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首页剑桥大学多学科设计项目数据库:实用工程资源宝典
剑桥大学多学科设计项目数据库:实用工程资源宝典
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更新于2024-07-18
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剑桥大学的多学科设计项目数据库(MDPDatabook,英文版)是一个集合了广泛工程领域实用信息的综合资源,版本为0.0.3,发布于2009年11月20日。这个数据库旨在为实际工程领域的专业人士提供一个便捷的参考工具,包含各种关键的设计数据和建议,涵盖了从基础组件选择(如螺钉、电线等)到复杂结构建造(如砌墙技术)等多元化主题。 数据库的创建得到了多个行业合作伙伴的支持,包括但不限于Corus、Farnell、Huntsman等公司的协助。这些公司不仅提供了专业知识,还确保了信息的准确性。数据库中的数据来源多样,有的由MDP团队直接收集整理,有的则是与原始作者或公司共享,并保留了原作者的版权。 值得注意的是,尽管数据库尽力确保所有信息的可靠性,但用户在处理关键项目时仍被强烈建议进行核实,因为可能存在需要更新或补充的信息。此外,为了方便用户获取更详细的内容,数据库中包含了指向各个子主题索引的热点链接,这些链接直接导向各个原作者或贡献公司的官方网站,这些网站通常提供更为详尽的数据和附加资料。 剑桥大学多学科设计项目数据库不仅是一个工具,也是工程师们持续学习和更新知识的平台。它体现了工程实践中的跨学科合作精神,鼓励工程师们在项目实施过程中灵活运用多种学科的知识,提升工作效率和项目成功率。通过这个数据库,工程师们可以节省寻找和验证相关信息的时间,专注于核心设计工作,从而推动创新和科技进步。
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Limitations
Temperature resistance: Adhesives are drawn from the
class of materials which we know as ‘polymers’, ‘plastics’
or ’synthetic resins’. They have the limitations of that class.
They are not as strong as metals. (the difference is offset
by the increased surface contact area provided by the
bonded joints). With increasing temperature the bond
strength decreases, and the strain properties of the
adhesive move from elastic to plastic. This transition is
usually in the temperature range 70 – 220
0
C: the transition
temperature depends on the particular adhesive.
Chemical resistance: The resistance of bonded joints to
the in-service environment is dependent on the properties
of the polymer from which the adhesive is made. Possible
exposure of the bonded structure to oxidising agents,
solvents, etc., must be borne in mind when selecting the
adhesive type to use.
Curing time: With most adhesives maximum bond
strength is not produced instantly as it is with mechanical
fastening or with welding. The assembled joint must be
supported for at least part of the time during which the
strength of the bond is building up. The quality of the bond
may be adversely affected if, in the bonding process, the
surfaces are not readily wetted by the adhesive.
Process controls: Ensuring consistently good results may
necessitate the setting up of unfamiliar process controls. A
badly made joint is often impossible to correct.
In service repair: Bonded assemblies are usually not
easily dismantled for in-service repair.
Modern adhesives: types and main
characteristics
Modern adhesives are classified either by the way they are
used or by their chemical type. The strongest adhesives
solidify by a chemical reaction. Less strong types harden
by some physical change. Key types in today’s industrial
scene are as follows.
Anaerobics: Anaerobic adhesives harden when in contact
with metal and air is excluded, e.g. when a screw is tight
in a thread. Often known as ‘locking compounds’ or
‘sealants’, they are used to secure, seal and retain turned,
threaded, or similarly close-fitting parts. They are based on
synthetic resins known as acrylics. Due to the curing
process, anaerobic adhesives do not have gap-filling
capability but have advantage of relatively rapid curing.
Cyanoacrylates: A special type of acrylic, cyanoacrylate
adhesives cure through reaction with moisture held on the
surfaces to be bonded. They need close-fitting joints.
Usually they solidify in seconds and are suited to small
plastic parts and to rubber. Cyanoacrylate adhesives have
relatively little gap-filling capability but can be obtained in
liquid and thixotropic (non-flowing) versions.
Toughened Acrylics/Methacrylates: A modified type of
acrylic, these adhesives are fast-curing and offer high
strength and toughness. Supplied as two parts (resin and
catalyst), they are usually mixed prior to application, but
specialised types are available which are applied by
separate application: resin to one bond surface, catalyst to
the other. They tolerate minimal surface preparation and
bond well to a wide range of materials. The products are
available in a wide range of cure speeds and as liquids or
pastes which will gap-fill up to 5mm.
UV curable adhesives: Specially modified acrylic and
epoxy adhesives, which can be cured very rapidly by
exposure to UV radiation. Acrylic UV adhesives cure
extremely rapidly on exposure to UV but require one
substrate to be UV transparent. The UV initiated epoxy
adhesives can be irradiated before closing the bondline,
and cure in a few hours at ambient temperature or may be
cured at elevated temperature.
Epoxies: Epoxy adhesives consist of an epoxy resin plus a
hardener. They allow great versatility in formulation since
there are many resins and many different hardeners. They
form extremely strong durable bonds with most materials.
Epoxy adhesives are available in one-part or two-part form
and can be supplied as flowable liquids, as highly
thixotropic products with gap-filling capability of up to
25mm, or as films.
Polyurethanes: Polyurethane adhesives are commonly
one part moisture curing or two-part. They provide strong
resilient joints, which are resistant to impacts. They are
useful for bonding GRP (glassfibre-reinforced plastics) and
certain thermoplastic materials and can be made with a
range of curing speeds and supplied as liquids or with
gap-filling capability of up to 25mm.
Modified Phenolics: The first adhesives for metals,
modified phenolics now have a long history of successful
use for making high strength metal-to-metal and metal-to-
wood joints, and for bonding metal to brake-lining
materials. Modified phenolic adhesives require heat
pressure for the curing process.
The above types set by chemical reactions. Types that
are less strong, but important industrially, are as
follows:
Hot Melts: Related to one of the oldest forms of adhesive,
sealing wax, today’s industrial hot melts are based on
modern polymers. Hot melts are used for the fast
assembly of structures designed to be only lightly loaded.
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Plastisols: Plastisol adhesives are modified PVC
dispersions which require heat to harden, The resultant
joints are often resilient and tough.
Rubber adhesives: Based on solutions of latexes, rubber
adhesives solidify through loss of solvent or water. They
are not suitable for sustained loading.
Polyvinyl Acetates (PVAs): Vinyl acetate is the principal
constituent of the PVA emulsion adhesives. They are
suited to the bonding of porous materials, such as paper
or wood, and general packaging work.
Pressure-sensitive adhesives: Suited to use on tapes
and labels, pressure-sensitive adhesives do not solidify but
are often able to withstand adverse environments. They
are not suitable for sustained loading.
No one company supplies all these types of adhesives.
Each supplier specialises in particular types.
Huntsman Advanced Materials supplies many
industries with epoxy, polyurethane, modified phenolic,
toughened methacrylate and UV curable acrylic
adhesives under the tradenames Araldite
®
, Epibond
®
,
Epocast
®
and Uralane
®
.
Designing a bonded joint
It is important that bonded articles are designed with
bonding in mind, rather than simply bonding a design
made for welding or mechanical joining. When designing
bonded joints the considerations include:
• Joint geometry
• Adhesive selection
• Mechanical properties of adhesive and adherent
• Stress in the joint
• Manufacturing conditions
Bonded joints may be subjected to tensile, compressive,
shear or peel stresses, often in combination. (See Figure
3). Adhesives are strongest in shear, compression and
tension. They perform less effectively under peel and
cleavage loading. A bonded joint needs to be designed so
that the loading stresses will be directed along the lines of
the adhesive’s greatest strengths.
To indicate the performance of an Araldite, Epibond,
Epocast or Uralane adhesive, the Huntsman Advanced
Materials Instruction
Sheet for the particular adhesive quotes the shear
strengths and peel strengths obtained by standard test
methods. For example, the standard test method for shear
(ISO4587) uses a simple lap joint made from metal sheet,
usually an aluminium alloy, 25mm wide with 12.5mm
overlap. The mean breaking stress at room temperature
will be in the range 5 to 45 N/mm
2
depending on the
adhesive. At the top end of this breaking stress range,
joints made from aluminium alloy sheet of up to 1.5mm
thickness will yield or break in the metal. (The lap joint is
only one of sveral different types of bonded joint).
Tension Compression Shear Cleavage Peel
Tension Compression Shear Cleavage Peel
stress stress stress stress stress
component component component component component
Fig.1 Loading conditions
A bonded joint can be loaded in five basic ways (shown in the diagram). Cleavage and peel loading are the most taxing: they concentrate the applied force into
a single line of high stress. In practice a bonded structure has to sustain a combination of forces. For maximum strength, cleavage and peel stresses should be
as far as possible designed out of the joints.
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7
The breaking load of a lap joint is proportional to its width,
but not to its overlap length. Although the breaking load
will increase as overlap length is increased, the mean
breaking stress will be reduced.
A method of determining the best dimensions for a simple
lap joint is described in Simple Lap Joints: Determination
of dimensions (page 10).
The strength of a joint is a complex function of the stress
concentrations set up by the load. In a simple lap joint
made from thin metal sheet there are two sorts of stress:
shear and peel. Both the shear and peel stresses vary
along the length of the joint, with concentrations at the
ends. Alternative joint designs are shown in Figure 4 where
these stresses are more evenly distributed. The efficiency
gained results in joints of greater strength.
A peel joint can be designed such that the forces acting
upon it become compression forces, making a much
stronger joint.
Weak cleavage joints can be strengthened through design,
in this instance by adding a U-section to the previously
bent sheet.
By adding reinforcing plates to this butt joint, the forces
run along a much stronger shear joint.
A similar effect is produced by sleeving this cylindrical butt
joint.
Simple lap joint good
Tapered lap joint very good
Scarf joint excellent
Stepped lap joint very good
Double strap joint/double lap joint very good
Tapered double strap joint excellent
Fig.4 Basic bonded joints between strip/sheet metals
The basic types of bonded joints are shown diagramatically. In practical
structures two or more basic types may be used in combination – and the
relative dimensions (and areas of bonded surface) of the joints may vary
from those shown in the diagrams.
Tapering of the ends of lap joints or scarf joints serves to distribute the
stress more uniformly and reduce stress concentration.
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8
Fig.5 Practical bonded joints between sheet materials
Certain metals, especially mild steel, are easily bent or folded to form
advantageous joints. (a) Shows a development from the simple lap
joint: a toggled joint. (b) and (c) show further developments.
Closed box structures (d) from formed sheet metal are easily produced
using this folding and bonding technique to join the edges.
Fig.6 Bonding of multi-layer structures
Multi-layer structures may be built up by adhesive bonding and may
also be bonded to other parts. In (a) a multi-layer fibre-reinforced
plastics laminate is joined to its neighbour by a multi-stepped lap joint.
In (b) an edge member is bonded into a sandwich panel. On loading,
the stresses will be transferred into the panel. The honeycomb core is
itself assembled and bonded to the facing sheets with adhesives.
Fig.7 Joints using profiles
Sheets or plates that cannot be bent and folded may be bonded
together by means of purpose-made profiles. Tapering removes the
high stress concentrations caused by abrupt change in section.
(a)
(b)
(c)
(d)
(a)
(b)
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9
Fig.8 Stiffening of large thin sheets
Large sheets of thin-gauge material (metal or plastics) may be
stabilised by bonding stiffeners made of the same material in similar
gauge. The diagram shows a ‘top hat’ stiffener.
Towards the edge of the sheet, the stiffener may be cut away (as
shown) in order to reduce stress concentrations. The effect is similar
to that of the scarf joint in Fig.4.
Fig.9 Bonded frameworks
Framework structures of square or round tubes, or simple profiles,
may utilise plugs (a), angles (b), or bosses (c) at the joints. Use of
these additional pieces greatly increases the area of bond surface at
the joint.
(a)
(b)
(c)
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