Papers by Author: H. Hadavinia

Abstract: One of the primary applications of polymer based cellular solids is to act as an energy absorbing material during impact where compressive strain rates may reach 500-800/s. In reality, impacts occur over a wide range of temperatures and velocities at different angles of incidence. Understanding and modelling the behaviour of the polymer foams requires characterisation of the material response in detail. The stress-strain response that covers both compressive and tensile behaviour for a wide range of strain rates and temperatures are needed to characterize the mechanical performance of polymer foams as polymeric foams are highly nonlinear materials that undergo large deformation in crashworthiness related cases. It is reported in literature that any increase or decrease in temperature over the glass transition region can cause changes by order of magnitude in elastic modulus of polymeric foams. However, creation of cross linking at high temperature can affect the elastic modulus. In this work, the behaviour of two, polyamide-6 (PA-6) based closed cell foams at elevated temperatures were investigated covering the glass transition temperature. This work presents the variation of elastic and tangent modulus of two low densities PA-6 and PA-6/polyolefin (Nylon alloy) based foams. Empirical equations have been proposed to allow the prediction of modulus over a temperature range of 23°C to 120°C for these materials.

Abstract: The mixed-Mode interlaminar fracture toughness, GI/IIC, of z-pinned hybrid laminated composites is studied to investigate the effect of 3D-composites on the crack propagation resistance of delaminated composite structures. In this regard, the mixed-Mode interlaminar fracture toughness, GI/IIC, was measured using asymmetric double cantilever beam (ADCB) test method. The hybrid ADCB and z-pinned hybrid composite beams were laid-up from [G0/C0]4, [G0/C90]4, [G90/C0]4 and [G90/C90]4 to study the effect of z-pinning on the interlaminar fracture toughness. From the obtained results from test it was found that the resistance of z-pin fibres against the crack propagation in z-pinned hybrid composites can significantly increase the mixed-mode interlaminar fracture toughness.

Abstract: It is a fundamental response of any polymeric foam material to undergo non-recoverable deformation following the application of a defined compressive strain, exacerbated by temperature and humidity. This process is commonly referred to as compression set. The ability to predict recovery after the application of a compressive strain is crucial to both the manufacturers and end users of foam materials. Specific compression set test procedures have been established to quantify the extent of non-recoverable deformation in specific foam types but to date no general predictive approach exists. In this work, compression set (fixed strain) tests were undertaken on a cellular polyamide-6 material at various temperatures (-5°C to 90°C) and the foam recovery monitored over time periods in excess of those dictated by standard methods (ISO 1856 [1]). An empirical formula has been proposed to allow the prediction of recovery after compressive strain, covering recovery periods from 10 minutes to 24 hours (up to 168 hours at 23°C).

Abstract: This study is focused on modification of a conventional bumper damper to increase its energy absorbability. This bumper damper device is currently used on passenger vehicles for reducing structural damage, as well as part of the pedestrian protection system. A hollow aluminium cylinder with 2mm in thickness was subjected to a range of experiments. The energy absorption takes place as a ram displaces along the cylinder’s bore, where the outer diameter of the ram is slightly wider than the inner diameter of this cylinder.

Abstract: The present paper investigates specific sustained crushing stress (SSCS) of various composite laminate designs and stiffened boxes under axial crushing test. In this regard, an optimum composite crash box design is sought by studying the effect of laminate design and stiffeners on SSCS. Crash boxes were fabricated from carbon/epoxy twill-weave fabrics of [0]4, [45]4 and [0,45]2. The progressive failure with three distinct crushing modes of transverse shearing, lamina bending and brittle fracture was observed for three laminate designs. Two new assembled composite boxes were made from channels and V-shape stiffener and tested in quasi-static condition. Adhesive bonding was used in joining the channelled and stiffened boxes. Measured amount of SSCS for all models were compared to find an optimum crash box. It was found SSCS increases with increasing proportion of 0° plies in the laminate.

Abstract: In this paper the energy absorption of thin-walled aluminium tubes used as crash boxes in the body structure of a vehicle has been optimized. In order to achieve this, various cross-sections of extruded aluminium were chosen and their behaviour under dynamic impact loading was investigated. The crash boxes were made from aluminium alloy 6060 temper T4. Finite element software LS-DYNA in ANSYS was used for modelling. For each cross-section, the results of dynamic crushing load versus crushing distance was obtained from the FE simulation and the results were compared with the experimental and numerical work on a square crash box in the literature. Parameters such as the crush force efficiency and the specific energy of various crash boxes were compared with the relevant ones for the square crash box and the most efficient crash box was recommended.

Abstract: The effect of transverse shear on the deformation of thick laminated sandwich plates under cylindrical bending is studied, based on the first order shear deformation theory (FSDT) with the application of shear correction factor (SCF). It is shown that depending on the mechanical and geometrical properties of the layers, the contribution of the transverse shear stress to the maximum deflection of the plate is variable and in some cases accounts for up to around 88% of the total deflection. The analytical results are compared and verified with finite element analysis.