Prediction Of Failure Modes Research Articles

Microstructure-based constitutive modeling of TRIP steel: Prediction of ductility and failure modes under different loading conditions

We study the ultimate ductility and failure modes of a commercial transformation-induced plasticity (TRIP) 800 steel under different loading conditions with an advanced microstructure-based finite-element analysis. The representative volume element (RVE) for the TRIP 800 under examination is developed based on an actual microstructure obtained from scanning electron microscopy. The ductile failure of the TRIP 800 under different loading conditions is predicted in the form of plastic strain localization without any prescribed failure criteria for the individual phases. This indicates that the microstructure-level inhomogeneity of the various constituent phases can be the key factor influencing the final ductility of the TRIP 800 steel under different loading conditions. Comparisons of the computational results with experimental measurements suggest that the microstructure-based modeling approach accurately captures the overall macroscopic behavior of the TRIP 800 steel under different loading and boundary conditions.

Predicting failure modes and ductility of dual phase steels using plastic strain localization

Ductile failure of metals is often treated as the result of void nucleation, growth and coalescence. Various criteria have been proposed to capture this failure mechanism for various materials. In this study, ductile failure of dual phase steels is predicted in the form of plastic strain localization resulting from the incompatible deformation between the harder martensite phase and the softer ferrite matrix. Microstructure-level inhomogeneity serves as the initial imperfection triggering the instability in the form of plastic strain localization during the deformation process. Failure modes and ultimate ductility of two dual phase steels are analyzed using finite element analyses based on the actual steel microstructures. The plastic work hardening properties for the constituent phases are determined by the in-situ synchrotron-based high-energy X-ray diffraction technique. Under different loading conditions, different failure modes and ultimate ductility are predicted in the form of plastic strain localization. It is found that the local failure mode and ultimate ductility of dual phase steels are closely related to the stress state in the material. Under plane stress condition with free lateral boundary, one dominant shear band develops and leads to final failure of the material. However, if the lateral boundary is constrained, splitting failure perpendicular to the loading direction is predicted with much reduced ductility. On the other hand, under plane strain loading condition, commonly observed necking phenomenon is predicted which leads to the final failure of the material. These predictions are in reasonably good agreement with experimental observations.

Structural Integrity Assessment of Steam Generator Tube by the Use of Heterogeneous Finite Element Method

Aging steam generator tubes have been experiencing a variety of degradations such as pitting, fretting wear, erosion-corrosion, thinning, cracking, and denting. To assist with steam generator life cycle management, some defect-specific flaw models have been developed from burst pressure testing results. In this work, an alternative approach; heterogeneous finite element model (HFEM), is explored. The HFEM is first validated by comparing the predicted failure modes and failure pressure with experimental measurements of several tubes. Several issues related to the finite element analyses such as temporal convergence, mesh size effect, and the determination of critical failure parameters are detailed. The HFEM is then applied to predict the failure pressure for use in a fitness-for-service condition monitoring assessment of one removed steam generator tube. HFEM not only calculates the correct failure pressure for a variety of defects, but also predicts the correct change of failure mode. The Taguchi experimental design method is also applied to prioritize the flaw dimensions that affect the integrity of degraded steam generator tubes such as the defect length, depth, and width. It has been shown that the defect depth is the dominant parameter controlling the failure pressure. The failure pressure varies almost linearly with defect depth when the defect length is greater than two times the tube diameter. An axial slot specific flaw model is finally developed.

Application of an Extended Stress-Based Forming Limit Curve to Predict Necking in Stretch Flange Forming

An extension of the stress-based forming limit curve (FLC) advanced by Stoughton (2000, “A General Forming Limit Criterion for Sheet Metal Forming,” Int. J. Mech. Sci., 42, pp. 1–27) is presented in this work. With the as-received strain-based FLCs and stress-strain curves for 1.6-mm-thick AA5754 and 1-mm-thick AA5182 aluminum alloy, stress-based FLCs are obtained. These curves are then transformed into extended stress-based forming limit curves (XSFLCs), which consist of the invariants, effective stress, and mean stress. By way of application, stretch flange forming of these aluminum alloy sheets is considered. The AA5754 stretch flange displays a circumferential crack during failure, whereas the AA5182 stretch flange fails through a radial crack at the edge of the cutout. It is shown that the necking predictions obtained using the strain- and stress-based FLCs in conjunction with shell element computations are inconsistent when compared with the experimental results. By comparing the results of the shell element computations with those in which the mesh comprises eight-noded solid elements, it is demonstrated that the plane stress approximation is not valid. The XSFLC is then used with results from the solid-element computations to predict the punch depths at the onset of necking. Furthermore, it is shown that the predictions of failure location and failure mode obtained using the XSFLC are in accord with the differences observed between the two alloys/gauges.

Numerical analysis of water inrush from working-face floor during mining

Abstract Confined water in the Ordovician limestone is one of the hidden troubles that threaten safe production of mines in north China. A numerical model of the key strata was developed. It included the structural characteristics and mechanical properties of the floor rock at the working face of a particular coal mine. The model was used to predict failure modes and to help establish rules for safe mining above the aquifer. The distribution of deformation, failure and seepage was simulated by using Dilian Mechsoft's Realistic Failure Process Analysis (RFPA2D) program. The stress distribution, the deformation and the flow vectors were also obtained. The results indicate that: 1) The original balance of the stress and seepage fields is disturbed due to coal mining; and 2) As the working face advances different deformation, or failure, appears in the surrounding rocks, the water-resisting strata in floor may be destroyed and the passage of water from the aquifer into the mine may occur. The combined action of mining stress and water pressure ultimately lead to water inrush from the floor.

Erratum: “Deformation and failure of a film/substrate system subjected to spherical indentation: Part II. Prediction of failure modes in a thin TiN film deposited on a compliant elastic substrate” [J. Mater. Res. 21, 783 (2006)

With reference to the above mentioned article [J. Mater Res. 21, 783 (2006)], we would like to correct two equations and a reference

Prediction of Ultimate Behaviors in Cold-formed Steel Bolted Connection by the Introduction of Initial Geometric Imperfection in FE Modeling

Experimental research and nonlinear finite element analysis for the structural behaviors of single shear test on bolted connections fabricated with cold-formed stainless steel have been conducted. Failure criteria for prediction of failure mode of bolted connections under static shear and out-of-plane deformation, i.e., curling criteria were proposed based on experimental data for calibration of FE modeling. Failure mode and ultimate strength predicted by recommended FEA procedures with solid element showed a good correspondence with those of previous test results and the validation of FEA method was verified. The previous numerical analyses of bolted connection were carried out on geometrically perfect specimens. However, it has been known that geometric imperfection of thin-walled members must be considered in a FE model to simulate the actual shape of specimen. Therefore, in this paper, parametric studies were carried out based on the validity of numerical modeling of bolted connections in cold-formed stainless steel so that authors investigated the influence of initial geometric imperfection of connected plate on its structural behavior. Solid element and shell element with reduced integration were introduced as an element type and included two types of geometric imperfection. Consequently, FE modeling technique of cold-formed stainless steel bolted connection introducing initial imperfection to compensate the function of shell element and to induce the curling was proposed.

Non-parametric prediction of unobserved failure modes

This paper presents the application of a recently developed non-parametric predictive inferential approach for multinomial data to the problem of prediction of occurrence of new failure modes. These inferences are in terms of lower and upper probabilities for the next observation. The lower probability of occurrence of a new failure mode is zero in all cases, as the data never suggest strongly that there have to be further failure modes. The main interest is in the upper probability that the next observed failure is caused by a new failure mode.

On the analysis of adhesively bonded structures: A high order semi-elastic adhesive layer model

Several fundamental questions remain open regarding the behavior of adhesive layers in axially loaded lap shear joints. These issues are: the existence and magnitude of longitudinal tensile stresses, the appropriateness of an assumed state of plane stress versus a state of plane strain and the relevance of these two states with regards to failure mode prediction. To investigate these issues, a high-order approximate elasticity solution for the adhesive layer is developed. This linear elastic model satisfies all natural and essential boundary conditions for the adhesive layer. Using this model, it is found that the longitudinal tensile stresses are negligible and that the shear stress state does not vary through the thickness of the adhesive layer. It is also found that the prevailing plane stress adhesive layer models underestimate the adhesive peel stresses by nearly 10%.

The Analyses of Extended Shear Tab Steel Connections Part II: Stiffened Connections

The objective of this paper is similar to the companion paper published in the same issue of the Engineering Journal, to develop a viable, comprehensive 3-D finite element model capable of predicting the nonlinear behavior of shear tab connections, except this paper addresses in detail the analyses and failure prediction of the stiffened shear tab connection versus the unstiffened connections discussed in Part I. Correlation between the results of the computational finite element method and the experimental investigation is established and verified. Past experimental investigations have shown that the unstiffened shear tab connections are prone to twisting failure and low load-carrying capacity. Therefore, the use of stiffened shear tab connections is a viable design approach to overcome these problems. This paper compares various predicted failure modes from a finite element analysis with those observed in a recent experimental study performed by Sherman and Ghorbanpoor. In this paper, three, six, and eight-bolt stiffened extended shear tab connections are analyzed and compared. In addition, five FE models of 2-bolt and deep connections are analyzed in the plastic range to predict their failure modes. These models are two-bolt beam-to-column, 10-bolt beam-to-girder connection, 10-bolt beam-to-column connection, 12-bolt beam-to-girder connection, and 12-bolt beam-to-column connection.The three- and the five-bolt connections failed primarily in shear yielding, bearing failure of the holes around bolts, and in bolt shear. Secondary failure was observed in the form of girder web mechanism and shear tab twist. The eight-bolt connection failed primarily in bolt shear and bearing of holes around bolts. The failure modes predicted by the FE analysis were in agreement with those from the experimental investigation. The locations of high plastic strain, bearing failure of holes, plate twisting mechanism, and the web deterioration were identical in the FE model and the experimental observations. The FE model generated in this analysis proved to be accurate in predicting the failure mechanism of the extended shear tab connections.

Effect of weld nugget size on overload failure mode of resistance spot welds

In the present paper, effects of welding current, welding time, electrode pressure and holding time on the weld nugget size were studied. A failure mechanism was proposed to describe both interfacial and pullout failure modes. This mechanism was confirmed by SEM investigations. In the light of this mechanism, the effect of welding parameters on static weld strength and failure mode was studied. Then, an analytical model was proposed to predict failure mode and to estimate minimum nugget diameter (critical diameter) to ensure pullout failure mode in shear tensile test. On the contrary to existing industrial standards, in this model, critical nugget diameter is attributed to metallurgical characterisation of material (weld nugget hardness to failure location hardness ratio), in addition to sheet thickness. For a given sheet thickness, decreasing HWN/HFL increases interfacial failure mode tendency. The results of this model were compared with experimental data and also with the literature.

Finite element modeling of bolted connections in thin-walled stainless steel plates under static shear

The recently performed experimental study indicates that the current Japanese steel design standards (AIJ) cannot be used to predict accurately the ultimate behavior of bolted connections loaded in static shear, which are fabricated from thin-walled (cold-formed) SUS304 austenite stainless steel plates and thus, modified formula for calculating the ultimate strength to account for the mechanical properties of stainless steel and thin-walled steel plates were proposed. In this study, based on the existing test data for calibration and parametric study, finite element (FE) model with three-dimensional solid elements using ABAQUS program is established to investigate the structural behavior of bolted shear connections with thin-walled stainless steel plate. Non-linear material and non-geometric analysis is carried out in order to predict the load–displacement curves of bolted connections. Curling, i.e., out of plane deformation of the ends of connection plates which occurred in test specimens was also observed in FE model without geometric imperfection, the effect of curling on the ultimate strength was examined quantitatively and the failure criteria which is suitable to predict failure modes of bolted connections was proposed. In addition, results of the FE analysis are compared with previous experimental results, failure modes and ultimate strengths predicted by recommended procedures of FE showed a good correlation with those of experimental results and numerical approach was found to provide estimates with reasonable accuracy.

Flexural Behavior of Concrete Beams Strengthened with Near-Surface-Mounted CFRP Strips

Engineers have proposed relocating externally bonded strengthening fiber reinforced polymer (FRP) material from the unprotected exterior of the concrete to the protected interior. This technology is known as near-surface mounted (NSM) strengthening. In NSM reinforcement, the FRP is surrounded by concrete on three sides so the bond and damage problems associated with externally bonded FRP strengthening systems are reduced or eliminated. This paper presents experimental results from 12 full-scale concrete beams strengthened with NSM carbon FRP (CFRP) strips. Three companion unstrengthened specimens were also tested to serve as a control. Experimental variables include three different ratios of steel reinforcement and two different ratios of CFRP reinforcement. Yield and ultimate strengths, flexural failure modes, and ductility are discussed based on measured load, deflection, and strain data. Test results show measurable increases in yield and ultimate strengths in all beams strengthened with CFRP as well as predictable nominal strengths and failure modes. Force transfer between the CFRP, epoxy grout, and surrounding concrete was able to develop the full tensile strength of the CFRP strips. Energy and deflection ductilities were reduced for CFRP strengthened beams. Future research needs are addressed.

지진하중을 받는 철근콘크리트 원형교각의 전단성능곡선 모델

Reinforced concrete bridge columns with relatively small aspect ratio show flexure-shear behavior, which is flexural behavior at initial and medium displacement stages and shear failure at final stage. Since the columns with flexure-shear failure have lower ductility than those with flexural failure, shear capacity curve models shall be applied as well as flexural capacity curve in order to determine ultimate displacement for seismic design or performance evaluation. In this paper, a modified shear capacity curve model is proposed and compared with the other models such as the CALTRANS model, Aschheim et al.`s model, and Priestley et al.`s model. Four shear capacity curve models are applied to the 4 full scale circular bridge column test results and the accuracy of each model is discussed. It may not be fully adequate to drive a final decision from the application to the limited number of test results, however the proposed model provides the better prediction of failure mode and ultimate displacement than the other models for the selected column test results.

Deformation and failure of a film/substrate system subjected to spherical indentation: Part II. Prediction of failure modes in a thin TiN film deposited on a compliant elastic substrate

We have demonstrated previously, using nanoindentation, that the film thickness and substrate plasticity, the important two external variables in the film layer, control the failure of the film in a mutually exclusive way. In this work, we used a non-iterative Hankel transform method to analyze the stresses in an elastic film bound to an elastic substrate by a no-slip boundary condition and subjected to a Hertzian traction. We vary the substrate compliance by two orders of magnitude to generate interfacial mismatch stresses, which mimic the corresponding changes found in a real-life elastic film on an elastic-plastic substrate when the hardness of the substrate is changed. The analysis is found to reproduce faithfully the experimental trends, which showed that normal load and interfacial stresses generated by strain mismatch drive different modes of fracture depending on the film thickness in a mutually exclusive way. This validation paves the way for this theoretical technique to be used to design multilayered film structures.

Efficient 3D and 2D failure conditions for UD laminae and their application within the verification of the laminate design

Abstract The paper presents a set of three-dimensional (3D) and 2D strength failure conditions for unidirectional (UD) laminae made from fibre-reinforced plastics (FRP). The conditions are based on a concept termed the failure mode concept (FMC) which provides failure conditions formulated on UD lamina level that allows for a prediction of the critical lamina failure mode, and finally of laminate failure. Following the FMC there are five independent conditions: for inter fibre failure (IFF) three and for fibre failure (FF) two conditions. These are based on averaged lamina stresses. The IFF conditions were basically developed in 1996 and later contributed [Cuntze RG, Freund A. The predictive capability of failure mode concept-based strength criteria for multidirectional laminates. Part A. Compos Sci Technol 2004;64:343–77; Cuntze RG. The predictive capability of failure mode concept-based strength criteria for multidirectional laminates. Part B. Compos Sci Technol 2004;63:487–516] to a ‘World-Wide Failure Exercise (WWFE) on failure theories’ [Hinton MJ, Soden PD. Predicting failure in composite laminates: the background to the exercise. Compos Sci Technol 1998;58(7):1001–10]. In this paper a single but effective modification of one IFF condition is highlighted. It consists in the replacement of the ‘shear mode IFF 2’ by a numerically advantageous simplified formulation for the 3D and for the 2D case. A very satisfying verification of the 3D IFF2 condition, reduced to 2D, as well as for a directly formulated 2D IFF2 condition has been achieved when judging them versus the 2D experiments provided by the WWFE and other sources. As design verification demands for an accurate determination of the reserve factor. Its determination is presented for both the linear and the non-linear case. Values are given for one example.

An exploratory study of loading and morphometric factors associated with specific failure modes in fatigue testing of lumbar motion segments

There is currently little information regarding factors associated with specific modes of motion segment failure using a fatigue failure model. Thirty-six human lumbar motion segments were fatigue tested using spinal compressive and shear loads that simulated lifting a 9 kg weight in three torso flexion angles (0 degrees, 22.5 degrees, and 45 degrees). Twenty-five segments failed via fatigue prior to the 10,000 cycle maximum. These specimens were visually inspected and dissected so that the mode(s) of failure could be determined. Failure modes included endplate fractures (classified into nine varieties), vertebral body fractures, and/or zygapophysial joint disruption. Logistic regression analyses were performed to determine whether certain morphometric variables, amount of motion segment flexion, disk degeneration scores, and/or loading characteristics were associated with the occurrence of specific failure modes. Results indicated that stellate endplate fractures were associated with increased posterior shear forces (P < 0.05) and less degenerated discs (P < 0.01). Fractures running laterally across the endplate were associated with motion segments having larger volumes (P < 0.01). Endplate depression was more common in smaller specimens (P < 0.01), as well as those experiencing increased posterior shear force (P < 0.05). Zygapophysial joint damage was more likely to occur in a neutral posture (P < 0.01). These results suggest that prediction of failure modes (e.g., specific endplate fracture patterns) may be possible (at least for older specimens) given knowledge of the spinal loads along with certain characteristics of the lumbar spine.

A Study of Anisotropic Creep Behaviour of a 9CrMoNbV Weld Metal Using Damage Analyses with a Unit Cell Model

The deformations and failure life ratios, obtained from creep damage analyses using a ‘unit cell’ model, are used to describe the ‘bulk’ anisotropic creep behaviour of a 9CrMoNbV weld metal. The model consists of a columnar region and a weaker equiaxed (fine-grained and coarse-grained) region, and is based on the assumption that the structure of the weld metal has a regular geometric pattern. The creep damage analyses were conducted using a single-variable Kachanov type damage law, for which the material constants are based on experimental data. The bulk material ‘uniaxial’ creep rupture lives in the three principal directions were investigated taking account of the effects of the length to width ratio of the unit cell, the material property mismatch for the two regions, and the volume fraction of the columnar region. Results from the analyses indicate that the predicted failure modes are the same as those observed in an experimental study of the weld metal.

Shakedown of Unbound Pavements

ABSTRACT The performance of a pavement material beyond the elastic limit has substantial influence on the bearing capacity of a pavement and its serviceability. But in all the extant design methods for unbound pavements, only the elastic strain or stress at the top of the subgrade of the pavements is used as a design parameter. Although experimental data and practical experience have been introduced into the design method through empirical parameters, there is not a simple relationship between the elastic strain and the long-term plastic behaviour of pavement materials. Furthermore, these methods assume that pavements deteriorate indefinitely, whereas in practice pavements are frequently observed to “shakedown” to a steady state. In this paper, we will review progress in developing a new mechanistic approach to unbound pavement design, based on the shakedown theory, which incorporates the plastic as well as elastic properties of the pavement layers, provides predictions of various failure modes, such as rut formation and subsurface slip, all of which are consequences of unlimited accumulation of plastic deformation. The minimum shakedown loads are found by varying the geometry of the proposed failure mechanisms, and this is done for an unbound pavement overlying a clay subgrade.

Flexural analysis and design of reinforced concrete beams with externally bonded FRP reinforcement

This work addresses the flexural analysis of reinforced concrete beams with externally bonded FRP (Fibre Reinforced Polymer) reinforcement. A numerical method has been developed for the computation of the bending moment capacity of FRP-plated reinforced concrete beams and prediction of the flexural failure modes. The expressions for the upper and lower values of the characteristic plate reinforcement ratios are derived for rectangular and T-sections using the Eurocode 2 model for concrete. A flow-chart of the numerical procedure, suitable for computer implementation, is included and its accuracy is validated with available experimental results. Some of the novel features of the numerical analysis are demonstrated through a brief investigation of the effects of loads acting at the stage of strengthening on the ultimate flexural capacity and deformation behaviour of FRP-plated R.C. beams.