Spiral Notch Torsion Test Research Articles

SAE 4340 Fracture Toughness Evaluation Following Wang’s Formulation for Spiral Notch Torsion Test Specimens

Abstract The fracture toughness parameter (fracture toughness in plane strain-state [KIC], fracture toughness near the onset of crack growth, or crack tip open displacement) is essential information for understanding fracture behavior in mechanical and structural components. To determine this property, several methods have been developed; these require specimens with sufficient thickness to ensure a plane-strain condition at the crack front, which depending on materials mechanical properties, may lead to the need of large amounts of material. To overcome this problem, the spiral notch torsion test (SNTT) was proposed by Wang in 2000, and over these years, methodologies for testing many different materials have been presented, as well as the elastic compliance equations for assessing crack size (essential for introducing the fatigue precrack), also presented in 2012. Although the SNTT presented advantages and potentialities, its application is still quite restricted, and information on its use is limited. Implementation of the SNTT is a challenge, mainly because of the inexistence of a specific standard, thus producing ambiguities when dealing with the application of the test, methodology, and limitations because the SNTT implementation is principally based on previously published theoretical and experimental studies. The study presented herein aims at organizing already existing information based on work conducted by Wang while presenting in detail a test chart and the necessary techniques for obtaining a fracture toughness parameter, in mode I loading and in the domain of a plane strain-state. Determining KIC using the SNTT was performed on a quenched and tempered AISI 4340 steel (45 HRC) in air at room temperature. The obtained results proved the great potential of SNTT methodology for KIC determination. The KIC values obtained by the SNTT methodology exhibited a maximum error of 3.5 % and were statistically equal to those provided by the conventional method compact tension (C(T)) that follows ASTM E399-20, Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness KIC of Metallic Materials. The fracture surfaces observed in SNTT and C(T) specimens showed similar fractographies and mechanisms.

J-integral measured from experimental torque load versus torque-ends rotation angle obtained from a spiral notch torsion test

Spiral Notch Torsion Test (SNTT) was developed to measure the intrinsic fracture toughness of structural materials using small specimens. The SNTT system operates by applying pure torsion to cylindrical specimens with a notch line that spirals around the specimen at a 45° pitch. 3-D finite-element analysis has been used to estimate the fracture toughness obtained from SNTT test specimen with 3-D non-coplanar crack front. Here a newly developed SNTT protocol to estimate J-integral directly from the experimental torque load versus torque-ends rotation angle trend curve without the aid of 3-D finite element analyses is presented and its applicability is illustrated by applying it for the evaluation of fracture toughness of SS304/308 weldment. The estimated JQ’ for the SS304/308 weld is 144.2 kJ/m2, which is consistent with that obtained from CT specimen.

Fracture toughness evaluation for Zr-4 clad tubing structure with pellet inserts

Applying fracture mechanics approach to spent nuclear fuel (SNF) system reliability investigation is warranted due to the inherent flaws and hydride structures existed in a SNF system after nuclear reactor operation. However, none of the existing fracture toughness data deal with fuel cladding specific geometry or spent fuel material conditions, such as cladding structure with the pellet-inserts and the associated pellet clad mechanical interactions induced mixed-mode damage mechanisms. Thus, the development of an intrinsic fracture mechanics approach that is suitable for SNF materials is needed. Due to thin wall and small dimension of clad tubing structure, the spiral notch torsion test (SNTT) method of small specimen approach was used to estimate the clad tubing structure fracture toughness. The estimated fracture toughness for Zr-4 cladding with alumina-pellet inserts are presented in this report. For SNTT samples with a short or medium crack length, between 5.4-mm and 8-mm, the estimate JIQ upon fracture initiation for the baseline Zr-4 cladding is at 50 kJ/m2 with 2-sigma uncertainty of 3.26 kJ/m2, and the associated KIQ is at 67.46 MPa√m. For SNTT samples with a long crack length, around 13-mm, the crack initialization is deviated from that of the Mode-I tensile fracture and appears to be a mixed-mode fracture of Mode I – tensile stress and Mode III – out of plane shear stress; the estimated JMQ is at 18.9 kJ/m2, the associated KMQ is at 41.4 MPa√m.

Fracture toughness evaluation for thin-shell stainless steel weldment

Integrity of spent nuclear fuel (SNF) interim storage canisters are very important to the safety of the back-end nuclear fuel cycle. Stress corrosion cracking (SCC) potential of interim storage canister has been considered as a high priority. Because no post-weld heat treatment was required for forming these canisters, the high tensile residual stress existed within these canister welds. This can change the fracture resistance capacity significantly as well as increase SCC potential. Due to relative thin shell thickness of a canister weldment, the spiral notch torsion test (SNTT) method of small specimen approach was used to estimate the canister weldment fracture toughness. Most SNTT weldment samples fracture initiation sites are at heat-affected zone (HAZ) regions. The estimated fracture toughness JQ’ for the baseline SS304 steel is at 283 kJ/m2. The estimated JQ’ for the SS304/308 weld from HAZ and weld specimens are 126 kJ/m2 and 148.9 kJ/m2, respectively.

A method for evaluating the fatigue crack growth in spiral notch torsion fracture toughness test

The spiral notch torsion test (SNTT) has been a recent breakthrough in measuring fracture toughness for different materials, including metals, ceramics, concrete, and polymers composites. Due to its high geometry constraint and unique loading condition, SNTT can be used to measure the fracture toughness with smaller specimens without concern of size effects. The application of SNTT to brittle materials has been proved to be successful. The micro-cracks induced by original notches in brittle materials could ensure crack growth in SNTT samples. Therefore, no fatigue pre-cracks are needed. The application of SNTT to the ductile material to generate valid toughness data will require a test sample with sufficient crack length. Fatigue pre-crack growth techniques are employed to introduce sharp crack front into the sample. Previously, only rough calculations were applied to estimate the compliance evolution in the SNTT crack growth process, while accurate quantitative descriptions have never been attempted. This generates an urgent need to understand the crack evolution during the SNTT fracture testing process of ductile materials. The newly developed governing equations for SNTT crack growth estimate are discussed in the paper.

The development of in situ fracture toughness evaluation techniques in hydrogen environment

Fracture toughness and fatigue properties of pipeline steels play a critical role in developing advanced high-pressure hydrogen infrastructure for alternative fuel pipelines program. The reliability of structure components, particularly resistance to damage and failure in the intended service environment, is highly dependent on the selected materials. An effective surveillance program is also necessary to monitor the material degradation during the course of service. Steels have been proven to be desirable for hydrogen infrastructure. However, hydrogen embrittlement is an important factor that limits steel performance under high-pressure hydrogen conditions. Furthermore, many conventional fracture testing techniques are difficult to be realized under the presence of hydrogen, in addition to the inherent specimen size effect. Thus it is desired to develop novel in situ fracture toughness evaluation techniques to study the fracture behavior of structural materials in hydrogen environments. In this study, a torsional fixture was developed to utilize Spiral Notch Torsion Test (SNTT) methodology. A fatigue pre-crack procedure of SNTT approach was also developed and demonstrated for weldment. The in situ testing results indicated that the exposure to H2 significantly reduces the fracture toughness of 4340 high strength steels by up to 50 percent. Moreover, in-air simulated heat-affected zone specimen by Gleeble demonstrated a significant fracture toughness reduction of 75 percent in samples which illustrated the effect of welding on the fracture toughness.

Investigating fracture behavior of polymer and polymeric composite materials using spiral notch torsion test

Wind turbine blades are usually fabricated from fiber reinforced polymeric materials, which are subject to complex loading conditions during service. The reliability of the blades thus depends on the mechanical behaviors of these composites under various loading conditions. Specifically, the composite fracture behavior is of great importance to both the scientific research community and the wind industry.In the current study, a new testing technique is proposed based on the spiral notch torsion test to study the fracture behavior of composite structures under Mode I or mixed mode loading conditions, particularly under combined Mode I (flexural or normal tensile stress) and Mode III (torsional shear stress) loading. For this test method, round-rod specimens with spiral V-groove are subjected to pure torsion. Depending on the pitch angle of the spiral lines, pure Mode I, pure Mode III, or mixed Mode I/Mode III loading conditions can be simulated. A three dimensional finite element analysis was used to estimate the fracture toughness.In the current study, both epoxy and fiberglass reinforced epoxy materials are investigated using spiral notch torsion test. This paper will discuss the fracture behaviors of Mode I and mixed loading conditions with and without fatigue pre-crack. In addition, results from fractographic study and finite element analysis will be presented and discussed in detail.

Fractographic study of epoxy under mode I and mixed mode I/III loading

Abstract Fiber-reinforced polymeric matrix composites are widely used in structural components such as wind turbine blades, which are typically subject to complicated loading conditions. Thus, the response of these materials under mixed-mode loading is of great relevance to the reliability of these structures. In this study, a novel fracture testing methodology, the spiral notch torsion test (SNTT), was applied to study the fracture behavior of an epoxy resin. SNTT samples were tested using either monotonic loading or cyclic loading, while both mode I and mixed mode I/III loading conditions were used. Fractographic examination indicated the epoxy samples prevalently failed in mode I even when the test specimens were subject to mixed mode loading. Different fatigue precracks were observed on mode I and mixed mode samples, i.e. precracks appeared as a uniform band under mode I loading, and a semi-ellipse under mixed mode loading. Fracture toughness was also estimated using quantitative fractography.

A new test method for determining the fracture toughness of concrete materials

The Spiral Notch Torsion Test (SNTT) determines the intrinsic fracture toughness (KIC) of structural materials by applying pure torsion to cylindrical specimens having a notch line that spirals around the specimen at a 45° pitch. KIC values are obtained with the aid of a three-dimensional finite-element computer code, TOR3D-KIC. The SNTT method is suitable for testing a wide variety of materials used extensively in pressure vessel and piping structural components and weldments, as well as ceramic and graphite materials. One important characteristic of SNTT is that neither a fatigue precrack nor a deep notch is required for evaluation of brittle materials, significantly reducing the sample size requirement. Results are reported for a Portland cement-based mortar demonstrating applicability of the SNTT method to cementitious materials. The estimated KIC of the tested mortar samples with compressive strength of 34.45 MPa was found to be 0.360 ± 0.017 MPa √m.

A New Approach to Evaluate Fracture Toughness of Structural Materials

A new method, designated as Spiral Notch Torsion Test (SNTT), is developed recently to measure the intrinsic fracture toughness KIC of structural materials. The SNTT overcomes many of the limitations inherent in traditional techniques and makes it possible to standardize fracture toughness testing. It is uniquely suitable for testing a wide variety of materials used extensively in pressure vessel and piping structural components and weldments, including others such as ceramics, their composites, and concrete. The SNTT system operates by applying pure torsion to uniform cylindrical specimens with a notch line that spirals around the specimen at a 45 deg pitch. The KIC values are obtained with the aid of a three-dimensional finite-element computer code, TOR3D-KIC, developed at Oak Ridge National Laboratory (ORNL). The fundamental mechanism of SNTT approach is also described in the paper.

Oak Ridge National Laboratory (ORNL) spiral notch torsion test (SNTT) system

he Oak Ridge National Laboratory (ORNL) spiral notch torsion test (SNTT) system measures the intrinsic fracture toughness (K Ic) of structural materials, overcomes many of the limitations inherent in traditional techniques, and introduces new possibilities for standardizing fracture toughness testing. The system is uniquely suited to test a wide variety of materials, such as metals and alloys, ceramics, composites, polymers, carbon foam, and concrete. The SNTT system operates by applying pure torsion to cylindrical secimens machined with a notch line that spirals around the specimen at a 45° pitch (Fig. 1). Results are obtained with the aid of TOR3D-KIC, a three-dimensional finite-element computer code developed at ORNL, which may be obtained by contacting the author.