3.1  RECENT ADVANCES IN SANDWICH COMPOSITES.

Sandwich  construction  composites  are used in a wide variety of structural applications largely
because of their relative advantages over other structural materials in terms of improved stability
and weight savings as well as a number or other factors cited previously.  While the initial design
of  structures comprised of sandwich construction composites is at a fairly mature stage of
development [2, 3], less progress has been made in understanding the long term response of such
structures subjected to adverse in-service impact events or environmental influences.  Such an
understanding is critical if there is to be widespread use of sandwich composites in applications
where structural durability and damage tolerance is  a primary consideration  (e.g.,  aerospace or
automotive  applications).  The  focus  of  this section is to provide a broad overview of recent
research  aimed  at characterizing the thermomechanical response of various sandwich
construction composites subjected to low-to-moderate energy impact  events  as  well  as
environmental influences.  Such impacts may induce  localized damage in sandwich composites
(fiber breaks, resin cracking, face sheet-core delamination, core crush, puncture, etc.) and can be
attributable to a number of fairly common discrete sources (hail, tool drops, runway projectiles, bird strikes, or other unintentional impacts).  Any reference  to  impact damage used herein will
suggest  the  damage  associated  with low-to-moderate energy impact events, unless stated
otherwise.  The effect of intrinsic processing induced defects  (porosity,  voids,  small  disbonds,
etc.) and catastrophic damage associated with high-energy impacts resulting from  airplane
crashes or similar events are not considered in this discussion.  Widespread processing induced
defects,  perhaps, may  best be addressed using continuum damage mechanics [4]; Abbott [5]
noted that evolution of processing induced defects is likely not an issue for many  structural
applications involving sandwich composites.

Recent efforts aimed at clarifying the  thermomechanical  response  of  sandwich  composites
subjected to low-to-moderate energy impacts can be loosely categorized into three areas:

a.  Analysis of the impact dynamics between the indentor (projectile)  and  the  target
(sandwich panel).

b.  Characterization of impact damage modes and mechanisms.

c.  Application of durability and damage tolerance principles to sandwich composites.

A summary of key research in each of these areas is presented in the following discussion.  See
Abrate  [6]  for  an excellent review of recent investigations concerning the effect of impact on
sandwich structures with laminated facings.

3. 损伤容限之前调查

3.1 夹层复合材料的最新进展

由于夹层复合材料相对于其他结构材料更先进的稳定性、质轻性和大量之前论述过的相对优势，它被广泛应用在一系列的结构之中。尽管夹层复合材料的原型设计已经有一个比较成熟的发展，但是这些结构在负面的工作冲击和环境影响中的长期方应的理解并没有一个较深入的研究。假如这些夹层复合材料被广泛应用在结构耐久性和损伤容限是首要考虑的地方（比如宇航空间和汽车应用），那么这项研究就是至关重要的。本章的重点就是要对各种夹层结构复合材料在受到低到中等能量冲击和环境影响下的热力反应的研究提供一个大概的综述。这些因素有可能来之于一些常见的相互之间没有联系的原因（冰雹、工具掉落、跑道弹、鸟击或其他意外事件），从而引发在夹层复合材料中的局部破坏（纤维断裂、树脂裂化、表面薄板中心分层、核心粉碎、穿刺等等）。除非特别注释，不然本章中引用的冲击事件都是暗指低至中等能量冲击事件。由于固有的处理导致的缺陷（孔隙度、空隙、小电桥等等）和因为飞机坠毁或类似事件的高能量冲击带来的灾难性破换在此处不讨论。广泛的处理导致的缺陷也许最好用连续损伤力学来解决；Abbott指出处理的发展导致的缺陷不会是夹层复合材料应用在许多结构中的问题。

旨在说明在低至中等能量冲击下的夹层复合材料的热力性能的最新研究大致可以分为以下三个方面：

A、 压头（抛射体）和目标（夹层面板）冲击动力学的研究。

B、 冲击破换模式和机制特点

C、 夹层复合材料中耐久性和损伤容限公差原则的应用

各种方面的重点综述会再下面的讨论中呈现。关于冲击在层压面板的夹层结构的影响的最新研究，参看Abrate的优秀综述。