Axial cutting of AA6061-T6 circular extrusions under impact using single- and dual-cutter configurations

Experimental and numerical axial cutting of AA6061-T6 circular extrusions under both dynamic and quasi-static loading conditions were completed using single- and dual-cutter configurations to investigate load/displacement and collapse behaviour of the extrusions. Circular specimens with various wall thicknesses were considered for impact and quasi-static testing in this research. A steel cutter (AISI 4140) with four blades, having blade tip widths of 1.0 mm or 0.75 mm and blade lengths of 7 mm or 26.1 mm were used to cut through the extrusions. Straight and curved deflector profiles were used to flare the cut petalled sidewalls and facilitate the cutting system. Further quasi-static cutting tests using dual cutters were completed with or without the presence of a spacer to examine the load/displacement response as an adaptive energy absorption system. Results from the experimental impact tests illustrated that a higher peak cutting force, with a magnitude of approximately 1.09–1.98 times that of the force necessary under quasi-static testing conditions, was needed to initiate the cutting deformation mode. After this initial high force, the load/displacement responses were observed to be similar to those from the quasi-static tests with the exception of minor variations which resulted from material fracture that occurred on the petalled sidewalls during dynamic testing. Larger lengths of cutter blades and the curved deflector eased the flaring of the petalled sidewalls and reduced the occurrence of material fracture. The blade tip width had minor effects on the initial peak cutting force and mean cutting forces for extrusions under impact loading. The mean cutting force from the dynamic tests was determined to be 0.82–1.2 times that from the quasi-static experimental tests. Finally, quasi-static axial crushing of extrusions was completed to compare crashworthiness measures with the adaptive energy absorption system under the cutting deformation mode. A finite element model incorporating an Eulerian formulation was selected for the numerical model to simulate the cutting process. Simulation results generally agreed well with the experimental tests with a maximum over prediction of approximately 33% and 18% for the cutting force under impact and quasi-static loading, respectively.     

Static and dynamic axial crushing of square thin-walled aluminium extrusions

An experimental investigation was carried out to study the behaviour of square thin-walled aluminium extrusions in alloy AA6060 subjected to axial loading. Both static and dynamic tests were performed and the primary variables were the wall thickness and temper of the square tubes and the impact velocity of the projectile. The mass of the projectile in the dynamic tests was 56 kg, while the impact velocity was in the range 8–20 m/s. The experimental results show that a symmetric deformation mode is formed for the static tests with a lobe number that is a function of the temper. In the dynamic tests a mixture of modes is found. The experimental results also show that the dynamic mean force is significantly higher than the corresponding static force for the same axial displacement, which indicates a strong inertia effect. For initially straight square tubes, the mean load ratio between a dynamic and a static test is a decaying function with respect to the axial displacement. However, by introducing initial geometrical imperfections prior to dynamic testing an almost constant ratio is found.     

Crashworthiness characteristics of a carbon fiber reinforced dual-phase epoxy–polyurea hybrid matrix composite

The crashworthiness characteristics of rectangular tubes made from a Carbon-fiber reinforced Hybrid-Polymeric Matrix (CHMC) composite were investigated using quasi-static and impact crush tests. The hybrid matrix formulation of the CHMC was created by combining an epoxy-based thermosetting polymer with a lightly crosslinked polyurea elastomer at various cure-time intervals and volumetric ratios. The load–displacement responses of both CHMC and carbon-fiber reinforced epoxy (CF/epoxy) specimens were obtained under various crushing speeds; and crashworthiness parameters, such as the average crushing force and specific energy absorption (SEA), were calculated using subsequent load–displacement relationships. The CHMC maintained a high level of structural integrity and post-crush performance, relative to traditional CF/epoxy. The influence of the curing time and volumetric ratios of the polyurea/epoxy dual-hybridized matrix system on the crashworthiness parameters was also investigated. The results reveal that the load carrying capacity and total energy absorption tend to increase with greater polyurea thickness and lower elapsed reaction curing time of the epoxy although this is typically a function of the loading rate. Finally, the mechanism by which the CHMC provides increased damage tolerance was also investigated using scanning electron microscopy (SEM).     

An experimental study of square tube crushing under impact loading using a modified large scale SHPB

This paper presents an experimental study on square tubes made from a rate insensitive material under static and impact loading. Rate insensitivity of the base material (Cu–Zn alloy) is confirmed by static and dynamic tests on small samples cut from the tubes. A direct impact large scale Hopkinson bar (80 mm diameter, 10 m length) system is used to perform tube crushing tests. A two-point measurement method is applied to extend measuring duration of the pressure bar, which is usually limited by its length. The proposed method permits to monitor the whole tube crushing process.Static and impact tests (7–15 m/s) on these square tubes reveal that there is a significant increase under impact loading of both initial and successive peak loads with respect to quasi-static loading. Such a study is useful for the understanding of strength enhancement under impact loading observed for cellular materials such as honeycombs.     

Investigation of the response to low velocity impact and quasi-static indentation loading of particle-toughened carbon-fibre composite materials

This work investigates damage caused by low velocity impact and quasi-static indentation loading in four different particle-toughened composite systems, and one untoughened system. For impact tests, a range of energies were used between 25 and 50 J. For QSI, coupons were interrupted at increasing loading point displacement levels from 2 to 5 mm to allow for monitoring of damage initiation and propagation. In both loading cases, non-destructive inspection techniques were used, consisting of ultrasonic C-scan and X-ray micro-focus computed tomography. These techniques are complemented with instrumentation to capture force–displacement data, whereby load-drops are associated with observed damage modes. Key results from this work highlight particular issues regarding strain-rate sensitivity of delamination development and an earlier onset of fibre fracture associated with particle-toughened systems. These issues, in addition to observations on the role of micro-scale events on damage morphology, are discussed with a focus on material development and material testing practices.     

Comparison of the crushing performance of hollow and foam-filled small-scale composite tubes with different geometrical shapes for use in sacrificial cladding structures

This paper presents the quasi-static crushing performance of nine different geometrical shapes of small-scale glass/polyester composite tubes filled with polyurethane closed-cell foam for use in sacrificial cladding structures. The effect of polyurethane foam on the crushing characteristics and the corresponding energy absorption is addressed for each geometrical shape of the composite tube. Composite tubes with two different thicknesses (1 mm and 2 mm) have been considered to study the influence of polyurethane foam on the crushing performance. From the present study, it was found that the presence of polyurethane foam inside the composite tubes suppressed the circumferential delamination process and fibre fracturing; consequently, it reduced the specific energy absorption of composite tubes. Furthermore, the polyurethane foam attributed to a higher peak crush load for each composite tube. However, the presence of polyurethane foam inside the composite tubes significantly increased the stability of the crushing phenomena especially for the square and hexagonal cross-sectional composite tubes with 1 mm wall thickness. The results from this study are compared with our previous results for composite tubes without polyurethane foam [1].     

Influence of temperature and thickness on the off-axis behaviour of short glass fibre reinforced polyamide 6.6 – cyclic loading

This paper investigates the anisotropic properties of short glass fibre reinforced polyamide 6.6 (PA66-GF35) under tension–tension and tension–compression cyclic loading. Tensile fatigue tests were carried out on dog-bone specimens, machined out from injection-moulded plates 80 × 80 mm, of three different thicknesses t (1 and 3 mm) at three different nominal fibre orientation angles θ (0°, 30° and 90°). The tests were carried out at RT as well as at 130 °C.The Tsai–Hill failure criterion, modified to account for cyclic loading, is applied to the fatigue data for estimating the fatigue strength parameters of the material under investigation. Results are compared to the strength parameters obtained under quasi-static loading in a previous part of this work [De Monte M, Moosbrugger E, Quaresimin M. Influence of temperature and thickness on the off-axis behaviour of short glass fibre reinforced polyamide 6.6 – quasi-static loading. Composites: Part A, 2010;41(10):1368–79]. The experimental results highlight how specimen thickness remarkably affects mechanical properties: the thinner the specimen the higher will be the degree of anisotropy. Also temperature strongly reduces the fatigue strength under cyclic loading. The Tsai–Hill criterion allows for an adequate fitting of experimental data at the investigated temperatures and load ratios.     

Numerical Study on Hybrid Tubes Subjected to Static and Dynamic Loading

The commercial finite element program LS-DYNA was employed to evaluate the response and energy absorbing capacity of cylindrical metal tubes that are externally wrapped with composite. The numerical simulation elucidated the crushing behaviors of these tubes under both quasi-static compression and axial dynamic impact loading. The effects of composite wall thickness, loading conditions and fiber ply orientation were examined. The stress–strain curves under different strain rates were used to determine the dynamic impact of strain rate effects on the metal. The results were compared with those of a simplified analytical model and the mean crushing force thus predicted agreed closely with the numerical simulations. The numerical results demonstrate that a wrapped composite can be utilized effectively to enhance the crushing characteristics and energy absorbing capacity of the tubes. Increasing the thickness of the composite increases the mean force and the specific energy absorption under both static and dynamic crushing. The ply pattern affects the energy absorption capacity and the failure mode of the metal tube and the composite material property is also significant in determining energy absorption efficiency.     

Quasi-static and dynamic axial compression of glass/polyester composite hemi-spherical shells

An experimental investigation has been carried out to study the collapse mechanisms and the energy absorption capacities of the glass/polyester composite hemi-spherical shells under both quasi-static and drop hammer loading. The shells were made of randomly oriented glass fibre mats and polyester resin. Quasi-static tests were conducted at speed of 2 mm/min. and the impact velocities varied from 5 to 9 m/s. The radii of the shells varied from 53.5 to 106.1 mm and their thicknesses from 1.10 to 2.84 mm. Influence of these variables on the mechanism of deformation has been discussed. Experiments on 45 shells showed that their progressive crushing occurred due to the formation of successive zones of fracture. Based on these observations an analytical model has been developed for the prediction of load-deformation and energy-compression curves. The results thus obtained are found to match well with the experiments. It is seen that the ratio of the mean collapse loads recorded in impact and quasi-static tests for a given shell is greater than one but it decreases with the increase in thickness of the shell.     

Energy absorption and load carrying capability of woven natural silk epoxy–triggered composite tubes

This study investigated the energy absorption response and load carrying capability of woven natural silk/epoxy–triggered composite rectangular tubes subjected to an axial quasi-static crushing test. The rectangular composite tubes were prepared by hand lay-up technique. The tubes consisted of 12, 24, and 30 layers of natural woven silk/epoxy laminate and were 50, 80, and 120 mm long. The crashworthiness of the tubes was evaluated by measuring the specific energy absorption in quasi-static axial compression. Specific energy absorption was obtained from the load–displacement curve during testing. The failure mode of the tubes was analyzed from high resolution photographs obtained. Overall, the tube with 50 mm length and 30 layers showed the best crashworthiness among the tubes. The failure morphology showed that the specimens failed in two distinct modes: local and mid-length buckling. The triggered composite tubes exhibited progressive failure.     

Axial crushing of thin-walled high-strength steel sections

Quasi-static and dynamic axial crushing tests were performed on thin-walled square tubes and spot-welded top-hat sections made of high-strength steel grade DP800. The dynamic tests were conducted at velocities up to 15 m/s with an impacting mass of 600 kg in order to assess the crush behaviour, the deformation force and the energy absorption. Typical collapse modes developed in the sections and the associated energy absorbing characteristics were examined and compared with previous studies on high-strength steel. A significant difference was observed between the quasi-static and the dynamic crushing tests in terms of the deformation force and impact energy absorption. As this difference is attributed to strain-rate and inertia effects, material tensile tests at elevated strain rates have been carried out. A comparison is made with analytical methods and the response was under-predicted. In addition, numerical simulations of the axial crushing of the thin-walled sections were performed and comparisons with the experimental results were satisfactory. The validated numerical model was used to study the energy absorption capacity of thin-walled sections with variations in the yield strength, sheet thickness, flange width and spot-weld spacing. Structural effectiveness differences have been captured through simulations between spot-welded top-hat sections made of mild steel and high-strength steel.     

Failure of AA 6061 and 2099 aluminum alloys under dynamic shock loading

Microstructural aspects of the deformation and failure of AA 6061 and AA 2099 aluminum alloys under dynamic impact loading are investigated and compared with their responses to quasi-static mechanical loading in compression. Cylindrical specimens of the alloys, heat-treated to T4, T6 and T8 tempers, were subjected to dynamic compressive loading at strain rates of between 2800 and 9200 s⁻¹ and quasi-static compressive loading at a strain rate of 0.0032 s⁻¹. Plastic deformation under the dynamic impact loading is dominated by thermal softening leading to formation of adiabatic shear bands. Both deformed and transformed shear bands were observed in the two alloys. The shear bands offer preferential crack initiation site and crack propagation path in the alloys during impact loading leading to ductile shear fracture. While cracks propagate along the central region of transformed bands in AA 6061 alloy, the AA 2099 alloy failed by cracks that propagate preferentially along the boundary region between the transformed shear bands and the bulk material. Whereas the AA 2099 alloy shows the greatest propensity for adiabatic shear banding and failure in the T8 temper condition, AA 6061 alloy is most susceptible to formation of adiabatic shear bands and failure in the T4 temper. Deformation under quasi-static loading is dominated by strain hardening in the two alloys. Rate of strain hardening is higher for naturally aged AA 6061 than the artificially aged alloy, while the strain hardening rate for the AA 2099 alloy is independent of the temper condition. The AA 2099 alloy shows a superior mechanical behaviour under quasi-static compressive loading whereas the AA 6061 shows a higher resistance to impact damage.     

Dynamic ice forces of slender vertical structures due to ice crushing

The dynamic ice forces and structure vibrations generated by crushing failure of ice sheet were investigated by full-scale tests conducted on a cylindrical compliant monopod platform in Bohai Bay, China. The load panels recorded three ice force modes distinguished by ice speed, which cause structure quasi-static, steady-state and random vibrations respectively. Based on the tests data, the physical process of dynamic ice forces in crushing failure was probed, by analysis of mechanical behaviour of ice under compressive loading related to relative loading speed of ice-structure interaction. It is proved that the three ice force modes takes place in loading speeds which make ice fail in ductile, ductile–brittle transition and brittle range respectively. In addition, the key problem from the structure design point of view involved in each ice force mode is discussed.     

Crashworthiness of self-piercing riveted connections

This paper presents an experimental and numerical study on the behaviour of self-piercing riveted connections under quasi-static and dynamic loading conditions. The specimens were made from aluminium sheets in alloy AlMg3.5Mn and AlMg3.0Mn with a nominal thickness of 3 mm. The dynamic tests were performed using a viscoelastic split Hopkinson pressure bar (SHPB). Two different specimen geometries were designed in order to test the riveted connections under pull-out and shear loading conditions. The dynamic tests were performed under three different velocities, i.e. approximately 10, 15 and 20 m/s, and compared with tests performed under static conditions. The influence of the loading rate on the behaviour of the riveted connection was investigated in terms of force–displacement curves and failure mode. Furthermore, a 3D numerical model of the riveted connection was generated using the explicit finite-element code LS-DYNA and numerical simulations of the quasi-static and dynamic tests were performed.     

Fiber/matrix interphase characterization using the dynamic interphase loading apparatus

The dynamic interphase loading apparatus (DILA) developed to perform microdebonding push-out tests directly characterizes the fiber/matrix interphase properties of composites as a function of loading rate. The use of a piezoelectric transducer allows one to input a variety of displacements and loading rates (quasi-static to 50 mm/s) to the indenter. Transient force and displacement values, recorded during the test, are then used to determine the average shear strength and energy absorbed during debonding and frictional sliding during the microdebonding process. An E-glass/vinyl ester composite was tested under single microdebonding as well as fatigue loading. Test results showed that the strength and energy-absorbing capability of the interphase was sensitive to loading rate.     

Quasi-brittle fracture during structural impact of AA7075-T651 aluminium plates

The stress–strain behaviour of the aluminium alloy 7075 in T651 temper is characterized by tension and compression tests. The material was delivered as rolled plates of thickness 20 mm. Quasi-static tension tests are carried out in three in-plane directions to characterize the plastic anisotropy of the material, while the quasi-static compression tests are done in the through-thickness direction. Dynamic tensile tests are performed in a split Hopkinson tension bar to evaluate the strain-rate sensitivity of the material. Notched tensile tests are conducted to study the influence of stress triaxiality on the ductility of the material. Based on the material tests, a thermoelastic–thermoviscoplastic constitutive model and a ductile fracture criterion are determined for AA7075-T651. Plate impact tests using 20 mm diameter, 197 g mass hardened steel projectiles with blunt and ogival nose shapes are carried out in a compressed gas-gun to reveal the alloy's resistance to ballistic impact, and both the ballistic limit velocities and the initial versus residual velocity curves are obtained. It is found that the alloy is rather brittle during impact, and severe fragmentation and delamination of the target in the impact zone are detected. All impact tests are analysed using the explicit solver of the non-linear finite element code LS-DYNA. Simulations are run with both axisymmetric and solid elements. The failure modes are seen to be reasonably well captured in the simulations, while some deviations occur between the numerical and experimental ballistic limit velocities. The latter is ascribed to the observed fragmentation and delamination of the target which are difficult to model accurately in the finite element simulations.     

On the response of thin-walled CFRP composite tubular components subjected to static and dynamic axial compressive loading: experimental

In this work the crushing response and crashworthiness characteristics of thin-wall square FRP (fibre reinforced plastic) tubes that were impact tested at high compressive strain rate are compared to the response of the same tubes in static axial compressive loading. The material combination of the tested specimens was carbon fibres in the form of reinforcing woven fabric in epoxy resin, and the tested tubes were constructed trying three different laminate stacking sequences and fibre volume contents on approximately the same square cross-section. Comparison of the static and dynamic crushing characteristics is made by examining the collapse modes, the shape of the load–displacement curves, the peak and average compressive load and the absorbed amount of crushing energy in both loading cases. In addition, the influence of the tube geometry (axial length, aspect ratio and wall thickness), the laminate material properties-such as the fibre volume content and stacking sequence-and the compressive strain rate on the compressive response, the collapse modes, the size of the peak load and the energy absorbing capability of the thin-wall tubes is extensively analysed.     

Experimental and theoretical study of the lateral compression process on the empty and foam-filled hexagonal columns

In this article, a new analytical model of plastic deformation during the flattening process on hexagonal metal columns under the lateral loading in the quasi-static condition is presented. Based on the introduced model, some theoretical relations are derived to forecast the average and instantaneous lateral load of the hexagonal column in two different conditions: empty and polyurethane foam-filled. Then, some lateral compression tests were carried out on the empty and foam-filled metal columns and the experimental results were compared with the theoretical predictions by the present formulas that showed an admissible correlation. The theoretical relations estimate variations of the lateral load in terms of lateral displacement. The theoretical analysis shows that the lateral load on the hexagonal column during the flattening process is dependent on the column wall thickness, column length, and material properties of the column and polyurethane foam-filler. Finally, the effects of geometrical characteristics of the hexagonal columns and material properties of the columns and foams on the lateral load are investigated, experimentally.     

Influence of temperature and thickness on the off-axis behaviour of short glass fibre reinforced polyamide 6.6 – Quasi-static loading

This paper investigates the anisotropic behaviour of mechanical properties of a short glass fibre reinforced polyamide 6.6 (PA66-GF35) under quasi-static loading. For this purpose tensile tests were carried out on dog-bone specimens, machined out from injection moulded plates 80 × 80 mm, of three different thicknesses t (1–3 mm) at eight different orientation angles. The tests were performed at room temperature as well as at 130 °C. Material elastic constants were estimated from fitting experimental tensile moduli according to the theory of elasticity for orthotropic materials. A fit on geometrical tensile strengths with the Tsai–Hill failure criterion provided instead the material strength parameters. Both specimen thickness and temperature appear to have a strong influence on mechanical properties and degree of anisotropy.     

Effect of triggering and polyurethane foam-filler on axial crushing of natural flax/epoxy composite tubes

In this study, empty and polyurethane-foam filled flax fabric reinforced epoxy composite tubes were longitudinally crushed under quasi-static compression. The effects of foam-filler (density of 160 kg/m³, two diameters of 64 and 86 mm), tube thickness (2, 4 and 6 plies of laminate), and triggering (45° edge chamfering) and the combination of triggering and foam-filler on the crushing characteristics and energy absorption capacity of these tubes were investigated. The test results indicate that the observed primary failure mode was progressive crushing for all the specimens. Foam-filled tubes have better crashworthiness than empty tubes in total absorbed energy, specific absorbed energy and crush force efficiency. The presence of triggering has no significant effect on total absorbed energy and specific absorbed energy of the empty tubes. However, the crush force efficiency of triggered tube is significantly larger compared to the non-triggered one. In addition, the triggering minimises the force variation of the tubes from the average crush force and in turn a more stable progressive crushing is achieved. The foam-filled and triggered tubes have better crashworthiness than the empty tubes in all the aspects. Compared with either triggered or foam-filled tubes, the triggered and foam-filled tubes have larger values in average crush load and crush force efficiency. In terms of total absorbed energy and specific absorbed energy, the triggered and foam-filled tubes have values always larger than those of the tubes with triggering only, but these values are either larger or smaller than the tubes with foam-filler only.