強度的不均質をもつ溶接継手の動的変形・破壊特性に関する研究 ２ 動的負荷を受ける軟質継手の強度および破壊特性

Recently, research on the impact resistance of transparent hybrid glass-polymer systems (layered and particle-matrix systems) has been conducted at the Netherlands Organisation for Applied Scientific Research (TNO), showing the potential of these materials for the application in protection concepts. A soft, transparent polyurethane elastomeric polymer, Clear Flex 75 (CF 75 in short) was selected as matrix material. Knowledge about the impact resistance of particle(s)-polymer systems is also relevant for propellants, consisting of a polymer binder and solid energetic particles. The impact-induced damage will change the combustion properties and the performances of the propellant. Against this background, a combined research programme of experimental tests and computational modelling was defined on the mechanical response of hybrid particle(s)-polymer systems under dynamic loading. In this programme, CF 75 polymer is the matrix material, which is embedded with single and multiple particles. In the computational part, multi-scale modelling techniques for dynamic conditions are developed. In the experimental part, a step-wise approach with increasing complexity is conducted for studying the dynamic response in compression and tension of (i) a monolithic polymer matrix, (ii) a single-particle-polymer system and (iii) a multiple-particles-polymer system. The used apparatus is a small scale split Hopkinson bar (SHB) device developed at TU Delft, which can conduct the dynamic compression (SHPB) and tension (SHTB) tests at various strain rates. The experimental results are important for (i) the evaluation of a transparent impact-resistant device of a soft polymer material backed by rigid glasses, (ii) the development of a particle(s)-polymer system for propellants and (iii) the calibration of computational models for designing a rigid-particle-modified composite material system for protective applications. Firstly, the experimental setups were built for high-strain-rate tension (SHTB) and compression (SHPB) tests. These two setups were carefully evaluated and proved to be reliable for dynamic tests on a soft polymer material at different strain rate levels. The details on the technology for casting the specimens of the monolithic CF 75 polymer material, the single-glass-polymer system and the multiple-particles-polymer system were given, as well as the basic physical properties of CF 75 polymer material. Afterwards, the dynamic tension and compression experiments were conducted at various strain rate levels. During these tests, a high-speed camera was employed to record the dynamic loading process and a scanning electron microscopy (SEM) was used to carry out the post-test analysis. From the dynamic tension tests, the damage, fracture and toughness mechanisms of the monolithic CF 75 polymer material were revealed. The rate dependency of the mechanical properties and dynamic deformation behaviour were characterized. Then, the bonding of the glass-polymer interface was also characterized by dynamic tension loading on a single-glass-polymer system of the CF 75 polymer matrix with an embedded single 3mm-diameter glass particle. Debonding, cavitation and necking were illustrated, as well as the effects on the stress-strain response at various strain rates. Dynamic compression tests were carried out to characterize the impact-resistant performance of the monolithic CF 75 polymer material. They revealed the deformation behaviour, damage mechanisms, fracture characteristics, rate dependency of mechanical properties and temperature rise. Then, the single-glass-polymer system was tested by dynamic compression tests. The results clarify the particle effect on the dynamic compressive response. The debonding-induced decrease of yield stress and the contribution of glass particle failure to fracture energy are quantitatively characterized at various strain rates. Finally, the impact-resistant performance of the multiple-particles-polymer systems of CF 75 polymer matrix with embedded 25wt.% and 50wt.% 0.5mm-diameter PMMA particles was characterized by dynamic compression tests. For the multiple-particles-polymer systems, an improved static stiffness is explained. Multiple cracking from the particle-polymer interface is observed, which induces the decrease of yield stress. A high particle density results in the decrease of maximum stress and strain energy. However, crushing of the particles causes a faster increase of maximum stress and strain energy with increasing strain rate (indicated by a higher strain rate sensitivity index). In this thesis, a systematical investigation about the dynamic mechanical response of soft polymers was carried out. It includes the monolithic polymer material (as matrix material in the particle(s)-polymer system), the single-glass-polymer system (for studying the interfacial bonding and single particle effect), and the multiple-particles-polymer system (as a representative of the real particle(s)-polymer system for impact-resistant applications).

Deformational and breakage behaviors of concrete and cement mortar greatly influence various engineering structures, such as dams, river bridges, ports, tunnels, and offshore rig platforms. The mechanical and petrophysical properties are very sensitive to water content and are controlled by the liquid part in pore spaces to a large extent. The objective of this paper is to investigate the water saturation effect on the strength characteristics and deformability of cement mortar under two loading conditions, static and dynamic compression. A set of cement mortar samples was prepared and tested to study the mechanical behavior in dry and saturated states. The first part of the research incorporates the study of static mechanical properties for dry and brine-saturated cement mortar through uniaxial compressive strength tests (UCS). Second, drop-weight impact experiments were carried out to study the dynamic mechanical properties (impact resistance, deformation pattern, and fracture geometry) for dry and saturated cases. The comparative analysis revealed that water saturation caused substantial changes in compressive strength and other mechanical characteristics. Under static loading, water saturation caused a reduction in strength of 36%, and cement mortar tended to behave in a more ductile manner as compared to dry samples. On the contrary, under dynamic loading conditions, water saturation resulted in higher impact resistance and fracture toughness as compared to dry conditions. In addition, fractures could propagate to smaller depths as compared to dry case. The study will help resolve many civil, mining, and petroleum engineering problems where cement structures undergo static as well as dynamic compression, especially in a hydraulic environment where these structures interact with the water frequently. To the best of our knowledge, the effect of water saturation on the dynamic mechanical properties of cement mortar has not been well understood and reported in the literature.

The microstructures and mechanical properties of hypoeutectoid U- Nb alloy laser welded joint were investigated by optical microscopy(OM), X- ray diffractometer(XRD), transmission electron microscopy(TEM), split hopkinson pressure bar(SHPB) and other analysis apparatus. The results show that the microstructure of hypoeutectoid U-Nb alloy base metal is α-U+γ-U lamellar pearlite under isothermal heat treatment, while the laser welding seam is composed of α' lath martensite for pre-heated or α' twin martensite for no pre-heated with orthogonal crystal structure. The quasi-static tensile strength of welded joint(about 400 MPa) is much less than base metal and microstructures of weld, for the main reason of incomplete penetration weld and low fracture toughness. Between dynamic impact loading for base and welded joint, the strain rate of welded joint is lower than base metal, and the yield strength of welded joint is higher. Also, the compressive stress-strain curves indicated that the flow stresses for welded joint increased with the increase of strain rate and the obvious effect of strain rate hardening has been observed. At strain rate of 2000 s-1, selected plastic deformation taking place in welded joint is due to the tremendous difference mechanic properties between weld seam and base metal, and the adiabatic shear band(ASB) only appears in the rest of welded joint.

Some resin polymers available for three-dimensional (3D) printing are slightly elastic, which may be advantageous when used for full crown coverage of the primary teeth. This study was performed to evaluate the mechanical properties of two types of 3D-printed resin crowns in terms of strength and surface characteristics. Polymer resins used for temporary crowns (TCs) and temporary flexible dentures (TFDs) were tested. Digitally designed crowns with different thicknesses (0.4 and 0.6 mm) were 3D-printed. Milled zirconia crowns were used as the control. The static and dynamic fracture loads of the crowns were measured. The crown surface was evaluated using scanning electron microscopy. The average strength did not differ between the types of crowns. The differences between the dynamic and static fracture loads were insignificant. In the TC group, thicker crowns showed lower strength both under static and dynamic loads. After thermomechanical loading, microcracks and dropouts of macrofillers were detected on the surface of all types of resin crowns. The deposition of abraded debris occurred more in the TFD group. The 3D-printed resin crowns were thought to endure biting forces in children. However, some limitations of the material itself should be improved for consideration as a new treatment option in pediatric dentistry.

Due to the influence of the welding thermal cycle, the toughness of structural steel generally degenerates. Recently, the intercritically reheated coarse-grained heat-affected zone (IC CG HAZ) was found to demonstrate the worst toughness in welded joint, which was associated with its fracture mechanism. In this article, two IC CG HAZs of a structural steel were prepared by welding thermal-cycle simulation techniques. For the two IC CG HAZs, the static and dynamic fracture toughness were evaluated; the fracture mechanism was also studied. Under both static and dynamic loading, cracks in the IC CG HAZ were found to initiate at the intersection of bainitic ferrite α /0 packets with different orientations, followed by propagation in cleavage. In some crack propagation regions, adjacent cleavage facets are connected by shear, thus producing dimple zones. Though the brittle fracture initiation mechanism remains unchanged, the cleavage facet size, the proportion of the dimple zones between facets, and the distance from the cracking initiation site to the crack tip vary with loading speed and welding conditions. These changes were found to be related to the variations caused by strain rate and welding conditions in fracture toughness for the IC CG HAZs.

The response of metallic foams to a high strain or high stress rate loading has received increased attention in recent years due to their potential to absorb large amounts of energy during plastic deformation and crushing (see, e.g., Thornton and Magee [1]). Research of the mitigation of blast effects indicates that the high-energy absorption characteristic of metallic foams makes them very useful as protective layers of critical structural elements. Consequently, understanding the material dynamic properties of metallic foams will enable engineers to better utilize their energy absorption characteristics. Aluminum foam (Al-foam) is a lightweight material with excellent plastic energy absorbing characteristics [2]. The implementation of bare Al-foam as a protective layer is not practical. The material needs to be a part of a multilayer structure (see, e.g., Seitzberger et al. [3]. The foam layer can be exploited as a protective layer in military vehicles where both lightweight and good energy absorption are needed. The ability of aluminum foams to reduce the explosion-generated blast-induced damage from concrete slabs has been demonstrated in the course of high-explosive (HE) field experiments (see, e.g., Hanssen et al. [4, 5] and Sadot et al. [6]). Several studies have been conducted in order to investigate the constitutive model parameters of Al-foams during the past three decades. The effect of the strain rate was one of the important issues needed to be resolved [7]. A constitutive numerical model was validated in the work of Hanssen et al. [4, 5]. The validation procedure was based on three levels: (a) material calibration; (b) non-uniformed compression test at the material level; and (c) numerical validation at the structural interaction level. Several material models from the LS-DYNA library were calibrated. However, discrepancies between the models were found even for relatively simple load configurations. The most important conclusion noted by the authors was the need for further development of more robust fracture models for the Al-foam. This conclusion is crucial especially since there are increasing numbers of Al-foam manufacturers. Various experimental facilities were used to dynamically load the Al-foam at large ranges of strain and stress rates. In the work of Dannemann and Lankford Jr. [8], closed-cell Al-foams were assessed under static and dynamic loads in the strain rate range of 400–2500 s−1. This range was achieved by using split Hopkinson bar apparatuses. It was found that the strain rate effect is significant in high density Al-foams. Deshpande and Fleck [9] suggested that the initial elastic modulus was lower than that of fully dense alloys. Deformation in the cell walls led to stress concentration around the deformation zones, which resulted in a decrease of the modulus. Some inconclusive results regarding the dependence of the stress–strain curve on the strain rate were presented. Deshpande and Fleck [9] and Paul and Ramamurty [10] did not notice any strain rate dependency, in contrast to the findings of Dannemann and Lankford [8] and Paul and Ramamurty [10]. In a later work by Wang et al. [11], experiments were done using an Instron compression machine at strain rates ranging from 10−3 s−1 up to 450 s−1. Strain–stress curves constructed and distinct strain rate dependency was noted. In the work of Bastawros et al. [12] efforts were made to understand the morphology of the Al-foam during its collapse. Explanation was given to the cell deformation. However, some observations have been ignored and not fully explained even though some key elements of the deformation were identified. The dynamic behavior of Hydro/Cymat Al-foam material was investigated by Tan et al. [13] under different load conditions. The plastic collapse, the plateau range, and the strain at which the deformation occurred were found. It was demonstrated that the dynamic response depends on the direction of the load with respect to the plate manufacturing orientation. Some load enhancement was observed and was explained by micro-inertial effects. Postimpact observation of partly crushed specimens revealed that the deformation is through crush bends. Feng et al. [14] conducted experiments to investigate the rate dependence of Al-foams having different relative densities. They found that the effect of strain rate increases while increasing the Al-foam density as was found by others. For Al-foams with a relative density of 15 % there was little effect of the strain rate while for heaver foams significant strain rate effect was noted.

Materials and structures are commonly subjected to dynamic loading. The behavior of materials and structures under dynamic loading, however, can be significantly different from that under static loading. This is especially true for so-called strain rate sensitive materials, such as polymeric materials and fiber reinforced polymer-matrix composite materials. Based on a previous study, this paper continues on the characterization of low strain rate effects, such as those up to 100/s, on glass-fiber-reinforced polymer-matrix composite materials. Testing techniques based on slip Hopkinson's pressure bar have been constantly used for investigation of strain rate effects. However, because of the low strain rate range, the commonly used slip Hopkinson's pressure bar should become over-qualified. On the other hand, the commonly used drop-weight impact testers have been found to be useful for low strain rate characterizations. The constitutive relations of the composite material at low strain rates are presented in this study based on the testing technique for lower strain rates. Besides, the effect of fiber orientation on the strain rate effects is also discussed.

Soft ferromagnetic alloys are often utilized in electromagnetic applications due to their desirable magnetic properties. In support of these applications, the ferromagnetic alloys are also required to bear mechanical load under various loading and environmental conditions. In this study, a Fe–49Co–2V alloy was dynamically characterized in tension with a Kolsky tension bar and a Drop–Hopkinson bar at various strain rates and temperatures. Dynamic tensile stress–strain curves of the Fe–49Co–2V alloy were obtained at strain rates ranging from 40 to 230 s−1 and temperatures from − 100 to 100 °C. All dynamic tensile stress–strain curves exhibited an initial linear elastic response to an upper yield followed by Luders band response and then a nearly linear work-hardening behavior. The yield strength of this material was found to be sensitive to both strain rate and temperature, whereas the hardening rate was independent of strain rate or temperature. The Fe–49Co–2V alloy exhibited a feature of brittle fracture in tension under dynamic loading with no necking being observed.

Soft ferromagnetic alloys are often utilized in electromagnetic applications due to their desirable magnetic properties. In support of these applications, the ferromagnetic alloys are also desired to bear mechanical load at various environmental temperatures. In this study, a Permendur 2V alloy manufactured by Metalwerks Inc. (but referred to Hiperco 50A, a trademark of Carpenter Technologies Inc.) was dynamically characterized in tension with a Kolsky tension bar and a Dropkinson bar at various strain rates and temperatures. Dynamic tensile stress-strain curves of the Hiperco 50A alloy were obtained at the strain rates ranging from 40 to 230 s -1 and temperatures from -100 to 100degC. All tensile stress-strain curves exhibited an initial linear elastic response to an upper yield followed by a Eiders banding response and then a nearly linear work-hardening behavior. The yield strength of this material was found to be sensitive to both strain rate and temperature; whereas, the hardening rate was independent of strain rate or temperature. The Hiperco 50A alloy exhibited a feature of brittle fracture in tension under dynamic loading with no necking being observed. ACKNOWLEDGEMENTS The authors acknowledge Kyle Johnson, Jefferey Dab ling, Donald Susan, Jay Carroll, Adam Brink, Scott Grutzik, and Andrew Kustas for the valuable discussion of test plan and results. Thanks Donald Susan for specimen preparation for this project. The authors also thanks Randy Everett for his support to the operation of dynamic tests in this proj ect.

Dynamic ultimate strength analysis of stiffened plate based on Idealized Structural Unit Method

Introduction. Modern wooden structures are mostly connected by mechanical working joints. To increase the reliability of nodal joints, various kinds of washers can be pressed, inserted, or glued into the wood of the elements, thus ensuring the transmission of forces from one element to another. Joints on glued washers can transfer significant forces on a relatively small area of mutual contact, which is due to their high loadbearing capacity. Thus, the gluing of steel washers in places of increased stress concentration during force transfer ensures a significant redistribution of buckling/cracking stresses over a larger area of the connected parts. However, steel washers are highly corrosive, so additional measures are required to protect metal parts from corrosion or to replace the material with the composites. The results of full-scale tests of the samples with glued-in plastic washers are used to consider the ways of increasing the strength and deformation characteristics of the plastic washer material by applying additive technologies.Aim. To increase the load-bearing capacity of the wooden structure joints by increasing the strength and deformation characteristics of the glued washer material.Materials and methods. A method for full-scale tests of wooden specimens with glued fiberglass washers is presented. The wooden elements are made of second grade pine, while the washers are of REC Formax and REC Friction plastics. The test was performed in compression along the fibers, with control of vertical shear deformations. The methods of increasing the strength and deformation characteristics of plastic washers by using additive technologies are considered.Results. Using the data of full-scale tests the diagrams of sample deformations on glued plastic washers are drawn, the obtained results are analyzed. The sufficiently high plasticity of washer materials is established. The ways for increasing the strength and deformation characteristics of plastic washers, particularly by reinforcing the plastic washers is suggested.Conclusions. In order to increase the load-bearing capacity of the joints on the glued fiberglass washers, the reinforcement of plastic using additive CFC printing technologies and the use of different reinforcing fiber materials is adopted.

To reduce fuel consumption and greenhouse gas emission, dual phase (DP) steels have been considered for automotive applications due to their higher tensile strength, better initial work hardening along with larger elongation compared to conventional grade of steels. In such applications, which would create potential safety and reliability issues under dynamic loading, the mechanical behavior of DP steel considering the strain rate must be examined. In the present study, the dynamic tensile behavior of DP600 steel sheets was investigated using a high-speed tensile testing machine at various strain rates. And the quasi-static tensile testing was also conducted on the steel to understand the effect of the strain rate on the tensile property. The fracture mechanisms of the steel were also analyzed. The results show that the mechanical properties of DP600 steel are noticeably influenced by the strain rates. As the strain rate increases, the strength of the steel increases and the obvious yield phenomenon can be observed when the strain rate is above 0.01 s-1. The fracture elongation of DP600 steels decreases with increasing strain rate from 0.001 to 1 s-1, then increases up to the strain rate of 100 s-1 and reaches the lowest value at the strain rate of 1000 s-1. DP600 steel sheet exhibit typical ductile fracture characteristics with dimples morphology of the facture surface when tensile deformed at various strain rates.

The effects of rate of strain on strength and deformation characteristics of soil–lime were investigated. Five strain rates (0.1, 0.8, 2.0, 4.0 and 7.0 %/min), five lime contents (0, 3, 6, 9 and 12 %) by dry soil weight and three cell pressures (100, 200 and 340 kN/m2) were carried. Triaxial tests, under unconsolidated condition, were used to study the effect of strain rate on strength and initial modulus of elasticity of soil and soil–lime mixture after two curing periods 7 and 21 days, respectively. A total of 405 triaxial specimens have been tested, where 225 specimens have been tested with first curing period (7 days). The testing program includes nine specimens for each strain rate, and each lime content was carried out, including natural soil with zero lime content. Another set of triaxial tests with a total of 180 specimens for the second curing period (21 days) was prepared at optimum moisture content, and the corresponding maximum dry density was also tested. The effects of strain rate and curing period on each of stress–strain behavior, type of failure, deviator stress at failure, cohesion and angle of internal friction and initial modulus of elasticity were studied thoroughly for the natural soil as well as soil–lime mixtures. For natural soil, the test results showed that the undrained shear strength, the initial modulus of elasticity and the cohesion increase significantly as the strain rate increase, while for soil–lime mixture at different curing periods, the undrained shear strength, initial modulus of elasticity and the cohesion increases to a maximum and then decreases as the strain rate and lime content increase. Also, the same variables and angle of internal friction increase with increasing curing period.

Worldwide, there is a growing level of interest to develop sustainable cement-based products and processes in which the usage of natural resources such as sand and limestone are reduced from the current levels. One of the ways to achieve this is by replacing them with suitable inclusions of recycled granular materials from municipal wastes where possible. However, to understand the effects of such inclusions in concrete structures, research advancements are needed to sense and characterise the distribution of stresses (/strains) at the local scale and to establish their links with the fracture and bulk strength characteristics under external loading environments, which is the focus here. In this research, polyethylene (PE)-based granular materials derived from municipal wastes and fly ash obtained from the incineration of municipal solid wastes are used together as secondary raw materials in preparing the concrete mixtures. Photo stress analysis (PSA) is performed here, making non-contact and whole-field digital measurements of maximum shear stress distribution and the directions of the principal stresses at any point of interest on the surface of the samples under external loading. Their links with the fracture toughness and flexural strength of the samples cured at different times are presented. The novel PSA-based stresssensing helps to establish new understandings of the strength characteristics of composites across scales in the applications involving recycling and reusing conventional wastes and possibly in otherengineering applications in the future.

When measuring the compressive strength of rock, size and strain rate are the two main influencing factors. To study the rock strength size effect, rock specimens with length-to-diameter ratios of 0.5, 0.6, 0.7, 0.8, 0.9 and 1 were subjected to static loading tests using the RMT rock mechanics test system and dynamic loading with the split Hopkinson pressure bar, respectively. Based on the Weibull size-effect formula, the experimental results were compared with the improved formula obtained. The results show that rock strength is influenced by size and strain rate. Both the dynamic increase factor and rock strength are proportional to strain rate. The different failure modes of rock with size variation and strain rate variation are described according to the failure process of the specimens. The same length-to-diameter ratio specimens produced more fragments with a strain rate increase. Under the same strain rate of impact, the larger the rock specimen, the finer the broken fragments. Considering the factor of strain rate in the Weibull size-effect formula, the calculated result is accurate. The improved size-effect formula could be used to better elaborate the potential mechanisms of dynamic rock strength. In the unified theoretical formula containing static and dynamic loads, the relationship of rock strength, size and strain rate is well described.