Various admixtures to mitigate the long-term strength retrogression of Portland cement cured under high pressure and high temperature conditions

In order to investigate the problem of long-term strength retrogression in oil well cement systems exposed to high pressure and high temperature(HPHT)curing conditions,various influencing factors,including cement sources,particle sizes of silica flour,and additions of silica fume,alumina,colloidal iron oxide and nano-graphene,were investigated.To simulate the environment of cementing geothermal wells and deep wells,cement slurries were directly cured at 50 MPa and 200?C.Mineral compositions(as determined by X-ray diffraction Rietveld refinement),water permeability,compressive strength and Young’s modulus were used to evaluate the qualities of the set cement.Short-term curing(2e30 d)test results indicated that the adoption of 6 m m ultrafine crystalline silica played the most important role in stabilizing the mechanical properties of oil well cement systems,while the addition of silica fume had a detrimental effect on strength stability.Long-term curing(2e180 d)test results indicated that nano-graphene could stabilize the Young’s modulus of oil well cement systems.However,none of the ad-mixtures studied here can completely prevent the strength retrogression phenomenon due to their inability to stop the conversion of amorphous to crystalline phases.

10℃条件下纳米C-S-H掺量对水泥早期水化的影响

目的探究10℃条件下不同掺量纳米C-S-H对水泥早期水化进程及强度发展的影响,并分析其早强剂作用机理。方法通过溶液共沉淀法制备纳米C-S-H悬浮液,将其掺入到普通硅酸盐水泥中,在10℃条件下进行养护,分析其对水泥凝结时间、抗折强度、抗压强度的影响;通过XRD、TG等检测手段分析水泥的水化进程及对水化产物的影响。结果掺入0.05%纳米C-S-H,可使水泥初凝时间缩短9.9%,终凝时间缩短10.4%;24 h时抗折强度和抗压强度分别提高了34%和33.5%。纳米C-S-H的掺入造成水泥3 d抗折强度略有降低。水化产物中Ca(OH)2质量分数在12 h、16 h和24 h时较参比样分别提高了17.6%、21.3%和29.6%。结论在10℃条件下,纳米C-S-H的加入能够有效促进C 3S和C 2S的水化过程,形成更多的水化产物,从而缩短水泥凝结时间,提高24 h内水泥砂浆强度,28 d强度没有显著降低。

纳米C-S-H的合成及其对普通硅酸盐水泥早期水化的影响

为提高水泥基材料的早期强度,制备了纳米C-S-H悬浮液,并通过XRD和Zeta电位对其组成和粒度分布、稳定性进行了分析。通过水化放热、热重分析和扫描电子显微镜扫描电镜(SEM)研究了纳米C-S-H对硅酸盐水泥早期强度发展、水化放热、水化产物的影响。结果表明:合成了具有半结晶结构的稳定的纳米C-S-H悬浮液。纳米C-S-H的加入促进了C3S的水化,尤其是在前24 h,并使水泥的水化诱导期明显缩短。含有0.05%纳米C-S-H砂浆的8 h、16 h和24 h抗压强度分别提高了176.0%、145.6%和43.9%,与此同时,砂浆的3 d抗压强度没有明显减少。

纳米C⁃S⁃H晶种与掺和料及早强剂复配对混凝土强度的影响

为了提高混凝土早期强度,根据不同混凝土配方的组成差异,将纳米C‐S‐H晶种与掺和料以及早强剂复配,成型不同龄期的混凝土试件,分析复配后混凝土初终凝时间、抗压强度和抗折强度受到的影响。结果表明:粉煤灰和硅灰会降低混凝土的早期抗压和抗折强度,纳米C‐S‐H晶种与粉煤灰、硅灰复配后可减弱粉煤灰、硅灰对强度下降的影响;纳米C‐S‐H晶种可缓解石膏的缓凝作用,并且可以与石膏、硫酸钠、硫铝酸盐水泥发挥协同作用,进一步提高混凝土的早期强度。

C-S-H晶核早强剂制备及性能研究

针对常规早强剂可能引起混凝土后期强度倒缩、泛霜等问题,采用聚羧酸减水剂作为分散剂,并以四水硝酸钙和五水合硅酸钠作为原料制备了C⁃S⁃H晶核早强剂,研究了不同反应条件对晶核早强剂稳定性能及强度性能的影响。结果表明:钙硅比为1.5、分散剂相对分子质量为17000时所制备的晶核早强剂具有良好的水泥适应性,其18、24 h的抗压强度比空白组分别提高160%、130%,对后期强度也有一定的促进作用。

C-S-H-PCE对基准水泥体系水化的影响

传统的早强剂在实际应用过程中会对混凝土的早强性能和耐久性能产生一定的影响,因此开发新型早强剂尤为重要。其中新开发的纳米C-S-H-PCE新型早强剂能够更好地加速水泥基材料的早期水化,有利于水化产物的快速生成。探究C-S-H-PCE早强剂对基准水泥体系水化的影响,对C-S-H-PCE的研发和应用具有重要意义。本文基于水泥水化的溶解-沉淀理论,研究C-S-H-PCE对基准水泥体系水化各阶段的影响,通过水化热测试对比分析C-S-H-PCE、纳米SiO_(2)对水化历程的影响,并测定各阶段的凝结时间和抗压强度。结果表明,C-S-H-PCE的加入显著缩短水化诱导期、提高水化最大放热速率、促进未水化颗粒的溶解;C-S-H-PCE缩短了初凝和终凝时间,1d和3d的强度得到明显提升。由此可知C-S-H-PCE对基准水泥水化的作用规律和影响机制,从而建立了水化微结构和宏观力学性能之间的联系。

A Model of the Fatigue Life Distribution of Composite Laminates Based on Their Static Strength Distribution

The reasons of the static strength dispersion and the fatigue life dispersion of composite laminates are analyzed in this article. It is concluded that the inner original defects, which derived from the manufacturing process of composite laminates, are the common and major reason of causing the random distributions of the static strength and the fatigue life. And there is a correlative relation between the two distributions. With the study of statistical relationship between the fatigue loading and the fatigue life in the uniform confidence level and the same survival rate S-N curves of material, the relationship between the static strength distribution and the fatigue life distribution through a material S-N curve model has been obtained. And then the model which is used to describe the distributions of fatigue life of composites, based on their distributions of static strength, is set up. This model reasonably reflects the effects of the inner original defects on the static strength dispersion and on the fatigue life dispersion of composite laminates. The experimental data of three kinds of composite laminates are employed to verify this model, and the results show that this model can predict the random distributions of fatigue life for composites under any fatigue loads fairly well.

RELIABILITY OF AEROCRAFT GEAR FATIGUE STRENGTH

With the method of group test, fourty pairs of carburization-quenching gears made from 16NCD13 steel for aerocraft were tested to research the contacting fatigue strength on tooth flank. As a result, the samples of fatigue life at the moments when the pitting appears and reaches failure criterion were obtained at four stressing levels respectively. The distribution rule of fatigue life were distinguished, and the distribution parameters were estimated by statistical analysis. Based on that, the R-S-N curves with confidence 95% of contacting fatigue on gear tooth flank were evaluated. Therefore, the basic data were provided for the reliability design of the gears and prediction of their life.

Effect of Friedel's salt on strength enhancement of stabilized chloride saline soil

In the field of soil stabilization, only calcium silicate hydrate(CSH) and ettringite(AFt) as hydration products have been reported to directly contribute to the strength enhancement of the soil. A chloride dredger fill, an artificial chloride saline soil, and a non-saline soil were stabilized by Portland cement(PC) and PC with Ca(OH)_2(CH) with different contents. A series of unconfined compressive strength(UCS) tests of stabilized soil specimen after curing for 7 d and 28 d were carried out, and the hydration products and microstructure of the specimens were observed by X-ray diffractometry(XRD), scanning electronic microscopy(SEM), and energy-dispersive X-ray analysis(EDXA). The results showed that the strengths of PC+CH-stabilized chloride saline soils were much higher than those of PC-stabilized soils. A new hydration product of calcium aluminate chloride hydrate, also known as Friedel's salt, appeared in the PC+CH-stabilized chloride saline soils. The solid-phase volume of Friedel's salt expanded during the formation of the hydrate; this volume filled the pores in the stabilized soil. This pore-filling effect was the most important contribution to the significantly enhanced strength of the PC+CH-stabilized chloride saline soils. On the basis of this understanding, a new optimized stabilizer was designed according to the concept that the chloride in saline soil could be utilized as a component of the stabilizer. The strength of the chloride saline soils stabilized by the optimized stabilizer was even further increased compared with that of the PC+CH-stabilized soils.

Strength and damage of marble in ductile failure

To study the effects of loading paths and stress states on rock strength and deformation, marble specimens were axially compressed to various displacements under a confining pressure (CP) firstly, and then the damaged specimens were recompressed under another CP. The bearing capacity of a marble specimen depends merely on CP at the stage of ductile deformation, and it has no relation with the loading history when CP keeps constant or increases. However, the damaged specimen turns into brittle when it is recompressed uniaxially or at a lower CP, and the Young’s modulus and strength are lower than those of a dense specimen. The increasing ratio of triaxial strength to CP has a close relation with the areas of fissures in the damaged specimens but not the internal friction angle. Material strength and bearing capacity are two different conceptions for rocks. Material strength decreases continually as the plastic deformation increases; however, the bearing capacity is determined by both the stress state and the material strength.

An improved method to assess the required strength of cemented backfill in underground stopes with an open face

Backfill is increasingly used in underground mines to reduce the surface impact from the wastes produced by the mining operations. But the main objectives of backfilling are to improve ground stability and reduce ore dilution. To this end, the backfill in a stope must possess a minimum strength to remain self-standing during mining of an adjacent stope. This required strength is often estimated using a solution proposed by Mitchell and co-workers, which was based on a limit equilibrium analysis of a wedge exposed by the open face. In this paper, three dimensional numerical simulations have been performed to assess the behavior of the wedge model. A new limit equilibrium solution is proposed, based on the backfill displacements obtained from the simulations. Comparisons are made between the proposed solution and experimental and numerical modeling results. Compared with the previous solution, a better agreement is obtained between the new solution and experimental results for the required cohesion and factor of safety. For large scale(field) conditions, the results also show that the required strength obtained from the proposed solution corresponds quite well to the simulated backfill response.

Improve regression-based models for prediction of internal-bond strength of particleboard using Buckingham's pi-theorem

Internal bond （IB） strength is one of the most important me- chanical properties that indicate particleboard quality. The aim of this study was to find a simple regression model that considers the most important parameters that can influence on IB strength. In this study, IB strength was predicted by three kinds of equations （linear, quadratic, and exponential） that were based on the percentage of adhesive （8%, 9.5%, and 11%）, particle size （＋5, -5 ＋8, -8 12, and -12 mesh）, and density （0.65, 0.7, and 0.75 g/cm3）. Our analysis of the results （using SHAZAM 9 software） showed that the exponential function best fitted the experi- mental data and predicted the IB strength with 18~,/0 error. In order de- crease the error percentage, the Buckingham Pi theorem was used to build regression models for predicting IB strength based on particle size,

Effect of Plastic Deformation and H_2S on Dynamic Fracture Toughness of High Strength Casing Steel

The effects of plastic deformation and H2 S on fracture toughness of high strength casing steel（C110 steel） were investigated. The studied casing specimens are as follows： original casing, plastic deformation（PD） casing and PD casing after being immersed in NACE A solution saturated with H2S（PD＋H2S）. Instrumented impact method was employed to evaluate the impact behaviors of the specimens, meanwhile, dynamic fracture toughness（JId） was calculated by using Rice model and Schindler model. The experimental results show that dynamic fracture toughness of the casing decreases after plastic deformation. Compared with that of the original casing and PD casing, the dynamic fracture toughness decreases further when the PD casing immersed in H2 S, moreover, there are ridge-shaped feature and many secondary cracks present on the fracture surface of the specimens. Impact fracture mechanism of the casing is proposed as follows： the plastic deformation results in the increase of defect density of materials where the atomic hydrogen can accumulate in reversible or irreversible traps and even recombine to form molecular hydrogen, subsequently, the casing material toughness decreases greatly.

High-cycle Fatigue Fracture Behavior of Ultrahigh Strength Steels

The fatigue fracture behavior of four ultrahigh strength steels with different melting processes and therefore different inclusion sizes were studied by using a rotating bar two-point bending fatigue machine in the high-cycle regime up to 107 cycles of loading. The fracture surfaces were observed by field emission scanning electron microscopy （FESEM）. It was found that the size of inclusion has significant effect on the fatigue behavior. For AtSI 4340 steel in which the inclusion size is smaller than 5.5 μm, all the fatigue cracks except one did not initiated from inclusion but from specimen surface and conventional S-N curve exists. For 65Si2MnWE and Aermet 100 steels in which the average inclusion sizes are 12.2 and 14.9 μm, respectively, fatigue cracks initiated from inclusions at lower stress amplitudes and stepwise S-N curves were observed. The S-N curve displays a continuous decline and fatigue failures originated from large oxide inclusion for 60Si2CrVA steel in which the average inclusion size is 44.4 pro. In the case of internal inclusion-induced fractures at cycles beyond about 1×10^6 for 65Si2MnWE and 60Si2CrVA steels, inclusion was always found inside the fish-eye and a granular bright facet （GBF） was observed in the vicinity around the inclusion. The GBF sizes increase with increasing the number of cycles to failure Nf in the long-life regime. The values of stress intensity factor range at crack initiation site for the GBF are almost constant with Nf, and are almost equal to that for the surface inclusion and the internal inclusion at cycles lower than about 1×10^6. Neither fish-eye nor GBF was observed for Aermet 100 steel in the present study.

A Method for Calculating Bottom Backscattering Strength Using Omnidirectional Projector and Omnidirectional Hydrophone

A method of obtaining bottom backscattering strength by employing an omnidirectional projector and omnidirectional hydrophone is proposed. The backscattering strength is extracted from monostatic backscattering data. The method was adopted in an experiment conducted in the South Yellow Sea of China. The seafloor surface was relatively smooth and covered by a small quantity of shell fragments, as observed through a digital camera system. Sampling data showed that the main component of the sediment at this experimental site was fine sand. In this paper, we detail the calculation method. Preliminary results of backscattering strength as a function of grazing angle(20?–70?) in the frequency range of 6–24 kHz are presented. The measured backscattering strength increased with the grazing angle and changed more rapidly at large grazing angles(60?–70?). A comparison of the data at different frequencies reveals that the measured backscattering strength substantially rises with the increase of acoustic frequency. A fitting curve of Lambert's law against the measured data shows that the backscattering strength deviates from Lambert's law at large grazing angles.

Strength criterion of porous media: Application of homogenization techniques

The present paper aims at giving some general ideas concerning the micromechanical approach of the strength of a porous material. It is shown that its determination theoretically amounts to solving a nonlinear boundary value problem defined on a representative elementary volume(REV). The principle of nonlinear homogenization is illustrated based on the case of a solid phase having a Green’s strength criterion. An original refinement of the so-called secant method(based on two reference strains) is also provided. The paper also describes the main feature of the Gurson’s model which implements the principle of limit analysis on a conceptual model of hollow sphere. The last part of the paper gives some ideas concerning poromechanical couplings.

The Strength Characters of C/C Composite Material with Opening Hole and Typical Application

Research on the opening hole and connecting problem of C/C composite material was conducted. The strength characteristics of plate with opening hole were tested and the applicability of strength criteria focused on particular point was analyzed. Conclusion is obtained that obviously conservative to evaluate open hole and joint strength by hole-edge stress. Based on these, high and nor-mal temperature strength test of typical circular shaft was completed, proving that comprehensive joint performance can be significantly improved by appropriately optimized design.

-10℃条件下掺氯化镁溶液的γ-C_(2)S碳化性能研究

γ-C_(2)S是一种高活性的碳化固结胶凝材料,在与CO_(2)反应过程中对环境温度敏感性低,可用于制备负温建筑材料。本工作研究了-10℃条件下掺氯化镁溶液γ-C_(2)S碳化养护过程中的温度变化和硬化体的碳化产物组成、强度与反应程度发展规律以及微观形貌。结果表明:未掺MgCl_(2)的γ-C_(2)S样品在-10℃碳化养护后强度发展缓慢,24 h后仅有10.83 MPa。掺入不同浓度MgCl_(2)溶液后,碳化体早期与后期强度均提升明显,0.5 h和24 h抗压强度最高可分别增至22.14 MPa和178.56 MPa。γ-C_(2)S在-10℃下碳化产物以方解石为主,并含有少量文石,掺氯化镁溶液后由于Mg^(2+)对Ca^(2+)的取代形成部分镁方解石。MgCl_(2)一方面通过降低溶液冰点使体系内在-10℃下仍存在液态水,为碳化过程提供反应环境,另一方面能促进Ca^(2+)溶出和碳化反应的进程。两者的协同效应保证γ-C_(2)S在-10℃条件下碳化养护仍能表现出优异的力学性能。

Solution of Singular Integrals in Mathematical Model of Mode I Crack near Strength Mismatched Interface

Characteristics of Mode I crack near the interface of elasticity matched but plasticity and strength mismatched materials differ from those of the crack in a homogenous body. Interface body of different strength influences the plastic or cohesive zone at the crack tip in parent body. The mathematical model for load line opening of the crack near the interface in linear elastic regime involves singular integrals. The paper presents explicit solution of these integrals with the help of Cauchy’s principal value theorem. Cases of thin and thick welds between the materials are investigated. Solutions of the integrals are well substantiated. Final results are provided in a consolidated form.

Shear behavior of two-order morphology in rock joints

Two-order morphology of rock joints named as waviness and unevenness can be separated by morphology classification method,which plays a decisive role in the evolution of shear stress during the shear test.The joint morphology is obtained by using 3D printing and 3D laser scanning techniques and the joint model samples in two-order morphology are produced by cement mortar.Then,shear tests are performed under different normal loads.Results shows that the waviness is dominant in the total morphology during the shear test,and the shear contribution of unevenness mainly occurs in the climbing phase of shearing process.Comparing the failure modes of two-order morphology,waviness mainly embodies shear dilation characteristics and unevenness mainly shows shear wear characteristics.Based on this,a quantitative parameter is proposed to represent the ratio of the peak shear strength of the two-order morphology to that of total morphology.The functional relationship between the peak shear strength of total and two-order morphologies is determined,providing a theoretical method for further in-depth study on the shear strength of the interaction with two-order morphology of rock joints.