Small Beam Specimens Research Articles

Effect of Material Property Difference on the Creep Behavior of Bending Specimen

The application of small specimen testing techniques in the evaluation of creep properties of materials in-service arise. In order to acquire the creep data accurately and conveniently, the bending test with small beam specimens has been proposed and validated for the metal materials. Initially, the fact that material behaves different creep rates under tension and compression is ignored for simplification. Thus, the effect of material property difference on the creep behavior of bending specimen is analyzed in the present paper. On the basis of Norton creep law, the deformation behavior of three type’s specimens under tension and compression is theoretically described. Assumed different creep exponents and constants, finite element models of these beam bending specimens are established. The creep response is simulated. Meanwhile, the effect of material property under different stress state is further investigated. The results show that the stress exponent has a significant effect on the creep curves. Usually, the stress exponent can be evaluated based on the displacement rate or strain rate. However, if large discrepancy of creep properties under tension and compression exits, it will yield disparate results for the steady-state stress exponent. It is suggested that the stress exponent determined solely by bending test should be accepted with a certain degree of reliability, especially for the non-metal materials.

Performance of CFRP-Strengthened Concrete Beams after Exposure to Wet/Dry Cycles

This paper reports on the effect of exposure to cycles of wetting and drying in fresh and salt water on the performance of carbon fiber-reinforced polymer (CFRP)-bonded concrete beams. The test program included 3, 8, and 12 months of exposure of normal and high-strength concrete specimens strengthened with one and two layers of CFRP reinforcement. The results indicated that the exposure caused a reduction in the tensile strength and modulus of elasticity of the epoxy adhesive. The ultimate loads of the small-beam specimens were not significantly affected by the exposure owing to the redistribution of shear stress along the CFRP plates. The degradation of the CFRP-concrete interface, however, decreased the bond strength and fracture energy of CFRP strengthening systems in the small-beam test, especially in the one-layer high-strength concrete configuration. Insights on bond characteristics and failure mechanisms of CFRP-strengthened concrete under the tested exposure conditions are presented.

Simple Approach to the Investigation of Asphalt Mixture Strength on Small Beam Specimens at Low Temperature

AbstractIn this research, a simplified approach based on size effect theory is used to extrapolate the strength values obtained on small asphalt mixtures specimens at low temperature with a modified bending beam rheometer (BBR). First, nine asphalt mixtures having different mix designs are prepared. Then, Weibull statistics and factorial design are used to evaluate the statistical distribution of the BBR measurements and the effect of conditioning time, loading rate, and testing temperature on material response, respectively. Finally, the BBR strength measured at three different temperatures is compared to the corresponding values obtained from two conventional test methods: direct tension (DT) and indirect tension (IDT) tests. It is observed that BBR strength values are statistically equivalent to those of IDT and DT when the size effect theory and the weakest link model are used to take into account the difference in volume, geometry, and stress field across the three testing methods.

Microstructural Analysis and Rheological Modeling of Asphalt Mixtures Containing Recycled Asphalt Materials

The use of recycled materials in pavement construction has seen, over the years, a significant increase closely associated with substantial economic and environmental benefits. During the past decades, many transportation agencies have evaluated the effect of adding Reclaimed Asphalt Pavement (RAP), and, more recently, Recycled Asphalt Shingles (RAS) on the performance of asphalt pavement, while limits were proposed on the amount of recycled materials which can be used. In this paper, the effect of adding RAP and RAS on the microstructural and low temperature properties of asphalt mixtures is investigated using digital image processing (DIP) and modeling of rheological data obtained with the Bending Beam Rheometer (BBR). Detailed information on the internal microstructure of asphalt mixtures is acquired based on digital images of small beam specimens and numerical estimations of spatial correlation functions. It is found that RAP increases the autocorrelation length (ACL) of the spatial distribution of aggregates, asphalt mastic and air voids phases, while an opposite trend is observed when RAS is included. Analogical and semi empirical models are used to back-calculate binder creep stiffness from mixture experimental data. Differences between back-calculated results and experimental data suggest limited or partial blending between new and aged binder.

Behavior of High Ductility Cement Composite Beams under Low Impact

This paper describes part of a study that examines possible material compositions aiming at increasing and controlling the ductility of cementitious composities under dynamic loading. The work described herein focuses on the influence of adding micro or macro scale steel fibers to the cementitious composite mix on the hardened specimen's mechanical properties. Small beam specimens made of 14 types of mixes were tested under static and dynamic loads, where the low-velocity dynamic loading was generated in a free-fall hammer. Variables that were examined included compression strength, single and hybrid usage of different types of steel fibers and the fibers volume ratios. The main conclusions indicate enhanced mechanical properties of the hybrid HSC specimens under dynamic loading. The results show that an enhanced material behavior, and therefore also structural response, can be engineered by designing different fiber reinforced cement composite mixes with similar compressive strengths.

Steel fibers as only reinforcement for flat slab construction – Experimental investigation and design

This paper presents the investigation on the bearing behavior of concrete flat slabs with steel fibers as only reinforcement. In a first step, experimental investigation on large-scale plates with symmetrical loading around the column is presented. The results show an absence of punching shear failure and give information on fiber distribution and orientation in steel fiber reinforced concrete (SFRC) elements with growing thickness. In general, a decreasing fiber orientation with an increasing plate height can be noticed. Furthermore, a size effect with lower tensile strength at cracked state for large scale elements in comparison to small beam specimens is shown. As bending failure occurred for all large-scale plate specimens, yield line theory is applied for both results discussion and the development of a design concept. Design abaci eventually show possible spans and slab thickness for satisfactory structural behavior at ultimate load state.

A Simple Methodology to Measure the Dynamic Flexural Strength of Brittle Materials

A simple methodology is proposed for measuring the dynamic flexural strength of brittle materials. The proposed technique is based on 1-point impact experimental setup with (unsupported) small beam specimens. All that is needed is a measurement of the prescribed velocity as a boundary condition and the fracture time for a failure criterion, both to be input in a numerical (FE) model to determine the flexural strength. The specimen was modeled numerically and observed to be essentially loaded in bending until its final inertial failure. The specimen’s geometry was optimized, noting that during the very first moments of the loading, the specimen length does not affect its overall response, so that it can be considered as infinite. The use of small beam specimens allow large scale testing of the flexural strength and comparison between static and dynamic loading configurations. Preliminary experiments are presented to illustrate the proposed approach.

Dynamic Properties of NiTi Shape Memory Alloy and Classic Structural Materials: A Comparative Analysis

Shape Memory Alloys (SMA) are smart materials that have attracted increasing attention due to their superior damping properties when compared to conventional structural materials. These functional materials exhibit high damping capacity during phase transformation as well as in the low temperature martensitic state. In this work NiTi SMA, commercial aluminum, stainless steel and brass were submitted to dynamic mechanical analysis (DMA) in a single cantilever mode. Small beam specimens were manufactured to accomplish the DMA tests. The studied NiTi presented a damping capacity peak during phase transformation, being much higher than damping of conventional materials. NiTi SMA also showed an increase of storage modulus after conversion of low temperature phase to high temperature phase while an almost linear decrease is observed for the conventional materials studied.

Analysis of crack propagation in nuclear graphite using three-point bending of sandwiched specimens

The aim of this paper was to assess the suitability of the sandwiched beam in three-point bending as a technique for determining fracture toughness and R-curve behaviour of nuclear graphite using small beam specimens. Surface displacements of the cracked beam specimen were measured using Electronic Speckle Pattern Interferometry (ESPI) and Image Correlation in order to accurately monitor crack propagation and frictional contact between the test specimen and the sandwiching beams. The results confirmed that solutions based on the simple beam theory could overestimate the fracture toughness of graphite. Finite element analysis using a Continuum Damage Mechanics failure model indicated that both friction and shape of the notch played an important part in providing resistance to crack growth. Inclusion of these factors and the use of more accurate load vs. crack length curves derived from the FE model would provide a satisfactory measure of fracture toughness in small beam specimens under such a loading configuration. The particular graphite tested, IG-110, showed a decrease in fracture toughness with increasing crack length.

Engineering properties of inorganic polymer concretes (IPCs)

This paper presents the engineering properties of inorganic polymer concretes (IPCs) with a compressive strength of 50 MPa. The study includes a determination of the modulus of elasticity, Poisson's ratio, compressive strength, and the splitting tensile strength and flexural strength of IPCs, formulated using three different sources of Class-F fly ash. Six IPC mix designs were adopted to evaluate the effects of the inclusion of coarse aggregates and granulated blast furnace slag into the mixes. A total of 90 cylindrical and 24 small beam specimens were investigated, and all tests were carried out pursuant to the relevant Australian Standards. Although some variability between the mixes was observed, the results show that, in most cases, the engineering properties of IPCs compare favorably to those predicted by the relevant Australian Standards for concrete mixtures.

A method for dynamic fracture toughness determination using short beams

This paper deals with dynamic fracture toughness testing of small beam specimens. The need for testing such specimens is often dictated by the characteristic dimensions of the end product. We present a new methodology which combines experimentally determined loads and fracture time, together with a numerical model of the specimen. Calculations are kept to a minimum by virtue of the linearity of the problem. The evolution of the stress intensity factor (SIF) is obtained by convolving the applied load with the calculated specimen response to unit impulse force. The fracture toughness is defined as the value of the SIF at fracture time. The numerical model is first tested by comparing numerical and analytical solutions (Kishimoto et al., 1990) of the impact loaded beam. One point impact experiments were carried out on of commercial tungsten base heavy alloy specimens. The robustness of the method is demonstrated by comparing directly measured stress intensity factors with the results of the hybrid experimental-numerical calculation. The method is simple to implement, computationally inexpensive, and allows testing of large sample sizes, without restriction on the specimen geometry and type of loading.

Improved Impulse-Frequency Response Techniques for Measurement of Dynamic Mechanical Properties of Composite Materials

Abstract The dynamic mechanical properties of composite material specimens are rapidly determined by using two new computer-aided impulse techniques. Small beam specimens are excited in either flexural or extensional vibration by an electromagnetic hammer with a force transducer in its tip, while specimen response is measured with an eddy current probe or accelerometer. A desktop computer/Fast Fourier Transform analyzer system is then used for rapid data acquisition and computation of the complex modulus by curve-fitting to the frequency response function.

Acoustic emission from irradiated nuclear graphite

Abstract Measurements of acoustic emission (AE) from a range of four unirradiated nuclear graphites during three-point bend tests are reported. Results are in agreement with the trends found in earlier work using different AE apparatus. The technique is applied to the testing of small beam specimens cut from irradiated Civil Advanced Gas-cooled Reactor (CAGR) graphite fuel sleeves after discharge from the reactor. The AE information is explained by considering separately the known changes in graphite microstructure that occur in the reactor due to radiolytic oxidation and fast neutron irradiation. Coarsening of the material due to radiolytic oxidation increases the total number of AE events and the proportion of events of low amplitude. Fast neutron irradiation increases the fracture stress and makes the stress-strain curve more linear. As a consequence, the number of AE events is reduced along with the proportion of events of low amplitude.