ASTM E399 Linear-Elastic Plane Stress Fracture Toughness of Metallic Materials

The ASTM E399 standard, developed by the American Society for Testing and Materials (ASTM), describes a test method for determining the fracture toughness of metallic materials under primarily linear elastic, plane stress conditions, using fatigue-pre-cracked specimens of 1,6 mm or greater thickness.

In this method, samples are subjected to crack displacement forces that increase slowly or, in special (selective) cases, rapidly. Details of the testing apparatus, sample configuration, and experimental procedure are given in this standard. Two procedures for calculating fracture toughness values ​​using experimental data are outlined:

• KIc test procedure: This test procedure relies on crack growth up to 2% of the sample width. This can lead to an increasing fracture toughness-resistance curve depending on the sample size, with larger samples producing higher fracture toughness results.
• KIsi test procedure: This test procedure is based on a constant crack elongation amount of 0,5 mm, and as a result, KIsi is less sensitive to sample size than KIc. This less size-sensitive fracture toughness (KIsi) is referred to as size-insensitive throughout this test method.

Plane tensile fracture toughness tests for materials thinner than 1,6 mm and sufficiently brittle can be performed using other sample types. There is no standard test method for materials this thin.

ASTM E399 is a widely used fracture mechanics standard developed to determine the linear-elastic plane tensile fracture toughness of metallic materials. The fundamental property measured by the test is KIc, which describes the metal's resistance to unstable crack growth when a sharp crack is present under severe tensile stress.

Fracture toughness is particularly important because a component can fracture even if its nominal stress is below the material's conventional tensile or yield strength, if a sufficiently large crack or defect is present.

The ASTM E399 standard specifies requirements for measuring fracture toughness under predominantly linear elastic, plane stress conditions. This method uses a specimen containing a pre-fatigue crack and applies an increasing crack displacement force until fracture occurs.

This standard is primarily designed for metallic materials where the plastic region of the crack tip is sufficiently small compared to the relevant sample dimensions. This allows the fracture behavior to be characterized using linear elastic fracture mechanics.

The ASTM E399 standard covers KIc and also provides an optional procedure for determining KIsi under specified conditions.

Fracture toughness is a measure of a material's ability to resist crack propagation. It differs from tensile strength. While tensile strength describes how much stress a material can withstand before tensile fracture, fracture toughness takes into account the combined effect of:

• Applied voltage
• Crack size
• Crack geometry
• Material properties
• Stress concentration at the crack tip.

A material with high fracture toughness can withstand relatively large cracks without catastrophic fracture, compared to a material with low fracture toughness.

The symbol KIc represents the plane tensile fracture toughness of a material. This value characterizes the material's resistance to fracture in a neutral environment when subjected to a sharp crack and conditions approaching linear-elastic, plane tensile behavior. This standard specifies that for the intended KIc interpretation, the crack tip plastic zone should remain small relative to the crack size, specimen thickness, and remaining connective tissue.

When the stress concentration reaches the critical fracture toughness of the material under the prevailing plane stress conditions, unstable crack growth can occur.

The ASTM E399 standard procedure generally follows these basic steps:

• Sample preparation: A metallic sample is prepared using appropriate processing according to the suitable sample configuration.
• Notch preparation: The sample includes a sharp initial notch.
• Fatigue pre-cracking: Cyclic loading is used to elongate the initial notch and create a sharp fatigue crack.
• Dimensional measurements: Key sample dimensions and crack dimensions are measured.
• Fracture loading: Force and displacement are recorded while the sample is loaded under controlled conditions.
• Load-displacement record analysis: The characteristic load used to calculate temporary toughness is determined.
• KIq (or Kq) calculation: A temporary fracture toughness value is calculated.
• Validation assessment: Sample dimensions, loading behavior, crack geometry, and other requirements are checked.
• Reporting: If the validity requirements are met, the result can be reported as KIc.

The ASTM E399 standard provides requirements for various specimen configurations. The latest version specifically includes the following:

• Compact tensile specimen: This is one of the most commonly used configurations in fracture toughness testing. Loading is applied through holes near the crack opening of the specimen, creating tensile loading that promotes crack opening.
• Single-edge bending specimen: The specimen is typically subjected to bending using a three-point loading arrangement. This is another commonly used geometry for determining plane tensile fracture toughness.
• Disc-shaped compact specimen: This provides an alternative configuration that can be useful when the specimen material or geometry supports a disc-shaped specimen.
• Spring-shaped specimens: This standard also includes requirements for spring-shaped tensile and spring-shaped bending specimens.

The chosen sample configuration depends on the material, the volume of material available, the component geometry, and laboratory requirements.

One of the most important aspects of the ASTM E399 standard is to create a sufficiently sharp crack before the final fracture test. A machined notch does not necessarily reproduce the sharpness and stress concentration of a naturally occurring crack. Therefore, the specimen is subjected to controlled cyclic loading to widen the notch and create a pre-fatigue crack. This provides a more realistic crack tip condition for fracture mechanics tests.

Several measurements are critical in the ASTM E399 standard:

• Applied load: The force applied to the sample during the fracture test is continuously monitored.
• Crack length: Since stress intensity calculations depend heavily on crack geometry, the length of the initial fatigue crack must be accurately determined.
• Sample dimensions: Dimensions such as thickness and width are important for both fracture toughness calculation and validity assessment.
• Fracture mouth opening displacement: Displacement near the fracture mouth provides important information about the sample's behavior during loading.

The ASTM E399 standard is important because fracture toughness is a critical engineering property for structures that contain, or potentially contain, cracks and defects. This standard provides a controlled and repeatable method for evaluating this property. Application areas include:

• Aerospace structures
• Aircraft components
• Automotive components
• Pressure vessels
• Energy production equipment
• Nuclear components
• Oil and gas equipment
• Bridges and structural components
• Heavy machinery
• Defense equipment
• High-strength metal structures
• High-strength metallic structures

For many years, our organization, with its competent and experienced team, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting them in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services in accordance with the ASTM E399 standard.