Effects of anisotropy and pre-strain on the shear fatigue behaviour of DP600 steel

A novel elastic strain energy density approach for fatigue evaluation of welded components

In this paper, the solder joint reliability under -40 °C - 125 °C thermal cycling loading of different package sizes of mold-first FOWLP and redistribution layer first (RDL-first) FOWLP was studied by finite element simulation considering the viscoelastic material property of epoxy molding compound (EMC), dielectric and underfill. The critical solder joint is located at the die corner for the designed mold-first and RDLfirst FOWLP. Volume average creep strain energy density range of critical solder joint increases with the package size from 12 mm×12 mm×0.2 mm to 18 mm×18 mm×0.2 mm for mold-first FOWLP. The distance to neutral point (DNP) becomes invalid when the RDL-first FOWLP package size increases to 18 mm×18 mm×0.2 mm. Volume average creep strain energy density range of package corner solder joint is overestimated without considering the viscoelastic material properties. However, the volume average creep strain energy density range of die corner solder joint is underestimated without considering the viscoelastic material properties. Low CTE PCB can help to improve the reliability of the critical solder joint at die corner of the designed mold-first FOWLP. The effects of the low CTE PCB for improving solder joint reliability the designed RDL-first FOWLP is not significant. Thinner PCB can help to improve the reliability of the critical solder joint at die corner of both mold-first and RDL-first FOWLP.

Oil and gas pipelines may be subject to high plastic strains, either intentionally as a result of the method of installation, or the requirements of the design and operation, or accidentally (due to mechanical damage), before they enter service (transportation, construction/installation, etc.) and during operation. Pre-strain is introduced by denting, cold bending, land slides, subsidence, frost heave, ice gouging, earthquake induced ground movement, reeling, installation in deep water, and wrinkling or buckling. Material subjected to pre-strain will have different material properties to that of the virgin material. Previous experimental studies have indicated that pre-strain has a detrimental effect on the fracture toughness of steel: it reduces the resistance to crack initiation, reduces the resistance to crack growth, and increases the transition temperature. To investigate the effect of pre-strain on the fracture toughness of line pipe steel a programme of tests and numerical analyses has been undertaken. The results of tensile, notched tensile, fracture toughness (J-integral and CTOD) and Charpy V-notch impact tests of virgin (not pre-strained) material, prestrained material and artificially strain aged material are reported. It is shown that the effect of pre-strain can be simulated numerically using a finite element model incorporating the influence of material damage through a Gurson-Tvergaard constitutive model. The properties of the virgin material that influence the effect of pre-strain on toughness are discussed. The role of material damage (void nucleation and growth, etc.) during the introduction of pre-strain is shown to be less significant than the changes to the tensile properties and ductility caused by pre-strain. The effect of tensile pre-strain on fracture toughness can be characterised in terms of the effect of pre-strain on the stress-strain characteristics of the material, the critical fracture strain, and several parameters that relate to the conditions for ductile fracture (or cleavage fracture). A simple, engineering approximation to the effect of pre-strain on fracture toughness for application to pipeline design and fitness-for-purpose assessment is proposed in terms of the true strain at the tensile strength of the virgin material.

In this paper, a new time-domain method for detecting structural local damage has been developed, which is based on the measured strain signals. The “pseudo strain energy density (PSED)” is defined and used to build two major damage indexes, the “average pseudo strain energy density” (APSED) and the “average pseudo strain energy density changing rate” (APSEDR). A probability and mathematical statistics technique is utilized to derive a standardized damage index. Afterwards, these indexes are used to establish the damage identification strategies for beam structures and plate structures respectively. Furthermore, the wavelet packet transform is used to pre-process the measured dynamic strain signals. Then, the effectivity of the new damage identification method is confirmed by numerical simulations. Finally, a laboratory beam model experiment is conducted to verify this method examine the feasibility and applicability of the new method.

New and existing pipelines can be subjected to high plastic strains. Denting a pipeline causes permanent plastic deformation. Onshore pipelines subject to subsidence, frost heave or earthquake loading can experience significant plastic strain during service. Offshore pipelines that are reeled prior to laying, or are laid in deep water, or are operating at high temperatures and high pressures, can experience significant plastic strain both prior to, and during, service. Experimental studies have indicated that pre-strain (permanent plastic deformation) has a detrimental effect on the fracture toughness of steel; it reduces the resistance to crack initiation, reduces the resistance to crack growth, and increases the transition temperature. Consequently, there is a need for a thorough understanding of the effect of pre-strain on the fracture toughness of line pipe. Accordingly, a theoretical model for predicting the effect of tensile pre-strain on the ductile fracture toughness has been developed using the local approach. The effect of pre-strain is expressed in terms of an equation for the ratio of the fracture toughness of the pre-strained material to that of the virgin (not pre-strained) material. The model indicates that the effect of tensile pre-strain on the material’s fracture toughness can be characterised in terms of the effect of pre-strain on the stress-strain characteristics of the material, the critical fracture strain for a stress state corresponding to that during pre-strain, and several parameters that relate to the conditions for ductile fracture (or cleavage fracture). The implications of the model are that it may be possible to estimate the reduction in toughness caused by pre-strain simply from a full stress-strain curve of the virgin material. The model has been validated against the results of crack tip opening displacement (CTOD) tests conducted by Tokyo Gas on two line pipe steels subject to uniaxial tensile pre-strain. It is shown that the predictions and trends of the theoretical model are in broad agreement with the test results.

Understanding strain controlled low cycle fatigue response of P91 steel through experiment and cyclic plasticity modeling

This research presents some experimental, numerical, and theoretical results on brittle fracture of disk-type test specimens weakened by V-notches with end-holes under mixed mode I/II loading with negative mode I contributions. First, 54 fracture tests are conducted on VO-notched Brazilian disk specimens made of the general-purpose polystyrene under mixed mode I/II loading with negative mode I contributions. Then, two energy-based brittle fracture criteria, namely the averaged strain energy density and averaged strain energy density based on the equivalent factor concept are proposed to predict the experimentally obtained fracture loads of the tested general-purpose polystyrene specimens. Additionally, the fracture initiation angles of the tested VO-notched Brazilian disk specimens are predicted by using averaged strain energy density criterion. The finite element analyses, as well as the experimental observations, show that although brittle fracture in the specimens under mixed mode I/II loading takes place from the applied load side of the notch border by local tensile stresses, the notch bisector line and the other sides of the notch border sustain compressive stresses. In fact, this phenomenon states the concept of mixed mode I/II loading with negative mode I contributions. Finally, it is shown that good agreement exists between the experimental results and the theoretical predictions of the two energy-based fracture criteria.

A previously developed energy based high cycle fatigue (HCF) life assessment framework is modified to predict the low cycle fatigue (LCF) life of aluminum 6061-T6. The fatigue life assessment model of this modified framework is formulated in a closed form expression by incorporating the Ramberg–Osgood constitutive relationship. The modified framework is composed of the following entities: (1) assessment of the average strain energy density and the average plastic strain range developed in aluminum 6061-T6 during a fatigue test conducting at the ideal frequency for optimum energy calculation, and (2) determination of the Ramberg–Osgood cyclic parameters for aluminum 6061-T6 from the average strain energy density and the average plastic strain range. By this framework, the applied stress range is related to the fatigue life by a power law whose parameters are functions of the fatigue toughness and the cyclic parameters. The predicted fatigue lives are found to be in a good agreement with the experimental data.

This paper provides a complete overview of the applicability of the Equivalent Material Concept in conjunction with the Average Strain Energy Density criterion, to provide predictions of fracture loads in structural materials containing U-notches. The Average Strain Density Criterion (ASED) has a linear-elastic nature, so in principle, it does not provide satisfactory predictions of fracture loads in those materials with nonlinear behaviour. However, the Equivalent Material Concept (EMC) is able to transform a physically nonlinear material into an equivalent linear-elastic one and, therefore, the combination of the ASED criterion with the EMC (EMC–ASED criterion) should provide good predictions of fracture loads in physically nonlinear materials. The EMC–ASED criterion is here applied to different types of materials (polymers, composites and metals) with different grades of nonlinearity, showing the accuracy of the corresponding fracture load predictions and revealing qualitatively the limitations of the methodology. It is shown how the EMC–ASED criterion provides good predictions of fracture loads in nonlinear materials as long as the nonlinear behaviour is mainly limited to the tensile behaviour, and how the accuracy decreases when the nonlinear behaviour is extended to the material behaviour in the presence of defects.

Thermal load-induced notch stress intensity factors derived from averaged strain energy density

NSIFs estimation based on the averaged strain energy density under in-plane mixed mode loading

Experiences and recommendations for numerical analyses of notch stress intensity factor and averaged strain energy density

The average strain energy density (ASED) criterion applied to design of notched mechanical components has been achieved a considerable success over the years thanks to its numerous advantages including the need for a relatively coarse mesh, the use of a scalar rather than vector quantity and the ability to compare the strength of parts with different notch angles. It is known that adhesively bonded joints mechanical strength is governed by the stress singularity arising near the intersection between the adhesive‐substrate interface and the free surface of the bonded joint, even without notches. This is due to the different elastic material properties of the adhesive compared to those of the substrate that induces a constitutive stress singularity. This phenomenon makes the strain energy density criterion particularly suitable for the design of bonded joints, as well. This contribution is aimed at applying the ASED criterion to the static strength assessment of bonded joints. Experimental results taken from literature were found in good agreement with those predicted by the ASED approach.

J integral and local strain energy density approach to characterize the cracks in anisotropic hyperelastic skin type composite materials

Analysis of high temperature and strain amplitude effects on low cycle fatigue behavior of pitting corroded killed E350 BR structural steel