Residual Strength Measurements Research Articles

Effect of acidic media on flexural strength and fatigue of CAD-CAM dental materials.

To investigate the effect of acidic media, including beverages and gastric fluids on flexural strength and fatigue of CAD-CAM materials. Four CAD-CAM materials (high-translucency zirconia (Ceramill Zolid HT+), lithium disilicate (IPS e-max CAD), hybrid ceramic (Vita Enamic), and nanohybrid resin composite (Grandio Blocs) were prepared and immersed in one of five media (gastric HCl, white wine, Coca-Cola, orange juice, and artificial saliva) in an incubator (37℃, 24h). Surface topography and roughness were obtained using a scanning electron microscope (SEM) and a stylus contact profilometer, respectively. Initial 3-point flexural strength was measured for half of the bars (n=20/gp) using a universal testing machine (0.5mm/min). The other bars underwent 106 cyclic fatigue loadings before measurement of residual 3-point flexural strength. Data were statistically analyzed (two-way and three-way ANOVA, Tukey's post-hoc, p<0.05). Weibull distributions were plotted for reliability analysis. Zirconia bars has the highest initial flexural strengths followed by lithium disilicate, while resin composite and hybrid ceramic groups had the lowest strength regardless of the erosive medium. Cyclic fatigue significantly reduced initial flexural strengths for all materials except for hybrid ceramic and resin composite. Weibull moduli were the highest for zirconia, lithium disilicate and resin composite and lowest for hybrid ceramic. Erosive media significantly changed surface roughness of CAD-CAM materials except for zirconia and resin composite without jeopardizing the flexural strength of the CAD-CAM materials. Despite the higher flexural strengths for zirconia and lithium disilicate, resin composite and hybrid ceramic were more resistant to cyclic fatigue.

Residual Strength of Liquefied Soil: The Effect of Induced Partial Saturation

Abstract Post-liquefaction response and residual strength play important roles in stability assessment of liquefied ground. Considering the recent advancements in the application of induced partial saturation for liquefaction mitigation, the state of knowledge in estimating the residual strength should be extended for liquefied desaturated soils. In this paper, the residual strength response of a clean sand at different saturation levels was investigated using a ring shear device. Direct air injection was used to desaturate the soil, which helped with mitigating the liquefaction under cyclic loading. However, by raising the shear strain level, both saturated and partially saturated soils were liquefied, followed by residual strength measurement. Results indicate that the residual strength increased with a reduction of the saturation level because of the change in compressibility and consequent volume reduction. In addition, the strain-rate dependency of the residual strength was confirmed because an increase of shear strain rate resulted in an increase in residual strength under both saturated and partially saturated conditions.

Investigation of Newly Developed PCM/SiC Composite Aggregate to Improve Residual Performance after Exposure to High Temperature.

High temperature conditions, such as fire, are detrimental factors to the mechanical and chemical properties of concrete. In this paper, the authors developed a new type of coarse aggregate, named PCM/SiC composite aggregate (enhanced aggregate: EA), to improve fire-resistance performance. To investigate the validity of EA for construction materials, a compressive strength test, static modulus of elasticity, X-ray diffraction (XRD), and scanning electron microscopy (SEM) were conducted. In addition, this EA has been developed to improve residual performance after exposure to high temperature, with residual compressive strength and internal temperature measurement tested at 1000 °C. Furthermore, chemical properties after heating were investigated by XRD and SEM-EDAX. The results show that the percentage of residual compressive strength of heated concrete with EA is higher than plain concrete. The concrete with EA exhibited primary cement composites such as C-H and C-S-H after exposure to high temperature through XRD and SEM-EDAX. On the other hand, major hydration products could not be observed in plain concrete. PCM and SiC offer an opportunity to delay the increase in concrete temperature. From evaluation of the results, we can see that EA enhanced the residual performance of concrete after exposure to high temperature conditions.

Experimental evaluation of fire resistance performance of cement mortar with PCM/Mg(OH)2-based composite fine aggregate

When concrete structures are exposed to fire and high temperatures for an extended period of time, they become significantly less durable due to the decomposition of majorhydrationproducts and the evaporation of capillary water. A change in internal properties does not guarantee the stability of concrete structures and may reduce their life cycle. To preserve the durability and stability of concrete structures upon exposure to fire and high temperatures, fire-resistant mortar for exterior walls was developed in this work using zeolite, a phase change material (PCM), and magnesium hydroxide (MH). Zeolite was first coated with paraffin wax. Primary coated aggregates were then coated with MH, which was mixed with dissolved polyvinyl acetate to enhance adhesion on the surface of pre-coated aggregates. The physical and chemical properties of mortar mixed with different percentages of coated aggregates as a replacement for normal aggregates were evaluated by compressive strength tests, X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). Fire-resistant properties were investigated by residual compressive strength measurement tests, mass loss calculations, and mock-up tests to compare the internal temperature of mortar covered with normal and coated aggregate when heated in an electric furnace at 1000 °C. The residual compressive strength results showed a decrease in strength with a larger percentage of coated aggregates in the mortar. However, according to the mock-up test, control mortar covered with plain mortar took 5430 s (90.5 min) to reach a maximum temperature of 842 °C, while the mortar covered with 100% coated aggregate took 7170 s (119.5 min) to reach a maximum temperature of 735 °C. The obtained results indicate that the coated aggregate is a proper replacement for conventional aggregate in the development of fire-resistant mortar.

Effect of moulding water content on strength characteristics of a cement-stabilized granular lateritic soil

Performance of the cement-stabilized soils primarily depends on the soil type, cement quantity, moulding moisture content and ambient temperature. Control of the moulding moisture content and curing temperature are the challenges for most of the pavement engineers as these play a significant role in the strength development of stabilized soils. In the work presented here, an effort has been made to study the effect of the moulding water content on compressive strength and flexural strength of a stabilized granular lateritic soil collected from the eastern part of India. A detailed experimental programme was conducted, which involves various strength tests such as unconfined compressive strength test, California bearing ratio test and four-point flexure test. Microstructural analysis was also conducted using a scanning electron microscope and X-ray diffraction techniques to study the mechanism behind the variation of strength with varying moisture contents. Effect of moisture was also investigated through measurement of residual compressive strength. It is observed that compressive strength decreases with an increase in the moulding moisture content; however, the flexural strength increases with an increase in moulding moisture content within the testing range of moisture content around optimum moisture content (OMC), i.e. OMC-2% to OMC+2%.

Estimation of residual stress of metal material without plastic plateau by using continuous spherical indentation

This paper presents a method to allow the measurement of residual stress (σr), yield strength (σy) and strain hardening exponent (n) of metal materials through continuous spherical indentation test. Based on the analysis of finite element simulation results, a new fitting equation between the indentation response parameter and the material properties was given in this paper. The influence of friction and the mesh size of finite element model was also discussed when the residual stress is included in the material. Only three load values are required at each different indentation depths, and the three unknown parameters (σr, σy and n) can be obtained through corresponding dimensionless functions based on the reverse analysis. Finally, the validation of the method presented in this paper was verified by simulating the material (SUS304) with the known material parameters in the literature. Results show that the method proposed in this paper can accurately predict the residual stress, yield strength and strain hardening exponent of metal materials from the indentation load-depth curve.

Estimation of Residual Stress and Plastic Properties of a Material without Plastic Plateau by Using Continuous Spherical Indentation

This paper presents a method to allow the measurement of residual stress (), yield strength () and strain hardening exponent () of metal materials through continuous spherical indentation test. Based on the analysis of finite element simulation results, a new fitting equation between the indentation response parameter and the material properties was given. The influence of friction on finite element model simulation was also discussed when the residual stress is included in the material. Only three load values are required at each different indentation depths, and the three unknown parameters (, and ) can be obtained through corresponding dimensionless functions. Finally, the validation of the method presented in this paper was verified by simulating the material (SUS304) with the known material parameters in the literature.

The Application of DIC Technique to Evaluate Residual Tensile Strength of Aluminum Alloy Plates with Multi-Site Damage of Collinear and Non-Collinear Cracks

This paper introduces an aviation industrial application of digital image correlation (DIC) technique on the measurement of residual tensile strength (RTS). In order to investigate multi-site damage (MSD) that is common in the fuselage of aging aircraft, RTS of 2024-T4 aluminum alloy sheet with MSD was evaluated using DIC technique. Firstly, the four-factor and three-level orthogonal experiment was designed to optimize the DIC method to control the strain calculation error by considering subset size, interpolation tap, calibration score and step size. Secondly, RTS and strain fields were generated to analyze the path of crack propagation. The results show the optimal factor combination is 0.018 of calibration score, 23 pixels of subset size, step size is 1/4 of subset size and the filter size of interpolation calibration is 8 pixels. With the increase of spacing between adjacent holes, the RTS increases and the collinear cracked specimen becomes more perilous than that of non-collinear cracked while the hole spacing is 25 mm from the statistical analysis. Based on the Net section yield criterion, the RTS was calculated, which can give a conservative prediction of RTS.

Microstructure, ablation behavior and thermal retardant ability of C/C-HfB2 composites prepared by precursor infiltration pyrolysis combined with chemical vapor infiltration

C/C-HfB2 composites were fabricated by precursor infiltration pyrolysis combined with chemical vapor infiltration. Ablation behavior, oxidation performance and thermal retardant ability of the prepared composites were investigated. The results showed that the oxidation products of HfB2 could enhance the oxidation performance and ablation property of C/C-HfB2 composites. During ablation in the heat flux of 2.38 MW/m2 under oxyacetylene torch, a protective HfO2 layer was covered on ablated surface, acting as an efficient barrier to heat transfer and denudation of the oxyacetylene torch. As the heat flux increased to 4.18 MW/m2, thermal energy concentration and denudation enhancement of the oxyacetylene torch resulted in the spalling of the HfO2 layer. Based on microstructure evolution, thermal retardant ability test, thermogravimetric analysis and measurement of residual flexural strength, oxidation as well as ablation mechanism of the C/C-HfB2 composites was discussed.

Electrospun nanofibrous interleaves for improved low velocity impact resistance of glass fibre reinforced composite laminates

This study analyses the damage tolerance of nanofibre interleaved composites when subjected to low velocity impact. Cross-ply glass/epoxy composite laminates are produced. Drop-weight impact and residual compressive strength measurements are performed on these laminates according to the ASTM D7136 and ASTM D7137 standards for a range of impact energies around the Barely Visible Impact Damage energy limit. Polyamide 6, polyamide 6.9 and polycaprolactone nanofibrous veils with two different veil densities are selected to assess their effect on the damage tolerance. The low velocity impact resistance of nanofibre interleaved laminates increases considerably compared to the virgin material. The (projected) damage area decreases up to 50–60%, especially at higher impact energies where the virgin material shows widespread delamination. As more energy is absorbed in the interleaved laminates by the nanofibres, less damage to reinforcing fibres and matrix resin is produced. Analysis of fracture surfaces shows that the development of nanofibre bridging zones is the main reason for the improved impact damage tolerance.

Research of the Load Bearing Capacity of Shape-Optimized Metal Inserts Embedded in CFRP under Different Types of Stresses

An efficient implementation of lightweight design is the use of continuous carbon fiber reinforced plastics (CFRP) due to their outstanding specific mechanical properties. Embedded metal elements, so-called inserts, can be used to join metal-based attachments to structural CFRP parts in the context of multi-material design. They differ from other mechanical fasteners and have distinctive benefits. In particular, drilling of the components to be joined can be avoided and, depending on the preforming, fiber continuity can be maintained using such elements. Thus, no local bearing stress is anticipated. Previous work published by the authors [1] dealt with a systematic research of the influence of different types of stresses on the load bearing capacity of welded inserts. This contribution aims at the investigation of the performance of shape-optimized inserts under the same types of loading to compare with the results of the welded inserts serving as a reference. For that purpose, the respective load bearing capacities were evaluated after preinduced damages from impact tests and thermal cycling. In addition, dynamic high-speed tensile tests (pull-out) were conducted under different loading velocities. It is shown that the load bearing capacities increased up to 19% for high velocities (250 mm/s) in comparison to quasi-static loading conditions (1.5 mm/min) showing an obvious strain rate dependency of the CFRP. Quasi-static residual strength measurements under tensile loading identified the influence of the respective preinduced damages of the insert. Influence of the thermal loading condition was evaluated by placing the specimens in a climate chamber and exposing it to various numbers of temperature cycles from-40 °C to +80 °C with a duration time of 1.5 hours each. Here, it turned out that already 10 temperature cycles decreased the quasi-static load bearing capacity up to 31%. According to DIN EN 6038 the specimens were loaded with different impact energies and the residual strength were measured carrying out pull-out tests. It could be shown that the damage tolerance is significantly lower for the shape-optimized insert due to failure-critical delamination. The optimized insert also endured lower impact energies and the influence on the performance was higher.

Behavior of Sandstones Under Heat Treatment

Knowledge of materials behavior under heat treatment is of high importance in construction and safety engineering; tunnels represent a special field because of their specific safety issues. In the case of fire, tunnel structure and surrounding rock are subjected to extreme temperatures which induces irreversible changes in the material’s microstructure and consequently its mechanical properties. Significant portion of the Earth’s crust is formed by sandstones; this group of sedimentary rocks is highly variable in structure, composition and engineering properties. Quartz grains (alternatively together with other minerals) form the clastic part of sandstones; the space between clasts is filled by variable amount of cement and matrix which can contain particularly clay minerals, quartz and calcite. The porosity of sandstones is again highly variable from a nearly compact material to a highly porous one. The paper aims to find out and explain differences in response of various kinds of sandstones to heat treatment. The behavior of a representative set of sandstones under heat treatment was studied by TG/DSC, thermodilatometry and residual strength measurement. These experiments were accompanied by SEM and porosimetry measurement. The effect of increased temperature on the compressive strength was found to be crucially dependent on the nature of the cement and matrix present in the individual rock. The rocks with calcite cement which had high initial strength and low porosity were damaged by calcite decomposition. The siliceous sandstones were damaged by cracking due to thermally induced volume changes. In contrary, the strength of the clayey sandstones was even improved after the heat treatment. It can be concluded that behavior of sandstone under heat treatment is controlled by its composition and diagenesis.

Impact and post impact behavior of fabric reinforced geopolymer composite

Impact and post impact behavior of fabric reinforced geopolymer composites is investigated in this study. Carbon, E-glass and basalt fabrics are incorporated in the geopolymer matrix. The geopolymer composites are tested for impact resistance to an out-of-plane low velocity impact test, followed by post impact performance testing using residual strength measurement on the sample. Impact properties such as impact energy, energy absorption capacity, and damaged area of the composites were examined for fabric reinforced geopolymer composite samples. The drop weight impact tests were performed on composite samples of 15×15cm. The quality of the samples was examined using C-scan in ultrasonic vibration mode and μCT before and after impact test. The post impact behavior of the composite samples was characterized using four point bending tests. Fiber orientation and alignment was observed using SEM. The results obtained showed that E-glass fabric reinforced composites show slightly better performance than carbon reinforced composites. Especially the damaged area is smaller for E glass composites. Basalt fabric reinforced composites have low bending strength and very low impact resistance. The energy absorption in E-glass is due to post-debond fiber sliding phenomena and in carbon fiber to fiber pull out from the composite. The smaller damaged area is attributed to the higher elastic energy absorption of E-glass fibers. The residual strength of carbon and E-glass composites decreased in relation to the size of the visually damaged area, meaning that no invisible damage took place.

Measurement and optimisation of residual tensile strength and delamination damage of drilled flax fibre reinforced composites

The advent of natural fibre as reinforcement material for polymer composites has gained much attention in recent years. Since composite materials made of natural fibres exhibited good and comparable mechanical properties, they are being applied in automotive, packaging and general appliances. These applications would inevitably require the natural fibre composites to be drilled, if not machined. However, open hole in the composites induces discontinuity and strength reduction in the composite structure. Results from literature search showed that experimental study on evaluating the effect of hole on the drilled natural fibre composites, namely, flax fibre, has not been attempted or reported. Therefore, the objective of this study is to experimentally evaluate and measure the residual tensile strength and delamination factor of the drilled flax fibre composites. In particular, a series of drilling experiments were performed for collection and measurement of the aforementioned outputs through Taguchi design of experiment. The results suggest that both residual strength and delamination factor were more sensitive to the change in feed rate, followed by spindle speed and tool geometry. Indication from this study is that flax fibre composites could be a potential substitute of their synthetic fibres counterpart in the load-bearing as well as non-structural applications.

An empirical model to predict the lifetime of braided HMPE handling ropes under cyclic bend over sheave (CBOS) loading

Repeated bending over sheaves is one of the main causes of failure of synthetic fibre ropes used in marine operations. There are few published results available and even fewer models allowing lifetime to be estimated. A large new set of data from cyclic bend over sheave (CBOS) tests on 250kN break load braided HMPE synthetic ropes is presented first, both tests to failure and interrupted tests followed by residual strength measurements. These data are analysed in order to propose an empirical lifetime model. This is identified using constant load tests, then evaluated for variable load sequences. A methodology to include rope lifetime prediction in handling system design is then discussed.

Long Term Investigation on Sulphate Resistance of Concrete Containing Laterite Aggregate

Utilization of laterite rock at 20% as partial aggregate replacement in concrete production is seen as one of the alternative to reduce the high dependency of concrete industry on granite aggregate supply and preserve this natural resource for future generation. This paper reports the durability performance of concrete containing laterite aggregate upon sulphate attack. A reference mix, plain concrete consisting 100% granite aggregate and another one mix containing 20% laterite aggregate as partial coarse aggregate replacement were used. All specimens were prepared in cubes of (100×100×100mm) and subjected to water curing for 28 days. After that, the specimens were immersed in Magnesium Sulphate solution for a period of 50 weeks. The durability of the concrete mixes was determined through measurement of mass change and residual compressive strength. The compressive strength test was conducted following the procedures in BSEN 12390 – 3. Upon exposure in sulphate environment, reaction between sulphate ions and portlandite which formed gypsum contributes to concrete expansion and disintegration causing strength reduction in both specimens. Owing to the small difference in total mass loss and residual compressive strength between the two mixes, it is concluded that the sulphate resistance of laterite concrete is comparable to plain concrete.

S0420103 微視破壊過程のAE評価に基づいたセラミックスの破壊シミュレーション([S042-01]セラミックスおよびセラミックス系複合材料(1),機械材料・材料加工部門)

Microfracture process in ceramics was analyzed using Monte-Carlo simulation. It was supposed in the simulation that the microfracture process consists of the maincrack formation resulted from the growth and coalescence of pre-existing microcracks, and the propagation of maincrack to final failure. To determine the crack growth parameters for simulation, microfracture process in alumina ceramics was characterized by acoustic emission technique, and the critical stress for maincrack formation and final strength were determined and compared with the results of simulation. The Monte-Carlo simulation assuming the 3 microcracks array with random size was adapted to the fracture analysis during the proof test with various proof stresses and holding times as well as residual strength measurements. The time-to-maincrack formation, yields and residual strength were calculated and it was found that they showed good agreement with the several experimental results. Consequently, the proof test with the wide range of proof stresses and holding times were examined and their influence on the yields, residual strength and lifetime was discussed.

Research on Mullite-Corundum-Aluminium Titanate Composite

Mullite-aluminium titanate-corundum composite was prepared at 1300°C with refractory clay, aluminium titanate and high alumina grog as raw material, molded at pressure of 50MPa. Effect of raw material ratio on sintering and themal shock resistance of the mullite-aluminium titanate-corundum composite was researched by measurements of apparent porosity, bending strength and residual strength after water-cool, and analyses of XRD and SEM. The results showed that as refractory clay content, apparent porosity of samples decrease, bulk density and bending strength increase. When the weight ratio of refractory clay, aluminium titanate and high alumina grog is 60/10/30, themal shock resistance of sample is excellent, The XRD and SEM analysis results indicated that the mechanical and thermal proprieties are relative to the microstructure and crystal phases of the composite materials.

The Influence of Through-Thickness Material Property Variation on Stretch Forming Springback

Stretch forming is commonly used in the manufacture of low-volume convex aircraft panels. The gentle curvature and high degree of stretch leads to relatively low levels of elastic recovery post forming. Previous investigations [1] have shown that there is a large difference in the measured springback depending upon which side of an Alclad aluminium sheet is in contact with the tool surface. Initial investigation into the clad layer properties revealed no accountable variation in either thickness or strength. Instead the asymmetric springback behaviour was attributed to material property variations through the alloy core of the sheet. An analytical method for the prediction of the asymmetric springback incorporating the through thickness property variation is presented. Material strengths at specific through thickness positions were determined using a procedure based on residual strength measurements. The predicted results correlate well with the experimental data, which had initially demonstrated the asymmetric springback

Experimental studies on the performance of novel layered materials under highly dynamic loads

This paper focuses on the experimental observations of the performance of different layered composite material systems subjected to blast loadings. These material systems include layered composites and sandwich composite materials. A controlled blast loading of pre-defined pressure magnitude and rise time were obtained using a shock tube apparatus. Rectangular plate elements of the desired material system were subjected to such a controlled blast loading and the effects of the blast loading on these elements were studied using optical and residual strength measurements. A high speed imaging technique was utilized to study the damage modes and mechanisms in real time. It was observed that layering of a conventional composite material with a soft visco-elastic polymer provided better blast resistance and sandwiching the polymer greatly enhanced its survivability under extreme air blast conditions. Aside from layering the conventional composite material with a soft visco-elastic polymer, it was observed that layering or grading the core can successfully mitigate the impact damage and thus improve the overall blast resistance as well. In addition to these, three dimensional (3D) woven skin and core reinforcements were introduced in the conventional sandwich composites and their effects on the blast resistance were studied experimentally. It was observed that these reinforcements also enhance the blast resistance of conventional sandwich composites by changing the mechanism of failure initiation and propagation in these sandwich structures.