STUDY THE RESIDUAL STRESSES IN COMPOSITES MATERIALS USING X-RAY TECHNIQUE

This study was conducted to evaluate the curing temperature effect on the mechanical properties of high-strength strain-hardening cementitious composite (SHCC) containing waste supplementary cementitious materials (SCMs) and polyethylene (PE) fibers. High-strength SHCC is developed to extend the strain-hardening interval by simultaneously inducing multiple cracks and ensuring the durability and strength of high-strength concrete. The starting point of this study was to enhance the tensile performance and durability of high-strength SHCC by utilizing various SCMs. In addition, the optimal curing conditions were investigated to derive the maximum material potential of each SCM, which aims to advance the performance of high-strength SHCC. The temperatures employed for the curing process were 20, 40, and 90°C. Moreover, ground granulated blast-furnace slag (GGBS), silica fume (SF), and cement kiln dust (CKD), were used as a partial replacement for cement to determine the best mix for achieving optimal tensile performance. Four mix designs were prepared, including a plain test specimen composed entirely of cement as binder; therefore, a total of 12 types of specimens were set considering the three curing temperatures. A compressive strength test was conducted with cube specimens, and a direct tensile test was performed with dog-bone-shaped specimens. Derivative thermogravimetry (DTG) and energy dispersive X-ray spectroscopy (EDS) mapping were conducted to identify the microstructures. The SF-containing SHCC cured at 90°C exhibited the best tensile performance in terms of deformability and energy absorption capacity by achieving the highest strain capacity of 4.37% and g-value of 294.5 kJ/m3. In addition, the performance of each specimen was reconfirmed based on the DTG, EDS mapping, and crack pattern results. Through these results, the optimal SCM mixing amount and curing conditions that led to noticeable performance improvement of high-strength SHCC were identified.

It is pointed out by the authors as the result of studies on the residual stresses of stretched and quenched carbon steel specimens by using X-ray method: 1) that thermal residual stress in the (211) crystal plane represents the macroscopic residual stress itself, 2) that it is considered that the compressive residual stress obtained by using the Cr-Kα beams in stretched carbon steel specimen includes the macroscopic residual stress due to the surface effect, and also the microscopic one due to the anisotropy of plastic deformation.It has been reported by many researchers that X-ray compressive residual stresses are observed on the plastically stretched carbon steel specimens. However, considering from the recently obtained experimental results, some contradictions to the discussions are often found since the past studies treated only the residual stress due to the uniaxial stretching, and scarcely dealt with the residual stresses caused by other processes. For this reason, the authors carried out experiments to examine the nature of X-ray residual stress in order to extend the method of X-ray stress measurement to practical use.The crystal plane dependence of residual stress in stretched or quenched specimen was investigated, and the biaxial residual stresses in the thin-walled tubular specimens caused by plastic deformation due to the internal pressure were measured.Iron and five sorts of carbon steels with carbon contents ranging between 0.06 and 1.10 percent were prepared as test materials. All the specimens were annealed after machining. Some of them were water-quenched from 600°C after being annealed, and these specimens include the residual stresses of purely thermal effect.The X-ray diffraction apparatus used was a parallel beam type X-ray stress analyser, and the diffraction lines of Co-Kα and Cr-Kα beams from (310) and (211) crystal planes, respectively, were automatically recorded.The results obtained in this study indicate interesting features as follows:1) The crystal plane dependence of residual stress is not seen in the quenched specimens, and the thermal residual stress can be accurately measured by means of X-rays.2) On the other hand, it is clearly observed in plastically deformed specimens. This result indicates that presence or absence of crystal plane dependence may be effected according to the difference of mechanism causing the residual stress.3) In addition, for plastically deformed specimens, compressive residual stress tends to approach a constant value at the center of the specimen. It is important to note that even in the iron specimen which consists of sole ferrite structure, residual compressive stress was observed in the core of the specimen.4) The plastic deformation under a uniaxial or multiaxial load stresses induces a uniaxial or a multiaxial residual stress system, respectively, of microscopic nature.Based on these results, the authors suppose at present that the cause of X-ray compressive residual stress of microscopic nature lies in the anisotropy of plastic deformation at room temperature, consequently, the diverse values of residual stress will be observed in different crystal planes.

In this study, the bonding reliability of the COG devices was studied. A finite element analyses model was established to study the curing process of COG module. The equivalent stress of the different locations of the package structure and the change of the temperature distribution with time were studied. The heat transfer process and the conductive particle deformation process were displayed through the simulation. The results show that the curing process is the heat transformation and particle deformation process. The residual stress generated by the temperature difference between the curing temperature and the operated temperature. The results show that the maximum residual stress is in the most distorted places of the conductive particles. The maximum residual thermal stress was studied with different bump pitch (35μm 30μm 25μm and 20μm) and the size of the particles (5μm, 4μm and 3.5μm). It shows that for a certain size of the particles, the maximum residual thermal stress will decrease when the bump pitch decreases. For a certain bump pitch, the maximum residual thermal stress will decrease when the size of the particles decrease.

The residual stress and hardness generated by the nitriding process are important parameters for increasing the bending fatigue strength to Ti-6Al-4V. Therefore, this research is focused on the analysis of residual stress and hardness, including surface morphology and microstructure generated by the nitriding process at different times and temperatures. The plasma nitriding at temperatures of 750 °C and 800 °C with times of 5 h and 10 h were selected in this research. After plasma nitriding, the material would have residual compressive stress and higher hardness, including changes in the surface morphology and microstructure. The results also indicated that higher temperature and processing times generated more surface roughness and thickness in the compound layer, resulting in higher surface hardness. Moreover, higher time and temperature could generate deeper residual compressive stress and case depth hardness. This research revealed maximum hardness in the cross-sectional analysis of 643 HV and residual compressive stress of −65.3 MPa. In conclusion, the depth of the residual stress and case depth hardness were well compatible with the depth of the diffusion layer of plasma-nitrided Ti-6Al-4V, which confirmed the effect of plasma nitriding.

High-temperature phosphate adhesives are widely used in the aerospace and nuclear power industries. However, complex residual stresses can result when the curing temperature parameters are unreasonable due to the brittleness of the adhesive. To reveal the curing temperature mechanism affecting the bonding strength of the phosphate adhesives, several curing temperature curves (CT-1~6) were designed for the single lap joint (SLJ) using phosphate adhesive. The residual stress helped to reveal the relationship between the curing temperature parameters and the bonding performance. In this process, the residual stress of the silicon carbide joint was measured using micro-Raman spectroscopy, and the tensile strength of the joint was tested. A cohesive zone model (CZM) was established with Abaqus® to verify the results, and the numerical results from the model agreed well with the experimental values. The residual stress and adhesive strength were obviously affected by curing temperature. The reasonable curing temperature curves have the benefits of reducing the residual stress and improving the bonding strength.

Ti2AlNb-based alloys are developed from conventional titanium aluminides (TiAl-based alloys and Ti3Al-based alloys). Comparing with TiAl and Ti3Al-based alloys, Ti2AlNb-based alloys have better mechanical properties of strength density ratio, high temperature fracture toughness and creep resistance. It is expected to replace traditional high temperature materials such as nickel-based alloys for the manufacture of aero-engine components. Laser shock peening (LSP) is an advanced surface modification technology which induces high amplitude, large depth residual compressive stress on the surface of materials. It can effectively enhance mechanical properties of material such as micro-hardness, fatigue resistance, high temperature oxidation performance. Compared to other surface modification technologies such as shot peening and rolling, LSP can not only induce deeper compress residual stress (1–2 mm), which is about five to ten times deeper than that of shot peening, but it can refine the grain or even form nanocrystals which plays a significant role in enhancing properties of material. For these reasons, this surface treatment technology has been widely used in the aircraft and nuclear energy industries. In this paper, the surface modification of Ti2AlNb alloy was carried out by laser shock peening, and the microstructure evolution, residual stress and the performance at high temperature were investigated. The results show that laser shock peening can significantly reduce the grain size of the surface of Ti2AlNb alloy. Dislocation lines, dislocation tangles and dislocation walls were generated after LSP. The micro-hardness is increased from 350 HV to 395 HV and the effect layer is about 1.2 mm. The residual compressive stress of −337 MPa is generated on the surface of the sample. In addition, residual stress is released obviously at high temperature. The residual stress on the surface of sample reduces to −65 MPa after heat treatment at 720°C for 4 h. The residual stress is almost completely released after heat treatment at 720°C for 10 h. However, the release rate of residual stress is slowed down at 600°C. The effect of laser shock peening is still presence after heat treatment for 100 h.

In-depth residual stress analysis considering manufacturing process and cyclic loading of bolts

TiN and TiCN thin films are used as antiwear layer on parts such as metal cutting tools. It is reported that the thin films that are deposited ceramics and metals have preferred orientation, and that the residual stresses generate by the difference in the coefficients of thermal expansion between the thin films and the substrate. Tools coated with TiN and TiCN films are heated to high temperature during service due to the frictional heat. Under this condition, the texture and the residual stress should change due to the heat. In the material engineering, it is well known that the residual stress in the material affects the mechanical strength of the industrial products. The purpose of this study is to examine the effect of the residual stress on the mechanical strength of TiN and TiCN thin films having preferred orientation.In this study, specimens that are deposited TiN and TiCN respectively by Physical Vapor Deposition (PVD) were annealed in a furnace at temperature of 573K, 798K, 843K and 893K. Using X-ray diffraction technique, the crystallite orientation was evaluated by the pole figure and the crystallite orientation distribution function (ODF). The films exhibited {111} fiber texture. After that, the residual stress of thin films was measured using CoKα radiation with the two-exposure method at Ψ=39° and 75°. Since the relationship between the mechanical strength and the residual stress has not been revealed about the thin film materials, the effect of the residual stress on the mechanical strength of the TiN and TiCN thin films having preferred orientation was investigated by the dynamic hardness (DH) and the scratch test.As a result, although the ODF little changed due to the heat treatment, the residual compressive stress relaxed at higher temperatures than 573K. As for the hardness of thin films, the residual stress did not affect the hardness. However, the residual stress influenced the behavior of the scratch test. The abrasive wear and the micro cracks were decreased by highly residual compressive stress.

A method for determining residual surface stresses is presented. For their determination, it is necessary to remove small layers from the part which has been bent by the residual stresses in the unrestrained state and to measure the resulting change in deflection and thickness of metal removed. Etching with a weak nitric acid and water solution was found to be a satisfactory method for removing surface metal without introducing additional stresses. Mild-steel bars were ground with a medium-soft grinding wheel in a surface grinder. The depth of surface layer containing residual stresses extended to approximately 0.012 in. to 0.018 in. below the surface for grinding cuts ranging in depth from 0.0003 in. to 0.003 in. The thickness of the layer containing residual stresses increases with increasing depth of grind. The maximum residual stress occurs on the surface and, for all depths of grinds, was considerably above the yield point of the original material. The maximum surface stresses when grinding mild steel within the foregoing range of depth of cut decreases with increasing depth. It is believed that this phenomenon can be explained by the possible partial recrystallization of the surface grains because of the higher surface temperatures obtained with heavy cuts.

Thermal residual stressesinduced by the bonding of composite patch repairs causeadverseeffectson the fatigue performance of patch repairs. The reduction of residual stresses was attempted by modifying the cure cycle of the adhesive, FM73M. An effective temperature drop D Teff was determined to quantify thermal residual stresses. Variouscurecyclesfortheadhesivewereinvestigated empirically to determinethe curetimeand curetemperature combination that can achieve a 100% degree of cure and the desired mechanical properties. An efe cient two-step curecyclewasdevelopedforbonding compositepatchestocracked aluminumplates. Theeffectofreduced residual stresson the fatiguelifeof cracked aluminum plates repaired with symmetrical compositepatcheswasinvestigated experimentally and numerically. I. Introduction L IKE other load-bearing structures, aging aircraft may contain damaged or cracked structural components resulting from fatigue andcorrosionduringservice,and thedegradedstructuralcomponents must be replaced or repaired to extend their service lives. Because of thesuperiorpropertiesofadvancede ber composites and the maturing adhesive bonding technology, composite patch repair isnowrecognizedasanefe cientandeconomicalrepairtechnology. 1 Because bonded repairs using high-performance composite materials offer high efe ciency and enhanced structural integrity, extensive works of composite patching on metallic aircraft components have been performed and reported. Examples include Hercules aircraft wings, Mirage wing skins, an Orion fuselage skin, an F-111 wing pivot e tting, a B-767 lower fuselage, and an MD-82 leading-edge slat that were repaired with boron/epoxy patches. 1;2 Because highperformance adhesives used in bonding require elevated cure temperatures, signie cant thermal residual stresses in the repair result from the mismatch of thermal expansion coefe cients between the composite patch and the cracked aluminum structure. The induced thermal residual stresses in the aluminum structure are tensile and tend to open the crack, thus inducing a substantial level of stress intensityatthecracktip.Evidently,theresidualstressescanreducethe effectiveness of the patch on the fatigue performance of the patched structure. 3 In general, thermal residual stresses in the metallic structure repaired with adhesively bonded composite patches can be reduced by lowering the bonding (curing) temperature. However, lowering the cure temperature of the adhesive can yield adverse effects on the degree of cure of the adhesive and, as a result, yield poorer mechanical propertiesofthe repair.Moreover,lowercure temperatures require longer cure time to achieve, if ever, the same degree of cure with a high cure temperature. In this study, an experimental investigation of various cure cycles for the adhesive was performed to e nd an efe cient cure time and cure temperature combination that can achieve a 100% cure and the desired mechanical properties. Atwo-step bonding cycle was selected toreduce thermalresidual stressesandtostudyitseffectivenessonthefatiguelifeofcomposite patch repairs. Fatigue lives of double-sided patch repairs fabricated with the bonding cycle recommended by the manufacturer (Cytec Industries, Inc. ) of FM73M and by the two-step cycle, respectively, were modeledandcompared.Theexperimental result indicated that lowering thermal residual stresses can be achieved by the two-step

In order to study (i) how the in-plane stress develops in alkoxide-derived oxide gel films on heating and (ii) how the residual stress changes with firing temperature, we prepared titania and silica gel films on Si(100) wafers from alkoxide solutions. The intrinsic stress was previously measured in situ during heating up to 500 °C for the gel films, where the stress was calculated from the radius of curvature of the substrate bending. Because the commercialized apparatus that measures the substrate bending has an upper limitation of temperature at 500 °C, we devised a method for estimating the intrinsic stress over 500 °C in the present work. The gel films were heated from 100 °C up to 900 or 1000 °C successively and cumulatively. Then we estimated the intrinsic stress over 500 °C by monitoring the changes in stress in situ during heating and cooling for the cumulatively fired films. Combining the estimated data over 500 °C with those measured below 500 °C, we found that the intrinsic stress increases and then decreases around to zero with temperature for both titania and silica gel films, which we recognized as a general trend in alkoxide-derived oxide thin films in conjunction with our previous work on yttria stabilized zirconia (YSZ) films. Regarding the residual stress, the titania and silica films exhibited different behaviors. The titania film showed an increase followed by a slight decrease, and finally a significant increase in residual tensile stress with increasing firing temperature. The silica films also exhibited an increase and a decrease in residual tensile stress with increasing firing temperature, but the stress continued to decrease and turned into compressive one at ca 800 °C. The last decrease in residual tensile stress that turned into compressive one resulted from the development of compressive thermal stress during cooling, which is due to the smaller thermal expansion coefficient of silica than silicon. It was thus clarified that the difference in thermal expansion coefficient between the film and the substrate becomes prominent in residual stress when the films are fired at high temperatures like 800 °C. Thus we could successfully obtain a general view on how the residual stress changes with firing temperature as well as on how the in-plane stress develops during heating thanks to the information on the stress development in the sol–gel-derived titania and silica films that was added to the information on the YSZ films.

Residual stresses in high strength steel welded box sections

This work focusses on thin carbon-based films, deposited by magnetron sputtering with additional argon ion bombardment (0 eV to 800 eV) without extra adhesive layer on hard metal inserts. As one possibility of increasing the reduced adherence of hard carbon films the deposition of films with additions of titanium and silicon is studied. The aim of this work is to examine the influence of a modification of the transition between substrate and film by realizing three different types of deposition gradients. The gradient in structure and properties is made by stepwise increasing ion energy during the deposition process. The continuously gradient in chemical composition is realized by using segmented targets, with a steady movement of the substrates from the carbide-rich half to the carbon-rich half during deposition. Additionally the continuous gradient in chemical composition is combined with the gradient in structure and properties. The ion flux to atom flux ratio is for all deposited films between 0,02 and 0,21 and is influenced by the ion energy. The pure carbon films are amorphous, the dominant network of atoms is formed by sp 2 bonded atoms. The amount of sp 3 bonded atoms is up to 30 % and is influenced by the bombarding argon ion energy. Carbon films with additions of silicon are amorphous, only in films with a high amount of titanium (approx. 20 at%) nanocomposites of titanium carbide crystals with diameters of less than 5 nm in an amorphous carbon matrix were found. The mechanical properties and the behavior of single layer carbon films strongly depend on the argon ion energy. An increase of this energy leads to higher film hardness and higher residual stress and results in the delamination of superhard carbon films on hard metal substrates. The adhesion of single layer films for ion energies of more than 200 eV is significantly improved by additions of titanium and silicon, respectively. The addition of 23 at% silicon and titanium, respectively leads to a high reduction of the residual stress. Different gradients in structure and properties by stepwise increase of argon energy are suitable for reducing the residual stress and to improve adhesion and hardness of the films. It is possible to deposit thick (9,3 μm) pure carbon films with a good adhesion (critical load of failure in scratch test: 31 N), extremely low residual stress (-0,7GPa), and very high hardness (5300 HV0,05). Especially for carbon/titanium carbide films these gradients in structure and properties lead to a significant improvement of the critical load of failure in the scratch test from 14 N to 39 N. In a non-reactive PVD process thin films were deposited with a continuously gradient in chemical composition. The results of the investigations of the films with two different concentrations of titanium and silicon, respectively show that carbon-based films with a good adhesion could be deposited. The combination of the two gradients in structure and properties and in chemical composition leads in the system with carbon and silicon carbide to hard and very adhesive films. Especially for carbon films with a high amount of silicon very low friction coefficients (0,08 to 0,05) were found by investigations in the pin-on-disc arrangement for unlubricated sliding friction against AISI 52100. These low friction values were found for single layer films and also for films deposited with a gradient. In contrast to the friction coefficients of single layer films and of graded films in the other systems (pure carbon and carbon/titanium) the small friction coefficient of the films in the system carbon/silicon does not rise for increasing ion energy.

Effect of Growth Rate and Wafering on Residual Stress of Diamond Wire Sawn Silicon Wafers

Shot peening is widely used for springs and other automobile parts in order to improve their fatigue strengths. In fact, their endurance limits are considerably raised and their fatigue durabilities are also noticeably increased by this treatment. Two causes are considered for these improvements: (1) work hardening at and near the surface and (2) residual compressive stresses produced in the surface layers. In contrast to these favourable effects, the indentations produced by shots are assumed to act unfavourably, exerting notch effects for the specimens. To investigate the contribution of residual stresses on the fatigue strength, their changes due to repeated stressing must, first of all, be made clear. This time, mild steel (S 40C) was used and the plate specimens with the thickness of 3.2mm were shot peened by the centrifugal type machine. Four stress amplitudes, two being below and the other two above the endurance limit, were employed in repeated stressing and the number of cycles were selected to be the same as much as possible for different stress amplitudes. Residual stresses were measured by the etching method as before, but this time, considering the aventurine surfaces, weight measurements were also employed in determining the depths of etch. According to other investigators, the favourable effects of shot peening are reduced considerably by heating. To examine this, shot peened specimens were heated at 100°, 200°, 300°, 400°C and 500°C for 1hr respectively and then air-cooled. Residual stress and Micro-Vickers hardness were measured on these specimens. The results obtained were as follows:(1) Residual stresses produced by shot peening are of the thermal stress type, being compressive at the surface and tensile in the core and the value of surface residual stress may reach a very high amount as compared to the yield strength of the material as the result of work hardening.(2) The fading of residual stresses is noticeable during the very early stage of repetition of cycles (about 2×104 cycles) and thenceforward proceeds gradually. This early fading is called the first stage of fading of residual stresses and the following one the second stage.(3) Surface residual stresses do not fade completely after the repetition of ten-million cycles. They have nearly 65% of the initial value under the stress amplitude of 70% of the endurance limit and 55% under the stress amplitude of 95% of the endurance limit.(4) The fading of surface residual stresses after the same number of cycles is larger for the higher stress amplitudes.(5) Plotting the ratio of surface residual stress σr/σ0, where σr is the surface residual stress after repeated stressing and σ0 that of the initial state, against the logarithm of cycle ratio n/N, nearly straight line relation was obtained in each stress amplitude. The gradients of these straight lines are larger for the higher stress amplitudes. Assuming that this gradient is proportional to the stress amplitude, the following empirical formulas concerning the fading of surface residual stresses in the second stage were obtained;For the stress amplitudes less than the endurance limitσr/σ0=1-0.0156σa-0.0035σa·log10n/N (σa<0.7σw)σr/σ0=1.088-0.02σa-(0.01+0.003σa)log10n/N (σw≥σa≥0.7σw)