Mechanism Of Damage Formation Research Articles

Study on nanomechanical properties of 4H-SiC and 6H-SiC by molecular dynamics simulations

This article aimed to study the nanomechanical properties of silicon carbide under nanoindentation and nanoscratching by the molecular dynamics (MD). First, the MD models were established and experimentally verified. Then, the MD results were used to analyze the subsurface morphology and crystal defects occurring during the processes, which are comparatively difficult to study through experiments. For comparison, the C face and the Si face of 4H-SiC and 6H-SiC were selected for research. The results indicate that 4H-SiC shows greater normal resistance than 6H-SiC for both faces, and the C face may be easier to machine than the Si face for both 4H- and 6H-SiC. In addition, the hexagon amorphous patterns were found in the horizontal cross-sections views after the indentation, which were deemed to make up of two mutually central symmetric triangles. The results show that these amorphous patterns may be caused by the slip systems of 4H-SiC and 6H-SiC, which are mainly take place on the (0001) basal plane and along the <11–20> directions, depending on their special lattice structures. Through MD simulations, the formation mechanism of sub-surface damage of SiC substrates is much clearer.

Understanding the formation mechanism of subsurface damage in potassium dihydrogen phosphate crystals during ultra-precision fly cutting

Potassium dihydrogen phosphate (KDP) crystals play an important role in high-energy laser systems, but the laser damage threshold (LDT) of KDP components is lower than expected. The LDT is significantly influenced by subsurface damage produced in KDP crystals. However, it is very challenging to detect the subsurface damage caused by processing because a KDP is soft, brittle, and sensitive to the external environment (e.g., humidity, temperature and applied stress). Conventional characterization methods such as transmission electron microscopy are ineffective for this purpose. This paper proposes a nondestructive detection method called grazing incidence X-ray diffraction (GIXD) to investigate the formation of subsurface damage during ultra-precision fly cutting of KDP crystals. Some crystal planes, namely (200), (112), (312), (211), (220), (202), (301), (213), (310) and (303), were detected in the processed subsurface with the aid of GIXD, which provided very different results for KDP crystal bulk. These results mean that single KDP crystals change into a lattice misalignment structure (LMS) due to mechanical stress in the subsurface. These crystal planes match the slip systems of the KDP crystals, implying that dislocations nucleate and propagate along slip systems to result in the formation of the LMS under shear and compression stresses. The discovery of the LMS in the subsurface provides a new insight into the nature of the laser-induced damage of KDP crystals.

Effect of the cutting speed on the cutting mechanism in machining CFRP

Cutting mechanism of CFRP is different from that of a homogeneous metal. For metal, high speed cutting can improve machining quality and efficiency. However, there is no agreed conclusion on the effect of cutting speed on the cutting mechanism of CFRP. This paper studies the effect of cutting speed on cutting mechanism of CFRP and damage formation by using finite element and experiment methods. A three-dimensional finite element model of orthogonal cutting CFRP is established considering the strain-rate-dependent mechanical properties of resin and the anisotropy of fiber. By this model, cutting processes at micro level are simulated. Fracture evolution processes of fiber and resin are characterized. Furthermore, experimental data of cutting forces are obtained, and the machined surface microstructure are observed based on orthogonal cutting experiments. Subsurface morphology and degree of resin cracking extension in simulation and experiment are consistent with each other. Results show that with the increase of cutting speed, the fiber deformation and the affected area before fiber fracture obviously deceases. Accordingly, the resin cracking and extension degree caused by the excessive deformation of fiber also be greatly reduced. In addition, there is a trend that the specific energy gap for different chip formation modes becomes smaller and smaller.

Distinct differences in the mechanisms of mucosal damage and γ-H2AX formation in the rat urinary bladder treated with o-toluidine and o-anisidine.

Although aromatic amines are widely used as raw materials for dyes, some of them have been concerned about carcinogenicity in the urinary bladder. We examined early changes in histopathology and the formation of γ-H2AX, a biomarker of DNA damage, in the urinary bladder of rats to investigate the mechanisms of mucosal damage induced by monocyclic aromatic amines. 6-week-old male F344 rats were administered 0.4% or 0.8% o-toluidine, 0.3% or 1.0% o-anisidine, 0.4% 2,4-xylidine, 0.2% p-toluidine, or 0.6% aniline in the diet for 4 weeks. Animals were sequentially killed from day 2 to after 2 weeks of recovery, and histopathological and immunohistochemical analyses were performed. In the 0.8% o-toluidine group, there was sequential progression of bladder lesions, characterized by edematous changes and intramucosal hemorrhage at day 2 and formation of granulation tissue with mononuclear cell infiltration at week 1, followed by diffuse hyperplasia at weeks 2 and 4. In the 1.0% o-anisidine group, simple hyperplasia only with slight inflammation was detected from week 1. Whereas γ-H2AX-positive bladder epithelial cells in the 1.0% o-anisidine group were significantly increased in a time-dependent manner, transient increases in γ-H2AX-positive cells were detected at day 2 and week 1 in the 0.8% o-toluidine group. No apparent bladder lesions or increases in γ-H2AX formation were observed in any other groups. These results revealed different mechanisms of bladder mucosal damage associated with o-toluidine and o-anisidine. Moreover, immunohistochemical analysis for γ-H2AX suggested that both compounds may induce DNA damage in epithelial cells, mainly basal cells, of the bladder mucosa.

Study on grinding damage of high chromium alloy based on molecular dynamics

The new cast high chromium alloy is the conventional material of the nuclear main pump thrust-bearing with good wear and corrosion resistance. According to the structural characteristics of high chromium alloy, the simulation model and the coupling potential function were constructed to study the grinding damage layer using molecular dynamics method. The simulation results show that the crystal lattice distortion caused by carbon atoms in the formation of interstitial solid solution leads to the occurrence of amorphous structure after full relaxation. The break and recombination of metal bond and non-metal bond between atoms in the alloy result in the occurrence of damaged layer under grinding. And the bond angles between the atoms in the damage layer are less than that in the alloy matrix. The damage layer is mainly composed of atoms in front of the abrasive particle and the atoms of extrusion deformation in the bottom. Moreover, with the increase of grinding depth, the cutting force and the damage layer thickness increase. The study is conducive to understand the damage formation mechanism of high-chromium alloy materials in micro-nano processing, and provides a theoretical reference value for the actual processing.

Analysis of Surface Damage Formation Mechanism in Milling of CFRPs

Analysis of Surface Damage Formation Mechanism in Milling of CFRPs

Analytical and numerical investigation on the mechanical behavior of subsurface damage during orthogonal cutting of carbon fiber reinforced polymer composites

Rejection of parts at the assembly stage due to poor machining quality is major problems in the manufacturing of structural components from carbon fiber reinforced polymer composites. On-line monitor of the multi-scale failure modes during machining to reveal the damage formation mechanisms is a great challenge for industries. Therefore, there is a great need for multi-scale mechanical modeling method. This study will address this problem through a combined analytical–numerical method. A microscale analytical model considering the cutting edge of tool is established to clarify the effect of cutting edge radius on the fiber deformation and failure mechanism. In addition to the analytical model, a user-defined subroutine based on maximum stiffness degradation model of composite is implemented into the two-dimensional macroscopic anisotropic finite element code to characterize the dynamic processes of subsurface damage initiation and propagation. It is found that the extent of uncut fibers increases as the cutting edge radius increases when the fiber orientation is less than 90°. The subsurface damage is relatively serious when the fiber orientation is greater than 90°, and the cutting edge radius has no obvious effect, while the tool rake angle has a significant impact. The results provide theoretical basis for the development of new tools and new technologies to the composite machining.

Evaluation of Impact Damage in Composite Structures Using Ultrasonic Testing

Abstract Barely visible impact damage is one of the problems commonly occurring in composite elements during an aircraft operation. The authors described the mechanisms of impact damage formation and propagation in composite structures. The paper presents a performed analysis of an influence of impact parameters on the resulting damage, i.e. its detectability by means of visual observation as well as its extent determined based on ultrasonic tests results. The tests were conducted on the CFRP specimens with a wide range of impact damage cases obtained with combinations of variable impact energy and shapes of impactors. Additionally, an algorithm based on image processing and image analysis methods is proposed for the purpose of the effective evaluation of the ultrasonic data obtained.

Australian forensic textile damage examinations – Finding a way forward since PCAST

Textile damage examinations are requested in a range of crime types such as assault, sexual assault and homicide. They typically involve the examination of clothing for damage such as cut, tear or thermal damage, often then followed by experimental scenario testing to help ascertain the cause of the damage. Understanding the underpinning science is central to the accurate interpretation of the complex mechanism of damage formation. In a stabbing incident for example, an understanding of the dynamic relationship between the knife blade, fabric and skin (or skin simulant) is critical.Recent reports, including the President's Council of Advisors on Science and Technology (PCAST) report, have scrutinised forensic feature-based comparison techniques. Whilst textile damage was not a focus area, it can be considered a feature-based evidence class, and one which is currently largely reliant upon a practitioner's opinion, experience and professional judgement.This paper will review the current state of textile damage examinations in Australia and survey research being conducted to address the issues raised in the context of the PCAST report. The central contribution of observational data to the evidence class of textile damage will also be explored, as well as some practical measures to counter the effects of cognitive bias.

Investigation of Ga ion implantation-induced damage in single-crystal 6H-SiC

In order to investigate the damage in single-crystal 6H-silicon carbide (SiC) in dependence on ion implantation dose, ion implantation experiments were performed using the focused ion beam technique. Raman spectroscopy and electron backscatter diffraction were used to characterize the 6H-SiC sample before and after ion implantation. Monte Carlo simulations were applied to verify the characterization results. Surface morphology of the implantation area was characterized by the scanning electron microscope (SEM) and atomic force microscope (AFM). The ‘swelling effect’ induced by the low-dose ion implantation of 1014−1015 ions cm−2 was investigated by AFM. The typical Raman bands of single-crystal 6H-SiC were analysed before and after implantation. The study revealed that the thickness of the amorphous damage layer was increased and then became saturated with increasing ion implantation dose. The critical dose threshold (2.81 × 1014−3.26 × 1014 ions cm−2) and saturated dose threshold (˜5.31 × 1016 ions cm−2) for amorphization were determined. Damage formation mechanisms were discussed, and a schematic model was proposed to explain the damage formation.

Grinding Characteristics, Material Removal, and Damage Formation Mechanisms of Zirconia Ceramics in Hybrid Laser/Grinding

Zirconia ceramics which are sometimes called “ceramic steel” have gained significant interest because of their excellent properties. However, it is desired to maintain the surface quality while increasing the economics of ceramics grinding process. A hybrid laser/grinding (HLG) process was utilized to grind zirconia ceramics which was irradiated with continuous wave laser before grinding in the hybrid process. The feasibility of hybrid laser/grinding of zirconia ceramics was investigated in terms of grinding force and energy, material removal, and damage formation mechanisms. The results show that laser irradiation can induce lateral cracks, which can help material removal and prevent further crack propagating into the base. The results of grinding tests indicate that grinding force and energy decrease significantly as compared with conventional grinding of ceramics. The combinations of the fractured area, the plowing striations, and seldom debris on the ground surfaces in this work indicate the combined material removal mechanism of both brittle mode and ductile mode.

Damage Features and Formation Mechanism of the Strong Water Inrush Disaster at the Daxing Co Mine, Guangdong Province, China

The largest water disaster in the history of coal mining in China with respect to maximum water inrush quantity (≈ 13,600 m3/min) occurred at the Daxing underground mine in 2005. Triggered by accumulated water in a higher-level, mined out (goaf) area, it caused great damage, including 121 deaths and direct economic losses of over 47 million yuan. Damage to the roof and floor rock caused by mining-induced redistribution of ground stress is believed responsible for the caving of the roof and the subsequent inrush, though accumulated water in the goaf area, the weight of the roof pillar, fractures due to minor faults, the comprehensive influence of groundwater, including the pore water pressure and the water’s chemical action, and the steeply inclined coal seam all contributed to the scale of the disaster. The inrush site was located near a fault and had been observed as an anomaly during geophysical testing. Recommendations made based on this tragedy include: mining of steeply inclined coal seams beneath water bodies must be forbidden due to the lack of effective mitigation measures; the integrity of waterproof pillars must be a major priority; and underground mining requires a thorough understanding of hydrogeological conditions, which must be regularly updated through the mining process.

Textile damage examinations on the cutting edge – an Australian perspective

Requests for textile damage examinations can result from a variety of crime types. While a reasonable proportion involve an edged implement, a broad spectrum of damage types can be encountered. The examination requires an understanding of the complex mechanism of damage formation to enable an accurate interpretation and reliable conclusion to be formulated. The report recently published by the President’s Council of Advisors on Science and Technology (PCAST) scrutinized numerous forensic feature-based comparisons, and highlighted deficiencies in their validity and reliability. Textile damage was not a focus area of this report, but is a subjective feature-based technique. It is, by necessity, largely reliant upon a practitioner’s opinion, experience and professional judgement, and is therefore subject to the report’s commentary. Significant progress has been made to strengthen the discipline since examinations were conducted during the investigation of the death of Azaria Chamberlain nearly 40 years ago. The purpose of this paper is to review the current state of textile damage examinations in Australia in light of the PCAST report, and to survey the work being conducted to improve the foundational science and standardization of damage analysis. Certain areas requiring future attention and research, such as empirical studies, will be discussed.

Mechanisms of damage formation in glass in the process of femtosecond laser drilling

The development of femtosecond lasers has renovated micromachining technology, enabling the microdrilling of transparent materials at high speed. Although effective, this technique is still imperfect because irregular microscopic damage occurs during processing. Several factors in the formation of damage induced by a single laser pulse have been suggested; however, the entire mechanisms of damage formation in the process of drilling are not fully understood. Here we investigated the causes of microscopic damage both by experimentally carrying out laser drilling on chemically strengthened glass samples and by numerically analyzing the propagating stress wave and temperature distribution. We have revealed that the damage at the sidewall and bottom of a generated hole is mainly due to the stress wave, while the damage around the hole entrance is mainly due to relaxation of thermal stress. In particular, we have analyzed these processes quantitatively, demonstrating the time courses of the passage of the stress wave through the material and the temperature distribution generated by the excited electrons. Our analyses are useful in suggesting processing techniques, or suggesting glass materials suitable for laser drilling such as glass with high heat resistance, which may lead to an improvement in the quality of micromachining.

Secondary cutting edge wear of one-shot drill bit in drilling CFRP and its impact on hole quality

Drilled hole quality of carbon fiber reinforced polymer (CFRP) deteriorates quickly with rapid tool wear, which ultimately influences service performance of CFRP parts. And for one-shot drill bit, it is the secondary cutting edge that directly determines hole surface quality. This work first carries out an investigation on wear evolution of secondary cutting edge of one-shot drill bit in drilling CFRP. The tool wear is characterized by the cutting edge rounding (CER) and the flank wear (VB), and the tool wear measurement interval is shrunk down to every two holes to obtain detailed wear mechanisms. The secondary cutting edge is found prone to dulling according to CER and cutting edge profile. However, flank wear has regrinding effects on the cutting edge, which in turn decreases CER. Then combined with chip formation mechanisms, tool wear impacts on cutting capabilities of secondary cutting edge under various fiber cutting angles are investigated. And CER is proved to determine cutting capabilities under acute fiber cutting angles. Based on above analyses, initiation of hole surface damages and damage formation mechanisms are further discussed. Damages induced by degradation of cutting capabilities are of great difference within and across prepreg laminates under various fiber cutting angles.

Crystalline damage in silicon wafers and 'rare event' failure introduced by low-energy mechanical impact

We used X-ray diffraction imaging to detect and characterize mechanical damage introduced to 300mm silicon wafers by low impact energy exerted on the wafer edge. Maps of crystalline damage show a correlation between the damage size, the magnitude of the impact energy and the location of the impact point. We demonstrate the existence of crystalline non-visual defects; crystalline defects that appear in the X-Ray diffraction images but not in optical microscopy or scanning electron microscope. We propose a mechanism of crystalline damage formation at low impact energies based on finite element analysis and high-resolution synchrotron white beam transmission X-ray topography. Finally, we propose the concept of 'rare-event' to described relatively low rate of occurrence of wafer failure by fracture within semiconductor manufacturing facilities.

Thermally induced variation of primary wave velocity in granite from Yantai: Experimental and modeling results

This work analyzes the results from experiments for studying the effect of high temperature on primary wave velocity of granite. Test samples are preheated to different temperatures (from 25° to 900 °C) and then cooled down by air. The primary wave velocity test is performed on the heated samples. Lastly, the microstructure test is carried out to study the mechanism of thermal damage and development process of micro cracks. The results show that the primary wave velocity reduces gradually while the temperature increases, i.e. the higher the heating temperature is, the faster the wave velocity reduces. Basing on the waveform, the quality factor (Q) of primary wave velocity is calculated and is decreased with the increase of heating temperature. From the thermal damage factor (D(T)) model of primary wave velocity and the measured data, the D(T) of granite samples is found to increase with heating temperature rising following the S-logistic function. Then, according to the change of microstructure, the formation mechanism of thermal damage is analyzed and the damage process of rock verified by the deterioration of mechanical parameters. For rock heated to different temperatures and then cooled down, the main reason for damage at lower temperature is dehydration and that at higher temperature is the increase of cracks which destroy the structure and weaken the bonding between particles.

Unlawfulness of determination of the mechanism of an injury by the forensic medical examiner

The article presents an analysis of the meanings of the corresponding definitions («the mechanism of an injury», «the specialized knowledge», «competence of forensic expert»), describes requirements of the existing criminal procedural legislation of Ukraine relating to the performing of examinations and procedural status of forensic expert, showing that that the forensic medical examiner without specialized knowledge, skills and training in the field of Biomechanics - one of the section of Biophysics, is not authorized, so he should not and has no procedural law right to give an expert opinion on the non-medical question of investigator (detective), prosecutor or the court about the mechanism of damage formation in the injured person.

Understanding and simulating the material behavior during multi-particle irradiations.

A number of studies have suggested that the irradiation behavior and damage processes occurring during sequential and simultaneous particle irradiations can significantly differ. Currently, there is no definite answer as to why and when such differences are seen. Additionally, the conventional multi-particle irradiation facilities cannot correctly reproduce the complex irradiation scenarios experienced in a number of environments like space and nuclear reactors. Therefore, a better understanding of multi-particle irradiation problems and possible alternatives are needed. This study shows ionization induced thermal spike and defect recovery during sequential and simultaneous ion irradiation of amorphous silica. The simultaneous irradiation scenario is shown to be equivalent to multiple small sequential irradiation scenarios containing latent damage formation and recovery mechanisms. The results highlight the absence of any new damage mechanism and time-space correlation between various damage events during simultaneous irradiation of amorphous silica. This offers a new and convenient way to simulate and understand complex multi-particle irradiation problems.

Damage layer thickness and formation mechanism of shotcrete with and without steel fiber under sulfate corrosion of dry–wet cycles by ultrasound plane testing method

In recent years, nondestructive testing, especially ultrasound testing method, has been increasingly used to detect concrete strength, compactness, cracks, and other aspects. In this work, the damage layer, thickness measured by ultrasound plane testing method and the formation mechanism of the damage layer were studied. Simultaneously, the damage process and microstructure of shotcrete under multiple coupling effects of sulfate attack and cyclic drying and wetting were also investigated. The results indicated that ultrasound plane testing accurately characterizes the damage layer thickness of shotcrete with and without steel fiber. The variation in performance of the original specimen was significant with increased thickness. An exponential distribution existed between the damage layer thickness and the original performances expected weight loss ratio. In the corrosion process, sulfate ions were transported into micropores and microcracks layer by layer, and velocity was slowly diffused from a distance from the surface. For ordinary shotcrete, sulfate attack is divided into three stages: ettringite action, combined ettringite and gypsum action, and gypsum action. However, for steel fiber-reinforced shotcrete, sulfate attack only occurred at the combined ettringite and gypsum action.