Strain For Initiation Research Articles

Influence of processing parameters on dynamic recrystallization and the associated annealing twin boundary evolution in a nickel base superalloy

Dynamic recrystallization (DRX) kinetics and its association with annealing twins in alloy 617 has been studied in a range of strain rates (0.001s−1 to 10s−1) and temperatures (1173–1473K). In both low (< 0.1s−1) and high (≥ 1s−1) strain rate domains, DRX fraction is high as compared to intermediate strain rate (~ 0.1s−1). The lower critical strain for DRX initiation with higher growth of DRX grains is responsible for large DRX fraction with coarse grains microstructure in the low strain rate domain. In contrast, DRX at higher strain rates is mainly controlled by the large stored energy along with adiabatic temperature rise which leads to higher DRX nucleation. It is also noticed that the evolution of twins is greatly influenced by deformation temperature at all imposed strain rates. Here, the evolution of twins is quantified by measuring twin boundary length fraction and length density. The twin-length density increases with increase in DRX fraction at low temperatures (< 1423K) but decreases with increasing DRX fraction at high temperatures (≥ 1423K). An exponential rise in recrystallized grain size with DRX fraction is observed at a temperature ≥ 1423K. This suggests that rapid migration of high-angle boundaries happened at ≥ 1423K which may not be favorable for nucleation of twins during DRX.

Critical stress and thermal activation of crystal plasticity in polyethylene: Influence of crystal microstructure and chain topology

The influence of microstructure and temperature on the initiation of yield stress and strain of high-density polyethylene are examined using a set of linear and branched polyethylenes. The polymers were crystallized in different ways in order to get samples covering the range of crystallinity 0.5 ≤ Xc ≤ 0.8 and crystallite thickness 8 nm ≤ Lc ≤ 29 nm. In contrast to the conventional macroscopic yield strain and stress, the initiation strain εyi and stress σyi were estimated from the macroscopic stress-strain curves at the onset of local plasticity as judged from in situ WAXS experiments upon tensile deformation. Phenomenological linear relation was observed between σyi crystallinity at each draw temperature Td. The dislocation model was applied to check the correlation between σyi and crystal thickness. In order to also account for the chain topology, namely the concentration of stress transmitters ST, a modified Eyring's approach was proposed. This modelling provides a good prediction the σyi dependence on Lc and ST in the context of thermally activated rate processes. Finally, anelastic stress gauge, σccr, was determined from local strain measurements in crystals at the same local strain as for σyi. This critical elastic stress at initiation of crystal plasticity displayed a good correlation with σyi at high crystallinity. However, σccr was found to deviate from σyi with increasing Td particularly at low Xc values. This finding was attributed to the activation of the crystalline mechanical relation that involves a significant drop of σyi with increasing Td in the crystalline lamellae under shear yielding whereas it does not affect the theoretical σccr elastic stress.

Effects of Ag Addition on Hot Deformation Behavior of Cu–Ni–Si Alloys

Hot deformation behavior of Cu–Ni–Si and Cu–Ni–Si–Ag alloys is investigated using the Gleeble‐1500D simulator in the 600–800 °C deformation temperature and 0.01–5 s−1 strain rate ranges. Dynamic recrystallization (DRX) mechanism is a feature of high temperature flow stress–strain curves of the alloy. Microstructure is observed by optical microscopy. Ag addition can refine the grains and accelerate dynamic recrystallization. Characteristic points of the flow stress curves, including critical strain for DRX initiation (ϵc), are determined by employing strain hardening rate analysis. Processing maps are developed and analyzed based on the dynamic material model (DMM). Ag addition can optimize the alloy processing workability.

Simulation accuracy of crack-tip parameters with extended GP methods

The success in using the GP, short for the generalized particle dynamics method (Fan, 2009), with the cohesive zone method (CZM) to bridge crack propagation at the atomistic and mesoscopic scale (Xu et al., 2016) brings more attention to its central issue: simulation accuracy of crack-tip parameters. This issue is addressed in this work by an extension of the GP method in which finite element (FE) nodes are connected with the outermost particles, thus it can reduce artificial effects on the most interesting atomic regions to obtain accurate crack-tip parameters. The accuracy of the XGP, short for this extended GP method, is confirmed by comparisons of simulation results with classic analytical solutions of an edge-crack and a central cylindrical hole in a two dimensional plate. After developing a series of models from 60nm to 5μm, used in an asymptotic analysis for model size effect, it is applied to the atomistic crack-tip simulation of a Mode-I edge crack of iron under plane strain condition. Results show that there exists a problem-dependent critical model size, LCR, above which may not be necessary under a given error tolerance; below which the simulation result is inaccurate in underestimating crack-tip displacement, the range of the traction-separation (TS) curve and the critical energy release rate GIC and overestimating the initiation strain for the crack-tip BCC to FCC phase transformation.

Effect of Heat Input on the Tensile Damage Evolution in Pulsed Laser Welded Ti6Al4V Titanium Sheets

The present paper is focused on studying the effect of heat input on the tensile damage evolution of pulsed Nd:YAG laser welding of Ti6Al4V alloy under monotonic loading. To analyze the reasons that the tensile fracture site of the pulsed-laser-welded Ti6Al4V sheet joints changes with the heat input under monotonic loading, the microstructure of the sample with different nominal strain values was investigated by in situ observation. Experiment results show that the tensile ductility and fatigue life of welded joints with low heat input are higher than that of welded joints with high heat input. Under tensile loads, the critical engineering strain for crack initiation is much lower in the welded joint with high heat input than in the welded joints with low and medium heat input. And the microstructural damage accumulation is much faster in the fusion zone than in the base metal for the welded joints with high input, whereas the microstructural damage accumulation is much faster in the base metal than in the fusion zone for the welded joints with low input. Consequently, the welded joints fractured in the fusion zone for the welds with high heat input, whereas the welded joints ruptured in the base metal for the welds with low heat input. It is proved that the fine grain microstructure produced by low heat input can improve the critical nominal strain for crack initiation and the resistance ability of microstructural damage.

Strain-induced phase transformation of a thin Co film on flexible substrates

The strain-induced FCC to HCP phase transformation of a two phase Co film on polyimide was investigated by performing a tensile test in an X-ray diffractometer. During straining of the 2 μm thick film, the intensity of the (002)FCC peak continuously decreases at engineering strains between 2 and 8% and remains constant at higher strains. Complementary in situ tensile tests under an optical light microscope showed crack formation at 6.7% and crack saturation at around 10% engineering strain. The strain-induced phase transformation starts before the first cracks form leading to a maximum lattice strain of approximately 0.9% as initiation strain measured from the (101¯1)HCP peak with the sin2ψ method which converts to a film stress of approximately 1270 ± 150 MPa. It could be revealed that a strain-induced phase transformation can enhance the ductility and therefore delay the crack onset of a thin cobalt film.

Prediction of Single-Peak Flow Stress Curves at High Temperatures Using a New Logarithmic-Power Function

In this study, using a nonlinear estimation of strain hardening rate versus strain, a new phenomenological constitutive equation is developed. Utilizing the presented model, three new equations were presented to determine the peak strain, critical strain for initiation of dynamic recrystallization (DRX), and transition strain associated with the maximum softening rate of DRX. Also, two temperature and strain rate-sensitive parameters were introduced to generate flow stress curve at any desired deformation conditions. The predicted results were found to be in a good agreement with the ones measured experimentally. Maximum errors in prediction of peak strain, critical strain, and transition strain were about 8, 11, and 4%, respectively. In addition, evaluation of maximum errors in prediction of flow stress indicates that the presented constitutive equation gives a more precise estimation of flow stress curves in comparison with the previous models pertaining modeling of single-peak flow stress curves.

Indentation of Polytetrafluoroethylene (PTFE) Thin Film: Simulations by Using Continuum Damage Mechanics

ABSTRACTThin compliant films on relatively hard substrates have a wide range of applications. In this work, continuum damage mechanics is used to simulate indentation of a 10-μm-thick polytetrafluoroethylene (PTFE) film deposited on glass for different load levels by finite element analysis. The results, compared to experiments, are useful in investigating the mechanics of wear and friction of soft thin films. The material model is elastic–plastic before damage initiation and includes linear damage progression thereafter. The effects of ductile and shear damage criteria and two parameters pertinent to the damage model, the equivalent plastic strain for damage initiation and the bulk fracture toughness, on the indentation are investigated. It is shown that the shear damage model is more suitable to characterize the indentation of the PTFE thin film. The bulk fracture toughness has greater significance with regard to damage compared to equivalent plastic strain at the onset of damage initiation. Comparison of simulation and experimental results shows that bulk fracture toughness of the thin PTFE film is approximately 20 J/m2. This value is lower than that for the bulk PTFE, and the difference is attributed to the thin-film nature of the case considered here.

Bending strength of adhesive joints in microelectronic components: Comparison of edge-bonding and underfilling

The bending strength of underfilled and edge-bonded ball grid array (BGA) microelectronic packages assembled on printed circuit boards (PCBs) was compared using double cantilever beam (DCB) specimens. All specimens with fillets of the same size and shape failed at the same load, with cracks initiating and propagating within the PCB. This was consistent with measurements of the crack initiation strain energy release rate for PCB interfacial failure, which was significantly smaller than that of cohesive failure within the adhesives. Finite element analysis (FEA) indicated that the stress state in the PCB near the PCB-fillet interface in both underfilled and edge-bonded specimens was only a function of the adhesive fillet size and shape, and independent of the extent of the adhesive layer between the PCB and the BGA, and independent of the adhesive mechanical and thermal properties over the broad range of properties of the tested adhesives. This explained why decreasing the fillet curvature in edge-bonded specimens produced a significant increase in the joint strength. The crack path in the PCB of the edge-bonded specimens was found to change with the adhesive cure temperature; however, this had a negligible effect on the failure load.

Crack propagation speed in ultra high performance concrete (UHPC)

This paper investigates crack speed in ultra-high performance concrete (UHPC) using pre-notched three-point bending specimens. The experimental parameters are fiber volume fraction and rate of loading. A hydraulic servo-controlled testing machine is used to apply lower notch tip strain rates, in the range of 0.025–1.01/s, while a newly developed impact testing system is used to achieve higher notch tip strain rates, ranging from 6.8 to 41.11/s. A high-speed camera is used to record images of the UHPC specimens during testing. Notch tip strain and crack speed are computed from the images, which show that crack speed increases asymptotically as the crack initiation strain rate increases. Crack speeds of up to 514m/s were achieved at the lower notch tip strain rates and up to 1454m/s for the higher notch tip strain rates. The achieved relationships are incorporated into a recently proposed crack-velocity dependent dynamic fracture model. The model is validated using published experimental data and used to show that, like conventional concrete, the strain rate sensitivity of UHPC is strongly associated with the characteristics of dynamic crack growth.

The variation in elastic modulus throughout the compression of foam materials

We present a comprehensive experimental study of the variation in apparent unloading elastic modulus of polymer (largely elastic), aluminium (largely plastic) and fibre-reinforced cement (quasi-brittle) closed-cell foams throughout uniaxial compression. The results show a characteristic “zero-yield-stress” response and thereafter a rapid increase in unloading modulus during the supposedly “elastic” regime of the compressive stress–strain curve. The unloading modulus then falls with strain due to the localised cell-wall yielding or failure in the pre-collapse stage and the progressive cell crushing in the plateau stage, before rising sharply during the densification stage which is associated with global cell crushing and foam compaction. A finite element model based on the actual 3D cell structure of the aluminium foam imaged by X-ray computed tomography (CT) predicts an approximately linear fall of elastic modulus from zero strain until a band of collapsed cells forms. It shows that the subsequent gradual decrease in modulus is caused by the progressive collapse of cells. The elastic modulus rises sharply after the densification initiation strain has been reached. However, the elastic modulus is still well below that of the constituent material even when the “fully” dense state is approached. This work highlights the fact that the unloading elastic modulus varies throughout compression and challenges the idea that a constant elastic modulus can be applied in a homogenised foam model. It is suggested that the most representative value of elastic modulus may be obtained by extrapolating the measured unloading modulus to zero strain.

Effect of microstructure on oxygen rich layer evolution and its impact on fatigue life during high-temperature application of α/β titanium

The near alpha titanium alloy, Ti-6424S, is utilized in many critical high-temperature aerospace components due to its unique properties. However, oxygen ingress during elevated-temperature exposure induces formation of a subsurface brittle oxygen-rich layer (ORL), resulting in a deterioration of mechanical performance. This paper, for the first time, establishes the effect of the underlying microstructure on the formation and evolution of the ORL in α/β titanium alloys. In addition, models were developed to predict (i) the evolution of ORL as a function of the material microstructure, (ii) the effect of ORL on the critical strain for in-service crack initiation, and (iii) estimates of fatigue life of components made from a specific microstructure during in-service high temperature exposure and formation of ORL. In particular, five different microstructures were produced by tailored heat-treatments and thermally exposed at 650 °C up to 420 h. The base metal and the ORL were quantified using microhardness indentations, optical microscopy, and scanning electron microscopy (electron backscatter diffraction (EBSD), backscattered electron (BSE), and secondary electron (SE) imaging). The effective diffusion coefficients (Deff) for each microstructure were calculated and then integrated into a critical strain model to predict crack initiation strain as a function of exposure time. The predicted ORL thickness was used to estimate fatigue life using experimentally measured crack growth data. The largest Deff coefficient was observed in a colony microstructure, while a basketweave microstructure showed the smallest Deff. For several bimodal microstructures, Deff was noted to increase with increasing area fraction of secondary alpha colonies.

Pressure, Temperature and Dwell Time Effects on Fatigue Life in 304 Stainless Steel Using a R5-based Mechanistic Fatigue Model

This paper aims to evaluate the effects of pressure, temperature, pipe wall thickness and dwell time on fatigue life in 304 stainless steel. For a given dwell time and temperature-internal pressure combination, fatigue life is calculated using the mechanistic fatigue model (MFM) presented. In addition, the influence of pipe wall thickness is also examined. The MFM uses Tresca strain range for initiation and Rankine strain range to account for crack growth rate up to a crack limiting length. The results showed that fatigue life was generally lower for a given load combination in the thinner pipe considered given its smaller surface area compared to the thicker pipe. This led to higher plastic strains and consequently, faster crack growth rates. Also, dwell time influences fatigue life with longer dwell times found to be more damaging. However, the influence of dwell time is tightly coupled with pipe wall thickness as it determines the nature of the thermal gradients developed.

The Initiation and Propagation of Dynamic Abnormal Grain Growth in Molybdenum

Plastic straining can initiate and propagate abnormal grains at temperatures significantly lower than is possible by static annealing. This phenomenon is termed dynamic abnormal grain growth (DAGG). Experiments that produce DAGG in commercial-purity molybdenum sheet materials are used to study the initiation and propagation of abnormal grains by plastic straining at temperatures from 1673 K to 2073 K (1400 $$^\circ $$ C to 1800 $$^\circ $$ C). The minimum strain necessary to initiate DAGG, termed the critical strain, decreases approximately linearly with increasing temperature. The variation in critical strain values observed at a single temperature and strain rate is well described by a normal distribution. An increased fraction of grains aligned with the $$\left\langle 110\right\rangle $$ along the tensile axis, a preferred orientation for DAGG grains, appears to decrease the critical strain for DAGG initiation. DAGG grains preferentially grow into the finest-grained polycrystalline regions, which suggests that the driving force for DAGG propagation is primarily from grain-boundary curvature. No effects of local crystallographic texture variation on growth are evident in microstructures containing DAGG grains. Together, these observations support the hypothesis that plastic straining during DAGG acts primarily to increase boundary mobility, rather than to increase the driving force for boundary migration.

Transition between mild and severe wear in titanium alloys

The mild to severe wear transition of two titanium alloys, Ti–10V–2Fe–3Al (a near-β titanium alloy) and Ti–6Al–4V (an α+β titanium alloy) was investigated and the correlation between the microstructure and wear micromechanisms was studied. Mild wear was dominated by oxidation via the formation and spallation of mechanically mixed layers (MMLs). The near-β alloy exhibited lower wear compared with α+β alloy in the mild wear region due to more stable MMLs as evidenced by 16% higher thickness of layer. Severe wear was characterized by strain localization and subsurface crack growth. Ti–6Al–4V showed lower transition load and lower resistance to unstable plastic deformation. However, Ti–10V–2Fe–3Al exhibited higher critical shear strain for subsurface crack initiation.

Twinning Assisted Crack Propagation of Magnesium-Rare Earth Casting and Wrought Alloys under Bending

Due to their high specific strength, good corrosion resistance and high temperature strength Magnesium alloys containing Rare Earth additions are promising candidates for structural and engine applications in the transportation industry. Also medical applications, like bone screws and nails, benefit from their moderate corrosion rate and biocompatibility. All applications need materials which show a high strength, ductility and fracture toughness in case a crack has formed during service to keep safety against rupture. In this study four extruded Mg10Gd based alloys modified with Nd and La have been 3-point-bend tested at low a deformation speed to evaluate the influence of the microstructure on crack growth. A comparison to the cast material (subjected to T4 to increase ductility and to reduce the dendritic microstructure) shows an increase in strength and ductility due to the fine grained microstructure as a result of recrystallization during extrusion. The maximum bending strength and outer strain to crack initiation is also strongly influenced by the alloying system itself. The influence of Nd and La to the binary alloy Mg10Gd is discussed in using tensile, compression and bending tests. The increase in strength results in reduced elongation to fracture in tension loading as well as the outer strain for the crack initiation during bending tests. Tensile tests are often discussed to be not a reliable method for determining the Young’s modulus of magnesium. Therefore resonance frequency damping analysis has been applied to determine the dynamic modulus of elasticity, which is compared with the flexural (bending) modulus. Crack growth is discussed using light microscopy and correlated with bending stress-strain curves. The crack growth rate of the extruded, fine grained material is many times higher than of the cast, coarse grained material. Crack propagation is mostly transgranular and assisted by twinning.

Combined effect of strain-rate and mode-ratio on the fracture of lead-free solder joints

Abstract The critical strain energy release rate for the solder joint fracture was measured as a function of the strain rate and the mode ratio of loading. These data are useful in predicting the fracture of solder joints loaded under arbitrary combinations of tension and shear during the impact conditions typical of falling portable electronic devices. In this study, strain rates from quasi-static (close to 0 s − 1 ) to 61 s − 1 were investigated at phase angles from 0 to 60°, typical of the range found in microelectronic devices. Copper–solder–copper double cantilever beam (DCB) model specimens were prepared using SAC305 solder at cooling rates and times above liquidus typical of actual ball grid arrays (BGAs). A drop tester was designed and built to achieve different strain rates at various mode ratios. The critical initiation strain energy release rate, J ci , increased about 70% from quasi-static to intermediate strain rates, before decreasing by more than 67% from intermediate strain rates to 42 s − 1 .

Prestress Driven Improvement in Fracture Behavior of in Situ Sputtered Zinc Oxide Thin Films on Stretched Polymer Substrates.

Flexible electronic devices need to survive bending or stretching operation without mechanical failure. If inorganic thin films are involved in the device structure, the evolution of cracks is a major challenge to overcome. Here, we report a novel way to substantially improve the fracture behavior of films that are based on intentional utilization of residual stress on the films by in situ sputtering on a stretched polymer substrate. The in situ sputtering combined with a stabilization stage yielded ZnO:Al thin films with a nearly 2-fold improvement in crack initiation strain, which indicates greater resistance to bending. The critical strain of the optimal ZnO:Al films was ∼1.83%, which is a significant improvement compared to the current tolerance value of ∼1%. This was accompanied by a ∼300% improvement in fracture energy. We attributed the improved fracture behavior to the presence of residual compressive stresses, which creates a barrier for crack formation by acting opposite to the applied bending strain.

Hot Deformation and Dynamic Recrystallization of Ti-6Al-7Nb Biomedical Alloy in Single-Phase β Region

The aim of this investigation is to evaluate the hot deformation behavior and critical conditions for onset of dynamic recrystallization in Ti-6Al-7Nb biomedical alloy. The effects of temperature and strain rate on the hot deformation behavior of Ti-6Al-7Nb alloy were studied by employing hot compression tests. The experiments were conducted at temperatures of 1050, 1100, and 1150 °C under strain rates of 0.0025, 0.025, and 0.25 s−1 up to a true strain of 0.65. The evolution of compressive flow stress during deformation of the experimental alloy in the single-phase β region was investigated. The obtained microstructures showed the occurrence of partial dynamic recrystallization along the prior grain boundaries, which was discussed relying on the continuous recrystallization process. Using the strain-hardening rate curves (dσ/de versus σ), two characteristic parameters including the critical strain for DRX initiation (e c) and the strain for peak stress (e p) were identified. The normalized critical stress and strain for initiation of DRX were, respectively, found to be 0.9 and 0.67. True stress-true strain curves have been used to construct proper constitutive equations for the experimental alloy. The obtained activation energy for hot deformation was discussed considering the dominant restoration processes. Moreover, the flow stress was modeled for different strain levels, where good agreement with the experimental alloy was found.

A physically-based constitutive model for SA508-III steel: Modeling and experimental verification

Due to its good toughness and high weldability, SA508-III steel has been widely used in the components manufacturing of reactor pressure vessels (RPV) and steam generators (SG). In this study, the hot deformation behaviors of SA508-III steel are investigated by isothermal hot compression tests with forming temperature of (950–1250)°C and strain rate of (0.001–0.1)s−1, and the corresponding flow stress curves are obtained. According to the experimental results, quantitative analysis of work hardening and dynamic softening behaviors is presented. The critical stress and critical strain for initiation of dynamic recrystallization are calculated by setting the second derivative of the third order polynomial. Based on the classical stress–dislocation relation and the kinetics of dynamic recrystallization, a two-stage constitutive model is developed to predict the flow stress of SA508-III steel. Comparisons between the predicted and measured flow stress indicate that the established physically-based constitutive model can accurately characterize the hot deformations for the steel. Furthermore, a successful numerical simulation of the industrial upsetting process is carried out by implementing the developed constitutive model into a commercial software, which evidences that the physically-based constitutive model is practical and promising to promote industrial forging process for nuclear components.