Evaluation of CTOA from load vs. load-line displacement for C(T) specimen

A method to evaluate crack length and crack tip opening angle (CTOA) using only load vs. load-line data (P vs. Δ) is proposed for sheet materials for the C(T) specimen. The method has been applied to high-strength steel sheet and reasonable agreement was found with results from the surface optical method.     

A new method to estimate the plane strain fracture toughness of materials

Although the testing method for fracture toughness K_IC has been implemented for decades, the strict specimen size requirements make it difficult to get the accurate K_IC for the high‐toughness materials. In this study, different specimen sizes of high‐strength steels were adopted in fracture toughness testing. Through the observations on the fracture surfaces of the K_IC specimen, it is shown that the fracture energy can be divided into 2 distinct parts: (1) the energy for flat fracture and (2) the energy for shear fracture. According to the energy criterion, the K_IC values can be acquired by small‐size specimens through derivation. The results reveal that the estimated toughness value is consistent with the experimental data. The new method would be widely applied to predict the fracture toughness of metallic materials with small‐size specimens.     

An interpretation of the Ct parameter for increasing load conditions

The prior assessment equations for C_t , which is a well-known fracture parameter for characterizing creep and creep–fatigue crack growth rates, have applicability to constant loading conditions only. However, crack growth due to creep can also occur under varying load conditions during a fatigue cycle when the loading (or unloading) rate is slow enough such that creep deformation can occur near the crack tip. Hence, the applicability of the C_t parameter should be extended to varying load conditions.
In this study, a method of extending the use of the C_t parameter to increasing load conditions is proposed. Based on the concept of Irwin's effective crack size, new equations for estimating C_t under increasing load conditions are derived and denoted as ( C_t )_r . Finite element analyses were also performed under various increasing load conditions. From the analysis, the variation of ( C_t )_r values during the load rise period is obtained and the difference between the ( C_t )_r value at the end of the load rise period and the C_t value at the beginning of the succeeding load hold period is discussed. A generalized creep–fatigue crack growth model which employs ( C_t )_r as a parameter characterizing crack growth rate during the rise time is also discussed.     

Comparison of using the crack mouth displacement (CMOD) and load line displacement (LLD) methods in the determination of critical J integral in SENB specimens

Various methods for direct and indirect determination of LLD and CMOD were used to determine J from SENB specimens in three different steels. The influence of the displacement measurement on J is discussed, and shows that the values of J using LLD determined from clip gauge methods to the ASTM E1820 or ISO 12135 standards are consistent with values of J determined from CMOD (either directly or using clip gauge methods), as defined in ASTM E1820. From this work it is recommended that standard methods such as ISO 12135 should permit load‐CMOD and load‐LLD as alternative methods to determine J. Methods to determine LLD by corrections to the ram displacement were also shown to be effective in determining J, for applications where the use of clip gauges may be challenging, such as fracture toughness testing in sour environments, dynamic tests, or testing at very high temperature.     

Plastic limit load and its application to the fracture toughness testing for heterogeneous single edge notch tension specimens

The current investigation pursues the confirmation of the applicability of the limit load solutions in determination of the η factors necessary for fracture toughness testing protocols. The procedure begins with the correct calculation of limit load values in welded single edge notch tension (SE(T)) fracture specimens containing centreline cracks. Hence, the η factor is inferred through the principle of potential energy. Additionally, such results are compared with those obtained from finite element analyses, including strain hardening effects available in the literature. SE(T) specimens subject to pin‐loading display that the η factors are insensitive to the configurational effects and hardening properties. On the other hand, in clamped SE(T) specimens, such effects become meaningful, making its usage in fracture toughness experiments questionable. This work provides an alternative methodology to compute fully plastic proportionality coefficients (η) based on limit load solutions for heterogeneous cracked SE(T) specimens. These analyses also consider the limitations and potentialities of such an approach in experimental measurements of ductile crack growth.     

FRACTURE TOUGHNESS CALIBRATION OF THE Si3N4/Fe JOINT SYSTEM: A PARAMETRIC STUDY

Abstract— A fracture toughness calibration of the Si₃N₄/Fe joint system has been performed for various mixed-mode loading conditions and for different thicknesses of the metal interlayer. The asymmetric four-point-bend loading geometry was used. The values for the calibration function ( Y ) as well as the mode mixity (ψ)of the system increase on increasing the thickness of the metal. As the loading conditions change from mode II to mode I the dependence of both parameters on metal thickness is more intensive.     

A study to estimate crack length using the separability parameter Spb in steels

The separability property with S_pb parameter was used in this work to estimate the instantaneous crack length in pre-cracked specimens. A test matrix, pre-cracked ASTM C(T), SE(B) specimens and non-standard A(B) arc-shaped geometry was prepared. Materials were ASTM 387-Gr.22 2.25Cr-1.0Mo steel, an API Gr.N80 and HSLA welded joints. Initial and final crack lengths were measured on the crack surface and instantaneous crack length was determined by the compliance method to compare against the values of crack length estimated using the S_pb parameter. The difference between the resulting values was less than 15%Δa suggested as reference in ASTM E1820-96.     

A simplified improved beam analysis of the DCB specimen

Large deviations from the simple cantilever beam model have been observed when analysing double cantilever beam tests in composite materials. In the present paper a simplified improved beam model is proposed. The cracked part of the specimen is analysed by means of shear-corrected classical beam theory. The uncracked part is analysed by considering Saint Venant effects and deformation of a beam on an elastic foundation. Superposition of compliances results in a simple closed form expression which can be used to isolate the effects of different material parameters and analytical simplifications. Crack length corrections are discussed as well as the corrections that have to be made when calculating the strain energy release rate. An extensive numerical comparison is made with the more elaborate solutions of Whitney and Williams. Finally, a discussion is made on the relevance of one-dimensional analysis as compared with two- and three-dimensional analyses. The discussion is supported by numerical comparisons.     

Numerical prediction of notch bluntness effect on fracture resistance of SM490A carbon steel

The present work investigates the notch radius effect on fracture resistance using the finite element (FE) damage analysis based on the multiaxial fracture strain model. The damage model was determined from experimental data of notched bar tensile and fracture toughness test data using a sharp‐cracked compact tension specimen. Then, the FE damage analysis was applied to simulate fracture resistance tests of SM490A carbon steel specimens with different notch radii. Comparison of simulated results with experimental data showed good agreement. Further simulation was then performed to see effects of the specimen size, thickness, and side groove on J‐R curves for different notch radii. It was found that effects of the specimen size and thickness became more pronounced for the larger notch radius. Furthermore, it was found that without side groove, tearing modulus for notched specimens was similar to that for cracked specimens, regardless of the notch radius.     

Cyclic fracture behaviour of 304LN stainless steel under load and displacement control modes

Attempts have been made to understand cyclic fracture behaviour of AISI 304LN stainless steel used for nuclear piping materials under load vis‐à‐vis displacement controlled fracture tests; the former closely simulate the seismic loading conditions. The load controlled tests indicate that a material fails in a limited number of cycles even when the load amplitudes are sufficiently below the maximum load in a monotonic J–R test. The displacement controlled tests, on the other hand, show that the energy absorbing ability of a material gets severely reduced under cyclic loading conditions. The obtained results on standard laboratory specimens have been compared with similar available results on components in order to provide guidelines for maximum load bearing capability of engineering components under cyclic loading.     

Influences of flux-cored arc welding consumables on dynamic fracture toughness (J1d) of armour grade Q&T steel joints

Quenched and tempered (Q&T) steels are prone to hydrogen-induced cracking in the HAZ after welding. Austenitic stainless steel (ASS) welding consumables are traditionally used for welding of high hardness Q&T steels as they have higher solubility for hydrogen. The use of stainless steel consumables for a non-stainless steel base metal is not economical. In recent years, the developments of low hydrogen ferritic steel (LHF) consumables that contain no hygroscopic compounds are utilized for welding of Q&T steels The use of ASS and LHF consumables for armour grade Q&T steel will lead to formation of distinct microstructures in their respective welds. This microstructural heterogeneity will have a drastic influence in the dynamic fracture toughness of the armour grade Q&T steel welds. Hence, an attempt was made in this paper to study the influence of flux-cored arc welding consumables on dynamic fracture toughness ( J _1d) of armour grade Q&T steel joints. The flux-cored arc welding consumables have a significant effect on the dynamic fracture toughness of the armour grade Q&T steel welds. The joints fabricated using ASS flux-cored wires showed superior J _1d values than the joints fabricated using LHF consumables and the base metal.     

Measuring residual stress and the resulting stress intensity factor in compact tension specimens

The measurement of residual stress through the remaining ligament of a compact tension specimen was studied. In the crack compliance method, a slot or notch is successively extended through the part, and the resulting strain is measured at an appropriate location. By using a finite element simulation of a specimen preloaded beyond yield, three techniques for determining the original residual stress from the measured strains were compared for accuracy and sensitivity to measurement errors. A common beam-bending approximation was substantially inaccurate. The series expansion method proved to be very versatile and accurate. The fracture mechanics approach could determine the stress intensity factor caused by the residual stresses with a very simple calculation. This approach offers the exciting possibility of determining the stress intensity factor prior to a fatigue or fracture test by measuring strains during the specimen preparation.     

A new method for determining the fracture toughness of main pipeline steels

One of the fundamental aims of fracture mechanics is to define fracture toughness K_IC of a material. Hence, the ASTM E399 standard was developed. However according to the standard, large‐sized specimens are required to determine the fracture toughness of low alloy carbon steels. ASTM E1921 standard was developed on the fracture toughness of ferritic steels. In this study, a new method was proposed to determine the fracture toughness of ferritic steels. The purpose of the present paper is to compare the results of the method with the experimental results. Two steels that are used in gas and oil main pipelines were investigated in this study.     

A critical investigation of the use of a mandrel peel method for the determination of adhesive fracture toughness of metal-polymer laminates

The application of peel tests for the measurement of adhesive fracture toughness of metal-polymer laminates is reviewed and the merits of a mandrel peel method are highlighted. The mandrel method enables a direct experimental determination of both adhesive fracture toughness (G_A) and the plastic bending energy (G_P) during peel, whilst other approaches require a complex calculation for G_P. In this method, the peel arm is bent around a circular roller in order to develop a peel crack and an alignment load attempts to ensure that the peel arm conforms to the roller.The conditions for peel arm conformance are thoroughly investigated and the theoretical basis for conformation are established. Experimental investigations involve the study of the roller size (radii in the range 5-20 mm are used), the peel arm thickness (varied from 0.635 to 2.0 mm) and the magnitude of the alignment load. In addition, the plane of fracture is studied since fractures can vary from cohesive to interfacial and this has a profound influence on the value of G_A and on interpretation of results.A test protocol for conducting mandrel peel is developed such that the roller size for peel arm conformance can be established from preliminary fixed arm peel tests.The work is conducted on two epoxy/aluminium alloy laminates suitable for aerospace applications. Comparative results of adhesive fracture toughness from mandrel peel and multi-angle fixed arm peel are made with cohesive fracture toughness from a tapered double cantilever beam test.     

Separable functions in load separation for the ηpl and ηpl CMOD plastic factors estimation

The objective of this paper is to develop the load separation method for evaluating the _pl and _pl ^CMOD plastic factors used in the J estimation approach based on load versus displacement records. Appropriate forms for the geometry and deformation functions have been suggested from the EPRI Handbook solutions to produce the separable form for the load. The obtained functions are applied to evaluate the _pl and _pl ^CMOD plastic factors for center cracked tension specimen. The present load separation method gave results which are somewhat different from the estimated values of _pl given in the literature. For shallow cracks, the _pl and ^CMOD _pl plastic factors show considerable variation with crack size and the strain hardening exponent. For a deeply cracked CCT specimen, the ^CMOD _pl factor tends to the _pl factor and equals approximately unity. Abbreviations: CCT – center cracked specimen; CMOD – Crack Mouth Opening Displacement; EPRI – Electric Power Research Institute; FEM – Finite Element Method; LLD – Load Line Displacement.     

A method to evaluate ductile fracture toughness based on load separation principle

In combination with load separation principle, the P‐V curves of blunt‐notched specimens with different stationary cracks can be explicitly expressed as a unified formula. Further, a load separation‐based direct calibration (LSDC) method to predict instantaneous crack size and J‐resistance curve of growing cracked specimen has been developed. Two specimen configurations, compact tension and single edge‐notched bending, which are made of Cr2Ni2MoV and Q345B, respectively, are employed to verify the validity of LSDC method. The results show that the estimated J‐resistance curves are more reasonable in comparison with those obtained by unloading compliance and normalization method.     

Residual strength of unidirectional fibre-metal laminates based on JC toughness of C(T) and SE(B) specimens: comparison with M(T) test results

Fibre‐metal laminates (FMLs) are structural composites designed with the aim of producing very low fatigue crack‐propagation rate, damage‐tolerant and high‐strength materials, if compared to aeronautical Al alloys. Their application in aeronautical structures demands a deep knowledge of a wide set of mechanical properties and technological values, including both fracture toughness and residual strength. The residual strength of FMLs have been traditionally determined by using wide centre‐cracked tension panels M(T). The use of this geometry requires large quantities of material and heavy laboratory facilities. In this work, fracture toughness ( J_C) of some unidirectional FMLs laminates was measured using a recently proposed methodology for critical fracture toughness evaluation on compact tension C(T) and single‐edge bend SE(B) specimens. Additionally, residual strength values of wider M(T) specimens with different widths (W from 150 to 200 mm) and several crack to width ratios (2a/W) were experimentally obtained. Some experimental residual strength values of M(T) specimens (W from 150 to 400 mm and different 2a/W ratios) of Arall were also obtained from the bibliography. Based on J_C results from C(T) and SE(B) specimens, and either using or not using crack‐tip plasticity corrections, the residual strengths of the M(T) specimens were predicted and compared to the experimental ones. The results showed good agreement, especially when crack‐tip plasticity corrections were applied.     

Procedure for accurate calculation of the J-integral from digital volume correlation displacement data

The application of digital image and volume correlation techniques to obtain displacement fields from images has become ubiquitous in experimental fracture mechanics. In this paper, a procedure to extract the J-integral (J) from three-dimensional displacement fields obtained using digital volume correlation is presented. The procedure has been specially adapted to allow for experimental noise and errors, such as poorly defined crack front displacements, smearing of the displacement field across the crack faces, and knowledge of the imprecise crack front location. The implementation is verified using analytical crack-tip fields perturbed with synthetic image correlation errors to characterise the response of J. The method is then applied to experimental results using a Magnesium alloy WE43 loaded elastically in mixed mode. The steps outlined are intended as a guideline for the application of the volume integral from displacement fields to allow for accurate calculation of J along a crack front embedded within the volume.     

On the use of constraint parameter A2 determined from displacement in predicting fracture event

The constraint based fracture mechanics methodology, J–A₂ method, has been used to interpret cleavage fracture recently. In all previous studies, the constraint parameter A₂ was determined by stresses analytically calculated from finite element analyses (FEA). In the current paper, it is first demonstrated that A₂ can be measured during a fracture test using the crack tip opening displacement (CTOD). A single-edge-notched specimen under bending (SENB) is used to compare the A₂ values determined from δ₅ displacement and the stress components. Finally, cleavage fracture toughness values for A533-B reactor pressure vessel (RPV) steel at −40°C obtained from test programs at Oak Ridge National Laboratory (ORNL) and the University of Kansas (KU) are interpreted using the J–A₂ analytical model. Particular emphasis is placed on using the A₂ determined from CTOD to characterize the fracture event. It is demonstrated that the effects of crack depth (shallow vs deep) and specimen size (small vs large) on the fracture toughness from the test programs can be interpreted and predicted using J and the constraint level A₂ measured from the displacement.