Fracture Of Sheet Research Articles

Comparison between Explosive Welding and Roll-Bonding Processes of AA1050/Mg AZ31B Bilayer Composite Sheets Considering Microstructure and Mechanical Properties

In this article, the effect of the manufacturing process on the microstructure and mechanical properties of AA1050/Mg-AZ31B bilayer composite sheets has been studied experimentally. In multilayer composite sheets, the interface bond strength plays a significant role in the mechanical properties of the composite. The most important factor affecting the bond strength is the manufacturing process of these sheets. So, two processes of explosive welding and roll bonding have been used to manufacture the bilayer composite sheets. The results show that the atomic diffusion in the interface is 6.5 microns in the explosive welding and 6.7 microns in the roll bonding. No intermetallic compounds have been observed in the interface of the studied bonding methods. The wavy and straight interface morphologies have been observed for explosive welding and roll bonding, respectively. Also, recrystallization has been observed in the microstructure of both methods. In the magnesium microstructure of rolled sample, twinning and deformation bands have been seen, while adiabatic shear bands have been formed in the explosive-welded sample. The ultimate tensile strength, elongation, and work of fracture (WOF) of explosive-welded sheets have been increased by 145%, 31%, and 407% relative to the rolled sheets, respectively. Besides, considering the separation of layers in the tensile test of rolled sheets compared to explosive welded ones, it can be stated that the structural failure in explosive welding occurs simultaneously for all layers. However, for roll bonding, the layer failure is independent of the entire structure failure. In this case, first, necking and fracture of the magnesium layer occur, and then, after the separation of the interface, the fracture of the aluminum layer happens.

Numerical simulation for deformation characteristics and fracture prediction of perforated sheet in incremental sheet punching

Based on the numerical simulation, the deformation characteristics of incremental sheet punching (ISP) of perforated sheet with circular holes was studied. Combined with the experimental results, the feasibility of using the normalized Cockroft-Latham ductile fracture criteria to predict fracture of perforated sheet during the ISP process was verified. On the basis of above, the influence of the geometric parameters of perforated sheet on the formability was studied. The main conclusions are the major strain is much greater than the minor strain at the interval of holes on the side wall of the perforated sheet; the thickness at the short-axis edge of the hole is smaller than that at the long-axis edge obviously; and the maximum value of major strain of perforated plate decreases with the increase of straight diameter center distance ratio. It is feasible to predict fracture of perforated sheet during the ISP process by numerical simulation with normalized Cockroft-Latham ductile fracture criteria.

Exploring the critical role of grinding modification on the flotation recovery of electrode materials from spent lithium ion batteries

The growing demand for high-quality batteries has promoted frequent upgrading of Lithium-Ion Batteries (LIBs), resulting in a large number of spent LIBs entering into the waste stream. Grinding flotation may be a promising physical recycling method to help dispose of this waste. This study provides theoretical support for this technology by exploring the physical and chemical changes in the grinding modification process. To study physical grinding behavior, an interaction model between LiCoO2 and graphite particles was proposed; it shows that a mixed grinding process successively undergoes peeling of the graphite lamellar structure, abrasion of the LiCoO2 particles, and bending fracture of the graphite sheets. This process effectively avoids excessive pulverization of feed particles and guarantees a good flotation environment. As for surface hydrophobic changes, the contact angle difference significantly increased, from 5° to 53.34°, after mixed grinding. Further chemical composition analysis suggests that the active lithium element robs the F element from the C–F bond to form LiF, which reduces about 25% surfactant organic impurities and ultimately increases the hydrophobicity differential in the electrode materials. It is the synergistic effect of a good feeding environment and great hydrophobicity differential that contributes most to excellent flotation separation (LiCoO2 94.38%).

Plastic instability and fracture of ultra-thin stainless-steel sheet

A new approach was used to characterise the hardening, instability and fracture behaviour of ultra-thin (0.1 mm) stainless steel sheets under stress triaxiality ranging from 0.37 to 0.66. The Swift and the linear hardening laws were used to capture the stress–strain relationship for the complete level of plastic deformation that exceeds that achievable by a uniaxial tensile test. For this, the Virtual Field Method (VFM) was applied, which uses the strain field measured on the surface of the notched samples, to output the material hardening parameters. To determine one set of hardening parameters that fits all conditions of stress triaxiality, an upper bound of major strain was selected for the VFM fitting that optimises a single set of hardening parameters across all of the stress triaxiality conditions. The void coalescence parameter in a Gurson-Tvergaard-Needleman (GTN) fracture model was calibrated and the model results for fracture initiation were validated experimentally with a quasi-biaxial stretching test.

Investigation on Friction Coefficient Considering Al-Si Coating Layer Fracture of Boron Sheets in Hot and Cold Deep Drawing

In this paper, hot and cold deep drawing processes are determined with direct deep drawing process and indirect deep drawing process. To predict the friction coefficient, the finite-element method, which can predict deformation behavior until the fracture of a blank sheet, was proposed using the forming limit diagram (FLD) curve. The effect of fracturing of the coating layer on the friction coefficient during the hot and cold deep drawing processes was investigated. The deformation behavior of the coating layer of the boron steel sheet that affects the friction coefficient in the hot and cold deep drawing processes was also proposed. A forming method that can control the surface condition of the formed product is further proposed by explaining the fracture of the coating due to the forming process.

Comparative study on forming limit prediction of AA7075-T6 sheet with M−K model and Lou−Huh criterion

Abstract In order to effectively predict the fracture of AA7075-T6 sheet, the forming limit curves of AA7075-T6 high-strength sheet were drawn according to Morciniak−Kuczyski (M−K) model and Lou−Huh criterion, respectively. The errors between the predicted values of the two theoretical prediction models and experimental values were calculated by error analysis. The forming limit curves were verified by the punch−stretch test to evaluate the prediction accuracy of M−K model and Lou−Huh criterion. The error analysis results show that the mean error of Lou−Huh criterion with the optimal parameters for all tensile specimens is 25.04%, while the mean error of M−K model for all tensile specimens is 74.24%. The prediction accuracy of Lou−Huh criterion in predicting the fracture of AA7075-T6 sheet is higher. The punch−stretch test results show that the forming limit curve drawn by Lou−Huh criterion can effectively predict the fracture of AA7075-T6 sheet, but the prediction accuracy of M−K model is relatively poor.

A combined theoretical-experimental approach for modelling ductile fracture of cold-reduced G450 steel sheet

A thermodynamics-based coupled damage-plasticity model for high strength cold-reduced steel is developed on the basis of a generic form, allowing the use of any yield criterion and damage evolution rule. The model possesses essential features of ductile failure, including large plastic deformation prior to fracture and the effects of both triaxiality and Lode angle on the failure. In conjunction with theoretical development, experimental work for both furthering the understanding of ductile fracture in high strength cold-reduced steel and validation of the proposed model under different loading conditions is also performed. In particular, advanced non-contact measurement techniques based on Digital Image Correlation (DIC) are used not only for characterising the material failure/data in both pre- and post-peak phases but also for calibration of parameters and validation of the proposed model. While the pre-peak hardening responses with homogeneous deformation can be described by a nonlinear hardening rule, in the post-peak regime, localisation of deformation before complete fracture requires the use of energy in both characterisation of the material failure and calibration of the model. In this sense, the essential work of fracture obtained from the experiments is linked with parameters controlling the fracture of the material and also used for a simple regularisation technique based on the concept of smeared deformation. The model responses are validated against experimental data in terms of both macro responses and evolving failure patterns, demonstrating good potential for applications in failure analysis of structures made of high strength cold-reduced steel.

Influence of welding parameters on morphology and mechanical performance of friction spot pressure welded thin monomer cast nylon 6 sheets

In this paper, 1-mm-thickness monomer cast nylon 6 sheets were joined under the pressure and the frictional heat generated between the pinless tool and thin stainless steel plates, which was named friction spot pressure welding technology. Influences of welding parameters on spot weld diameter, morphology, and mechanical performance were investigated. Results showed that incomplete interfacial bonding was observed under insufficient plunge depth. Increased rotational speed, plunge depth, and dwell time caused severer extrusion ring and reticular cracks on the joint surface. Material overheating occurred under rotational speed of 1000 rpm and material adhesion on the back of steel plate happened under plunge depth of 0.25 mm. The weld-bonded area diameter increased with larger welding parameters, but the corresponding joint fracture loads increased and then decreased. Four failure modes including lap interface tearing (IT), upper sheet fracture (UF), outer bonding area separation (BS), and the mixed UF and BS were observed. Spot welds failed with UF possessed the maximum tensile load while joints fractured with IT showed low performance. Three different morphologies were observed on the fracture surface of the IT joints while the fracture surfaces of the joints failed with other fracture modes showed elongated strips and flocculent structures.

Calibration of ductile fracture criterion from shear to equibiaxial tension using hydraulic bulge test

Phenomenological ductile fracture criteria have defining parameters to be determined from experimental fracture tests. For a precise calibration of a ductile fracture criterion, fracture experiments should cover a wide range of stress states. To this end, sheet metal specimens with different geometries are tested using various experimental setups, such as uniaxial tension, Nakazima, and butterfly tests. Usually, a combination of these tests is performed to capture a wide range of stress states, which makes the calibration procedure more difficult and tedious. In contrast, in the present research, only the hydraulic bulge test with a circular die is utilized to calibrate a ductile fracture criterion with three defining constants. To cover a wide range of stress triaxiality from the pure shear to the equibiaxial tension, circular blanks with different geometries are designed and developed with the aid of the finite element (FE) simulation. Among the presented geometries, there is an innovative specimen for reproducing the in-plane shear state using the hydraulic bulge test. In the present research, the ductile fracture of the AA6061-T6 aluminum alloy sheet is investigated. By comparing results of the FE modeling and the experiment of the hydraulic bulging, it is shown that the pressure, the bulging height, and the sheet thickness at the instant of fracture onset can be predicted with a reasonable accuracy by the ductile fracture model calibrated using the proposed test. Therefore, the designed specimens allow the hydraulic bulge test to be implemented for investigating the ductile fracture of sheet metals under a wide range of stress conditions.

Investigation on Fracture of a 6014-T4 Aluminum Alloy Sheet in the Flanging and Hemming Process Based on Numerical and Experimental Methods

The fracture of a flat-surface straight-edge hemmed component of aluminum alloy sheets was investigated in this study. The specimen was made of 1 mm thick 6014-T4. Natural aging characteristics of 6014-T4 were studied via uniaxial tensile tests. The results show that the yield stress and ultimate tensile strength increased while the uniform elongation, strain hardening exponent, and plastic strain ratio decreased during the natural aging period, which worsened the formability. The sheet was biaxially stretched to obtain a pre-strain before the flanging and hemming operation. The influence of the flanging radius on the fracture was evaluated using experimental and numerical methods, and the optimum values were obtained. The comparison between the roller hemming and die hemming process proved that the former tends to produce better formability.

Fatigue characterization and crack propagation mechanism of self-piercing riveted joints in titanium plates

The fatigue characterization and failure modes of self-piercing riveted joints in titanium plates were studied experimentally. The self-piercing riveting (SPR) process was optimised to improve the forming quality, mechanical property and failure behaviour of titanium joints through the comparative trials. The crack propagation mechanisms and fretting behaviours are quite different for different failure modes and analysed in detail from macro- and micro-perspective. The static and fatigue properties are both enhanced and the failure mode of rivet fracture is avoided for the optimised joints. The failures of upper sheet fracture and lower sheet fracture occur in the fatigue tests. Heavy fretting wear exists precisely at the critical faying interfaces and results in the fatigue crack initiations. The fatigue crack initiates at the faying interface between both sheets for the upper sheet fracture and propagates to both sides of the upper sheet simultaneously. Concerning the lower sheet fracture, the fatigue crack initiates at the faying interface between the rivet and the lower sheet, and then propagates to the close side of the lower sheet in the first stage, extends to the far side in the second stage.

Characterization of plasticity and fracture of an QP1180 steel sheet

Experiments are designed to experimentally characterize the plastic behaviour and fracture limits of an QP1180 steel sheet under complex loading conditions. The plastic behaviour is modelled by the Drucker yield function, while the fracture behaviour is illustrated the DF2016 [1] fracture criterion. The pressure-coupled Drucker yield function is calibrated by inverse engineering approach. The predicted load-stroke curves are compared with experimental results, which shows that the prediction matches with the experimental results with good agreement. Then the fracture data are then extracted from experiments and numerical simulations to calibrated two fracture criteria. The predicted fracture loci are compared with experimental results. The comparison demonstrates that both fracture criterion matches with the experimental results very well. Therefore, the Drucker function together with the Swift-Voce hardening law is recommended to model plastic deformation up to large plastic strain for various loading conditions. The fracture behaviour from shear to plane strain tension is recommended to be modelled by the DF2016 criterion and the pressure-coupled Drucker functions of sheet metal forming processes.

Fracture resistance of imperfect filler particles within nuclear graphite during irradiation

Although there have been many in-reactor experiments of nuclear graphite in history, its microstructure evolution during irradiation have not been well studied. One filler particle within nuclear graphite was investigated during ion irradiation. Remarkable dimensional changes and distortion of graphite filler were observed. Different from the well-known Mrozowski crack, cracks formed by graphite sheets fracture were found in filler and showed continuous opening during irradiation, resulting in porosity increase and decline in integrity of nuclear graphite. But evidences show that these cracks did not grow along the c-axis, indicating a strong resistance to irradiation-induced fracture of un-fractured graphite sheets. Although the dimensional evolution of graphite crystallite continues at the highest dose of 10 dpa, the Raman spectra showed that the structure of damaged graphite lattice became very stable at doses above 0.84 dpa during room-temperature irradiation.

Ductile fracture of AISI 304L stainless steel sheet in stretching

The present study focuses upon the numerical investigation of fracture initiation of austenitic stainless steel, AISI 304L, during a forming process leading to an expansion strain path. Hence, a combined experimental and numerical approach was tested. Monotonic tensile tests up to fracture were carried out. Load-extensometer displacement curves and a major strain distribution were measured. Numerical simulations of the tensile test up to rupture were carried out to calibrate damage parameters of several macroscopic fracture criteria.Numerical simulations of the Erichsen test were also carried out to verify the validity of the determined parameters of the ductile fracture criteria. The numerical predictions of the strain field, the onset of fracture, and the fracture location were compared with the experimental results. The simulation results show that the Rice-Tracey or Brozzo fracture criteria are reliable predictors of the onset of fracture of AISI 304L in the Erichsen test. More precisely, the Rice-Tracey criterion is the most effective to predict the ductile fracture in this case.

Quantitative analysis and kinetic modeling of ultrasound-assisted exfoliation and breakage process of graphite oxide

Ultrasonic exfoliation is the most widely used delamination technique for producing graphene oxide. In this work, the exfoliation of graphite oxide suspension is conveniently monitored via real-time measurement of viscosity and then quantitatively described by using the relative single-layer fraction (α) for the first time, by which the full exfoliation is achieved with a time scale of minutes. Lateral size evolution is demonstrated using lognormal distribution based on statistical analysis. With the quantization of relative single-layer fraction and lateral size distribution, a prototype database has been built to efficiently integrate experimental data tested under different process conditions (suspension concentration, ultrasonic frequency, and ultrasonic power density), which can be used as reference guides for process optimization and product control. The selection and breakage functions (Si and Bij) of ultrasound-assisted exfoliation are calculated and the breakage mechanisms of graphite oxide are found attributed mainly to sheet fracture.

Ductile fracture of dual-phase steel sheets under bending

Ductile fracture involving nucleation, growth and coalescence of microscale voids limits the performance, safety, reliability and manufacturability of a variety of metallic components and structures. This phenomenon is affected by length-scales induced by the geometry of deformation, loading conditions and microstructure of the material. For example, under uniaxial tension, dual-phase (DP) advanced high strength steel sheets exhibit similar flow response along rolling (RD) and transverse directions (TD) with ductility along RD being either equal to or greater than TD. However, the bendability of sheet specimens with bend axis parallel to RD is less than the bendability of sheet specimens with bend axis parallel to TD. The objective of this work is to model the interplay of length-scales induced by bending and microstructure on ductile fracture of DP steel sheets. To this end, microstructure-based finite element calculations of ductile fracture in DP steel sheets under bending have been carried out. In the calculations, DP microstructures in a bend specimen are discretely modeled. Our results show that the microscopic state of stress/strain, and hence, damage evolution in DP steel sheets under bending are highly heterogeneous. The extent to which length-scales induced by bending and DP microstructure affects crack nucleation and early stage crack growth is discussed. Parametric studies to quantify the effect of initial porosity, susceptibility to secondary void nucleation and energy dissipated in damage evolution prior to crack nucleation on the bendability of DP steel sheets have also been carried out.

Fatigue response of electromagnetic riveted joints with different rivet dies subjected to pull-out loading

Electromagnetic riveting (EMR) as a green manufacturing and efficient process has extensive application prospect in engineering fields. In this study, the fatigue properties of electromagnetic riveted joints with different rivet dies under pull-out loading were studied. The results showed that rivet dies had a significant effect on the pull-out fatigue performance. The pull-out fatigue life of the electromagnetic riveted joints with special dies was higher than that with the flat die. The reason was that the special rivet dies restricted the radial flow of the material in driven head. More material flowed into the riveted hole and rivet shaft became larger. The joints were reinforced by larger interference fit. Particularly, the joints with 80° rivet die had the best pull-out fatigue properties due to more uniform and moderate interference fit. Fracture analysis showed that there were two typical failure modes for the joints under pull-out loading: rivet manufactured head and upper sheet fracture. Due to the effect of cyclic impact and fretting wear damage, the joints with a flat die at high stress levels (194.8 MPa and 173.2 MPa) failed in rivet manufactured head, while the joints with special dies at all stress levels and with a flat die at low stress levels (151.5 MPa and 129.87 MPa) failed in upper sheet.

Ductile fracture of an aluminum sheet under proportional loading

The ductile fracture of an AA6111 aluminum sheet after a thermal cycle typical of auto-body paint-baking is investigated with the hybrid experimental-numerical method. The plastic flow of the material is examined by uniaxial tension, plane-strain tension, disk-compression and notched-tension experiments, that are used to calibrate the Yld2004-18p anisotropic yield criterion and the combined Swift-Voce hardening model. Then, the fracture behavior under equibiaxial and plane-strain tension, as well as uniaxial tension and shear, is characterized using a specially-developed cruciform specimen, along with center-hole and shear specimens, respectively. The cruciform fracture specimen proposed here contains two shallow hemispherical depressions (dimples) in the test-section, to initiate fracture. For the fracture characterization, special emphasis is put on specimen design, so that the stress states developed at the neighborhood of the fracture initiation point remain proportional throughout the loading history. In all experiments, the surface strain fields are measured by a stereo-type digital image correlation system. This information is used to validate finite element simulations of the fracture experiments. It is found that the Yld2004-18p model provides a better agreement with experiments than von Mises does, which underscores the sensitivity of the hybrid method to the plasticity models adopted. Once validated, these simulations are used to obtain the fracture loci in terms of two stress-state metrics, i.e., the stress triaxiality and Lode angle parameter.

Residual flexural properties of CFRP sandwich structures with aluminum honeycomb cores after low-velocity impact

This paper aims to evaluate the influences of impact-induced damage on the residual flexural strength of honeycomb core sandwich panels with different structural configurations by combining the experimental, numerical and theoretical methods. Low-velocity impact tests and three-point bending tests after impact are carried out to determine and quantify the effects of structural configuration and impact energy on the impact damage and residual flexural strength of such structures. Subsequently, an integrated FE model with the VUMAT subroutine is developed to further investigate the damage states and failure mechanisms for the impact and bending simulation. The numerical results match well with the experimental ones in terms of impact load, absorbed energy, residual flexural strength and failure mechanisms. Results indicate that increasing cell wall thickness or decreasing side length of honeycomb core has significant effects on peak load, while increasing core height has little effect. Specimens with lower core stiffness fail through core buckling and crushing under the bending load, while specimen with higher core stiffness fails by top face sheet fracture. The residual flexural strength reduces markedly even through the impact damage is barely visible, indicating that it has a strong correlation with impact energy and structural configuration of cores.

Effect of foam copper interlayer on the mechanical properties and fretting wear of sandwich clinched joints

Fretting wear has a considerable effect on the fatigue properties of clinched joints and is the most important cause of fatigue failure in them. Foam metal is a new type of structural material that can be sandwiched into a clinched joint to improve its mechanical properties with respect to fretting wear.In the work reported here, clinched single-lap and cross-lap joints with and without foam copper layers were prepared and subjected to static load testing. Fatigue tests were conducted on the single-lap joints to analyse the fatigue properties and failure modes of the aluminium-alloy clinched and foam metal sandwich clinched joints. The damage caused in single-lap joints by fretting wear was assessed by examining joint cross-sections before completely fatigue broken. Scanning electron microscopy and energy spectrometry were used to examine the failure fractures of the two kinds of joints and to investigate the fretting wear failure mechanism. The incorporation of the foam copper layer increased the shear strength of the clinched joint by 5% but had little effect on the peeling strength. It also increased the energy absorption capacity of the joint under a shear force by 33%, but reduced it under a peeling load by 27%.The fretting wear of the clinched joint occurred mainly at the contact surface of the two sheets in the lap area and at the clinched neck. Fretting wear generated oxide wear debris and caused micro-cracks on the sheet surface and these micro-cracks then continuously expanded until sheet fracture failure occurred.The foam copper layer effectively reduced the fretting wear between the upper and lower sheets and greatly enhanced the fatigue life of the aluminium-alloy clinched joint.