Plastic compressive strength testing is a fundamental mechanical evaluation that determines how rigid and semi-rigid plastic materials perform under axial compressive loads. This guide explains the complete testing procedure in accordance with internationally recognized standards such as ASTM D695 and ISO 604, covering specimen preparation, equipment setup, test execution, and result calculation. Whether you are a quality engineer, materials scientist, or product designer, understanding these steps ensures accurate and reproducible compressive property data for material qualification and product development.
Technical Overview
Compressive strength testing measures the ability of a plastic material to withstand loads that tend to reduce its size under compression. During the test, a specimen is placed between two parallel compression platens on a universal testing machine, and an axial load is applied at a constant rate until the material yields, deforms to a specified strain, or fractures. The test generates a load-deformation curve from which compressive strength, compressive modulus, and compressive yield strength are derived.
The most widely adopted standards for this test include ASTM D695 (Standard Test Method for Compressive Properties of Rigid Plastics) and ISO 604 (Plastics — Determination of Compressive Properties). In China, the equivalent national standard is GB/T 1041. These standards define specimen geometry, loading rates, platens configuration, and calculation methods to ensure inter-laboratory comparability. Compressive testing is particularly important for plastics used in structural supports, load-bearing housings, fasteners, and packaging components, where compressive failure could compromise product integrity.
Testing Samples
Standard Prismatic Specimens
For ASTM D695, the preferred specimen is a right cylinder or prism measuring 12.7 mm × 12.7 mm × 25.4 mm (0.5 in × 0.5 in × 1 in). For ISO 604, preferred specimens include prisms of 10 mm × 10 mm × 4 mm, or rods and tubes with suitable length-to-diameter ratios. The preferred geometry ensures stable, uniform load distribution and minimizes buckling during compression.
Molded or Machined Specimens
Specimens may be directly injection-molded, compression-molded, or machined from sheets, rods, tubes, or finished parts. Machined specimens must be prepared carefully to avoid introducing residual stresses, overheating, or surface irregularities that could affect results. All surfaces should be flat, parallel, and free from visible defects.
Specimen Conditioning
Prior to testing, specimens should be conditioned in a controlled environment, typically 23 °C ± 2 °C and 50% ± 10% relative humidity for a minimum of 40 hours, in accordance with ISO 291 or ASTM D618. A sufficient number of replicate specimens—normally at least five per material condition—should be prepared to ensure statistical validity.
Testing Items
The compression test provides several key mechanical parameters:
- Compressive Strength: The maximum compressive stress carried by the specimen, calculated as the maximum load divided by the original cross-sectional area, expressed in MPa or psi.
- Compressive Yield Strength: The stress at which the stress-strain curve shows a defined yield point or a specified offset (commonly 0.2% strain offset for materials without a distinct yield point).
- Compressive Modulus: The ratio of stress to strain within the initial linear elastic region of the stress-strain curve, typically measured using an extensometer or between strain points such as 0.05% and 0.25%.
- Compressive Strain at Yield or at Maximum Load: The deformation level corresponding to yield or peak stress, expressed as a percentage.
- Deformation at Break: For brittle materials that fracture under compression, the strain at the moment of specimen failure.
Testing Methods
Step 1: Equipment Preparation and Calibration
Verify that the universal testing machine has been calibrated and is within its calibration interval. Attach polished, parallel compression platens that are flat within the tolerance specified by the standard. Ensure the load cell capacity is appropriate for the expected failure load, ideally so that the measured force falls within 15% to 85% of the cell range for optimal accuracy.
Step 2: Specimen Measurement
Measure the width, thickness, and length of each specimen to the nearest 0.02 mm using a calibrated micrometer or caliper. Record the dimensions at multiple points and use the average cross-sectional area in subsequent calculations. Confirm that specimen end faces are perpendicular to the loading axis within the tolerance allowed by the standard.
Step 3: Test Setup
Place the specimen centrally between the compression platens, ensuring the load is applied along the specimen's principal axis. For slender specimens prone to buckling, use a lateral support jig (an anti-buckling guide) as specified in ASTM D695. If compressive modulus is required, attach a compressometer or extensometer to measure strain directly on the specimen.
Step 4: Loading Procedure
Apply the compressive load at the standard crosshead speed. For ASTM D695, the standard speed is 1.3 mm/min (0.05 in/min) until yield; after yield, the speed may be increased to permit testing to fracture. For ISO 604, speeds of 1 mm/min are typically used for modulus determination, with higher speeds permitted for strength measurement depending on the material behavior. Maintain consistent speed throughout the test to ensure valid comparisons.
Step 5: Data Acquisition
Continuously record load and deformation data throughout the test using the machine's data acquisition system. Observe the specimen for signs of yielding, barreling, cracking, or fracture, and note the failure mode. Terminate the test when the specimen fractures, yields beyond the specified limit, or reaches the required strain level.
Step 6: Calculation of Results
Calculate compressive stress by dividing the recorded load by the original cross-sectional area. Determine compressive strength from the maximum load, compressive yield strength from the yield point or offset method, and compressive modulus from the slope of the linear portion of the stress-strain curve. Average the results from all valid specimens and report the standard deviation.
Step 7: Reporting
The test report should include: material identification and specimen preparation method, conditioning details, specimen dimensions, testing machine and load cell details, crosshead speed, ambient temperature and humidity, individual and average values for all calculated properties, failure mode descriptions, and the specific standard followed.
Applications
Compressive strength data for plastics supports a broad range of engineering and commercial applications:
- Structural components: Load-bearing plastic brackets, spacers, bushings, and insulating supports in construction and machinery.
- Electrical and electronic products: Housings, connectors, terminal blocks, and switch components where sustained compressive contact forces occur.
- Automotive engineering: Clips, seals, vibration mounts, and interior structural parts evaluated for long-term compressive loading.
- Packaging industry: Foams, cushioning materials, and rigid containers assessed for stacking strength and distribution durability.
- Medical devices: Sterilizable plastic components, implantable polymer elements, and device housings requiring verified mechanical performance.
- Material research and development: Comparison of resin formulations, filler content, and processing conditions during new material qualification.
- Quality control: Batch acceptance testing and supplier verification performed by manufacturers and accredited third-party laboratories.
FAQ
What is the difference between ASTM D695 and ISO 604?
ASTM D695 uses a 12.7 × 12.7 × 25.4 mm prismatic specimen with a standard speed of 1.3 mm/min and permits anti-buckling guides for modulus measurement. ISO 604 specifies several preferred specimen geometries and uses strain-rate-based loading speeds. Results obtained under the two standards are not directly interchangeable due to differences in specimen size, speed, and strain measurement methods.
Why do some plastics not show a clear compressive failure point?
Ductile thermoplastics often deform by barreling rather than fracturing under compression, producing a stress-strain curve without a distinct break. For such materials, compressive strength is reported at yield or at a specified strain limit instead of at fracture.
What causes buckling during the test and how can it be prevented?
Buckling occurs when the specimen is too slender relative to its cross-section, or when the ends are not parallel and properly aligned. Using the standard specimen aspect ratio, ensuring flat and parallel end faces, and employing a lateral support fixture as allowed by ASTM D695 all help prevent buckling and ensure valid compressive data.
How many specimens should be tested?
Both ASTM D695 and ISO 604 recommend testing a minimum of five specimens per material condition. Testing more specimens improves statistical confidence, particularly for materials with inherent variability such as fiber-reinforced or foamed plastics.
Which factors most commonly affect the accuracy of results?
Key factors include specimen dimensional accuracy, surface flatness and parallelism, load cell calibration, crosshead speed consistency, ambient conditioning, strain measurement technique, and correct alignment of the specimen within the platens. Strict adherence to the selected standard minimizes these sources of error.