Failure Behaviour Of Steels Research Articles

Experimental analysis and modelling of dynamic deformation and failure behaviour of steel

New specimen geometries with various stress states are designed and applied for dynamic loading tests. Oscillation-free force is measured during multiaxial loading tests in the strain range of 10-4 – 103 s-1. The deformation and local strain fields of specimens have been measured using high-speed camera and evaluated by digital image correlation (DIC) techniques. It is found that the strain rate effects on fracture strain are stressstate dependent. To model the material plasticity and ductile fracture behaviour during dynamic tests, an extended damage mechanics model (eMBW model) is used. In this work, the model is enhanced and implemented into LS-DYNA. To cover the strain rate effects on plasticity at a large strain rate range, a modified Johnson–Cook-type rate-dependency and exponential temperature-dependency are used. In addition, the influences of both stress state and strain rate on fracture locus are considered. The enhanced damage mechanics model successfully predicts the deformation and fracture behaviour of the investigated steel under dynamic multiaxial loading.

Exploiting the Property Profile of High Strength Steels by Damage Mechanics Approaches*

Abstract Although high strength low alloy steels offer the potential for lightweight design, their application is often rather limited due to existing design rules. A further development of these standards requires precise prediction of realistic limit states. The damage model of Bai and Wierzbicki has been extended in order to improve the description of the ductile failure behaviour of steels, especially when crack initiation in dynamic loading is to be simulated. Application examples of the extended model are given in order to show that it can be used for limit state analysis. An outlook presents the application of the model to derive improved safety factors for probabilistic safety concepts.

The Effect of Post Fatigue Tensile Loading on Mechanical Properties and Failure Behaviour of Steel

AbstractIn this research work the early fatigue damage characteristics up to 50% of life has been investigated at near‐endurance stress amplitude in a medium carbon steel with hardened & tempered and normalized conditions by application of post fatigue tensile loading. During fatigue loading microvoids were generated at the interphase‐interface through vacancy clustering. Directional localized coalescence of these microvoids generated ‘void sheets’ in the normalized steel and ‘craters’ in the hardened & tempered steel. These defects acted as the potential sites for failure during post fatigue tensile loading deteriorating the mechanical properties. This was, therefore, clearly indicated that when a high tensile load is superimposed on near‐endurance fatigue loading, material may fail much earlier than its actual fatigue life. The normalized steel suffered from more reduction in ductility than the hardened & tempered steel indicating more susceptibility to fatigue damage.

Effects of the constraint parameter triaxial stress on the failure behaviour of steels

The constraint parameter triaxial stress which results from loading types and component geometries plays an important role in the failure behaviour of materials. Especially the local failure initiation of materials depends very strongly on triaxial stress. The failure behaviour of a material under different stress states (i.e. different constraint) differs. In this paper effects of triaxial stress on the failure behaviour of steels are presented by means of theoretical analysis, experiments and finite-element-calculations. A theoretical model describing the effects of triaxial stress on failure initiation of materials has been developed based on theoretical analysis. Corresponding experiments were conducted to determine the material constants in the failure model of materials. In order to obtain local damage parameters finite-element calculations have been conducted. According to the results from theoretical analysis, experiments and finite-element calculations the damage curves of materials, in which the critical equivalent plastic strain is plotted as a function of triaxial stress, have been obtained. Furthermore, the failure behaviour of materials under different triaxial stresses has been discussed. Die Spannungsmehrachsigkeit, die von der jeweiligen Belastungsart und der Bauteilgeometrie abhängig ist, hat einen großen Einfluß auf das Versagensverhalten von Werkstoffen. Insbesondere die Rißinitiierung eines Werkstoffs hängt deutlich von der Mehrachsigkeit des Spannungszustandes ab. Das Bruchverhalten eines Werkstoffs unter verschiedenen Spannungszuständen ist unterschiedlich. Im Rahmen dieser Arbeit wird der Einfluß der Spannungsmehrachsigkeit auf das Versagensverhalten von Stählen untersucht. Ein auf theoretischen Analysen basierendes Modell beschreibt die Abhängigkeit des Werkstoffversagens von der Spannungsmehrachsigkeit. Zusätzlich wurden Experimente durchgeführt, um die werkstoffabhängigen Konstanten im Versagensmodell zu bestimmen. Des weiteren dienten Finite-Element-Untersuchungen zur Ermittlung der lokalen Schädigungszustände. Unter Berücksichtigung der theoretischen und experimentellen Ergebnisse sowie der Finite-Element-Untersuchungen sind sogenannte Versagenskurven ermittelt worden. Diese Kurven zeigen den Zusammenhang zwischen der kritischen plastischen Vergleichsdehnung und der Spannungsmehrachsigkeit. Abschließend wird das Versagensverhalten von Werkstoffen unter Beachtung verschiedener Spannungsmehrachsigkeiten diskutiert.

Failure behaviour of steels with different levels of strength and toughness

The failure behaviour of tension loaded structures is studied with wide plates of steels with different levels of strength and toughness. Procedures for the derivation of toughness requirements are presented. A procedure using fracture mechanics values on the material's side and Finite-Element-calculations on the applied side is introduced and used to quantify toughness requirements.