ASTM D695 Compression Testing Machine: Rigid Plastic Compression Test Method, Specimen Preparation, and Compressive Properties

1. Introduction to ASTM D695

Compression testing is an important method for evaluating the mechanical performance of rigid plastic materials. Many plastic components are subjected to compressive loads during normal use, including housings, supports, structural parts, mechanical components, and engineering products. ASTM D695 is a standard test method for determining the compressive properties of rigid plastics. It covers unreinforced and reinforced rigid plastics, including high-modulus composites, when subjected to compression under relatively low uniform rates of straining or loading. During a typical ASTM D695 test, a prepared specimen is placed between two compression platens of a Universal Testing Machine. The machine applies a controlled compressive force, causing the specimen to shorten or deform. The load and deformation are recorded throughout the test. The resulting data can be used to determine compressive modulus, compressive yield stress, deformation beyond the yield point, and compressive strength when the material exhibits an appropriate failure condition. Unlike tensile testing, where a specimen is pulled apart, compression testing evaluates how a material responds when force is applied along its length or thickness to shorten it. Rigid plastics may respond differently under compression depending on their polymer structure, reinforcement, geometry, and manufacturing history. Some materials show a distinct yield point, while others continue deforming without a clear fracture. A suitable Universal Testing Machine is essential for obtaining reliable results. It should provide accurate force measurement, stable compression movement, suitable platens, and appropriate deformation measurement.

2. Scope and Purpose of the Standard

ASTM D695 covers the determination of compressive properties of unreinforced and reinforced rigid plastics, including high-modulus composites. The method uses standard-shaped specimens subjected to compressive loading at relatively low uniform rates of straining or loading. The standard is used to evaluate the mechanical response of rigid plastic materials under compression. It supports research and development, quality control, material acceptance, and specification testing. The main purposes of ASTM D695 include:

• Determining compressive modulus and stiffness.
• Measuring compressive yield stress when a yield point is present.
• Evaluating deformation beyond the yield point.
• Determining compressive strength when the material exhibits a suitable failure condition.
• Comparing the compression performance of different rigid plastics.
• Monitoring material quality during manufacturing.
• Supporting material selection and product development.

The standard recognizes that not all plastics fail in the same way under compression. Some materials fracture or shatter, producing a relatively clear compressive strength value. Others flatten or deform continuously without a distinct fracture. For materials that continue to deform without a well-defined failure point, the reported compressive strength may depend on the specified deformation or failure criterion. In these cases, the test result must be interpreted carefully. ASTM D695 also requires attention to the applicable material specification. If that specification provides different requirements for specimen preparation, conditioning, dimensions, or testing parameters, those requirements take precedence.

3. Principle of Compression Testing

The principle of ASTM D695 is to apply a controlled compressive load to a standardized rigid plastic specimen while measuring the applied force and deformation. The specimen is positioned between two compression platens. The upper platen moves toward the lower platen, applying force along the specimen's loading axis. The Universal Testing Machine records the applied force and the corresponding deformation. These measurements are used to calculate compressive stress and strain. Engineering compressive stress is calculated as:

Compressive Stress = Applied Compressive Force / Original Cross-sectional Area

Engineering compressive strain is calculated as:

Compressive Strain = Change in Specimen Length / Original Specimen Length

The original cross-sectional area and original specimen length are used as the reference dimensions for the corresponding calculations. When the compressive load increases, the specimen may initially show elastic deformation. As the load continues to increase, it may reach a yield point, undergo permanent deformation, buckle, crack, or continue flattening. The failure mode depends on the material, specimen geometry, loading alignment, and test conditions. For a reliable test, the compressive force should be applied along the intended specimen axis. Misalignment can introduce bending stresses and create nonuniform deformation.

4. Materials Covered by ASTM D695

ASTM D695 is used for evaluating the compressive properties of rigid plastic materials, including unreinforced and reinforced plastics and certain high-modulus composites.

4.1 ABS

ABS is used in appliance housings, automotive components, electronic enclosures, and industrial products. Compression testing can help evaluate its resistance to compressive deformation and its stiffness under loading.

4.2 Polycarbonate (PC)

Polycarbonate is used in applications requiring toughness and dimensional stability. Compressive testing helps characterize its deformation response and mechanical behavior under compressive loads.

4.3 Polyamide (PA)

Polyamide materials are used in gears, bearings, fasteners, mechanical components, and automotive parts. Their properties may be influenced by moisture content and conditioning.

4.4 Polyoxymethylene (POM)

POM is widely used in precision mechanical components, gears, sliding parts, and industrial products. Compression testing helps evaluate its resistance to deformation under compressive loading.

4.5 Rigid PVC

Rigid PVC is used in pipes, profiles, building products, and industrial components. Compressive testing can provide information about stiffness and deformation behavior.

4.6 Fiber-Reinforced Plastics

Reinforced plastic materials may be evaluated under compression when their reinforcement structure and specimen configuration are suitable for ASTM D695. The test response depends on fiber orientation, reinforcement content, matrix properties, and the way the specimen is manufactured. For resin-matrix composites with oriented continuous, discontinuous, or cross-ply reinforcements, other specialized compression methods may be more appropriate depending on the material system and test objective. The applicable composite specification should be reviewed before selecting ASTM D695.

4.7 High-Modulus Composites

ASTM D695 includes high-modulus composites within its scope up to the stated modulus limit of the method. The specimen configuration and material characteristics should be checked against the current standard before testing.

5. Specimen Preparation and Geometry

Specimen preparation is essential for reliable compression test results. A specimen with incorrect dimensions, uneven end surfaces, or poor alignment may produce misleading data. ASTM D695 uses standard-shaped specimens. The specimen geometry must be selected according to the applicable standard and material specification. Important specimen considerations include:

• Specimen length: The original length is used when calculating compressive strain.
• Cross-sectional area: Accurate width, thickness, or diameter measurements are required to calculate compressive stress.
• End surface condition: The specimen ends should be prepared appropriately to provide stable contact with the compression platens.
• Parallelism: The end surfaces should be sufficiently parallel to support uniform loading.
• Surface quality: Cracks, machining defects, and surface damage may influence the compression response.
• Specimen orientation: The cutting direction may affect the behavior of molded, extruded, and reinforced plastics.

The specimen's length-to-width or length-to-diameter relationship is important because slender specimens may be more susceptible to buckling. If a specimen is too slender for the selected configuration, it may bend or buckle rather than undergo the intended uniform compression. The specimen dimensions and preparation method should be selected using the current ASTM D695 edition and the relevant material specification. Laboratories should not assume that one specimen geometry is suitable for every rigid plastic.

6. Universal Testing Machine Requirements

A Universal Testing Machine for ASTM D695 must be capable of applying compressive force in a controlled and measurable manner.

6.1 Suitable Load Capacity

The machine capacity should be selected according to the material strength and specimen cross-sectional area. Common machine configurations for plastic compression testing may include 5 kN, 10 kN, 20 kN, 50 kN, and higher capacities. These are examples of available equipment options rather than fixed ASTM D695 requirements. The expected maximum compressive force should be estimated before selecting the machine.

6.2 Accurate Load Cell

The load cell measures the compressive force applied to the specimen. It should be suitable for the expected force range and maintained through an appropriate calibration and verification program.

6.3 Stable Compression Movement

The machine should provide smooth and controlled crosshead movement. Stable movement helps maintain the selected loading conditions and improves repeatability.

6.4 Rigid Load Frame

The load frame must be suitable for the expected compression force. Excessive structural deformation may influence displacement measurement and test interpretation.

6.5 Compression Platens

The machine should be equipped with suitable upper and lower compression platens. Their dimensions, alignment, and contact surfaces should be appropriate for the specimen.

6.6 Deformation Measurement

The system should provide a suitable way to measure specimen deformation. Depending on the test requirements, this may involve machine displacement or a separate deformation measurement device.

6.7 Data Acquisition and Software

The control software should record force and deformation continuously and support the calculations required for compressive properties.

7. Compression Platens and Accessories

Compression platens and related accessories play an important role in ASTM D695 testing. They transfer the applied force from the testing machine to the specimen.

7.1 Compression Platens

The upper and lower platens provide the contact surfaces used to compress the specimen. They should be sufficiently rigid and suitable for the expected load. The platen surfaces must be maintained in suitable condition to provide stable contact.

7.2 Platen Alignment

The compression platens must be aligned so that the applied force acts along the intended loading axis. Poor alignment may create uneven contact and introduce bending stresses.

7.3 Spherical Seating

Some compression test configurations use a spherical seating arrangement to accommodate small alignment variations. Whether this is appropriate depends on the specimen and applicable test requirements. It should not be assumed that every ASTM D695 configuration requires a spherical seat. The fixture arrangement must follow the standard and material specification.

7.4 Compression Fixtures for Different Specimens

Different specimen geometries may require different fixtures or contact arrangements. The selected accessories should support the specimen without causing unwanted instability.

7.5 Extensometers and Deformation Measurement

An appropriate deformation measurement system may be used when accurate strain or modulus measurement is required. The measurement method should account for the difference between machine movement and actual specimen deformation.

7.6 Safety Protection

Some rigid plastics may crack or fracture suddenly during compression. A suitable safety shield can help protect the operator from fragments. The laboratory should also establish safe procedures for handling specimens and operating the machine.

8. ASTM D695 Test Procedure

The following workflow describes a typical rigid plastic compression test. Exact specimen dimensions, loading rate, conditioning, and calculation requirements must follow the current ASTM D695 edition and the applicable material specification.

Step 1: Identify the Material

Record the polymer type, material grade, manufacturer, batch number, and relevant product specification.

Step 2: Select the Specimen Configuration

Determine the appropriate specimen shape and dimensions according to ASTM D695 and the material specification.

Step 3: Prepare the Specimen

Prepare the specimen with the required dimensions and suitable end surfaces. Check for cracks, machining damage, and visible defects.

Step 4: Condition the Specimen

Condition the specimen when required. Record the environmental conditions and conditioning history.

Step 5: Measure Initial Dimensions

Measure the original cross-sectional dimensions and specimen length. Record the values needed for stress and strain calculations.

Step 6: Configure the Universal Testing Machine

Select a suitable load cell and install the compression platens. Confirm that the machine capacity is appropriate for the expected maximum force.

Step 7: Position the Specimen

Place the specimen centrally on the lower platen. Ensure that its loading axis is aligned with the machine axis.

Step 8: Check Platen Contact

Bring the upper platen into contact with the specimen using the appropriate setup procedure. Avoid introducing unintended preload or misalignment.

Step 9: Configure Test Parameters

Set the loading rate or strain rate according to the applicable ASTM D695 procedure and material specification.

Step 10: Apply Compressive Load

Start the test and apply compressive force under the selected conditions. The machine records force and deformation continuously.

Step 11: Continue to the Required Endpoint

Continue loading until the applicable endpoint is reached. Depending on the material, this may involve yielding, a defined deformation level, fracture, or another specified condition.

Step 12: Calculate the Compressive Properties

Use the recorded data and original specimen dimensions to calculate compressive stress, strain, modulus, yield stress, or compressive strength as applicable.

Step 13: Prepare the Test Report

Record material identification, specimen dimensions, conditioning, machine configuration, test conditions, deformation measurement method, and test results.

9. Compressive Properties and Calculations

ASTM D695 testing can provide several important mechanical properties of rigid plastics.

9.1 Compressive Stress

Compressive stress is calculated by dividing the applied compressive force by the original cross-sectional area of the specimen.

Compressive Stress = Compressive Force / Original Cross-sectional Area

The original area is used as the reference area for engineering compressive stress.

9.2 Compressive Strain

Compressive strain describes the relative shortening of the specimen during loading.

Compressive Strain = Change in Length / Original Length

The strain is dimensionless and may be expressed as a percentage.

9.3 Compressive Modulus

Compressive modulus describes the material's resistance to deformation under compressive loading. It is determined from the stress-strain relationship within the specified measurement region. A higher compressive modulus generally indicates greater stiffness under comparable test conditions.

9.4 Compressive Yield Stress

Some rigid plastics exhibit a recognizable yield point during compression. Compressive yield stress represents the stress associated with the specified yielding behavior. Materials with limited ductility may not show a clearly defined yield point.

9.5 Deformation Beyond Yield

The test can also provide information about how the specimen continues deforming after reaching the yield point. This behavior can be important when evaluating the ability of a material to withstand sustained compressive deformation.

9.6 Compressive Strength

Compressive strength is determined from the relevant maximum or failure load according to the applicable test conditions. For brittle materials that fracture suddenly, compressive strength may have a relatively clear value. For ductile plastics that continue flattening without a distinct fracture, the meaning of compressive strength depends on the selected deformation or failure criterion.

9.7 Permanent Deformation

Some plastics undergo permanent deformation after the applied load is removed. The amount of permanent deformation can provide additional information about material behavior, although it is not automatically a standard reported property in every ASTM D695 test.

10. Understanding Plastic Compression Behavior

The compression response of rigid plastics can vary significantly depending on the polymer type, reinforcement, specimen geometry, and loading conditions.

10.1 Initial Elastic Deformation

At the beginning of compression, many rigid plastics show a relatively linear relationship between stress and strain. In this region, deformation is mainly elastic. If the load is removed before the material exceeds its elastic limit, much of the deformation may recover.

10.2 Yielding

As the applied stress increases, some materials reach a yield point. Beyond this point, permanent deformation begins to develop. The exact shape of the yield response varies between polymer types and material formulations.

10.3 Plastic Deformation

After yielding, the specimen may continue to shorten under increasing compressive load. The amount of deformation depends on the material's ductility and internal structure.

10.4 Brittle Fracture

Some rigid plastics fail suddenly under compression. They may crack, split, or shatter when the stress exceeds their resistance. For such materials, the maximum force before failure can provide a useful compressive strength value.

10.5 Continuous Flattening

Other plastics may not fracture under the test conditions. Instead, they may continue deforming until they become a relatively flat shape. In this situation, a single compressive strength value may not fully describe the material's behavior. The deformation criterion and test objective must be clearly defined.

10.6 Buckling and Instability

If the specimen is too slender or poorly aligned, it may bend or buckle during compression. This can produce a result that does not represent the intended material compression behavior. Specimen geometry and alignment are therefore critical.

11. Universal Testing Machine for ASTM D695 Testing

A Universal Testing Machine equipped with suitable compression platens can be used to evaluate the compressive properties of rigid plastic materials. Electronic Universal Testing Machines are commonly used for laboratory compression testing because they provide controlled crosshead movement and digital data acquisition. A suitable ASTM D695 configuration may include:

• A load cell selected for the expected compressive force.
• Upper and lower compression platens.
• A suitable deformation measurement system.
• Control software for force and displacement recording.
• Calculation and reporting functions for compressive properties.
• Appropriate safety protection for specimens that may fracture.

For laboratories that also perform tensile and bending tests, a multi-purpose Universal Testing Machine can provide a flexible solution. The machine can be equipped with different grips and fixtures according to the test method. The required machine capacity should be determined by the expected force, material strength, and specimen dimensions. A higher capacity is not automatically better if the selected load cell is poorly matched to the actual test range.

12. Factors Affecting Test Accuracy

Several factors can influence ASTM D695 test results. Controlling these factors helps improve consistency and repeatability.

12.1 Specimen Dimensions

Incorrect specimen dimensions affect the calculated cross-sectional area and original length. These errors influence stress and strain calculations.

12.2 End Surface Quality

Uneven or damaged specimen ends can create nonuniform contact with the compression platens. This may cause localized stress and inconsistent deformation.

12.3 Platen Alignment

Misaligned platens may introduce bending forces in addition to compression. The specimen should be loaded along the intended axis.

12.4 Specimen Slenderness

A specimen that is too slender may buckle rather than undergo uniform compression. The geometry must follow the applicable requirements.

12.5 Loading Rate

Plastic materials may show rate-dependent mechanical behavior. The loading rate must be selected and controlled according to the standard.

12.6 Temperature and Moisture

Environmental conditions can influence polymer stiffness, strength, and deformation. Consistent specimen conditioning is important when comparing test results.

12.7 Load Cell Selection

The load cell must be suitable for the expected force range. An unsuitable range may reduce measurement quality.

12.8 Deformation Measurement

Crosshead movement may include deformation from the machine frame, platens, and other components. When precise specimen strain is required, an appropriate deformation measurement system should be considered.

12.9 Machine Verification

The force measurement system and relevant measuring devices should be maintained and verified according to applicable requirements.

12.10 Operator Procedure

Inconsistent specimen positioning, incorrect test settings, or incomplete recording of test conditions can reduce repeatability. Standardized operating procedures help minimize these differences.

13. Industrial Applications

ASTM D695 compression testing is used in industries where rigid plastic materials must withstand compressive forces.

• Automotive manufacturing: Evaluating rigid plastic supports, housings, brackets, and mechanical components.
• Electrical and electronics: Testing plastic enclosures, insulating parts, and structural components.
• Construction: Evaluating rigid plastic profiles, panels, pipes, and building products.
• Machinery manufacturing: Testing polymer gears, bearings, spacers, and mechanical parts.
• Consumer products: Evaluating plastic components used in household appliances and everyday products.
• Industrial equipment: Studying the compressive behavior of plastic supports, protective components, and load-bearing parts.
• Composite manufacturing: Evaluating suitable reinforced plastic materials under compression.
• Research and development: Comparing polymer formulations, additives, reinforcement systems, and processing conditions.

Compression testing provides useful information for material selection and quality control. However, the results should be interpreted in relation to the actual loading conditions of the final product. For applications involving long-term compressive loads, impact, cyclic loading, or elevated temperatures, additional tests may be required. A short-duration compression test does not automatically predict creep or fatigue performance.

14. How to Select a Compression Testing Machine

Selecting a suitable Universal Testing Machine for ASTM D695 requires consideration of the expected compressive force, specimen geometry, measurement requirements, and future laboratory needs.

14.1 Estimate the Expected Maximum Force

Calculate the approximate maximum force using the expected compressive stress and specimen cross-sectional area. Choose a machine capacity suitable for the expected load.

14.2 Select an Appropriate Load Cell

The load cell should match the expected force range. Laboratories testing different plastic grades may require different measurement ranges.

14.3 Check Compression Platen Dimensions

The platens must provide suitable contact with the specimen and withstand the expected force. Their dimensions should be compatible with the specimen configuration.

14.4 Evaluate Alignment Requirements

Check the machine and fixture arrangement to ensure that the specimen can be loaded along the intended axis.

14.5 Review Deformation Measurement

Determine whether crosshead displacement is sufficient for the laboratory's objectives or whether a separate deformation measurement system is necessary.

14.6 Check Software Functions

The software should support force and displacement acquisition, stress-strain calculations, and test report generation.

14.7 Consider Multi-Function Testing

If the laboratory also performs tensile and bending tests, a multi-purpose Universal Testing Machine may provide better equipment utilization. Different grips and fixtures can be installed for different testing applications.

14.8 Evaluate Future Capacity Requirements

Consider whether future materials may require higher forces or different specimen configurations. Selecting a suitable machine platform can reduce the need for equipment replacement.

14.9 Confirm ASTM D695 Suitability

Before purchasing, verify that the complete configuration meets the applicable ASTM D695 requirements. Consider the machine, load cell, compression platens, measurement system, and software together.

15. Frequently Asked Questions

Q1: What is ASTM D695 used for?

ASTM D695 is used to determine the compressive properties of rigid plastics, including unreinforced and reinforced materials and certain high-modulus composites.

Q2: Which properties can ASTM D695 measure?

The method can provide compressive modulus, yield stress, deformation beyond yield, and compressive strength when the material exhibits an appropriate failure condition.

Q3: Which Universal Testing Machine capacity is suitable for ASTM D695?

The required capacity depends on material strength and specimen cross-sectional area. Common equipment configurations include 5 kN, 10 kN, 20 kN, and 50 kN systems, but the correct capacity must be calculated for the intended application.

Q4: What is the difference between tensile and compression testing?

Tensile testing pulls a specimen apart, while compression testing applies force to shorten the specimen. The two methods evaluate different loading behaviors and may produce different mechanical property values.

Q5: Can ASTM D695 test reinforced plastics?

Yes. The method covers unreinforced and reinforced rigid plastics, including certain high-modulus composites. Some specialized continuous-fiber composite systems may require other compression methods.

Q6: Why is specimen alignment important?

Misalignment can introduce bending stress and uneven contact. This may cause premature failure or inaccurate compressive property measurements.

Q7: Why do some plastics flatten instead of breaking?

Some ductile plastics continue to deform under compression without a distinct fracture. In such cases, compressive strength may depend on the specified deformation or failure criterion.

Q8: Are compression platens necessary for ASTM D695?

Yes. The specimen must be loaded through a suitable compression arrangement. The platen configuration and alignment should be appropriate for the specimen and test requirements.

Q9: Can one Universal Testing Machine perform tensile and compression tests?

Yes. A suitably configured Universal Testing Machine can perform both tests by using compatible grips, compression platens, load cells, and software settings.

Q10: Is ASTM D695 suitable for long-term compression testing?

ASTM D695 is a standard compression test method, but it does not by itself replace long-term creep testing. Additional methods may be necessary when evaluating deformation under sustained loads.

16. Conclusion

ASTM D695 is an important standard for evaluating the compressive properties of rigid plastic materials. It provides a controlled method for measuring how unreinforced and reinforced plastics respond when subjected to compressive loading. A properly configured Universal Testing Machine is essential for reliable results. The testing system should provide suitable force capacity, accurate load measurement, stable compression movement, appropriate platens, and reliable deformation measurement. Specimen geometry, end surface quality, platen alignment, loading rate, and conditioning must also be controlled. By selecting the correct specimen configuration and machine accessories, laboratories can use ASTM D695 testing for quality control, material comparison, research, and product development. Always consult the current official ASTM D695 edition and the relevant material specification before conducting tests or issuing formal test reports.