Impact strength is one of the most important structural properties for a designer to consider, but is often the most difficult to quantify or measure. A constant concern in the field of composites is the effect of foreign object impact damage because it is often undetectable by visual inspection. An impact can create interlaminar damage that often results in severe reductions in strength and instability of the structure. The main objective of this study is to determine the effectiveness of a damage arrestment device (DAD) on the mechanical behavior of composite sandwiches, following a low-velocity impact. A 7.56-lbf crosshead dropped from a height of 37.5-inches was considered for the low-velocity impact testing. In this study, the experimental and numerical analysis of composite sandwiches were investigated, which included static 4-point bend and vibration testing. Composite sandwiches were constructed utilizing four-plies of Advanced Composites Group LTM45EL/CF1803 bi-directional woven carbon fiber face sheets with a General Plastics Last-A-Foam FR-6710 rigid polyurethane core. Specimens were cured in an autoclave, using the manufacturer’s specified curing cycle. In addition to the experimental and numerical analysis of composite sandwiches, developing and building a data acquisition (DAQ) system for the Dynatup 8250 drop weight impact tester was accomplished. Utilizing National Instruments signal conditioning hardware, in conjunction with LabView and MATLAB, complete testing software was developed and built to provide full data acquisition for an impact test. The testing hardware and software provide complete force vs. time history and crosshead acceleration of the impact event, as well as provide instantaneous impact velocity of the projectile. The testing hardware, software, and procedures were developed and built in the Aerospace Structures/Composites laboratory at Cal Poly for approximately 15% of the cost from the manufacturer. In the first study, static 4-point bend testing was investigated to determine the residual flexural strength of composite sandwich beams following a low-velocity impact. Four different specimen cases were investigated in the 4-point bend test, with and without being impacted: first a control beam with no delamination or DAD, second a control beam with a centrally located 1-inch long initial delamination, third a DAD key beam with two transverse DADs centrally located 1-inch apart, and finally a DAD key beam with a centrally located initial delamination between two transverse DADs. The specimens used followed the ASTM D6272 standard test method. The specimens were 1-inch wide by 11-inch long beams. The experimental results showed that the presence of DAD keys significantly improved both the residual stiffness and ultimate strength of a composite sandwich structure that had been damaged under low-velocity impact loading, even with the presence of an initial face-core delamination. In the second study, vibration testing was investigated as a means to detect a delamination in the structure and the effect of impact damage on the vibrational characteristics, such as damping, on composite sandwich plates. Four different specimen cases were investigated in the vibration test, both with and without being impacted: first a control plate with no delamination or DAD, second three control plates with varying 1-inch initial delamination locations at the 1st, 2nd, and 3rd bending-mode nodes, third a DAD key plate with one DAD running the entire length longitudinally along the center of the plate, and finally three DAD key plates with one DAD running the entire length longitudinally along the center of the plate and varying 1-inch delamination locations at the 1st, 2nd, and 3rd bending mode-nodes. The response accelerometer location was varied at 1-inch increments along the length of the plate. From the experimental results, it was determined that varying the location of the accelerometer had a significant effect on the detection of face-core delamination in a composite sandwich structure. Additionally, it was shown that damping characteristics significantly degraded in control case plates after a low-velocity impact, but they were better retained when a DAD key was added to the structure. Numerical analysis utilizing the finite element method (FEM) was employed to validate experimental testing, as well as provide a means to examine the stress distribution and impact absorption of the structure. The impact event was modeled utilizing the LS-Dyna explicit FE solver, which generated complete force vs. time history of the impact event. Static 4-point bending and vibration analysis were solved utilizing the LS-Dyna implicit solver. Finally a damaged mesh was obtained from the explicit impact solution and subjected to subsequent static 4-point bending and vibration analysis to numerically determine the residual mechanical behavior after impact. All cases showed good agreement between the numerical, analytical, and experimental results.

[ES] Las cargas de impacto son la fuente primaria de lesiones en la cabeza y pueden resultar en un rango de traumatismo desde leve hasta severo. Debido a la existencia de múltiples entornos en los que se pueden desencadenar lesiones por impacto (accidentes automovilísticos, deportes, caídas accidentales, violencia), éstas pueden afectar potencialmente a toda la población independientemente de su estado de salud. Pese al creciente esfuerzo en investigación para comprender la biomecánica de las lesiones por traumatismo en la cabeza, todavía no es del todo posible realizar predicciones precisas ni prevenir estos eventos. En esta Tesis, se han estudiado algunos aspectos del comportamiento ante impacto de los diferentes tejidos biológicos involucrados mediante el desarrollo de un modelo numérico de cabeza humana a partir de imágenes de tomografía computerizada (TAC). Se han realizado simulaciones en elementos finitos (EF) de ensayos experimentales de la literatura con el fin de validar el modelo numérico desarrollado, estableciendo unas propiedades mecánicas adecuadas para cada uno de sus constituyentes. De esta manera se puede adquirir una predicción adecuada del riesgo de sufrir daños. Parte de esta Tesis se centra en el entorno balístico, específicamente en cascos de combate antibalas, los cuales son susceptibles de causar traumatismo craneoencefálico debido a la elevada deformación que sufren durante el impacto. Previamente al estudio de estos fenómenos de alta velocidad, se han realizado ensayos experimentales y numéricos para caracterizar la respuesta mecánica de algunos materiales compuestos ante impacto de baja velocidad. Al principio de esta Tesis se ha realizado una revisión del estado del arte acerca de los criterios existentes para cuantificar el trauma craneoencefálico.Este es un aspecto clave para las simulaciones numéricas, ya que la idoneidad de algunos de estos criterios para la predicción de lesiones cerebrales todavía es un debate abierto. Mediante EF se han realizado simulaciones de impactos balísticos en una cabeza protegida con un casco de combate. Mediante la posterior aplicación de diferentes criterios de daño sobre los resultados obtenidos se ha evaluado el nivel de protección que aseguran los protocolos de aceptación de cascos de combate, así como las estrategias para determinar su tallaje. Se ha demostrado que las normativas existentes para cascos de combate son capaces de mitigar algunos mecanismos de trauma pero no logran prevenir otros como los gradientes de presión intracraneales. Además, se ha demostrado que algunas de las estrategias de tallaje más comúnmente adoptadas por los fabricantes, como producir un solo tamaño de calota, deberían ser reconsideradas ya que existe un mayor riesgo de traumatismo cuando la distancia entre la cabeza y la calota del casco no es suficiente. Siguiendo la línea de protecciones personales, algunos de los materiales compuestos comúnmente empleados en la industria armamentística se han combinado para crear distintas configuraciones de calota para optimizar la relación entre peso del casco y protección para la cabeza. Materiales ligeros como el UHMWPE han resultado en un comportamiento menos eficiente que el de los apilados de tejido de aramida a la hora de limitar la BFD (deformación máxima en la calota del casco en la zona de impacto). Hacia el final de la Tesis se presenta un modelo numérico de cabeza humana detallado, que incluye treinta y tres de las estructuras anatómicas principales. Dicho modelo se ha desarrollado para la simulación de un accidente ecuestre en el que aparecen múltiples lesiones craneoencefálicas. Principalmente, se pretende establecer un criterio mecánico para predecir el hematoma subdural (HS) basado en la ruptura de los vasos sanguíneos intracraneales. Se ha propuesto un valor umbral de ruptura en tensiones de 3.5 MPa, pero tanto este límite como la localización del vaso dañado son altamen / [CAT] Les càrregues d'impacte son la font primària de lesions al cap i poden resultar en un rang de severitat des de lleu a greu. Degut als múltiples entorns en que poden desencadenar-se lesions per impacte (accidents automobilístics, esports, caigudes accidentals, violència), aquestes poden afectar potencialment a tota la població independentment del seu estat de salut. Malgrat el creixent esforç en investigació per comprendre la biomecànica de les lesions per traumatisme al cap, encara no és del tot possible realitzar prediccions precises ni prevenir aquestos esdeveniments. En aquesta Tesi, s'han estudiat alguns aspectes del comportament a impacte dels diferents teixits biològics involucrats mitjançant el desenvolupament d'un model numèric de cap humà a partir d'imatges de tomografia computeritzada (TAC). S'han realitzat simulacions en elements finits (EF) d'assajos experimentals de la literatura amb la finalitat de validar el model numèric desenvolupat, establint unes propietats mecàniques adequades per a cadascun dels seus constituents. D'aquesta manera es pot aconseguir una predicció del risc de sofrir danys traumàtics. Part d'aquesta Tesi es centra en l'entorn balístic, específicament en cascs de combat antibales, els quals són susceptibles de causar traumatisme degut a l'elevada deformació que sofrixen durant l'impacte. Previament a l'estudi d'aquests fenòmens d'alta velocitat, s'han realitzat assajos experimentals i numèrics per a caracteritzar la resposta mecànica d'alguns materials compostos en condicions d'impacte a baixa velocitat. Al començament d'aquesta Tesi s'ha realitzat una revisió de l'estat de l'art sobre els criteris existents per quantificar el trauma cranioencefàlic. Aquest és un aspecte clau per a les simulacions numèriques, ja que l'utilitat d'alguns d'aquestos criteris per a la predicció de lesions cerebrals és encara un debat obert. Mitjançant EF s'han realitzat simulacions numèriques d'impactes balístics en un cap protegit amb un casc de combat. Gràcies a la posterior aplicació de diferents criteris de dany sobre els resultats obtinguts s'ha evaluat el nivell de protecció que asseguren els protocols d'acceptació de cascs de combat, així com les estratègies per a determinar les seues talles. S'ha demostrat que les normatives existents són capaces de mitigar alguns mecanismes de trauma però no aconseguixen prevenir altres com els gradients de pressions intracranials. A més, s'ha demostrat que algunes estratègies per determinar les talles més comunament adoptades pels fabricants (com produir només un tamany de calota i adaptar el gruix de les escumes interiors a les diferents dimensions dels subjectes) haurien de ser reconsiderades ja que existeix un major risc de traumatisme quan la distància entre el cap i la calota del casc no és suficient. Seguint la línia de proteccions personals, alguns dels materials compostos comunament utilitzats en la indústria de l'armament s'han combinat per a crear distintes possibles configuracions de calota amb la finalitat d'optimitzar la relació entre pes i protecció. Materials lleugers com l'UHMWPE han resultat en un comportament menys eficient que el d'apilats de teixit d'aramida a l'hora de limitar la BFD (deformació màxima a la calota del casc a la zona d'impacte). Cap al final de la Tesi es presenta un model numèric detallat de cap humà, que inclou trenta-tres de les estructures anatòmiques principals. Aquest model s'ha desenvolupat per a la simulació d'un accident eqüestre en el qual apareixen múltiples lesions cranioencefàliques. Principalment, es pretén establir un criteri mecànic per a la predicció de l'hematoma subdural (HS) basat en la ruptura dels vasos sanguinis intracranials. S'ha proposat un valor umbral de ruptura en tensions de 3.5 MPa, pero tant aquest límit com la ubicació del vas danyat són altament dependents de l'anatomia específica de cada subjecte. / [EN] Impact loading is the primary source of head injuries and can result in a range of trauma from mild to severe. Because of the multiple environments in which impact-related injuries can take place (automotive accidents, sports, accidental falls, violence), they can potentially affect the entire population regardless of their health conditions. Despite the increasing research effort on the understanding of head impact biomechanics, accurate prediction and prevention of traumatic injuries has not been completely achieved. In this Thesis, some aspects of the impact behaviour of the different biological tissues involved have been analysed through the development of a numerical human head model from Computed Tomography (CT) images. FE simulations of experimental tests from the literature have been performed and enhanced the validation of the head model through the establishment of proper material laws for its constituents, which enable adequate prediction of injury risks. Part of this Thesis focuses on the ballistic environment, especifically in bulletproof composite helmets, which are susceptible to cause blunt injuries to the head because of their large deformation during impact. Prior to the study of these high-speed impacts, experimental tests and finite element (FE) models have been performed to characterise the mechanical response of composite materials subjected to low velocity impact. The implementation of a continuum damage mechanics approach coupled to a Hashin failure criterion and surface-to-surface cohesive relations to the numerical model provided a good matching with the impact behaviour obtained experimentally, capturing the principal damage mechanisms. A review of the head injury criteria currently available in the literature has been performed at the beginning of this Thesis. This is a key issue for the numerical simulations, as the suitability of some criteria to predict head injuries is still an open question. Numerical simulation of ballistic impacts on a human head protected with a combat helmet has been conducted employing explicit FE analysis. The level of protection ensured by helmet acceptance protocols as well as their sizing strategies have been studied and discussed by means of the application of different mechanical-based head injury criteria. It has been demonstrated that current helmet testing standards do mitigate some specific forms of head trauma but fail to prevent other injury mechanisms such as the intracranial pressure gradients within the skull. Furthermore, it has been demonstrated that some well-established helmet sizing policies like manufacturing one single composite shell and adapting the thickness of the interior pads to the different head dimensions should be reconsidered, as there is a great risk of head injury when the distance between the head and the helmet shell (stand-off distance) is not sufficient. Following the line of personal protections, some composite materials commonly employed in the soft body armour industry have been combined into different helmet shells configurations to optimise the ratio of weight-to-head protection. Light materials like UHMWPE appear to be less efficient than integral woven-aramid lay-ups in the limitation of the backface deformation (BFD), the maximum deformation sustained by the helmet at the impact site. A detailed head numerical model including thirty-three of its main anatomical structures has been developed for the simulation of an equestrian accident that resulted in many head injuries. Above all, the establishment of a mechanical criterion for the prediction of subdural hematona (SDH) based on the rupture of the head blood vessels is intended. A stress threshold for vein rupture has been set on 3.5 MPa, but both this limit and the location of vessel failure are highly dependent on the specific anatomy of the subject's vascularity. / Palomar Toledano, M. (2019). Assessment of head injury risk caused by impact using finite element models [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/135254 / TESIS

A research group at University of Skövde has developed an interphase element for implementation in the commercial FE-software Abaqus. The element is using the Tvergaard & Hutchinson cohesive law and is implemented in Abaqus Explicit version 6.7 using the VUEL subroutine. This bachelor degree project is referring to evaluate the interphase element and also highlight problems with the element. The behavior of the interphase element is evaluated in mode I using Double Cantilever Beam (DCB)-specimens and in mode II using End Notch Flexure (ENF)-specimens. The results from the simulations are compared and validated to an analytical solution. FE-simulations performed with the interphase element show very good agreement with theory when using DCB- or ENF-specimens. The only exception is when an ENF-specimen has distorted elements. When using explicit finite element software the critical time step is of great importance for the results of the analyses. If a too long time step is used, the simulation will fail to complete or complete with errors. A feasible equation for predicting the critical time step for the interphase element has been developed by the research group and the reliability of this equation is evaluated. The result from simulations shows an excellent agreement with the equation when the interphase element governs the critical time step. However when the adherends governs the critical time step the equation gives a time step that is too large. A modification of this equation is suggested.

<p>A research group at University of Skövde has developed an interphase element for implementation in the commercial FE-software Abaqus. The element is using the Tvergaard & Hutchinson cohesive law and is implemented in Abaqus Explicit version 6.7 using the VUEL subroutine. This bachelor degree project is referring to evaluate the interphase element and also highlight problems with the element.</p><p>The behavior of the interphase element is evaluated in mode I using Double Cantilever Beam (DCB)-specimens and in mode II using End Notch Flexure (ENF)-specimens. The results from the simulations are compared and validated to an analytical solution.</p><p>FE-simulations performed with the interphase element show very good agreement with theory when using DCB- or ENF-specimens. The only exception is when an ENF-specimen has distorted elements.</p><p>When using explicit finite element software the critical time step is of great importance for the results of the analyses. If a too long time step is used, the simulation will fail to complete or complete with errors. A feasible equation for predicting the critical time step for the interphase element has been developed by the research group and the reliability of this equation is evaluated.</p><p>The result from simulations shows an excellent agreement with the equation when the interphase element governs the critical time step. However when the adherends governs the critical time step the equation gives a time step that is too large. A modification of this equation is suggested.</p>

Forklifts that are capable of lifting heavy loads and reaching high lift heights are required by stan-dards to have an overhead guard to protect the operator from falling objects. The same standardsspecify a standardized procedure for testing the strength of these overhead guards. The test in-volves dropping ten 45 kg wooden cubes and a heavy timber load onto the overhead guard. Thesedestructive tests are time-consuming and expensive, and it is the purpose of this master’s thesis todevelop a methodology for simulating this kind of test using the finite element method with a largedisplacements, explicit scheme using the solver RADIOSS by Altair. This was achieved by firstdesigning, constructing, and testing a physical prototype of an overhead guard to use as a referencefor a finite element methodology to be validated against. The work has also included tensile testingof the overhead guard material, and this was done both to obtain material data from the sametype of material as the prototype, and to get Johnson-Cook material parameters, which are hardto come by in the literature. Next, a basic finite element model was created which showed a verylarge discrepancy compared to the physical test results. An extensive investigation into aspectssurrounding finite element modeling and material modeling was undertaken, and resulted in a fi-nal model which overestimated the displacements by about 40 % only. The remaining inaccuracyis believed to mostly stem from inadequate strain-rate sensitivity data, caused by limitations inavailable resources for material testing.