Oblique Impact Tests Research Articles

Critical evaluation of the SHARP motorcycle helmet rating

The Department for Transport introduced a safety helmet assessment and rating program (SHARP) for motorcycle helmets sold in the United Kingdom. The mechanics behind the part of the scheme that uses a rigid sphere head-plus-helmet model to estimate the peak headform acceleration in oblique helmet impact tests is assessed and the approximations exposed. When finite element analysis (FEA) of a deformable helmet with a realistic shape was used, the predicted levels of oblique peak acceleration were much smaller than those estimated by the SHARP scheme. The statistical data of motorcyclists’ impact velocities, sites and impact type, used to weight the SHARP impact results, were shown to overemphasise impacts at the sides of the helmet. It was concluded that the derived oblique impact test results, based on an inappropriate mechanics model, are meaningless. Consequently, the helmet star ratings are not related to head protection levels.

A door sub-system sled test methodology for simulation of occupant responses in FMVSS 214 side impact oblique pole test

This paper describes the development of a new door subsystem sled test methodology to simulate the occupant responses in NHTSA's 75° oblique pole side impact test mode using SID-IIs and ES-2re dummies. This sled subsystem test methodology consists of a door sled oriented at 75°, with the dummy on a seat located at the same distance from the door as in full-vehicle test, to simulate gap closure for evaluating side airbags. The door sled is accelerated to an initial velocity of 20 mph and then impacts the stationary pole. From the onset of door contact with the pole, the velocity of door sled is decelerated following the non-struck side velocity of the vehicle by means of VIA decelerator. The validation of the test methodology with both SID-IIs and ES-2re dummies is presented by comparing their respective responses between the sled subsystem and full-vehicle tests.

FEA of oblique impact tests on a motorcycle helmet

In accidents, motorcycle riders full-face helmets often make oblique impacts with road surfaces. Finite element analysis was used to predict the rotational and linear acceleration of a Hybrid II headform, representing a motorcyclist's head, in such impacts, considering the effects of friction at the head/helmet and helmet/road interfaces. Simulations of the oblique impact test in British Standard BS 6658 were validated by comparison with published data. This showed that COST 327 experimental data was largely determined by the friction coefficient (0.55) between the helmet shell and abrasive paper, and hardly affected by that between the head and helmet. Slip was predicted at the shell/paper interface throughout the impact, due to the high angular inertia of the helmet, and the normal force remaining below 3.5 kN. Simulations of more severe motorcycle helmet impacts explored the effects of impact site and direction, impact velocity components, helmet fit and the scalp. In these impacts, the higher velocity component normal to the road caused high frictional forces on the helmet shell, eventually causing it to roll on the road. The peak headform rotational accelerations, at some impact sites, were potentially injurious. The most effective method of reducing head rotational acceleration could be a reduction in the linear acceleration limit of the helmet standards.

The Influence of Reduced Friction on Head Injury Metrics in Helmeted Head Impacts

Objective. Reducing surface friction has been shown to reduce head injury risk in some scenarios. However, rigid body mechanics suggests that other scenarios may exist in which reducing surface friction increases head injury risk. The goal of this study is to demonstrate that reducing friction on the surface of a helmet decreases the rotational acceleration of the head in some scenarios and increases it in other scenarios and to discuss the implications for helmet design. Methods. A series of drop tests were conducted to simulate normal and oblique impacts and assess head injury risk with conventional helmets and helmets that had been modified to reduce friction. Results. The low friction modification had little influence on peak linear head acceleration or HIC. However, it decreased peak rotational acceleration by 55% in one oblique impact scenario and increased it by 83% in another oblique impact scenario. In normal impacts, the low friction modification had no effect on peak rotational acceleration. Conclusions. The relationship between rotational acceleration and surface friction is sensitive to the impact scenario. A single oblique impact test may be misleading when assessing the overall performance of a low friction helmet. The implications of these findings for helmet design are discussed.

Experimental testing and phenomenological modelling of the fragmentation process of braided carbon/epoxy composite tubes under axial and oblique impact

A new simulation technique is presented for the phenomenological modelling of stable fragmentation in fibre reinforced composite structures under dynamic compressive loading. An explicit crash code is used for implementation of a hybrid modelling technique, in which two distinct material models act simultaneously. The first model is implemented in a multi-layered shell element and uses a unidirectional composites fracture criterion to predict potential ply fracture mechanisms on a macroscopic scale. This model is, however, unable to represent the complex localised fracture mechanisms that occur on a meso (sub-ply) scale under compression fragmentation loading. Therefore, a second constitutive model is added to capture the energy absorbing process within the fragmentation zone, utilising an Energy Absorbing Contact (EAC) formulation between the composite structure and the impacting body. The essential benefits of the procedure are that it requires minimal input data that can be obtained from simple fragmentation tests, and that the procedure is computationally efficient enabling application to large scale industrial structures. The EAC theory is discussed, together with the required material model parameters. A series of dynamic axial and oblique impact tests and simulations of cylindrical continuous carbon fibre reinforced composite tubes have been performed to validate the approach. Furthermore, the application to more complex load cases including combinations of fragmentation and global structural fracture have also shown a good correlation with test results.

Oblique impact testing of bicycle helmets

The performance of bicycle helmets was investigated in oblique impacts with a simulated road surface. The linear and rotational accelerations of a headform, fitted with a compliant scalp and a wig, were measured. The peak rotational accelerations, the order of 5 krad s −2 when the tangential velocity component was 4 m s −1, were only slightly greater than in comparable direct impact tests. Oblique impact tests were possible on the front lower edge of the helmet, a site commonly struck in crashes, without the headform striking the ‘road’. Data characterizing the frictional response at the road/shell and helmet/head interfaces, were generated for interpretation via FEA modelling.

Some studies on scuffing in boundary lubricated sliding contact with subsurface plastic deformation

PurposeTo study the nature of scuffing in boundary lubricated sliding contacts with subsurface plastic deformation, as it occurs in plastic deformation processing.Design/methodology/approachLow speed oblique plastic impact testing (LOSOPIT) has been conducted on copper specimen with a hard En31 ball in a test rig that has facility to measure the coefficient of friction. Based on the findings of friction coefficient in these experiments, friction power has been estimated and was found to be in the typical range. Scuffing studies were undertaken both by observation of the slid surface of En31 sphere in a ferrographic microscope with camera facility as well as by calculation of the friction power.FindingsThe boundary lubricant was found to have profound role in safeguarding the surface from severe deformation and micro‐cracks. Scanning electron microscope (SEM) examination of the craters produced by LOSOPIT has given evidence that using the boundary lubricant resulted in smooth transfer of shear stress from the sphere to the specimen surface through the boundary lubricant layer. Owing to this, the asperities have been found flattened in a smooth manner instead of metal at the surface being scuffed. A limited amount of reduction was found in the coefficient of friction due to the use of boundary lubricant from that in the dry testing.Research limitations/implicationsThe model used to estimate the friction power is predominantly governed by the friction coefficient itself rather than either the normal load or the sliding speed. Friction coefficient itself may be contributed by various mechanisms all of which may not equally contribute to scuffing. Study is underway to carefully glean out those components of friction that exactly result in scuffing, and to use more effective criteria for scuffing.Practical implicationsThe knowledge and data developed in the paper give a clear explanation of conditions under which scuffing can take place in sliding contacts operating under boundary regime. The most important applications are metalforming and metal cutting. It is relevant to mechanical engineering machinery in which intense contact pressures are expected.Originality/valueThis paper fills the gap of lack of scuffing studies in plastic deformation processing. All earlier studies focused on elastic conditions prevailing at the contact. Since, industry has been witnessing a need to tackle the severe problems related to formed product quality and certain defects hitherto unexplained, this paper gives a new direction to explain the defects in products from scuffing point of view. In this paper, it has been shown that friction power can be a good criterion to represent scuffing intensity in boundary lubrication.

4134 斜め衝突時の破片分布計測(S77 宇宙ごみ問題とその解決策,S77 宇宙ごみ問題とその解決策)

We have developed measurement system of fragment distribution after hypervelocity impact so far. In this paper, we tried to improve accuracy of fragment distribution by increasing the number of control points in Direct Linear Transformation Method, and successfully applied this to oblique impact tests.

A STUDY ON DAMAGE BEHAVIOR OF GLASS BY OBLIQUE IMPACT OF STEEL BALL

The damage behavior of soda-lime glass was studied by oblique impact test with a steel ball, and in order to investigate the effect of roughness damage on glass, variable damages and strength degradation of smooth and Al 2 O 3 sand blasted for roughness glass was investigated by the ball impact test. After the test, the crack patterns were investigated using a stereo-microscope. In addition, the finite element method was performed to analyze the stress distribution and variation in the glass impacted obliquely by the steel ball. As a result of the impact test, the crack length of 90° impacted glass was the largest and that of 50° impacted glass was the smallest. In particular, as the impact velocity and diameter of the steel ball increased, the difference in crack length was prominent. The finite element analysis showed the maximum principle stresses distribution in the contact area of glass specimen. The result of analysis was accorded with the crack growth behavior by the oblique impact test.

Bicycle helmet design

Finite-element analysis (FEA) was used to consider the improvement of bicycle helmet performance, in oblique impacts with the road. The events, predicted by FEA, were described and the peak head accelerations interpreted in terms of a simplified model. FEA shows that thicker foam liners of lower compressive yield stress can protect the head against linear acceleration in 150 J impacts. The peak rotational acceleration of the head was shown to be only slightly affected by the friction coefficient on the road and, in general, to be insufficient to cause serious diffuse brain injuries. Improved protection of the forehead and temple was investigated; this required extending the liner downwards at the sides and front, and the inclusion of an oblique impact test in the BSEN 1078 standard.

A New Laboratory Rig for Evaluating Helmets Subject to Oblique Impacts

Current requirements and regulations governing motorcycle helmets around the world are based on test results of purely radial impacts, which are statistically rare in real accidents. This study presents a new impact rig for subjecting test helmets to oblique impacts, which therefore is able to test impacts of increased statistical relevance to real motorcycle accidents. A number of different head-helmet interfaces have been investigated. A test rig was constructed to produce oblique impacts to helmets simulating those occurring in real motorcycle accidents. A Hybrid III dummy head was fitted with accelerometers to measure the accelerations arising during impact testing. The equipment used for data collection was validated in both translational and rotational acceleration. In order to better resemble the human head, an artificial scalp was fitted to the hybrid dummy. The same test rig was used to investigate the performance of a number of different helmets. Impact velocities ranging from 7.3 to 9.9 m/s were tested using a number of different impact angles and impact areas. This study shows that the new test rig can be used to provide useful data at speeds of up to 50 km/h and with impact angles varying from purely tangential to purely radial. The rotational accelerations observed differ greatly depending on both helmet and scalp designs. For example, a helmet with a sliding outer shell placed on an experimental head fitted with an artificial scalp (made to resemble the human scalp) reduces rotational accelerations of the head by up to 56%, compared with those of an experimental head fitted with a fixed scalp and conventional helmet. The degree of slippage between the skull and the scalp, and between the scalp and the helmet, leads to considerable variation in the results. This innovative test rig appears to provide an accurate method for measuring accelerations in an oblique impact to a helmet. In order to obtain a good level of repeatability in oblique impact testing, it is crucial that the helmet be fixed to the head in the exact same way in each individual test. Both the position and the angle of impact must be reproduced identically in each test. The test rig used here has shown that this type of rig can be used to compare different helmet designs, and it therefore is able to contribute to achieving safer helmets.

Finite Element Analysis and Experimental Study About Damage Behavior of Glass by Oblique Impact of Steel Ball

The damage behavior of soda-lime glass was studied due to a steel ball of 1mm and 2mm at oblique impact test. The thickness of glass specimen were 3mm and 5mm and oblique degrees of impact were 90,70 and 50. After the steel ball impact test, the crack patterns were investigated using a stereo-microscope. In addition, the finite element method was performed to analyze the stresses distribution and variation in the oblique impacted glass by steel ball. As a result of the impact test, the crack length of 90impacted glass was the largest and that of 50impacted glass was the smallest. In particular, as the impact velocity and diameter of the steel ball increased, the difference of crack length was prominent. The finite element analysis showed the maximum principle stresses distribution in contact area of glass specimen. The result of analysis was accorded with the crack growth behavior by the oblique impact test.

Ballistic performance of Ceramic Faced Composite Plates at Oblique Impact Test

Ballistic performance of Ceramic Faced Composite Plates at Oblique Impact Test

A new oblique impact test for motorcycle helmets

A new oblique impact test for motorcycle helmets is described, simulating a fall from a motorcycle on to the road surface or the windshield of a car. An instrumented headform falls vertically to impact a horizontally moving rigid rough or deformable surface. Both the impact site on the helmet, and the vertical and horizontal velocities, can be varied, while the headform linear and rotational accelerations are measured. The rig was used to compare a new helmet design with current helmets, which are designed to pass impact tests in which the impact force is perpendicular to the helmet surface. The new design, which has a low friction layer between the shell and the liner, reduced, by up to 50%, the rotational acceleration of the head compared with conventional designs.

Normal and oblique hypervelocity impacts on carbon fibre/peek composites

A series of normal and oblique hypervelocity impact tests were performed on carbon fibre/PEEK composite specimens using the Light Gas Gun facilities at the University of Kent at Canterbury. The tests were conducted on 16 ply and 24 ply targets, using 1 mm and 2 mm aluminum projectiles, at velocities of approximately 5 km/s, and at impact angles of 0, 30 and 45 degrees. The primary objective of this investigation was to add oblique impact data to the University of Toronto Institute for Aerospace Studies (UTIAS) database on hypervelocity impact damage to composites. The parameters investigated in this study were debris cloud (primary and secondary) dispersion cone angles, (equivalent) entry and exit crater diameters, and front and rear surface delamination damage zones. The results of the analysis showed that the cone angles were not symmetric about the projectile velocity vector, maxima for both angles occurring at a normalized energy of approximately 55 J, followed by an asymptotic decrease. The results obtained for the entry crater diameter and equivalent entry damage diameter were found to be in good agreement with the existing UTIAS database. For a given impact energy, the damage area in the 24 ply targets was found to be nearly twice that of the damage in the 16 ply targets. Results were found to be in good agreement when compared, using an empirical equation, to predictions of impact damage of Al projectiles on aluminum targets (when a scaling factor of 0.9 was applied to the PEEK data to account for the greater damage in composites).

Hypervelocity impact on carbon fibre reinforced plastic / aluminium honeycomb: Comparison with whipple bumper shields

Normal and oblique incidence hypervelocity impact tests (velocity range 4-6 km s(-1)) were carried out to determine the ballistic limit of a 1.6 mm carbon fibre reinforced plastic facesheet bonded to 45 mm aluminium honeycomb core, as typically used in Low Earth Orbiting spacecraft. The internal honeycomb damage was determined as a function of the impactor parameters. The ballistic limit data showed a strong dependence with impact angle. The internal honeycomb damage was found to be independent of impact angle for constant impact energy for Theta<50 degrees. An empirically-determined damage equation linking honeycomb damage to impact energy was developed. For the highest impact energy perforating impacts, the debris cone angles for the primary and secondary debris cones were determined. As the impact angle increased, the centre of the damage cones rotated away from the line of flight. The data have been compared with the ballistic limit curve defined by the modified Cour-Palais aluminium Whipple bumper equation and show broad agreement with the equation predictions. A reduced value of the rear facesheet thickness is required to bring the normal and 15 degrees incidence data into agreement with the ballistic limit curve.

Penetration and ricochet phenomena in oblique hypervelocity impact

An experimental investigation of phenomena associated with the oblique hypervelocity impact of spherical projectile on multisheet aluminum structures is described. A model that can be employed in the design of meteoroid and space debris protection systems for space structures is developed. The model consists of equations that relate crater and perforation damage of a multisheet structure to parameters such as projectile size, impact velocity, and trajectory obliquity. The equations are obtained through a regression analysis of oblique hypervelocity impact test data. This data shows that the response of a multisheet structure to oblique impact is significantly different from its response to normal hypervelocity impact. It was found that obliquely incident projectiles produce ricochet debris that can severely damage panels or instrumentation located on the exterior of a space structure. Obliquity effects of high-speed impact must, therefore, be considered in the design of any structure exposed to the meteoroid and space debris environment.

An empirical relation for the volume of the crater formed during high velocity oblique impact tests

An empirical equation relating the volume of the crater formed during high velocity oblique impact tests to the velocity and angle of impact and to the target material hardness has been proposed. It is further demonstrated that this equation can predict the volume of the crater formed quite accurately over a wide range of impact velocities (50–350 m s -1), impact angles (up to 60°) and target hardnesses (1–4 GPa).