Imperial College London

ProfessorKamranNikbin

Faculty of EngineeringDepartment of Mechanical Engineering

Professor of Structural Integrity
 
 
 
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Contact

 

+44 (0)20 7594 7133k.nikbin Website

 
 
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Location

 

721City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

386 results found

Xi J, Hu Y, Xing H, Han Y, Zhang H, Jiang J, Nikbin Ket al., 2021, The Low-Cycle Fatigue Behavior, Failure Mechanism and Prediction of SLM Ti-6Al-4V Alloy with Different Heat Treatment Methods., Materials (Basel), Vol: 14, ISSN: 1996-1944

Selective laser melting (SLM) is a promising additive manufacturing (AM) process for high-strength or high-manufacturing-cost metals such as Ti-6Al-4V widely applied in aeronautical industry components with high material waste or complex geometry. However, one of the main challenges of AM parts is the variability in fatigue properties. In this study, standard cyclic fatigue and monotonic tensile testing specimens were fabricated by SLM and subsequently heat treated using the standard heat treatment (HT) or hot isostatic pressing (HIP) methods. All the specimens were post-treated to relieve the residual stress and subsequently machined to the same surface finishing. These specimens were tested in the low-cycle fatigue (LCF) regime. The effects of post-process methods on the failure mechanisms were observed using scanning electron microscopy (SEM) and optical microscopy (OM) characterization methods. While the tensile test results showed that specimens with different post-process treatment methods have similar tensile strength, the LCF test revealed that no significant difference exists between HT and HIP specimens. Based on the results, critical factors influencing the LCF properties are discussed. Furthermore, a microstructure-based multistage fatigue model was employed to predict the LCF life. The results show good agreement with the experiment.

Journal article

Naraghi T, Najib MF, Nobari AS, Nikbin Ket al., 2021, Fitness for Service Assessment Approach for Ageing Pipeline Section Based on Sparse Historical Data, JOURNAL OF MULTISCALE MODELLING, Vol: 12, ISSN: 1756-9737

Journal article

Bagnoli KE, Cater-Cyker ZA, Hay CA, Holloman RL, Hioe Y, Wilkowski G, Rollins BC, Nikbin KMet al., 2021, Assessment of flaws in non-stress relieved carbon steel welds caused by hydrogen attack, ISSN: 0277-027X

Cracking due to high temperature hydrogen attack (HTHA) has been observed in non-PWHT'd carbon steel process equipment at conditions of temperature and hydrogen partial pressure below the original design limits recommended in API RP 941, necessitating changes to that standard. Consequently, flaw assessment procedures are needed to manage defects detected during inspection, or to establish appropriate inspection frequency. The latter typically involving estimation of the time for a detected or postulated crack to reach a critical size. This type of evaluation has been difficult to perform owing to the scarcity of data of fracture toughness as well as crack growth rate for steels in high temperature hydrogen. To address this gap, an experimental program was undertaken to help describe the ductile tearing characteristics of steel removed from service with various levels of HTHA damage. Near full thickness single edge notched tension SEN(T) specimens were machined from field samples and tested using existing "natural" cracks as the starter-crack. This provided insight into the behavior of real flaws subject to constraint conditions closely matching circumferential flaws in piping. Tests of undamaged steel were also performed in hydrogen at conditions designed to produce HTHA and compared with tests run in nitrogen. Crack growth tests obtained from the literature have been used to develop an empirical crack growth law for use in fitness for service assessments. The C*integral was also explored as a parameter for describing crack growth rate due to the strong similarity of HTHA damage to creep. The key results show substantial reduction in tearing resistance resulting from HTHA damage. A crack growth law similar to the Nikbin-Smith-Webster (NSW) model using the C*integral was found to show promise in describing the combined effects of creep and HTHA on crack propagation, although additional testing is needed to validate the correlation.

Conference paper

Zhao H, Xi J, Zheng K, Shi Z, Lin J, Nikbin K, Duan S, Wang Bet al., 2020, A review on solid riveting techniques in aircraft assembling, Manufacturing Review, Vol: 7, Pages: 1-18, ISSN: 2265-4224

Solid riveting is the most widely used joining technique in aircraft assembly, and the current key problems affecting practical application and reliable lifting are concentrated on static strength and fatigue. This paper aims to present a practical review on current practice and novel techniques of solid riveting for aircraft applications in order to obtain a thorough understanding of the underlying mechanisms of defect development to assist industrial users to find pragmatic solutions for safe life extension of components. At first, the current status of solid riveting processes is reviewed, and the key influencing factors on static/fatigue failure of riveted joints are identified. Effects of solid riveting design parameters, manufacturing parameters, residual stress, load transfer and secondary bending on static and fatigue strengths of riveted lap joints are discussed, followed by a review of the state-of-the-art solutions that deal with static/fatigue failures. Furthermore the new development in solid riveting techniques, including the use of different materials and riveting processes, is addressed. Finally, future research perspective and applications industrial riveting is presented.

Journal article

Chavoshi SZ, Hill LT, Bagnoli KE, Holloman RL, Nikbin KMet al., 2020, A combined fugacity and multi-axial ductility damage approach in predicting high temperature hydrogen attack in a reactor inlet nozzle, Engineering Failure Analysis, Vol: 117, ISSN: 1350-6307

High temperature hydrogen attack (HTHA) and creep damage and cracking are time-dependent phenomena which can occur at relatively low temperatures (300–500 °C) in low alloy steel components used in petroleum refinery and petrochemical plant. The present study combines a novel multiaxial ductility creep model with a sub-grain level fugacity partial pressure strain-based failure criterion due to a build-up of methane gas to predict progressive damage accumulation and HTTA degradation in an aging nozzle component. Finite element (FE) simulations using appropriate subroutines for the coupled approach allow progressive failure predictions combining HTHA and creep deformation in a C-0.5Mo steel inlet nozzle which had seen 80,000 h operation. A sensitivity study has been carried out to quantify the effects of different operating temperatures, failure strains, hydrogen concentration and pressure and material hydrogen diffusivity on the levels of damage in the nozzle over its lifetime. The predictions accurately bound the measured damage observed in the nozzle component and highlight the sensitivity of the model with respect to the input variables.

Journal article

Chavoshi S, Tagarielli V, Shi Z, Lin J, Wang S, Jiang J, Dear J, Nikbin Ket al., 2020, Predictions of the mechanical response of sintered FGH96 powder compacts, Journal of Engineering Materials and Technology, Vol: 142, ISSN: 0094-4289

This paper presents predictions of the response of sintered FGH96 Ni-based superalloy powder compacts at high temperature, obtained by analysis of 3D representative volume elements generated by both X-ray tomography and a virtual technique. The response ofthe material to a multiaxial state of stress/strain for porosities as large as 0.3 is explored, obtaining the yield surfaces and their evolution as well as scaling laws for both elastic and plastic properties. The two modelling approaches are found in good agreement. The sensitivity of the predictions to particle size, inter-particle friction, applied strain rate,and boundary conditions is also examined.

Journal article

Bagnoli KE, Cater-Cyker ZA, Holloman RL, Hay CA, Chavoshi S, Nikbin KM, Hill LTet al., 2020, Volumetric damage modeling of high temperature hydrogen attack in steel using a continuum damage mechanics approach, ISSN: 0277-027X

Hydrogen attack is a degradation phenomenon that affects process equipment operated at elevated temperatures in an environment containing a high hydrogen partial pressure. It has been the subject of numerous studies over the years prompted by damage discovered during routine inspections, or incidents that have occurred in service. As non-destructive evaluation (NDE) techniques have improved, damage is being detected during earlier stages where safe operation may still be possible for some time period. This work focuses on the fitness for service evaluation of equipment containing high temperature hydrogen attack (HTHA) using a continuum damage mechanics (CDM) approach. The model can be employed to assess the loss in load bearing capacity due to damage in the form of widespread microfissuring and voids (i.e. up to the point of macro-crack coalescence). Experimental data from literature sources have been used to develop a relationship between damage rate and operational loading conditions. The predictions are compared to field experience to illustrate key aspects of this approach.

Conference paper

Nikbin K, Liu S, 2019, Multiscale-constraint based model to predict uniaxial/multiaxial creep damage and crack growth in 316-H steels, International Journal of Mechanical Sciences, Vol: 156, Pages: 74-85, ISSN: 0020-7403

A new failure ductility/multiscale constraint strain-based model to predict creep damage, rupture and crack growth under uniaxial and multiaxial conditions is developed for 316H Type stainless steels by linking globally uniform failure strains with a multiaxial constraint factor. The model identifies a geometric constraint and a time-dependent local constraint at the sub-grain level. Uniaxial and notched 316H steel as-received and pre-compressed data at various load levels and temperatures with substantial scatter were used to derive the appropriate constitutive equations by using the proposed empirical/mechanistic approach. Constrained hydrostatic development of creep damage at the sub-grain level is assumed to directly relate to the uniform lower-bound creep steady state region of damage development measured at the global level. Uniaxial and notched bar rupture at long terms is predicted based on the initial short-term creep or a representative tensile strength and a multiaxial constraint factor. The model is consistent with the well-known NSW remaining multiaxial ductility creep crack growth model which predicts crack growth bounds over the plane strain/stress states. This model, therefore, unifies the creep process response over the whole range of uniaxial, notched and crack growth processes which is extremely consequential to simple long term failure predictions of components at elevated temperatures.

Journal article

Jones MD, Nikbin KM, Davies CM, 2019, LOAD LINE DISPLACEMENT PARTITIONING IN CREEP CRACK GROWTH ANALYSES OF 316H STAINLESS STEEL, ASME Pressure Vessels and Piping Conference (PVP 2018), Publisher: AMER SOC MECHANICAL ENGINEERS

Conference paper

Jäschke R, Weidlich M, 2019, Preface

Book chapter

O'Connor AN, Davies CM, Nikbin KM, 2019, FRACTURE TOUGHNESS OF DEFECTS ORIENTATED PARALLEL TO A DISSIMILAR METAL WELD BOUNDARY, ASME Pressure Vessels and Piping Conference (PVP 2018), Publisher: AMER SOC MECHANICAL ENGINEERS

Conference paper

Tarnowski K, Nikbin K, Dean DW, Davies Cet al., 2018, A unified potential drop calibration function for common crack growth specimens, Experimental Mechanics, Vol: 58, Pages: 1003-1013, ISSN: 0014-4851

Calibration functions, used to determine crack extension from potential drop measurements, are not readily available for many common crack growth specimen types. This restricts testing to a limited number of specimen types, typically resulting in overly conservative material properties being used in residual life assessments. This paper presents a unified calibration function which can be applied to all common crack growth specimen types, mitigating this problem and avoiding the significant costs associated with the current conservative approach. Using finite element analysis, it has been demonstrated that Johnson’s calibration function can be applied to the seven most common crack growth specimen types: C(T), SEN(T), SEN(B), M(T), DEN(T), CS(T) and DC(T). A parametric study has been used to determine the optimum configuration of electrical current inputs and PD probes. Using the suggested configurations, the error in the measurement of crack extension is <6% for all specimen types, which is relatively small compared to other sources of error commonly associated with the potential drop technique.

Journal article

Zhao L, Nikbin KM, 2018, Characterizing high temperature crack growth behaviour under mixedenvironmental, creep and fatigue conditions, Materials Science and Engineering A: Structural Materials: Properties, Microstructure and Processing, Vol: 728, Pages: 102-114, ISSN: 0921-5093

Components in high temperature plant could undergo failure due to combinations of fatigue, creep or oxidation/corrosion depending on the loading, temperature and environmental conditions. A novel and robust approach for a progressive failure modelling is presented in this paper which for the first time attempts to combine these failure mechanisms as time or cycle dependent processes. In this study, a combined multiaxial inter/transgranular crack growth model at the meso-scale level was proposed to conveniently deal with the various failure scenarios that may exist in plant components. The simulated crack under the combinations of time dependent creep and oxidation mainly propagated along grain boundaries initiating from the notch surface, exhibiting an irregular shapes with crack branching. Whereas under fatigue/oxidation condition, the crack grew in a transgranular manner. Furthermore, the role of creep, fatigue and oxidation on the failure life was dependent on the applied duration period at peak loads. Cracks were prone to nucleate in transgranular and then propagate in intergranular. There existed competitions between creep, fatigue and oxidation damage. Finally, the failure modes due to different damage mechanisms and loading conditions in the cases of creep-fatigue-oxidation were proposed. The calculated failure modes corresponded with those observed in engineering alloys.

Journal article

Tarnowski KM, Nikbin KM, Dean DW, Davies CMet al., 2018, Improvements in the measurement of creep crack initiation and growth using potential drop, International Journal of Solids and Structures, Vol: 134, Pages: 229-248, ISSN: 0020-7683

To predict the residual life of components operating in the creep regime, it is vital to accurately identify crack initiation, and measure subsequent crack growth, in laboratory tests. Potential drop (PD) measurements, used for this purpose, are susceptible to errors caused by the accumulation of creep strain. For creep ductile materials, this can result in highly conservative crack initiation models and the implementation of unnecessary inspection and maintenance programmes that can cost millions of pounds in lost revenue. Conversely, the crack growth models can be non-conservative.Using a novel combination of interrupted creep crack growth (CCG) tests and sequentially coupled structural-electrical finite element analyses a new method of interpreting PD data has been developed and validated. It uses an increase in gradient on a plot of PD vs. load-line displacement to accurately identify crack initiation. This has been compared to the current method in ASTM E1457-15 by reanalysing data from CCG tests performed on a range of materials at various temperatures and loads. The initiation times, measured using the current ASTM method, were underestimated by factors of up to 23 and the subsequent crack growth rates were underestimated by factors of up to 1.5.

Journal article

Ab Razak N, Davies CM, Nikbin KM, 2018, Testing and assessment of cracking in P91 steels under creep-fatigue loading conditions, ENGINEERING FAILURE ANALYSIS, Vol: 84, Pages: 320-330, ISSN: 1350-6307

Journal article

Alang NA, Nikbin K, 2018, An analytical and numerical approach to multiscale ductility constraint based model to predict uniaxial/multiaxial creep rupture and cracking rates, INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES, Vol: 135, Pages: 342-352, ISSN: 0020-7403

Journal article

Zhao L, Xu L, Nikbin K, 2017, Predicting failure modes in creep and creep-fatigue crack growth using a random grain/grain boundary idealised microstructure meshing system, Materials Science and Engineering A: Structural Materials Properties Microstructure and Processing, Vol: 704, Pages: 274-286, ISSN: 0921-5093

An idealised microstructure mesh model combined with a novel creep and creep-fatigue damage accumulation model was employed to simulate crack growth behaviour under creep/fatigue conditions for a modified 9Cr-1Mo steel. The influence of microstructures on the crack growth behaviour was studied using a random grains separated by idealised grain boundaries. For accurately representing the damage accumulation in creep-fatigue regime, a non-linear damage model was employed. In this case, creep damage was determined by multiaxial ductility exhaustion approach and fatigue damage was reliant on maximum stress and plastic range ahead of crack tip per loading cycle. When creep dominated, cracks mainly propagated along grain boundaries in steady crack growth stage. As an exception, the crack growth in the tertiary crack growth stage gradually changed from intergranular to mixed model and finally transgranular fracture. In contrast, in creep-fatigue regime, the crack growth behaviour was greatly reliant on the dwell time. For short duration period, the crack mainly propagated in a transgranular manner. But as the dwell time increased to greater than 600 s, the creep intergranular fracture dominated once again. Furthermore, the grain size gradient had little influence on the crack growth model and only affected the fracture life in creep and creep-fatigue regimes.

Journal article

Tarnowski KM, Dean DW, Nikbin KM, Davies CMet al., 2017, Predicting the influence of strain on crack length measurements performed using the potential drop method, Engineering Fracture Mechanics, Vol: 182, Pages: 635-657, ISSN: 0013-7944

The potential drop (PD) crack growth measurement technique is sensitive to strain accumulation which is often erroneously interpreted as crack extension. When testing ductile materials these errors can be significant, but in many cases the optimum method of minimising or suppressing them remains unknown because it is extremely difficult to measure them experimentally in isolation from other sources of error, such non-ideal crack morphology. In this work a novel method of assessing the influence of strain on PD, using a sequentially coupled structural-electrical finite element (FE) model, has been developed. By comparing the FE predictions with experimental data it has been demonstrated that the proposed FE technique is extremely effective at predicting trends in PD due to strain. It has been used to identify optimum PD configurations for compact tension, C(T), and single edge notched tension, SEN(T), fracture mechanics specimens and it has been demonstrated that the PD configuration often recommended for C(T) specimens can be subject to large errors due to strain accumulation. In addition, the FE technique has been employed to assess the significance of strain after the initiation of stable tearing for a monotonically loaded C(T) specimen. The proposed FE technique provides a powerful tool for optimising the measurement of crack initiation and growth in applications where large strains are present, e.g. J-R curve and creep crack growth testing.

Journal article

Tarnowski KM, Davies CM, Dean DW, Nikbin KMet al., 2017, Experimental determination of C* using a hot unload and a modified reference stress method, 14th International Conference on Fracture, Pages: 466-467

© 2017 Chinese Society of Theoretical and Applied Mechanics. All Rights Reserved. During creep crack growth (CCG) testing, large errors in the calculated elastic and plastic load-line displacement (LLD) rates, associated with assumptions of ideal crack geometry and power-law hardening material behavior, significantly influence the experimentally determined value of the crack tip characterizing parameter, C*. To mitigate these errors a novel method of interpreting CCG test data is proposed which combines experimental observations at the start and end of the test, with a modified version of the reference stress method. Using finite element analysis it has been demonstrated that this method can significantly improve the accuracy of the elastic and plastic LLD rates predicted throughout a CCG test.

Conference paper

Nikbin K, 2017, A unified multiscale ductility exhaustion based approach to predict uniaxial, multiaxial creep rupture and crack growth, Engineering Fracture Mechanics, Vol: 179, Pages: 240-259, ISSN: 0013-7944

Numerical and analytical methods for predicting uniaxial damage have largely depended on the constituent components of the stress/strain measured data which have inherent scatter. Models developed for this purpose have also attempted, with some degree of success, to address the fundamental issues of failure mechanisms within a multiaxial stress state context. This paper presents a new analytical/empirical/a postpriori unifying approach to predict creep damage and rupture under uniaxial/multiaxial and crack growth conditions by deriving a multiscale based constraint criterion. Essentially, the model links the global constraint due to geometry in a globally isotropic material with a microstructural constraint arising from creep diffusional processes occurring in a sub-grain locally anisotropic microstructure. Furthermore, it is shown that the model is consistent with the established NSW crack growth model (Nikbin et al., 1984, 1986; Tan et al., 2001) which is routinely used to determine the plane stress/strain bounds for cracking rates in fracture mechanics geometries and cracked components. The concept assumes that at very short times an initial upper shelf material tensile strength and global plasticity and power law creep control creep damage failure and sub grain multiaxial axial stress state dependent failure strain dominates the long term diffusion/dislocation controlled creep response. It is established that the material yield strength in the short term and a measure of creep failure strain at the creep secondary/tertiary transition region described at the limits by the Monkman-Grant failure strain (Monkman and Grant, 1963), are the important variables in both the uniaxial and multiaxial failure processes. For verification creep constitutive properties from long term data from uniaxial and multiaxial and crack growth tests on Grade P91/92 martensitic steels from various databases (EPRI, private communications; NIMS data base), are used to establish the procedur

Journal article

Wei Z, Nikbin K, 2017, Statistical Characterization, Pattern Identification, and Analysis of Big Data, SAE International Journal of Materials and Manufacturing, Vol: 10, ISSN: 1946-3979

© 2017 SAE International. In the Big Data era, the capability in statistical and probabilistic data characterization, data pattern identification, data modeling and analysis is critical to understand the data, to find the trends in the data, and to make better use of the data. In this paper the fundamental probability concepts and several commonly used probabilistic distribution functions, such as the Weibull for spectrum events and the Pareto for extreme/rare events, are described first. An event quadrant is subsequently established based on the commonality/rarity and impact/effect of the probabilistic events. Level of measurement, which is the key for quantitative measurement of the data, is also discussed based on the framework of probability. The damage density function, which is a measure of the relative damage contribution of each constituent is proposed. The new measure demonstrates its capability in distinguishing between the extreme/rare events and the spectrum events. Several case studies including vehicle reliability, vehicle road test score, warranty, salary distribution of an institution, the city population distribution in 3 countries, and the earthquake distribution worldwide and in the USA, are provided to demonstrate the role of the statistical and probabilistic approaches in the characterization and analysis of the big data.

Journal article

Kapadia P, Davies C, Pirling T, Hofmann M, Wimpory R, Hosseinzadeh F, Dean D, Nikbin Ket al., 2017, Quantification of residual stresses in electron beam welded fracture mechanics specimens (vol 106, pg 106, 2017), INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, Vol: 113, Pages: 255-255, ISSN: 0020-7683

Journal article

Najib MF, Nobari AS, Nikbin K, 2017, Modification and evaluation of a FRF-based model updating method for identification of viscoelastic constitutive models for a nonlinear polyurethane adhesive in a bonded joint, International Journal of Adhesion and Adhesives, Vol: 74, Pages: 181-193, ISSN: 0143-7496

In this study, a Frequency Response Function (FRF) -based model updating method, was modified for the purpose of the identification of viscoelastic constitutive models. A steel beam, bonded to a heavy rigid steel block by a layer of Sikaflex-252 polyurethane adhesive, was employed as the test setup. Using the concept of Optimum Equivalent Linear FRF (OELF), accelerance FRFs were measured at different random excitation levels which demonstrated the nonlinear behavior of the adhesive. Using a finite element model, the sensitivity analysis showed that the selected FRFs are more sensitive to the storage and loss moduli of the adhesive near the resonances. Therefore, firstly, both of the storage and loss moduli were identified near each resonance separately and the results have been compared with the results based on Inverse Eigen-sensitivity Method (IEM). In continuation, five viscoelastic constitutive models were utilized and identified to characterize the dynamic mechanical properties of the adhesive at different excitation levels. Applying the identified models, the correlation between the FRFs of the FE models and the experimental ones were tested. The results show that amongst the identified models, The Standard Linear Solid (SLS) model in parallel with a viscous or constant structural damper (stiffness proportional) results in better correlation with experiments. Increasing the excitation level, the storage modulus of the adhesive decreases, whereas the loss modulus increases, especially at high frequencies.

Journal article

Nasser M, Davies CM, Nikbin K, 2017, THE INFLUENCE OF AGR GAS CARBURISATION ON THE CREEP AND FRACTURE PROPERTIES OF TYPE 316H STAINLESS STEEL, ASME Pressure Vessels and Piping Conference, Publisher: AMER SOC MECHANICAL ENGINEERS

Conference paper

Wei Z, Nikbin K, McKeighan PC, Harlow DGet al., 2017, Foreword, astm, Pages: iii-iii, ISSN: 0066-0558

Conference paper

Larrimbe L, Nikbin K, Pettina M, Katz AP, Jones E, DeCerbo J, Hawkins C, Brown P, Vandeperre LJMet al., 2017, High heat flux laser testing of HfB2 cylinders, Journal of the American Ceramic Society, Vol: 100, Pages: 293-303, ISSN: 1551-2916

Hafnium diboride (HfB2) is one of a family of ultra-high temperatureceramics (UHTCs) which are being considered forapplication in environments with a substantial heat flux such ashypersonic flight. In order to characterize transitions in thematerial response with heat flux and therefore predict the inservicebehavior of UHTCs, a range of tests were conducted inwhich small cylindrical bars of HfB2 were laser heated usingheat fluxes from 25 to 100 MW/m2. After testing, the externaldamage as well as damage observable in cross sections throughthe cylinders was characterized using photography, optical, andscanning electron microscopy. Experimental results were comparedwith finite element modeling of the heat flow, temperaturedistribution, and phase transition. Heat flux rather thantotal deposited heat was found to be the strongest determinantof the way in which damage develops in samples; for lower heatfluxes, the main damage mechanism is oxidation, progressingto oxidation-induced melting and finally, at the highest heatfluxes, substantial ablation by melting irrespective of oxidation.The agreement between calculations and experimental observationsindicates that such calculations can be used with confidenceto guide the design of components.

Journal article

Tarnowski KM, Davies CM, Nikbin KM, Dean DWet al., 2017, EXPERIMENTAL DETERMINATION OF ELASTIC AND PLASTIC LLD RATES DURING CREEP CRACK GROWTH TESTING, ASME Pressure Vessels and Piping Conference, Publisher: AMER SOC MECHANICAL ENGINEERS

Conference paper

Kapadia P, Davies C, Pirling T, Hofmann M, Wimpory R, Hosseinzadeh F, Dean D, Nikbin Ket al., 2016, Quantification of residual stresses in electron beam welded fracture mechanics specimens, International Journal of Solids and Structures, Vol: 106-107, Pages: 106-118, ISSN: 1879-2146

Residual stress measurements have been made in a range of electron beam welded samples to study how the weld induced residual stresses redistributed during fabrication of compact tension, C(T), specimens. The samples were manufactured from Type 316H stainless steel in the ex-service material condition and in material which had been preconditioned by inducing 8% plastic strain. Measurements made using neutron diffraction, slitting and the contour method were generally in good agreement and showed residual stress components of up to three times the base material's yield strength existed in the samples. When sectioning a sample to perform the contour method, large elastic deformations occurred at the cut tip due to the large residual stresses present. A correction was applied to the measured surface displacements to account for this deformation. Neutron diffraction measurements were made at various stages of the fabrication process, which showed significant stress redistribution occurred as the welded samples were machined into C(T) specimens. However the tensile stresses near the crack tip of the C(T) specimens remained large and could significantly influence subsequent crack growth tests.

Journal article

Nasihatgozar M, Daghigh V, Lacy TE, Daghigh H, Nikbin K, Simoneau Aet al., 2016, Mechanical characterization of novel latania natural fiber reinforced PP/EPDM composites, Polymer Testing, Vol: 56, Pages: 321-328, ISSN: 0142-9418

© 2016 Elsevier Ltd Novel natural fiber-reinforced polypropylene (PP)/ethylene-propylene-diene-monomer (EPDM) composites containing 0–30 wt% of short latania fibers (SLFs) were fabricated and tested. Measured ASTM standard tensile and Charpy impact properties were compared to those for PP/EPDM composites containing short jute fibers (SJFs) [1] . SLF composites displayed somewhat better tensile properties than analogous SJF composites. Moreover, as the SLF weight fraction was increased, there was a profound increase in composite energy absorption capability (EAC). In contrast, increasing SJF weight fraction resulted in a marked decrease in the composite EAC. Good qualitative agreement was found between measured EAC values for both PP/EPDM/SLF and PP/EPDM/SJF composites and calculated fracture toughness values. Scanning electron microscopy images of PP/EPDM/SLF composite fracture surfaces were used to characterize the relevant failure mechanisms and damage morphology. This study suggests that latania fibers hold promise as a composite reinforcement phase for applications where both tensile properties and energy absorption are important.

Journal article

Behnia S, Daghigh V, Nikbin K, Fereidoon A, Ghorbani Jet al., 2016, Influence of Stacking Sequence and Notch Angle on the Charpy Impact Behavior of Hybrid Composites, Mechanics of Composite Materials, Vol: 52, Pages: 489-496, ISSN: 0191-5665

The low-velocity impact behavior of hybrid composite laminates was investigated. The epoxy matrix was reinforced with aramid, glass, basalt, and carbon fabrics using the hand lay-up technique. Different stacking sequences and notch angles were and notch angles considered and tested using a Charpy impact testing machine to study the hybridization and notch angle effects on the impact response of the hybrid composites. The energy absorption capability of specimens with different stacking sequences and notch angles is compared and discussed. It is shown that the hybridization can enhance the mechanical performance of composite materials.

Journal article

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