Tensile Creep Tests Research Articles

Evaluation of Creep Deformation by Indentation Test with a Constant Depth - Applicability for Inhomogeneous Materials

In design of the electronic device, FEM analyses considering the creep deformation of solder joints in-situ should be conducted to evaluate the strength reliability. The indentation test is one of effective method to evaluate the creep deformation in microscopic region. However, the result obtained by the indentation test does not coincide with that obtained by the tensile creep test. To solve the problem, the method to determine the suitable area for the indentation test had also been proposed by using the numerical test. To apply the proposed method for the actual indentation test, the homogeneity of specimen should be considered. In this paper, the analyses of the proposed indentation tests were conducted by using the homogeneous and inhomogeneous specimen models of Sn-3.0Ag-0.5Cu. Three types of the deformation behavior were given for the initial phase and the indenter was pushed into in the specimen at the three locations. As a result, it was found that there was not difference in the distribution of the principal stress caused by variation in the indent location. However, the proposed method can successfully determine the reference area except for the case when the deformation behaviors of the constituent phases have a large difference.

English

This paper presents the effect of matrix type on creep behavior at 80% loads of ultimate tensile strength of two laminated composites at different fiber and matrix system. For this, two types of laminated composites were manufactured based on two types of fibers (carbon fabric and glass fabric) with different matrix systems EPOCAST 50-A1 (EP50-A), Epoxy STR (STR) and Epoxy INJ812 (INJ). The tensile and creep behavior of each laminated composite was studied in the same test conditions. A microstructural study was also investigated by SEM-microscopy on the morphology of composite specimens after creep rupture failures at 80% of applied load. The obtained results showed clearly the influence of matrix type used on the tensile and creep behavior of studied composites. Indeed, it was noted that no creep rupture failures were observed in short-term (less than 4 h) for INJ/Carbon composite at tensile creep tests at 80% loads  of ultimate tensile strength. At the same ultimate tensile strength, EP50/Carbon composite  showed a best creep behavior up to 30 h and had a creep modulus higher than other laminated composites. The observation of rupture facets of all composite samples showed clearly that the rupture will take place in the direction of loading, creating voids at the interface resin/fiber. These are observed based on the nature of the matrix used.   Key words: Epoxy, aliphatic amine, cross-linked, carbon fabric, glass fabric, creep test, cyclic tensile test.

Comparison of tensile and compressive creep of fly ash concretes in the hardening phase

Tensile and compressive creep tests of fly ash concretes in the hardening phase have been performed in the Temperature-Stress Testing Machine (TSTM) at NTNU. All tests were performed under nominally identical conditions, e.g. dimensions, curing conditions, measurement set-up and load-regime. Two concretes with 17% and 33% fly ash (as % by weight of cement and fly ash content) were investigated. Contradictorily to most of the compressive versus tensile creep comparisons found in the literature, both the investigated concretes showed similar creep behaviour in compression and tension throughout the hardening phase. It was also seen that for the investigated time-span, the specific creep development over time was found to increase (i.e. soften) with increasing amount of fly ash.

Viscoelastic braided stent: Finite element modelling and validation of crimping behaviour

Braided stents are fabricated at their relaxed dimensions and need to be crimped in order to fit into the catheter for deployment. Once the covering sheath is removed, the braided stent self-expands at the diameter of the diseased lumen, the radial force restoring the luminal patency. The mechanical properties of the viscoelastic monofilament used in the fabrication of the braided stents are analysed. The monofilament is subjected to both uniaxial tensile and extensional creep testing, with the purpose of predicting the material's time dependent characteristics. An accurate finite element model is required to enable correct estimation of the mechanical behaviour of braided stents subjected to crimping. In this report, Abaqus (v6.14) was used to develop a linear viscoelastic constitutive material model, where the creep compliance function and stress relaxation function were derived from experimental data. The material model was validated by performing simulation of radial compression for a braided stent and a comparison between the experimentally evaluated radial forces of the braided stent and the finite element prediction is presented. The regression analysis performed for the viscoelastic material model led to an R-squared value of 94.4%, showing a very good agreement between the predicted values and the experimental test data.

The characteristics of hot mixed asphalt modified by nanosilica

ABSTRACTThe objective of this research is to investigate the characteristics of the hot mixed asphalt modified by 2%, 4%, 6%, and 8% nanosilica by weight of binder. Marshall stability, dynamic creep, and indirect tensile tests were used to study effects of nanosilica on performance characteristics of the mixtures. Results showed that nanosilica can improve Marshall stability, resilient modulus, indirect tensile strength, and fatigue life compared to unmodified mixtures. However, the tensile strength ratio parameter in the moisture sensitivity tests indicated a decrease in the resistance of the modified mixtures against moisture damage.

Effect of texture on anisotropy at 600 °C in a near-α titanium alloy Ti60 plate

To study the anisotropy of mechanical properties at 600°C in a near-α titanium alloy Ti60, a highly T-type textured plate was prepared, and tensile tests, creep tests and creep rupture tests were carried out at 600°C. Notable anisotropies were observed in all three kinds of mechanical tests. The deformation in TD was much harder than in RD，no matter high or low was the loading stress. The anisotropy of tensile properties was concluded to be related to the different Schmid Factors distribution caused by T-type texture, leading to prismatic <a> slips easily being activated in RD and both prismatic <a> and basal <a> slips hard to be activated in TD. The restriction of creep deformation in TD samples was believed resulting from the effect of T-type texture on self- and solute diffusion. For the anisotropy of creep rupture properties, it was deduced to be a result of the combined anisotropy of diffusion and dislocation slipping.

Mechanical characterization of 3D printed polymers for fiber reinforced polymers processing

This paper presents an investigation in the thermo-mechanical creep properties of additive manufactured (AM) polymeric materials for the processing of AM parts with fiber reinforced polymers (FRP). Commercially available Acrylonitrile Butadiene Styrene ABSplus-P430 (ABS) for applications in Fused Deposition Modeling (FDM), Polyamide 12 (PA12) and DuraForm HST Composite (HST) made by Selective Laser Sintering (SLS) are investigated for in- and out-of-plane building orientations. The materials were subjected to tensile and three-point bending creep tests to assess tensile Young's modulus, tensile yield strength, and flexural creep modulus at a creep time of 180min and temperatures ranging from to 110°C. The material properties were then used in the identification of the optimal processing pressure and temperature for the autoclave curing of an AM honeycomb. The results indicated that the mechanical properties decreased as the temperature increased. Building orientations significantly impact the mechanical properties for ABS and HST. Processing recommendations of the AM honeycomb reflect the variation of the mechanical properties over temperature: the pressure decreases as the temperature increases. Numerical results correlate well with experiments conducted for an AM honeycomb made with HST and processed in an autoclave at 100°C and pressures of 2.26 and 4bar.

Investigation on Reinforced Mechanism of Fiber Reinforced Asphalt Concrete Based on Micromechanical Modeling

Short fibers have been widely used to prepare the fiber reinforced asphalt concrete (FRAC). However, internal interactions between fiber and other phases of asphalt concrete are unclear although experimental methods have been used to design the FRAC successfully. In this paper, numerical method was used to investigate the reinforced mechanism of FRAC from microperspective. 2D micromechanical model of FRAC was established based on Monte Carlo theory. Effects of fiber length and content on stress state of asphalt mortar, effective modulus, and viscoelastic deformation of asphalt concrete were investigated. Indirect tensile stiffness modulus (ITSM) test and uniaxial creep test were carried out to verify the numerical results. Results show that maximum stress of asphalt mortar is lower compared to the control concrete when the fiber length is longer than 12 mm. Fiber reduces the stress level of asphalt mortar significantly. Fiber length has no significant influence on the effective modulus of asphalt concrete. Fiber length and content both have notable impacts on the viscoelastic performance of FRAC. Fiber length should be given more attention in the future design of FRAC except the content.

Investigations on tensile creep of CNT-epoxy shape memory polymer nanocomposites

In this work, epoxy-based shape memory polymer nanocomposites (SMPnCs) were prepared using 0.5 wt% and 1 wt% multi walled carbon nanotubes (MWCNTs) pre-dispersed in epoxy resin. This was cured with triethylene tetraamine (TETA) curing agent. The glass transition temperatures (Tg) were determined using Advanced Rheometric Expansion System (ARES). The creep behaviour of the 0 wt% CNT SMPnC as well as the 0.5 wt% and 1 wt% MWCNT SMPnCs were studied through short term tensile creep test at different temperatures. Master curves of creep compliance were derived using a time-temperature superposition principle (TTSP) based on Williams-Landel-Ferry (WLF) equation. A Findley power-law model was used to predict the creep deformation behaviour of 0, 0.5 and 1 wt% CNT SMPnCs. Good correlations between experimental data and the predictive model were obtained for both SMPs particularly at lower temperature and above Tg.

直径1mm丸棒引張型ミニチュア試験片を用いたCrMoV鍛鋼のクリープ損傷評価

Creep damage assessment of high temperature components in aged thermal power plants is necessary to maintain reliable operation. Although an accurate creep damage assessment result is obtained by a destructive assessment method in which creep tests are performed, it is difficult to cut a standard size specimen from actual components. Therefore, development of testing and damage assessment methods by using a miniature specimen are expected. In this study, creep tests of a CrMoV forging steel were conducted by using a solid bar tensile type miniature creep specimen with 1mm diameter at 600℃. It was demonstrated that equivalent creep deformation and rupture properties to those obtained from the standard size specimen are obtained by the miniature specimen under stress level below 200MPa. Creep damage materials were produced by interrupting creep tests performed by using a standard sized smooth bar specimen and two types of round notch bar specimen with radiuses at notch root of 0.5mm(R0.5) and 2.0mm(R2.0). Uniaxial tensile creep tests were performed by using the miniature creep specimens taken from the plain specimens and the notch roots of the notch specimens. Although accurate creep damage assessment results were obtained from the creep tests with the miniature creep specimens taken from the plain specimens, it should be noted that the results may give under estimation for higher creep damaged condition. Creep damage assessment results based on the creep tests with miniature specimens also gave under estimation for the creep damaged material of R0.5 due to lower axial creep strain accumulation under multiaxial stress states before interrupting the creep tests. On the other hand, accurate creep damage assessment results were obtained for the creep damage materials of R2.0 indicating that creep tests with the miniature specimens may be applicable to creep damage assessment of components under certain range of stress multiaxiality.

A complete laboratory assessment of crumb rubber porous asphalt

Besides the many advantages of porous asphalt (PA), the permanent deformation resistance, fatigue strength, stripping and moisture susceptibility are not as good as for dense graded mixtures. In this research, the effectiveness of adding crumb rubber (CR) by dry method to PA mixtures was assessed. Rubberized porous asphalt (RPA) mixtures manufactured incorporating CR into SBS modified asphalt concrete were produced and compared with a control mixture by means of Indirect Tensile Strength (ITS), Indirect Tensile Stiffness Modulus (ITSM), moisture susceptibility, permeability, Cantabro, rolling bottle, draindown and creep tests. Results proved that although the application of CR reduces the vertical permeability and permanent deformation resistance, it improves the bitumen/aggregate affinity and it controls the draindown rate without adding fibres. On the other hand, the use of CR decreased the ITSM value at low temperature, which represents a lower susceptibility to thermal cracking. Other complementary test results in this research support the effectiveness of RPA mixtures.

Combination of Experiments and Continuum Damage Mechanics Approach for Prediction of Creep Fracture in an Advanced 9%Cr Steel

As candidate materials for high-temperature components, most attention has been paid to improving tempered martensitic creep-resistant 9-12%Cr steels. In this work, creep damage and fracture behaviour of an advanced W-modified P92 steel (ASTM Grade P92) was investigated at 600 and 650°C. Tensile creep tests were followed by fractographic analysis of crept and broken specimens. Besides experimental investigations, the creep damage tolerance parameter λ has been used to assess the creep fracture mode. In accordance with experiments the values of λ indicate variety in the fracture mode and provide some evidence on accelerated degradation of the creep strength. The SEM investigations of creep fracture surface revealed substantial changes in microfractographic features of creep fracture. At high applied stress level, the fracture was frequently transgranular due to local loss of a stability of plastic deformation. The fracture ductility drops with decreasing applied stress, demonstrating ductile dimple (transgranular) to brittle (intergranular cavitation) transition of the fracture mode. It was suggested that both the creep deformation and fracture processes are controlled by the same processes and the rate controlling mechanism is most probably climb of intergranular mobile dislocations.

Effects of Grain Refinement by ECAP on the Deformation Resistance of Al Interpreted in Terms of Boundary-Mediated Processes

Results of a large set of tensile and compressive creep tests on pure Al were reanalyzed for the influence of low- and high-angle grain boundaries on the deformation resistance at the temperature T = 473 K = 0.51Tm where Tm is the melting point. Thermomechanical treatment by equal channel angular pressing followed by heating to T led to strong increase of areal fraction of high-angle boundaries in a structure of subgrains of ≈10−6 m in size, accompanied by significant reduction of subgrain strengthening and of the stress sensitivity of the deformation rate. (Sub)grain strengthening by low-angle boundaries is most effective; the strengthening effect virtually disappears during creep as the boundary spacings coarsen toward their stress-dependent, quasi-stationary size wqs. The same type of coarsening is found for (sub)grain structures with large fraction of high-angle boundaries; in the quasi-stationary state they lead to softening at low and strengthening at high stresses, and a significant increase in tensile fracture strain to values up to 0.8. The results are analogous to former results for Cu and are explained in the same way by the influence of boundaries on storage and recovery of crystal defects and the homogenization of glide.

Creep fracture behaviour of SUS304H steel with vanadium addition based on small punch creep testing

The high-temperature creep fracture behaviour and creep strength of SUS 304H containing a minor addition of V were investigated in this study. A series of small punch (SP) creep and uniaxial creep tests were performed at 700 °C. The load and punch displacement rates obtained from the SP creep tests were used to derive conversion equations to determine the equivalent stress and creep strain rates. A converting equation is suggested in this study so that Norton’s secondary power law creep constants obtained from the SP creep testing can be in agreement with those obtained from the uniaxial tensile creep tests. The creep strength of the modified SUS 304H steel containing V was shown to be superior to that without V based on the current results and other available results for type 304H steel.

Viscoelastic response of an epoxy adhesive for construction since its early ages: Experiments and modelling

Abstract Epoxy adhesives have been extensively used in strengthening of reinforced concrete structures. In spite of that, research on the viscoelastic behaviour of these adhesives and its influence on the mechanical behaviour of strengthened concrete structures is scarce, particularly at the early ages of curing. The main goal of present work is to better understand the tensile creep behaviour of a specific commercial epoxy adhesive both at early ages and at long term. For this purpose, an experimental program comprising tensile creep tests was performed, in which epoxy specimens were subjected to: (i) equal stress levels but loaded at different ages, and; (ii) two different stress levels for a specific loading age. The results obtained have confirmed the viability of the principles of homogeneity and superposition, thus allowing the classification of this epoxy as linear viscoelastic. In addition, analytical simulations with Burgers model were carried out, and its predictive performance was considered insufficient at early ages, particularly upon unloading. In order to surpass some disadvantages of this model, a new framework is proposed to analytically evaluate the creep curing behaviour of this type of viscoelastic epoxy adhesives. The obtained results evidenced the capability of the proposed framework to predict the creep behaviour of an epoxy adhesive since its early ages.

Short-term creep and recovery behavior of medical grade ultra-high molecular weight polyethylene (UHMWPE)

Purpose: In this study, short-term tensile creep and recovery behaviors of medical gradeultra-high molecular weight polyethylene (UHMWPE) were investigated to contributedeformation behaviour of UHMWPE components in knee and hip prosthesis during daily lifeactivities of patients.Design/methodology/approach: Tensile test specimens were machined fromcompression molded UHMWPE sheets having commercial brand name: Chirulen 1020 andthey were prepared according to ASTM 527-2. The tensile creep tests were performed atconstant stress levels of 5, 9, 13, 18 and 21 MPa as long as 1 hour for each test. Then, thespecimens were allowed to recover unloaded for 1 hour. Automatic extensometer was usedto measure the deformations precisely for each test.Findings: Results show that creep rate linearly increased with increasing the stress levels.Permanent deformations were observed after recovery. Recovery of the material becamedifficult with increasing the applied load at intended time interval.Research limitations/implications: UHMWPE components used in prosthesis havebeen subjected to complex loading conditions during service life. Polymeric materials showthe viscoelastic material properties like strain rate sensitivity, relaxation, creep and recoveryat room temperature. Because of the viscoelastic material properties of the UHMWPE,it makes difficult to predict the failure of the UHMWPE components in hip and knee prosthesis.Therefore, deformation behavior of medical grade UHMWPE should be investigated in manydifferent loading conditions.Practical implications: Medical grade ultra-high molecular weight polyethylene(UHMWPE) have been used commonly in total hip replacements as acetabular cup and intotal knee replacements as tibial insert since early 1960s.

Effect of Creep Aging Process on Microstructures and Properties of the Retrogressed Al-Zn-Mg-Cu Alloy

The creep aging behaviors of retrogressed Al-Zn-Mg-Cu alloy were studied by uniaxial tensile creep tests at 140 °C. The effects of creep aging time and applied stress on microstructures and properties of the studied alloy were investigated by using transmission electron microscope (TEM), hardness, and corrosion resistance tests. Results show that the effects of the creep aging process on microstructures and properties are significant. The size of matrix precipitate (MPt), distance between MPts, width of precipitate-free zone (PFZ), and distance between grain boundary precipitates (GBPs) increase with the increase of creep aging time and applied stress. With the increase of creep aging time and applied stress, the corrosion resistance of the studied alloy improved. After creep aging for 20 h, the electrical conductivity varied with different applied stress from 35.99% to 37.24% International Annealed Copper Standard (IACS), and the exfoliation corrosion (EXCO) resistance increased to the corrosion rating of “EB”, which express slight surface corrosion. Compared with the traditional retrogression and re-aging process (RRA), the retrogression and creep aging process (RCA) can increase the MPt size, widen the precipitates distribution, narrow the PFZ width, and enhance the corrosion resistance while offering the hardness comparable to that of the RRA process.

Microstructural degradation and its corresponding mechanical property of wrought superalloy GH4037 caused by very high temperature

Overheating exposures of turbine blades during service increased the risk of service safety in aircraft engines. Nevertheless, limited investigations about microstructural degradation induced by the overheating temperature and the effects on mechanical property of wrought superalloys were reported. In this paper, wrought superalloy GH4037 sectioned from an un-used 1st stage turbine blade of an aircraft engine was adopted to investigate the microstructural degradation caused by short-time thermal exposure at very high temperature (higher than normal service temperature) and the microstructural evolution during the mechanical tests after the short-time thermal exposures. The effects of microstructural degradation on mechanical properties including hardness, high temperature tensile properties and creep properties were also analyzed. The results indicate that gradual dissolutions of γ′ phase and grain boundary (GB) carbides were the typical manifestation of microstructural degradation in GH4037 alloy during the short-time thermal exposures at 1000–1140 °C for 3 min and 5 min. The dissolved γ′ phase and GB carbides re-precipitated in a very short time (less than 12 min) as ultra-fine particles and continuous/cellular GB carbides during 850 °C tensile tests and creep tests at 850 °C/196 MPa. Due to the re-precipitated ultra-fine γ′ phase, the tensile strength at 850 °C after exposing to 1140 °C for 3 min was analogous to those of specimens without the thermal exposure. But the re-precipitated continuous/cellular GB carbides would cause the significant decrease of tensile ductility at 850 °C and the creep elongation at 850 °C/196 MPa. On the other hand, the dissolution of γ′ phase and GB carbides during the short-time thermal exposures reduced the creep properties at 700°C/471 MPa significantly, as it took much longer time (more than 18.1 h) for the re-precipitation of γ′ phase when the overheated alloys were exposed at 700°C/471 MPa. The work provides the guidance for overheating-inspection and diagnosis of service safety for blades made of wrought superalloys.

Intermittent plasticity associated with the spatio-temporal dynamics of deformation bands during creep tests in an AlMg polycrystal

Many structural Al-based alloys, and AlMg alloys, in particular, exhibit an intermittent plasticity on the macroscopic scale also known as the Portevin-Le Chatelier (PLC) effect which is associated with repetitive stress drops related with the formation of the bands of localized plastic deformation in tests with a constant imposed strain rate. In the current work we present a comprehensive investigation of a discontinuous creep in commercial AlMg6 alloy used in the aerospace industry. This phenomenon manifests itself as strain bursts during creep tensile tests, where the creep curves contain macroscopic strain jumps in the magnitudes of several percent. The experimental investigation employs the measurements of the load and strain sensors synchronized with a high-speed video camera to capture the deformation band formation and propagation. Unexpectedly, an individual strain burst is found to generate a complex jerky load response which consists of a number of stress drops (similar to the PLC effect) due to inertia of the specimen – creep machine mechanical system. Each stress drop is associated with a fast evolution stage of a single deformation band which takes the shape of a widening neck in a flat specimen. During the widening of the band its “center of gravity” remains stationary. We have established that the main mechanism responsible for the development of the strain burst is the multiplication of the deformation bands, when the boundary of each band generates a secondary band and so on. As a result of the relay-race transfer of the deformation from band to band, a macrolocalized plastic deformation propagates along the specimen. Complementary statistical, spectral, fractal and dynamical analyses are performed on the stress-time series recorded during a single strain burst evolution. It is shown that when the initial specimen length increases the intermittent load response exhibits a tendency to the state of self-organized criticality. Furthermore, transitions between distinct dynamical PLC-like regimes of type A and B during strain burst are explored. We found that the time-periodic stress oscillations in form of a limit cycle can be spontaneously generated in short-time range during dynamical regime of type A. Nonlinear aspects of crystal plasticity conformably to discontinuous creep are discussed.

Performance of warm asphalt mixtures using Sasobit

ABSTRACTThe performance of limestone warm mix asphalt mixtures using Sasobit over control mixtures was investigated. The Superpave mixture design method was used to prepare control and Sasobit-modified mixtures. The indirect tensile and dynamic creep tests were used for fatigue and rutting evaluation, respectively. The main findings of this research were the noticed lower accumulated strains and resilient modulus for Sasobit-modified mixtures over control mixtures in rutting performance testing. On the other hand, higher creep stiffness values in fatigue testing for Sasobit-modified mixtures were observed over control mixtures for deviator loads that ranges from 1 to 2 KN. Moreover, better fatigue resistance at lower temperatures was also observed for both control and Sasobit-modified mixtures.