Tension Specimens Research Articles

Equivalent conversion of different multiaxial stress rupture criteria for creep materials based on skeletal point stresses

Multiaxial creep life prediction of components has been challenging. To cope with the challenge, efforts have been made to develop suitable multiaxial stress rupture criteria (MSRC) based on representative stress concept. Of all MSRCs, the ones due to Sdobyrev and Hayhurst-Leckie (denoted as SHL MSRC), and to Cane (denoted as Cane MSRC) are commonly-used. In this work, the equivalent conversion between the SHL MSRC and the Cane MSRC is investigated based on skeletal point stresses. Equivalent conversion formulae are proposed based on the skeletal point stresses of circumferentially-notched tension (CNT) specimen, and then validated by comparison with the experimental data available in the literature. The equivalent conversion formula between the two MSRCs is found to be dependent on the signs of principal stresses. Therefore, different conversion formulae should be chosen depending on the signs of principal stresses. Afterwards, the applicability of different equivalent conversion formulae is demonstrated for the CNT specimen when skeletal point stresses are available, and for creep crack growth specimen when skeletal point stresses are not available. Finally, the equivalence of the two MSRCs is discussed.

Parametric characterization of the Christopher–James–Patterson model for crack propagation in welded zone of A7N01 Aluminum alloys

AbstractAluminum alloy is a widely used material in railway vehicle structures. In order to accurately analyze the crack propagation mechanism of Aluminum alloy welding structures and predict their crack propagation life, this study focuses on the A7N01 Aluminum alloy and proposes a full‐field strain solution method based on the least‐squares method. For the first time, digital image correlation (DIC) experimental measurements are combined with the finite element analysis method to determine the shape and size of the plastic zone at the crack tip of the compact tension (CT) specimen. And it also calculates the crack propagation driving force parameters of the Christopher–James–Patterson (CJP) model using traditional crack propagation driving parameters. The research results revealed that the plastic zone at the crack tip captured by DIC experiments is in good agreement with the finite element simulation results. Additionally, the crack growth rate curve of the A7N01 Aluminum alloy, fitted based on the CJP model, is insensitive to the stress ratio. The results offer an effective approach to utilizing the da/dN‐∆KCJP curve in analyzing A7N01 Aluminum alloy and welded structural failures, broadening the scope of engineering applications for the CJP model.

Fracture toughness and microstructural analysis of rotary friction welded S355J2 and SS316L steels for critical applications

Dissimilar metal welding is seeing growing adoption across industries to enhance structural functionality and efficiency. Achieving high-quality, defect-free dissimilar weld joints requires a comprehensive understanding of the interrelationships between the welding-induced microstructural changes and the material's performance characteristics, particularly its fracture-related properties. This study investigates the impact of microstructural changes on the fracture toughness of dissimilar welds between structural low-carbon steel (S355J2) and austenitic stainless steel (SS316L) prepared using the Rotary Friction Welding (RFW) technique. Welding preforms were created from respective pipe pup pieces. The evaluation involves microstructural analysis, tensile testing, hardness testing, and fracture toughness testing using compact tension specimens derived from various zones of the weld joints. Results revealed significant microstructural differences across the weld joint. The weld region exhibited stable hardness with a maximum of 208 HV1 in S355J2′s thermo-mechanically affected zone (TMAZ). High tensile strength (Ultimate Tensile Strength 540 MPa, Yield Strength 367 MPa) with failures mainly on the S355J2 side. The fracture toughness (KQ) matched parent metal values, with the RFW weld centre line (WCL) showing superior crack tip opening displacement (CTOD) of 0.35 mm. Fractography generally indicates ductile failure.

Phase field modeling of underloads induced fatigue crack acceleration

This study proposes a novel methodology to model the fatigue crack growth acceleration under underloads using a phase field fracture framework. In this model, fatigue crack growth is characterized by the degradation of fracture toughness, with an emphasis on employing a representative loading strategy instead of explicit cyclic loading, thus accelerating simulations of high-cycle fatigue. The model integrates a zone-based crack acceleration approach that responds to single-cycle underload. Notably, the model adeptly captures the dynamics described by the Paris-Erdogan law. Post-underload crack growth acceleration is simulated by identifying an acceleration zone near the crack tip, inspired by existing models based on the plastic zone. This zone is defined by a strain energy density threshold, and the underload ratio governs the rate of fatigue damage accumulation attenuation within this area. The implementation leverages the UMAT user subroutine in Abaqus, utilizing coupled temperature-displacement elements where temperature analogously represents the phase field parameter. Experimental validation of the model confirms its ability to accurately reflect the loss of fatigue life and the acceleration of crack growth rates in compact tension and middle tension specimens. Additionally, the model shows promise for extension to periodic underloads, highlighting its potential for simulating real-world fatigue scenarios effectively.

Performance of bolted steel laminated bamboo lumber connections under axial cyclic loading

Bolted connections are well known for their simplistic design and installation, and have been used in bare frame structures of various types of materials including steel and timber. Compared with monotonic loading, cyclic loading is more dangerous. Bamboo has excellent toughness and deformation recovery ability but a quantitative description of the mechanical performance of bolted steel laminated bamboo lumber (LBL) connections under cyclic loading has not yet been determined. As a result, tension and compression tests were carried out separately under axial cyclic loading. Obtained experimental results indicated that the tensile specimens suffer brittle failure because of the premature cracking of LBL due to cyclic loading; the bolts remained almost straight after failure. On the other hand, cyclic compressive specimens showed considerable ductile behavior as the bolts underwent plastic deformation since LBL sustained considerable load prior to failure due. Key mechanical connection parameters such as elastic stiffness, yield and ultimate displacement, yield and ultimate load, ductility ratio obtained from the skeleton curves were carefully analyzed to study the influence of bolt number on various properties. The increase in elastic stiffness from two to four bolts is about 60 % for the cyclic tension specimens, whilst that was about 25 % for the cyclic compression specimens. Compared with the cyclic tensile specimens, cyclic compressive specimens have higher ductility ratio, which is mostly greater than 2. In addition, theoretical models for both loading and unloading curves have been proposed, which showed good agreement with test results.

Experimental study on hybrid fiber reinforced high performance concrete properties: Investigatingcomprehensive energy consumption capacity

The brittleness of concrete in tension increases with higher compressive strength, necessitating the study of cementitious composites that combine high strength and ductility. This study investigated the fresh and mechanical properties of high-strength concrete (HPC) incorporating fibers: steel, basalt, and polyethylene (PE). Hybrid fiber reinforced high performance concrete (HyFHPC) was created by tailoring mix proportions, with eight series of specimens cast, each carrying different fiber combinations alongside plain HPC as a reference. Compressive, tensile, and flexural tests were conducted, and results were analyzed. The electron microscopy techniques were employed to observe the microstructural morphology and perform elemental analysis of the composites. Findings revealed a dense HPC matrix with 1.5 % total fiber content, exhibiting uniform dispersion and ideal bonding. Digital image correlation (DIC) technique was utilized to analyze strain distribution and crack development of specimens in tension and bending, proving effective in tracking the strain and crack evolution. Statistical analysis of five influencing elements—cubic compressive strength, axial compressive strength, tensile energy dissipation, tensile fracture energy, and energy release rate—revealed Series F9 (S0.5P1) as optimal for comprehensive energy consumption performance. HyFHPC demonstrated enhanced strength, ductility, and toughness compared to HPC with mono fiber or none. The hybridization achieved positive synergy and improved reinforcing efficiency while conserving resources.

Phase field modelling of elastic-plastic fatigue fracture of oil and gas pipeline

This paper established a fatigue fracture phase-field model (PFM) to evaluate fatigue damage evolution and crack propagation in oil and gas pipeline. To address inaccuracies in damage evolution, a threshold of the elastic-plastic fracture energy was introduced in the proposed PFM. Using the finite element method, the PFM was applied to simulate fatigue crack growth. Results from compact tension (CT) specimen of the X56 gas pipeline steel demonstrated that the da/dN-ΔK curve from the current PFM, accounting for plasticity, aligned more closely with experimental results than the elastic PFM. The fatigue crack propagation and fatigue life of the X80 gas pipeline with different defects of the same depth were also analyzed. The results indicated that triangular defects significantly impacted the fatigue life of the X80 gas pipeline. Finally, a model of X60 pipeline with various initial defects was developed to validate the effectiveness of the proposed PFM for full-scale pipeline fatigue fracture by comparing it to experimental a-N curves. The simulation results indicated that the distance and angle between two initial defects in the pipeline significantly influenced the propagation of fatigue cracks and the pipeline’s service life. These findings of this paper can serve as a reference for estimating the service life of gas and oil pipelines.

Investigating the effect of stress ratio on fatigue crack growth rate behaviour in friction stir welded AA7075-T651 aluminium alloy plates

This study examines the characteristics of Fatigue Crack Growth Rate (FCGR) in AA 7075-T651 Friction Stir Welded (FSW) joints with a thickness of 16 mm. Compact tension (CT) specimens from both the base and the FSW joints were utilized to assess FCGR under various stress ratios (0.2 to 0.5). Additionally, the study involved analyzing the transverse tensile characteristics of both the based and the welded joints. The microstructures of the welds were inspected using optical and transmission electron microscopes. Findings indicate that the FSWjoint critical stress intensity factor range (ΔKcr)is lower than the unwelded base, that can be related to precipitates dissolving in the weld zone when the friction stir welding process. Consequently, in contrast to the unwelded base, the fatigue life of AA 7075-T651 aluminium alloy FSW joints is significantly reduced. A scanning electron microscope carried out microstructural analyses of the fracture surfaces of the FSW and base under various stress ratioconditions. The results showed that, in contrast to the parent material, the FSW joint has ultra-fine grains resulting in intergranular cracks in the propagation zone.

On comparison of fracture toughness of irradiated and thermally aged decommissioned nuclear weld metals with miniature specimen test technique

This paper investigates the fracture toughness of thermally aged and irradiated weld metals extracted from head and beltline regions of a decommissioned nuclear reactor pressure vessel. Miniature compact tension specimens are fabricated from the thermally aged reactor pressure vessel head weld as well from the circumferential and axial beltline welds having fluences of 2.90 × 1016n/cm2 and 7.94 × 1017n/cm2, respectively. Master curve approach in accordance with the ASTM E1921 is applied to determine the reference temperature T0 and it is found to be − 113.5 °C, −104.4 °C, and − 89.3 °C for the reactor pressure vessel head, axial beltline, and circumferential beltline welds, respectively. In addition, the multimodal reference temperature Tm to assess the homogeneity of welds as well as key curve analysis for the quality assurance of testing are also provided. Fractographic analysis identified variations in crack initiation sites and microstructural inhomogeneities, particularly in the irradiated beltline welds. Overall, fracture mechanical testing of the materials demonstrated that miniature compact tension specimens are effective for characterizing the fracture toughness of limited surveillance weld materials, revealing differences due to irradiation and weld location.

Analysis of crack propagation of PLA fabricated by the additive manufacturing technique

Understanding crack propagation is crucial for evaluating the structural integrity and reliability of additive manufacturing components, as cracks can compromise mechanical properties and potentially lead to catastrophic failures. The study of crack propagation in additive manufacturing components is used to develop strategies for mitigating crack initiation and growth, improving material properties, and optimising the design and manufacturing processes. Crack propagation in additive manufacturing components can be influenced by various factors, including material properties, design considerations, manufacturing defects, and loading conditions. Due to the identified issue, the study was carried out to investigate the crack propagation of the Fused Deposition Modeling (FDM) component using compact tension fracture testing. The experimental work started with fabricating the samples using PLA material, followed by a fracture test based on the compact tension specimen test to get a response of the structure and its crack propagation under tensile. Material properties were also collected using the dog bone tensile test. The material properties of the testing were then imported to Finite Element Analysis for further investigation of fracture mechanics. It was found that the maximum force of the sample was 141.7 ± 28 N at 1.70 ± 0.26 mm displacement, and cracks initiated around the tip and propagated upward or downward based on the initial crack location. The deformation patterns of PLA material have shown it to be brittle plastic deformation and low energy absorption before fracture.

Effect of thermal ageing on fatigue crack growth behaviour of forged alloy 617M at elevated temperatures

ABSTRACT Fatigue crack growth (FCG) studies have been conducted on compact tension specimens of alloy 617M for as-received and aged for 1000, 5000 and 10,000 h at 710°C. Effects of thermal ageing on FCG behaviour of alloy 617M forged has been characterised at 27°C, 650°C, 710°C and 750°C. Results reveal that threshold stress intensity factor ( Δ K th ) decreases as temperature increases upto 710°C and then increases at 750°C for all the ageing conditions except for 1000 h aged. The Δ K th value also decreases with an increase in ageing time till 5000 h, but at 10,000 h a reverse trend is observed. The influence of thermal ageing and temperature reflects major deviations for Paris constant (C) and exponent (m). Additionally, thermal ageing and temperature influences on FCG, the effects of dynamic strain ageing, Cr-rich carbides and γ’-phase precipitations have been discussed and corroborated with fractographic and microstructural changes.

Experimental Characterization and Phase-Field Damage Modeling of Ductile Fracture in AISI 316L

(1) Modeling and characterization of ductile fracture in metals is still a challenging task in the field of computational mechanics. Experimental testing offers specific responses in the form of crack-mouth (CMOD) and crack-tip (CTOD) opening displacement related to applied force or crack growth. The main aim of this paper is to develop a phase-field-based Finite Element Method (FEM) implementation for modeling of ductile fracture in stainless steel. (2) A Phase-Field Damage Model (PFDM) was coupled with von Mises plasticity and a work-densities-based criterion was employed, with a threshold to propose a new relationship between critical fracture energy and critical total strain value. In addition, the threshold value of potential internal energy—which controls damage evolution—is defined from the critical fracture energy. (3) The material properties of AISI 316L steel are determined by a uniaxial tensile test and the Compact Tension (CT) specimen crack growth test. The PFDM model is validated against the experimental results obtained in the fracture toughness characterization test, with the simulation results being within 8% of the experimental measurements. (4) The novel implementation offers the possibility for better control of the ductile behavior of metallic materials and damage initiation, evolution, and propagation.

A Machine Learning Boosted Data Reduction Methodology for Translaminar Fracture of Structural Composites

AbstractThis work explored a machine learning (ML) algorithm as a fast data reduction method for translaminar fracture energy in composite laminates. The method was validated with translaminar fracture tests on compact tension (CT) specimens on AS4/8552 and IM7/8552 cross-ply lay-ups. Experimental fracture energy and R-curves for both materials were determined using the most common data reduction methods, such as the compliance calibration (CC), the area (AM) and the Irwin relationship (IM). Our new data reduction method uses a surrogate model based on an artificial neural network (ANN) trained with synthetic data generated with the cohesive crack finite element model. Such a surrogate model maps the cohesive properties with the corresponding load–displacement, crack-displacement and energy-displacement curves with interrogation times in the order of 20 ms and relative errors in the load–displacement and crack growth less than 2%. Such performance enabled its encapsulation to approximate the inverse problem to infer the cohesive parameters with the maximum likelihood estimator (MLE) directly from the experimental load–displacement and crack-displacement curves. The results demonstrated the ability of the model to deliver cohesive parameter inference directly from the macroscopic tests carried out at the laboratory level.

Tension anisotropy of rolled AA7075-T6 auminium alloy: Experiments, BP-Neural network modeling and orientation dependent failure mechanisms

This paper presents the investigations on anisotropic tension mechanical responses of AA7075-T6 based on experiments, classical strength theory, BP-neural network modeling and fracture morphology characterization. Results show that the tension strength anisotropy weakens with deformation degree. Compared to the traditional method, the machine learning model exhibits more flexible in solving the anisotropic plastic responses of AA7075-T6 auminium alloy sheet and provides more accurate predictions. Through analyzing the fracture surface of tension specimen at various orientations, the failure mechanism is sensitive to orientation. Specifically, the irregular distribution of dimples zones and cleavage steps can be observed at lower material orientation. As the orientation increases, the alternative occurrence of ductile and brittle features dominates the failure mechanism. The medium-size dimple caused by coalescence of small-size dimples represents a transition between ductile features zone and brittle characteristics region.

A remaining multiaxial ductility-based fracture toughness prediction model for metallic alloys

Predicting fracture toughness is an important part of developing an overall structural integrity methodology for components. This study therefore tries to establish a fracture toughness prediction model based on the concept of remaining multiaxial ductility exhaustion. Numerical simulation model which takes into account the level of constraint at the crack tip and the material’s inherent multiaxial ductility is established to research fracture initiation and propagation. The core of the model is based on increasing load to consume the remaining multiaxial ductility of metallic alloys until final fracture. The presence of grains (G) and grain boundaries (GB) in the simulation model is used to highlight a sensitivity analysis on the local interaction of microstructural at meso-scale with regards to crack initiation and growth. The model has been validated with fracture toughness tests by using additively manufactured Ti6Al4V alloy compact tension (CT) specimens. It is shown that the predicted plane strain KIC compares well with results from both wrought and heat-treated additively manufactured test specimens. Future tests on different other materials using low/high constraint cracked geometries should strengthen the model’s predictive capabilities for different fracture scenarios.

Effects of multiple overloads and high-low sequence loading on the fatigue crack propagation of AZ31B magnesium alloy

An experimental study was conducted to examine the initiation of cracks in the AZ31B magnesium (Mg) alloy for tensile overload effects, exploring the influence of overload cycle number, magnitude, and multiplicity of the applied overloads (OL). The number of cycles to failure increased from 8 % to 15 % for overload ratio (OLR) of 1.5 and 2.0 respectively applied as first cycle when compared with constant amplitude loading (CAL). However, the number of cycles to failure increased from 25 % to 43 % for OLR of 1.5 and 2.0 respectively when overload was applied at 5000th cycle. The onset of cracks was delayed when two overloads were applied as the first and 5000th loading cycles. The longitudinal-transverse (L-T) specimens were tested using a two-step high-low sequence loading procedure by varying maximum and minimum load. The fractography results also depict variations in the initial crack formations under different overload conditions. The effects of different overloads on the fracture surfaces of Compact Tension specimens were examined using the Field Emission Scanning Electron Microscopy, indicating substantial reduction in both the number and size of microcracks. 3D profilometer tests were performed, indicating a surface roughness of 0.20 μm to 0.30 μm.

Laser shock peening for enhanced fatigue resistance in steel structures: Insights from Q960 steel study

Fatigue damage of steel structures is often likened to the “cancer” of structural engineering. Fatigue failure, characterized by the absence of apparent signs before occurrence, usually leads to significant losses. To investigate the potential of laser shock peening (LSP) as an effective method for improving the fatigue performance of steel structures, square plates and compact tension (CT) specimens made of Q960 structural steel were designed. The surface integrity, fatigue crack growth (FCG) performance, microstructural grain size, and fracture morphology of specimens were compared at different laser shock parameters. The results show that LSP implanted a high level of beneficial residual compressive stress in a mild way, and no obvious surface defects were introduced. A noticeable retardation effect was found in the FCG of LSPed specimens, and this effect becomes more pronounced with the increase of laser pulse energy. Compressive residual stress is a more dominant factor in enhancing fatigue performance compared to grain refinement for the small-grained metals studied. LSP can effectively reduce the FCG rate of Q960 high-strength steel, showing promising application prospects in strengthening the fatigue hazard zone of steel structures.

The optimal design of double-edge wedge-splitting tensile test for ultra high performance concrete

The double-edge wedge-splitting (DEWS) test is a new indirectly tensile test, those of stress distribution similar with the uniaxial tension state. However, there are no standard rules regarding the splitting specimen and groove dimensions suitable for the UHPC tensile properties. In this paper, DEWS test, conventional splitting test, and direct tensile test were performed to measure the tensile strength and tensile stress vs. crack opening response of UHPC. The digital image correlation (DIC) technique was utilized to detect the crack propagation paths and fracture modes. Two specimen shapes, three groove angles and three fiber types are utilized to this investigation. The experimental results of 18 DEWSs under tensile loading, including peak strength, tensile stress-COD curves, fracture modes and properties, and the crack paths, are presented. The results show that the UHPC tensile performance used with cubic specimen is less sensitive to groove angle than in the cylinder. An important finding is that DEWS test used with 45° grooved cube specimen presents the lowest variation in peak strength and stress-COD curve for UHPC tensile behaviors. After comparison, the tensile strengths using DEWS test and direct tension test are essential equal in first-cracking and peak strength. The crack paths and fracture mode are basically the same inside the DEWS and direct tension specimens. Accordingly, the DEWS test used with 45° grooved cubic specimen is suggested for measuring the UHPC tensile property.

Determination of the fracture toughness of carburized Pyrowear 53 steel for planetary gears by the small punch test method

The aim of the current study was to determine the fracture toughness of different zones in carburized Pyrowear 53 steel using the small punch test (SPT) method. Firstly, Pyrowear 53 steel was quenched and tempered using different processing parameters to obtain core materials with varied microstructures and fracture toughness. The results obtained for the core material in standard fracture toughness tests were then compared with the SPT results, which allowed the determination of a formula correlating the fracture energy integral, JSPT, from the SPT with JQ integrals obtained from standardized compact tension specimens. In the next stage, Pyrowear 53 steel was carburized at 925 °C and divided into the following zones: (1) a carburized layer (up to 0.5 mm from the surface), (2) a transition layer (from 0.5 to 1.5 mm), and (3) a core zone (more than 1.5 mm). Each zone was characterized in terms of its microstructure and tensile properties using miniaturized test specimens. Finally, the fracture toughness values of the core zone (JSPT = 78–102 kJ/m2), the transition layer (JSPT = 71–80 kJ/m2), and the carburized layer (JSPT = 8.1–9.1 kJ/m2) were determined based on the obtained SPT results. It was shown that the use of such a relatively simple SPT method with the proposed energy-based approach seems to be a promising way of determining the fracture toughness of thin layers or local changes in the fracture behavior of surface-treated materials.

Microstructural causes and mechanisms of crack growth rate transition and fluctuation of additively manufactured titanium alloy

Wire and arc additive manufacturing (WAAM) enables rapid near-net-shape fabrications of large-size parts and in-situ remanufacturing in many industry sectors. A comprehensive understanding of the fatigue failure mechanism of WAAM titanium alloys is a prerequisite for their widespread use in critical structural components subject to fatigue load. Here, the fatigue crack growth behavior of WAAM TA15 material is investigated. Fatigue crack growth tests are performed using compact tension specimens sampled from different locations and with different crack orientations of the WAAM TA15 block. The fatigue crack growth rate (FCGR) data exhibit two governing rates separated by a transition stress intensity factor value, ΔKn, and the degrees of fluctuation of the FCGR data in the two regimes are notably different. A piecewise log-linear model is first proposed by incorporating the Heaviside step function and ΔKn into the classical Paris’ model, allowing for the transition ΔKn to be determined by the data. The potential causes of the transition ΔKn are phenomenologically inferred via fractography and surface roughness profiling results. The critical microstructure affecting the value of ΔKn is identified by relating the crack tip cyclic plastic zone size at ΔKn to the sizes of main microstructures. The cause of different degrees of fluctuations in the two regimes separated by ΔKn is inferred by examining the microstructures within the plastic zone. The microstructural mechanisms of the local FCGR reduction and fluctuation are further identified and explained.