Full-field analysis of shear test on 3D orthogonal woven C/C composites

In this work, the digital image correlation (DIC) technique was used as full-field measurement to analyze the shear properties of the 3D orthogonal woven C/C composites. Both the in-plane and the through-the-thickness specimens were tested and the macroscopic average strain was obtained. The composites were composed of lots of periodic units and the macroscopic average strain was dependent on these meso-structures. There were three regions within one unit, which showed different mesoscopic strain. The relationship between the shear test region and the macroscopic average strain was systematically studied. Finally, the accuracy of conventional strain-gauge rosette measurement was also discussed.     

Recent Developments in Measuring Creep Strain in High Temperature Plant Components

Accurate measurements of creep strain are necessary to evaluate the condition and predict the remaining life of power plant constituent materials. Optical techniques are appropriate for this purpose as they are a non‐contact method and can therefore be used to measure strain without requiring direct access to the surface. Within this class of techniques, the Auto‐Reference Creep Management And Control (ARCMAC) camera system can be used to calculate the strain between two points using a series of silicon nitride (SiN) target spheres (the ARCMAC gauge). There are two iterations in system design, the Conventional ARCMAC and Digital Single‐Lens Reflex (DSLR) ARCMAC. Experiments are conducted to determine the absolute limit of accuracy of the systems in comparison to a strain gauge, and the relative accuracy across several orders of magnitude until specimen failure. In addition, tests have been performed using the ARCMAC gauge at elevated temperatures to evaluate the effect of temperature on the gauges and to investigate whether its accuracy diminishes in creep conditions. It was found that both conventional and DSLR ARCMAC systems can be accurate to 60 με or less. In accelerated creep tests, the ARCMAC gauge produced similar agreement to a linear variable displacement transducer when used to measure creep strain. Strain variations (under 500 με) were noted on a steel plate subjected only to operational temperature and no stress. This error is very reasonable compared to a critical strain value of 93 000 με in a given high temperature‐service material. Digital image correlation (DIC) results using the DSLR ARCMAC system show approximately 4% error in measurement for plastic strains in the specimen. The two measures of strain measurement (using ARCMAC and DIC) can serve to complement each other.     

Nonlinear Shear Behavior of Poly(vinyl acetate) Material

Because of the strong environmental sensitivity of poly(vinyl acetate), PVAc, especially with respect to moisture, and the fact that shear deformation is essentially equivoluminal up to moderate strain levels, little has been reported in the literature on the nonlinear mechanical creep behavior of this polymeric material loaded in shear. This paper presents the results of torsional tests which establish the shear response through the linear zone and well into the nonlinear region. Test specimens were thin-walled cylinders giving an approximately uniform deformation field. Because of carefully chosen wall thickness to length ratio, it is considered that these measurements represent some of the most accurate nonlinear shear results to date in the strain range above 1%. Measurements of stress, strain and creep compliance were made at temperatures near the glass transition temperature and somewhat below it. Isochronal shear stress-strain dependence into the nonlinear range was used to establish limits of viscoelastic linearity during creep. As temperature is increased toward the glass transition, the limit shows a greater dependence on stress than on strain. The stored distortional strain energy at the limit of linearity was not a constant but varied with temperature and load. Thus, these results appear not to support the concept of stored energy as a material property defining the threshold for nonlinear viscoelastic behavior. Strain during the short-time load-up period gives evidence that PVAc is also subject to nonlinear elasticity in the glassy response region.     

Constitutive laws for ice

A theoretical discussion of models which describe the transient and secondary creep response of polycrystalline ice is presented, including a hypothesis which incorporates temperature dependence in the rate laws by the introduction of a reduced time scale. The secondary response is described by a general non-linear incompressible viscous fluid law, and it is shown that bi-axial stress experiments are insufficient but combined shear and compression experiments are sufficient to determine, in principle, the general response functions. Transient creep can be described qualitatively by a viscoelastic fluid model, and the most simple material memory influence is given by dependence on the current creep acceleration which leads to a first order differential relation between stress and strain rate. The secondary creep response is incorporated as a steady asymptotic limit with the time scale of significant transient creep governed by the response coefficients. Quantitative tests of such transient response require data from experiments at short time intervals, and in particular the determination of an initial strain rate to complement the differential law.     

Assessing the Feasibility of Monitoring Strain in Historical Tapestries Using Digital Image Correlation

Abstract:  Digital image correlation (DIC) is used to monitor strain in a representative textile material and an historic tapestry. The validity of a 'map function' that allows 3D DIC displacement measurements to be obtained when the reference data are collected with a camera set-up different from that of the deformed data is assessed. An experiment was devised to study the effects of DIC processing parameters (interrogation cell size and overlap) on strain measurements, and to investigate if the textile contains adequate contrast for DIC to operate. The study shows that the textile's weave pattern can be used as the device for correlation. Long-term tests for monitoring creep strain using DIC both in the laboratory and in situ are presented. The results show good correspondence between strain changes in the tapestry and relative humidity.     

Effect of loading rate on the creep behaviour of epoxy resin insulators by nanoindentation

In the present work, the effect of loading rate on indentation creep was studied. Indentation creep tests were conducted on epoxy resin to provide creep deformation under constant load, contact creep compliance and cut-off time using a Berkovich indenter. Several loading rates, ranging from 0.25 to 6 mN/s, were used to perform the tests. The results showed that there is a strong loading rate dependence on creep response of the epoxy resin under indentation. Contact creep compliance and cut-off time decreased with increasing loading rate. In contrast, an increase in reduced modulus, hardness, displacement variation and contact creep compliance variation during the holding time was noticed. The loading rate sensitivity on creep response under indentation can be attributed to viscoelastic response prior to holding segment and strain rate effect on yield stress of the epoxy resin. This study provided an insight to understand the loading rate dependence on creep behaviour of epoxy resin under indentation.     

考虑一阶和二阶位移梯度的数字图像相关方法在剪切带测量中的比较

当剪切带中存在二阶位移梯度或应变梯度时,研究了两种数字图像相关(DIC)方法在测量位移、应变及应变梯度中的表现。以基于梯度塑性理论的剪切带的位移解作为基础,通过Matlab仿射变换制作了具有不同平均剪切应变和应变梯度的虚拟剪切带。通过对其计算和分析得到了下列结果：当剪切带的平均剪切应变和应变梯度较高时,与只考虑一阶位移梯度的DIC方法相比,考虑二阶位移梯度的DIC方法优势明显,获得的位移、应变及应变梯度结果与理论解比较吻合,由于DIC方法测量的是平均应变,因此,剪切带中心的峰值应变将被低估;当剪切带的平均剪切应变和应变梯度较低时,剪切带中心的峰值应变可能被高估,受标准偏差的影响,考虑二阶位移梯度的DIC方法没有优势。基于上述研究结果,在单向压缩应力控制加载条件下,对砂样从开始加载至宏观裂纹出现之前变形过程中的3种应变场的4种结果进行了分析。由于应变不超过0.25,因而考虑二阶位移梯度的DIC方法的结果并无优势。     

Consistent Identification of CFRP Viscoelastic Models from Creep to Dynamic Loadings

Rate‐dependent models require creep or mechanical tests at various strain rates in order to be identified and validated. Different geometries coexist for creep and static tests (normative geometry) and for dynamic tests. Therefore, due to geometrical sample considerations, experimental results could be inconsistent for identification or validation procedures, inducing, for example, differences on the shear modulus only due to the change of geometry. The objective of this work is to present an improved sample geometry that allows to obtain consistent mechanical tests results at various strain rates highlighting the rate dependencies of laminates. In particular, a complete mechanical validation of the sample geometry for dynamic tests is successfully performed in order to avoid inconsistency. Results of static and dynamic tests on the validated geometry are analysed, and the rate dependency of the elastic properties of the UD T700GC/M21 mesoscopic ply is highlighted on a wide strain rate range (10^?3 to 10² s^?1). Finally, the identification of a non‐linear viscoelastic model is performed on dynamic and creep tests results in order to obtain a representative model for dynamic, static and creep loadings, and to demonstrate the importance of introducing the improved geometry for the dynamic tests.     

基于PPP-BOTDA技术的PBL剪力键群应变测试研究

为研究混合结构PBL剪力键群各剪力键的剪力传递,在开孔板界面摩擦效应测试中引入了光纤传感测试技术。阐述了PPP-BOTDA (Pulse-PrePump Brillouin Optical Time Domain Analysis)技术的测量原理,根据多中间层的剪滞理论,建立了基体-光纤间的应变传递模型,推导了表贴式光纤的应变传递公式,基于4个四排PBL剪力键群试件的静载破坏试验,实现了对PBL剪力键开孔钢板及混凝土应变的空间分布式测量,并对分布式传感光纤的应变传递进行了敏感性分析。测试结果及分析表明：在仪器的空间分辨率一定时,传感光纤的粘贴宽度、光纤护套的厚度对应变传递影响显著,粘结层胶体的剪切模量对应变传递影响不甚明显;对比剪力键试验中应变片与分布式光纤的应变数据,试验采用的两种光纤均具有良好的应变测量精度,试验结果与理论敏感性分析较为一致;基于PPP-BOTDA的分布式光纤测试方法,克服了传统应变片测试中的不足,在既定的光纤铺设工艺下,适用于结构的大应变、高精度、分布式测量。     

Time and temperature dependent mechanical characterization of polymer‐based materials in electronic packaging applications

Abstract

The thermo‐mechanical testing of high performance polyimide films Type HPPST supplied by Dupont^® was conducted at different strain rates and in different temperature environments. The stress‐strain behavior of materials was investigated, and the dependence of Young's modulus on temperature and strain rate is reported. In view of the uncertainty of the Young's modulus determination, the specimens were tested with unloading‐reloading to verify the test results. Constant strain rate uniaxial tensile tests and long‐time creep tests at various temperatures were performed to characterize the time‐temperature‐dependent mechanical property precisely. Cyclic loading tests were also implemented on specimens to investigate cyclic stress‐strain behaviors. This research is expected to enhance finite‐element‐modeling accuracy and characterize material properties precisely.     

Uneven intensity change correction of speckle images using morphological Top-Hat transform in digital image correlation

By comparing two digital speckle images recorded before and after deformation, two-dimensional digital image correlation (DIC) method can accurately determine the in-plane displacement fields and strain fields. In a practical measurement, however, the variance of light source intensity, location and direction will cause the random uneven intensity change of the random speckle images and will lead to the obvious measurement error. Numerical simulation experiment is first carried out to analyse the influence of the recorded speckle images undergoing uneven light variation on DIC measurement accuracy. Then, a correction method for speckle images with uneven intensity change is proposed based on morphological Top-Hat transform. In addition, quantitative measurements of both in-plane rotation of a rigid body and three-point bending beam are investigated experimentally by DIC to verify the feasibility of the correction method. Experimental results show that the measurement accuracy of DIC is improved dramatically after the procedure of uneven light variation correction.     

Creep and creep-fatigue behaviour of continuous fibre reinforced glass-ceramic matrix Composites

The flexural creep and creep strain recovery behaviour during creep-fatigue tests of a cross-ply SiC fibre reinforced Barium Magnesium Aluminosilicate glass-ceramic matrix composite was investigated at 1100°C in air. Only heat-treated samples (1 h at 1100°C) were tested. Stress levels of 90, 105 and 120 MPa were examined to produce low strains (?0.4”?). A continuously decreasing creep strain rate with values between 1.6 × 10^?6 s^?1 to 4.7 × 10^?8s^?i at 120 MPa was observed with no steady-state regime. Extensive viscous strain recovery was found upon the unloading period during the short-duration cyclic creep (creep-fatigue) experiments. The creep strain recovery was quantified using strain recovery ratios. These ratios showed a slight dependence on the stress and cyclic loading frequencies investigated. The crept composites retained their ?graceful”? fracture behaviour after testing indicating that no (or limited) damage in the matrix was induced during creep and creep-fatigue loading.     

A method for measurement of multiple constitutive properties for composite materials

Accurate stress–strain constitutive properties are essential for understanding the complex deformation and failure mechanisms for materials with highly anisotropic mechanical properties. Among such materials, glass-fiber- and carbon-fiber-reinforced polymer–matrix composites play a critical role in advanced structural designs. The large number of different methods and specimen types currently required to generate three-dimensional allowables for structural design slows down the material characterization. Also, some of the material constitutive properties are never measured due to the prohibitive cost of the specimens needed. This work shows that simple short-beam shear (SBS) specimens are well-suited for measurement of multiple constitutive properties for composite materials and that can enable a major shift toward accurate material characterization. The material characterization is based on the digital image correlation (DIC) full-field deformation measurement. Two key elements show advantage of using DIC in the SBS tests. First, tensile, compressive, and shear stress–strain relations are measured in a single experiment. Second, a counter-intuitive feasibility of closed-form stress and modulus models, normally applicable to long beams, is demonstrated for short-beam specimens. The modulus and stress–strain data are presented for glass/epoxy and carbon/epoxy material systems.     

Digital Image Correlation and Noise‐filtering Approach for the Cracking Assessment of Massive Reinforced Concrete Structures

This paper describes the use of Digital Image Correlation (DIC) techniques for the cracking assessment of reinforced concrete (RC) massive beams and walls. DIC is known to provide accurate and detailed information on displacement and strain fields. Non‐contact measurements can be used to evaluate concrete cracking of destructive tests carried out on a wide range of specimen scales. When applied to large RC structures tested outdoors or in difficultly controllable conditions, DIC‐based methods may lead to erroneous results. In this study a post‐processing procedure is presented to cope with noisy full‐field measurements. The proposed cracking assessment approach is validated on a large experimental campaign. Four points bending tests are carried out on RC beams: firstly on full‐scale rectangular beams and then on mock‐ups scaled down by 1/3. In addition, fours RC walls are tested under in‐plane cyclic shear up to failure. Digital images taken throughout the tests are processed by DIC techniques to provide in‐plane displacement and strain fields. Full‐field measurements are post‐processed by the noise‐filtering technique and the cracks patterns are identified. Crack widths are measured and compared with measurements obtained from conventional point‐based sensors (linear variable differential transformer LVDT and fibre‐optic FO transducers). The proposed DIC‐based post‐processing provides accurate estimation of cracks width for most of the tests. The analyses carried out on the two groups of RC beams show a scale‐effect on the cracks width.     

Shear Creep Deformation of the Super Alloy Single Crystal CMSX‐4 at High Temperatures and Low Stresses

In the present paper we investigate the shear creep behavior of the single crystal super alloy CMSX‐4 at temperatures between 950 and 1100 °C and shear stresses ranging from 80 to 155 MPa. A double shear creep test technique is used to study the shear creep behavior of four specific macroscopic crystallographic shear systems defined by a specific crystallographic shear plane and a specific crystallographic shear direction (systems investigated: {001}<110>, {100}<010>, {011}<01‐1>, and {111}<01‐1>). The shear creep behavior is analyzed in terms of the shape of individual creep curves and in terms of the stress and the temperature dependence of the secondary shear creep rate. Individual creep curves are generally characterized by a pronounced primary creep range where creep rates decrease by up to three orders of magnitude. A sharp creep rate minimum is not observed. The secondary creep range starts at shear stresses of the order of 0.02 and is followed by a secondary creep range which extends over shear strain ranges of the order of 0.1. No pronounced increase of shear creep rate in the later stages of creep is observed. Norton plots yield power law stress exponents ranging from 5.5 to 9.7. The temperature dependence of the secondary creep rate is of an Arrhenius type and apparent activation energies between 549 and 690 kJ/mol were found. There is a clear influence of crystallography on shear creep rates, which vary between different macroscopic crystallographic shear systems; this effect decreases with increasing temperature. The shear creep results obtained in the present study are discussed in the light of results from uniaxial testing and in the light of underlying microscopic deformation processes.     

Physical aging behavior of high-performance composites

The effect of physical aging on the viscoelastic creep properties of a thermoplastic-toughened cyanate ester resin (Fiberite 954-2) and its IM8/954-2 composites, and a semi-crystalline thermoplastic (Fiberite ITX) and its IM8/ITX composites was investigated. The study was carried out by using dynamic mechanical analysis (DMA) and tensile creep tests. Tests were performed on plain resin, [90 °]_4s, and [± 45 °]_2s composite specimens. Creep tests were conducted up to an aging time of 54 h with the logarithmic aging shift rate, μ, and its dependence on sub-glass transition aging temperature, being determined. The results showed significant physical aging in both material systems. To study the effect of long-term aging on creep behavior, momentary creep tests were conducted on the [± 45 °]_2s composites of both material systems at temperatures between 140 and 200 °C. Master curve plots were drawn from these momentary creep tests using the time/temperature superposition principle (TTSP). Effective time theory (ETT) was then used to modify TTSP and incorporate physical aging effects.     

Viscoplastic–plastic modelling of IN792

At high temperatures metallic materials behave in a viscous manner exemplified by strain rate dependence, stress relaxation and creep deformation. At low temperatures however, these effects are extremely small, and the behaviour is strain rate independent and shows no or very small relaxation effects. Finally there exists an intermediate region, in which the material behaviour is close to strain rate independent for high strain rates but at the same time shows time dependent inelastic effects, such as stress relaxation and creep. For IN792 this occurs at temperatures around 650 °C. The article describes the extension of a power-law viscoplastic model describing the behaviour of IN792 at 850 °C, also to describe the behaviour at 650 °C, by bounding the elastic–viscoplastic stress-space by a plastic yield surface. The model parameters have been estimated using data from creep test and tailored step relaxation tests, and the model fits well to both the step relaxation data aimed at resembling relevant component conditions and long term creep data.     

Nonlinear Viscoelastic,Viscoplastic Characterization of Unidirectional GF/EP Composite

Viscoplastic strains of unidirectional continuous fiber composite (HEXCEL GF/EPprepreg system) are studied experimentally and theoretically. Creep and strainrecovery tests are used. Schapery's nonlinear viscoelastic viscoplasticconstitutive equations are used and generalized to describe inelastic behavior ofunidirectional composite under isothermal creep and strain recovery conditions. Themethodology to quantify the viscoplastic strains with respect to applied stress isproposed. Viscoplastic strains of composite are described by plastic shear strain inmaterial symmetry axis. Assumptions has been used and validated that the functiondescribing the stress and time dependence of viscoplastic strain can be presented asa product of two, time and correspondingly stress dependent, master curves.     

Creep Behavior in Interlaminar Shear of a SiC/SiC Ceramic Composite with a Self-healing Matrix

Creep behavior in interlaminar shear of a non-oxide ceramic composite with a multilayered matrix was investigated at 1,200 °C in laboratory air and in steam environment. The composite was produced via chemical vapor infiltration (CVI). The composite had an oxidation inhibited matrix, which consisted of alternating layers of silicon carbide and boron carbide and was reinforced with laminated Hi-Nicalon? fibers woven in a five-harness-satin weave. Fiber preforms had pyrolytic carbon fiber coating with boron carbide overlay applied. The interlaminar shear properties were measured. The creep behavior was examined for interlaminar shear stresses in the 16–22 MPa range. Primary and secondary creep regimes were observed in all tests conducted in air and in steam. In air and in steam, creep run-out defined as 100 h at creep stress was achieved at 16 MPa. Larger creep strains were accumulated in steam. However, creep strain rates and creep lifetimes were only moderately affected by the presence of steam. The retained properties of all specimens that achieved run-out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated.