ASTM D256 Izod Notched Impact Testing: Standard Method for Determining Impact Resistance

ASTM D256 is the standard test method for determining the resistance of plastics to impact from a pendulum, commonly known as the Izod notched impact test. This critical evaluation quantifies the energy required to break standardized notched specimens, providing essential data for material selection and quality control in engineering applications. Understanding the procedures and requirements of ASTM D256 is vital for manufacturers ensuring the structural integrity and durability of plastic components under sudden loading conditions.

Technical Overview

ASTM D256 is a widely recognized standard that measures the impact resistance of notched plastic specimens using a pendulum-type impact tester. The test involves striking a notched sample with a swinging pendulum released from a fixed height. The primary objective is to determine the amount of kinetic energy absorbed by the material during fracture. This energy value is then used to calculate the impact strength, which indicates the material's toughness and its ability to resist brittle fracture when subjected to rapid stress concentrations. The test results are highly sensitive to the notch geometry, making specimen preparation a critical factor in obtaining accurate data.

Testing Samples

Specimen Geometry and Dimensions

The standard specimen for ASTM D256 is typically a rectangular bar. The most common dimensions for the test specimen are 80 mm in length, 10 mm in width, and 4 mm in thickness. However, the standard allows for variations depending on the specific material being tested. The most critical feature of the sample is the notch, which serves as a stress concentrator to ensure the fracture occurs in a predictable location and manner.

Notch Preparation

The quality of the notch is paramount to the reliability of the test results. The standard specifies a V-notch with a specified depth and radius. The notch is usually machined into the specimen using a specialized cutting tool or a milling machine. The radius of the notch tip must be precise, typically 0.25 mm radius, as even minor deviations can significantly alter the stress concentration factor and lead to misleading impact strength values. Dull cutting tools or improper feed rates can induce residual stresses or plastic deformation at the notch root, affecting the outcome.

Conditioning

Before testing, samples must be conditioned according to specific environmental parameters. Since the impact properties of plastics are sensitive to temperature and humidity, specimens are typically conditioned at a standard laboratory atmosphere, such as 23 degrees Celsius and 50% relative humidity, for a minimum period defined by the material specification. This ensures consistency and comparability of data across different testing laboratories.

Testing Items

The primary output of the ASTM D256 test is the impact energy absorbed by the specimen, recorded in units of energy such as joules or foot-pounds. From this energy value, the impact resistance or impact strength is calculated by dividing the absorbed energy by the cross-sectional area at the notch base. This value is expressed in units such as joules per square meter or foot-pounds per inch. The test report typically includes the average impact strength, standard deviation, and a description of the fracture surface, noting whether the failure was brittle, ductile, or a combination of both.

Testing Methods

Izod Impact Principle

The Izod impact test utilizes a pendulum swinging in a vertical plane. The specimen is clamped vertically in a fixed vise, acting as a cantilever beam. The notch faces the direction of the striking hammer. When the pendulum is released from its latched position, it swings down, strikes the sample, and breaks it. The pendulum continues its swing to a lower height than its starting point, with the difference in height corresponding to the energy absorbed to break the specimen.

Test Procedure

• Calibration: The impact tester must be verified and calibrated to ensure it meets the requirements of ASTM D256 regarding pendulum velocity and friction losses.
• Specimen Mounting: The notched specimen is placed in the vise, ensuring the notch is aligned precisely with the top of the vise jaws.
• Pendulum Release: The pendulum is released without any initial velocity imparted to it.
• Reading the Scale: The energy absorbed is read directly from the scale or digital display on the machine.
• Correction Factors: Corrections for friction and windage are applied to the final calculations to ensure the energy attributed solely to the fracture is recorded.

Comparison with Charpy Test

While ASTM D256 refers to the Izod test, it is often compared to the Charpy impact test (ASTM D6110). The primary difference lies in the specimen support: in the Izod test, the sample is clamped as a cantilever, whereas in the Charpy test, the sample is supported horizontally as a simple beam. ASTM D256 focuses specifically on the Izod configuration, which is more susceptible to errors caused by improper clamping torque or specimen thickness variations compared to the Charpy method.

Applications

The data generated from ASTM D256 testing is crucial for a wide range of industries and applications. Material engineers utilize these results to select appropriate materials for components that may experience sudden impact or shock loads. For instance, in the automotive industry, this test helps evaluate materials for bumpers, dashboards, and interior trims. In the consumer goods sector, it is used to assess the durability of plastic housings for electronics and power tools. Furthermore, the test is essential for quality control, allowing manufacturers to verify that production lots meet the required toughness specifications and to detect degradation in material properties due to processing conditions.

FAQ

Why is the notch necessary in the ASTM D256 test?

The notch serves as a stress concentrator. Without the notch, many ductile plastics would deform plastically rather than fracture, making it difficult to measure brittleness. The notch ensures a tri-axial state of stress, promoting brittle fracture and providing a more sensitive measure of the material's notch sensitivity and toughness.

What is the difference between ASTM D256 and ASTM D4812?

ASTM D256 is specifically for notched specimens. ASTM D4812 is a related standard for un-notched cantilever beam impact testing, used when the material is too brittle to be tested with a notch or when the specific application involves smooth surfaces without stress concentrators.

What factors influence the test results?

Several factors can influence results, including the radius of the notch tip, the sharpness of the cutting tool, the clamping force of the vise, the temperature at the time of testing, and the orientation of the sample (especially for anisotropic materials like fiber-reinforced composites).