Drop-weight Test Research Articles

GFRP laminates modified with a viscoelastic layer for improvement of the response under low velocity impacts

High speed crafts made of glass fibre reinforced polymers (GFRP) suffer in-service structural damage due to the slamming impacts, when the bottom surface impacts against the water. It is important to quantify the extend of the damage and progression during the service life of the ship. In this work, it is presented a methodology using non-destructive evaluation for the assessment of the level of damage in the laminated panels. It is also investigated the effect of the introduction of a viscoelastic layer, composed of two polymeric material with different stiffnesses, designed specifically to dissipate the impact energy. The combination of these polymers with different deformation response under compression, and due to the confinement of the elastomer in small cell surrounded by the rigid polymer, constitutes an effective mechanism of energy dissipation due to impacts, protecting the plies beneath the viscoelastic layer and minimizing the damaged generated in the laminates due to slamming. Test specimens have been prepared using GFRP prepregs cured Out-of-Autoclave (OoA) and impacted using a specifically designed device that apply pressure peaks in the laminates that resemble very closely those that appears in the hull due to slamming loads, and compared with and without viscoelastic layer in the laminate. Changes in panel compliance are used for monitoring the evolution of damage in the material, as a function of the number of repeated impacts imposed on the panel for every load condition. Drop weight tests have been also used to understand the percentage of the income energy that is generating damage in the material, and to establish threshold levels for damage initiation both for unprotected panels and those with viscoelastic shielding layer.

Consecutive and constant low-velocity drop-weight impact loading tests of cushion rubber set RC beams

Consecutive and constant low-velocity drop-weight impact loading tests of cushion rubber set RC beams

Drop-weight impact loading tests of cushion rubber set AFRP sheet strengthened RC beams reaching ultimate state with sheet failure in case of varying rubber thickness

Drop-weight impact loading tests of cushion rubber set AFRP sheet strengthened RC beams reaching ultimate state with sheet failure in case of varying rubber thickness

Effect of different hostile solutions on mechanical properties of composite materials

Due to their excellent performances in terms of high specific strength and stiffness, good static and dynamic properties, good corrosion resistance, adjustable properties, competitive cost, and quick manufacture, fibre-reinforced composites have been used in a variety of engineering fields, and this trend is expected to continue. In this context, these materials are subjected to a variety of environmental conditions, which have a major impact on their mechanical characteristics. Although several studies have been published on the influence of hostile solutions on low velocity impact response, the flexural fatigue performance of composite laminates exposure to corrosive solutions still has some lack of studies. As a result, the goal of this research is to investigate the low velocity impact and flexural fatigue evaluation of Kevlar/epoxy laminates after immersion in Sulphuric acid (H2SO4), diesel and seawater. For this purpose, static three-point bending tests were performed at a displacement rate of 2 mm/min. The low velocity impact tests were made using a drop weight testing machine IMATEK-IM10. Flexural fatigue tests were carried out in flexural mode using an Instron servohydraulic machine. Constant amplitude loading tests were performed at room temperature, under a stress ratio of R = 0.05 and frequency of 2 Hz. It was concluded that the different solutions affect flexural and impact behaviour of the studied materials, but the exposure time was determinant on the mechanical properties’ degradation.

An Experimental Investigation of the Behavior of Single-Particle Breakage on Polymetallic Nodules

Abstract Underwater mining systems for mining polymetallic nodules use various transport systems through submersible pumps that can span across subsea platforms (i.e., flexible risers and subsea structures), despite the role of concentration of the medium, shape, and size of the particle and conduit. The crushing unit determines the particle's shape and size, which plays a vital role in transportation (slurry ores) in the subsea environment. The National Institute of Ocean Technology is currently working on a flexible riser transport system. The collected nodules vary in size, and the majority of the nodules are nearly spherical/spheroid in shape with sizes ranging from 3 to 7 cm (Vineesh et al., 2009). The planned vertical slurry transport pipe/flexible hose has an internal diameter of 10 cm for better handling, favorable flow rate, and peak pressure capabilities. The collected nodules are fragmented using a crusher to less than 3 cm (1/3 of the hose inner diameter) to form narrow particle size distribution to prevent layering of the different particle sizes and plug formation by the larger particles during vertical slurry transport. Impact tests were conducted with actual wet nodules of various sizes, using different striker shapes in a drop weight tester to assess the size of the crusher, design the impact surfaces, and estimate the power required. The tests were conducted in submerged conditions to assess the actual load and breakage results. The objective was to maintain low crusher power and have minimal fragmentation to the desired sizes. Single-particle breakage tests were conducted on irregular polymetallic nodules using customized drop-weight impact equipment and a uniaxial compression testing machine to analyze the effect of shape and size on the specific comminution energy (SCE) of three different particle sizes (i.e., 4‐5 cm, 5‐ 6 cm, 6‐7 cm). Furthermore, to understand the deformation behavior of the nodules, experiments are performed with different types of strikers (i.e., hemisphere, conical, and flat). The breakage behavior of ore particles with different particle arrangements and SCE were studied in the tests. Experimental studies indicate that the shape of particles significantly affected the impact-loading mode and the breakage progress. With the increase in the size of particles, the SCE for wet particles decreases for a fixed fraction of particles. Regulating the input power of the crushing machinery and optimizing the crushing technology, considering the influence of particle shape and size, can reduce the SCE consumption and improve energy efficiency. Results reveal that the shape of the striker has a pronounced effect on the SCE in dry, wet, and submerged water test conditions. Compared to wet particles, submerged water conditions show a noticeable increase in SCE consumption due to the associated drag inside water conditions. Our results develop reference data sets required to understand the fragmentation of particles due to intricate crushing and transporting in a submerged environment.

Impact-Initiation Sensitivity of High-Temperature PTFE-Al-W Reactive Materials

Drop-weight tests were conducted to investigate the impact-initiation sensitivity of high-temperature PTFE-Al-W reactive materials. The test results show that the impact-initiation sensitivity of the materials more than doubles with increasing the sample temperature from 25 to 350 °C. Combined with the impact-induced initiation process recorded by high-speed video and the difference between reacted and unreacted residues, the crack-induced initiation mechanism was revealed. The rapid propagation of crack provides a high-temperature and aerobic environment where Al reacts violently to PTFE, which induces the initiation. Moreover, the influence of sample temperature on the sensitivity was discussed and analyzed. The analysis results indicate that the sensitivity shows a temperature interval effect, and 127 and 327 °C are the interval boundaries where the sensitivity changes significantly. The sensitivity may leaps at 127 °C and increases more rapidly in the temperature interval from 127 to 327 °C, but hardly changes after the temperature reaches 327 °C.

Impact identification on concrete panels using a surface-bonded smart piezoelectric module system

Timely identification of collision damage, especially in aging bridges, is critical for the safety of commuters. However, there is no efficient, cost-effective, in-situ technique to serve this purpose. Wave propagation-based structural health monitoring (SHM) using piezoelectric material is a promising alternative for remote sensing. To that end, this study aims to develop a wave propagation-based monitoring technique using surface-bonded smart piezoelectric modules (SPM) to determine the impact force, location, and projectile properties of low-velocity impacts on concrete panels. An impact source localization algorithm used in composite structures is adapted and simplified for concrete structures. This technique is validated using a combined experimental and numerical investigation, which shows good agreement with the actual impact source location. The impact force, projectile mass, and velocity are determined using a semi-theoretical-experimental technique based on Reed contact model. A special contact-SPM is fabricated and calibrated to determine the contact force at the impact location. The relationship between contact-SPM response and distributed-SPM response is determined using a drop-weight test with steel sphere. The peak contact force and contact duration are in good agreement with Reed contact model, although the latter overpredicts the given parameters. A simplified formula based on Reed contact model is used to inversely estimate the projectile velocity of a known mass and vice versa. Then, using the pre-calibrated data, the impact force, projectile properties, and impact force-time distribution are determined using the response of distributed-SPM system. The technique is validated using an arbitrary steel sphere mass. As demonstrated in the combined experimental, theoretical, and numerical study, the proposed surface-bonded SPM system is capable of effectively identifying low-velocity impact incidents on concrete structures, which could potentially facilitate inexpensive, in-situ, real-time condition assessment.

Experimental and numerical investigation of impact behavior of RC slab with different opening size and layout

The present study intends to investigate the dynamic responses and failure modes of two-way reinforced concrete (RC) slabs that have openings in different sizes and locations under the impact load effect. RC slabs may include the openings due to the various architectural imperatives. Stair and elevator openings, ventilation openings, the openings left for the electricity, and the water installations can be given as examples of openings taken place in the RC slabs. Many studies related to RC slabs subjected to the static and the reversed-cyclic dynamic loads have existed in the literature. However, at the end of the detailed literature review conducted, the authors have not encountered a comprehensive study focused on the RC slabs' impact behavior that has openings in different sizes and locations. In the present experimental study's scope, the impact loading was applied to two-way RC slabs with the openings via the drop-weight test setup designed by the authors. The time-histories of acceleration and displacements measured from the RC slabs and impact forces acting on the RC slabs have been recorded during the tests to investigate the effect of opening sizes and locations on the RC slabs' impact performance. Furthermore, a finite element model (FEM) has been developed using ABAQUS software to calculate dynamic response and failure modes of the RC slabs with openings. Good agreement between experimental and numerical results has proven that the proposed FEM could be used to evaluate the impact behavior of the RC slabs that have the openings.

Advances on the Fragmentation-Energy Fan Concept and the Swebrec Function in Modeling Drop Weight Testing

The breakage index equation (BIE), or t10 model from drop weight testing (DWT) data for rocks and ores is used in the design of crushers and mills. Such models are becoming increasingly difficult to visualize as the number of variables increases. The so-called double fan BIE, combined with the Swebrec distribution’s accurate description of the sieving curves, is applied to the modelling of drop-weight test fragmentation. The key parameters are geometric properties visible in the fan plot; slopes of straight lines and their point of convergence. The ability of the double fan BIE to reproduce DWT data had been previously established for 8 rocks with 480 DWT data sets. Here the fidelity of the double fan BIE is further evaluated for 18 new materials, based on 281 data sets. The fidelity of the double fan BIE with three fan lines is on par with the fidelity of the current state-of-the-art models for the new materials. Besides the breakage index equation, the new double fan BIE’s t10 equation produces, without additional parameters or fitted constants, the general breakage surface equation tn for an arbitrary n value as a bonus. The specific sieving curve for any combination of particle size and impact energy is also contained in the same formula. The result is an accurate, compact and transparent model.

Energy Requirement for Rock Breakage in Laboratory Experiments and Engineering Operations: A Review

Based on the review of a wide range of literature, this paper finds that: (1) the average specific surface energy of various single crystals is only 0.8 J/m2. (2) The average specific fracture energy of the rocks with a pre-crack under static cleavage tests is 4.6 J/m2. (3) The average specific fracture energy of the rocks with a pre-cut notch but with no pre-crack under static tensile fracture (mode I) tests is 4.6 J/m2. (4) The average specific fracture energies of regular rock specimens with neither pre-made crack nor pre-cut notch are 26.6, 13.9 and 25.7 J/m2 under uniaxial compression, tension and shear tests, respectively. (5) The average specific fracture energy of irregular single quartz particles under uniaxial compression is 13.8 J/m2. (6) The average specific fracture energy of particle beds under drop weight tests is 74.0 J/m2. (7) The average specific fracture energy of multi-particles in milling tests is 72.5 J/m2. (8) The average specific energy of rocks in percussive drilling is 399 J/m3, that in full-scale cutting is 131 J/m3, and that in rotary drilling is 157 J/m3. (9) The average energy efficiency of milling is only 1.10%. (10) The accurate measurements of specific fracture energy in blasting are too few to draw reliable conclusions. In the last part of the paper, the effects of inter-granular displacement, loading rate, confining pressure, surface area measurement, premade crack, attrition and thermal energy on the specific fracture energy of rock are discussed.

Static and dynamic crushing of polymeric lattices fabricated by fused deposition modelling and selective laser sintering – an experimental investigation

Lightweight polymeric lattices, which display good energy absorption characteristics during crushing, are increasingly employed in impact protection applications. Lattices based on an octet, rhombic dodecahedron (RD) and a novel hybrid structure (HS), were fabricated by Selective Laser Sintering (SLS), and their dynamic responses were investigated via a drop-weight tester. Their stress-strain responses and crushing patterns were compared with quasi-static responses obtained using a universal testing machine. The experimental results show that SLS-fabricated lattices display good energy absorption performance under quasi-static compression, but collapse catastrophically when subjected to impact. To investigate the cause for the distinct responses under different deformation speeds, the quasi-static and dynamic tensile properties of the cell strut material were also examined. SLS-fabricated specimens are notably more brittle than those fabricated by another commonly used technique – Fused Deposition Modelling (FDM). Consequently, lattices fabricated by FDM were also tested. Unlike their SLS counterparts, FDM-fabricated lattices exhibit good energy absorption characteristics under both quasi-static and dynamic compression, as well as typical rate dependence, whereby the stress level increases with deformation speed. Compared with the traditional octet and RD, the novel HS lattice design shows superior energy absorption characteristics for all tests.

Lizard osteoderms – Morphological characterisation, biomimetic design and manufacturing based on three species

Osteoderms (OD) are mineralised dermal structures consisting mainly of calcium phosphate and collagen. The sheer diversity of OD morphologies and their distribution within the skin of lizards makes these reptiles an ideal group in which to study ODs. Nonetheless, our understanding of the structure, development, and function of lizard ODs remains limited. The specific aims of this study were: (1) to carry out a detailed morphological characterisation of ODs in three lizard species; (2) to design and manufacture biomimetic sheets of ODs corresponding to the OD arrangement in each species; and (3) to evaluate the impact resistance of the manufactured biomimetic sheets under a drop weight test. Skin samples of the anguimorphs H. suspectum and O. ventralis, and the skink C. zebrata were obtained from frozen lab specimens. Following a series of imaging and image characterisations, 3D biomimetic models of the ODs were developed. 3D models were then printed using additive manufacturing techniques and subjected to drop weight impact tests. The results suggest that a 3D printed compound of overlapping ODs as observed in Corucia can potentially offers a higher energy absorption by comparison with the overlapping ODs of Ophisaurus and the non-overlapping ODs of Heloderma. Compound overlapping ODs need to be further tested and explored as a biomimetic concept to increase the shock absorption capabilities of devices and structures.

Qualifying carbon nanotube reinforced polyurethane foam as helmet inner liner through in-situ, static and low velocity impact testing

Helmet inner liner (HIL) is a critical component of helmets which ensures safety and comfort of the wearer. Efforts are made in this work to qualify polyurethane (PU) foams for HIL application. Processing temperature and weight fraction of carbon nanotubes (CNTs) nanofillers is varied to optimize the microstructure and hence average properties of PU foams. Experimental results show that PU foam processed at −5 °C and reinforced with 1.6 wt% of oxidized CNTs shows 40% higher specific elastic modulus and 11% better recovery under compression testing, absorbs 97% more energy per unit volume in comparison to EPS under low velocity impact in drop weight tests. The 34% higher thermal conductivity than EPS implies that it will provide better comfort by efficiently dissipating the heat generated in helmet. The superior combination of properties makes CNT reinforced PU foams a better alternative as HIL in comparison to currently used EPS foam.

Evaluation of Impact-Critical RC Beams Strengthened with a Bottom Layer of HPFRCC

One of the most hazardous consequences of impacts on concrete structures is the high tendency to develop shear or punching failures, with debris generation and associated flying fragments split out. In order to reduce such hazards, concrete structures can be strengthened with a thin bottom layer of high performance fiber-reinforced cement composites (HPFRCCs). As strengthening and anti-spalling layers, the most demanding application is for shear-critical elements, where extensive diagonal cracking leads to multiple fragments, besides concrete cover spalling typical of bending-critical members. An experimental campaign has been carried out on 21 beams to investigate their failure mode. Different configurations were studied for the HPFRCC to determine the influence of the fiber amount (2-4%) and the introduction of embedded steel bars in the strengthening layer. Specimens were subjected to drop weight tests and quasi-static reference tests. The tests have shown that a membrane-type resisting mechanism can be activated by the HPFRCC layer, which can retain potentially falling generated fragments and improve the carrying capacity. Embedding steel bars in the strengthening layers can avoid HPFRCC rupture. Moreover, members strengthened using tensile strain-hardening HPFRCC mixes have shown enhanced strength compared with unstrengthened members, while mixes without strain-hardening did not.

Experimental and numerical investigations on the dynamic response of woven carbon fiber reinforced thick composite laminates under low-velocity impact

This paper represents experimental and numerical investigations into the dynamic response of woven carbon fiber reinforced thick composite laminates under low-velocity impact. Experiments with nine impact energies are carried out by the drop weight testing machine. The force-displacement curves are extracted from experimental data, and the influence of impact energy on impact response is discussed. The relationships between impact energy and impact performance such as peak force, maximum displacement, and absorbed energy are analyzed. The damage morphology obtained by digital microscope and ultrasonic C-scan is used to analyze the damage mechanisms. Experimental results show that woven composite laminates considered here exhibited several failure modes, including indentation, delamination, matrix cracks and fiber breakage. The finite element model is established to study the complex damage and failure mechanisms by employing ABAQUS/Explicit. Good consistency is achieved between the experimental and numerical results by comparing the force-time curves, impact resistance and damage status. Finally, the impact damage behavior of woven composite laminates with helicoidal layups is explored.

Experimental Study on Mechanical Properties of Pervious Concrete Containing Recycled Aggregate

This investigational study aims to examine the influence of using recycled coarse aggregate (RCA) instead of natural coarse aggregate (NCA) on the fresh properties (workability) and mechanical properties of pervious concrete (PC). The mechanical properties include compressive strength, splitting tensile strength and flexural strength and the impact resistance of PC was also assessed using drop weight test. In total, four mixes were prepared (including control mix with 100% NCA) in which various ratios of RCA (30%, 70% and 100%) replaced the NCA in the preparation of PC mixes without using any fine aggregate and fixing the water-cement ratio (w/c) to 0.4. The outcomes of the study reveal that the content of the RCA has significant adverse effects on the workability and mechanical performance of the investigated mixtures. This adverse effects increase with the increase of the content of RCA. The strength reduction, compared to the mix with NCA, reached to 59% for compressive, 28% of the splitting tensile, 38% for flexural strength and 50% for the impact resistance when 100% RCA is used.

Impact of crack propagation path and inclusion elements on fracture toughness and micro-surface characteristics of welded pipes in DWTT

Despite fundamental differences between the characteristics of base and weld metal and higher accuracies obtainable using original-size specimens in drop-weight tear test (DWTT), there still exists a research gap in fracture surface examinations of DWTT welded specimens. This study investigates the microscopic characteristics of the fracture surface of spirally welded API X65 steel with chevron notch (CN) and pressed notch (PN) DWTT specimens. Microstructures of different sub-zones were investigated, including, weld metal (WM), heat affected zone (HAZ), and base metal (BM). Scanning electron microscopy (SEM) investigations revealed the numerous inclusions in the WM that cause stress concentration. Micro-cracks are formed at the beginning of the fracture process when the energy level is high; nevertheless, in the shear fracture area, where energy is reduced, micro-cracks were not observed. Inverse fracture was located in the HAZ and BM in the PN and CN specimens, respectively. Comparison of the weight percentages of inclusion elements obtained by energy-dispersive x-ray spectroscopy (EDS) with API 5L standard target values showed that in most inclusions, Mn, Ti, and S were higher than standard values. After metallurgical and mechanical investigations, a framework for a new prospect has been introduced to conduct statistical analyses, based on which, the confidence intervals of the weight percentages of inclusion elements were carefully determined. Changes in the weight percentages of constituent inclusion elements in the WM are smaller compared to the BM and HAZ that would be reasons for better mechanical properties of the WM compared to the BM. Considering the impact of inclusions on weld-ability and toughness, industrial guidelines are presented to carefully control the elements within the confidence intervals of this research, during welding to minimize the formation of inclusions, in turn, reducing the formation and growth of micro-cracks, and significantly improving mechanical properties.

Analysis of Energy Absorption Distribution for Circular Concrete-Filled Steel Tubular Members under Lateral Impact

A nonlinear finite element model of the circular concrete-filled steel tubular (CFST) member under lateral impact is established based on the existing drop weight tests, and the accuracy of numerical simulation is validated by comparing with the test results. Then the segmented numerical model of the circular CFST member is divided evenly to obtain the distribution of energy consumption along its length. The results indicate that the prime areas of energy consumption for the circular CFST member are distributed at the impact position (within 1/6 of the effective length) and the support position (within 1/12 of the effective length). On this basis, the affect of impact position on energy absorption distribution is carried out to reveal the changes of the prime areas of energy consumption. The results of this investigation can provide a reference for the impact resistant design and damage reinforcement of circular CFST members under lateral impact.

Plastic collapse model and drop-weight test of aluminum honeycomb under the out-of-plane compression

Firstly, according to the two dimensionless parameters proposed, the collapse stress factor and the hinge length factor, the unified expressions of the plastic collapse stress and the plastic hinge length are obtained from three theoretical formulas for honeycomb under the out-of-plane compression, which are the semi empirical formula, the Tresca yield criterion formula and the Mises yield criterion formula. Then, the different aluminum honeycombs are used in the out-of-plane compression to obtain load-displacement curves, stress-strain curves, and the energy absorption characteristics. Comparing the plastic collapse stress and the plastic hinge length of the measured values with those of theoretical values, they are found sometimes quite different. Finally, in order to reduce the differences between the measurement and the theory, the correction coefficient is introduced to establish the correction model of the plastic collapse stress and the plastic hinge length for honeycomb under the out-of-plane compression, which can provide new ideas for the selection and structural design of aluminum honeycomb.

Energy absorption and damage assessment of non-hybrid and hybrid fabric epoxy composite laminates: experimental and numerical study

The energy absorption due to low-velocity impact on non-hybrid and hybrid (Kevlar-K, carbon-C, and glass-G) fabric reinforced epoxy composite laminates were studied by conducting standard drop weight tests. The experimental energy absorption results were compared with finite element analysis results. Both values were in good agreement with a maximum variation of about 6%. Kevlar/epoxy (K/K/K) specimen recorded the highest energy absorption (18.67 J) followed by Kevlar specimen sandwiched with glass fabric in the middle layer (K/G/K) (17.24 J). Specimens consisting of glass or carbon layers in the middle, sandwiched by Kevlar layers at the top and bottom (K/G/K and K/C/K) exhibited better energy absorption compared with laminates consisting of all three fabric reinforcements (K/C/G (13.38 J), G/C/K (16.11 J) and C/G/K (16.04 J), respectively). Considering the cost of these composite laminates, K/G/K composite offers 21% reduction in material cost when compared to K/K/K specimen and also offered reasonably enhanced impact resistance and energy absorption as compared to other hybrid composites. Hence, this data may be useful for developing cost effective impact resistant components for defense (protective helmets and body armor) and aerospace (aircraft body components) sectors. The morphological study of the drop weight impact tests revealed that, delamination, fiber breakage, fiber splitting, and matrix cracking are the major damage mechanisms observed in the study. The cost was found to be a critical factor to compare between the different composites.