Biaxial Impact Research Articles

Development of a True-Biaxial Split Hopkinson Pressure Bar Device and Its Application.

Although highly desirable, the experimental technology of the dynamic mechanical properties of materials under multiaxial impact loading is rarely explored. In this study, a true-biaxial split Hopkinson pressure bar device is developed to achieve the biaxial synchronous impact loading of a specimen. A symmetrical wedge-shaped, dual-wave bar is designed to decompose a single stress wave into two independent and symmetric stress waves that eventually form an orthogonal system and load the specimen synchronously. Furthermore, a combination of ground gaskets and lubricant is employed to eliminate the shear stress wave and separate the coupling of the shear and axial stress waves propagating in bars. Some confirmatory and applied tests are carried out, and the results show not only the feasibility of this modified device but also the dynamic mechanical characteristics of specimens under biaxial impact loading. This novel technique is readily implementable and also has good application potential in material mechanics testing.

Fracture evaluation of plasticized polylactic acid / poly (3-HYDROXYBUTYRATE) blends for commodities replacement in packaging applications

Nowadays, scientific and technological efforts are being carried out to diminish serious ecological problems caused by indiscriminate use of non-biocompostable polymers in the packaging industry. In this sense, novel biodegradable blends of different composition based on poly(lactic acid) (PLA), poly(3-hydroxybutyrate) (PHB) and tributyrin (TB) are developed and here proposed as an eco-friendly alternative. Materials are characterized by fracture experiments under quasi-static and biaxial impact loading. Fracture behavior is analyzed together with thermal, tensile and water permeation properties to evaluate their potential in-service performance. TB_PLA/PHB blends with 15 wt% TB exhibit better permeation and fracture toughness than currently used bio-based polymers, being in the range of polyethylene properties. Results highlight the potential of these new blends broadening the current application field of PLA.

Deformation and failure of semicrystalline polymers under dynamic tensile and biaxial impact loading

We propose a constitutive model to aid in the engineering design of semicrystalline polymer components that may be subjected to biaxial impact loading. To this end, we investigate the thermomechanical and failure behaviour of high density polyethylene (HDPE) under dynamic loading, both experimentally and analytically. We have carried out dynamic tensile tests at 101, 102 and 103 mm/s displacement rates. Digital image correlation (DIC) and infrared thermography were used to measure full 2D true strain fields and determine specimen temperature rise during tensile testing. The results were used to calibrate the constitutive parameters. To analyse the biaxial impact response, we have carried out falling weight impact (FWI) tests at a 4 m/s impact velocity. We assessed the model prediction capabilities by comparing numerical predictions with experimental results and good agreement was observed. The proposed model, which aims to achieve a compromise between prediction accuracy and formulation simplicity, shows that initial linear elastic response coupled with a temperature-dependent power-law viscoplastic flow element and a non-linear strain-hardening element are sufficient to model biaxial stress scenarios.

Mechanics of polycarbonate in biaxial impact loading

• Biaxial impact of Polycarbonate (PC) plates is investigated numerically and experimentally. • A thermomechanical constitutive model to predict PC dynamic response is proposed. • In biaxial impact, high hydrostatic stresses at low triaxiality levels are developed. • Effective plastic stretch is the invariant that best represents ductile failure. • An extension for criteria modeling ductile/brittle failure competition is proposed. We investigate the deformation and failure behavior of Polycarbonate (PC) under low-velocity biaxial impact loading. For this, we have conducted experiments and numerical simulations of the falling weight impact test (FWT) for different testing configurations. To model PC behavior in dynamic situations, we employed a 3D thermomechanical constitutive model that captures the strain rate, pressure and temperature dependence of deformation response. The finite element model also incorporates adiabatic heating and contact friction effects. Numerical predictions of the FWT force–displacement curves are in good agreement with experimental observations. An assessment of the developed stress–strain fields indicates that the effective plastic stretch λ p is the invariant that most suitably represents the point of material failure in all different test configurations. We examine the relation of a maximum λ p fracture criterion with existing criteria for PC on the grounds of the triaxiality conditions specific to the biaxial impact test. Finally, we propose an extension of these criteria to enhance their capability of predicting failure in part geometries that may become subjected to more complex multiaxial loading scenarios.

Starch/multi-walled carbon nanotubes composites with improved mechanical properties

In this work, novel starch based nanocomposites containing very small quantities of multi-walled carbon nanotubes (MWCNTs) (0.027 wt% and 0.055 wt%) were developed. These materials exhibited highly improved tensile and impact properties as a consequence of wrapping the MWCNTs with a starch–iodine complex composed by the same starch of the matrix. Thus, good dispersion of the filler in the matrix and excellent adhesion between phases (as shown in FE-SEM micrographs) were achieved. Increments up to almost 70% in stiffness and 35% in ultimate tensile strength, keeping deformations higher than 80% without break were found. Therefore, tensile toughness also increased up to ∼50%. Enhancements of up to ∼100% in biaxial impact parameters (thickness related perforation energy and disc maximum strength values) were also observed. The significant improvements in all uniaxial tensile and biaxial impact properties obtained for such significantly low contents of filler, as a result of the type of functionalization used, have not been already reported in the literature and point out these biodegradable composites as a very appealing alternative to traditional materials for different applications.

Aircraft accident reconstruction: comparison of data

This paper presents results from the reconstruction of nine airplane accidents that occurred during the period of 1992 through 2000. The purpose of this paper is to re-examine previously compiled accident and test data for small airplanes. Comparing accident data from the present study with previous accident data shows the need to include the roll and yaw angles of an aircraft at impact when calculating the acceleration of the aircraft during impact. Comparing accident data from the present study with previous test data shows the need to include the bearing characteristics of an impacted surface when extrapolating aircraft test response to aircraft accident response. The need to compile more accident data and resolve differences between triaxial and biaxial impact conditions is discussed.

Impact properties and microhardness of double‐gated glass‐reinforced polypropylene injection moldings

AbstractInjection moldings with weld lines were produced in glass reinforced polypropylene grades differing in filler content using a two‐gated hot runner injection mold. The skin‐core microstructure developed during injection molding was qualitatively analyzed by means of optical and scanning electronic microscopy techniques. The load bearing capacity of the moldings was assessed by uniaxial tensile‐impact and biaxial instrumented falling dart impact tests. Microhardness was also used to ascertain the possibility of using it as a simple nondestructive technique for characterizing glass fiber‐reinforced injection moldings. The properties were monitored at various points to evaluate their variation at the bulk and the knit region. The biaxial impact test highlights the 10‐fold reduction of the impact strength caused by the weld line. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Thermal Degradation Behavior, Permeation Properties and Impact Response of Polyethylene/Organo-montmorillonite/(Ethylene Methacrylic Acid) Ternary Nanocomposites

Through this work we explored the effect of melt compounding a commercial grade of HDPE with organoclays of different precedence using EMAA as compatibilizing agent on the thermal behavior, barrier properties and biaxial impact response of composites. Morphology was examined by XRD and TEM. Crystalline structure was examined by DSC. Thermal behavior was evaluated by TGA. Barrier properties to low-molecular-weight penetrants were experimentally determined employing a gravimetric technique. Mechanical properties under impact conditions were evaluated by instrumented puncture tests. Intercalated nanocomposites were obtained. Throughout the thermal degradation of the nanocomposites in oxidant atmosphere a charring process of the PE, which is normally a non-char-forming polymer, was observed. The addition of OMMT improves barrier properties due to its contribution to tortuosity path and to the reduction of molecular mobility. Impact properties were only slightly reduced by nanocomposite formation. Results demonstrate that EMAA did not improve exfoliation, but it enhanced polymer–organoclay interactions giving rise to better thermal and permeation properties, without detriment of impact response.

Poly(acrylic acid) surface grafted polypropylene films: Near surface and bulk mechanical response

Radical photo-grafting polymerization constitutes a promising technique for introducing functional groups onto surfaces of polypropylene films. According to their final use, surface grafting should be done without affecting overall mechanical properties. In this work the tensile drawing, fracture and biaxial impact response of biaxially oriented polypropylene commercial films grafted with poly(acrylic acid) (PAA) were investigated in terms of film orientation and surface modification. The variations of surface roughness, elastic modulus, hardness and resistance to permanent deforma- tion induced by the chemical treatment were assessed by depth sensing indentation. As a consequence of chemical modifi- cation the optical, transport and wettability properties of the films were successfully varied. The introduced chains generated a PAA-grafted layer, which is stiffer and harder than the neat polypropylene surface. Regardless of the surface changes, it was proven that this kind of grafting procedure does not detriment bulk mechanical properties of the PP film.

Impact Fatigue Behaviour of Rubber-Modified Glass Fibre Reinforced Polyamide 6

The impact fatigue behaviour of glass fibre reinforced polyamide 6 with and without impact modifier was studied. The accumulated damage was investigated by means of light microscopy, confocal laser scanning microscopy and scanning electron microscopy. The impact modifiers were found to increase the ductility of the matrix. In particular, the increased resistance to crack initiation and growth during biaxial impact fatigue was demonstrated. The rubber particles cavitate during impact testing and initiate local shearing, thereby reducing the amount of debonding at the fibre ends, as compared to the sample without toughness modifiers. The shearing of the matrix suppresses debonding, matrix flaws at the fibre ends and fibre pull-out.

Adiabatische Schervorgänge

When materials are subjected to high strain rate or impact loading, respectively, the stress-strain behaviour is changed significantly by adiabatic shearing in comparison with quasistatic loading. If amounts of shear stress additionally to the compression stress acts on, the maximum of compression stress as well as deformability are reduced. Under laboratory condition it is possible to produce shear failures and to stop the test at any point of deformation in order to investigate the initiation and propagation of the failure process under biaxial impact loading by metallographic examination.

Biaxial impact fatigue of polycarbonate

An impact fatigue test has been designed for studying the repeated biaxial impact load response of polymers. In the test geometry, a thin plate-like sample is rigidly held by an annular clamp and is repeatedly impacted by a hemispherically-tipped plunger. Results have been obtained for polycarbonate. Under impact fatigue, cracks initiate on the bottom surface where high biaxial tensile stresses exist. These cracks propagate along radial lines on the sample surface and also into the sample normal to the surface. The load-bearing capability and the residual impact strength decrease as the cracks propagate. The development of impact fatigue cracks, the lifetime behaviour and the change in mechanical properties after partial impact fatigue are discussed.

Densification of a porous solid during hot forging

1. For a porous, viscous, compressible solid whose solid phase is incompressible and obeys a non linear law of flow, an analysis is made of the extent of densification during hot forging under uniaxial and biaxial impact compression conditions. Expressions are derived for the variation of density and densification time as functions of impact parameters and flowability of the solid phase of the porous solid. 2. It is shown that densification conditions in the hot forging of a porous solid are less favorable than in dynamic hot pressing. 3. It is demonstrated that, in both forging and dynamic hot pressing, increasing the mass of the striking tool improves densification conditions to a greater extent than does increasing the impact velocity.