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Often, the how and why of material failure is as critical as the when. Crack propagation in 3D bodies will undoubtedly be proportional to the force applied and the size/shape of the flaws. However, loading is complex, with a stress-strain matrix developing depending on the distance in space between the applied loading and material flaws, the material’s performance in tension versus compression, and other variables. Fracture toughness testing looks to quantify performance by destroying materials with increasing loading forces, studying the progression of cracks through failure.

Fracture Toughness Testing at LTI

The fracture toughness test determines how well a material can resist the growth of a crack under an increasing load. During a fracture toughness test, a specimen with a pre-existing crack experiences a rising tensile load. By measuring the combination of load and displacement as the crack lengthens, the material’s resistance to further crack propagation reveals itself. Fracture toughness is an important material property in design applications since the occurrence of flaws is not entirely avoidable. Flaws may appear as cracks, voids, inclusions, weld defects, or design discontinuities. The fracture toughness test is also valuable in determining whether component failure is dangerous when a flaw is revealed in an existing structure.

Test Specimens Preparation by LTI
  • Single Edge Bend (referred to as SE(B) or SENB) – a rectangular specimen with a slot cut in the long side, loaded in three-point bending
  • Compact Tension (C(T)) – the most commonly used geometry
  • Disk-Shaped Compact Tension (DC(T)) – necessary when a fracture toughness result is essential for the C-R direction of a round bar
  • Specimen sizes ranging from Charpy-sized SE(B) bars up to 4.5” W C(T)

Capabilities

  • Test Methods: KIC per ASTM E399 / E1820, JIC per ASTM E1820
  • Force: Up to 55,000 lbs
  • Temperature: -150°F~350°F
  • Fixturing: Single-edge bend [SE(B)] and compact tension C(T)
  • Machined Specimen: Support for EDM notching

Standards

  • ASTM E399 / KIC Test

    According to ASTM E399, fracture toughness testing applies a continuously increasing load to the specimen and determines the critical stress intensity KIc. This method is known as the KIC, KIC, or K1C fracture toughness test. It is a widely used test, particularly in aerospace industry standards.

  • ASTM E1820 / JIC Testing and More

    During testing to ASTM E1820, a rising load with periodic partial unloading applies to measure the crack length as the test progresses. Since this method takes measurements throughout the test, it provides much data on the material’s behavior as the crack opens. It is often possible to get valid results where it would be impossible or unfeasible with a KIc test.

  • ASTM E1921 / Master Curve Test

    ASTM E1921 is a fracture test method to determine certain steels’ reference temperature (To). To statistically characterize fracture toughness as a function of the temperature of ferritic steels that experience cleavage fracture mechanisms, requiring a minimum of 6 results taken at varying temperatures. The process is similar to the ASTM E1820 test method.

  • JIc, KJIc, and CTOD

    ASTM E1820 provides KJIc and JIc. The data analysis also provides crack-tip opening displacement (CTOD, δ). The “J” integral describes elastic-plastic fracture toughness in more ductile materials that can better deform and resist crack growth under load. A Roman numeral subscript added to “K” and “J” indicates which of the three fracture modes occurs during the test. Mode I fracture is the most common condition in which a tensile load is applied normal to the direction of the crack plane. When a cracked material under Mode I plane-strain conditions reaches a critical value, denoted as KIc or JIc, the crack will begin to grow. Experimental values of KIc and JIc can apply to the design of structures to ensure that a component does not fail by brittle or ductile fracture; KJIc derives from the JIc result.

Having a Tough Time With Material Testing Logistics? Talk to LTI

Material defects like voids, cracks, and crystalline inclusions/exclusions are often the reality for development materials and product testing. Moreover, materials’ wear and aging can lead to these defects forming in carefully grown isotropic crystals and other controlled material manufacturing. Fracture toughness testing shows how failure events build over time, giving product developers an excellent reference and comparison tool to detect the early effects of defects on materials before failure. LTI offers a full slate of material testing services along with experts in respective fields; talk to us to see how your material testing process can accelerate approvals without sacrificing quality.

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