Elastic-plastic Failure Research Articles

A successful combination of the equivalent material concept and the averaged strain energy density criterion for predicting crack initiation from blunt V-notches in ductile aluminum plates under mixed mode loading

Crack initiation from blunt V-notch borders in ductile A16061-T6 plates is investigated experimentally and theoretically under mixed mode I/II loading. Experimental observations with naked eye during loading indicated large plastic deformations around the notch tip at the onset of crack initiation, demonstrating large-scale yielding failure regime for the aluminum plates. To theoretically predict the experimentally obtained value of the maximum load that each plate could sustain, i.e. the load-carrying capacity, without performing elastic-plastic failure analyses, the equivalent material concept (EMC) is combined with a well-known brittle fracture criterion, namely the averaged strain energy density (ASED) criterion. It is shown that the combined EMC-ASED criterion could successfully predict the experimental results for various V-notch angles and radii.

Evaluation of the load-carrying capacity of notched ductile plates under mixed mode loading

The load-carrying capacity (LCC) of V-notched plates made of Al 6061-T6 is experimentally measured under mixed mode I/II loading conditions for various notch geometries. Large amount of plasticity is observed around the notch at crack initiation instance, demonstrating large-scale yielding failure regime. To avoid elastic–plastic failure analysis and theoretically predict the experimental failure loads by means of the linear-elastic notch fracture mechanics (LENFM), the Equivalent Material Concept (EMC), proposed originally by the first author, is utilized in conjunction with the V-notch maximum tangential stress (VMTS) and V-notch mean stress (VMS) failure models, which both are stress-based brittle fracture criteria. It is shown that while the EMC-VMS criterion is always more conservative than the EMC-VMTS criterion, both criteria provide very good predictions to the experimental results.

Elastic-plastic failure assessment of cold worked stainless steel pipes

The change in fracture strength due to cold working was investigated for cracked stainless steel pipes. The cold working was introduced into Type 316 stainless steel plates. Then, material properties were identified in order to make elastic–plastic fracture assessments for a pipe with a circumferential crack under bending load. The two-parameter method was employed for deriving fracture strength considering the change in the flow stress (yield and ultimate strengths), fracture toughness and stress–strain curve caused by the cold working. It was found that the fracture strength was not reduced even if 40% cold working was introduced, while the failure mode was altered from the plastic collapse mode to the elastic–plastic failure mode. Although cold working decreased the fracture toughness, the change in the stress–strain curve reduced the driving force for fracture (J-integral) and increased the fracture strength. It was shown that the failure assessment curve of cold worked material can be derived using the yield and ultimate strengths of the material that is free of cold working.

Ductile tearing predictions with Wellman’s failure model

Predictions for the Sandia National Laboratories fracture challenge (Boyce et al. in Int J Fract 2013) were generated using a transient dynamic finite element code with a multi-linear elastic plastic failure model developed by Wellman (Simple approach to modeling ductile failure. Sandia National Laboratories, Albuquerque 2012). This model is a conventional, rate independent, von Mises plasticity model for metals with user-prescribed hardening as a function of equivalent plastic strain. In addition to conventional plasticity, this model has empirical criteria for crack initiation and growth. Ductile tearing predictions generated with this model were found to be in good agreement with experimental measurements and observations.

Nanotribological behavior of ZnO films prepared by atomic layer deposition

We used atomic layer deposition to form ZnO thin-film coatings on Si substrates and then evaluate the effect of pile-up using the nanoscratch technique under a ramped mode. The wear volume decreased with increasing annealing temperature from room temperature to 400°C for a given load. Elastic-to-plastic deformation occurred during sliding scratch processing between the groove and film for loading penetration of 30nm. The onset of non-elastic behavior and greater contact pressure were evident for loading penetration of 150nm; thus, full plastic deformation occurred as a result of a substrate effect. We suspect that elastic–plastic failure events were related to edge bulging between the groove and film, with elastic–plastic deformation attributable to adhesion discontinuities and/or cohesion failure of the ZnO films.

The dynamic compressive behavior of armor structural materials in split Hopkinson pressure bar test

High strain rate–dependent deformation behaviors are important in design and optimization of armor structural materials. Herein, the static tensile and the dynamic compressive behaviors of the practical materials including Al5083, rolled homogeneous armor steel and tungsten heavy alloy were investigated by means of a universal testing machine and a split Hopkinson pressure bar apparatus, respectively. The test was performed at high strain rates (1200–3100 s−1) to obtain a detailed understanding of the responses of the materials. A finite element analysis was then carried out using an elastic–plastic failure material model considering a user-defined parameter determined from the split Hopkinson pressure bar tests. Both flow and peak stresses of the materials were different corresponding to the mechanical properties and strain rates. The dynamic yield stresses are generally larger than static yield stresses, particularly in Al5083. As shown in the results, the experimentally obtained true stress–strain behaviors give a good agreement with those from finite element analyses. In addition, observations of the impacted zone in the specimen showed that a few cracks propagated along the specimen’s original periphery. In order to determine the level of deformation, strain rate sensitivity, m, at strain of 0.1 was also calculated for all materials; rolled homogeneous armor steel and tungsten heavy alloy had lower strain rate sensitivity than Al5083. The proposed procedure shows proper agreements between numerical predictions and experimental results such as stress–strain relations, peak stresses and deformation. The methodology coupled with the experimental data reflecting the dynamic compressive properties provides a more accurate prediction of the strain rate–dependent behavior of armor structural materials.

Elastic-Plastic Analysis and Failure Assessment of the Pipeline Steel with Surface Crack

A detailed elastic-plastic analysis on different semi-elliptical surface crack geometries under uniaxial and biaxial load was conducted using 3D finite element analysis for pipeline steel X100. After quantitating fracture driving force J-integral and plastic yielding load, the effects of biaxial load and crack geometry were investigated by using the Option 3 of R6 failure assessment diagram approach for the surface cracks with the different crack sizes ( aspect ratios c/a = 1, 1.5 and 3 ) and the various biaxial loading ratios λ from -1 to 2. And three option curves of the R6 assessment were compared to evaluate the applicability of R6 assessment method for the X100 steel.

Study on the Ductility of T-Shaped Short-Limb Shear Walls (I)

In this paper, the author takes on the experiment of low cyclic load and analyzes the tenacity and ductility characteristic of the specimen, and simulates the elastic-plastic failure modes of T-shaped short-limb shear walls by using the finite element software ABAQUS and applying the damage plasticity model to select the proper unit type. Results indicate that the mechanical behavior of T-shaped short-limb shear walls under the condition of horizontal load force may be accurately simulated by the method proposed in this paper.

D114 軸方向き裂を有する鋳造ステンレス鋼管の破壊評価方法(OS6 保全・設備診断技術)

Since the ductility of cast austenitic stainless steel pipes decreases due to thermal aging embrittlement after long term operation, not only plastic collapse failure but also unstable ductile crack propagation (elastic-plastic failure) should be taken into account for the structural integrity assessment of cracked pipes. In the fitness-for-service code of the Japan Society of Mechanical Engineers (JSME), Z-factor is used to incorporate the reduction in failure load due to elastic-plastic failure. However, the JSME code does not provide the Z-factor for axial cracks. In this study, Z-factor for axial cracks in aged cast austenitic stainless steel pipes was derived. Then, a comparison was made for the elastic-plastic failure load obtained from different analysis procedures. It was shown that the obtained Z-factor could derive reasonable elastic-plastic failure loads, although the failure loads were more conservative than those obtained by the two-parameter method.

Fracture Analysis Procedure for Cast Austenitic Stainless Steel Pipes with an Axial Surface Crack

Since the ductility of cast austenitic stainless steel pipes decreases due to thermal aging embrittlement after long term operation, not only plastic collapse failure but also unstable ductile crack propagation (elastic-plastic failure) should be taken into account for the structural integrity assessment of cracked pipes. In the fitness-for-service code of the Japan Society of Mechanical Engineers (JSME), Z-factor is used to incorporate the reduction in failure load due to elastic-plastic failure. However, the JSME code does not provide the Z-factor for axial cracks. In this study, Z-factor for axial cracks in aged cast austenitic stainless steel pipes was derived. Furthermore, an optimized failure assessment curve (FAC) for the two-parameter method, which can also derive elastic-plastic failure load, was obtained. Then, a comparison was made for the elastic-plastic failure load obtained from different analysis procedures. It was shown that the optimized FAC as well as the obtained Z-factor could derive reasonable elastic-plastic failure loads, although the failure loads were more conservative than those obtained by the two-parameter method using FAC obtained J-value solutions. The reference stress method was found to be applicable to evaluation of the FAC. It was concluded that elastic-plastic failure loads could be derived by the Z-factor and two-parameter methods in which FAC was obtained using J-value solutions, the optimized curve and the reference stress method.

The mechanical properties of a transverse isotropic voided material

The mechanical properties of a transverse isotropic honeycomb porous material were investigated by the experiment and the finite element method. The relative density dependence of the macroscopic elastoplastic constants along the in-plane and out of plane directions (e.g., Young’s modulus, Poisson’s ratio, yielding stress, etc.) was systematically studied. Good agreement between the experimental and numerical results was observed. In addition, the three dimensional numerical results were compared with simplified foam models and self-consistent estimates, showing the necessity to improve available models for foams in order to achieve a better predictive capability. At the same time, the elastic–plastic failure mechanism was discussed in detail.

Buckling of Cylindrical Tunnel Liners

Buckling of thin cylindrical shell liners used for the stabilization of soft ground tunnels is treated as the buckling of an elastic ring restrained radially and tangentially by an infinite surrounding elastic medium. Stiffness components for the elastic medium are derived and used to provide various levels of approximation for the elastic critical loads and associated modes of the liner when subject to overburden pressure loading. For most practical tunnel liners, elastic buckling is found to occur in modes having relatively short circumferential wavelengths. In these circumstances an approximation introduced into the critical pressure analysis allows both the lowest critical pressure and its associated mode shape to be represented explicitly in terms of a single “soft ground tunnel buckling parameter”; this single composite parameter encapsulates all the relevant ground and liner geometric and material properties. It is this closed-form analytical representation of elastic critical buckling that provides a particularly convenient basis for predicting elastic-plastic failure. When imperfections are introduced, liner collapse and the various forms of ground failure can be modeled by methods analogous to the Ayrton-Perry expression for columns. Two generalized imperfection parameters emerge: one for liner collapse and the other for each of three possible, soft ground, failure modes. It is suggested that the analytical simplicity of the approach should make it an attractive alternative basis for at least the initial, rational, design of soft ground tunnel liners.

K-0807 R6法におけるパラメータの確率論的影響度評価(S11-2 構造健全性評価(2))(S11 構造健全性評価)

This paper presents a parameter sensitivity study on the probabilistic elastic-plastic failure analysis of a circumferential through-wall-cracked pipe subject to bending. The analysis is performed by the FORM based on the R6 option 1 methodology. Both the sensitivity of a reliability index due to a standardized variation of the mean value of the basic variable and the approximate measure for the stochastic importance of the basic variable are evaluated considering a side riser pipe made of Type 304 stainless steel from a boiling water reactor plant.

Elastic-Plastic Failure Analysis of Pressure Burst Tests of Thin Toroidal Shells

This paper provides a comparison between test and analysis results for bursting of thin toroidal shells. Testing was done by pressurizing two toroidal shells until failure by bursting. An analytical criterion for bursting is developed based on good agreement between structural instability predicted by large strain-large displacement elastic-plastic finite element analysis and observed burst pressure obtained from test. The failures were characterized by loss of local stability of the membrane section of the shells consistent with the predictions from the finite element analysis. Good agreement between measured and predicted burst pressure suggests that incipient structural instability as calculated by an elastic-plastic finite element analysis is a reasonable way to calculate the bursting pressure of thin membrane structures.

Reliability of Welded Structures Containing Fatigue Cracks

Abstract This study investigates the reliability of a cracked fillet welded T-joint typically found in offshore structures. A formulation for the aspect ratio (a/c) of a propagating semi-elliptical fatigue crack located at the toe of the weld is developed using Newman and Raju’s stress intensity factor for a cracked flat plate in conjunction with a weld magnification factor. The reliability in terms of fatigue lifetime is then calculated using the aspect ratio and Paris’s law of crack propagation with both fracture toughness and elastic-plastic failure criteria. The variation in crack aspect ratio in the T-joint is compared to that in a cracked flat plate, and examples are provided of reliability calculations for tension and bending loads.

Elastic-Plastic Axisymmetric Failure of Swedge-Stiffened Cylindrical Pressure Hull Under External Pressure

An elastoplastic and geometrically nonlinear analysis of two of Ross's swedge-stiffened pressure hulls subjected to hydrostatic pressure loading is presented. Three commonly used swedge forms—trapezoidal, triangular, and sinusoidal-shaped—are considered. Comparisons on the responses of these three swedge-stiffened shapes are included. The load/displacement relations as well as the distribution of stresses with plastic zone spread phenomena are also illustrated.

Tests of Model Reinforced Concrete Circular Slabs

Six small-scale circular reinforced concrete slabs with variable thicknesses and reinforcement ratios were tested to establish their load-deflection relationships. The slabs were made of microconcrete and miniaturized reinforcements. An analysis based on equilibrium equations, kinematic relations, yield conditions, a flow rule, and an assumed elastic-plastic failure mechanism was performed and the results of the model tests were compared with the analysis. The influence of membrane forces, concrete compressive strength, specimen thickness, reinforcement ratio, rate of loading, and degree of end restraint were studied.

Extension of a one-dimensional finite element model program for analysing elastic-plastic stresses and progressive failure of adhesive bonded joints

Abstract A program for stress analysis of adhesive bonded joints within an elastic range was extended to consider the elastic-plastic stress state in an adhesive layer and its progressive failure. The program is based on the one dimensional finite element method. The von Mises yield criterion and the Mohr-Coulomb failure criterion are used in the program. Numerical analysis of a single lap joint subjected to four-point bending load was conducted and its result was compared with the experimental result. Good agreements were obtained between both results except for the final failure load. The present extension has some advantages. The stress singularity in the adhesive layer at the lap end or crack tip can be avoided due to the simple assumption for adhesive strains. Shorter computing time by the present method than by other general two- or three-dimensional finite element model programs should be much emphasized as one of the advantages.

Formula omitted] crack growth—Modification of [formula omitted] testing and failure assessment diagrams

Incorporating modern, generalized integral criterion of fracture mechanics into elastic-plastic failure and J-R testing is presented here. Firstly the J integral is expressed in a more appropriate failure assessment diagram. Secondly previous criteria for stable crack growth based on crack tip energy absorbed with crack extension are confirmed. Finally the engineering analysis of J- resistance curves is reformulated.

Probabilistic elastic-plastic fracture mechanics analysis based on the R6 methodology

An analysis is presented for calculating elastic-plastic failure probabilities. The approach is unified so that linear elastic and plastic collapse failures are covered by the same procedure. The main statistical variables considered are fracture toughness, flow stress and defect size. The analysis allows a sophisticated modelling of the failure process to be performed. A method is presented for calculating the service failure probability for a structure following a successful proof test. The method is general and may include the possibility of a change in dominant failure mode between plastic collapse and fast fracture as a result of crack growth, thermal stressing, radiation and temperature changes. Separate sections cover the basic analysis, the effect of prior loading and the effect of work hardening. The basic analysis includes a simple technique for determining the fit parameters for candidate materials property distributions. In each section a description of the technique is followed by a detailed numerical example.