Cyclic Strain Rate Research Articles

Cyclic creep strain rate criterion for short-fibre reinforced thermoplastics: Validation of creep and fatigue performance across a wide range of load ratios

The fatigue and creep responses of short-fibre reinforced thermoplastic matrix composites are influenced by numerous parameters such as load ratio for fatigue. Fatigue criteria, based solely on the evolution of the cyclic creep of the mean strain, are effective for load ratios greater than zero (tension/tension) but have limitations for negative load ratios (tension/compression).The aim of this study was to propose a unified creep and fatigue criterion, based on the evolution of the cyclic creep of the maximum strain, that allowed both load ratio domains to be covered. To investigate a wide range of load ratios (-1 to 0.8), an existing creep and fatigue database was augmented with purely alternated tension/compression tests. This database, which included 489 tests, considered the variations in parameters that influence the creep and fatigue responses, such as fibre orientation, water content and specifically for fatigue load ratio and frequency. This was used to validate a unified criterion based on the cyclic creep of the maximum strain. This criterion improved fatigue life prediction compared to existing criteria and allowed fatigue and creep results to be unified with 87% of estimated lifetimes within a scatter band of factor two.

Cyclic strain rate dependent behaviour of Alloy 617

This paper investigates the effect of strain rate on the properties of Alloy 617 under low-cycle fatigue (LCF) in room temperature (RT). The LCF life properties and the damage mechanism are studied for this Alloy over a range of strain rate (5E-4 ∼ 1E-2 s−1) under fully-reversed controlled total strain range of 1.2%. It is found that LCF life of Alloy 617 is strongly influenced by the time-dependent mechanism, in terms of slow strain rate test (SSRT). The relationship between total strain, plastic strain, and time to failure with strain rates are established and expressed by means of power law function to describe the fatigue life. Metallography examination is carried out on the fractured specimens with the help of electron microscope and the fractography is discussed to distinguish the effect of SSRT on the physical damage characteristic under LCF loadings.

Muscle fiber strain rates in the lower leg during ankle dorsi-/plantarflexion exercise.

Static quantitative magnetic resonance imaging (MRI) provides readouts of structural changes in diseased muscle, but current approaches lack the ability to fully explain the loss of contractile function. Muscle contractile function can be assessed using various techniques including phase-contrast MRI (PC-MRI), where strain rates are quantified. However, current two-dimensional implementations are limited in capturing the complex motion of contracting muscle in the context of its three-dimensional (3D) fiber architecture. The MR acquisitions (chemical shift-encoded water-fat separation scan, spin echo-echoplanar imaging with diffusion weighting, and two time-resolved 3D PC-MRI) wereperformed at 3 T. PC-MRI acquisitions and performed with and without load at 7.5% of the maximum voluntary dorsiflexion contraction force. Acquisitions (3 T, chemical shift-encoded water-fat separation scan, spin echo-echo planar imaging with diffusion weighting, and two time-resolved 3D PC-MRI) were performed with and without load at 7.5% of the maximum voluntary dorsiflexion contraction force. Strain rates and diffusion tensors were calculated and combined to obtain strain rates along and perpendicular to the muscle fibers in seven lower leg muscles during the dynamic dorsi-/plantarflexion movement cycle. To evaluate strain rates along the proximodistal muscle axis, muscles were divided into five equal segments. t-tests were used to test if cyclic strain rate patterns (amplitude > 0) were present along and perpendicular to the muscle fibers. The effects of proximal-distal location and load were evaluated using repeated measures ANOVAs. Cyclic temporal strain rate patterns along and perpendicular to the fiber were found in all muscles involved in dorsi-/plantarflexion movement (p < 0.0017). Strain rates along and perpendicular to the fiber were heterogeneously distributed over the length of most muscles (p < 0.003). Additional loading reduced strain rates of the extensor digitorum longus and gastrocnemius lateralis muscle (p < 0.001). In conclusion, the lower leg muscles involved in cyclic dorsi-/plantarflexion exercise showed cyclic fiber strain rate patterns with amplitudes that varied between muscles and between the proximodistal segments within the majority of muscles.

Assessment of the capability of Zr, Y, La, Gd, Dy, C, and Si to enhance the creep strength of gamma titanium aluminide alloys based on their effect on flow stress and thermal activation parameters

The present work examines the capacity of the elements Zr, Y, La, Gd, Dy, C, and Si to increase the structural stability and creep strength of γ titanium aluminide alloys. The elements were alloyed for this purpose to the base material Ti-44at.%Al-5at.%Nb, and were transformed into four different states of precipitate dispersions via thermomechanical treatment. The resulting materials were deformed in compression at room temperature as well as at 1023 and 1173 K. Strain-rate cycling was performed during the tests to determine the associated flow stresses and the reciprocal activations volumes of plastic deformation. This information has been used to estimate the impact of the precipitates on the athermal component of the flow stress, which significantly influences the creep properties. The results indicate that only the elements C and Si are able to effectively enhance the creep strength in the long term. The element C forms Ti3AlC precipitates, which lead to a considerable increase of the athermal stress component, whereas Si forms Ti5Si3 precipitates leading to a moderate increase of the athermal stress component but a comparatively stable microstructure. Moreover, it appears that the strengthening effects could be increased by using higher additions of C or Si.

A modified energy‐based fatigue parameter for short fiber reinforced polymers: Performance analysis with varying thicknesses, load ratios, and fiber orientations

AbstractThe durability of short fiber reinforced polymers (SFRPs) is affected by several variables. Accurate unifying fatigue parameters are thus essential for an efficient characterization campaign. This research investigated the fatigue behavior of a Polyphtalamide PA6T/6I reinforced with 50% of glass fibers. Two thicknesses (1.6 and 3 mm), two orientations from the injection molding direction (0° and 90°), and three load rations (−0.5, 0.1, 0.5) were investigated. A new fatigue parameter, called alternating energy density (AED), was presented; its ability to correlate the fatigue tests results was compared to other known fatigue parameters—cyclic mean strain rate and cyclic creep energy density. All the fatigue parameters were found to be independent of specimen thickness. Furthermore, the prediction of the fatigue lifetime based on AED showed greater accuracy with respect to the other known investigated methods. AED could thus be used for accurate and efficient lifetime prediction of SFRPs.

Study on X-Ray-Induced Defects as Barriers for Dislocation Motion by Strain-Rate Cycling Tests Combined with Ultrasonic Oscillation

Study on X-Ray-Induced Defects as Barriers for Dislocation Motion by Strain-Rate Cycling Tests Combined with Ultrasonic Oscillation

Non-local continuum damage model for poro-viscoelastic porous media

We present a novel poro-damage-viscoelastic model for predicting the failure response of fluid-saturated porous geomaterials. The Generalized Maxwell model is introduced for the representation of the viscoelastic behavior of the solid skeleton, which is achieved by a standard Prony-series type expansion. Damage regularization is obtained by an non-local integral-type formulation and damage behavior is described by Mazars model with the modified von Mises-type equivalent strain measure. The poromechanics parameters (Biot’s coefficient, Biot’s modulus) are functions of damage, and the fluid flow obeys Darcy’s seepage law in the entire domain, while the permeability is assumed to be anisotropic and strain dependent. The coupled system is discretized in time using a backward Euler scheme. The non-linear system is linearized using a Newton Raphson scheme and solved monolithically every time step. A consistent Jacobian matrix and residual vector are derived analytically. Several numerical examples are studied in order to investigate the performance of the proposed approach, including (i) a column undergoing hysteresis from cyclic loading, stress relaxation, creep and variable strain rate loading tests and (ii) fluid-driven fracturing in a 2D poro-viscoelastic domain. The numerical time-dependent results exhibit mesh insensitivity for all field variables, and confirm the feasibility and applicability of the proposed non-local damage model for simulating hydraulic fracture. • A non-local damage model for poro-viscoelastic saturated porous media is proposed. • Von-Mises strain measure is used to drive the non-local integral type damage growth. • Damage-response includes nonlinear Biot’s coefficients and anisotropic permeability. • A mixed FEM solution is developed, including a monolithic iterative solution. • Analytically derived Jacobian is used to drive the non-linear solution of equations. • Benchmark problems include poro-viscoelastic column and hydraulic fracture problems. • Objective mesh-independent and oscillation free numerical results are presented. • The model has the potential to represent time-dependent damage in several rock types.

Study on Thermal Activation Barrier for Dislocation Motion by Strain-Rate Cycling Tests Combined with Ultrasonic Oscillations

Study on Thermal Activation Barrier for Dislocation Motion by Strain-Rate Cycling Tests Combined with Ultrasonic Oscillations

The effect of dwell on thermomechanical fatigue behaviour of Ni-base superalloy Inconel 713LC

Ni-base superalloy Inconel 713LC has been subjected to in-phase and out-of-phase thermomechanical fatigue loading in the temperature range from 500 °C to 900 °C. Both constant strain rate cycling and cycling with 10 min dwell at peak temperature were applied. The effect of phase shift and dwells on the cyclic stress response and damage mechanism was studied. The cyclic stress-strain curves and fatigue life curves were evaluated. The microstructure was investigated in SEM and TEM and the effect of dwells on thermomechanical fatigue characteristics is documented. The acquired results further discussion on the damage mechanism operating under specific loading conditions.

Undrained response of geocell-confined pond ash samples under static and cyclic loading

In this study, the effect of geocell confinement on strength, deformation, dynamic properties, and liquefaction characteristics of pond ash samples were investigated. Pond ash was collected from the Panki thermal power station, India. The geocells were handmade with high-density polyethylene sheets. Each geocell has three equal diameter circular cells. Consolidated undrained triaxial tests were conducted on both remolded and geocell-confined ash samples of size 100 × 200 mm under static and cyclic loading conditions. The confining pressures were chosen as 50, 75, and 100 kPa, cyclic strain rates were chosen as 0.2, 0.3, and 0.4% and the frequency of cyclic loading was 1 Hz. From this experimental study, it is observed that the increase in peak deviatoric stress of geocell-confined pond ash samples is about 21% to 32% higher than the unconfined samples. The liquefaction resistance of ash samples increases 51% to 87% due to geocell confinement. Result shows that ash samples exhibit better response in terms of cohesive strength, dynamic properties, and liquefaction resistance with geocell confinement, which proves the applicability of geocell-confined pond ash as a suitable material for construction of road embankments and slopes.

High-temperature plastic deformation of langle 110rangle-oriented BaTiO3 single crystals

BaTiO3 single crystals were deformed in compression along the langle 110rangle crystal axis to study the plastic deformability and dislocation structures at high temperatures under different loading conditions. The yield strength is determined from stress–strain curves under strain rate control, load control, strain rate cycling tests, and under step-wise loading conditions to elucidate the impact of measurement approach in yield strength behavior. A comparison between the chosen methods based on stress-dependent strain rate plots indicates that load control measurements are a suitable alternative to the commonly used strain rate-control experiments in metals. This allows avoiding overloading and providing an estimate of the overall achievable strain rates in a ceramic. Activation energies and activation volumes in the temperature range of 1100–1170 °C indicate a similar mechanical deformation behavior to SrTiO3.Graphical abstract

Multimodal analysis of the differential effects of cyclic strain on collagen isoform composition, fibril architecture and biomechanics of tissue engineered tendon

Tendon is predominantly composed of aligned type I collagen, but additional isoforms are known to influence fibril architecture and maturation, which contribute to the tendon’s overall biomechanical performance. The role of the less well-studied collagen isoforms on fibrillogenesis in tissue engineered tendons is currently unknown, and correlating their relative abundance with biomechanical changes in response to cyclic strain is a promising method for characterising optimised bioengineered tendon grafts. In this study, human mesenchymal stem cells (MSCs) were cultured in a fibrin scaffold with 3%, 5% or 10% cyclic strain at 0.5 Hz for 3 weeks, and a comprehensive multimodal analysis comprising qPCR, western blotting, histology, mechanical testing, fluorescent probe CLSM, TEM and label-free second-harmonic imaging was performed. Molecular data indicated complex transcriptional and translational regulation of collagen isoforms I, II, III, V XI, XII and XIV in response to cyclic strain. Isoforms (XII and XIV) associated with embryonic tenogenesis were deposited in the formation of neo-tendons from hMSCs, suggesting that these engineered tendons form through some recapitulation of a developmental pathway. Tendons cultured with 3% strain had the smallest median fibril diameter but highest resistance to stress, whilst at 10% strain tendons had the highest median fibril diameter and the highest rate of stress relaxation. Second harmonic generation exposed distinct structural arrangements of collagen fibres in each strain group. Fluorescent probe images correlated increasing cyclic strain with increased fibril alignment from 40% (static strain) to 61.5% alignment (10% cyclic strain). These results indicate that cyclic strain rates stimulate differential cell responses via complex regulation of collagen isoforms which influence the structural organisation of developing fibril architectures.

Tendons exhibit greater resistance to tissue and molecular-level damage with increasing strain rate during cyclic fatigue

Musculoskeletal soft connective tissues are commonly injured due to repetitive use, but the evolution of mechanical damage to the tissue structure during repeated loading is poorly understood. We investigated the strain-rate dependence of mechanical denaturation of collagen as a form of structural microdamage accumulation during creep fatigue loading of rat tail tendon fascicles. We cycled tendons at three strain rates to the same maximum stress relative to their rate-dependent tensile strength. Collagen denaturation at distinct points during the fatigue process was measured by fluorescence quantification of collagen hybridizing peptide binding. The amount of collagen denaturation was significantly correlated with fascicle creep strain, independent of the cyclic strain rate, supporting our hypothesis that tissue level creep is caused by collagen triple-helix unfolding. Samples that were loaded faster experienced more creep strain and denaturation as a function of the number of loading cycles relative to failure. Although this increased damage capacity at faster rates may serve as a protective measure during high-rate loading events, it may also predispose these tissues to subsequent injury and indicate a mechanism of overuse injury development. These results build on evidence that molecular-level collagen denaturation is the fundamental mechanism of structural damage to tendons during tensile loading. Statement of significanceThis study is the first to investigate the accumulation of denatured collagen in tendons throughout fatigue loading when the maximum stress is scaled with the applied strain rate. The amount of denatured collagen was correlated with creep strain, independent of strain rate, but samples that were cycled faster withstood greater amounts of denaturation before failure. Differential accumulation of collagen damage between fast and slow repetitive loading has relevance toward understanding the prevalence of overuse musculoskeletal injuries following sudden changes in activity level. Since collagen is a ubiquitous biological structural component, the basic patterns and mechanisms of loading-induced collagen damage in connective tissues are relevant for understanding injury and disease in other tissues, including those from the cardiovascular and pulmonary systems.

An investigation into the effects of cyclic strain rate on the high cycle and very high cycle fatigue behaviors of wrought and additively manufactured Inconel 718

Additive Manufacturing (AM) has increasingly been used to fabricate parts in aerospace applications, which may require service lives beyond ten-million cycles due to the imposed high loading frequencies. Understanding the very high cycle fatigue (VHCF) behavior of these additive manufactured (AM) parts is an important step towards their design and qualification processes. This study focuses on the high cycle fatigue (HCF) and VHCF behaviors of both wrought and laser beam-powder bed fusion (LB-PBF) fabricated Inconel 718 in machined/polished surface condition, emphasizing on the influence of test frequency (i.e., cyclic strain rate). Uniaxial, fully-reversed force- and stress-controlled fatigue tests were conducted utilizing a servo-hydraulic and an ultrasonic test system operating at 5 Hz and 20 kHz, respectively, on wrought as well as LB-PBF vertically and diagonally built specimens. Fatigue cracks in the majority of the specimens were found to initiate from intra-granular slip bands near or at the surface, which gives rise to strong anisotropy in fatigue resistance in LB-PBF specimens due to the presence of columnar grains along the build directions. Longer fatigue lives were obtained at 20 kHz, which was ascribed to possibly lower-than-intended stresses applied in the ultrasonic tests. The corrected stress-life fatigue data at 20 kHz were found to converge to the one obtained from conventional testing at 5 Hz, implying no effect of cyclic strain rate on the fatigue behavior of Inconel 718 regardless of the fabrication process. The findings of this work confirm the use of ultrasonic fatigue testing to expedite generation of AM materials data to keep up with the current demand; however, the applied stress may need to be corrected.

An overstress elasto-viscoplasticity model for high/low cyclic strain rates loading conditions: Part II – Numerical analyses

The present paper aims to show the ability of the extended overstress subloading surface EOSS theory to model the rate-dependency of metallic materials subjected to different loading conditions. The numerical results were carried out by means of Finite Elements FE simulations reproducing pure creep, stress relaxation, monotonic tensile and cyclic loading conditions. A particular emphasis is given to show the model response under quasi-static and high strain rates loading conditions describing the evolution of the main variables and investigating the convergence rate of the implicit integration scheme. The validation of the EOSS model is carried out against experimental tests, found in the literature, on a lead-free solder material and a SS316L stainless steel. The numerical results are in good agreement with the experimental data proving the ability of the EOSS model to catch a realistic material description.

An overstress elasto-viscoplasticity model for high/low cyclic strain rates loading conditions: Part I – Formulation and computational aspects

Plastic deformations of metallic materials depend on the rate of loading or deformation, implying rate-independent constitutive theories cannot catch a realistic response of the material when the loading condition is not quasi-static. Considering the conventional rate-dependent algorithms, the description of irreversible deformations does not show a smooth transition of the so-called viscoplastic strain from the elastic domain to the viscoplastic domain. In fact, a sudden generation of irreversible deformation takes place whenever the stress state exceeds the macroscopic yield stress during loading. Moreover, some models that adopt the overstress concept are not suitable for the description of impact loadings, since they predict an unrealistic elastic response, implying that the material could bear an infinite load. The present paper adopted the overstress subloading surface theory, which overcame the previous drawbacks. The constitutive equations were modified to take into account the movement of the similarity centre with the viscoplastic deformations to extend the description of the rate-dependency of the material to cyclic loading conditions. Moreover, aspects of the finite step accuracy of the algorithm and the local convergence are discussed.

Cyclic plastic response and damage mechanisms in superaustenitic steel Sanicro 25 in high temperature cycling – Effect of tensile dwells and thermomechanical cycling

The cyclic plastic response and damage evolution in isothermal high temperature constant strain rate cycling and during in-phase thermomechanical fatigue in superaustenitic Sanicro 25 steel have been studied both with and without introduction of dwells at maximum strain. Fatigue hardening/softening curves and fatigue life curves are reported for all types of fatigue tests. Rapid hardening and a tendency to saturation has been found primarily in isothermal cycling with dwells and in thermomechanical cycling. Study of the surface damage evolution using SEM observations and FIB cutting revealed the preferential oxidation of grain boundaries perpendicular to the stress axis. Introduction of dwells in maximum tension leads to the enlargement of the plastic strain amplitude and to additional creep damage in the form of cavities along grain boundaries and internal cracks. Fatigue hardening rate in thermomechanical cycling is higher than in constant strain rate cycling and fatigue life decreases substantially in in-phase thermomechanical cycling. These findings are discussed in the perspective of effectiveness of dominant damage mechanisms relevant to individual types of cyclic straining.

Hot deformation behavior of Hastelloy-X preforms built using directed energy deposition based laser additive manufacturing

Hastelloy-X (Hast-X) preforms built using Laser Directed Energy Deposition (LDED) are investigated for hot workability using hot deformation tests (temperature: 1223–1373 K, strain rate: 0.01–10 s−1). Strain hardening is observed at lower strain due to dislocation pileup and flow softening is observed at higher strain. Cyclic strain rate jump test and jump temperature tests are performed to calculate strain sensitivity factor and activation energy (Q), respectively. The derived constitutive equations are found to be in good agreement with experimental results with maximum deviation of 20%. Average Q value is found to be 280 kJ·mol−1, which is lower than that for similar alloys in wrought condition. The lower ‘Q’ value confirms the suitability of hot working of LDED built Hast-X as a potential method to build high strength engineering components.

Effect of boron on microstructure evolution and hot tensile deformation behavior of Ti-5Al-5V-5Mo-1Cr-1Fe alloy

Abstract Ti-5Al-5V-5Mo-1Cr-1Fe (Ti-55511), one of the near β-Ti alloys is considered to be the candidate material for high strength applications. However, the tendency to excessive grain growth under high temperatures limits its potential applications due to drastic reduction in strength with temperature and poses difficulties during forming. In present study, based on the understanding established for the workhorse Ti-6Al-4V and other Ti alloys, the near-β, Ti-55511 alloy was modified with 0.06 and 0.12 wt.% B to investigate the effect of boron on its microstructure and high temperature tensile behavior. The boron addition led to significant grain refinement and changes in morphology of α-phase from lamellar to globular. The high temperature tensile tests conducted over a range of temperatures and strain rates showed that the alloy with 0.06 wt% B displays superplastic characteristics with maximum elongation to failure of 385% at 1123 K and an initial strain rate of 1 × 10−3 s−1. The bare alloy and the alloy with higher boron content (0.12 wt.%) exhibited relatively higher flow stresses and consequently reduced superplastic elongations. Cyclic strain rate tests indicated that the alloy exhibits high strain rate sensitivity (m) values in the range of 0.3–0.8, which suggests that grain boundary sliding (GBS) is the dominant strain contributing mechanism for the observed superplasticity at elevated temperatures. Further, the detailed microstructural examination revealed that the coarser and elongated grain structure, which decreases GBS contribution and the presence of large fraction of titanium borides, which leads to an increased cavitation, respectively are the responsible factors for the observed reduced superplasticity characteristics in the bare alloy and the alloy with 0.12 wt.% B, respectively.

Evolution of fabric in spherical granular assemblies under the influence of various loading conditions through DEM

Fabric of a granular assembly represents the topology of the contact network. This paper investigates the evolution of contact anisotropy (fabric) and average coordination number for a granular assembly subjected to uniaxial compression through the Discrete Element Method (DEM). A monosize three-dimensional random close-packed granular assembly with periodic boundary conditions under uniaxial compression is considered in this work. The fabric evolution is studied by post-processing the output data of the DEM simulation. The influence of cyclic loading, strain rate, and Young’s modulus on the evolution of contact anisotropy and average coordination number is presented. The Young’s modulus of the particle shows a significant influence on the particle contact creation during compression of the granular assembly with high strain rate. Effect of inertia on the contact anisotropy is observed to be significant during the compression of granular assemblies with different Young’s modulus under high strain rate. The paper concludes with a semi-empirical model to predict the evolution of contact anisotropy as a function of the macroscopic stress state of the assembly during quasi-static uniaxial compaction. The model also introduces two microscopic non-dimensional parameters that are independent of friction between the particles and can be used to relate the macroscopic stresses with the contact anisotropy.