Instrumented Drop Weight Impact Tester Research Articles

Energy Absorption and Dissipation Characteristics of Pultruded Glass-Graphite/Epoxy Hybrid Composite Beams

The response of pultruded glass-graphite/epoxy hybrid composites has been evaluated under two different incident impact energy conditions. A high incident energy (HIE) and a low incident energy (LIE) of impact are chosen to cause either complete fracture or induce delamination, respectively, for assessing the energy absorption characteristics and delamination fracture toughness of these hybrid composites. From the HIE instrumented drop-weight impact tests, the energies required for damage initiation, propagation, as well as total absorbed energies and other parameters necessary for evaluating the impact performance of composites were derived from the load-deformation behavior of each sample. The influence of hybridization on the energy absorption characteristics of pultruded composites was investigated. A parametric study also was performed to correlate the energy absorption characteristics with the energy dissipative mechanisms attributed to the inherent material damping capacity of these composites. The qualitative assessment of the impact performance using inherent damping capacity as a non-destructive testing parameter is highlighted. The samples that were subjected to LIE impact testing were examined using ultrasonic C-scan tests to ascertain changes in the extent of delamination crack propagation as a result of hybridization. Finite element modeling was performed to simulate delamination crack propagation at various levels through the thickness. The strain energy release rate computed using the virtual crack closure technique was monitored to determine the likelihood of delamination crack propagation with increment in crack growth. The results obtained from LIE impact tests and ultrasonic C-scan tests were used to corroborate the predicted delamination fracture toughness of pultruded hybrids. Graphite-outside hybrids exhibited high flexural stiffness, propagation energy, ductility and failure index, but had lower initiation energy along with a greater tendency to delaminate.

Impact response and damage tolerance characteristics of glass–carbon/epoxy hybrid composite plates

Impact behaviour and post impact compressive characteristics of glass–carbon/epoxy hybrid composites with alternate stacking sequences have been investigated. Plain weave E-glass and twill weave T-300 carbon have been used as reinforcing materials. For comparison, laminates containing only-carbon and only-glass reinforcements have also been studied. Experimental studies have been carried out on instrumented drop weight impact test apparatus. Post impact compressive strength has been obtained using NASA 1142 test fixture. It is observed that hybrid composites are less notch sensitive compared to only-carbon or only-glass composites. Further, carbon-outside/glass-inside clustered hybrid configuration gives lower notch sensitivity compared to the other hybrid configurations.

Cooling rate influences in carbon fibre/PEEK composites. Part III: impact damage performance

The effect of cooling rate on impact damage performance of carbon fibre/polyether ether ketone (PEEK) matrix composite is characterised based on the instrumented drop-weight impact test, scanning acoustic microscopy (SAM) damage evaluation and compression-after-impact (CAI) test. Judging from the incipient impact load, incipient impact energy and total damage area, the ability to resist damage initiation upon impact was higher in the order of fast-cooled carbon/PEEK, slow-cooled carbon/PEEK and carbon/epoxy laminates. Furthermore, the threshold impact energy was higher and the CAI strength reduction rate was lower for the fast-cooled specimen than the slow-cooled counterpart, strongly indicating higher impact damage tolerance of the former system. The present study demonstrates that the impact damage performance and other important properties of carbon/PEEK composites can be optimised, if not maximised, by proper control of processing conditions, especially the cooling rate.

Low velocity impact response of GFRP laminates subjected to cycling moistures

AbstractThe low velocity impact response of laminated composites in ambient hygrothermal environments has been extensively investigated. This response, however, is not well understood when subjected to moisture cycling at elevated temperatures. In this current study, two types of glass fiber reinforced plastic (GFRP) laminates, unidirectional and crossply, were conditioned in a conditioning chamber for a maximum of eight moisture cycles at conditioning temperatures from 50°C to 100°C. Low velocity impact tests were conducted on the conditioned specimens and control specimens via an instrumented drop‐weight impact testing machine. The tension after impact (TAI) strength was investigated using an MTS machine. The equivalent damage size is obtained using an average stress criterion found in the literature. The effect of moisture cycling and conditioning temperatures on the low velocity impact response and residual load carrying capacity of GFRP laminates are evaluated via the test results.

Fiber Reinforced Wet-Mix Shotcrete under Impact

As a result of excavation blasting, traffic, sudden ground movements, rock bursts, and seismic activity, shotcrete is often subjected to impact and other dynamic loads. Limited data exist, however, with regard to the resistance of shotcrete to such dynamic loads. Impact resistance of wet-mix shotcrete reinforced with 10 different types of fibers was investigated using instrumented drop weight impact tests. A direct comparison was made with the quasi-static response. Both beam and plate specimens were tested and the data were correlated. Fiber reinforcement was found to be highly effective in improving the fracture energy absorption and toughness under impact loading, but the improvements under impact loading were found to be substantially different than those seen in quasi-static tests, and were also highly dependent on the type of fiber used. When beams and plates were compared, the relative improvements between fiber types were found not necessarily in agreement.

Effect of thermal degradation on the impact properties of PVC compounds

An instrumented drop weight impact test was used to study the effect of thermal degradation on the impact properties of PVC compounds. The impact resistance of the aged compounds related well with their weight loss and hence, with thermal degradation. Each compound showed a specific weight loss percentage that correlated with a 50% loss in its impact properties (failure point), irrespective of the aging temperature. The results were also used to estimate a thermal index (TI) of each compound in a rapid and reliable way.

Measurement of dynamic interlaminar fracture toughness of FRP laminates using dynamic displacement measuring apparatus

Dynamic mode II and mixed-mode interlaminar fracture toughnesses of CFRP laminates were measured using an instrumented impact testing system at a low impact speed (0.9 m/s). The system consisted of an instrumented drop weight impact tester and dynamic displacement measuring apparatus constructed from an optical fiber, position sensing detector and laser light source. The results showed that if dynamic interlaminar fracture toughness is estimated on the basis of load data obtained from a strain gage signal measured on the falling dart, the dynamic toughness could be overestimated, resulting in an erroneous understanding of the experimental results. It was also shown that the dynamic toughness can be evaluated accurately on the basis of the displacement data measured by the dynamic displacement measuring apparatus. It is thus concluded that the displacement-based analysis is more accurate and reliable than the load-based analysis in the measurement of the dynamic interlaminar fracture toughness of FRP laminates.

平織ＣＦＲＰ積層板の落錘衝撃による微視的損傷

CFRP (Carbon Fiber Reinforced Plastics) have been increasingly used aerospace structures because of their weight saving effect. They are used in the form of multidirectional laminates. It is well known that delamination easily occurs in the CFRP laminates under transverse low velocity impact loading, and that the compressive strength of CFRP laminates with delamination reduces substantially. Therefore, the CAI (compressive strength after impact) problem has been studied extensively. However, most previous studies focused on the relationship between the compressive strength and the delamination area. For the design of performance materials, it is important to understand the microscopic damage behavior. There is few study on the microscopic damage progress in plain weave CFRP laminates under transverse loading. In the present study, instrumented drop weight impact tests and static transverse loading tests are conducted on plain weave CFRP laminates, and microscopic damage onset and propagation are observed by an optical microscope and a scanning acorstic microscope. It is found that fiber bundle fracture is dominant in impact loading, and delamination is dominant in static loading.

Microscopic Damage Induced by Dropweight Impact Associated with Attributed Residual Compressive Strength in CFRP Laminated Composites.

Instrumented dropweight impact tests are conducted for CFRP laminates. Impact-induced damage, transverse cracks and delamination are investigated microscopically. Transverse crack density and delamination area are measured as functions of impact energy. In addition, the residual compressive strength is measured, and the correlation between the impact energy and the microscopic damage with the attributed residual compressive strength is discussed. It is found that the residual compressive strength decreases as the impact energy increases but above a certain impact energy, the decrease in the strength becomes gradual.

Morphological behaviour and instrumented dart impact properties of β-crystalline-phase polypropylene

Abstract Specimens consisting of high-purity β-phase polypropylene were prepared by adding a bicomponent β-nucleator consisting of equal amounts of pimelic acid and calcium stearate. Scanning electron microscopy (SEM), differential scanning calorimetry (d.s.c.), dynamic mechanical analysis and instrumented drop weight impact tests were used to characterize the morphology, thermal behaviour and the impact properties of the β-phase polypropylene. SEM examinations show that the β-spherulites exhibit a sheaf-like structure with no clear boundaries between them. This distinct spherulitic morphology results in a substantial improvement in the falling weight impact resistance. Fractographic analysis reveals that microfibrils and voids were formed in the fracture induction area of the β-form specimen. The greater impact strength observed in the β-form material is due to the larger energy dissipation which is associated with the formation of microfibrils. Furthermore, d.s.c. analysis showed that there is no β → α phase conversion during the impact test.

Mechanical behavior of injection molded β‐crystalline phase polypropylene

AbstractHigh purity and a large amount of β‐modification can be produced in polypropylene (PP) by adding a bicomponent β‐nucleator consisting of equal amounts of pimelic acid and calcium stearate. Wide angle X‐ray diffraction (WAXD), scanning electron microscopy (SEM), static tensile tests, Izod impact tests, and instrumented drop weight impact testing were used to characterize the structure, morphology, and mechanical behavior of the β‐phase PP. The WAXD patterns show that the relative content of the β‐phase (K value) in the skin layer of the injection molded specimen is 0.54, whereas the K value in the core section is 0.92. SEM observations reveal that the β‐spherulite exhibits a sheaf‐like structure and there are no clear boundaries between the spherulites. Static tensile tests indicate that the elongation at break of pure PP is enhanced by the addition of a β‐nucleator. The impact and drop weight tests also indicate that the impact resistance of PP is improved by adding the β‐nucleating agent. The improvement in impact toughness is associated with the formation of microcrazes within the PP.

Effects of temperature and water content on impact properties of injection-molded glass nylon-6 composites

Instrumented drop-weight impact tests were conducted for injection-molded short glass fiber reinforced nylon-6 thermoplastic composites. The effects of fiber surface treatments, specimen temperatures and water contents on the impact fracture properties were studied. The first peak load P, or the corresponding absorbed energy E1 was found an appropriate parameter to evaluate the first cracking on specimen surfaces and also to compare the impact performance of thermoplastic composites. The effects of specimen temperatures and drying conditions were also found important in evaluating these materials. Microscopic observations revealed the microfracture mechanisms to explain the above macroscopic mechanical results. Static in-plane fracture toughness properties corresponded well with the impact properties.

Fracture toughness and impact behaviour of glass-fibre-reinforced polyamide 6,6 injection mouldings

The fracture toughness of injection-moulded long-glass-fibre-reinforced polyamide 6,6 (average fibre aspect ratio in mouldings, l/d ≈ 55) with fibre weight contents of 50 and 60 % was studied and compared with that of short-glass-fibre-reinforced polyamide 6,6 (l/d ≈ 20). Toughness was measured by using Mode I compact-tension specimens cut from the mouldings and also by means of instrumented drop-weight impact tests. The addition of fibres to a polyamide 6,6 matrix led to an improvement of fracture toughness. The improvement was marginally greater in the long-fibre grades. Fracture toughness values were dictated by the average in-plane fibre orientation angle ahead of the crack front and fibre length, particularly for the long-fibre grades. Instrumented drop-weight impact tests indicated brittle fracture for unreinforced polyamide 6,6, causing possible catastrophic shattering into several pieces. Short-fibre and long-fibre materials both remained intact after impact, with smaller dimensions of the damaged area for the long-fibre materials. Crack initiation force and total energy values were increased by 25–37% and 300–600%, respectively, with the inclusion of glass fibres. Negligible differences in these values were observed between the long- and short-fibre grades and with the long-fibre content increasing from 50 to 60% w/w.

Computational modeling for dynamic fracture process of FRP laminates under multiaxial impact loading1

Non-linear modeling for dynamic fracture process by the finite element method has been proposed to estimate the multiaxial impact strength of glass mat and glass, carbon, glass/carbon or aramid cloth laminates. The numerical results are compared with experimental data, which are obtained by instrumented drop-weight impact test and the validity of the proposed model is confirmed.

On the enhancement of impact damage tolerance of composite laminates

This paper examines the use of a thin layer of Ultra High Molecular Weight Polyethylene (UHMWPE) on the outer surface of carbon/epoxy composite materials as a method of improving impact resistance and damage tolerance through hybridization. Flat 16-ply laminates as well as honeycomb sandwich structures with eight-ply facesheets were tested in this study. Instrumented drop-weight impact testing was used to inflict damage upon the specimens. Evaluation of damage resistance included instrumented impact data, visual examination, C-scanning and compression after impact (CAI) testing. The results show that only one lamina of UHMWPE did not improve the damage tolerance (strength retention) of the 16-ply flat laminate specimens or the honeycomb sandwich beams, however, a modest gain in impact resistance (detectable damage) was found for the honeycomb sandwich specimens that contained an outer layer of UHMWPE.

ＣＲＴの衝撃時の動的挙動

Dynamic behavior of cathode ray tube (CRT) under the ball impact test condition was analyzed by computer simulation. Experiments were also carried out by using an instrumented drop-weight impact testing machine to investigate the impact load and stresses of CRT, and the phenomena of impact fracture of CRT were discussed. The following results were derived.(1) Dynamic behavior of CRT under several impact load conditions changing the impact speed and the hitting point was clarified by using the instrumented testing machine.(2) The results of computer simulation with the impact analysis program DYNA3D showed good agreement with the experimental dynamic behavior of CRT.(3) Static load due to vacuum in the tube or tightening force by the tension band had no effct on the dynamic characteristics of CRT before breaking.(4) It was explained quantitatively that hitting the corner of panel face at the impact test was more severe to the dynamic strength of CRT than the hitting the center of panel face.(5) The macroscopic impact fracture mode of CRT depended on the tensile stress at the edge of panel face and a criterion of the implosion was proposed.

アラミド／ガラスハイブリッド積層板の衝撃破壊解析

Various impact characteristics as well as fracture energy can be estimated by using an instrumented impact testing technique. The impact characteristics, such as a load versus deflection curve and maximum load, are used only for comparison among materials, but are seldom used for strength assessment in structural design. The analytical procedure which correlates the simple test data with structural strength under complex boundary conditions has not been established. In the present paper, a numerical method for dynamic strength analysis has been proposed and applied to estimate the multiaxial impact strength of aramid/glass hybrid composite plates.Dynamic fracture simulation based on the maximum stress criterion was carried out by using a finite element method and the numerical results were compared with the experimental data obtained by an instrumented drop-weight impact testing machine. Seven kinds of hybrid laminates were used for the tests, including roving-cloth reinforced composites, glass mat reinforced composite and mat/roving-cloth reinforced composites. The progressive fracture process under impact loading was well simulated and the numerical results showed close agreement with the experimental data which exhibit brittle fracture. The fracture controlling process of mat/roving-cloth reinforced laminates was also discussed quantitatively based on the present analysis.

Delamination failures in polymer composites

Delamination is one of the most important failure modes of composite structures. In the present paper, measurement of the interlaminar fracture toughness under static and impact loading as well as estimation of the compressive strength of delamination embedded composite plates have been investigated. A stabilized end-notched flexure test proposed by the present authors is introduced as a mode II interlaminar fracture toughness test, and some experimental data are presented. The interlaminar fracture toughness under impact loading is also estimated using an instrumented drop-weight impact test system, with which the effect of the loading rate on the mode II interlaminar fracture toughness is investigated. Instability-related delamination growth is examined experimentally and analytically by applying in-plane compressive load to an embedded through-width delamination sandwich column specimen.

Deformation and Fracture Mechanisms of Advanced Polymer Composites under Impact Loading

Instrumented drop weight impact testing has been used to study the impact performance of fiber reinforced polymer composites. Deformation processes and fracture mechanisms of thin-plate composite laminates were examined as a function of material parameters, plate thickness, stacking sequence, and impact loading rate. For the lami nates containing tough fibers, such as high-strength polyethylene, plastic deformation was found to be an important energy absorbing mechanism. High flexibility of these fibers allows the incident energy to be dispersed in a wider area and the impact load to be shared by a greater volume of material. A clean penetration crack was observed in the samples that contain primarily brittle fibers, such as graphite. This failure mode absorbed very lit tle incident energy. In addition, both matrix-fiber interfacial debonding and interlaminar cracking were shown to be effective in dissipating impact energy. Mechanistic study in dicated that the impact response of the laminates can be described by an initial stage of elastic deformation, followed by three stages of plastic deformation. Also, impact testing data showed that interlaminar hybridization with thermoplastic laminates was able to im prove the impact toughness of the otherwise brittle epoxy laminates. Fiber-matrix debon ding in thermoplastic laminates and interlaminar debonding between thermoplastic and epoxy laminates enhanced the impact energy absorption capacity of hybrid-matrix com posites. However, due to this poor interfacial adhesion, the result of post-impact three- point bending tests indicated an inferior damage tolerance for hybrid-matrix laminates. A simplified theory for impact failure mechanisms of composite laminates is proposed. It provides a mechanistic basis for the development of a practical model for the prediction of impact failure modes. This theory has proved to be useful in the studies on the impact penetration of more brittle composites.

Effect of testing speed on the crack growth resistance properties of nylon 6/ABS polymer blends

Crack growth resistance values are measured at three loading rates for blends of Nylon 6 and ABS with varying levels of compatibilizer using the generalized locus method [14]. Load against load point deflection curves are obtained at loading rates of 5 mm min−1 and 500 mm min−1 using a tensile testing machine. An instrumented drop weight impact testing machine is used to obtain data at a loading rate of 1.2×105 mm min−1. Dynamic effects can be considered negligible for the three testing speeds used and the effects of these speeds on crack growth resistance values can be observed. The results of these tests help determine the optimum compatibilizer level for high rate loading applications.