question: Why are plastics on one hand durable but on the other super easy to tear apart when a tiny incision is made?
comment: This is a very complex question without you even realizing it. It depends on what plastic it is too. But assuming you're saying durable I think you're talking about hard plastics. These have very strong bonds on a molecular level. But once the bond is broken it can tear way easier through the rest of the material basically.
passages: For example, aluminum which is stamped or forged exhibits plasticity in this sense, but is not plastic in the common sense. By contrast, some plastics will, in their finished forms, break before deforming and therefore are not "plastic" in the technical sense.
  
  Section::::Structure.
  
  Most plastics contain organic polymers. The vast majority of these polymers are formed from "chains of carbon atoms", 'pure' or with the addition of: oxygen, nitrogen, or sulfur. The chains comprise many repeat units, formed from monomers. Each polymer chain will have several thousand repeating units.
   Production of a good quality weld does not only depend on the welding methods, but also weldability of base materials. Therefore, the evaluation of weldability is of higher importance than the welding operation (see Rheological weldability) for plastics.
  
  Section::::Welding techniques.
  
  A number of techniques are used for welding of semi-finished plastic products as given below:
  
  Section::::Welding techniques.:Hot gas welding.
   Some plastics are more sensitive to wall thickness than others, where acetal and ABS plastics max out at around 0.12 in. thick (3 mm), acrylic can go to 0.5 in. (12 mm), polyurethane to 0.75 in. (18 mm), and certain fiber-reinforced plastics to 1 in. (25 mm) or more. Even so, designers should recognize that very thick cross sections can increase the likelihood of cosmetic defects like sink.
  
  Section::::Geometric considerations.:Draft angles.
   In the 1930s and 1940s, the plastics industry developed technologies to create a variety of plastics with broad applications. Once World War II began, the US military used these new plastic materials to improve weapons, protect equipment, and to replace heavy components in aircraft and vehicles. After WWII, manufacturers saw the potential plastics could have in many industries, and plastics were incorporated into new consumer product designs. Plastics began to replace wood and metal in existing products as well, and today plastics are the most widely used manufacturing materials.
   The most easily machined types of metals include aluminum, brass, and softer metals. As materials get harder, denser and stronger, such as steel, stainless steel, titanium, and exotic alloys, they become much harder to machine and take much longer, thus being less manufacturable. Most types of plastic are easy to machine, although additions of fiberglass or carbon fiber can reduce the machinability. Plastics that are particularly soft and gummy may have machinability problems of their own.
  
  Section::::For CNC machining.:Material form.
   There are two aspects to surface integrity: "topography characteristics" and "surface layer characteristics". The topography is made up of surface roughness, waviness, errors of form, and flaws. The surface layer characteristics that can change through processing are: plastic deformation, residual stresses, cracks, hardness, overaging, phase changes, recrystallization, intergranular attack, and hydrogen embrittlement. When a traditional manufacturing process is used, such as machining, the surface layer sustains local plastic deformation.
   BULLET::::- The primary embrittlement mechanism of plastics is gradual loss of plasticizers, usually by overheating or aging.
  
  BULLET::::- The primary embrittlement mechanism of asphalt is by oxidation, which is most severe in warmer climates. Asphalt pavement embrittlement can lead to various forms of cracking patterns, including longitudinal, transverse, and block (hexagonal). Asphalt oxidation is related to polymer degradation, as these materials bear similarities in their chemical composition.
  
  Section::::Glasses and Ceramics.
   Plastics and plastic-based composites may suffer swelling, debonding and loss of strength when exposed to organic fluids and other corrosive environments, such as acids and alkalies. Under the influence of stress and environment, many structural materials, particularly the high-specific strength ones become brittle and lose their resistance to fracture. While their fracture toughness remains unaltered, their threshold stress intensity factor for crack propagation may be considerably lowered. Consequently, they become prone to premature fracture because of sub-critical crack growth. This article aims to give a brief overview of the various degradation processes mentioned above.
  
  Section::::Stress corrosion cracking.
   Section::::Surface preparation of plastics.:Peel ply.
  
  For a peel ply, a thin, woven piece of material is applied to the adherend during fabrication. Because the material is woven, it will leave a torturous surface when removed, which will improve bonding by mechanical interlocking. Prior to adhesive bonding, the woven material acts to protect the surface of the adherend from contaminates. When an adhesive is ready to be applied, the material can be peeled off, leaving a rough and clean surface for bonding.
  
  Section::::Surface preparation of plastics.:Corona discharge treatment.
   To create a desirable surface for the adhesive bonding of plastics, there are three major requirements: the weak boundary layer of the given material must be removed or chemically modified to create a strong boundary layer; the surface energy of the adherend should be higher than that of the adhesive for good wetting; and the surface profile can be improved to provide mechanical interlocking. Meeting one of these major requirements will improve bonding; however, the most desirable surface will incorporate all three requirements. Numerous techniques are available to help produce a desirable surface for adhesive bonding.
  
  Section::::Surface preparation of plastics.:Degreasing.
   Some plastics are manufactured from re-cycled materials but their use in engineering tends to be limited because the consistency of formulation and their physical properties tend to be less consistent. Electrical and electronic equipment and motor vehicle markets together accounted for 58 percent of engineered plastics demand in 2003. Engineered plastics demand in the US was estimated at $9,702 million in 2007.
   Plastics have many uses in the medical field as well, with the introduction of polymer implants and other medical devices derived at least partially from plastic. The field of plastic surgery is not named for use of plastic materials, but rather the meaning of the word plasticity, with regard to the reshaping of flesh.
   BULLET::::- They are bad conductors of electricity.
  
  Section::::Disadvantages and limitations.
  
  BULLET::::- Plastics may be degraded under the action of direct sunlight which reduces their mechanical strength.
  
  BULLET::::- Many plastics are flammable unless treated.
  
  BULLET::::- High embodied energy content
  
  BULLET::::- Low modulus of elasticity: makes them unsuitable for load-bearing applications.
  
  BULLET::::- Thermoplastics are subject to creep and soften at moderate temperatures.
  
  BULLET::::- Thermal expansion for most plastics is high: adequate thermal movement has to be allowed in detailing.
  
  BULLET::::- Many types of plastics are not biodegradable thus cause pollution when they accumulate.
  
  Section::::Products.
   While many ideas from the linear elastic fracture mechanics (LEFM) models are applicable to polymers there are certain characteristics that need to be considered when modeling behavior. Additional plastic deformation should be considered at crack tips as yielding is more likely to occur in plastics.
  
  Section::::Fracture Mechanics in Polymers.:"Yielding Mechanisms".
   Hard plastic joints generally exhibit greater friction than polyvinyl joints, and are similarly more durable than styrene joints. ABS joints, however, require greater precision in tooling to ensure easy assembly, and in some cases, they require screws and a small gap between parts.
   Section::::Feature Based Rules.:Boss.
  
  Boss, a basic design element in plastics, is typically cylindrical and used as a mounting fixture, location point, reinforcement feature or spacer. Under service conditions, bosses are often subjected to loadings not encountered in other sections of a component. 
   BULLET::::- "Plasticity" or plastic deformation is the opposite of elastic deformation and is defined as unrecoverable strain. Plastic deformation is retained after the release of the applied stress. Most materials in the linear-elastic category are usually capable of plastic deformation. Brittle materials, like ceramics, do not experience any plastic deformation and will fracture under relatively low strain, while ductile materials such as metallics, lead, or polymers will plasticly deform much more before a fracture initiation.
   In structural engineering, the plastic moment (M) is a property of a structural section. It is defined as the moment at which the entire cross section has reached its yield stress. This is theoretically the maximum bending moment that the section can resist - when this point is reached a plastic hinge is formed and any load beyond this point will result in theoretically infinite plastic deformation. In practice most materials are work-hardened resulting in increased stiffness and moment resistance until the material fails. This is of little significance in structural mechanics as the deflection prior to this occurring is considered to be an earlier failure point in the member.
   BULLET::::- They do not rot or corrode.
  
  BULLET::::- Plastics are easy to install; many have a snap-fit kind of jointing procedures.
  
  BULLET::::- Plastics offer limitless possibilities in design achieved by extrusion, bending, moulding etc.
  
  BULLET::::- They can be given any range of colours by adding pigments.
  
  BULLET::::- The plastics are low conductors of heat and thus are used as insulation materials in green building concepts.
  
  BULLET::::- The plastics products can achieve tight seals.
  
  BULLET::::- They can be sawn and nailed employing standard carpentry tools and skills.
  
  BULLET::::- They can be easily removed and recycled.
   The critical stress intensity factor (K) can be defined as the threshold value of stress intensity base on the material properties. Therefore, the crack will not propagate so long as K is less than K. Since K is a material property it can be determined through experimental testing. ASTM D20 provides a standard testing method for determining critical stress of plastics. Although K is material dependent it can also be a function of thickness. Where plane stress is dominant in low thickness samples increasing the critical stress intensity. As your thickness increases the critical stress intensity will decrease and eventually plateau. This behavior is caused by the transitioning from the plane stress to plain strain conditions as the thickness increases. Fracture morphology is also dependent on conditions at the located at the crack tip.
   Plastic joining
  
  Plastic joining is the method of joining semi-finished products of plastic materials together or to other materials as a fabrication process or damage repart. Joining methods can be classified into three categories:
  
  BULLET::::1. Mechanical fastening,
  
  BULLET::::2. Adhesive bonding,
  
  BULLET::::3. Welding.
  
  Section::::Mechanical fastening.
  
  Mechanical fastening methods can offer an advantage of disassembly, but have drawbacks arising from stress concentrations, galvanic corrosion, mismatch of thermal expansion coefficients, etc. which rivets, screws and ropes can introduce (see fasteners).
  
  Section::::Adhesive bonding.
   Polycarbonate would be normally considered an engineering plastic (other examples include PEEK, ABS). Such plastics are valued for their superior strengths and other special material properties. They are usually not used for disposable applications, unlike commodity plastics.
  
  Specialty plastics are materials with unique characteristics, such as ultra-high strength, electrical conductivity, electro-fluorescence, high thermal stability, etc.
   BULLET::::- Plastic handles are more easily cared for than wooden handles and do not absorb microorganisms. However, plastics may also be less resistant to ultraviolet damage and may become brittle over time, resulting in cracking. Some plastics are also slippery in the hand. The material is lighter than most other materials, which may result in a knife that is off-balance or too light for some tastes.
   The dividing lines between the various types of plastics is not based on material but rather on their properties and applications. For example, polyethylene (PE) is a cheap, low friction polymer commonly used to make disposable bags for shopping and trash, and is considered a commodity plastic, whereas medium-density polyethylene (MDPE) is used for underground gas and water pipes, and another variety called ultra-high-molecular-weight polyethylene (UHMWPE) is an engineering plastic which is used extensively as the glide rails for industrial equipment and the low-friction socket in implanted hip joints.
  
  Section::::In industry.:Metal alloys.
   Section::::Self-healing polymers and elastomers.
  
  In the last century, polymers became a base material in everyday life for products like plastics, rubbers, films, fibres or paints. This huge demand has forced to extend their reliability and maximum lifetime, and a new design class of polymeric materials that are able to restore their functionality after damage or fatigue was envisaged. These polymer materials can be divided into two different groups based on the approach to the self-healing mechanism: intrinsic or extrinsic.
  
raw_presuppositions: All plastic is durable.
raw_corrections: Only certain types of plastic are durable.