The mechanism of ductile chip formation in cutting of brittle materials

A theoretical analysis for the mechanism of ductile chip formation in the cutting of brittle materials is presented in this paper. The coexisting crack propagation and dislocation in the chip formation zone in the cutting of ductile materials are examined based on an analysis of the geometry and forces in the cutting region, both on Taylor’s dislocation hardening theory and the strain gradient plasticity theory. It was found that the ductile chip formation was a result of large compressive stress and shear stress in the chip formation zone, which shields the growth of pre-existing flaws by suppressing the stress intensity factor K _I . Additionally, ductile chip formation in the cutting of brittle materials can result from the enhancement of material yield strength in the chip formation zone. The large compressive stress can be generated in the chip formation zone with two conditions. The first condition is associated with a small, undeformed chip thickness, while the second is related to the undeformed chip thickness being smaller than the radius of the tool cutting edge. The analysis also shows that the thrust force F _t is much larger than the cutting force F _c . This indicates that large compressive stress is generated in the chip formation zone. This also confirms that the ductile chip formation is a result of large compressive stress in the chip formation zone, which shields the growth of pre-existing flaws in the material by suppressing the stress intensity factor K _I . The enhancement of material yield strength can be provided by dislocation hardening and strain gradient at the mesoscale, such that the workpiece material can undertake the large cutting stresses in the chip formation zone without fracture. Experiments for ductile cutting of tungsten carbide are conducted. The results show that ductile chip formation can be achieved as the undeformed chip thickness is small enough, as well as the undeformed chip thickness is smaller than the tool cutting edge radius.     

Modeling of burr thickness in milling of ductile materials

Accuracy and surface finish play an important role in modern industry. The presence of undesired projections of materials, known as burrs, negatively affect the part quality and assembly process. To remove burrs, a secondary operation known as deburring is required for the post-processing and edge finishing of machined parts. The thickness of the burr is of interest as it describes the time and method necessary for deburring of the machined part. Burr thickness (B _t) measurements are costly and non-value-added operations that in most cases require the use of a scanning electron microscope for accurate burr characterization. Therefore, to avoid such expenses, the implementation of alternative methods for predicting the burr thickness is strongly recommended. In this research work, an analytical model for predicting the burr thickness in end milling of ductile materials is presented. The model is built on the geometry of burr formation and the principle of continuity of work at the transition from chip formation to burr formation that also takes into account the cutting force influence on burr formation. A very good correlation was found between the modeled and experimental B _t values. The model has shown a great sensitivity to material properties such as yield strength and specific cutting force coefficient (K _c). In addition, the sensitivity of the proposed model to the feed per tooth (f _t) and depth of cut (a _p) was considerably high. The proposed model allows the prediction of the thickness of the exit up milling side burr, without the need for experimental measurement and/or approximation of shear angle (Φ), friction angle (λ), and the tool chip contact length (L), unlike existing analytical burr size prediction models. Besides analytical modeling, statistical analysis is performed on experimental results in order to distinguish dominant process parameters on B _t. It is observed that the depth of cut and feed per tooth are the main parameters which significantly affect the B _t, while the speed has only a negligible effect on it.     

Investigation of mechanical properties of friction-welded AISI 304 with AISI 1021 dissimilar steels

Austenitic stainless steel and low alloy steels are extensively used in various automotive, aerospace, nuclear, chemical, and other general purpose applications. Joining of dissimilar metals is one of the challenging tasks and most essential need of the present-day industry. It has been observed that a wide range of dissimilar materials can be easily integrated by friction welding. The objectives of the present investigation were obtaining weldments between austenitic stainless steel (AISI 304) with low alloy steel (AISI 1021) and optimizing the friction welding parameters in order to establish the weld quality. In the present study, an experimental setup was designed in order to achieve friction welding of plastically deformed austenitic stainless steel and low alloy steel. AISI 304 and AISI 1021 steels were welded by friction welding using five different axial pressures at 1,430 rpm. The joining performances of friction-welded dissimilar joints were studied, and influences of these process parameters on the mechanical properties of the friction-welded joints were estimated. The joint strength was determined with tensile testing, and the fracture behavior was examined by scanning electron microscopy (SEM) and was supported and backed by energy dispersive spectroscopy (EDS) analysis. Furthermore, the proposed joints were tested for impact strength, and the microhardness across the joint was also evaluated.     

Fretting fatigue life estimations based on fretting mechanisms

Generally the fretting fatigue S-N curve has two regions: one is the high cycle (low stress) region and the second is the low cycle (high stress) region. In a previous paper we introduced the fretting fatigue life estimation methods in high cycle region by considering the wear process; with this estimation method the fretting fatigue limit can be estimated to be the crack initiation limit at the contact edge. In this paper we estimate the low cycle fretting fatigue life based on a new critical distance theory, modified for a high stress region using ultimate tensile strength σ_B and fracture toughness K_IC. The critical distance for estimating low cycle fretting fatigue strength was calculated by interpolation of the critical distance on the fretting fatigue limit (estimated from σ_w0 and ΔK_th) with critical distance on static strength (estimated from σ_B and K_IC). By unifying this low cycle fretting fatigue life estimation method with the high cycle fretting fatigue life estimation method, which was presented in the previous paper, we can estimate the total fretting life easily. And to confirm the availability of this estimation method we perform the fretting fatigue test using Ni-Mo-V steel.     

The singular stress field and stress intensity factors of a crack terminating at a bimaterial interface

For the fracture evaluation of inclined cracks terminating at the dissimilar material interface, not only the singularities, but also the detailed stress field and its stress intensity factors are necessary. However, though there are many researches reported on the singularity analysis, the stress field and its stress intensity factors are still not clear. This paper has deduced theoretically the singular stress and displacement fields near the tip of a crack terminating at the interface between bonded dissimilar materials, for both cases of real and oscillatory singularities. From the deduced singular stress field, the stress intensity factors are defined for such a crack, and the corresponding numerical extrapolation methods are also proposed. Through the numerical examinations, it is found that the theoretical stress distributions agree well with the numerical results obtained by the finite element method. Moreover, the proposed extrapolation method shows a good linearity, thus it can be used as an efficient way to determine the characteristics of the stress and displacement fields near the tip of a crack terminating at interface.     

Effects of heat input on microstructure and mechanical property of Al/Ti joints by rectangular spot laser welding-brazing method

A rectangular spot laser welding–brazing method was developed to join butted Ti/Al dissimilar alloys. In order to evaluate effects of heat input on mechanical property of the joints, microstructure of the joints were characterized. TiAl₃ intermetallic compounds (IMCs) were found at the joint interface in the case of low-heat input and TiAl₃, TiAl, Ti₅Si₃, and Ti₃Al IMCs were observed at high-heat input. Results of tensile test showed that the joints fracture in the fusion zone under the condition of low-heat input and in the interfacial reaction layer or the fusion zone with a mass of porosities at high-heat input. In addition, tensile strength of specimens broken at the fusion zone is higher obviously than that at the interface or the fusion zone with a mass of porosities, and tensile strength of the joints is up to 290 MPa.     

Analysis of edge slipping in a complete and adhesive contact of elastically dissimilar bodies subject to normal indentation

A slipping at the edge of a complete contact problem is studied. An asymptotic method is first used for the stress evaluation in the vicinity of the contact edge. The characteristics of the induced eigenvalue problem are solved. The influence of the contacting material’s dissimilarity on the eigensolutions is particularly investigated. Generalized stress intensity factors are defined with developing a method to select the mode separation angles. Assistance of a finite element analysis is explained, which is necessary to deduce the asymptotic solution of a semi-infinite body to a finite problem. Normalization of the stress equation with respect to the recently exploited parameters of length and stress dimensions (d _o and G _o , respectively) is also introduced. The condition of edge slipping is suggested by comparing the coefficient of friction with the ratio of eigenvectors of the shear and normal components. It was found that a leading edge slip will be formed in the vicinity of the contact edge. A trailing edge slip may take place inside the contact region. The size of the leading edge slip region is also evaluated. This dramatically decreases if the coefficient of friction increases as reasonably expected.     

An assessment of hardness, impact strength, and hot corrosion behaviour of friction-welded dissimilar weldments between AISI 4140 and AISI 304

Understanding the behavior of weldment at elevated temperatures and especially their corrosion behavior has recently become an object of scientific investigation. Investigation has been carried out on friction-welded AISI 4140 and AISI 304 under Na₂SO₄?+?V₂O₅ (60%) environment at 500 to 600°C under cyclic condition. The resulting oxide scales in the weldment have been characterized systematically using surface analytical techniques. Scale thickness on the low alloy steel side was found to be higher and was also prone to spalling. Weld area was found to be more susceptible to degradation than in base metals. The influences of welding parameters on the hot corrosion are discussed.     

Thickness dependence of signal/background ratio of inner-shell electron excitation loss in eels

The influence of the specimen thickness on the core-edge loss intensity has been studied using the relative specimen thickness (t_R) as the standard rule for the specimen thickness in EELS. The core-edge loss intensity monotonously increased with the increasing relative specimen thickness, 0 < t_R ? 1, and monotonously decreased with increasing relative specimen thickness, t_R ? 1, behind the maximum of core-edge intensity at t_R ? 1. “Optimum specimen thickness” to get the highest core-edge intensity is suggested for the thickness of t_R = 1. The thickness-dependent factor T(t_R) should be considered in a practical measurement in EELS.     

Manufacturing and mechanical characterization of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/β-TCP/TiO<Subscript>2</Subscript> biocomposite as a potential bone substitute

This paper focuses on the mechanical characterization of a bioceramic based on commercial alumina (Al₂O₃) mixed with synthesized tricalcium phosphate (β-TCP) and commercial titania powder (TiO₂). The effect of β-TCP and TiO₂ addition on the mechanical performance was investigated. After a sintering process at 1600 °C for 1 h, various mechanical properties of the samples have been studied, such as compressive strength, flexural strength, tensile strength, elastic modulus, and fracture toughness. The measurements of the elastic modulus (E) and the tensile strength (σ _t ) were conducted using the modified Brazilian test while the compressive strength (σ _c ) was determined through a compression test. Also, semi-circular bending (SCB) specimens were used to evaluate the flexural strength (σ _f ) and the opening mode fracture toughness (K _IC). From the main results, it was found that the best mechanical performance is obtained with the addition of 10 wt.% TCP and 5 wt.% TiO₂. Alumina/10 wt.% tricalcium phosphate/5 wt.% titania composites displayed the highest values of mechanical properties and a good combination of compressive strength (σ _c ?≈?352 MPa), flexural strength (σ _f ?≈?98 MPa), tensile strength (σ _t ?≈?86.65 MPa), and fracture toughness (K _IC?≈?13 MPa m^1/2).     

Analysis on annealing-induced stress of blind-via TSV using FEM

Copper-filled through silicon via (TSV) is a promising material owing to its application in high-density three-dimensional (3D) packaging. However, in TSV manufacturing, thermo-mechanical stress is induced during the annealing process, often causing reliability issues. In this paper, the finite element method is employed to investigate the impacts of via shape and SiO₂ liner uniformity on the thermo-mechanical properties of copper- filled blind-via TSV after annealing. Top interface stress analysis on the TSV structure shows that the curvature of via openings releases stress concentration that leads to ~60 MPa decrease of normal stresses, σ_xx and σ_yy, in copper and ~70 MPa decrease of σ_xx in silicon. Meanwhile, the vertical interface analysis shows that annealing-induced stress at the SiO₂/Si interface depends heavily on SiO₂ uniformity. By increasing the thickness of SiO₂ linear, the stress at the vertical interface can be significantly reduced. Thus, process optimization to reduce the annealing-induced stress becomes feasible. The results of this study help us gain a better understanding of the thermo-mechanical behavior of the annealed TSV in 3D packaging.     

异种耐热钢焊接接头低塑性蠕变失效力学因素的研究

用有限元法计算了异种耐热钢接头邻近焊缝界面低强度材料区的蠕变应变分布,建立了蠕变拘束区（CRZ）应力三轴度（σ(av)/σ）与蠕变应变速率的比值（εw/εb）之间的数学关系模型,并通过蠕变断裂试验证明了数学模型的正确性。理论分析和试验结果表明,焊缝界面低强度材料蠕变拘束区的应力三轴度是接头发生低塑性蠕变失效的主要力学因素。     

A study on optimal bonding angles of bi-material interfaces in dental crowns with porcelain fused to metal

Interfaces between two different materials exist in the dental crowns used in dental restoration work. A common failure mode observed in dental crowns is debonding/fracture that is initiated at the corners of the interface between two materials, where a stress concentration or a stress singularity can be created. In this paper, finite element modelling and analytical methods are used to analyse the stress singularity at the free edge corners at the interfaces between porcelain and metal and also metal and cement in porcelain-fused-to-metal (PFM) crowns. Optimal ranges of bonding angles at the corners of the interface, that result in the elimination of the stress singularity, are obtained for PFM crowns made from a precious-metal-based or non-precious-metal-based alloy, respectively. The findings presented in this paper can serve as guidelines for the design and manufacture of PFM dental crowns.     

Effects of substrate materials on fracture toughness measurement in adhesive joints

To understand the effects of substrate materials on the fracture behavior of adhesive joints, experimental studies and finite element analyses have both been conducted for double-cantilever-beams (DCB) with aluminum and steel substrates at different bond thickness (h). Numerical results show that the region dominated by the crack singularity is much smaller than by the bond thickness. Very small plastic deformation may hence violate the requirements for small-scale yielding where the crack-tip field can be characterized uniquely by the stress intensity factor. Both critical strain energy release rate and J-integral for the joints with steel substrate are lower than those for the joints with aluminum substrate. Compared to the critical strain energy release rate, the critical J-integral is less sensitive to the substrate material if small plastic deformation occurs before cohesive failure takes place through the adhesive layer. For the joints with aluminum substrate, the fracture toughness initially increases and then decreases with bond thickness. Elastic–plastic crack-tip analysis indicates that at the same level of loading, a higher opening stress is observed in the joint with a smaller bond thickness. A self-similar stress field can be obtained by the normalised loading parameter, J/hσ₀.     

Machining with tool-chip contact on the tool secondary rake face—Part I: a new slip-line model

Given the growing number of applications of groove-type chip breaker tools in modern machining, it is becoming increasingly important to study the tool-chip contact on the tool secondary rake face. This type of tool-chip contact significantly changes not only the state of stresses in the plastic deformation region, but also changes the distribution of forces and temperatures over the tool rake face. A new slip-line model accounting for the tool-chip contact on the tool secondary rake face is proposed in this paper. The model also takes into account chip curl and incorporates seven slip-line models developed for machining during the last six decades as special cases. Dewhurst and Collins's matrix technique for numerically solving slip-line problems and Powell's algorithm of nonlinear optimization are employed in the mathematical formulation of the model. The inputs of the model include (a) the tool primary rake angle γ₁, (b) the tool secondary rake angle γ₂, (c) the tool land length h, (d) the undeformed chip thickness t₁, (e) the ratio of hydrostatic pressure P_A to the material shear flow stress k, (f) the ratio of frictional shear stress τ₁ on the tool primary rake face to the material shear flow stress k, and (g) the ratio of frictional shear stress τ₂ on the tool secondary rake face to the material shear flow stress k. The outputs of the model include (a) the cutting force F_c/kt₁w and the thrust force F_t/kt₁w, (b) the chip up-curl radius R_u, (c) the chip thickness t₂, and (d) the natural tool-chip contact length l_n.     

Local fracture properties and dissimilar weld integrity in nuclear power plants

In this paper, the local fracture properties in a Alloy52M dissimilar metal welded joint (DMWJ) between A508 ferritic steel and 316 L stainless steel in nuclear power plants were investigated by using the single-edge notched bend (SENB) specimens, and their use in integrity assessment of DMWJ structures was analyzed. The results show that the local fracture resistance in the DMWJ is determined by local fracture mechanism, and which is mainly related to the microstructures and local strength mismatches of materials at the crack locations. The initial cracks always grow towards the materials with lower strength, and the crack path deviation is mainly controlled by the local strength mismatch. If the local fracture properties could not be used for cracks in the heat affected zones (HAZs), interface and near interface zones, the use of the fracture properties (J-resistance curves) of base metals or weld metals following present codes will unavoidably produce non-conservative (unsafe) or excessive conservative assessment results. In most cases, the assessment results will be potentially unsafe. Therefore, it is recommended to obtain and use local mechanical and fracture properties in the integrity assessment of DMWJs.     

Influence of glass substrate thickness in laser scribing of glass

The thickness of the glass substrate used in liquid crystal displays continues to be decreased from its original thickness of 1.1 mm for the purpose reducing size and weight. The aim of this study was to clarify the influence of the glass substrate thickness during laser scribing with crack propagation caused by laser heating followed by quick quenching. The laser scribe conditions for soda-lime glass substrates with thickness equal to or less than 1.1 mm were obtained in laser irradiation experiments. Two-dimensional thermal elasticity analysis was conducted with a finite element method based on the scribable conditions obtained in the experiment. The laser scribable conditions can then be estimated by the upper limit of the maximum surface temperature, T_max, and the lower limit of the maximum tensile stress, σ_tmax, in the cooling area, regardless of the glass substrate thickness. There is a substrate thickness with which the maximum tensile stress σ_tmax becomes the largest under each scribe condition. The substrate thickness with which σ_tmax becomes the largest is obtained at a faster scribe velocity for thinner glass substrate and at slower scribe velocity for thicker glass substrate. Owing to these relations, the crack depth also has almost the same tendency as σ_tmax.     

Stress concentration analyses of bi-material bonded joints without in-plane stress singularities

The problem of stress concentration in bi-material bonded joint is investigated under the condition of without stress singularities. Disappearance conditions of stress singularity near interface corners and edges are determined based on analyses of eigenvalue equations. Straight-side and curved interface of materials are designed for the bi-material models to avoid singular stress fields around the interface corner and edge. Assuming that one stress component or combined stresses are responsible for failure at or near the interface, the stress concentration becomes critical for the design of bi-material joints with higher interfacial strength. Numerical results show that the stress state near the interface depends strongly on both the interface geometry and the combination of materials, and stress concentration may always occurs at or near the interface. Emphasis is placed on the necessity for geometric optimization of an interface in order to design singularity-free junction with higher interfacial strength.     

Cutting surface quality analysis in micro ball end-milling of KDP crystal considering size effect and minimum undeformed chip thickness

Potassium dihydrogen phosphate (KH₂PO₄ or KDP) crystal is a typical soft-brittle optical crystal, and the size effect and brittle cutting mode are easy to appear in micro ball end-milling of KDP crystal. In this paper, micro-grooving experiments are conducted to study the size effect and brittle cutting in micro ball end-milling of KDP crystal with different feed rate and depth of cut. The cutting force, machined groove base quality and chip morphology are collected and analyzed carefully. The size effect is discovered by the phenomena of the existence of oscillations and relaxations in cutting force and hyper-proportional increase of specific cutting force, when the ratio of feed per tooth to cutting edge radius f_t/r_e is less than 1. While the brittle cutting mode is detected through the existence of sharp fluctuations in cutting force and cracks on the groove base when the ratio f_t/r_e is larger than 2. From the further comprehensive analysis of cutting force, specific cutting force, machined groove base quality and chip morphology, the cutting parameters with ratios of the maximum undeformed chip thickness in one cutting circle to cutting edge radius h_max/r_e around 0.14, 0.2 and 0.4 are regarded as size effect, optimal and brittle cutting points, respectively. The size effect, ductile cutting and brittle cutting zones are divided by the size effect and brittle cutting boundaries (points). Among the optimal points, the depth of cut of 2 μm with the ratio f_t/r_e of 1 is the optimal cutting parameter for micro ball end-milling of KDP crystal.     

Groove wear,built-up edge and surface roughness in turning

The variation of groove wear profile, built up edge adhering to the machined surface and surface roughness have been studied. The correlation between the surface roughness (R_a and R_t), groove wear and built up edge is discussed.