ASTM D638 Tensile Testing Machine: Test Method, Specimen Types, Procedure, and Material Properties

ASTM D638 Tensile Testing Machine: Test Method, Specimen Types, Procedure, and Material Properties

1. Introduction to ASTM D638 Tensile Testing

Plastic materials are widely used in automotive manufacturing, electronics, packaging, construction, medical devices, consumer products, and industrial equipment. Their mechanical properties directly affect product strength, flexibility, durability, and service life. Manufacturers therefore need reliable testing methods to evaluate how plastic materials behave when subjected to tensile forces. ASTM D638 is one of the most widely recognized test methods for determining the tensile properties of plastics. It provides a standardized procedure for evaluating how a plastic specimen responds when it is pulled under controlled conditions until a specified deformation or failure occurs. A Universal Testing Machine is commonly used to perform ASTM D638 tensile tests. The machine applies a controlled tensile force to a standardized plastic specimen while measuring the applied load and specimen deformation. The resulting data can be used to determine tensile strength, tensile modulus, elongation, and other mechanical characteristics. For plastic manufacturers, material suppliers, quality-control laboratories, and research institutions, ASTM D638 testing provides useful information for material comparison, product development, incoming material inspection, and production quality control. However, obtaining reliable results requires more than simply pulling a plastic specimen until it breaks. The specimen geometry, testing speed, gripping system, load cell, extensometer, environmental conditions, and data-processing method must all be considered. This article explains ASTM D638 in detail, including its testing principles, specimen types, equipment requirements, test procedures, mechanical properties, and practical considerations when selecting a Universal Testing Machine for plastic tensile testing.

2. What Is ASTM D638?

ASTM D638 is a standard test method published by ASTM International for determining the tensile properties of plastics. It is used to evaluate the behavior of plastic materials under tensile loading. During the test, a standardized specimen is held between two grips of a Universal Testing Machine. One grip remains fixed while the other moves at a controlled rate, stretching the specimen along its longitudinal axis. As the specimen is stretched, the machine continuously records the applied force and the corresponding deformation. The test may continue until the specimen breaks or until the required measurement condition is reached. The collected data can be used to determine several important properties, including: Tensile strength. Tensile strength at yield. Tensile strength at break. Tensile modulus of elasticity. Elongation at yield. Elongation at break. Stress-strain behavior. ASTM D638 is primarily intended for plastic materials in the form of standard test specimens. The method is commonly applied to rigid and semi-rigid plastics, although the suitability of a particular specimen and procedure depends on the material and its characteristics. It is important to distinguish ASTM D638 from other plastic tensile testing methods. For example, ASTM D882 is generally used for thin plastic sheeting and films, while ASTM D638 is commonly associated with molded or prepared plastic specimens having the dimensions required by the method. When selecting a testing procedure, laboratories should consult the current official edition of ASTM D638 to confirm the applicable specimen requirements, conditioning procedures, testing speed, and calculation methods.

2.1 Basic Principle of the ASTM D638 Test

The basic principle is to apply a tensile force to a plastic specimen at a controlled rate and measure the specimen's response. The machine records force and deformation throughout the test. Force is used to calculate tensile stress, while specimen deformation is used to calculate strain. The relationship between stress and strain provides information about the mechanical behavior of the material. For example, a rigid plastic may exhibit a relatively steep initial stress-strain slope, indicating high stiffness. A flexible plastic may show greater deformation under a similar level of stress. Some plastic materials display a clear yield point, while others deform continuously without a distinct yield point. Certain materials may stretch significantly before breaking, while brittle plastics may fracture at relatively low elongation. The test therefore provides more information than a single maximum-force value. It helps engineers understand the overall tensile behavior of a plastic material.

2.2 What Does ASTM D638 Evaluate?

ASTM D638 can be used to evaluate tensile properties that are important for engineering design and material selection. For example, tensile strength helps determine the maximum tensile stress a material can withstand under the specified test conditions. Tensile modulus indicates resistance to elastic deformation. Elongation provides information about how much a specimen can stretch before reaching a defined point. These properties are useful when comparing different plastic grades, evaluating formulation changes, and checking whether a material meets internal or customer specifications.

3. Why ASTM D638 Is Important for Plastic Manufacturers

Plastic materials are available in many formulations and grades. Even plastics with the same general chemical classification may have different mechanical properties because of differences in molecular structure, additives, fillers, reinforcement, processing conditions, and manufacturing methods. ASTM D638 provides a standardized approach for evaluating tensile behavior, helping laboratories generate comparable test data.

3.1 Material Quality Control

Plastic manufacturers need to confirm that their materials maintain consistent mechanical performance during production. A Universal Testing Machine can be used to test samples from different production batches. By comparing tensile strength, elongation, and modulus, quality-control personnel can identify changes in material performance. For example, if a new production batch shows significantly lower tensile strength than previous batches, further investigation may be necessary. Potential causes could include changes in raw material composition, processing temperature, moisture content, molding conditions, or material degradation.

3.2 Plastic Material Selection

Engineers often compare several plastic materials before choosing one for a specific product. A component used in automotive manufacturing may require high tensile strength and dimensional stability. A packaging component may require flexibility and sufficient elongation. An electronic housing may need a suitable balance between stiffness and other performance requirements. ASTM D638 test results provide useful tensile-property information for this comparison. However, tensile testing should not be treated as a complete evaluation of plastic performance. Other properties, such as impact resistance, hardness, heat resistance, creep, and environmental durability, may also need to be considered.

3.3 Product Development and Research

During research and development, manufacturers may modify polymer formulations by changing additives, fillers, reinforcing fibers, or processing conditions. ASTM D638 testing helps researchers evaluate whether these modifications improve or reduce tensile performance. By testing multiple specimens under consistent conditions, engineers can compare different formulations and study how changes in material composition affect mechanical behavior.

3.4 Supplier Evaluation

Plastic product manufacturers frequently purchase raw materials from different suppliers. Standardized tensile testing helps purchasing and quality-control departments compare materials from different sources. It can also provide supporting data when reviewing supplier specifications or investigating material-quality complaints.

3.5 Product Design and Engineering

Tensile properties are important when designing plastic components that experience pulling forces during normal use. Examples include: Plastic clips and fastening components. Industrial plastic connectors. Automotive interior components. Plastic housings and structural parts. Injection-molded components. Engineering plastic products. ASTM D638 data can support material selection and preliminary design evaluation. Engineers should still consider safety factors, loading conditions, temperature, aging, and other service-related factors.

4. Materials Covered by ASTM D638

ASTM D638 is used for evaluating the tensile properties of many plastic materials. The actual specimen preparation and testing conditions depend on the material and the purpose of the evaluation.

4.1 Thermoplastics

Thermoplastics soften when heated and can generally be processed repeatedly under suitable conditions. Common examples include: Polyethylene (PE). Polypropylene (PP). Polyvinyl chloride (PVC). Polystyrene (PS). Acrylonitrile butadiene styrene (ABS). Polycarbonate (PC). Polyamide (PA). Polyoxymethylene (POM). Tensile testing helps determine how these materials respond to stretching and provides useful information for material comparison and quality control. Different thermoplastics can have substantially different stress-strain curves. Some grades are relatively rigid and brittle, while others can exhibit significant plastic deformation before fracture.

4.2 Engineering Plastics

Engineering plastics are used in applications where mechanical performance, dimensional stability, and durability are important. Examples include polycarbonate, nylon, POM, and certain high-performance polymers. ASTM D638 testing can help evaluate tensile strength and stiffness when selecting materials for mechanical components, industrial parts, and precision products.

4.3 Reinforced Plastics

Some plastic materials contain reinforcing fibers or fillers to improve stiffness, strength, or dimensional stability. Glass-fiber-reinforced plastics and other filled polymer materials may show different tensile behavior from unreinforced plastics. When testing reinforced materials, specimen orientation, fiber distribution, preparation method, and failure mode can have a significant influence on the results.

4.4 Rigid and Semi-Rigid Plastics

Rigid plastics generally resist deformation more strongly than flexible plastics. Semi-rigid materials may combine moderate stiffness with greater deformation capability. ASTM D638 testing can provide information about tensile strength, modulus, and elongation for suitable specimen configurations.

4.5 Limitations for Thin Plastic Films

Although ASTM D638 is an important plastic tensile testing standard, it is not the default method for every plastic product. Thin plastic films and sheeting are commonly evaluated using ASTM D882. The appropriate standard depends on specimen thickness, product form, and the intended testing procedure. Using the correct method is important because specimen geometry and gripping conditions can influence tensile results.

5. ASTM D638 Specimen Types and Dimensions

Specimen geometry is one of the most important factors in ASTM D638 testing. A standardized specimen helps control the stress distribution and provides a consistent area for tensile deformation. ASTM D638 includes several specimen types designed for different material conditions and thickness ranges.

5.1 Dog-Bone Specimen Shape

ASTM D638 specimens generally have a reduced-width central section with wider ends. This shape is commonly described as a dog-bone or dumbbell specimen. The narrower central section is intended to concentrate tensile deformation within the gauge section rather than allowing the specimen to fail at the gripping ends. During testing, the grips hold the wider ends while the machine pulls the specimen in opposite directions. A correctly prepared specimen should have smooth edges and consistent dimensions. Surface defects, cutting damage, or dimensional irregularities may influence the failure location and measured tensile properties.

5.2 Specimen Type Selection

The standard includes different specimen types to accommodate different material and thickness conditions.

The dimensions, thickness requirements, gauge length, and other geometric details must be taken from the current official edition of ASTM D638. Laboratories should not assume that different specimen types are interchangeable.

5.3 Specimen Preparation

Specimens may be prepared through molding, machining, or another suitable process depending on the material and test requirements. Preparation quality can influence the final test result. Important considerations include: Specimen width and thickness. Consistency of the reduced section. Smoothness of the edges. Absence of visible cracks or notches. Correct specimen orientation. Consistent manufacturing and conditioning procedures. For molded specimens, processing conditions may influence molecular orientation, residual stress, and material structure. For machined specimens, cutting tools and machining procedures should avoid excessive heat or surface damage.

5.4 Specimen Measurement

Before testing, the specimen dimensions should be measured using suitable dimensional instruments. Width and thickness are particularly important because the original cross-sectional area is used in tensile stress calculations. For a rectangular cross-section: Original cross-sectional area = Width × Thickness If the specimen dimensions are recorded incorrectly, the calculated tensile strength may also be incorrect even when the machine measures force accurately.

6. Universal Testing Machine Requirements for ASTM D638

A Universal Testing Machine is the central piece of equipment used for ASTM D638 tensile testing. The machine must be capable of applying a controlled tensile force, measuring the applied load, recording deformation, and producing reliable test data.

6.1 Load Capacity

The appropriate load capacity depends on the plastic material, specimen geometry, expected tensile strength, and test requirements. Plastic tensile testing often uses lower forces than many metal tensile testing applications. Therefore, selecting a machine with a suitable force range is important. Common machine capacities used in different laboratory applications may include 1 kN, 5 kN, 10 kN, 20 kN, and other configurations. These values are examples of available machine capacities, not ASTM D638 requirements. The actual capacity should be selected according to the expected specimen force and measurement needs. A load cell that is too large for the expected force range may reduce the effective measurement resolution available for the test. Conversely, a sensor that is overloaded can be damaged or produce invalid results.

6.2 Speed Control

ASTM D638 testing requires the test speed to be selected according to the applicable procedure and material conditions. A Universal Testing Machine should provide suitable crosshead-speed control and maintain stable movement throughout the test. The required speed is not necessarily the same for every plastic material or test configuration. Users should refer to the current standard and the applicable material specification when setting the test parameters.

6.3 Force Measurement Accuracy

The load cell measures the force applied to the specimen. For reliable results, the measurement system must be suitable for the expected load range and maintained according to the laboratory's calibration and verification procedures. A stable force signal helps the software calculate tensile strength and construct an accurate force-displacement curve.

6.4 Displacement Measurement

The machine should record crosshead displacement throughout the test. Crosshead displacement is useful for observing overall machine movement and specimen extension. However, it may include deformation from the machine frame, grips, and other components. For accurate strain measurements, particularly when determining tensile modulus, a suitable extensometer may be required.

6.5 Testing Software

Testing software should allow operators to configure the test method, enter specimen dimensions, monitor force and displacement, display curves, calculate results, and generate reports. For ASTM D638 applications, useful software functions may include: Tensile test setup. Specimen dimension input. Real-time force-displacement curves. Stress-strain curve generation. Maximum force calculation. Tensile strength calculation. Elongation measurement. Modulus calculation where supported by the required measurement system. Test result storage. Report generation. The available functions depend on the machine controller and software configuration.

7. Essential Accessories for ASTM D638 Testing

The performance of an ASTM D638 testing system depends not only on the main machine but also on the accessories used to hold and measure the specimen.

7.1 Tensile Grips

Tensile grips secure the specimen during the test and transfer the tensile force from the machine to the specimen. Common grip configurations include manual grips, pneumatic grips, and other specialized gripping systems. The correct grip should hold the specimen securely without causing excessive damage near the gripping area. If the specimen slips during testing, the recorded displacement may not represent the actual specimen extension.

7.2 Extensometer

An extensometer measures the change in specimen length over a defined gauge length. It is particularly useful for measuring strain and determining tensile modulus. Different extensometer designs are available depending on specimen material, deformation range, and test requirements. A contact extensometer attaches directly to the specimen, while a non-contact extensometer uses optical or video-based measurement. For specimens that undergo large elongation, the extensometer must have an appropriate measurement range and should not interfere with the specimen's failure process.

7.3 Load Cell

The load cell should be selected according to the expected test force. Different load cells may be used for different material categories. A laboratory testing soft plastics and high-strength engineering plastics may need to consider a wider range of measurement requirements.

7.4 Pneumatic Gripping System

Pneumatic grips use air pressure to apply gripping force. They can help maintain consistent gripping conditions and reduce operator variation in suitable applications. However, the gripping pressure must be appropriate for the specimen. Excessive pressure may damage soft plastic specimens, while insufficient pressure may cause slippage.

7.5 Environmental Chamber

Some plastic materials need to be tested at controlled temperatures. An environmental chamber or temperature-control accessory may be required when the test program involves elevated or reduced temperatures. Temperature can influence polymer stiffness, deformation, and failure behavior. Therefore, environmental conditions should be controlled when required by the applicable procedure.

8. ASTM D638 Tensile Test Procedure

A consistent testing procedure helps improve repeatability and reduces errors caused by specimen preparation, installation, and machine operation. The following is a general overview of an ASTM D638 tensile testing workflow. The exact requirements, including specimen dimensions, conditioning, test speed, and calculation rules, must be confirmed in the current official standard.

Step 1: Prepare the Specimens

Prepare the plastic specimens according to the required specimen type and dimensions. Inspect each specimen for visible defects, cracks, surface damage, or irregular edges. Specimens should be prepared consistently so that differences in test results primarily reflect material behavior rather than specimen preparation errors.

Step 2: Condition the Specimens

Plastic materials may be affected by temperature and humidity. Where required, specimens should be conditioned under the environmental conditions specified by the test method or material specification. Conditioning helps reduce variation caused by differences in moisture content or temperature history.

Step 3: Measure Specimen Dimensions

Measure the width and thickness of the specimen's relevant section. Record the dimensions accurately because they are used to calculate the original cross-sectional area. Incorrect dimensions can lead to incorrect tensile stress and strength values.

Step 4: Configure the Universal Testing Machine

Select the appropriate test method in the software. Configure the required parameters, including: Test mode. Crosshead speed. Force measurement range. Specimen dimensions. Test termination conditions. Data recording settings. The selected parameters should correspond to the applicable ASTM D638 procedure.

Step 5: Install the Specimen

Place the specimen between the upper and lower tensile grips. Ensure that the specimen is aligned with the machine's loading axis. The specimen should be held securely without excessive initial tension or unnecessary deformation. Poor alignment can introduce bending stresses and affect the measured tensile properties.

Step 6: Install the Extensometer When Required

If accurate strain measurement is required, install a suitable extensometer according to its operating instructions and the test method. Check that the extensometer is correctly positioned and that its measurement range is suitable for the expected deformation.

Step 7: Start the Tensile Test

Start the test using the configured procedure. The moving crosshead applies tensile force to the specimen at the selected speed. The machine records force and the relevant displacement or strain measurements throughout the test.

Step 8: Observe the Test Process

Monitor the specimen and the real-time test curve. Check for: Specimen slippage. Abnormal force fluctuations. Incorrect specimen alignment. Unexpected failure near the grips. Measurement signal interruptions. If an abnormal condition occurs, the operator should determine whether the test remains valid according to the applicable procedure.

Step 9: Complete the Test

The test may end when the specimen breaks or when the programmed termination condition is reached. After the test, inspect the failure location and record any unusual observations. A specimen that breaks near a grip or shows obvious slippage may require further evaluation before the result is accepted.

Step 10: Analyze and Save the Results

The software processes the recorded data and calculates the required tensile properties. Review the force-displacement curve, stress-strain curve, maximum force, elongation, and other reported values. Save the results with the specimen identification and relevant test conditions to support traceability.

9. Mechanical Properties Measured by ASTM D638

ASTM D638 provides information about several tensile properties. Each property describes a different aspect of plastic material behavior.

9.1 Tensile Strength

Tensile strength describes the tensile stress corresponding to a specified force condition, commonly including the maximum tensile stress reached during the test. Engineering tensile stress is calculated using the applied force divided by the original cross-sectional area. Tensile stress = Force / Original cross-sectional area For example, if the measured force is 2,000 N and the original cross-sectional area is 40 mm²: Tensile stress = 2,000 / 40 = 50 MPa The result is an engineering stress value based on the original specimen area.

9.2 Tensile Strength at Yield

Some plastic materials exhibit a recognizable yield point during tensile deformation. Yield behavior indicates the beginning of significant permanent deformation under the defined test conditions. The software may identify the relevant force and calculate the corresponding tensile stress according to the applicable method. Not every plastic material has a distinct yield point. Some materials show continuous deformation without a clearly identifiable yield event.

9.3 Tensile Strength at Break

Tensile strength at break describes the tensile stress at the point where the specimen fractures. This value can provide useful information about the material's behavior near failure. For some materials, tensile strength at break may be lower than the maximum tensile strength reached earlier in the test.

9.4 Tensile Modulus of Elasticity

Tensile modulus describes the stiffness of a material in the specified region of its stress-strain curve. A higher tensile modulus generally indicates greater resistance to elastic deformation. The modulus is determined from the slope of the relevant stress-strain region. Accurate modulus measurement requires suitable strain measurement and appropriate data resolution. Crosshead displacement alone may not be sufficient for every application.

9.5 Elongation at Yield

Elongation at yield describes the percentage increase in specimen length at the yield point, where that point is defined and measurable. It helps characterize the deformation behavior of plastic materials before substantial plastic flow.

9.6 Elongation at Break

Elongation at break indicates how much the specimen has stretched when it fractures. It is usually expressed as a percentage of the original gauge length. A material with high elongation at break can undergo substantial deformation before fracture, while a brittle material may fail at a relatively low elongation.

10. Understanding the ASTM D638 Stress-Strain Curve

The stress-strain curve is one of the most useful outputs from an ASTM D638 tensile test. It shows how the plastic material responds as tensile stress increases and deformation develops.

10.1 Initial Elastic Region

At the beginning of the test, the specimen may deform approximately elastically. Within this region, the relationship between stress and strain may be relatively linear for certain materials. When the applied load is removed within the elastic range, the specimen may recover most or all of its deformation. The slope of this region is used to determine tensile modulus under the applicable method.

10.2 Yield Region

Some plastics exhibit a yield point or yield region after the initial elastic response. At this stage, the material begins to experience significant permanent deformation. The curve may show a peak, a transition, or another characteristic change depending on the material.

10.3 Plastic Deformation Region

After yielding, some plastic materials continue to stretch while undergoing permanent deformation. The material may exhibit strain hardening, necking, or other deformation behavior. The shape of the curve depends on the polymer structure, specimen geometry, test conditions, and material formulation.

10.4 Maximum Tensile Stress

The curve may reach a maximum stress during the test. This value is commonly used to determine ultimate tensile strength based on the original specimen cross-sectional area.

10.5 Fracture Region

Eventually, the specimen may fracture or reach the defined test endpoint. The force may drop sharply when fracture occurs. The recorded elongation and failure characteristics provide additional information about material behavior.

10.6 Comparing Different Plastic Materials

Stress-strain curves can help engineers compare different plastic materials. For example: A steep initial slope generally indicates higher stiffness. A higher maximum stress may indicate greater tensile strength. A longer deformation range may indicate greater elongation capability. A sudden fracture at low deformation may indicate brittle behavior. These observations should be interpreted together with the actual test conditions and material specifications.

11. Factors Affecting ASTM D638 Test Accuracy

Even when the same Universal Testing Machine is used, test results may vary because of differences in specimen preparation, testing conditions, equipment configuration, and operator technique.

11.1 Specimen Geometry

Incorrect specimen dimensions can directly affect calculated stress and strain. The specimen should meet the geometry requirements of the selected ASTM D638 configuration.

11.2 Specimen Preparation Quality

Surface defects, machining marks, cracks, and irregular edges may create local stress concentrations. These defects can cause premature fracture and affect the measured tensile strength.

11.3 Grip Slippage

If the specimen moves inside the grips, the measured crosshead displacement may not represent the actual specimen extension. The gripping system should provide sufficient holding force without damaging the specimen.

11.4 Specimen Alignment

The specimen should be aligned with the machine's loading axis. Misalignment can introduce bending forces and produce non-uniform stress distribution.

11.5 Test Speed

Plastic materials may exhibit rate-dependent behavior. A change in testing speed can affect tensile strength, elongation, and other measured properties. The test speed should follow the applicable method rather than being selected only to reduce testing time.

11.6 Temperature and Humidity

Environmental conditions can influence the mechanical behavior of many plastics. Temperature and humidity should be controlled or documented when required by the test procedure.

11.7 Load Cell Selection

The load cell should be suitable for the expected force range. A suitable measurement range helps improve the usefulness of the recorded force data.

11.8 Extensometer Accuracy

An unsuitable extensometer or incorrect installation may produce inaccurate strain measurements. When tensile modulus or accurate elongation is required, the measurement device should be selected according to the material and test requirements.

12. Common ASTM D638 Testing Problems and Solutions

12.1 Specimen Breaks Near the Grip

A specimen that breaks near the gripping area may indicate stress concentration, excessive gripping pressure, misalignment, or specimen preparation problems. Check the grip design, specimen installation, and specimen condition. The validity of the result should be evaluated according to the applicable test procedure.

12.2 Specimen Slips During Testing

Slippage can lead to incorrect displacement measurements and unreliable elongation results. Check the gripping surfaces and gripping force. Select a suitable grip type for the specimen material and geometry.

12.3 Inconsistent Tensile Strength Results

Large differences between specimens may be caused by material variation, specimen dimensions, processing conditions, environmental conditions, or testing errors. Review specimen preparation and machine settings before concluding that the material itself is inconsistent.

12.4 Incorrect Elongation Results

Elongation results may be affected by the measurement method, specimen slippage, gauge-length errors, or incorrect data-processing settings. For accurate strain measurement, use an appropriate extensometer when required.

12.5 Unstable Force Readings

Unstable readings may result from electrical interference, loose connections, sensor problems, mechanical vibration, or unsuitable measurement settings. Inspect the load cell, cables, controller, and machine installation. Calibration and verification should be performed according to the laboratory's procedures.

12.6 Incorrect Stress Calculations

Incorrect width or thickness values can lead to incorrect cross-sectional area calculations. Verify the specimen dimensions and confirm that the software uses the correct units and calculation settings.

13. How to Select a Universal Testing Machine for ASTM D638

Selecting a suitable Universal Testing Machine is an important decision for laboratories that perform plastic tensile testing. The machine should match the expected test force, material type, specimen geometry, required measurement accuracy, and laboratory workflow.

13.1 Determine the Required Capacity

First, estimate the expected tensile force based on the material's strength and specimen cross-sectional area. Select a machine and load cell that can cover the expected force range while providing suitable measurement performance. Do not choose a machine based only on the largest capacity available. For many plastic testing applications, an appropriately selected lower-capacity system may provide more useful force measurement.

13.2 Check the Speed Range

Confirm that the machine supports the test speeds required by the applicable procedure. The machine should maintain stable movement and provide appropriate control throughout the test.

13.3 Evaluate the Load Cell

The load cell should have suitable capacity and measurement characteristics for the expected plastic tensile forces. If the laboratory tests materials with substantially different strength levels, additional load cells may be useful.

13.4 Confirm Extensometer Compatibility

If tensile modulus or accurate strain measurement is required, check whether the machine supports a suitable extensometer. Consider the gauge length, measurement range, attachment method, and material elongation.

13.5 Select Suitable Tensile Grips

The grips should accommodate the specimen geometry and material characteristics. For routine plastic testing, reliable gripping is important for reducing slippage and preventing premature failure near the grip area.

13.6 Review Testing Software

The software should support the required tensile test procedure and calculations. It should allow operators to enter specimen dimensions, configure test parameters, observe curves, save results, and generate reports.

13.7 Consider Future Testing Requirements

A laboratory may initially perform only plastic tensile tests but later need compression, bending, or other mechanical tests. A Universal Testing Machine with compatible fixtures and software options may provide greater flexibility for future applications. However, users should confirm the machine's actual capabilities and accessory compatibility before purchasing.

13.8 Evaluate Manufacturer Support

Before purchasing a testing machine, consider the supplier's technical support, installation guidance, calibration services, spare parts, and operator training. A reliable supplier should be able to recommend a suitable configuration based on the actual material, test method, and laboratory requirements.

14. Frequently Asked Questions About ASTM D638

Q1. What is ASTM D638 used for?

ASTM D638 is used to determine the tensile properties of plastic materials. It can provide information about tensile strength, tensile modulus, elongation, and stress-strain behavior.

Q2. What machine is used for ASTM D638 testing?

A Universal Testing Machine or tensile testing machine with suitable force capacity, speed control, gripping fixtures, and measurement equipment is commonly used.

Q3. What is the difference between ASTM D638 and ASTM D882?

ASTM D638 is commonly used for tensile testing of standard plastic specimens, while ASTM D882 is generally used for thin plastic sheeting and films. The correct method depends on the product form and applicable specimen requirements.

Q4. Which specimen shape is used in ASTM D638?

ASTM D638 generally uses a dumbbell-shaped or dog-bone specimen with a reduced-width central section. Different specimen types are available for different material and thickness conditions.

Q5. What properties can be measured by ASTM D638?

The method can be used to determine tensile strength, tensile strength at yield, tensile strength at break, tensile modulus, elongation at yield, and elongation at break, as applicable.

Q6. What capacity Universal Testing Machine is suitable for ASTM D638?

The required capacity depends on the material strength, specimen dimensions, and expected maximum force. Common laboratory configurations may include 1 kN, 5 kN, 10 kN, or higher capacities, but the actual selection should be based on the test requirements.

Q7. Is an extensometer necessary for ASTM D638?

An extensometer is useful when accurate specimen strain or tensile modulus measurement is required. Whether it is necessary depends on the specific test property and applicable procedure.

Q8. Does ASTM D638 apply to rubber?

ASTM D638 is intended for plastic tensile testing. Rubber tensile testing is generally associated with ASTM D412, which has procedures and specimen requirements suited to rubber and elastomer materials.

Q9. Can ASTM D638 be used for reinforced plastics?

Suitable reinforced plastic materials can be evaluated using ASTM D638 where the material and specimen requirements are covered by the method. Specimen orientation, fiber distribution, and preparation should be considered.

Q10. Why do ASTM D638 test results vary?

Results may vary because of differences in specimen preparation, dimensions, conditioning, test speed, gripping, alignment, load measurement, and material consistency.

Q11. Can one Universal Testing Machine perform ASTM D638 and compression tests?

Yes. A suitably configured Universal Testing Machine can often perform both tensile and compression tests by using appropriate fixtures and test procedures. The machine's capacity and software must support the required applications.

Q12. How can I choose the correct ASTM D638 testing machine?

Consider the expected force range, load cell, speed range, extensometer requirements, grips, software, specimen dimensions, and future testing needs. The supplier should confirm the configuration against the applicable test procedure.

15. Conclusion

ASTM D638 is an important standard for evaluating the tensile properties of plastic materials. It provides a structured approach for measuring tensile strength, tensile modulus, elongation, and other mechanical characteristics under controlled testing conditions. A suitable Universal Testing Machine is essential for performing reliable ASTM D638 tests. The machine should provide appropriate force measurement, stable speed control, suitable tensile grips, and compatible displacement or strain measurement devices. However, accurate results depend on the complete testing process. Specimen preparation, conditioning, dimensions, alignment, gripping, calibration, and data analysis must all be properly controlled. For plastic manufacturers, quality-control laboratories, and research institutions, an appropriately configured Universal Testing Machine helps evaluate material performance, compare plastic grades, monitor production quality, and support product development. When purchasing an ASTM D638 tensile testing system, users should evaluate the complete configuration rather than focusing only on maximum load capacity. Selecting the right machine, accessories, and software can help establish a reliable and efficient plastic material testing process.