Standard Test Method for Tensile Properties of Plastics ASTM D638

Overview

The ASTM D638 test method is one of the most popular methods used to determine the tensile strength of reinforced and unreinforced plastics using a standard dumbbell or dog-bone shaped sample under consistent temperature, humidity and test speed. Materials up to 14 mm thick can be tested using this procedure. If your samples are greater than 14 mm in thickness they should be machined down to the proper thickness. If the material being tested is less than 1 mm thick ASTM D882 is the preferred test method.

Test Frame, Grips, and Accessories

United manufactures and supplies a wide range of Universal Test Machines, grips, and accessories to help you perform ASTM D638. Universal Test Machines are differentiated by capacity and choosing the proper test frame is determined by how much force is required to break the sample. If you are unsure as to how much capacity is required United will perform a free sample test to help you determine system capacity requirements.

United offers Universal Test Machines that range in capacity from our single-column 2.5kN (550 Lbf) up to 600kN (449,000 Lbf) dual-column frames. Our 50 kN and 100 kN dual-column frames are available in both table model and floor model configurations.

The standard itself specifies that the test apparatus, grips, and accessories provide the following:

Test Frame

• Constant rate of crosshead movement (Servo-controlled)
• A fixed or essentially stationary member carrying one grip
• A moveable member carrying a second grip
• displacement measurement capability
• Software and reporting capabilities

Grips

• Fixed or Self-aligning grips and proper grip jaws are required.
• A variety of different style grips will work for this test procedure including wedge grips, pneumatic grips, or vice grips, however certain types of grips are limited in terms of capacity, so the type of grip that is required is ultimately determined by how much force it takes for the sample to break.
• If the sample you are testing constantly slips out of the grip that is an indication that grips jaws or worn out, or that a larger capacity grip is required.
• Any grip with serrated grip jaws have proven to be a good choice for this test assuming they provide enough gripping force to not pinch the sample and cause premature breaks, while at the same time preventing the sample from slipping out of the grip.
• United A-Series wedge grips (shown below) are an ideal choice when perfroming ASTM D638.

Loadcell

A loadcell that is calibrated per ASTM E4 with an accuracy +/- 1% of the indicated value is required. This type of load cell is supplied with all United Universal Test Machines.

Extensometers

In order to obtain certain calculations such as Modulus of Elasticity, Secant Modulus and Poisson’s ratio the use of an extensometer is required. Calculating these results using only the grip separation at break does not provide enough precision or accuracy. When calculating Poisson’s ratio a Transverse Extensometer will be required. United supplies a wide variety of extensometers for the ASTM D638 test procedure including contact and non-contact extensometers such as Video or Laser extensometers. All extensometers are calibrated per ASTM E83 and are available in a wide variety of gauge lengths with varying strain measuring ranges, depending on the samples being tested.

Test Specimens

ASTM D638 references several different types of test specimens depending on the material being tested, material thickness, and characteristics displayed when it breaks. Methods of preparing test specimens include injection molding, machining operations, or die cutting from materials in sheet, plate, slab, or similar form. Please refer to the most recent published standard to obtain the exact dimensions of the samples.

Sheet, Plate, and Molded Plastics

• Type I specimen is the preferred specimen and shall be used where sufficient material having a thickness of 7 mm (0.28 in.) or less is available.
• Type II specimen is recommended when a material does not break in the narrow section with the preferred Type I specimen.
• Type III specimen must be used for all materials with a thickness of greater than 7 mm (0.28 in.) but not more than 14 mm (0.55 in.)
• Type IV specimen is generally used when direct comparisons are required between materials in different rigidity cases (that is, nonrigid and semirigid).Type V specimen shall be used where only limited material having a thickness of 4 mm (0.16 in.) or less is available for evaluation, or where a large number of specimens are to be exposed in a limited space (thermal and environmental stability tests, etc.).

Non-rigid Plastics

• The Type III specimen must be used for all materials with a thickness greater than 7 mm (0.28 in.) but not more than 14mm (0.55 in.)
• The Type IV specimen shall be used for testing nonrigid plastics with a thickness of 4 mm (0.16 in.) or less.

Reinforced Composites

Type I specimens shall be used

Rigid Tubes

Rigid tubes are hollow specimens and special preparation is required so that when the test is performed the tube is not crushed by the grips causing premature failure. The center portion of the tube is machined down to 60% of its original diameter so that when tension is applied this portion will break first since it is thinner. Steel or brass plugs should fit snugly inside the hollow tube ends and should be placed in the ends of the specimens to prevent crushing by the grips. Plugs can be located conveniently in the tube by separating and supporting them on a threaded metal rod.

Rigid Rods

Rigid rods are solid samples but the principal behind sample preparation is the same. The center portion of the sample is machined down to 60% of its original diameter making it thinner than the ends, so that when tension is applied the sample is more likely to break in the middle.

Specific dimensions for these types of samples are specified in the standard and the most recent version of the standard should be referenced to obtain these dimensions.

Test Calculations and Reporting

Perhaps the biggest challenge facing customers that are new to this standard are the calculations and reporting required.

United’s DragonFly UTS software along with one of our Universal Test Machines is fully capable of performing the ASTM D638 test procedure. Each UTM we sell includes the test frame, loadcell, Windows ® PC, DragonFly UTS software, and the pre-written test procedure to get you up and running quickly. Custom or modified test procedures can easily be written. Presentation quality test reports displaying all necessary calculations are available immediately and data is stored in a secure and easy to search Access ® database. Writing test procedures other than ASTM D638 using DragonFly UTS is easy to do and no additional fees or license is required to write or modify additional test procedures.

ASTM D638 requires the following to be calculated:

• Tensile Strength
• Percent Elongation
• Percent Elongation at Yield
• Percent Elongation at Break
• Nominal Strain
• Nominal Strain at Break
• Modulus of Elasticity
• Secant Modulus
• Poisson’s ratio (Transverse Extensometer Required)
• Average value of all samples tested
• Standard Deviation

Conclusion

United has done virtually all the work for our customers when it comes to performing the ASTM D638 test procedure. All customers need to do is determine which sample type they will be testing so we can choose the proper test frame capacity and grips.

View our United Testing System machines here