Modulus Degradation Research Articles

A THERMODYNAMIC CONSISTENT ELASTOPLASTIC FRACTIONAL TIME-DEPENDENT DAMAGE MODEL FOR ROCK-LIKE MATERIALS

This paper aims at presenting a thermodynamically consistent elastoplastic fractional time-dependent damage model for describing short- and long-term behaviors of rock-like materials. The model utilizes generalized potential theory with a yield criterion, a non-associated flow rule and an isotropic plastic hardening function for describing the evolution of plasticity. A time-dependent Lemaitre-type damage is introduced through fractional derivative considering the short- and long-term evolution of microstructure, which leads to progressive degradation of elastic modulus and failure strength of material. In this context, both instantaneous and delayed deformations shall be well described within the unique constitutive model. For practical application, an efficient and convergent semi-implicit return mapping (SRM) algorithm involving a plasticity-damage decoupling corrector is developed. The proposed model is finally adopted to predict the mechanical and deformation behavior of several types of rocks under different loading conditions in conventional or quasi-static (different loading strain rate) triaxial compression tests, creep tests and relaxation tests. Comparisons between model predictions and experimental data demonstrate that the proposed model has the capability to reproduce main features of short and long-term behaviors of rock-like materials.

Fatigue Life Prediction for Carbon-SMC and Carbon-FRP by Considering Elastic Modulus Degradation

In the automotive industry, being lightweight has become an important design factor with the enhancement of environmental regulations. As a result, many studies on the application of composite materials are in progress. Among them, interest in carbon materials, such as carbon sheet molding compound (C-SMC) and carbon-fiber-reinforced plastic (CFRP), which have excellent strength and stiffness, is increasing. However, CFRP is a material that makes it difficult to secure economic feasibility due to its relatively high manufacturing costs and limited mass production, despite its excellent mechanical strength and durability. As a result, many studies have been conducted on C-SMC as an alternative carbon composite material that can be easily mass-produced. In this regard, this study intended to conduct a study on evaluating the fatigue strength of C-SMC and CFRP among mechanical properties due to the lack of clear failure criteria for fatigue design. We investigated the tensile and fatigue strengths of C-SMC and CFRP, respectively. In the case of C-SMC, the mechanical strength tests were conducted for two different width conditions to evaluate the cutting effect and the machining methods to assess the effects of the edge conditions. To evaluate the fatigue failure assessment criteria, the stiffness drop and elastic modulus degradation criteria were applied for each fatigue test result from the C-SMC and CFRP. The results confirmed that the rationality of the failure criteria in terms of the stiffness drop and the application of the fatigue life prediction of C-SMC based on elastic modulus degradation demonstrated promising results.

Buckling Analysis of Curve Stiffened Fuselage Panel of Very Light Jet Aircraft

Buckling characteristic of curve stiffened fuselage panel of very light jet (VLJ) aircraft which use one of rivet or friction stir welding (FSW) joint on its stringer-frame-skin connections has been investigated by employing finite element methods. The linear and non-linear finite element analyses were used to determine the effects of stringer shapes and pitch on the buckling load of the fuselage panel. The buckling modes of all fuselage panels have also been observed. The material, boundary, and geometric non-linearity were applied on the fuselage panel in the analysis. Degradation of material elastic modulus around Heat Affected Zone (HAZ) due to friction stir welding processes has been taken into account. The results show that the buckling load of curve FSW stiffened fuselage panel slightly smaller than that curve rivet stiffened fuselage panel for all stringer shapes and stringer pitch. Besides, the curve stiffened fuselage panel with a smaller stringer pith has a higher critical buckling load and higher loading capacity. Both curve rivet and curve FSW stiffened fuselage panels with close hat stringer shape have a better loading capacity than with the open hat and the Z-stringer shape.

Evaluation of Demineralized Bone Matrix Particles Delivered by Alginate Hydrogel for a Bone Graft Substitute: An Animal Experimental Study.

BackgroundOur objective was to explore a synthetic alginate hydrogel delivery system for the delivery of demineralized bone matrix (DBM) particles for bone graft substitutes.Material/MethodsThe physiochemical properties of surface morphology, porosity measurements, in vitro degradation, equilibrium swelling, and mechanical testing of combined DBM powder and alginate in amounts of 0 mg/1 mL, 25 mg/1 mL, 50 mg/1 mL, and 100 mg/1 mL were detected. In vitro cell culture and in vivo studies using Sprague-Dawley rats were performed to evaluate the biocompatibility and osteoinductivity of DBM-alginate (ADBM) composites.ResultsDBM particles were uniformly scattered in all composites, and macro-scale pores were omnipresent. All composites showed a similar low degradation rate, with approximately 85% of weight remaining after 15 days. As the concentration of DBM particles in composites increased, degradation in collagenase and elastic modulus increased and the pore area and swelling ratio significantly decreased. No cytotoxicity of ADBM or alginate on mesenchymal stem cells (MSCs) was observed. Cell cultivation with ADBM showed greater osteogenic potential, evidenced by the upregulation of alkaline phosphatase and alizarin red staining activity and the mRNA expression level of marker genes RUNX2, OCN, OPN, and collagen I compared with the cells grown in alginate. Evaluation of ectopic bone formation revealed the osteoinductivity of the ADBM composites was significantly greater than that of DBM particles. Osteoinduction of the composites was demonstrated by a cranial defect model study.ConclusionsThe delivery of DBM particles using a synthetic alginate hydrogel carrier may be a promising approach in bone tissue engineering for bone defects.

Simulation of the compressive behaviour of a composite egg-box core with fibre orientation-mapping technique and progressively degraded material properties

The compressive characteristics of egg-box energy-absorbing cores composed of plain-weave carbon/epoxy composites were analysed via a finite-element analysis (FEA) and experimentally validated. Through draping experiments and simulations, material behavioural aspects, including wrinkle generation according to the initial draping orientation (0° ([0/90]T), 30° ([30/−60]T), and 45° ([±45]T)) of uncured prepregs were estimated. The compressive behaviour and energy-absorbing capability of the cured egg-box core were estimated via FEA and experimentally validated. To improve the simulation accuracy, the fibre orientation-mapping technique as well as models for modulus degradation and progressive failure were developed and applied in the FEA. Using the abovementioned simulation technique, the crushing behaviour of the egg-box energy-absorbing core was predicted, and the corresponding failure mode under compression was estimated successfully.

Insights into Mechanical and Thermal Properties of Additively Manufactured PLA Samples Triggered by Automotive Industry Demands

Paper explores the influence of the infill density (%), as a process parameter in additive manufacturing (3D printing), upon the mechanical (tensile, impact) and thermo-physical properties of PLA samples. The results indicated degradation of both tensile strengths and moduli with decrease of samples� relative density from 100% to 25% with 49.9% and 42.0%, respectively. Similar behaviour holds on impact strength values that degrades with 56.0% for the samples printed using a 25% infill density. The Young�s modulus variation with relative density values was approximated using a 3rd order polynomial function, in accordance with the expression for closed-cell thick edge rhombus cellular structures. All PLA samples revealed small difference on their coefficients of linear thermal expansion, irrespective of their infill densities, with a decrease of 6.34% in the lowest relative density value specimens, indicating enhanced stability within selected temperature range. Glass transition temperatures were approximately located at 65�C whereas cold-crystallization around 80�C, thus unaffected by selected process parameter.

Seismic Evaluation of a Profile of Volcanic Ash From the South American Andes

Although soils derived from volcanic ash are only a small portion of the soils that cover the world's surface, cities have developed on some of these soils. This makes their study interesting, especially since soils derived from volcanic ash have special characteristics including a high void ratio, liquid limits that can be greater than 100%, and shear wave velocities close to 150 m/s. Cities on these soils in the South American Andes are in areas with frequent and intense seismic activity making an understanding of how earthquakes affect these soils a matter of importance. This paper analyzes site response by the equivalent linear method, taking into account the variability of these soils. The Monte Carlo method was used for degradation of the shear modulus and damping curves. The results of this study show factors that amplify spectral acceleration can cause this measurement to become greater than four. In addition, the authors found that maximum spectral accelerations are functions of high plasticity indexes whose influence increases when shear wave velocities are between 175 and 225m/s.

The Effect of Functionally‐Graded Voids and GNPs on the Damage Tolerance of Polyurethane Foam Core

AbstractThe weight of sandwich composite structures with foam cores mainly depends on the volume fraction and the distribution of air voids in its porous structure. A considerable reduction of the weight can be achieved at the expense of mechanical and strength properties by increasing the fraction of air voids in the sandwich composite's core. Therefore, high stiffness nano‐fillers such as graphene nano‐platelets (GNPs) are frequently used to improve the specific stiffness and mechanical strength of sandwich composites without sacrificing from the lightweight requirements. In this contribution, the effect of the square‐ and circle‐shaped distributions of air‐voids and GNPs on damage tolerance properties of thermoplastic polyurethane (TPU) resin is investigated. TPU foams with randomly oriented air voids and GNPs are used as a baseline to evaluate the improvement of the properties mentioned above. For that purpose, finite element based micro‐mechanical models for representative volume elements (RVEs) are used to homogenize the properties of multi‐scale models of TPU foam. The analysis results show that the air pores increase the ductility of brittle TPU resin significantly when the pores are non‐linearly distributed in circular functionally graded (FG) shape. Besides, it is concluded that Young's modulus degradation associated with the use of linearly distributed air‐voids composed of the square‐shaped FG foam can be compensated by using GNPs. The results of the study suggest that the functional grading design of TPU foam has excellent potential for applications, which require meeting the challenging weight, strength, stiffness and toughness requirements at the same time.

Model of elastic modulus degradation and fracture of composite materials under cyclic loading

The aim of the work is to numerically simulate the degradation of elastic properties of carbon-fiber-reinforced polymers based on the previously developed model of damage in solids with mesoscopic defects. The paper presents a mathematical statement of cyclic loading of a representative volume of composite material with regard to damage accumulation. An analytical solution of the problem has been obtained, which made it possible to describe both the degradation of elastic properties and the Wöhler curve (fatigue curve). An identification procedure of unknown model constants has been performed. Comparison of modeling results with experimental data of other authors showed good agreement.

Modulus degradation of concrete exposed to compressive fatigue loading: Insights from lab testing

Modulus degradation of concrete exposed to compressive fatigue loading: Insights from lab testing

Predicting elastic modulus degradation of alkali silica reaction affected concrete using soft computing techniques: A comparative study

Alkali silica reaction (ASR) is a harmful distress mechanism which results in expansion and reduction of mechanical properties of concrete. The latter may cause loss of serviceability and load carrying capacity of affected concrete structures. Influences of ASR on concrete are known to be complex in nature, for which the traditional empirical and curve-fitting approaches are insufficient to provide adequate models to capture such complexity. Recent advancement in soft computing (SC) offers a new tool for tackling the complexity of ASR affected concrete. Most of previous experimental studies agreed that as a result of ASR, the elastic modulus suffers a significant reduction compared with other properties such as compressive and tensile strength of the affected concrete. In this study, an investigation has been conducted, utilising different SC models to quantify ASR-induced elastic modulus degradation of unrestrained concrete. Five SC techniques, namely support vector machine (SVM), artificial neural network (ANN), adaptive neuro-fuzzy inference system (ANFIS), M5P model and genetic expression programming (GEP), are investigated comparatively in this research. The models, on basis of SC techniques, are developed and tested using a comprehensive dataset collected from existing publications. In order to demonstrate the superiorities of SC techniques, the proposed approaches are compared to several empirical models developed using same dataset. The comparative results show that the developed SC models outperform empirical models in a wide range of evaluation indices, which indicates promising applications of the proposed approach.

Failure mechanisms of sand under asymmetrical cyclic loading conditions: experimental observation and constitutive modelling

In practical engineering, natural soil deposits often sustain an initial driving force prior to cyclic shear, owing to earthquakes, traffic and waves; such asymmetrical loading conditions may significantly affect the liquefaction susceptibility and failure mechanism of sand. To understand the typical cyclic liquefaction responses, comprehensive asymmetrical cyclic loading tests were conducted on sand samples subjected to either compressional or extensional static stress. The results indicated that different stress conditions can result in three distinct failure mechanisms: flow liquefaction, cyclic mobility and residual deformation accumulation. According to the experimental observations, an anisotropic sand model was developed within the framework of the anisotropic critical state theory. The model employed a fabric-dependent dilatancy, and accounted for the effects of the fabric evolution and accumulated loading index on the plastic hardening, in order to better reflect the cyclic degradation of the plastic modulus. The predictive capacity of the model was confirmed through undrained monotonic test results for samples with different densities. Comparisons between the model responses and experimental results indicated the excellent capabilities of the developed model in terms of capturing the typical deformation, strength and fabric characteristics of different cyclic failure mechanisms of sand under either symmetrical or asymmetrical loading conditions.

Laboratory Studies of Small Strain Stiffness and Modulus Degradation of Warsaw Mineral Cohesive Soils

The shear modulus and normalized shear modulus degradation curve are the fundamental parameters describing soil behavior. Thus, this article is focused on the stiffness characteristic of 15 different Warsaw cohesive soli represented by the parameters mentioned above. In this research, standard resonant column tests were performed in a wide shear strain range, from a small one, where soil behaves like an elastic medium, to a medium one, where soil has an unrecoverable deformation. Collected data allows the authors to create empirical models describing stiffness characteristics with high reliability. The maximum shear modulus calculated by the proposed equation for Warsaw cohesive soil had a relative error of about 6.8%. The formula for normalized shear modulus estimated G/GMAX with 2.2% relative error. Combined empirical models for GMAX, and G/GMAX allow the evaluation of Warsaw cohesive soil’s shear modulus value in a wide shear deformation range, with a very low value of the relative error of 6.7%.

Experimental investigation on the effects of fabric architectures on mechanical and damage behaviors of carbon/epoxy woven composites

The mechanical behaviors and damage evolutions of carbon/epoxy woven fabric composites with three different geometries, i.e., one plain weave and two twill weave patterns with different areal densities, are studied under tensile loading. The effects of weave patterns on mechanical properties are investigated by monotonic and cyclic tension tests. Remarkable variations in stress–strain curve, Poisson’s ratio, residual strain and strain map exist in the three composites. Crimp ratio is found to be a critical factor to govern the mechanical properties. With smaller crimp ratio, a quasi-linear stress–strain curve with higher elastic modulus and strength is observed. The stress–strain curves of composites with higher crimp ratio contain transition stages with significant tangent modulus degradation. Elastic modulus, strength and damage initiation are all correlated with the crimp ratio linearly regardless of the fabric pattern. Dramatic nonlinear evolution in Poisson’s ratio occurs in the composite with higher crimp ratio. Cyclic tension results indicate that the residual strain is a more appropriate damage indicator than the unloading elastic modulus. Microstructure examination shows that damage developments are essentially related to the fabric geometry, and result in various mechanical behaviors. This study provides important insights into the geometry-deformation mechanism-mechanical property relationship of the woven composites.

A thermodynamics-based hyperelastic-plastic coupled model unified for unbonded and bonded soils

A hyperelastic-plastic coupled constitutive model unified for bonded and unbonded soils is developed in this paper based on thermodynamics. An elastic potential function applicable for different kinds of soils is proposed to derive a hyperelastic model accounting for the pressure- and -density dependency, the stress-induced anisotropy and the bonding effects as well as their couplings with plasticity. From the perspective of elastic stability, state boundary and failure surfaces of different soils can be naturally predicted by the hyperelasticity without any additional definitions and parameters. Based on the classical nonequilibrium thermodynamics, novel plastic constitutive relations are derived and naturally coupled with the hyperelasticity. As a result, elasto-plastic coupling features such as the dissipative history effect on elastic stiffness, the cyclic shear behavior, the degradation of shear modulus under small strain conditions, the stress-induced anisotropy of plastic behavior and the cohesion degradation can be reproduced. The model is well validated by predicting the undrained/drained monotonic and cyclic shear behavior of unbonded and bonded sands, providing useful insights into their critical state behavior, irreversible shear-dilation/contraction and effects of bonding and cohesion degradation. It is also shown that the cohesion degradation in different shearing stages to a large extent determines both the monotonic and cyclic behavior of bonded soils.

Cyclic degradation and pore pressure dynamics of EICP treated hydrocarbon contaminated sands

Enzyme-induced carbonate precipitation (EICP) is an innovative bio-inspired approach that has emerged in recent years, offering a more environmentally sustainable option for soil improvement. The present article aims at extensively examining the effect of EICP treatment on the cyclic response of silty sands containing petroleum hydrocarbon compounds. Strain-controlled cyclic triaxial tests have been carried out on hydrocarbon contaminated sand with and without EICP treatment. Several independent variables such as hydrocarbon content (2, 4, 6, 8, and 10%), shear strain amplitudes (0.5% and 1%), and curing period (7 days and 14 days) were considered in the study. Prior to this, a baseline analysis was conducted to determine the effect of EICP treatment solution composition on the efficiency of carbonate precipitation. The effectiveness of the treatment was systematically investigated based on cyclic pore pressure response and degradation of cyclic shear modulus. The results revealed that petroleum contamination moderately increases the cyclic resistance up to a transition point beyond which it falls off sharply. The coupled effect of increasing oil content and curing time of EICP treatment has also been assessed. The variation of cyclic strength with crude oil content results in a single-peaked profile for both untreated and treated specimens. EICP treatment increases the cyclic resistance of oil-contaminated sand up to 1.5 times by suppressing the excess pore pressure build-up. A signification recovery in the degradation parameter, as high as 80%, was also observed. However, with increasing oil content and shear strain levels, the efficacy of EICP treatment exhibited a remarkable decline. The microstructural mechanism believed to be responsible for such a behavior has also been discussed.

Fatigue behavior of unbonded prestressed reactive powder concrete beams after creep

Abstract Failure modes of two kinds of unbonded prestressed reactive power concrete (UPRPC) beams were studied experimentally under static and fatigue loads, and compared. In the tests, deflections, crack widths, rebar strains, and beam strains were measured and analyzed. The fatigue test results show that the fatigue life of non-creeped beams is more than 2 million loading cycles and the creeped beam is damaged after 1.75 million loading cycles. The compression edge strain, mid-span deflection, and rebar strain are -814 μξ, 7.6 mm, and 484 μξ, respectively, for noncreeped UPRPC beams, and -1110 μξ, 11.2 mm, and 1169 μξ , respectively, for creeped UPRPC beams (μξ represents 1 × 10-6). These values are much smaller than the static load failure values. Considering the fatigue residual strain, fatigue elastic modulus degradation, and fatigue strength degradation, a simplified analysis method for a full fatigue process is proposed, which predicts values in good agreement with the measured results.

Corrosion effects on the granular nano mechanical properties of buried cast iron

Considerable research reported in literature shows that corrosive soil degrades the bulk mechanical properties of buried cast iron pipes due to cracks anddegradation of microstructure. The nanomechanical properties of the grains of cast iron pipes can also be affected by the corrosion before bulk mechanical properties. However, the influence of soil corrosivity on the nanomechanical properties at the granular level is remained unresolved to date. This paper addresses this gap and presents the nanomechanical measurements performed on the corroded cast iron specimens and the maximum degradation was quantified.Specimensafter 545 days of burial in corrosive soil conditions were tested by nanoindentation for determining the degradation of their nanomechanical properties. The analysis of the test results showed a significant reduction in the nano elastic and nano hardness of the grains of the cast iron due to the diffusion of hydrogen from the surrounding corrosive soil environments. The high corrosion rates were found proportional to inducing the maximum degradation of the nano elastic modulus of the grains. A relation for the prediction of maximum degradation of the nano elastic modulus of the grains as a function of corrosion rates, pH and moisture content of the clay soil was also developed. The finding of the current research is significant as it shows that apart from bulk mechanical properties, the nanomechanical properties of cast iron grains can be degraded by corrosive environments which in turn can affect the service life. Furthermore, understanding the mechanical science of the degradation of the grains can assist in manufacturing sustainable ferrous pipes resistive to corrosion soils.

Degradation of Mechanical Properties of Pine Wood Under Symmetric Axial Cyclic Loading Parallel to Grain.

The mechanical properties of wood, respectively the elastic, plastic, and strength properties, depend on a large number of factors, due both to its structural and physical characteristics, as well as to the size, direction, nature, and speed of application of forces. Wood, generally considered to be a viscous-elastic material, has creep deformations over time under the effect of a constant load. In this study the behavior of pine wood samples was investigated due to its large utilization in different finished products, such as roof construction, furniture, outdoor applications, garden furniture, and toys. The paper aims to analyze the viscoelastic behavior of pine wood subjected to cyclically loading to traction-compression with different loads (1 kN; 1.5 kN; 2 kN), applied at different speeds (1 mm/min; 10 mm/min). It was observed that, at low speeds (1 mm/min) and low intensities of the applied force, it was possible to distinguish the three creep regions specific to wood: the primary area (primary flow), the secondary area, and finally the tertiary creep. As the force increases, the law of variation of the wood flow changes. The degradation of longitudinal elasticity modulus occurs with the increase of the number of cycles, so after 20 alternating symmetrical cycles of traction-compression of the pine wood samples, there is a decrease of its values by 35%.

Nonlinear lateral response of offshore large-diameter monopile in sand

This paper presents a nonlinear analysis method for laterally loaded large-diameter monopiles. The method accounts for the effects of pile diameter and soil deformation on the modulus of horizontal subgrade reaction by considering a modified modulus of horizontal subgrade reaction. The solution considers the force and moment equilibrium of soil-pile system to calculate the response of the monopile to lateral loads. The influence of the soil side friction reaction on the ultimate horizontal resistance is evaluated considering the degradation of modulus of horizontal subgrade reaction due to large pile displacement. The predictions of the developed method compared favorably with the results obtained from full-scale field tests, centrifuge model tests and laboratory model tests as well as existing analytical methods. The developed analytical method is then applied to study the rigid rotation behaviour of offshore monopiles. The results demonstrated that under lateral loading at the pile head, the depth for rigid rotation point lies within 0.62L–0.73L (L is pile length), and the depth of the rotation point increases as the pile diameter increases. It was also found that the load eccentricity has a significant effect on the depth of the rotation point.