Understanding Fracture Toughness: A Critical Property for High-Performance Materials
Fracture toughness is a critical material property that quantitatively describes a material’s ability to resist the propagation of a crack. This property is especially crucial in aerospace and other high-performance applications where materials need to perform under high stresses despite the presence of small flaws introduced during service without experiencing sudden catastrophic failure.
What is Fracture Toughness?
Fracture toughness, denoted as KIc., is determined through standardized testing methods, with ASTM E399-22 [1] being the most recognized standard for metallic materials. During this test, a fatigue pre-crack is induced in the sample, which is then subjected to a gradually increasing force until rapid propagation of the crack occurs. The critical stress intensity factor at this juncture is the plane-strain fracture toughness KIc.
Testing Methodologies
Various specimen configurations can be employed for fracture toughness testing, with the ‘Compact C(T)’ specimen being a commonly used type. For materials produced through wrought processes, sample orientation is critical due to anisotropy. Orientation is described by a two-letter code, where the first letter denotes the direction normal to the crack plane, and the second letter denotes the expected direction of crack propagation. For plate materials, there are six possible sample orientations. Sample size influences the resulting value and should be held constant for any comparative or lot-release testing.
Importance in High-Performance Applications
In aerospace and other demanding fields, materials like AA7075 aluminum alloy are preferred for their high strength and good fracture toughness. This particular alloy is used in aircraft structures, M16 rifle receivers, and high-quality sporting goods, owing to its ability to resist crack propagation.
Secat, Inc. and Fracture Toughness Testing
At Secat, Inc., we understand the importance of accurate fracture toughness testing. As part of our certification process towards becoming NADCAP certified, we tested AA7075-T7351 aluminum alloy to validate our procedures. Samples with dimensions 3.75” x 3.6” x 1.5” were machined from a plate in the T-L orientation and tested on an MTS Landmark system. The results showed a KQ value of 27.8 ksi-in1/2, consistent with the literature value of 25-33 ksi-in1/2 [2], thus confirming the validity of our testing.
In the event that the validity checks after testing fail, a larger specimen is typically required to obtain a valid KIc value. The necessary specimen size increases with the square of the material’s toughness to strength ratio. In some cases, it will not be possible to obtain a large enough sample to get a valid KIc. In these situations, KQ can still be a useful indicator of the material’s fracture toughness.
Why Choose Secat?
Choosing Secat for your fracture toughness testing ensures that you receive accurate, reliable, and industry-standard results. Our commitment to quality and precision is reflected in our rigorous testing protocols and our continuous pursuit of industry certifications, such as NADCAP.
Contact Us
For more information on our testing services, or to discuss your specific requirements, reach out to us. Let Secat, Inc. be your trusted partner for all your fracture toughness testing needs.
References
[1] ASTM E399-22 Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness KIcKIc of Metallic Materials, ASTM International
[2] ASM Handbook Volume 2B – Properties and Selection of Aluminum Alloys, pp. 438, ASM International, 2019