Pendulum Hammer Research Articles

Evaluation of the compressive strength of autoclaved aerated concrete in existing masonry

AbstractSclerometric method, commonly used in reinforced concrete structures, have also found application in masonry structures, mainly in the diagnosis of brick masonry walls or testing mortar in joints. This article presents a proposal for determining the compressive strength of autoclaved aerated concrete masonry elements with the use of the Schmidt OS‐120 PT pendulum hammer. The test was carried out on masonry elements with a nominal density class of 400 and 600. Additional tests were also carried out on samples with different levels of humidity, in order to consider the influence of humidity on the change in rebound number and changes in compressive strength.

Evaluation of non-destructive techniques for mechanical characterisation of earth-based mortars in masonry joints

In this paper, the use of non-destructive tests for the mechanical characterisation of earth-based mortars in masonry joints is discussed. Four testing methods, namely the penetrometer, Schmidt hammer, pendulum hammer and scratch test, originally developed for other types of mortar, are reviewed. The methods are applied to the earth-based mortars at the Wupatki Pueblo archaeological site, in Arizona, US. The outcomes of the experimental programme allowed to assess the reliability of the methods and to identify their limitations. Finally, the methods are compared in terms of six qualitative indicators, namely easy-of-use, consistency of results, range and granularity of results, respect towards cultural value, depth of investigation under the visible surface and versatility in application. Overall, the penetrometer test is recommended as the preferable method to characterise the mechanical performance of earth-based mortars.

Multifunctional architected MWCNTs/PDMS composites with high sensing and energy absorption capability inspired by ant tentacle and pomelo peel

The multifunctional features of pressure sensitivity and energy-absorption capability of flexible electromechanical sensors are desirable for practical applications, such as personal protection, crash mitigation, and protective packaging of sensitive elements. However, there are still challenges in developing ultrasensitive pressure sensors with high energy absorption capability. Herein, a bio-inspired flexible piezoresistive MWCNTs/PDMS composite with a multilayered architecture mimicking the ant tentacle and the pomelo peel is innovatively constructed, which can simultaneously detect physical stimuli in an extensive pressure range and absorb impact energy. The composite architecture is composed of the bottom-layer interlocked tentacle-like conical micropillars and the top-layer pomelo peel-inspired hierarchically enclosed-porous microstructure, which has successfully wrapped air inside to form a solid-gas dual phase structure with enhanced energy absorbing capability and piezoresistivity. The composite architecture was manufactured by a laser-engraving method associated with the controlled solidification process of the composite by regulating the vacuum pumping rate in the pre-curing condition. Owing to the hollow-shaped tentacle-like conical micropillars, the contact area can be dramatically increased within a subtle pressure loading, leading to superb pressure sensitivity of ∼26.1 kPa−1. Simultaneously, the hierarchically enclosed-porous structure with trapped air (gas-phase), acting as an air cushion packaging, imitating the pomelo peel provides the composite architecture with exceptional mechanical energy absorption of ∼2.74 MJ/m3 and an extensive pressure sensing range from 0.1 Pa to 60 kPa. As a result, the as-proposed composite sensor was able to detect mechanical signals such human finger tapping, wrist flexion, and pendulum hammer impact. Numerical simulations also showed that the wrapped air inside the hierarchical enclosed-porous structure improved the mechanical damping properties of the composite architecture. It is envisioned that our finding could contribute to the development of multifunctional materials to meet the growing demands for next-generation electronic systems, inspired by sophisticated architectures from nature.

The influence of adhesive material properties on the impact strength of adhesive block connections

Adhesive connections are becoming increasingly popular in the construction of aircraft and other means of transport. Today, bonding is mainly used in the construction of helicopter fuselages, wings or lifting rotors. The increased popularity of bonding is forcing designers to seek new and improve research methods as well as enhancing the existing test methods which specify the static, fatigue or impact strength of joints. A variety of tests are used to determine the strength of structures, although new ones are constantly being sought so as to be applied more quickly and without specialised equipment. Current testing standards are also being modified in order to speed up and simplify the testing process, resulting in safer structures that use adhesive bonds. The aim of the research presented in this paper was to test whether there is a relationship between the mechanical properties of adhesive materials and the impact strength of adhesive block joints with a cylindrical top element. Steels S235, Raex 400 and PA6 aluminium alloy were used for the manufacture of the samples. From each material, 10 samples were prepared with upper elements of different diameters, namely: 17.8 mm, 12.6 mm and 8.9 mm. A pendulum hammer was used to determine the strength of the adhesive bond against dynamic load application. For the sake of the research, the authors used a modified PN-EN ISO 9653 with a mounted hammer equal to the maximum energy of 15 J. Lower failure energy was characteristic of samples made from material with a lower value of Young's modulus (dural) and from steel with a lower yield strength. The connection failure energy grew with increasing the joint area, which was approximately parabolic.

A Comparative Study on the Temperature Effect of Solid Birch Wood and Solid Beech Wood under Impact Loading.

In order to use wood for structural and load-bearing purposes in mechanical engineering, basic information on the impact behaviour of the material over a wide temperature range is needed. Diffuse porous hardwoods such as solid birch wood (Betula pendula) and solid beech wood (Fagus sylvatica) are particularly suited for the production of engineered wood products (EWPs) such as laminated veneer lumber (LVL) or plywood due to their processability in a veneer peeling process. In the frame of this study, solid birch wood and solid beech wood samples (300 × 20 × 20 mm3) were characterised by means of an impact pendulum test setup (working capacity of 150 J) at five temperature levels, ranging from −30 °C to +90 °C. The pendulum hammer (mass = 15 kg) was equipped with an acceleration sensor in order to obtain the acceleration pulse and deceleration force besides the impact bending energy. In both solid birch wood and solid beech wood, the deceleration forces were highest at temperatures at and below zero. While the average impact bending energy for solid birch wood remained almost constant over the whole considered temperature range, it was far less stable and prone to higher scattering for solid beech wood.

Analysis of the Effect of wind Loads on the dynamic response of the Suspended Crossing-Tower Auxiliary System

The operational impact analysis of Crossing-tower Auxiliary Fittings System is a performance impact analysis of the operational stability and performance indicators of the line and auxiliary towers under a variety of environmental conditions for the long-term operation of the auxiliary crossing-tower system. In this paper, the suspension type auxiliary tower fitting system is divided into two types: the hard rod connection and the soft line connection. The influence of the arrangement of the wind pendulum, ice coating and anti-vibration hammer on the mechanical performance is considered. At the same time, the influences of the length of the bridge, the span and sag of the ground wire on the displacement and stress-strain of the bridge over the tower are analyzed. On this basis, it lays a certain theoretical basis for the structural optimization of the suspended tower auxiliary system.

Elastic softening of sandstone due to a wideband acoustic pulse observed by dynamic acousto-elastic testing

Laboratory measurements of finite-amplitude, wideband elastic pulse propagation in a thin bar of Texas moss sandstone have been reported [Muir et al., JASA 147 (2020)] that utilized pulses generated by impact excitation with a pendulum hammer at one end of the bar. Each pulse is essentially unipolar in strain, with bandwidth of approximately 3 kHz and amplitude between 10 and 130 microstrain. Reduction of the sandstone elastic modulus by up to 17% resulted from propagation of the impact-generated pulse, as calculated from time-of-flight measurements of the pulse as it reflected between the ends of the bar. In the present work, elastic softening is observed locally using a Dynamic Acousto-Elastic Testing (DAET) configuration. During propagation of an impact-generated pulse, changes in the sandstone elastic properties are observed by tracking variations in the arrival time and amplitude of a continuous-wave 1 MHz P-wave signal that propagates perpendicular to the bar axis. Repetitive reflection of the pulse from the stress-free ends of the bar enables separate observation of effects from compressive and tensile strains. Reduction of the elastic modulus of up to 20% is observed, with almost all of the softening occurring during the tensile phase of the impact-generated pulse. a)Deceased.

Modified pendulum hammer in impact tests of adhesive, riveted and hybrid lap joints

This paper presents a modified pendulum hammer adapted to test the impact strength of lap joints. The impact tests of lap specimens exhibited greater repeatability in terms of the results, compared with the tests of block specimens (specimens used in Block Shear Test (ISO 9653)). The modified testing device also allowed for testing of rivet and hybrid rivet–adhesive joints. The comparative tests showed that the impact strength of the hybrid connections was greater than the sum of the impact strengths of the adhesive and rivet joints. Our research demonstrated the significant effect of the impact speed on the measured impact strength; this effect appeared to be related to the deformation energies of the test device and connected elements, which were more deformed at a lower load speed. We also showed a serious influence of adhesive forces on the impact strength of glued joints.

Influence of Local Frequent Dynamic Disturbance on Micro-structure Evolution of Coal-Rock and Localization Effect

This study aimed to investigate the influence of local frequent dynamic disturbance on micro-structure evolution in different zones of coal-rock. To do so, we carried out a systematic experimental research on the micro-structure evolution of briquette and raw coal samples under local impact load by using self-developed pendulum hammer dynamic impact loading test device of coal-rock and ultrasonic testing equipment, and analyzed the localization effect of local impact load. The results show that Mn (micro-structure cumulative change factor) of briquette coal samples presents an inclined M-shaped four-stage evolution mode along and perpendicular to impact direction with cyclic impact times under conventional full impact load, whereas it shows more obvious anisotropy and localization under local impact load. Mn for both conventional full impact and local impact shows a nonlinear increasing trend with the increase in impulse, but their increasing gradients are different. The critical zone is the most affected, the impact zone comes next, and the non-impact zone is the least affected with the increase in impulse under local impact load. Mn in the impact zone and critical zone decreases exponentially with the increase in the impact loading area, while it increases exponentially in the non-impact zone. The micro-structures evolution in briquette and raw coal samples is similar, but the anisotropy and localization effect of micro-structure evolution for raw coal samples are more significant and more sensitive to the impact loading area. The micro-structure evolution of coal-rock under local impact load shows obvious localization effect. Mn in the critical zone is usually the largest, Mn in the impact zone is slightly less than that in the critical zone, and Mn in the non-impact zone is the least. The larger the impact loading area, the wider the influence enhancement area, and the smaller the non-influence area, yet the smaller the impact zone and critical zone are affected by local impact load.

Laboratory study of nonlinear propagation of transient signals in a sandstone bar

Experiments are reported on finite-amplitude, impact propagation in a thin bar of Texas moss sandstone. Wideband, unipolar pulses were generated by excitation with a pendulum hammer at one end of the bar, in contrast to prior work with either tone-burst excitation or resonance methods. The bar was rectangular, 8 cm on each side and 175 cm in length, with a density of 2.0 g/cc. Impacts had a center frequency around 3 kHz, amplitude of 10 to 130 microstrain, and propagated repetitively between reflections at each end. Measurements were made with a laser Doppler vibrometer at multiple locations along the bar, while ultrasonic tone-burst probes were used at a central location, transverse to the axis. The sound speed and effective modulus were reduced in proportion to the impact amplitude as the finite-amplitude impact pulse propagated through the transverse and other measurement sites. The attenuation of the impact signal was also found to follow a power law in the frequency range of 1–6 kHz, in proportion to impact intensity. Although amplitude-dependent attenuation and softening of the elastic modulus are both found in micro-inhomogeneous materials, the present results were enhanced, up to an order of magnitude, compared to those in previously reported experiments.

Research on Design of Polycrystalline Diamond Composite Pendulum Wear-Resistant Belt

The torque impactor can effectively solve the stick-slip vibration of PDC bit in the deep formation, but the wear of the pendulum seriously restricted the service life of the impactor. In order to reduce the wear of the pendulum under complicated working conditions, the polycrystalline diamond composite material was used as the wear-resistant material to design the pendulum wear-resistant belt. Based on the analysis of the feasibility of using polycrystalline diamond as pendulum wear-resistant material, the finite element model of dynamic contact interaction between pendulum wear-resistant belt and impact cylinder was established, the influence of the arrangement of wear-resistant blocks on the stress of the contact area between the pendulum wear-resistant belt and the impact cylinder was studied. The results show: Polycrystalline diamond composite material has stronger wear resistance and better anti-friction performance than conventional wear-resistant materials, and can reduce the wear of the inner wall of the impact cylinder, which is feasible on the surface of the pendulum hammer. The use of the wear-resistant belt structure is beneficial to improve the variation of the shear stress of the area where the pendulum and the impact cylinder contact each other. The wear-resistant belt structure is beneficial to improve the shear stress variation amplitude of the pendulum and impact cylinder in contact with each other. When the wear-resistant block is arranged in a circular block staggered arrangement, the shear stress change amplitude of the interaction contact area is the largest, the triangular block staggered arrangement mode is second, and the trough block shape staggered arrangement is the smallest. The research results have important reference significance for the design and application of polycrystalline diamond composite pendulum wear-resistant belt.

Analysis of Ductile to Brittle Transition Temperature of Stainless Steel

All ferrous materials exhibit a transition from ductile to brittle when tested above and below a certain temperature, called as Transition Temperature. The Ductile-to-Brittle Transition Temperature is a phenomenon which is mainly observed in metals. Below the particular temperature the material suddenly losses its ductility and it changes from ductile to brittle material. This paper mainly deals with the analysis and comparison of Ductile to Brittle Transition Temperature of Stainless Steels TP 304 and 316. Work is carried out in the Charpy impact test experiment, The Charpy impact test is a standardized high strain test which determines the amount of energy absorbed by different materials. The amount of energy absorbed gives the material's toughness and this property is used to study the ductile-brittle transition. The specimens then keep in the dry ice for cooling for soaking time of 15 min. Then the sample pieces are loaded into the apparatus and the pendulum hammer strikes the notched sample. The notch is put in the sample to initiate the crack propagation. After the impact the sample is removed and examined. If the material breaks on a flat surface then it is classified as brittle, if it is a jagged surface then it is classified as ductile. The graph of energy absorbed vs temperature is plotted to get the range of transition temperature.

Experimental study on progressive collapse resistance of steel frames under a sudden column removal scenario

A comprehensive experimental apparatus for the progressive collapse testing of steel frames has been developed. The apparatus is suited for the testing of planar steel frames to study the load transfer process and the progressive collapse resistance of steel structures under a column removal scenario. In order to simulate a sudden removal of a middle column at the ground storey of the frames, a removable column unit has been designed to allow for an instantaneous knock-out by a pendulum hammer during the test. To avoid the out-of-plane instability of the planar steel frames, an out-of-plane restraining system has been designed and integrated into the test apparatus. Weights simulating the desired gravity loads were attached to the test frame through holding baskets, which were designed to minimize unwanted shaking and ensure that the suspended baskets moved together with the deformed steel frames during the tests. Experimental results showed that the column removal mechanism in the test apparatus was effective. Using this apparatus, the dynamic behaviour of three planer steel frames under a column removal scenario was investigated. Based on the measured deformations and strains, the dynamic response, collapse modes, load transfer path of the steel frames after the removal of the middle bottom column are studied.

Dynamic Characteristics of Green Sandstone Subjected to Repetitive Impact Loading: Phenomena and Mechanisms

A split Hopkinson pressure bar apparatus driven by a pendulum hammer was used to perform uniaxial compression tests to examine the degradation process of green sandstone subjected to repetitive impact loading. The acoustic characteristics, dissipated energy, deformation characteristics, and microstructure evolution were investigated. The representative stress–strain curve can be broken into five stages that were characterized by changes in the axial strain response during impact loading. Both the ultrasonic wave velocity and cumulative dissipated energy exhibited obvious three-stage behavior with respect to the impact number. As the impact number increased, more than one peak was observed in the frequency spectra, and the relative weight of the peak frequency increased in the low-frequency range. According to the evolution of the ultrasonic wave velocity, the degradation process was divided into three stages. By comparing the intact stage I and early stage II microcrack development patterns, the initiation of new cracks and elongation of existing cracks were identified as the main degradation mechanisms. Furthermore, a slight increase in the number of cracks was observed, and microcrack lengths steadily increased. Moreover, due to the low level of microcrack damage, the deformation mechanism was mainly characterized by volume compression during impact loading. In late stage II, the main degradation mechanism was the elongation of existing cracks. Additionally, as microcracks accumulated in the rock samples, cracks were arranged parallel to the loading direction, which led to volume dilation. In stage III, microcracks continued to elongate nearly parallel to the loading direction and then linked to each other, which led to intense degradation in the rock samples. In this stage, rock sample deformation was mainly characterized by volume dilation during impact loading. Finally, rock samples were split into blocks with fractures oriented subparallel to the loading direction. These results can improve the understanding of the stability evaluations of rock structures subjected to repetitive impact loading.

The Influence of the Joint Thickness Upon the Impact Strength of Block Adhesive Joints

Abstract The research described in this article relates to one of the aspects of methodology of investigating impact loading in block bonded joints. The authors checked experimentally the effect of the thickness of adhesive joints upon impact strength of the connection. The experimental research was completed with preliminary dynamic numerical calculations of the test cases. The experimental testing was conducted with a pendulum hammer designed to examine adhesive joints. The maximum energy of the pendulum used in the investigation equalled 15 J. In order to determine the impact strength of the examined connections, it was used a dependency according to which the energy used to detach the upper element of the sample in such a test, that is the energy lost by the pendulum, is the measure of impact strength of the adhesive joint. The elements of the samples were made of steel S235 or an aluminum alloy 2017A. In order to bond the samples, was used Epidian 57 epoxide resin with Z1 hardener. The models for numerical computations were prepared on the basis of samples used in the experimental research. The prepared models, taking into account the initial and boundary conditions were subjected to dynamic numerical computations by means of the finite element method with the use of the Explicit Dynamics module in the ANSYS programme. The aim of the numerical calculations was to compare the compliance of the results of the experimental and analytical investigations as well as the assessment of the impact of the size of boundary conditions, which were subjected to parametrization, upon the values of occurring stresses during the conducted impact loading simulations. The results of the experimental investigation indicate that along with increasing the thickness of the adhesive joint, impact strength of the connection is reduced, regardless of the bonded material.

Damage Assessment of Cultural Heritage Structures after the 2015 Gorkha, Nepal, Earthquake: A Case Study of Jagannath Temple

The Gorkha, Nepal, earthquake and the series of aftershocks that followed have damaged many heritage structures in and around Kathmandu Valley, including UNESCO World Heritage Sites (WHSs). This paper summarizes observed damage to the heritage structures of diverse typologies within the UNESCO WHSs of Kathmandu Valley. As a part of the investigation, inspection survey and damage assessment were carried out for Jagannath Temple, one of the partially damaged monuments in the Kathmandu Durbar Square WHS. Ambient vibration and in-situ tests using the pendulum hammer, the rebound hammer, and in-place push on masonry walls were performed. Finite-element models of the structure were developed, and the results were analyzed and compared with field observations. Based on the observed damages and the results obtained from numerical modeling, the primary causes of the damage are discussed.

The Coupled Effect of Loading Rate and Grain Size on Tensile Strength of Sandstones under Dynamic Disturbance

It is of significance to comprehend the effects of rock microstructure on the tensile strength under different loading rates caused by mining disturbance. So, in this paper, three kinds of sandstones drilled from surrounding rocks in Xiao Jihan Coal to simulate the in situ stress state, whose average grain size is 30 μm (fine grain, FG), 105 μm (medium grain, MG), and 231 μm (Coarse grain, CG), are selected with the calculation of optical microscopic technique and moreover processed to Brazilian disc (BD) to study the mechanical response of samples. The dynamic Brazilian tests of samples with three kinds of grain sizes are conducted with the Split Hopkinson Pressure Bar (SHPB) driven by pendulum hammer, which can produce four different velocities (V=2.0 m/s, 2.5 m/s, 3.3 m/s, and 4.2 m/s) when the incident bar is impacted by pendulum hammer. The incident wave produced by pendulum hammer is a slowly rising stress wave, which allows gradual stress accumulation in the specimen and maintains the load at both ends of the specimen in an equilibrium state. The results show that the dynamic strength of three kinds of BD samples represented loading rates dependence, and FG sandstones are more sensitive for loading rates than MG and CG samples. Moreover, the peak strength is observed to increase linearly with an increasing stress rates, and the relationship between the dynamic BD strength and stress rates can be built through a linear equation. Finally, the failure modes of different grain sizes are discussed and explained by microfailure mechanism.

Constant Strain Rate Uniaxial Compression of Green Sandstone during SHPB Tests Driven by Pendulum Hammer

During the Split-Hopkinson pressure bar (SHPB) tests driven by pendulum hammer, employing a proper special shape striker is an effective way to obtain dynamic stress equilibrium condition and to get constant strain rate of the rock specimen. To find the proper special shape striker, a striker with a cambered surface was introduced and eight geometrically different hammers were designed to analyze the effect of hammer geometry on the waveform of excited incident stress waves. Based on experiments and simulations, parameter effects, including the cambered hammer curvature radius and hammer diameter, length, and impact velocity, on the incident wave shape were examined. These parametric studies provided guidelines for achieving constant strain rates in rock specimens during SHPB tests. The use of different diameter hammers was noted for shaping stress-time curves to follow the stress-strain behavior of green sandstone. Finally, to examine the applicability of using hammer geometry for shaping incident waves to achieve constant strain rate, SHPB tests on green sandstone specimens were conducted. The results demonstrated that a constant strain rate (100 s−1) lasting for 70 μs was achieved with the 8# hammer (3.7 kg; curvature radius, diameter, and length of 100, 70, and 126.3 mm, resp.). In addition, dynamic experiments on green sandstone were carried out under various strain rates and the results showed that the initial tangential modulus was almost unaffected by strain rate. The strain at peak stress tended to increase with rising strain rate and the dynamic strength of green sandstone showed an apparent rate dependency.

Numerical Analysis of Impact Loading of Adhesive Joints

Due to small repeatability of results of impact strength of adhesive joints, particularly those performed on different test machines, the authors were inspired to conduct experimental and numerical research in this field. The investigation used the Block Shear Test. The authors compared impact strength of samples which were loaded in a normative and nonnormative manner and also the dependence in impact strength with regard to the distance between the impactor’s edge and the surface of the bonded joint. The numerical calculations were carried out in the program Ansys, using the Explicit Dynamics module. The authors proposed a way of modelling the impactor of the pendulum hammer. It was found that a change in the direction of applying the load to the sample and rotating the loaded sample piece in relation to the edge of the impactor results in a significant change in the Max Principal Stresses. Numerical investigations show that lower values of Max Principal Stresses occur in joints which are characterized by larger impact strength, determined experimentally. It was also noted that moving away the edge of the impactor from the surface of the adhesive joint increases normal stresses perpendicular to the surface of the joint.

Experimental and Numerical Study on Stress Relaxation of Sandstones Disturbed by Dynamic Loading

Time-dependent rheological deformation of rocks affects the stability of underground stopes or constructions. It may also be altered by dynamic disturbances, such as rock blasting. To study such effects, a new stress relaxation-disturbance testing machine was invented, capable of loading conditions for stress relaxation and dynamic disturbance. Effects of testing machine stiffness on rock deformation behavior were examined to confirm that rocks had undergone stress relaxation. Stress relaxation tests on specimens under uniaxial compression were carried out over 6 or more days. Under single-stage stress relaxation, axial stress relaxed within 5–7 days, stabilizing at constant strain. During two-stage stress relaxation, larger stress decay was observed under the higher strain level. A dynamic disturbance from a pendulum hammer was applied to specimens under stress relaxation to evaluate their response. In these tests, stress decline and strain increase were related to residual strain induced by the dynamic disturbance. The strain variation in specimens was found to be within 1.5 % of values before the disturbance. Finally, a damage-based constitutive model for rocks subjected to stress relaxation and dynamic disturbance is proposed. It reproduces the stress relaxation behavior of rock observed in our experiments. The model was used to quantify stress relaxation of rocks and the effects of dynamic disturbance on this process. The larger stress decay of rocks under higher strain is likely related to damage in the rock. Clearly, increases in strain and decreases in stress induced by dynamic loading, coupled with rock damage, are over-estimated in our numerical simulations.