring system is verified in accordance with ISO 7500-1 or ASTM E4 to class 1 accuracy or better, and crosshead displacement is monitored with calibrated instrumentation. Machine compliance correction may be applied when determining modulus values.

Specimen Geometry and Orientation

Specimen length, width and thickness are measured to 0.5 mm or better in accordance with ISO 1923, taking measurements at the center and near the corners and using the mean values to calculate the initial cross-sectional area A0. The loading direction, especially for polyurethane foams and sandwich panel cores, is recorded together with the specimen identification.

Test Procedure

• Center the conditioned specimen between the platens, ensuring that its axes are aligned with the loading axis
• Apply a small preload to guarantee full contact between platen and specimen
• Compress the specimen at a constant crosshead speed equal to 10% of the initial thickness per minute, as required by both ISO 844 and ASTM D1621
• Continuously record force and deformation until at least 10% compressive strain is reached, or further if the yield behavior must be fully characterized
• Identify the yield point as the first point on the curve where the slope reaches zero, or where strain increases without a corresponding increase in stress

Calculation and Expression of Results

Compressive stress is calculated as σ = F / A0, where F is the recorded force and A0 is the initial cross-sectional area, expressed in kilopascals (kPa). If the maximum force Fm occurs at a yield strain below 10%, the compressive strength σm = Fm / A0; otherwise the compressive stress at 10% deformation σ10 is reported. ISO 844 reports the median of the individual specimen values, while ASTM D1621 reports the mean, and both require the individual values, the loading direction and any deviations to be documented. Testing should be performed in the standard laboratory atmosphere because cellular plastics are sensitive to temperature and strain rate.

Applications

• Building and construction: verification of declared compressive stress classes for EPS, XPS and PUR/PIR insulation boards used under floor screeds, on flat roofs and in load-bearing wall systems
• External thermal insulation composite systems (ETICS): quality control of insulation boards and conformity assessment against EN 13163 and EN 13164 requirements
• Structural sandwich panels: evaluation of PUR/PIR and mineral foam cores loaded perpendicular to the facings in cold-store, industrial and modular buildings
• Protective packaging and logistics: characterization of EPS, EPE and EPP cushioning materials for the design of transport packaging and drop protection
• Civil engineering: testing of expansion joint fillers, void forms and underslab insulation boards used in road, bridge and railway construction
• Cold chain, HVAC, marine and flotation equipment: confirmation of mechanical reliability of foam components exposed to static and dynamic loads

FAQ

What is the difference between compressive strength and compressive stress at 10% deformation?

Compressive strength applies to foams that yield, fracture or plateau before reaching 10% strain; it is the maximum stress carried up to that point. If the material continues to harden without yielding up to 10% strain, the stress at exactly 10% deformation is reported instead. Insulation product standards designate these as CS(Y) and CS(10) classes respectively.

How do ISO 844, ASTM D1621 and GB/T 8813 compare?

All three standards share the same principle and the same compression rate of 10% of initial thickness per minute. GB/T 8813 is technically aligned with ISO 844. The main practical difference is specimen geometry: ISO 844 and GB/T 8813 favor 50 mm cubes, while ASTM D1621 specifies 50 mm × 50 mm × 25 mm specimens; result statistics also differ, with ISO 844 using the median and ASTM D1621 the mean.

Why is testing direction important for foam plastics?

Many foams, particularly poured and laminated polyurethane, are anisotropic because cells elongate in the rise direction. Compressive properties measured parallel to the rise direction can differ substantially from those measured perpendicular to it. Standards therefore require the test direction to be identified, and sandwich panel cores are tested perpendicular to the facings, matching service loading.

How does compressive creep differ from compressive strength?

Compressive strength characterizes short-term resistance measured in minutes, whereas compressive creep, tested to EN 1606 or ISO 7616, measures the long-term reduction in thickness under a sustained load over years-equivalent conditions. Insulation used under permanent loads must satisfy both requirements, with creep results declared as an Xt value.

When is third-party compressive strength testing required?

Independent verification by accredited third-party laboratories is required for product certification, CE marking under EN product standards, factory production control audits, regulatory compliance and dispute resolution. Manufacturers also commission compression testing during formulation development, density optimization and incoming quality inspection.