Development Of Residual Strains Research Articles

Local residual stresses and microstructure within recrystallizing grains in iron

The intragranular strains and the microstructure within recrystallizing grains in single phase pure iron have been systematically studied in 3D using synchrotron micro-diffraction. In contradiction to common knowledge we observe that residual elastic strains of up to 1–2 × 10−3 are present in the recrystallizing grains. The local strain variations within individual grains can be similar in magnitude to those between different grains. The effects of processing parameters, including rolling reduction, recrystallized volume fraction, and cooling rate, as well as microstructural parameters, including grain size, orientation and internal misorientation, on the development of local residual elastic strains and microstructure have been quantified. It is suggested that the difference in local defect density between recrystallized grains and the surrounding deformed matrix, as well as the redistribution of medium to long-range residual stresses in the deformed grains, are the main reasons for the development of such strains in the recrystallized grains. The local residual stress levels are calculated and their possible impact on the mechanical properties and recrystallization processes are discussed.

Experimental testing of residual strain and displacement development in pretensioned BFRP reinforced concrete beams

The utilisation of fibre reinforced polymers (FRPs) as reinforcement for concrete elements has attracted attention mainly due to their high tensile strength, light weight and corrosion resistance. Lately, there has been an interest in basalt FRPs as a more economically competitive and environmentally friendly option. Basalt FRP is manufactured from a widely available volcanic rock in a process which does not require any additives, giving it an edge over the currently more popular glass-fibre reinforced plastic (GFRP) reinforcement. Pretensioning of FRP reinforcement with low longitudinal modulus of elasticity, such as GFRP and BFRP has been proposed as an effective solution to the concerns regarding the serviceability performance of flexural elements reinforced with these composite materials. In previous research it has been demonstrated that prestressing even at low levels can significantly reduce deflections and postpone cracking of BFRP reinforced concrete elements. This research presents an experimental investigation of pretensioned BFRP reinforced concrete tested under quasi-static loading and unloading cycles at 5kN load increments until failure. A comparison with an unprestressed sample is also provided to examine the effectiveness of prestressing at improving the structural performance of the beams. The samples were equipped with internal strain gauges and linear displacement transducers to monitor the development of strains in the reinforcement, deflections and concrete surface strains during testing. Close monitoring of the anchorage zone and the development of cracks was also conducted. Based on the experimental results it can be concluded that the prestressing of BFRP reinforced beams delays the development of residual strains and residual displacements upon unloading. Furthermore, the increase in the prestress level further reduced the residual strain and displacements.

The effect of processing parameters and material properties on residual forces induced in Laser Powder Bed Fusion (L-PBF)

During the L-PBF process stresses are developed due to the large temperature gradients induced by the laser beam and the fast cooling rates which occur while the part is being built. In this study, the development of forces induced layer by layer from the cycles of thermal gradient during L-PBF are measured in-situ using a Force Transducer Device (FTD) for several processing parameters, including laser power and scanning strategy in various metal powders. Three scan strategies were studied: Stripe, Meander and Chessboard, and the results showed that Meander developed the lowest residual forces. All of the processing parameters had a strong influence on the development of residual strains in L-PBF; and, it was found that the laser power had relatively little effect on the residual forces above a minimum threshold and that above a critical value the point distance had little effect on the residual forces. Four materials were investigated: Ti–6Al–4V, Inconel 625, Inconel 718 and maraging steel. Maraging steel exhibited much lower residual forces than the other materials, which could be due to the low temperature phase transformation in this material. Evidence for this mechanism in the maraging steel was provided by changing the inert atmosphere in the L-PBF chamber. In addition, changing the inert gas atmosphere from argon to nitrogen for the maraging steel samples had an effect on the development of forces which changed the direction of the part deformation when it was released at the end of the build process.

Development of residual strains and their relaxation processes in atomically thin layers of core-shell structured nanoparticles

Residual elastic strains imposed on atomically thin films coated onto substrate materials cause changes in the electronic structures of such films. This knowledge has been employed in various industrial sectors as a means of tuning the optical, electrical, magnetic, and chemical properties of materials. Although the magnitudes of residual strains and properties of materials are closely related to the thickness of coating layers, the measurement of residual strains that develop in atomically thin films is challenging owing to the experimental complexities of conventional measurement methods. In this work, the residual strains developed in atomically thin Au overlayers grown epitaxially on Pd nanoparticles were measured using transmission electron microscopy based on nanobeam precession electron diffraction. Experiments revealed that a 1-nm-thick Au film can withstand an exceptionally high compressive strain of 4.3%, which is reduced to nearly zero as the film thickness increases beyond 5 nm. The microstructural evolution of Au films was monitored using high-resolution transmission electron microscopy and the large strain storage capacity and plastic relaxation behavior of Au films were interpreted by comparing them with the previous computer simulations and theoretical models.

The role of crystallographic orientations on heterogeneous deformation in a zirconium alloy: A combined experimental and modeling study

This study explores the effect of crystallographic orientations on the development of residual strains and local misorientations in a polycrystalline Zircaloy-4 during controlled tensile deformation. A combination of experiments (electron backscattered diffraction (EBSD), micro-Laue diffraction) and modeling (crystal plasticity finite element (CPFE)) are used. Data from two sets of grain orientations are analyzed and modeled: orientations within 5° of the exact (0002) basal pole (‘near basal’) and those within 5° of the exact (101‾0) prism pole (‘near prism’). In order to maintain similitude with experiments, the actual experimental microstructures (along with their crystallographic orientations) are used as an input for the CPFE simulations. Minor differences are observed for different measures of local misorientation developments. However, the ‘near basal’ grains show higher spread of grain reference orientation deviation, both in experiments and in CPFE simulations, and correlate with the intergranular strain heterogeneity predicted from CPFE simulations. Simulated trends of residual strains also compare favorably with the micro-Laue measurements. The ‘near basal’ grains have higher residual strains as compared to the ‘near prism’ grains. This observed behavior appears to originate from the elastic anisotropy of the hcp crystals and not from differences in the plastic deformation on the different family of slip systems.

Intervertebral disc swelling maintains strain homeostasis throughout the annulus fibrosus: A finite element analysis of healthy and degenerated discs

Tissues in the intervertebral disc have a large capacity to absorb water, partially due to the high glycosaminoglycan (GAG) content, which decreases linearly from the nucleus pulposus (NP) in the center to the outer annulus. Our recent work showed that fiber network and GAG distribution contributes to development of residual stresses and strains that were compressive in the inner annulus to tensile in the outer annulus. GAG loss in the inner annulus, as observed with early to moderate degeneration, reduced swelling capacity and circumferential-direction stress by over 50%. However, our previous model was not capable of evaluating interactions between the NP and annulus fibrosus (AF) during swelling. In this study, we evaluated the effect of degeneration (GAG content or swelling capacity) on residual stress development throughout the disc. Simulations of moderate to severe degeneration showed a 40% decrease in NP swelling capacity, with a 25% decrease in AF and cartilaginous endplate swelling. Together, these changes in tissue swelling resulted in a decrease in NP pressure (healthy = 0.21 MPa; severe degeneration = 0.03 MPa) that was comparable to observations in human discs. There was a 60% decrease in circumferential-direction residual deformations with early degeneration. Radial-direction stretch switched from compressive to tensile with degeneration, which may increase the risk for tears or delamination. Degeneration had a significant impact on residual stress/stretch and fiber stretch in the posterior AF, which is important for understanding herniation risk. In conclusion, degenerative changes in disc geometry and intradiscal deformations was recreated by only altering NP and AF GAG composition. Since most computational models simulate degeneration by altering material stiffness, this work highlights the importance of directly simulating biochemical composition and distribution to study disc biomechanics with degeneration. Statement of significanceTissues in the intervertebral disc have a large swelling capacity, due to its high glycosaminoglycan content. Our recent work demonstrated the importance of fiber network and glycosaminoglycan distribution residual stresses and strains development. In this study, we evaluated the effect of swelling on intradiscal deformations between the nucleus pulposus and annulus fibrosus. We also investigated the effect of degenerative glycosaminoglycan loss on swelling-based intradiscal deformations of the intact disc and its subcomponents. Decreases in nucleus glycosaminoglycan content resulted in morphological changes observed with degenerated discs and may help to explain mechanisms behind the increases in annular tears and mechanical dysfunction with degeneration.

Intervertebral Disc Swelling Maintains Strain Homeostasis Throughout the Annulus Fibrosus: A Finite Element Analysis of Healthy and Degenerated Discs

Tissues in the intervertebral disc have an excellent capacity to absorb water, largely due to the high glycosaminoglycan (GAG) content, which decreases linearly from the nucleus pulposus (NP) in the center to the outer annulus. Our recent work showed that fiber network and GAG distribution contributes to development of residual stresses and strains that were compressive in the inner annulus to tensile in the outer annulus. GAG loss in the inner annulus, as observed with early to moderate degeneration, reduced swelling capacity and circumferential-direction stress by over 50%. However, our previous model was not capable of evaluating interactions between the NP and annulus fibrosus (AF) during swelling. In this study, we evaluated the effect of degeneration (GAG content or swelling capacity) on residual stress development throughout the disc. Simulations of moderate to severe degeneration showed a 40% decrease in NP swelling capacity, with a 25% decrease in AF and cartilaginous endplate swelling. Together, these changes in tissue swelling resulted in a decrease in NP pressure (healthy = 0.21MPa; severe degeneration = 0.03MPa) that was comparable to observations in human discs. There was a 60% decrease in circumferential-direction residual deformations with early degeneration. Radial-direction stretch switched from compressive to tensile with degeneration, which may increase the risk for tears or delamination. Degeneration had a significant impact on residual stress/stretch and fiber stretch in the posterior AF, which is important for understanding herniation risk. In conclusion, relatively large changes in local strains occurred with degeneration, which may contribute to further degenerative remodeling.

PROCESS MODELING OF CARBON-EPOXY COMPOSITES: RESIDUAL STRESS DEVELOPMENT DURING CURE AND ANALYSIS OF FREE EDGE EFFECTS

Last decades, increased attention is paid to the deep understanding of the process induced residual stresses (locked-in) and their effect on shape distortion and fracture. Residual stresses evaluation is particularly important in multilayered composites with anisotropic thermo-mechanical properties, where the ply orientations and stacking sequences highly influence the appearing of manufacturing stresses. In the present study, the effect of free edge on residual stresses inherited during manufacturing of thermoset multilayered composites was investigated. In order to understand the stress-strain state in samples after free edge cut a simplified 2D plane strain finite element analysis was performed. Many phenomena, such as sensitivity of the results to the size of the numerical grid, stress redistribution and size of the area affected by free edge was analyzed. In the current research, the behavior of AS4/8552-1 carbon-epoxy composite during manufacturing cycle was studied by means of finite element modeling in ABAQUS. To describe the behavior of the composite material during the manufacturing process − including processes of formation, polymerization, development of residual strains and stresses was done by developed user subroutine-UMAT, describing the visco-elastic material behavior of the material. The program was implemented in ABAQUS and validated on the basis of literature data. The results of the study can be applied for prediction of residual stresses in composite structure by means of virtual simulation and further understanding the nature of fracture of composites.

Effect of aggregate shapes on local fields in 3D mesoscale simulations of the concrete creep behavior

Describing accurately the creep behavior of concrete is of significant importance for the evaluation of the long-term performance of structures. In this regard, a finer characterization of mesostructure effects and material non-linearity provides very insightful information. In this work, the effects of aggregate shapes on the creep response are studied using numerical simulations on 3D mesoscopic samples. The main focus is put on the assessment of the representativeness of generated samples versus real specimens obtained by tomography. Several mesostructures are generated by randomly distributing aggregates with different geometries, from simple spheres to realistic ones extracted from tomography. Creep simulations with finite element (FE) and Fast Fourier Transform (FFT) methods are then performed on different spatial refinements. Moreover, a classical linear viscoelastic (VE) and a viscoelastic-viscoplastic (VE-VP) behavior able to reproduce non-recoverable strains are adopted for describing the matrix behavior, to assess the relevance of a more accurate model. It is shown that numerical samples generated with tomographic aggregates may be regarded as a good approximation of the real specimen, while more ‘isotropic’ shapes, especially spherical, lead to significant differences at both local and macroscopic levels. Results obtained with FE and FFT methods are very close, indicating that while FFT is well adapted, FE remains attractive in this context. Finally, notable differences are observed between VE and VP response due to the development of residual strains in the matrix and correspondingly more limited strain redistribution, which indicates that VP-like models should be preferred to capture accurately the creep features at mesoscale.

Residual Strain and Stress in Biocrystals.

The development of residual strains within a material is a valuable engineering technique for increasing the material's strength and toughness. Residual strains occur naturally in some biominerals and are an important feature that is recently highlighted in biomineral studies. Here, manifestations of internal residual strains detected in biominerals are reviewed. The mechanisms by which they develop, as well as their impact on the biominerals' mechanical properties, are described. The question as to whether they can be utilized in multiscale strengthening and toughening strategies for biominerals is discussed.

Process-induced strains in RTM processing of polyurethane/carbon composites

The development of residual strains and stresses is critical to manufacture composite structures with the required dimensional stability and mechanical performance. This work uses Fiber Bragg Grating (FBG) sensors to monitor strain build-up in carbon fiber composites with a polyurethane (PU) matrix designed for high production volume applications. The PU matrix presents an initially low viscosity combined with a fast cure reaction, which makes it adequate to very short processing cycles. FBG sensors were incorporated into PU-matrix composites manufactured by vacuum assisted resin transfer molding (VARTM). The measured strains were compared with those obtained with different benchmark epoxy-matrix composites and with those obtained through micromechanical finite element simulations. Results showed that most of the residual strains were built-up during cool-down from the post-curing temperature and that stresses in the PU-matrix composites were comparable to those obtained for epoxies with similar Tg.

Effect of heat treatment temperature on nitinol wire

In-situ synchrotron X-ray diffraction has been used to study the influence of the heat treatment temperature on the subsequent micromechanical behavior of nitinol wire. It was found that increase in the heat treatment temperature rotated the austenite texture from the {332}B2 fiber towards the {111}B2 fiber, and the texture of the Stress-Induced Martensite phase changed from the (1¯40)B19' to the (1¯20)B19' fiber accordingly. Heat treatment at a low temperature reduces the internal residual strains in the austenite during super-elastic deformation and therefore improves the materials fatigue performance. The development of internal residual strains in austenite is controlled by transformation induced plasticity and the reversal martensite to austenite transformation.

Strain incompatibility and residual strains in ferroelectric single crystals

Residual strains in ferroelectrics are known to adversely affect the material properties by aggravating crack growth and fatigue degradation. The primary cause for residual strains is strain incompatibility between different microstructural entities. For example, it was shown in polycrystalline ferroelectrics that residual strains are caused due to incompatibility between the electric-field-induced strains in grains with different crystallographic orientations. However, similar characterization of cause-effect in multidomain ferroelectric single crystals is lacking. In this article, we report on the development of plastic residual strains in [111]-oriented domain engineered BaTiO3 single crystals. These internal strains are created due to strain incompatibility across 90° domain walls between the differently oriented domains. The average residual strains over a large crystal volume measured by in situ neutron diffraction is comparable to previous X-ray measurements of localized strains near domain boundaries, but are an order of magnitude lower than electric-field-induced residual strains in polycrystalline ferroelectrics.

Investigation of process-induced strains development by fibre Bragg grating sensors in resin transfer moulded composites

A comprehensive understanding of the development of residual strains in composite processing is essential to manufacture high quality composite parts. In this paper, the use of fibre Bragg grating (FBG) optical sensors was investigated to measure in situ the build-up of the process-induced strains in composite panels manufactured by resin transfer moulding. The FBG sensors, embedded in the composite laminate, successfully measured the evolution of the composite in-plane strains due to the temperature history. The sensors also captured a strain discontinuity during the cool-down related to the debonding of the composite from the mould. Finite element models were then proposed to simulate the strain development measured by the FBG sensors. Three different contact interactions between the tool and the laminate were investigated: no bonding, perfect bonding and frictional contact. The model using the frictional contact interaction described well the evolution of the measured strains.

Periosteal biaxial residual strains correlate with bone specific growth rates in chick embryos

It has been proposed that periosteal residual tensile strains influence periosteal bone apposition and endochondral ossification. The role of bone growth rates on the development of residual strains is not well known. This study examined the relationships between specific growth rate and residual strains in chick tibiotarsi. We measured length and circumference during embryonic days 11–20 using microCT. Bones grew faster in length, with longitudinal and circumferential specific growth rates decreasing from 17 to 9% and 14 to 8% per day, respectively. To calculate residual strains, opening dimensions of incisions through the periosteum were analysed using finite element techniques. Results indicate that Poisson's ratio for an isotropic material model is between 0 and 0.04. For the model with Poisson's ratio 0.03, longitudinal and circumferential residual strains decreased from 46.2 to 29.3% and 10.6 to 3.9%, respectively, during embryonic days 14–20. Specific growth rates and residual strains were positively correlated (p < 0.05).

Strain and texture evolution during mechanical loading of a crack tip in martensitic shape-memory NiTi

In situ synchrotron X-ray diffraction measurements are used to create two-dimensional maps of elastic strain and texture, averaged over a compact-tension specimen thickness, near a crack tip in a martensitic NiTi alloy. After fatigue crack propagation, the material ahead of the crack and in its wake exhibits a strong texture, which is eliminated by subsequent shape-memory heat treatment, indicating that this texture is due to detwinning, the main deformation mechanism of NiTi. Upon subsequent application of a static tensile stresses, the highly textured zone reappears and grows around the crack tip as the applied stress is increased. At the highest applied stress intensity of 35 MPa m 1/2, large tensile strains are measured ahead of the crack tip and considerable elastic anisotropy is observed. This detwinning zone is similar to the plastic zone produced by dislocation slip present around cracks in other metals. The texture in this zone is not significantly altered after mechanical unloading, despite the development of substantial triaxial compressive residual strains in this zone.

Residual stress development due to thermal cycling of the particle-reinforced alloy EN AW-6061– experiment and simulation

Abstract The development of residual stresses and strains in the matrix material of the extruded aluminium alloy EN AW-6061-T6 both with no reinforcement and also reinforced with 15vol.% and 22vol.% α-alumina particles was investigated in thermal cycling tests and additionally by finite-element-simulation for the non-reinforced material. For this purpose cylindrical specimens were heated by a 3kW diode laser and cooled by a water quench. Residual stresses in the surface area of the laser beam and temperature distribution inside the specimen were measured. The results were used to validate the finite-element model for calculating stress and strain distributions inside the specimens.

Time-dependent damage accumulation under stress relaxation testing of bovine trabecular bone

Time-dependent loading of trabecular bone consists of both cyclic and sustained static loads. Numerous studies have investigated the effects of fatigue and creep loading on the apparent and damage behavior of bone; however, microdamage formation in trabecular bone under stress relaxation tests has not been characterized. Bovine trabecular bone specimens (1.5–3 years old) were loaded in uniaxial compression to 0.9% strain and held for six different time points: 0 s, 250 s, 2000 s, 5000 s, 10,800 s, and 36,000 s. Total microdamage increased with hold time and leveled off after 10,800 s. The qualitative patterns of microdamage progressed from surface damage for short hold times to microcracks extending through the trabecular thickness as hold time increased. Residual strain behavior corresponded well with microdamage formation over time. These results indicate that under static compressive deformation, microdamage accumulation leads to the development of residual strains with time. This mechanism has important implications on reloading of specimens where degradations in bone strength may contribute to age-related skeletal fragility and traumatic fractures.

Development of residual strains in human vertebral trabecular bone after prolonged static and cyclic loading at low load levels

Development of irreversible residual strains in trabecular bone may be a mechanism by which age-related non-traumatic vertebral fractures occur. To investigate this concept, static and cyclic loading tests were conducted at low loading levels for cylindrical cores of cadaveric vertebral trabecular bone. Stresses were applied equivalent to elastic strains of either 750 or 1500 microstrain. Creep strains were measured during the tests, which lasted for 125,000 seconds (about 35 h), and for an additional 125,000 seconds after complete unloading. Emphasis was placed on the residual strains that developed, defined as the strain remaining at the end of the unloading phase. The results indicated that appreciable residual strains did develop, and were similar for static and cyclic loading. Irrespective of the applied load levels and loading modes, the residual strains that remained after the unloading phase were similar in magnitude to the originally applied elastic strain. Extrapolation of the observed residual strains to full recovery indicated that the time that would be required for full recovery was over 20 times longer than the duration of the applied loads. These results indicate that human vertebral trabecular bone does not creep in a linear viscoelastic fashion at low stress levels, and that creep mechanisms dominate the residual strains regardless of the loading mode. Taken together, these findings support the concept that non-traumatic vertebral fractures may be related to long-term creep effects because the trabecular bone does not have sufficient time to recover mechanically from creep deformations accumulated by prolonged static or cyclic loading.

A note upon the development of residual curing strains in carbon/epoxy laminates. Study by thermomechanical analysis

This work is directed towards analysing the development of process-induced strains in carbon/epoxy laminates during their curing. To this end, we have developed an original method using a thermomechanical analyser (TMA). This method makes possible the continuous follow-up of any longitudinal displacements. The longitudinal strains of unsymmetrical laminated strips are directly recorded by the thermomechanical analyser probe during their curing. This enables the out-of-plane deflections of unsymmetrical laminate strips to be continuously determined throughout the curing. Beyond this continuous measurement method, several preliminary experiments were carried out to study the changes in mechanical and rheological properties of the material during cure. Furthermore, the chemical shrinkage of the unreinforced polymeric matrix were accurately measured. Combination of the preliminary test results and measurements taken on various laminated strips while they are cured allows us to analyse the development process of residual strains and therefore stresses.