Combined Mechanical Loading Research Articles

A mesomechanical model for predicting the degradation in stiffness of FRP composites subjected to combined thermal and mechanical loading

The mechanical properties of Fiber Reinforced Polymer (FRP) composites decrease with increasing thermal exposure temperature and time. A mesomechanical model was presented to predict the degraded behavior of FRP composites supporting a static compressive loading under high temperatures. The thermal softening, thermal decomposition of the matrix material and phase transition of the reinforced fibers were considered in the developed model, which adversely affect the stiffness properties of the composite material. Also, in order to evaluate the effect of high internal pressure on stiffness property, the bulk modulus was applied in the formulation of the mathematical model. High temperature compression experiments were conducted to measure the temperature-dependent elastic modulus. The accuracy of the model was further assessed by comparing simulated and experimental modulus. The reduction in stiffness properties of FRP composites at high temperatures can be roughly divided into three stages by analyzing the predicted temperature-modulus curve.

Stress-based design of thermal structures via topology optimization

The design of thermal structures in the aerospace industry, including exhaust structures on embedded engine aircraft and hypersonic thermal protection systems, poses a number of complex design challenges. These challenges are particularly well addressed by the material layout capabilities of structural topology optimization; however, no topology optimization methods are readily available with the necessary thermoelastic considerations for these problems. This is due in large part to the emphasis on cases of maximum stiffness design for structures subjected to externally applied mechanical loads in the majority of topology optimization applications. In addition, while limited work in the literature has investigated thermoelastic topology optimization, a direct treatment of thermal stresses is not well documented. Such a treatment is critical in the design of thermal structures where excessive thermal stresses are a primary failure mode. In this paper, we present a method for the topology optimization of structures with combined mechanical and thermoelastic (temperature) loads that are subject to stress constraints. We present the necessary steps needed to address both the design-dependent thermal loads and accommodate the challenges of stress-based design criteria. A relaxation technique is utilized to remove the singularity phenomenon in stresses and the large number of stress constraints is handled using a scaled aggregation technique that has been shown previously to satisfy prescribed stress limits in mechanical problems. Finally, the stress-based thermoelastic formulation is applied to two numerical example problems to demonstrate its effectiveness.

Nonlinear Response of Piezoelectric Nanocomposite Plates: Large Deflection, Post-Buckling and Large Amplitude Vibration

This paper deals with nonlinear response of smart two-phase nanocomposite plates with surface-bonded piezoelectric layers under a combined mechanical, thermal and electrical loading. The governing equations of the carbon nanotube reinforced composite plate are derived based on first order shear deformation plate theory (FSDT) and von Kármán geometric nonlinearity. The material properties of the nanocomposite host are assumed to be graded in the thickness direction. The single-walled carbon nanotubes (SWCNTs) are assumed aligned, straight and a uniform layout. The Galerkin method is employed to derive the nonlinear governing equations of the problem. A perturbation scheme is employed to determine the nonlinear vibration response and the nonlinear natural frequencies of the plates with immovable simply supported boundary conditions. Post-buckling load–deflection and maximum transverse load–deflection relations have been obtained for the plate under consideration. The effects of the applied voltage, temperature change, plate geometry, and the volume fraction and distribution pattern of the SWCNTs on the linear and nonlinear natural frequencies of the smart two-phase composite plates are investigated through a detailed parametric study.

U.S. Naval Aviation: operational airframe experience with combined environmental and mechanical loading

Abstract Airframe structure is the core capability for all aviation operations, whether fixed or rotary wing, manned or unmanned, or ship-based or shore-based. Airframe materials are the underlying enabling technology for all air vehicle structures. Airframe maintainability is the primary availability and readiness driver for U.S. Naval Aviation. Traditionally, airframe structures are designed for immediate mechanical performance and loads-only structural response, and the degradation of properties over the life cycle and sustainment during operations are often an afterthought. Galvanic management and corrosion-resistant materials selection have never been done systematically as part of the structural analysis and lifing process. Additionally, the lack of true failure mechanism understanding of load path effects, crack initiation and growth behaviors, and nonuniform material response has often resulted in underdesign/overdesign, limits on new material insertion, costly life extension programs, and unexpected early in-service failures. Advances in structural and materials science for airframes offer significant opportunity for improvements in availability, readiness, reduced sustainment requirements, fatigue life enhancement, reduced weight and improved range, and enhanced design tools and standard practices. These science and technology advances would be realized in large part through the engineering and operational communities by facilitating mission profile-specific life prediction and operational requirement-tailored functionality, increasing operational readiness, reducing life-cycle costs, reducing logistics footprint, and lowering the maintenance burden on uniformed personnel. Durability can therefore be incorporated into the design and construction phase, where the largest return can be realized.

Comparison of R6 and A16 J estimation methods under combined mechanical and thermal loads with FE results

This paper compares elastic–plastic values of J calculated using the methods in the UK R6 and the French A16 fitness-for-service procedures with FE results for a vessel with a circumferential surface crack under axial tension and a radial thermal gradient. In the FE analyses, the relative magnitudes and loading sequence of mechanical and thermal loads are systematically varied, together with the material strain hardening exponent. Fully circumferential and semi-elliptical surface crack with two relative crack depths are considered. It is found that the R6 estimates are overall accurate but can be non-conservative at large Lr. The A16 estimates are more conservative than the R6 estimates at small Lr but are conservative even at large Lr. Possible sources of conservatism and non-conservatism in R6 and A16 are discussed.

Influence of cementation on in vitro performance, marginal adaptation and fracture resistance of CAD/CAM-fabricated ZLS molar crowns

ObjectivesThis study investigated the influence of conventional cementation, self-adhesive cementation, and adhesive bonding on the in vitro performance, fracture resistance, and marginal adaptation of zirconia-reinforced lithium silicate (ZLS) crowns. MethodsHuman molar teeth (n=40) were prepared and full-contour crowns of a ZLS ceramic (Celtra Duo, DeguDent, G, n=32) and a lithium disilicate ceramic (LDS; IPS e.max CAD, Ivoclar-Vivadent, FL, n=8) were fabricated and glazed. Four groups of ZLS crowns were defined (n=8/group) and cemented with different glass-ionomer cements, resin, and resin-modified self-adhesive luting materials. The LDS crowns served as reference group with adhesive bonding.A combined thermal cycling and mechanical loading (TCML: 3000×5°C/3000×55°C; 1.2×106 cycles à 50N) with human antagonists was performed in a chewing simulator. Fracture force of surviving crowns was determined. Marginal adaptation at the cement/tooth and cement/crown interface was investigated by scanning electron microscopy before and after TCML, and the share of perfect margins was determined. Data were statistically analyzed (one-way ANOVA; post hoc Bonferroni, α=0.05). ResultsOne crown of the adhesive group failed during TCML (879,000 cycles=3.7 years). No statistically significant (p=0.078) differences in fracture resistance were found between different cementations, although highest data in tendency were found for adhesive bonding. Shares of perfect margins at the cement/tooth (93.8±5.6–99.6±0.8%) and cement/crown (84.7±6.6–100.0±0.0%) interfaces did not differ significantly (p>0.05) between the different cementation groups. SignificanceMarginal adaptation and fracture forces of all tested groups are in a range, where no restrictions should be expected for clinical application.

Mechanical properties and real-time damage evaluations of environmental barrier coated SiC/SiC CMCs subjected to tensile loading under thermal gradients

SiC/SiC ceramic matrix composites (CMCs) require new state-of-the art environmental barrier coatings (EBCs) to withstand increased temperature requirements and high velocity combustion corrosive combustion gasses. The present work compares the response of coated and uncoated SiC/SiC CMC substrates subjected to simulated engine environments followed by high temperature mechanical testing to asses retained properties and damage mechanisms. Our focus is to explore the capabilities of electrical resistance (ER) measurements as an NDE technique for testing of retained properties under combined high heat-flux and mechanical loading conditions. Furthermore, Acoustic Emission (AE) measurements and Digital Image Correlation (DIC) were performed to determine material damage onset and accumulation.

Microscale insight into the influence of humidity on the mechanical behavior of mudstones

AbstractThe mechanical behavior of mudstones strongly depends on humidity. In this paper, we present some microstructural insights into this phenomenon gained from a microscale investigation using a novel experimental method. The experimental method consists of combined hydric and mechanical loading tests in environmental scanning electron microscopy, as well as full‐field strain measurement by digital image correlation techniques. The sample is subjected to a stepwise wetting (21%, 80%, and 99% relative humidity); for each equilibrium moisture state, a uniaxial compression test is performed. The microscale observation reveals that humidity‐induced changes in the mechanical behavior of mudstones are controlled by the deformation and microcracking upon wetting. With increasing relative humidity, expansion of pores causes the clay matrix to be softer. In addition, because of the reduction in shear modulus and the lessening of capillary effect, shear bands are prone to appear at a high humidity state. The microcracking upon wetting, which results in predamage of the material, also affects the mechanical behavior. Finally, the sample with more moisture exhibits a more ductile behavior that involves more pronounced microcracking at failure.

Carbon nanotube reinforced hybrid composites: Computational modeling of environmental fatigue and usability for wind blades

The potential of advanced carbon/glass hybrid reinforced composites with secondary carbon nanotube reinforcement for wind energy applications is investigated here with the use of computational experiments. Fatigue behavior of hybrid as well as glass and carbon fiber reinforced composites with and without secondary CNT reinforcement is simulated using multiscale 3D unit cells. The materials behavior under both mechanical cyclic loading and combined mechanical and environmental loading (with phase properties degraded due to the moisture effects) is studied. The multiscale unit cells are generated automatically using the Python based code. 3D computational studies of environment and fatigue analyses of multiscale composites with secondary nano-scale reinforcement in different material phases and different CNTs arrangements are carried out systematically in this paper. It was demonstrated that composites with the secondary CNT reinforcements (especially, aligned tubes) present superior fatigue performances than those without reinforcements, also under combined environmental and cyclic mechanical loading. This effect is stronger for carbon composites, than for hybrid and glass composites.

The dynamic performance of magnetic-sensitive elastomers under impact loading

This work presents the impact response of magnetic-sensitive elastomer (MSE) under combined mechanical and magnetic loads. Two types of MSE specimens with iron particles (spheres) embedded uniformly or in chain-like arrangement in a rubber matrix are prepared by our group. Their dynamic behaviors are studied by a drop hammer-based experimental set-up with magnetic field generator. A numerical simulation method based on magneto–structural coupling is also provided. Both the experimental measurement and the numerical simulation demonstrate that the peak acceleration increases, and oscillation period and the total mitigation time decreases with increasing magnetic field intensity, for any type of MSE. Also, the chain-like structure enhances the trend above further. The failure mode for both types is matrix crushing, which occur at the positions between adjacent spheres within a chain where the impact and magnetic forces are superimposed.

Hold-Time Effect on Thermo-Mechanical Fatigue Life and its Implications in Durability Analysis of Components and Systems

AbstractThermo-mechanical fatigue (TMF) resistance of engineering materials is extremely important for the durability and reliability of components and systems subjected to combined thermal and mechanical loadings. However, TMF testing, modeling, simulation, validation, and the subsequent implementation of the findings into product design are challenging tasks because of the difficulties not only in testing but also in results interpretation and in the identification of the deformation and failure mechanisms. Under combined high-temperature and severe mechanical loading conditions, creep and oxidation mechanisms are activated and time-dependent failure mechanisms are superimposed to cycle-dependent fatigue, making the life assessment very complex. In this paper, the testing procedures and results for high-temperature fatigue testing using flat specimens and thermal-fatigue testing using V-shape specimens are reported; emphasis is given to hold-time effects and the possible underlying mechanisms. The uncertainty nature and the probabilistic characteristics of the V-shape specimen test data are also presented. Finally, the impact of hold-time effect on current product design and validation procedure is discussed in terms of virtual life assessment.

Analysis of Mode II debonding behavior of fiber-reinforced polymer-to-substrate bonded joints subjected to combined thermal and mechanical loading

This paper presents a set of closed-form solutions for Mode II debonding process of fiber-reinforced polymer-to-substrate bonded joints subjected to combined thermal and mechanical loading. Six different bond–slip models are considered in deriving the closed-form solutions. For each bond–slip model, explicit expressions for the debonding load, effective bond length, interfacial shear stress, interfacial slip as well as the load–displacement response are presented. Provided the bond length is sufficiently long, the debonding load depends only on the interfacial fracture energy and the temperature variation. A temperature increase leads to an increase in both the debonding load and the effective bond length, and the rate of increase of the latter depends on the bond–slip model of the interface.

Influence of thermomechanical interaction effects on the failure behaviour of polymer foam cored sandwich panels

The paper presents an experimentally based and numerically supported investigation of the collapse behaviour of polymer foam cored sandwich beams subjected to combined mechanical and thermal loading. Recent analytical and numerical modelling results available in the literature have ascertained that collapse may be due to loss of stability induced by nonlinear interactions between mechanical loads and thermally induced deformations, when accounting for the reduction of the polymer foam core mechanical properties with increasing temperature. In the paper, experiments are devised whereby a thermal gradient is introduced into a sandwich beam specimen loaded in three-point bending. The experiments cover a range of temperatures where one face sheet is heated from room temperature (25℃) to just below the glass transition temperature of the polymer foam core (70℃) and the other face sheet remains at room temperature. Digital image correlation (DIC) is used to obtain the local displacement field of the sandwich beam and its temperature is monitored using an infrared detector and thermocouples. The experimental results are compared with the predictions of both a generally nonlinear finite element model and an analytical so-called high-order sandwich panel theory (HSAPT) model. It is important to note that the HSAPT model has clear limitations as it takes into account only the geometric nonlinearity and thermal degradation of the foam core elastic properties, and it further assumes that the sandwich constituents are linear elastic with infinite straining capability. The HSAPT model predicts the occurrence of a strongly nonlinear load response leading to loss of stability (limit point behaviour). However, the experiments show that for the investigated sandwich beam configuration, it is necessary to include the nonlinear material properties in the modelling, as the nonlinear beam response leading to failure and collapse is significantly influenced by plastic deformations in the constituent materials. Thus, core indentation and extensive plasticity precede the transition to loss of stability driven by geometric nonlinearity and thermomechanical interaction effects. Finite element analyses that include both geometric and material nonlinearities provide results that correlate closely with the experimental observations. The work presented lays the foundation of a methodology for validating complex thermomechanical behaviour in sandwich structures using non-contact full-field measurement systems, and it demonstrates that analytical or numerical models based on the assumption of linear elastic material behaviour (such as the HSAPT model referenced in the paper) generally cannot adequately describe the thermomechanical behaviour of foam-cored sandwich structures.

Shakedown and Creep Rupture Assessment of a Header Branch Pipe Using the Linear Matching Method

Many power plant components are subject to combined mechanical and thermal loading conditions during their operating lifetime. It is important that potential failure mechanisms of such components are extensively investigated in order to ensure sufficient confidence in their reliability. This paper presents shakedown and creep rupture analyses of a header branch pipe subjected to cyclic thermo-mechanical loading performed using the Linear Matching Method (LMM). The detailed investigation of failure mechanisms under the combined action of the internal pressure and the cyclic thermal load due to the temperature difference between the inner and outer pipe surfaces will be the primary focus of this paper. The header branch pipe considered here is composed of a single material with properties that are dependent upon both temperature and rupture life. A novel study investigating the effect that two geometric parameters – branch diameter and separation – have upon the failure mechanisms of the header branch pipe has also been carried out. The impact that these geometric parameters have upon the limit load, shakedown and creep rupture limits is one of the principal areas that is investigated in this work. In addition to this, an understanding of the dependency of the creep rupture limit upon the defined time to creep rupture is also studied. Verification of these results is then given by full elastic-plastic analyses performed within ABAQUS.

On Strength Reserve Assessment for Prismatic Folded Plate Roof Structures

The paper presents the results of experimental studies and numerical simulation of straining and failure of reinforced concrete folded-plate roof structures in limit and out-of-limit states performed on models and real structures, taking into account the combined mechanical loads and environmental actions. The results of the study show that in the process of reconstruction design for concrete prismatic roof structures of operated industrial and public buildings, along with the traditional limit state methods of calculation it is reasonable to carry out a residual strength reserve analysis.

Powder compaction. Experiments, modeling and simulation

AbstractField‐assisted sintering technology (FAST) is a combined thermal and mechanical loading process to compact and sinter a powder material within one process step. In this short essay a constitutive model of thermo‐viscoplasticity is proposed representing most of the phenomena observed in the experiments. The constitutive model is calibrated to the experimental data and some predicted experiments are compared with constitutive model showing appropriate results. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermo-mechanical buckling analysis of functionally graded plates with an elliptic cutout

In this paper, buckling of functionally graded plates (FG plates) with an elliptical cutout under combined thermal and mechanical loads is investigated using Finite Element Method. Unlike other studies in which the plates are exposed to thermal or mechanical loads, in this study it is assumed that the mechanical and thermal loads are applied simultaneously. The material properties are assumed to vary across plate thickness according to power law distribution of the volume fraction of constituents. The plate formulation is based on the First order Shear Deformation Theory (FSDT) and element stiffness matrices are derived based on the principle of minimum potential energy. A flexible mesh generation algorithm is prepared in which the mesh density around the hole can be controlled easily. After validating the results of developed FE code with those available in the literature, the effect of boundary condition, plate aspect ratio and cutout radius ratio on thermo-mechanical buckling behavior of FG plates is studied and stability diagrams are presented. Finally useful conclusions are presented.

Composition dependence of field induced phase transformations in [0 1 1]C PIN–PMN–PT relaxor ferroelectric single crystals with d322 piezoelectric mode

Relaxor ferroelectric single crystals are used in sonar, medical ultrasound, accelerometers and energy-harvesting applications due to their large electromechanical coupling. In this work, the compositional dependence of the electromechanical properties of [011]C cut relaxor ferroelectric lead indium niobate–lead magnesium niobate–lead titanate single crystals, xPb(In1/2Nb1/2)O3–(1-x-y)Pb(Mg1/3Nb2/3)O3–yPbTiO3 (PIN–PMN–PT) was determined. The effect of increasing the concentration of lead indium niobate (xPIN) and increasing the concentration of lead titanate (yPT) on compositions near the morphotropic phase boundary was explored. Specimens were subjected to combined electrical, mechanical and thermal loading. The electric field induced changes of polarization and strain were measured at a series of preload stresses and the stress induced polarization and strain changes were measured at a series of bias electric fields. This was repeated at several temperatures. Evidence of a phase transformation from ferroelectric rhombohedral to ferroelectric orthorhombic was observed in the form of jumps in strain and electric displacement at critical field levels. The linear piezoelectric constant, dielectric permittivity, elastic compliance, thermal expansion, pyroelectric coefficient and rate of change of the material properties with temperature were determined including a scalar measure of electromechanical work indicating the start of the field induced phase transformation.

Effects of composition and temperature on the large field behavior of [011]C relaxor ferroelectric single crystals

The large field behavior of [011]C cut relaxor ferroelectric lead indium niobate–lead magnesium niobate–lead titanate, xPb(In1/2Nb1/2)O3-(1-x-y)Pb(Mg1/3Nb2/3)O3-yPbTiO3, single crystals was experimentally characterized in the piezoelectric d322-mode configuration under combined mechanical, electrical, and thermal loading. Increasing the concentration of lead indium niobate and decreasing the concentration of lead titanate in compositions near the morphotropic phase boundary resulted in a decrease of mechanical compliance, dielectric permittivity, and piezoelectric coefficients as well as a shift from a continuous to a discontinuous transformation.

Homoclinic complexity in the localised buckling of an extensible conducting rod in a uniform magnetic field

We study the localised buckling of an extensible conducting rod subjected to end loads and placed in a uniform magnetic field. The trivial straight but twisted rod is described by a fixed point of a four-dimensional Hamiltonian system of equations previously shown to be chaotic. Localised solutions are given by homoclinic orbits to this fixed point and we explore the spatial complexity of localised rod configurations by means of shooting and parameter continuation methods that exploit the reversibility of the system of equations. Unlike in localisation studies of non-magnetic rods we find that for certain parameter values multiple Hamiltonian–Hopf bifurcations occur. Where these collide as parameters are varied, solutions exhibit delocalisation–relocalisation behaviour. Our results predict buckling instabilities and post-buckling behaviour of rods under combined mechanical and magnetic loads, which are relevant for electrodynamic space tethers and potentially for conducting nanowires in future electromechanical devices.