Predicting the Fatigue Life of Adhesively-Bonded Joints

The fatigue behaviour of adhesively-bonded joints, which consisted of an epoxy-film adhesive bonding fibre-composite substrates, has been studied. Using a double-cantilever beam specimen, the rate of crack growth per cycle has been measured as a function of the maximum strain-energy release rate, G_max. These data have then been modelled, and used to predict the fatigue lifetime of bonded single-lap joints. The agreement between the theoretical predictions and experimental results for the fatigue behaviour of the single-lap joints was found to be excellent.     

Creep effects in nanoindentation of hydrated phases of cement pastes

This paper focuses on the nanoindentation of cement pastes and their micromechanical response. Since cement paste is a heterogeneous material at microscale, micromechanical properties have to be assessed separately for its individual material phases, i.e. hydrated products and unhydrated phases. Our study addresses important issues concerning experimental loading paths and their effects on the evaluation of elastic properties using nanoindentation. The effect of creep is shown on a series of multicycle experiments. A wide range of maximum loads and corresponding final penetration depths (approximately 200–1200 nm) are covered in this study. Experimental measurements on both hydrated cement samples and on unhydrated clinker minerals have proved that significant creep deformation can be attributed purely to hydrated phases. Further, it has been found that cement paste exhibits a strong size effect on elastic properties measured by nanoindentation and evaluated by the standard Oliver–Pharr procedure for loading–unloading tests with no dwell period at the peak load. Such property was already reported for plastic materials like metals [Elmustafa AA., Stone DS., Indentation size effect in polycrystalline F.C.C. metals, Acta Materialia 2002; 50 (14): 3641–3650., Wei, Y., Wang, X., Zhao, M., Size effect measurement and characterization in nanoindentation test, Journal of Material Research 2004; 19 (1): 208–217.], due to various effects but creep. As shown in the paper, cyclic loading and using long dwell periods at peak load leads to minimizing of this spurious type of size effect. Moreover, comparison with results obtained for the COC/PE polymer (75% cycloolefin copolymer + 25% polyethylene) has been performed. This polymer has been found to exhibit similar time-dependent behavior as hydrated cement paste.     

Upscaling of viscoelastic properties of highly-filled composites: Investigation of matrix–inclusion-type morphologies with power-law viscoelastic material response

In this paper, homogenization schemes for upscaling of elastic properties in the framework of continuum micromechanics are extended towards upscaling of viscoelastic material properties. Hereby, the Laplace–Carson transform method is applied to the Mori–Tanaka scheme, the self-consistent scheme, and the generalized self-consistent scheme and solved numerically by the Gaver–Stehfest algorithm. The performance of the so-obtained upscaling schemes is: (i) illustrated for an academic example (a 2-phase composite with Maxwellian-type creep response of the phases) and (ii) assessed considering a polyester matrix/marble dust filler composite with respective experimental data taken from the literature. Hereby, for the investigated range of volume fractions of inclusions, ranging from 29 vol.% to 55 vol.%, and a matrix/inclusion-type morphology, the transformed generalized self-consistent scheme emerged as the most suitable scheme for determination of the effective viscoelastic properties of this highly-filled composite material, resulting in a sound representation of the experimental data.     

物理老化对环氧树脂蠕变性能的影响

本文采用动态机械分析仪ＤＭＡ９８３测得环氧浇铸体在不同物理老化处理条件下的蠕变曲线，计算出链段活化能，并利用时温等效原理、时温叠加原理和Ｄａｋｉｎ耐热长寿命预测原理推算出该材料在使用温度４０℃时模量随时间的变化曲线，从而预测其寿命。     

Influence of anisotropy on a limit load of weld strength overmatched middle cracked tension specimens

ABSTRACT A plane‐strain upper bound limit load solution for weld strength overmatched middle cracked tension specimens (M(T) specimens), is found. It is assumed that the weld material is isotropic, but the base material is orthotropic and its axes of orthotropy are straight and parallel to the axes of symmetry of the specimen. A quadratic orthotropic yield criterion is adopted. The solution is based on a simple discontinuous kinematically admissible velocity field and is an extension of the corresponding solution for the specimen made of isotropic materials. These two solutions are compared to demonstrate the influence of anisotropy on the magnitude of the limit load.     

应力环在氢脆控制试验中的应用

介绍了一种氢脆测定试验方法和试样，即采用应力环对V型缺口氢脆试样加载，考察在规定的时间内试样是否断裂来评定零件的氢脆性。该方法简单易行，只须一台持久强度试验机，便可同时进行多个试样的试验，过程易于控制，稳定性好，提高了工作效率。经近两年的实际使用，情况良好。     

Microstructural Evolution of 2.25Cr-1.6W-V-Nb Heat Resistant Steel during Creep

2.25Cr-i.6W-V-Nb developed in Japan, is a low alloy heat resistant steel with good comprehensive properties. Influence of long term creep at elevated temperature on the structure of 2.25Cr-I.6W-V-Nb steel was studied in this paper, and the micromechanism of creep strength degradation was elucidated, too. Both TEM observation and thermodynamic calculation reveal that during creep the transformation occurs from M7C3 and M23C6 to M6C, which can be cavity nucleation sites. Besides, creep at 600℃ also leads to the decrease of dislocation density, the coarsening and coalescence of M23C6, the nucleation of cavities and development of cracks. The strength decrease of 2.25Cr-1.6W-V-Nb steel after long term creep is related to the decrease of dislocation hardening,precipitation hardening,solution hardening,the nucleation of cavities and development of cracks.     

Plastic J-integral calculations using the load separation method for the center cracked tension specimen

The load separation method was used to determine the plastic work factor (η_pl) for the center crack tension geometry for power law hardening materials with Ramberg-Osgood hardening exponents n ranging from 2 to 20 and crack sizes a/W ranging from 0.4 to 0.8. The resulting expression for η_pl compares favorably to the analytical work of Rice, Paris, and Merkle and to Sharobeam and Landes. The results were also compared to η_pl calculated from the plastic J results in EPRI Handbook NP-1931. Unlike the EPRI results, η_pl from the load separation method are not a function of crack size.     

薄板点焊试件的应力强度因子计算

采用简化模型和边界配置方法,对薄板点焊试件进行了断裂分析,计算了焊接周围裂纹Ⅰ、Ⅱ、Ⅲ型的应力强度因子。本方法具有计算量小,精度高的优点,适用于不同几何尺寸薄板点焊试样的断裂计算。     

Creep and damage analysis of a serviced tee intersection in a boiler header: Comparison between numerical and experimental results

In the last few years modelling of damage phenomena under creep condition has been developed in order to take into account the microstructural material evolution in life predictions of high temperature components. The new analytical methods based on “Continuum Damage mechanics” and experimental creep and creep-rupture data aim at describing both stress-strain and damage field in structures in order to predict crack initiation. These models are implemented in computer finite element programs and should be subjected to rigorous experimental verification for a practical use in power plant assessment activities.

In the present paper the numerical results obtained from some creep and damagement analyses of a header component (10 CrMo 910 steel) are shown and compared with the experimental ones. The creep analyses have been performed by the computer code ABAQUS and the damage evaluation has been carried out by means of proper in-house developed user's subroutine and post processor.