Evaluation of Fatigue Strength Based on Dissipated Energy for Laser Welds

To optimize welding conditions that ensure the safety and reliability of laser welds, this study established an evaluation method of the fatigue strength for the laser welds of steel sheets over a short period of time. This study focuses on a fatigue limit estimation based on dissipated energy which is caused by micro plastic deformation. As a result, the area at which the temperature changes, due to dissipated energy, is locally high is the fracture origin of the laser welds. The fatigue limit of the laser welds is almost the same as the stress amplitude at which a temperature change occurs due to dissipated energy.

Cyclic micromechanical controls of transient crustal deformation

<p>Episodic brittle-ductile behaviour reflects the complex interplay of micromechanical hardening and softening, often with some type of fluid pressure associated with introduction of new material that acts as the switch from coseismic to interseismic response. Brittle features observed in nature can in general be characterized as discrete surfaces or narrow zones across which fast particle displacements have occurred, with or without dilatant behaviour; this descriptively meets the criteria for generation of earthquakes. Likewise, non-brittle flow is a priori associated with slower particle velocities. This reduces the problem to one of how and why rocks cycle between slow and fast displacements. Particle displacement in the solid-state is limited to three processes: individual atoms, glide of packets of atoms and frictional displacement across an essentially free surface. Each of these processes, however large the feature being studied or rapid the displacements, necessitates the sequential overcoming of extant atomic bonding energies. Within the rock record, evidence of seismic events are embedded as new or reconstituted material introduced to the deforming host as a consequence of brittle deformation; for example, veins and pseudotachylyte. This new material acts as an important sink for strain energy whereby brittle responses are suppressed until such time as a new critical state is reached. In turn, the strain rate softening abetted by the new material provides a ductile overprint of their syn-fracture origin. Consequently, rheological transitions within Earth’s crust are spatially and temporally transient, evidence for which may be routinely lost. As part of this cyclic behaviour, localization of deformation can be viewed as the default state, with macroscopic deformation a result of organization into required dissipative structures.</p>

Characterization of Crack Growth Behavior of Carburized SCM415 Steel under Cyclic Rotating Bending

Carburized steel was used in severe and cyclic loading conditions such as bearing and structural components. In this study rotating bending fatigue tests were carried out to observe the crack initiation and propagation behavior of carburized JIS SCM415 steel bar whose diameter was 10.0 mm. Transition area origin (TRO) crack on fracture surfaces were observed with scanning electron microscope. The depth of fracture origins was about 0.9 mm. TRO crack was observed around the fracture origin which was nucleated at the edge of carburized layer, and the crack propagated toward the surface and the inside core. Stress amplitude was modified with depth of crack origin, and S-N curve was corrected with modified applied stress amplitude σM. In order to reveal the crack propagation behavior around the boundary between the hardened and the soft core area, stress intensity factor (SIF) on crack front was also computed.

Effect of Hydrogen Content and Strain Rate on Hydrogen-induced Delay Cracking for Hot-Stamped Steel

Hot-stamped steel has been widely used in automobile bumper and other safety components due to its high strength. Therefore, this paper investigates the effect of hydrogen content and strain rate on hydrogen-induced delay cracking (HIDC) behavior. The results showed that the plasticity of the steel significantly decreased with an increase in hydrogen content or a decrease in the strain rate. Fractography was analyzed after tensile tests. It was found that all of the pre-charged specimens cracked at large-sized inclusions when stretched at a strain rate of 1 × 10−3 s−1, which indicates that, in this case, the defect itself in the material had great influence on the extend properties. No inclusions were found at the main fracture origin area for hydrogen steady-state specimens, when stretched at a strain rate of 1 × 10−6 s−1, which demonstrated that a slower strain rate causes greater influence by hydrogen. However, for the non-pre-charged samples, the fractures surface showed that cracking originated from the defect near the sample surface, which was independent of strain rates.

Failure Probability, Stress Distribution and Fracture Analysis of Experimental Screw for Micro Conical Abutment

The aim of this study was to evaluate the failure probability of two types of abutment screws after compressive load and to analyze the stress distribution with finite element method. Sixty (60) single-tooth implant restorations were assembled on titanium implants (e-fix, A.S. Technology - Titanium Fix). The groups were divided into Conventional screw (Screw neck 1.5 ø mm) and Experimental screw (Screw neck constricted with 1.2 ø mm). Specimens were subjected to single load to failure with compressive test according ISO 14801. The fractured specimens were subjected to stereomicroscopy for measurement of remaining screws inside the implant and characterization of fracture origin. Representative specimens were analyzed by scanning electronic microscopy. For finite element method (FEM), an identical 3D model of the two in vitro test groups were used with similar conditions (30º, 100 N load). The stress in the abutment screw was analyzed by von-Mises criteria. The results of strength means were 4132.5 ± 76 MPa and 4528.2 ± 127.2 for conventional and experimental groups, respectively. During microscopy, the mean (mm) of the remaining screw piece inside the implants were 0.97 ± 0.23 and 1.32 ± 0.12 for conventional and experimental groups, respectively. In FEM, the conventional group showed stress concentered in an unfavorable region (peak of 39.23 MPa), while the experimental group showed more stress areas but less concentration than the conventional group (36.6 MPa). In using the tested experimental geometry, the abutment screw can have its strength improved, and the origin of failure can be more favorable to clinical resolution.

Failure Analysis on 42CrMo Steel Bolt Fracture

Fracture behaviors of 42CrMo ultrahigh strength steel-based bolt have been experimentally studied including macroscopic and microscopic fracture observation, metallographic test, mechanical property testing, and energy spectrum analysis. The results show that a large amount of structure defects, such as sulfide inclusions, band, and carbon depletion, appear in the fracture origin region and matrix of the bolt. Such defects reduce fatigue strength of materials and easily yields fatigue fracture origin. In addition, sulfide inclusions provide easy access to crack growth, gradually reducing the effective cross section of the bolt accompanying with increasing stress and finally causes fracture when stress exceeds the material fracture strength. The fracture mechanism is also explored based on fracture failure criterion and fatigue crack growth curve.

Improved Fractographic Strength Estimates Based on Surface Profilometry

Fractography is a valuable method that uses post-mortem topographical information to estimate the stress field near the fracture origin and help establish the root cause of failures. Typically, in glass and ceramics the mirror radius is one of the features sought for by fractographers since its length could be empirically related to the sample’s strength. The mirror radius is usually subjectively estimated by fractographers though microscopy measurements. Nonetheless, variations in the estimates introduced by inconsistent viewing modes and the subjectivity of observers could lead to substantial errors even when standard protocols such as ASTM C1678 were followed. In this manuscript, a novel method combining a fracture mechanics model describing the mist formation in silicate glasses with profilometry data carried out by confocal laser scanning microscope is introduced. The new method was shown to be able to objectively establish the mirror-mist boundary. Furthermore, it was found that the proposed technique was repeatable within 2% regardless of the magnification or imaging mode used. Whereas the average strength estimated per ASTM C1678 by eight individual observers was influenced by both the magnification and the imaging mode used and displayed standard deviation of over 3%.

Using Fractography for Determining of Technological Reasons of Defect and Brittle Fractures Occurrence in Steels

The paper describes practical examples of identification of rupture defect on steel components and its connection with the violating of the production process. Every violation of optimal production technology causes irreversible changes in the material that are visible on the fracture after breaking. Characteristics of the fracture allow the backward identification of specific technological reasons for defect fracture origin and consequent degradation of utility properties of the component. Documented examples include wide spectrum of production technologies where defective fractures can occur as are casting, forming, heat treatment or surface treatment. Wider notice is dedicated to the brittle fractures caused by non-optimal heat treatment. Presented fractures are documented by macroscopical as well as microscopical study using SEM microscope and they represent handy atlas of defects fractures with usage in real industrial practice.