用于车辆前保险杠梁的替代轻质材料和部件制造技术【中文10100字】
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用于车辆前保险杠梁的替代轻质材料和部件制造技术【中文10100字】,中文10100字
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用于车辆前保险杠梁的替代轻质材料和部件制造技术G. Belingardi a, A.T. Beyene a, E.G. Koricho b, B. Martorana c意大利机械和航空航天工程系Politecnico di Torino美国密歇根州立大学复合材料车辆研究中心，Centro Ricerche FIAT，strada Torino 50，Orbassano Torino，Italy摘要：在轻量化设计中具有巨大优势的车辆子系统之一是保险杠子系统。保险杠子系统设计用于在共谋期间防止或减少乘客机动车辆的前端或后端的物理损坏。本文详细设计了方法和分析方法，特别参考了复合材料在汽车前保险杠子系统，碰撞箱和保险杠梁中的应用。集成式碰撞盒和保险杠梁的创新设计已被考虑用于更好的耐撞性;从子组件成本和有效生产过程的角度来看，所提出的解决方案也引起了极大的兴趣。在这种保险杠梁应用的准静态和冲击试验中已经对三种材料进行了表征：GMT，GMTex和GMT-UD。主要参数，如冲击能量，峰值载荷，抗撞击性，能量吸收和刚度，已被作为评估标准，以比较提出的材料解决方案与拉挤和钢解决方案。最后，通过有限元分析预测的结果与其他现有解决方案进行了评估和解释，以证明所提出的创新材料和设计概念解决方案的有效性。关键词：保险杠 耐撞性 轻量化设计 复合结构优化1.简介汽车保险杠子系统是车辆的前后结构，其目的是在低速冲击时吸收能量。通常，保险杠子系统包括保险杠横梁，支柱，连接到结构部件（通常是保险杠梁）的冲击吸收材料（例如泡沫或蜂窝）和盖子，其具有美观和保护目的。在这些元件中，保险杠梁是主要结构部件;预计它可以变形到足以吸收冲击能量，以减少行人和其他易受伤害的道路使用者受伤的风险，但同时，它还应该具有足够的强度和刚度以引起小的入侵发动机舱，因此，保护附近的车辆部件。复合材料的特点是在静态和冲击载荷条件下具有高的特定强度，并具有高的特定刚度;对于这种类型的部件来说，它们可能是一种有趣的候选材料，与现有的钢材解决方案相比，它具有轻量化的目标以及至少保持相同水平的安全性能。在使用复合材料进行设计时，不仅需要选择合适的材料，还要考虑复合材料（即不是简单地用新材料替换金属材料，而是重新设计零件）并选择生产技术的类型 - 将在制造业中使用的术语，因为这种选择将深刻影响结构性能和成本以及生产率1。因此，材料，设计和制造技术是相互严格联系的，应该一起考虑。从制造技术的角度来看，我们考虑了两种不同的类型：拉挤成型和模压成型。它们都具有成本效益且完全自动化，并且在几何精度和机械性能的一致性方面（主要由于过程自动化）提供高质量的部件。拉挤成型具有许多优点，例如完美的纤维排列和高纤维体积，因为在纤维处于张力状态时发生聚合，能够产生具有各种端部轮廓的封闭和开放截面等。然而，目前该技术强烈局限于直线和恒定截面。相反，模具成型复合材料制造技术也有其自身的优点，即它允许生产结构上集成的碰撞盒和梁，如图1所示，它提高了制造和装配速度，并消除了保险杠梁和碰撞盒之间的连接。然而，它主要局限于开放截面轮廓，其通常比闭合截面轮廓更少。图1.国际塑料复合材料开发的集成复合材料解决方案用于顶级车辆（b）的梅赛德斯。由于目标组件设计用于冲击载荷，因此在组件级别进行数值影响分析之前，评估复合材料的冲击性能。通常，与传统金属材料相比，整组复合材料的冲击响应和损伤机制更复杂，并且取决于许多不同的参数：纤维和基体类型，截面形状和尺寸，冲击速度，冲击角度，撞针的形状，目标几何形状和目标材料。开放文献表明，复合管能够通过材料碎裂吸收显着的冲击能量，并且当管经历大的弯曲变形时，管横截面几何形状发生大的变化2-7。在目前的研究中，考虑了六种材料。对于拉挤式保险杠梁解决方案，单向拉挤E玻璃/环氧树脂，双向织物E玻璃/环氧树脂和钢材料进行了比较。详细的机械性能记录8。对于模压成型集成碰撞箱梁解决方案的情况，考虑了三种材料：经典的玻璃纤维增强热塑性塑料（GMT），即无端玻璃纤维配合增强PP和随机取向的玻璃纤维，GMTex，即切碎的纤维玻璃纤维增强PP层压板随机取向的玻璃纤维，并在内部和外部加强织物GMT-UD，即切碎的玻璃纤维毡增强PP层压板采用随机取向的玻璃纤维，并采用单向薄玻璃纤维层加固。Quadrant提供的这三种材料被认为是用于前保险杠应用。 考虑到改性材料的新颖性，已经进行了广泛的材料表征以获得材料的主要机械性能并理解预期的装载情况的失效机理，并且最终他们对替代当前钢材料的能力进行了数值评估。2.材料表征复合材料在拉伸（纵向和横向），压缩（纵向和横向）和落镖试验下表征。 以下部分介绍了测试装置的简要概述以及所获得的测试材料的机械特性。2.1拉伸试验的实验装置每个材料类型的纵向和横向五个试样在拉伸载荷下用100 kN容量伺服液压试验机（INSTRON-8801）进行试验，如图2所示。每个试样用液压夹紧楔形夹子。该机器配备了标准称重传感器和十字头位移测量装置。图2.拉伸和压缩材料表征的实验装置。表1所考虑材料的拉伸性能。属性 GMT GMT UD GMTex准各向同性 纵向 横向 纵向 横向抗拉强度（MPa）80.7 180.4 59.2 174.7 170.6模量（GPa） 5.81 11.07 6.48 9.24 6.00泊松比为 0.284 0.307 0.217 0.389 0.150表2所考虑材料的压缩性能。属性 GMT GMT UD GMTex准各向同性 纵向 横向 纵向 横向压缩强度（MPa）65.8 82.2 58.5 69.0 57.1模量（GPa） 3.26 5.93 2.64 4.14 2.66泊松比为 0.338 0.43 0 0.178 0.237 0.168表3能量为准静态穿孔试验。材料 板厚mm 能量JGMT 4 38.9GMTex 3 31.1GMT-UD 4 41.2在试样的安装阶段，控制最大预载荷并设定低于0.2kN，以避免试样损坏。根据ASTM D3039，样品经受单调拉伸载荷，行程速率为2mm / min。通过应变计仪器测量样品以测量杨氏模量和泊松比。为了获得应变计数据，使用NI WLS-9163数据采集板并从机器获取负载和十字头位移数据，使用NI DAQCard-6062E。以等于1kHz的采样率采集所有数据。主要机械性能见表1。2.2。压缩测试的实验设置类似地，根据ASTM D6641 / D6641M在压缩载荷下测试每种材料在纵向和横向上的五个试样，并且发现的实验结果在表2中给出。2.3落镖测试的实验设置在冲击试验之前，在Zweck Roell 100万能试验机上进行准静态压痕试验，以研究所提出的复合材料层压板的穿孔能量，图3.主要结果见表3。图3.准静态压痕测试。根据ASTM标准3029使用仪器化的自由落体落镖试验机进行实验性冲击试验。冲击器的滑架质量为5.735千克，半球形头部半径为10毫米，测试机器的最大下落高度为2米（见图4）。落锤装置配备有电动升降轨道。收集的数据在每次撞击后存储，撞击器返回其原始起始高度。使用这种技术，在连续的冲击中始终如一地获得所选择的冲击速度。通过压电式称重传感器，获得了力 - 时间曲线，并且通过加速 - 时间曲线的双重积分，获得了力 - 位移曲线。将具有100mm边缘的方形样品板夹在具有76.2mm内径的样品架中，并固定到刚性基底上以防止样品滑动。夹紧系统设计用于在整个夹紧区域提供足够均匀的压力。获得三种材料穿孔所需的能量后，材料对动态加载的响应为图4.落镖试验机和试样配置。通过在相同能量水平下进行动态冲击试验，即GMTex准静态压痕能量的65进行比较。 通过为该特定测试场景开发的Lab VIEW Signal express环境，直接从测试机获取力 - 时间曲线（数据）和实际初始冲击速度，并且使用如上所述的自由体动方程12计算其他重要的动态变量。 在图5中。图5.跌落测试运动描述2.4复合材料的影响图3和图4中示出了力与时间，能量对时间和损伤模式的图像的代表性曲线，分别对于第一次和穿孔的影响。通过每次撞击一个病毒样本达到所需的冲击次数来监测穿孔板所需的冲击次数和通过连续冲击产生的损伤。图7和图9分别显示了根据指定的冲击次数对样品的受影响表面的图片，分别针对GMT和GMT-UD材料。 GMT-UD在第一次撞击时的平滑力与时间以及相对较低的吸收能量可以与GMT-UD板没有可见损伤的观察结果相关联，如图6e所示。这意味着大部分能量是由于材料内部断裂以外的机制而消散的。然而，GMT和GMTex的力与时间曲线显示出一个顶点，可以解释为裂缝的迹象，这可以与图6c和d中所示的撞击板上可见的可见损伤相关联。 图6.力与时间（a），能量与时间（b）和第一次撞击时的损伤（c-e）。图7.通过GMT和GMTex的连续影响发展损害。图8显示了反复冲击后穿孔的一些结果。我们可以观察到，在穿孔时，GMT和GMTex具有相似的失效行为，即撞击器穿透板破坏穿孔周围的有限区域，但没有大量的裂缝传播到板中（见图8c和d）。这是因为经典GMT具有平面准各向同性特性，即它在所有方向上具有几乎均匀的连续纤维，并且这防止了穿孔中心孔边缘处的裂缝传播。类似地，GMTex在经典GMT的中平面处具有织物层，因此裂缝的传播受到随机和织物纤维垂直于裂缝的共同作用的阻碍。相反，在GMT-UD的情况下，由单向纤维增强的经典GMT，有可能从穿孔中心孔的边缘产生的裂缝沿纤维方向传播（见图8e）。图8.力与时间（a），能量与时间（b）和穿孔损伤（c-e）。图9.通过GMT-UD的连续影响发展损害。众所周知，复合材料具有较差的塑性，因此，当使用这种类型的材料设计能量吸收部件（如保险杠梁）时，能量耗散主要通过材料的断裂来实现。因此，组分材料碎片越多，消耗的能量就越多。在这方面，能量与时间曲线和GMT-UD在穿孔冲击下的损伤模式显示出更好的断裂行为 - 即除了穿过板厚度的冲击器穿透之外，裂缝沿板宽度增加而增加能量吸收量。3.保险杠梁设计如引言段落中所示，已经考虑两种不同类型的制造技术，即拉挤成型和模具成型，以制造具有所需形状的梁。它们都具有成本效益和全自动化的制造技术，并在几何精度和机械性能的一致性方面提供高质量的零件。拉挤成型制造解决方案对于制造各种各样的端部轮廓非常有效，但目前仅限于直梁，而且不适合开发集成的碰撞箱梁解决方案。模具成型制造技术适用于开发集成的碰撞箱梁解决方案，并且对梁的曲率没有限制，但它仅限于开口型材，结构上比封闭型材更薄，并且有局限性。部分形状的形状。图10示出了拉挤保险杠梁（a）和模具形成的GMT / GMTex集成钱箱梁保险杠梁（b）的简化模型，其被考虑用于使用商业代码ABAQUS / Explicit version 6.12-1的非线性有限元件模拟。在这两种情况下，刚体都被建模为离散的刚性表面，以便在关键接触区域产生更高的网格密度。质量为1000千克与碰撞盒的两个后端刚性连接，以模拟车辆质量。边界条件也应用于车辆质量点，其朝向刚性壁的初始速度又设定为等于4,8和15km / h，以模拟与保险杠系统相关的三种不同的冲击情况。图10.简化的FEM保险杠模型（a）拉挤梁解决方案，（b）模具成型集成的梁碰撞盒解决方案。3.1.E-玻璃/环氧树脂拉挤光束解决方案许多学者已经研究了拉挤梁解决方案10,11，用于路边障碍结构，类似于车辆保险杠中的横向装载情况。路边障碍物通常设计用于保护驾驶者免受人为或自然危害，将错误车辆重新引导至道路并在发生碰撞时消散能量。这些研究表明拉挤复合材料可用于护栏系统，因为它们的假韧性特征主要来自材料碎裂（复合材料的破碎，分离和撕裂）以及管子横截面几何形状的大变化fl exural变形。提出的拉挤保险杠梁解决方案8旨在利用和优化拉挤复合梁的伪延性行为，以在低速车辆正面碰撞时有效地消散能量。通过梁截面结构的结构优化程序（可以通过适当形状的模具部分容易地获得）和曲率（目前主要制造商不提供该技术）来优化伪延性行为。旨在获得复合材料保险杠梁的渐进能量吸收和稳定的外部失效。根据8,9中开发的方法进行了数值研究，以探索用E-Glass /环氧树脂拉挤复合材料梁代替现有金属保险杠梁的可能性。所得到的结构在形状和能量吸收能力方面进行了比较，还与钢铁正常生产解决方案进行了比较。拉挤梁的伪延性行为源于材料破碎（复合材料的破碎，分离和撕裂）以及当管经历大的弯曲变形时管横截面几何形状的大的变化。因此，基于以下假设进行分析：正确优化和预先确定的应力集中区 - 即梁纵向槽（通过最终形状的优化过程） - 可用作碰撞触发机制，即启动裂缝形成并沿梁纵轴发展渐进撕裂。使用拉挤成型技术生产这种特定轮廓的能力，利用作为设计变量的梁端部轮廓的高度（h）上的凹槽数量和壁厚的分布进行了优化。通过对冲击事件特征数据的研究，如力 - 时间，力 - 位移，能量 - 位移和位移 - 时间曲线，对提出的最终结果进行了性能比较。优化的梁截面配置如图11所示。图11.拉挤解决方案的优化梁端部分剖面8即使当前的拉挤制造技术主要局限于直梁（弯曲拉挤成型技术仍处于婴儿阶段），也对梁的曲率半径（R）进行了优化。图12.大量的梁弯曲半径， 从直轴到较小的半径，被认为。 使用已经提到的冲击事件特征数据密切监视故障现象和保险杠梁性能。图12.考虑优化的保险杠梁曲率。3.2.GMT / GMTex模压成型一体化碰撞箱梁解决方案模具成型制造技术能够生产结构上集成的碰撞盒和梁作为单个部件。这是该技术的一个非常有趣的特征，因为从制造/组装速率的角度以及从应该生产和组装到的不同部件的数量的相关减少的角度来看，它导致显着的改进。构建前端结构。此外，由于连接是在汽车结构中使用复合材料部件的关键问题之一（因为结构通常具有将它们的部件连接在一起的薄弱点），所以模具成型技术适用于生产集成的保险杠梁和碰撞盒结构。消除了两者之间的关节需求。使用商业代码ABAQUS / Explicit version 6.12-1进行了非线性有限元模拟，其具有简化的保险杠梁模型，如图10b所示。该模型包括两个部分，一个刚性部件，即冲击刚性壁，以及一个可整合碰撞盒和横梁及其后盖的可变形部件。根据三种考虑的复合材料GMT，GMtx和GMT-UD，集成梁解决方案已经开发出三种替代方案。在碰撞盒的两个后端处牢固地连接1000千克的质量，为了模拟车辆质量，它以4或8千米/小时的初始速度朝向刚性壁移动。考虑到负载路径，在所提出的结构的不同部分使用了不同的部分，如图10b所示。为了获得渐进式失效，提出了用于碰撞盒的空心锥形三角形方形金字塔。使用所提出的材料的设计解决方案是从通过两种方法由钢制造的正常生产解决方案（即参考解决方案）开始开发的：直接替换现有钢梁，通过复合梁与带有轻微碰撞盒的集成梁的集成使用公司推荐的壁厚，即臼8毫米，仅用于连接目的的底板修改，和通过相等的弯曲刚度方法9，即对于参考材料的给定厚度和刚度，目标材料采用的厚度可近似地通过等式1计算。（1）。其中hs和hx分别是钢和目标材料溶液的壁厚，Es和Ex分别是钢和目标材料的弹性模量。在低速冲击期间，例如小的停车负载，预期保险杠梁仅会碰撞，即它必须在弹性极限内操作而没有任何形式的永久性损坏。因此，对于当前的研究，通过监测冲击能量曲线确定用于这种小负载的保险杠的允许最小厚度。获得阈值厚度后，它逐渐增加到一个值，其中光束与参考材料一样具有相似的冲击性能。最后，评估了质量减少。3.2.1集成保险杠梁的设计考虑因素当金属部件被复合材料部件替代时，考虑到两种材料的非常不同的失效模式，必须遵循新的设计假设，只有这样，新材料的重要特征所带来的优势才能得以实现。最大化。因此，在当前的集成保险杠系统中，进行了以下三种设计考虑：在传统的保险杠系统中，结构完整性之间通过将碰撞盒机械固定（焊接或螺栓连接）到梁上来获得保险杠横梁和碰撞盒。轴向加载复合金字塔管的失效模式是由前端触发机构启动并逐渐沿管长度传播，采用传统的复合保险杠系统组装连接方案将导致早期梁和碰撞箱脱离因此，不符合预期的能量吸收目标。这组材料必须遵循新的设计方法。如前所述，通过适当设计的集成保险杠系统，可以很好地解决问题。建议的解决方案有一个免费的正面碰撞盒端，带有所需的碰撞触发器。虽然可以通过优化连接边缘的棘手来获得结构完整性。特别地，必须优化轮辋厚度以承受由正面冲击产生的剪切载荷。如果小速度仅影响保险杠梁应该参与并且应该表现出完全弹性，没有碰撞盒的直接参与。因此，需要优化梁前部和碰撞盒前部之间的间隙C.坠毁物体的能量吸收是受力区域位移曲线，与挤压长度产生的力的乘积成正比。因此，挤压长度L是碰撞部件设计的重要参数。在集成保险杠系统设计期间，不正确地放置连接轮缘将影响碰撞长度并通过阻挡和控制碰撞盒的渐进故障来影响系统的能量吸收。因此，必须优化碰撞盒和保险杠梁边缘的相对位置。考虑到上述设计考虑因素，如图13所示，简化的集成保险杠梁模型在CATIA 5中建模，并且在ABAQUS中进行网格改造。该模型仅包括两个部分：可变形的集成式保险杠系统和刚性墙。刚体被建模为分析刚性表面。图13.考虑集成梁式碰撞盒设计的要点。4.结果和讨论4.1拉伸保险杠梁解决方案通过梁的高度（h）上的凹槽数量和壁厚的分布，优化了梁端配置。详细的优化过程已在8中报道。当保险杠梁受到正面碰撞时，在树丛顶点处产生集中应力;在与受影响表面等距离的折叠侧上的点具有相同的应力水平。这在直梁的情况下基本上是均匀的，而光束曲率的变化对沿梁的应力分布和应力值都有影响。图14示出了用于三种不同解决方案的保险杠梁的最终变形形状，其特征在于曲率半径从2400mm（情况a）到直梁（情况c）的不同值。图14d显示了这三种解决方案的反作用力历史。很明显，小曲率半径的情况产生靠近梁中跨的集中破坏铰链，并且出现非常大的载荷峰值;另外两种情况是提供更多的弥散能量吸收和更平滑的曲线;具有曲率半径的中间值的解决方案给出最小负载峰值。作为低速冲击分析的第一个一般观察，当梁曲率半径增加时，局部应力集中的形成减少。这是因为保险杠梁的较大区域同时与刚性壁处的fl接触。这导致更高的载荷峰值，其促进在具有相同应力水平的褶皱部分上形成弥散裂缝。图14.曲率半径的失效模式和峰值载荷（a）2400 mm，（b）3200 mm和（c）直线8。最坏的情况是保险杠梁是直的，图14c，其对应于一些车辆当前使用的解决方案。在这种情况下，刚好在碰撞盒前面的梁末端部分，其长度等于碰撞盒宽度，同时会破裂，因为梁的那部分处于相同的应力水平，并且不可能用于裂纹扩展和适当的能量吸收。另一方面，当光束曲率半径减小到某个临界曲率半径（在这种特殊情况下为2862 mm）以下时，不会发生裂缝传播，而是在光束的顶部产生高的局部应力线，导致不稳定的局部失效，如图14a所示。最后，所提出的拉挤复合材料保险杠梁解决方案的性能可以与钢和玻璃织物/环氧复合材料解决方案在冲击能量吸收和重量减轻方面进行比较。考虑材料比较，考虑三个参数，即吸收能量，峰值负荷值和失效模式。如图15所示，三种设计方案吸收了相同的能量，但峰值负荷值和故障模式完全不同。在车辆正面碰撞期间，峰值负载与车辆乘员风险相关，事实上较低的峰值负载会降低减速度，反之亦然，因此应仔细控制此参数。此外，通过比较两种复合材料解决方案（即拉挤和织物）的失效模式，得出拉挤梁的能量 - 位移曲线几乎是线性的，拉挤梁的载荷折射率曲线类似于弹塑性韧性材料的轴向应力 - 应变图，技术上称为伪韧性。因此，只要可以控制位移或将位移保持在设计极限内，拉挤解决方案的伪延性行为是保险杠部件的被动安全行为的重要特征。4.2.Die成型一体式保险杠梁碰撞盒解决方案图15.考虑的三种材料的反作用力与位移和能量与位移的关系8。根据我们先前的相关活动，已经了解到闭合截面梁比开放截面梁具有更好的结构完整性和能量吸收能力。因此，即使材料供应商公司考虑并推荐开口截面梁，为了生产的可行性和简单性，也已经对数值研究了闭合截面梁。第一次尝试是通过直接替换当前的钢梁进行的，其中集成了碰撞盒并且在底板上具有微小的修改，仅用于连接目的。根据材料供应商的建议，复合梁每种材料配置的壁厚设定为臼8毫米。如可以看出，在4 km / h的撞击事件模拟中，力与时间，力与位移曲线相比（见图16），所有三种材料解决方案在结构上都很薄弱。 GMT-UD和GMtx解决方案在梁的中心处显示出早期的急剧突破，而GMT解决方案显示出相对较高的弹性变形。图16.钢中设计解决方案的力与时间和力与位移曲线以及三种所考虑材料的相同厚度。通过增加截面尺寸，特别是底板，进行设计变更。基于参考材料溶液的数据确定集成的保险杠梁碰撞盒的壁厚。对于参考材料（钢）的给定壁厚和刚度，目标材料的厚度可以用方程式近似计算。 （1）。获得的近似壁厚和集成的保险杠梁碰撞盒解决方案的质量在表4中报告。表4组合梁式防撞箱的厚度和质量。材质 钢 GMT GMtx GMT-UD厚度mm 2.2 7.1 6.1 5.8质量kg 7.67 3.72 3.32 3.2如表1所示，GMT-UD的拉伸纵向模量分别比GMT和GMtx高约50和25。因此它具有更好的机械性能。此外，正如前面第2.4节所述，在经典GMT中引入单向纤维，使材料沿着板的宽度开裂，从而提高了能量吸收能力。在动态跌落飞镖测试的能量与位移曲线上也观察到这种失效行为。通过比较与目标四种材料相关的力 - 时间和位移 - 时间曲线（见图17），可以看出GMT-UD解决方案具有最小峰值载荷，即25kN，这是设计者必须控制的重要参数之一，并且具有类似于参考材料解决方案的失效模式，但具有最大侵入，即37mm，光束导致在所选载荷下断裂。这个结果是不可接受的，因为据说对于4 km / h的冲击情况，保险杠梁应该保持在弹性区域的行为而没有任何结构损坏。图17.修改的保险杠系统的力对时间和位移与时间的关系曲线。还可以使用载荷位移曲线和能量时间历史曲线来跟踪失效行为，如图4和图5所示。 18-20。图20a和b显示，将整个动能转化为内部变形能后的GMTex和GMT解决方案能够恢复大部分能量并发生弹性回弹。在图19b中可以看到完全不同的情况，对于GMT-D解决方案，恢复阶段缺失，这是由于梁裂缝造成的。GMT-UD解决方案的载荷和能量 - 位移曲线（显示能量吸收量和受影响系统在能量耗散过程中的行为）确定材料已经在选定的速度下断裂。在参考材料（钢）上也观察到类似的现象，这可能是由于所选钢的强度。由于金属材料具有较高的塑性范围，钢的能量曲线通过塑性变形显示出能量耗散。然而，复合材料的塑性范围非常有限，因此，能量曲线表明GMT-UD已经超过其弹性极限，因此，能量耗散是由材料碎裂引起的。图18.在4 km / h的冲击情况下，修改后的保险杠系统的能量与位移和力与位移曲线。图19.（4）钢和（b）GMT-UD解决方案在4 km / h冲击时的能量曲线。图20.（4）GMtx和（b）GMT解决方案在4 km / h冲击时的能量曲线。相反，GMT和GMtx溶液都保持在弹性范围内，这可以从图1和2中的载荷与位移和能量时间 - 历史曲线中观察到。失效模式如图21所示。对于接近停车负荷的4 km / h冲击速度，保险杠必须在弹性范围内运行，因此除了撞击之外，最终还会出现轻微的化妆品损坏，我们在GMT-UD解决方案中观察到的完全断裂是不可接受的。因此，利用所提出的梁配置和所考虑的载荷条件（即，在低速冲击下），从等式1获得的壁厚。（6）即使用等刚度方法，仅对GMT和GMtx产生可接受的结果，并且可以考虑更换材料并显着减轻重量，但对于GMT-UD，梁断裂，因此壁厚要求进一步改变。图21.在4 km / h的冲击情况下修改的保险杠系统的故障模式。上述提出的解决方案也在较高的冲击速度下，即以8km / h的速度进行。从图22的力 - 时间曲线可以看出，在弹性变形的初始阶段之后，在这个较高的速度下存在两种变形模式。图22.在8 km / h的冲击情况下，修改后的保险杠系统的力与时间和位移与时间的关系曲线。臼25-40 kN，这是一种类似于在较低处观察到的行为速度影响（4 km / h），GMT和GMtx梁在其中心部分完全断裂。最后，裂缝梁的其他部分撞击刚性墙，但没有进一步的材料破裂。这种现象产生相对较高的峰值载荷，即臼齿265kN，对于GMT和GMtx溶液分别为300kN。然而，GMT-UD解决方案在冲击的第二阶段期间显示出进一步破裂的症状，这无论是由于沿着板的宽度的破裂。这个导致相对较小的峰值载荷，即臼齿170kN。失败基于GMT-UD和GMTex的两种解决方案的行为可以在图23中观察到。图23.在8 km / h的冲击情况下，GMT-UD（a）和GMtx（b）解决方案的能量与时间曲线和失效模式。通常，在经典GMT上进行的改进确实改善了原始材料的拉伸和冲击性能，并且可以在某些应用中用于结构目的而不是钢和铝。然而，对于能量吸收组件，使复合材料具有与常规金属材料完全不同的失效行为，能量吸收性能受到组件几何形状的强烈影响。直接采用传统的金属吸收能量几何结构可能会导致灾难性故障并产生更高的峰值负荷。正如之前的研究8中指出的那样，在横向负载能量吸收复合材料部件（如汽车保险杠梁）的情况下，通过适当优化的梁端配置可以获得渐进式失效模式，使得梁角落可以作为应力集中区或裂缝触发点。通过这种方式，裂缝可以沿着梁纵轴开始并逐渐传播。然而，在三种用于模具成型集成保险杠梁碰撞盒解决方案的材料的情况下，经过适当的厚度和横截面优化，GMT-UD在机械性能和断裂行为方面具有更好的性能，导致有趣的解决方案在8 km / h冲击的情况下，但显示早期，4公里/小时的冲击情况下不可接受的裂缝。5. 结论在这项研究中，三种材料已经在保险杠梁应用的准静态和冲击试验中进行了表征：GMT，GMTex和GMT-UD。 主要参数，如冲击能量，峰值载荷，抗冲击性，能量吸收和刚度已被作为评估标准，以比较提出的材料解决方案与拉挤和钢解决方案。 主要获得的结果可归纳如下。GMT表现出最低的拉伸强度。 很明显，观察实验结果，在GMT内包含单向（UD）纤维，使纵向拉伸强度提高了125。 然而，横向拉伸强度降低了11。另一方面，GMT的织物修改版本，GMTex，表明纵向和横向拉伸增加约117。与抗拉强度，抗压强度相比较对材料配置的变化反应较小。实验影响测试表明，GMTex展示了在低速飞镖冲击试验中，与剩余候选材料相比，最高吸收能量和最小峰值反作用力。相反，数值预测揭示了拟议的在模具成型集成保险杠梁碰撞盒解决方案中，GMT-UD在最小峰值载荷和受控断裂行为方面表现出更好的性能，因此为8 km / h冲击速度的情况提供了有趣的解决方案。一般来说，实验低速观测冲击试验和数值预测可为在复合材料制成的轻型车辆中更好地设计创新和模块化燃烧子系统提供有价值的信息。最初提出的前保险杠子系统解决方案在轻量化（重量减轻量级为55）方面就目前的结构性解决方案提供了更好的结果，在影响的情况下具有攻击性 防止弱势道路使用者以及放置在发动机舱内的设备的保护。此外，通过利用所选材料的特定功能，并采用集成解决方案，将保险杠横梁和两个碰撞盒分成一个单独的部件，实现了制造过程的一个很好的简化。Contents lists available at ScienceDirectComposite Structuresjournal homepage: /locate/compstruct Composite Structures 120 (2015) 483495Alternative lightweight materials and component manufacturing technologies for vehicle frontal bumper beamG. Belingardi a, A.T. Beyene a, E.G. Koricho b, B. Martorana ca Politecnico di Torino, Department of Mechanical and Aerospace Engineering, Italyb Michigan State University, Composite Vehicle Research Center, USAc Centro Ricerche FIAT, strada Torino 50, Orbassano Torino, Italya r t i c l e i n f o Article history:Available online 25 October 2014Keywords: Bumper CrashworthinessLightweight design Composite structures Optimizationa b s t r a c t One of the vehicle subsystem where large advantage is expected in lightweight design is the bumper sub- systems. Bumper subsystems are designed to prevent or reduce physical damage to the front or rear ends of passenger motor vehicles during collusion.In this paper, detail design aspects and method of analysis with particular reference to the application of composite materials to automotive front bumper subsystem, crash box and bumper beam. Innovative design of integrated crash box and bumper beam has been considered for better crashworthiness; the proposed solution results to be of great interest also from the points of view of subassembly cost and effective production process.Three materials have been characterized under quasi static and impact tests for this bumper beam application: GMT, GMTex, and GMT-UD. Major parameters, such as impact energy, peak load, crash resis- tance, energy absorption and stiffness have been taken as evaluation criteria to compare the proposed materials solutions with pultruded and steel solutions. Finally, the results predicted by the nite element analysis have been evaluated and interpreted in comparison with other existing solutions to put in evidence the effectiveness of the proposed innovative materials and design concept solutions.。 2014 Elsevier Ltd. All rights reserved.1. IntroductionAutomobile bumper subsystem is the frontal and rear structure of the vehicle that has the purpose of energy absorption during low velocity impact. Usually, bumper subsystem consists of bumper transverse beam, stays, impact-absorbing materials (such as foam or honeycomb) connected to the structural components (generally the bumper beam) and a cover, that has both aesthetic and protec- tion purposes. Among those elements, the bumper beam is the main structural component; it is expected to be deformable enough to absorb the impact energy, in order to reduce the risks of injury for pedestrians and other vulnerable road users, but, at the same time, it should also have sufcient strength and stiffness to give place to small intrusion of the engine compartment and, therefore, to protect the nearby vehicle components.Composite materials are characterized by high specic strength, both in static and impact loading conditions, and high specic stiff- ness; they could be an interesting candidate material for this type of component, posing as targets the lightweight together with the Corresponding author.E-mail address: koricho (E.G. Koricho).maintenance of at least the same level of safety performance in comparison with the present steel solution.When designing with composite material, it is always needed not only to choice the appropriate material but to think composite (i.e. to not simply replace the metallic material with the new one, but to redesign the part) and to select the type of production tech- nology that will be used in manufacturing, as this choice will affect deeply both the structural performance and the cost and the pro- duction rate 1. Therefore material, design and manufacturing technology are strictly linked each other and should be considered all together.From the point of view of manufacturing technology we have taken into consideration two different types: pultrusion and die forming. Both of them are cost-effective and fully automated and give high quality parts in terms of geometry accuracy and degree of consistency of mechanical property (mainly due to process automation).Pultrusion has a number of advantages such as perfect ber alignment and high ber volume since polymerization takes place while the ber is under tension, capable of producing both closed and open section with a variety of end proles, etc. However, at the moment the technology is strongly limited to straight and/10.1016/pstruct.2014.10.007 0263-8223/。 2014 Elsevier Ltd. All rights reserved.484G. Belingardi et al. / Composite Structures 120 (2015) 483495constant section. Conversely, die forming composite manufactur- ing technology has also its own advantages, i.e. it allows producing structurally integrated crash box and beam, as shown in Fig. 1, that improve both manufacturing and assembling rate and eliminate connection between bumper beam and crash box. However, it is mainly limited to open section proles that are generally less per- forming than the closed section proles.As the targeted component is designed for impact loading, prior to conduct numerical impact analysis at the component level, the impact performance of composite material are assessed. In general, impact responses and damage mechanisms for the whole group of composite materials are more complex comparing with the con- ventional metallic materials and depend on a number of different parameters: ber and matrix type, section shape and dimensions, impact velocity, impact angle, shape of striker, target geometry and target material. Open literatures show that a composite tube is capable of absorbing signicant impact energy by material frag- mentation and large changes in the tubes cross-sectional geometry when the tube undergoes large exural deformation 27.In the current study six material were considered. For pultruded bumper beam solution, unidirectional pultruded E Glass/epoxy, a bidirectional fabric E Glass/epoxy and steel material were com- pared. The detailed mechanical properties documented 8. For the case of die formed integrated crash boxbeam solution, three materials were considered: A classic glass-mat-reinforced thermoplastics (GMT) i.e. an end- less ber glass mate reinforced PP with randomly oriented glass bers, GMTex, i.e. a chopped ber glass mat reinforced PP laminatewith randomly oriented glass bers and additionally reinforced with a fabric inside and GMT-UD, i.e. a chopped ber glass mat reinforced PP laminatewith randomly oriented glass bers and additionally reinforced with unidirectional oriented glass ber layers.These three materials, supplied by Quadrant, were considered for front bumper application. Considering the novelty of the modied material, extensive material characterization had been conducted to obtain the main mechanical properties of the mate- rial and to understand the failure mechanism for the intended loading case and nally their capability for substituting the current steel material were numerically assessed.2. Material characterizationThe composite materials were characterized under a tensile (both longitudinal and transverse direction), compressive (both longitudinal and transverse direction), and a drop-dart tests. A brief summary of the test set-up and of the obtained mechanicalcharacteristics of the tested materials are presented in the follow- ing sections.2.1. Experimental setup for tensile testFive specimens for each material type, in both longitudinal and transverse directions, were tested under tensile loading with a 100 kN capacity servo-hydraulic testing machine (INSTRON- 8801), as shown in Fig. 2. Each specimen was clamped by means of hydraulic wedge grips. The machine was equipped with a stan- dard load cell and a crosshead displacement measuring device.During the mount phase of the specimen, the maximum preload was controlled and set lower than 0.2 kN in order to avoid specimen damage. According to ASTM D3039, specimens were subjected to monotonic tensile loading with a stroke rate of 2 mm/min. The specimens were instrumented by strain gages to measure Youngs modulus and Poissons ratio. To acquire the strain gages data, a NI WLS-9163 data acquisition board was used and to acquire load and crosshead displacement data from the machine, a NI DAQCard-6062E was utilized. All data were acquired with a sampling rate equal to 1 kHz. The main mechanical properties are reported in Table 1.2.2. Experimental setup for compression testSimilarly, ve specimens for each material, in both longitudinal and transverse directions, were tested under compressive loading as per ASTM D6641/D6641M and the found experimental results are presented in Table 2.2.3. Experimental setup for the drop dart testPrior to impact test, quasi-static indentation tests were performed on Zweck Roell 100 universal testing machine, to inves- tigate perforation energy of the proposed composite laminates, Fig. 3. Main results are reported in Table 3.Experimental impact tests were performed according to ASTM standard 3029 using an instrumented free-fall drop dart testing machine. The impactor has a carriage mass of 5.735 kg and an hemispherical head with a radius of 10 mm and the maximum falling height of the testing machine is 2 m (see Fig. 4). The drop- weight apparatus was equipped with a motorized lifting track. The collected data were stored after each impact and the impactor was returned to its original starting height. Using this technique, the chosen impact velocity was consistently obtained in successive impacts. By means of a piezoelectric load cell, forcetime curves were acquired and, with a double integration of accelerationtime curve, forcedisplacement curves were obtained. Square specimen panels, with 100 mm edge, were clamped in the specimen holder with a 76.2 mm inner diameter, and xed to a rigid base to preventFig. 1. Integrated composite solution developed by Quadrant Plastic Composites International (a) and used on Mercedes for top class vehicle (b).G. Belingardi et al. / Composite Structures 120 (2015) 483495485Fig. 2. Experimental setup for tensile and compression material characterization.Table 1Tensile property of the material considered.PropertiesGMTGMT UDGMTexQuasi-isotropicLongitudinalTransverseLongitudinalTransverseTensile strength (MPa)80.7180.459.2174.7170.6Modulus (GPa)5.8111.076.489.246.00Poissons ratio0.2840.3070.2170.3890.150Table 2Compressive property of the material considered.PropertiesGMTGMT UDGMTexQuasi-isotropicLongitudinalTransverseLongitudinalTransverseCompression strength (MPa)65.882.258.569.057.1Modulus (GPa)3.265.932.644.142.66Poissons ratio0.3380.4300.1780.2370.168Fig. 3. Quasi-static indentation test.slippage of the specimen. The clamping system was designed to provide an adequate uniform pressure all over the clamping area. Having got the energy required for perforation of the three materials, the response of the materials for dynamic loading werecompared by conducting a dynamic impact test at equal energy level, i.e. 65% of GMTex quasi-static indentation energy. Forcetime curve (data) and the actual initial impact velocity were acquired directly from the test machine through Lab VIEW Signal486G. Belingardi et al. / Composite Structures 120 (2015) 483495Table 3Energy for quasi-static perforation test.MaterialPlate thickness mmEnergy JGMT438.9GMTex331.1GMT-UD441.2Fig. 4. Drop dart testing machine and specimen conguration.express environment developed for this particular test scenario and the other important dynamic variables were calculated using free body motion equation 12 as described in Fig. 5.2.4. Impact response of composite materialRepresentative curves of Force vs. time, Energy vs. time and pic- tures of the damage mode, respectively for the rst and perforation impacts, are presented in Figs. 6 and 8. The number of impacts needed to perforate the plate and the damage development through the successive impacts was monitored by impacting a vir- gin specimen every time to the desired number of impacts. Figs. 7 and 9 are presenting pictures of the impacted surfaces of theFig. 5. Drop test motion description.specimen according to the specied number of impacts, respec- tively for the GMT and GMT-UD materials. A smooth Force vs. time and the relatively lower absorbed energy of GMT-UD at the rst impact can be linked to the observation that the GMT-UD plate has no visible damage, as shown in Fig. 6e. This implies that most of the energy was dissipated due to mechanisms other than mate- rial internal fracture. Whereas, Force vs. time curve of both GMT and GMTex shows an apex that can be interpreted as a sign of frac- ture and this can be linked to the visible damages that can be observed on the impacted plate shown in Fig. 6c and d.Fig. 8 shows some results at the perforation after repeated impacts. We can observe that at perforation, GMT and GMTex have similar failure behavior i.e. the impactor penetrates the plate dam- aging a conned area around the perforation hole but without extensive crack propagation into the plate (see Fig. 8c and d). This is due to the fact that classical GMT has in plane quasi-isotropic properties, i.e. it has almost uniform continues bers in all direc- tion, and this prevented the cracks at the edge of the perforation central hole from being propagated. Similarly, GMTex has a fabric ply at the midplane of classical GMT, therefore the crack propaga- tion has been impeded by the joint effects of the random and fabric bers perpendicular to the crack. Conversely, in the case of GMT- UD, being the classical GMT reinforced by unidirectional bers, there was a chance for a crack originated at from the edge of the perforation central hole to propagate along the ber direction (see Fig. 8e).It is well known that, composite material has poor plastic properties, therefore, when energy absorbing components, like bumper beam, are designed using materials of this type, the energy dissipation can mainly take place through the material fracturation. Therefore the more the component material is frag- mented the larger amount of energy is dissipated. In this respect, Energy vs. time curve and the damage mode of GMT-UD at perfo- ration impact show a better fracture behavior i.e. in addition to the impactor penetration through the plate thickness, crack prop- agate along the plate width which increases the amount of energy absorption.3. Bumper beam designAs indicated in the introductive paragraph, two different types of manufacturing technology, namely pultrusion and die forming, have been considered to manufacture the beam with the desired shape. Both of them are cost-effective and fully automated manu- facturing technologies and give high quality parts in terms of geometry accuracy and degree of consistency of mechanical prop- erty. Pultrusion manufacturing solution is very effective to make a great variety of end proles but is currently limited to straight beams, moreover is not suitable to develop integrated crash box beam solutions. Die forming manufacturing technology is suitable to develop integrated crash boxbeam solution and with no limita- tion on the beam curvature, but it is limited to open section pro- les, that are structurally weaker than the closed section proles, and has limitations on the shape of the section proles.Fig. 10, shows a simplied model of pultruded bumper beam (a) and die formed GMT/GMTex integrated cashbox-beam bumper beam (b) considered for a nonlinear nite element simulation using commercial code ABAQUS/Explicit version 6.12-1. In both cases, the rigid bodies were modeled as discrete rigid surfaces in order to create higher mesh density at critical contact areas. A mass of 1000 kg was rigidly coupled with the two rear extremities of the crash boxes, in order to simulate the vehicle mass. Boundary condition was also applied on vehicle mass point whose initial velocities towards the rigid wall were set in turn equal to 4, 8 and 15 km/h, in order to simulate three different impact situations relevant for the bumper system.G. Belingardi et al. / Composite Structures 120 (2015) 483495487Fig. 6. Force vs. time (a), energy vs. time (b) and damage at the rst impact (ce).3.1. E-Glass/epoxy pultruded beam solutionPultruded beam solution have been studied by a number of scholars 10,11, for roadside barrier structures which is a similar to lateral loading case as in vehicle bumper. Roadside barrier are usually designed to shield motorists from man-made or natural hazards, to redirect errant vehicles back on to roadway and for energy dissipation in case crashing. These studies indicated thatpultruded composite materials are viable for use in guardrail sys- tem due to their pseudo-ductile characteristics that arise primarily from material fragmentation (crushing, separation and tearing of composite materials) and large changes in the tubes cross- sectional geometry when the tube undergoes large exural deformation.The proposed pultruded bumper beam solution 8 intended to utilize and optimize the pseudo-ductile behavior of pultruded488G. Belingardi et al. / Composite Structures 120 (2015) 483495Fig. 7. Damage development through the successive impact for GMT and GMTposite beam for effective energy dissipation at low velocity vehicle frontal crash. The pseudo-ductile behavior was optimized through a structural optimization procedure of the beam section prole (that can be easily obtained by means of a properly shaped die section) and of the curvature (that at present is not offered by main manufacturers with this technology) aimed to obtain a pro- gressive energy absorption and a stable exural failure of the com- posite bumper beam.A numerical study has been conducted according to the meth- odology developed in 8,9 in order to explore the possibility of substituting the current metallic bumper beam with E-Glass/epoxy pultruded composite beam. The resulting structures are compared in terms of shape and in terms of energy absorbing capability, com- parison is also established with steel normal production solution. The pseudo-ductile behavior of pultruded beams arise from material fragmentation (crushing, separation and tearing of com- posite materials) and large changes in the tubes cross-sectional geometry when the tube undergoes large exural deformation. Therefore, the analysis has been conducted based on the hypothe- sis that a properly optimized and predened stress concentration zone i.e. beam longitudinal groves (through an optimization process of the end prole shape) can serve as crash triggering mechanism, i.e. to initiate cracks formation and to develop pro- gressive tear along beam longitudinal axis. The optimization has been conducted using as design variables the number of groves on the height (h) of the beam end prole and the distribution of the wall thickness, taking advantage from the capability of the pultrusion technology to produce such a particular prole. The per- formance comparison among the proposed end proles was done through the investigation of impact event characteristic data, such as forcetime, forcedisplacement, energydisplacement and displacementtime curves. The optimized beam section prole isbeam and crash box structure thus eliminating the need of joints in between.A nonlinear nite element simulation, with a simplied bumper beam model, as shown in Fig. 10b, has been carried out using the commercial code ABAQUS/Explicit version 6.12-1. The model com- prises two parts, one rigid part, i.e. the impact rigid wall, and one deformable part that integrates crash-boxes and transverse beam and its back cover. The integrated beam solution has been developed in three alternatives according to the three considered composite materials, GMT, GMtx and GMT-UD. A mass of 1000 kg is rigidly attached at the two rear extremities of the crash boxes, in order to simulate the vehicle mass, it moves with an ini- tial velocity of 4 or 8 km/h towards the rigid wall. Considering the load path, different sections have been used at different portions of the proposed structure as shown in Fig. 10b. Hollow tapered trun- cated square based pyramids were proposed for crash boxes, in order to obtain a progressive failure.The design solutions that are using the proposed materials were developed starting from the normal production solution (that is the reference solution) made by steel by means of two approaches:- by direct substitution of the current steel beam, through inte- gration of the composite beam with crash boxes with minormodications to the base plate only for joining purpose, using the wall thickness recommended by the company i.e. 臼8 mm,and- through equal bending stiffness approach 9, i.e. for a given thickness and stiffness of the reference material, the thickness to be adopted with the targeted material can be approximately calculated by Eq. (1).s3 Espresented on Fig. 11.Even if the current pultruded manufacturing technology ishx hsEx1mainly limited to straight beam (curved pultrusion technology is still in infant stage) an optimization has also been conducted on beam curvature radius (R) Fig. 12. A large number of beam curva- ture radius, from straight axis to smaller radius, were considered. The failure phenomenon and the bumper beam performance were closely monitored using the already mentioned impact event characteristic data.3.2. GMT/GMTex die forming integrated crash boxbeam solutionDie forming manufacturing technology is capable of producing structurally integrated crash box and beam as a single component. This is an extremely interesting feature of this technology because it leads to remarkable improvements both from the point of view of the manufacturing/assembly rate and from the point of view of a relevant reduction of the number of different components that should be produced and assembled to construct the front end structure. Besides, since joining is one of the critical issue in using composite part in automotive structures (as structures often have their weak points where their parts are joined together), The die forming technology is suitable for producing an integrated bumperwhere hs and hx are respectively the wall thickness of steel and of the targeted material solutions and Es and Ex are the elastic modulus of steel and the targeted material respectively.During low velocity impact, such as small parking load, the bumper beam is expected only to bump i.e. it has to operate within elastic limit without any form of permanent damage. Therefore, for the current study, the allowable minimum thickness of the bumper for such small load was determined through monitoring impact energy curve. Having got the threshold value the thickness, it was gradually increased up to a value where the beam gives a sim- ilar impact performance as with the reference material. Finally the mass reduction has been evaluated.3.2.1. Design consideration for the integrated bumper beamWhen metallic components are substituted by composite com- ponents, taking into account the very different failure modes of the two materials, new design hypothesis has to be followed, only in this way the advantage that comes from the important features of the new material can be maximized. Therefore, in the current integrated bumper system the following three design consider- ations were made:G. Belingardi et al. / Composite Structures 120 (2015) 483495489Fig. 8. Force vs. time (a), energy vs. time (b) and damage during perforation (ce). In traditional bumper system, the structural integrity betweenthe bumper beam and crash box is obtained by mechanical fas- tening (welding or bolting) of the crash box to the beam. The failure mode of axially loaded composite pyramidal tube isinitiated by front end triggering mechanism and progressively is propagated along the length of the tube, the adoption of the traditional connection scheme for composite bumper system assembly will result in an early beam and crash box detachmentG. Belingardi et al. / Composite Structures 120 (2015) 483495490Fig. 9. Damage development through the successive impact for GMT-UD.Fig. 10. Simplied FEM bumper models:(a) pultruded beam solution, (b) die forming integrated beamcrash box solution.Fig. 11. Optimized beam end section prole for pultruded solution 8.and, therefore, will not meet the intended energy absorbing goal. A new design approach has to be followed for this group of materials. As previously pointed out, with a proper design of integrated bumper system the problem can be soundly addressed. The proposed solution has a free frontal crash box end with the required crash trigger. While the structural integ- rity can be obtained through optimization of the trickiness of the connecting rim. In particular the rim thickness has to be optimized to withstand the shearing load resulting from frontal impact. In case of small low velocity impact only the bumper beamshould be involved and should behave fully elastic, without the direct involvement of the crash boxes. Therefore, the clear- ance C between the front of the beam and the front of the crash box need to be optimized. Energy absorption for the crashed object is the area under forcedisplacement curve, that is proportional to the product of the force by the crush length. Hence, the crush length L is anFig. 12. Bumper beam Curvature considered for optimization.Fig. 13. Point considered for integrated beamcrash box design.G. Belingardi et al. / Composite Structures 120 (2015) 483495491important parameter for crash component design. During inte- grated bumper system design, improper placing of connecting rim will affect the crash length and affect the energy absorption of the system by blocking and controlling the progressive failure of the crash box. Thus, the relative position of the crash box and bumper beam rim has to be optimized.Having in mind the above stated design considerations, a sim- plied integrated bumper beam model, as shown in Fig. 13, was modeled in CATIA 5 and mesh renement was conducted in ABAQUS. The model comprises only two parts: a deformable inte- grated bumper system and the rigid wall. The rigid bodies were modeled as analytical rigid surfaces.4. Results and discussion4.1. Pultruded bumper beam solutionThe beam end prole has been optimized through the number of groves on the height (h) of the beam and the distribution the wall thickness. The detailed optimization process has been reported in 8. When the bumper beam is subjected to frontal impact, concentrated stresses develop at the grove vertexes; points on the fold sides at equal distance from the impacted surface have the same stress levels. This is substantially uniform in case of straight beam while a change in the beam curvature has an effect both on the stress distribution along the beam and on the stress values.Fig. 14 is showing the nal deformed shape of the bumper beam for three different solutions characterized by different values of the curvature radius from 2400 mm (case a) to straight beam (case c). Fig. 14d shows the reaction force histories for those three solu- tions. It is well visible that the case of the small curvature radius is generating a concentrated failure hinge close to the beam mid- span and a very large load peak comes out; the other two cases are giving more diffuse energy absorption and smoother curves; the solution with the intermediate values of the curvature radius is giving the minimum load peak. As a rst general observation on low velocity impact analysis, when the beam curvature radius is increased, the formation of local stress concentration is reduced. This is due to the fact that larger zones of the bumper beam are in contact with the at rigid wall at the same time. This leads to higher load peak that promote the formation of diffuse fractures on the portions of the folds which have the same stress level.The worst case is when the bumper beam is straight Fig. 14c, which corresponds to a solution currently used by some vehicles. In this situation the portion of the beam extremities just in front of the crash box, with length equal to the crash box width will frac- ture at the same time, since that portion of the beam is under equal stress level and there is not possibility for crack propagation and proper energy absorption.On the other hand, when the beam curvature radius is reduced below some critical curvature radius, 2862 mm in this particularcase, crack propagation is not taking place, but instead a high local stress line is developed at the apical portion of the beam, which results in unstable localized failure, as shown Fig. 14a.Finally, the performance of the proposed pultruded composite bumper beam solution can be compared with the steel and the glass fabric/epoxy composite solutions in terms of impact energy absorption and weight reduction. Three parameters, namely the amount of absorbed energy, the peak load value and failure mode, are considered for material comparison.A shown in Fig. 15, the three design solutions absorbed the same amount of energy, however the peak load values and the mode of failure are completely different. During vehicle frontal crash, peak load is relevant for the vehicle occupant risk, as a mat- ter of fact lower peak load yields to lower decelerations and vice versa, so this parameter should be carefully controlled. In addi- tions, by comparison of the failure modes of the two composite material solutions, i.e. pultruded and fabric, it comes out that the energydisplacement curve of pultruded beam is almost linear and the load deection curve of pultruded beam resembles the uni- axial stressstrain diagram of an elasto-plastic ductile material, that is technically termed as pseudo-ductile. Therefore, as far as it is possible to control the displacement or to keep the displace- ment within the design limits, the pseudo-ductile behavior of the pultruded solution is an important feature in the passive safety behavior of the bumper component.4.2. Die forming integrated bumper beamcrash box solutionFrom our previous related activity, it has been learned that a closed section beam has better structural integrity and energy absorbing capacity than an open section beam. Hence, even if an open section beam was considered and recommended by the material supplier company, for sake of production feasibility and simplicity, a closed section beam has also been numerically investigated.The rst attempt was conducted by direct substitution of the current steel beam with integration of the crash boxes and with minor modications on the base plate only for joining purpose. As recommended by the material supplier, the composite beamwall thickness for each material conguration was set 臼8 mm. Asit can be seen on force vs. time and force vs. displacement curves resulting from the simulation of impact events a 4 km/h (see Fig. 16), all the three material solutions are structurally weak. GMT-UD and GMtx solutions show an early sharp break at the cen- ter of the beam while GMT solution shows relatively higher elastic deformation.The design changes were made by increasing the section dimensions, particularly the base plate. The wall thickness of the integrated bumper beamcrash-box was determined on the basis of the data of the reference material solution. For a given wall thickness and stiffness of the reference material (steel), the thick- ness of the targeted material can be calculated approximately with Eq. (1). The obtained approximated wall thickness and the mass ofFig. 14. Failure mode and peak load for curvature radius (a) 2400 mm, (b) 3200 mm and (c) straight 8.G. Belingardi et al. / Composite Structures 120 (2015) 483495492Fig. 15. Reaction force vs. displacement and energy vs. displacement for the three materials considered 8.Fig. 16. Force vs. time and force vs. displacement curves for the design solutions in steel and in equal thickness of the three considered materials.the integrated bumper beamcrash box solutions are reported in Table 4.As presented in Table 1, GMT-UD has tensile longitudinal mod- ulus approximately 50% and 25% higher than GMT and GMtx, respectively. therefore it has better mechanical performance. Furthermore, as it was explained in the previous Section 2.4, the introduction of unidirectional bers in the classical GMT, makes the material to crack along the width of the plate, which is improv- ing the energy absorption capability. This failure behavior is also observed on energy vs. displacement curves of dynamic drop dart test.Through the comparison of force vs. time and displacement vs. time curves (see Fig. 17) related to the targeted four materials, it comes out that GMT-UD solution has the minimum peak load,i.e. 25 kN, this is one of the important parameters that the designer has to control, and has a failure mode similar to the reference material solution but has the maximum intrusion, i.e. 37 mm, the beam results to be fractured at the selected loading. This results is not acceptable since it was stated that for the 4 km/h impact cases the bumper beam should remain in the elastic region behav- ior without any structural damage.The failure behavior can also be tracked using load displace- ment curves and energy time history curves as shown respectively in Figs. 1820. Fig. 20a and b show that the GMTex and GMT solu- tions after having transformed the whole kinetic energy into inter- nal deformation energy, are able to restitute most of this energy and an elastic rebound takes place. A completely different situation is visible in Fig. 19b, for the GMT-D solution the restitution phase is missing and this is due to the beam crack.Table 4MaterialSteelGMTGMtxGMT-UDThickness mm5.8Thickness and mass of the combined beamcrash-box.Mass kg7.673.723.323.2Both load and energydisplacement curves of GMT-UD solution (which shows the amount of energy absorption and the behavior of the impacted system during energy dissipation) conrm that the material is already fractured at the selected velocity. Similar phe- nomenon is also observed on the reference material (steel), which might be due to the strength of the selected steel. As metallic materials have a higher plastic range, the energy curves of steel show the energy dissipation through plastic deformation. Whereas, composite materials have very limited plastic range, therefore, energy curve shows that GMT-UD has already passed its elastic limit and, as a consequence, the energy dissipation resulted from the material fragmentation.On the contrary, both GMT and GMtx solutions remain within elastic range, this can be observed from load vs. displacement and energy timehistory curves in Figs. 18 and 20. Failure mode are shown in Fig. 21. For 4 km/h impact velocity, which is close to parking load, the bumper has to operate within elastic range, therefore besides bumping and, eventually, a minor cosmetic dam- age, a complete fracture, as we observed on GMT-UD solution, is not acceptable. Therefore, with the proposed beam conguration and for the considered loading conditions (i.e. at low velocity impact) the wall thickness obtained from Eq. (6) i.e. using equal stiffness approach, leads to acceptable results only for GMT and GMtx and can be considered for material replacement with signif- icant weight saving but for GMT-UD the beam is fractured and therefore the wall thickness asks for further changes.The above proposed solutions have been also cheeked at higher impact velocity, i.e. at 8 km/h. From force vs. time curve of Fig. 22, it can be observed that there are two modes of deformation at this higher velocity, after the initial phase of elastic deformation up to臼2540 kN, which is a behavior similar that observed at lowervelocity impact (at 4 km/h), a complete fracture of GMT and GMtx beams at their central part comes out. Finally the other portions of the fractured beam impact against the rigid wall but without fur- ther material fracture. This phenomenon yields relatively higher peak loads i.e. 臼265 kN, and 300 kN for GMT and GMtx solutionsG. Belingardi et al. / Composite Structures 120 (2015) 483495493Fig. 17. Force vs. time and displacement vs. time curve for the modied bumper system.Fig. 18. Energy vs. displacement and force vs. displacement curves for the modied bumper system in case of impact at 4 km/h.Fig. 19. Energy curve for (a) steel and (b) GMT-UD solutions in case of impact at 4 km/h.Fig. 20. Energy curve for (a) GMtx and (b) GMT solutions in case of impact at 4 km/h.respectively. Whereas, GMT-UD solution shows symptom of further fractures during the second phase of the impact which are denitely due to fractures along the width of the plate. Thisresults in relatively smaller peak load i.e. 臼170 kN. The failurebehaviors of the two solutions based on GMT-UD and GMTex can be observed in Fig. 23.In general, the modication made on classical GMT is indeed improving both the tensile and impact performance of the original material and can be used for structural purposes in someapplication in place of steel and aluminum. However, coming to energy absorbing components, having the composite materials a completely different failure behavior than the conventional metal- lic materials, the energy absorbing performance is strongly affected by the geometry of the component. The direct adoption of the traditional metallic energy absorbing geometry may lead to a catastrophic fail
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