Today's KNOWLEDGE Share:Fatigue Striations (PC HEADLIGHT LENS)

Today's KNOWLEDGE Share:

Fatigue Striations

I completed a failure analysis on a polycarbonate automobile headlight lens. Cracking was identified within the lens during inspection conducted after completion of performance testing. This testing included exposure of the lens to vibratory stress.

The external macro examination indicated that the cracking lacked characteristics associated with micro ductility, and displayed features associated with brittle fracture. The crack was completed and the fracture surface was examined with the aid of a scanning electron microscope (SEM). The SEM examination revealed a single crack origin, positioned immediately adjacent to a threaded boss design feature. The origin corresponds to a design corner, which acted as a point of stress concentration, multiplying the applied vibratory load.

A characteristic feature present on the fracture surface was the presence of radiating band features. The bands presented features indicative of arrest markings associated with dynamic crack propagation. At high magnification, the bands displayed characteristics of fatigue striations, corresponding to crack propagation through alternating cycles of cracking and arrest. This was consistent with the stated stress loading which precipitated the failure.

The fatigue striations on this project were textbooks for plastic materials. Fatigue cracking generally initiates at inhomogeneities within the microstructure, particularly at points of stress concentration, as was the case in this instance. The imposed stresses typically produce a complex process of both interactions of the defects, resulting in the initiation of microscopic cracks. The presence of the crack under load creates a further condition of stress concentration around the crack tip. When the stress maximum within this region exceeds the yield strain, a zone of damage is formed immediately in front of the crack tip, resulting in craze formation. Continued cyclic stresses lead to disentanglement of polymer molecules chains through cumulative rupture of the craze fibrils and coalescence of micro voids. Ruptured crazes are evident in the images below representing the headlight lens project. This process represents crack propagation and corresponds to the formation of bands of fatigue striations, as indicated below.

Once again, fractography clearly tells the story of how the component failed.

Source:The Madison group

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Today's KNOWLEDGE Share: Pressure Relief Valve:

Today's KNOWLEDGE Share:

Pressure Relief Valve:
A pressure Relief Valve is a safety device designed to protect a pressurized vessel or system during an overpressure event. An overpressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure (MAWP). The primary purpose of a pressure Relief Valve is protection of life and property by venting fluid from an overpressurized vessel. Many electronic, pneumatic and hydraulic systems exist today to control fluid system variables, such as pressure, temperature and flow. Each of these systems requires a power source of some type, such as electricity or compressed air in order to operate.

A pressure Relief Valve must be capable of operating at all times, especially during a period of power failure when system controls are nonfunctional. The sole source of power for the pressure Relief Valve, therefore, is the process fluid. Pressure Relief Valve Once a condition occurs that causes the pressure in a system or vessel to increase to a dangerous level, the pressure Relief Valve may be the only device remaining to prevent a catastrophic failure.The importance of adding Pressure relief device in gas storage systems in the recent years get more priority than the others in the safety aspects of whole process.

Source:Technical Engineering portal

Today's KNOWLEDGE Share: The main properties of composite materials

 Today's KNOWLEDGE Share:

The main properties of composite materials

As you may know, the characteristics/properties of composite materials resulting from the combination of reinforcement and matrix depend on: the proportions of reinforcements and matrix, the form of the reinforcement, and the fabrication process. 

But what are the most remarkable properties of these materials? 

- Composite materials generally possess very high specific mechanical properties.

- Composite materials do not yield: their elastic limits correspond to the rupture limit.

- Composite materials have high strength under fatigue loads.

- Composite materials age under the action of moisture and heat.

- Composite materials do not corrode, except in the case of contact aluminum with carbon fibers in which galvanic phenomenon creates rapid corrosion.

- Composite materials are not sensitive to the common chemicals used in engines: grease, oils, hydraulic liquids, paints and solvents, petroleum. However, cleaners for paint attack the epoxy resins.

- Composite materials have medium- to low-level impact resistance (inferior to that of metallic materials).

- Composite materials have excellent fire resistance as compared with the light alloys with identical thicknesses. However, the smoke emitted from the combustion of certain matrices can be toxic.

Bibliographical Reference:

Composite Materials Design and Applications - Page 16

Source:#managingcomposites/#thenativelab

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Hydrogen engines to be mass produced by Hyundai by 2025

 

After the completion of its H2 internal combustion engines (ICE) design and rolling out the prototype, Hyundai Doosan Infracore (HDI) is revving up the development of its hydrogen engines, with the aim to mass produce these engines by 2025.

HDI’s H2 ICE is an 11-litre class engine

The hydrogen-powered internal combustion engine can produce a power output of 300 kW (402 HP) and a torque of 1700 NM at 2000 RPM. Fulfilling Tier 5/Stage 5/Euro7 regulation, the engine satisfies the emission requirements to be 90% decreased to the current level to meet Zero CO2 (below 1g/kwh) and Zero Impact Emission.

Low-purity hydrogen is used to power the hydrogen engines. This makes the engines not only strong, energy-dense and economical, but the most suitable engine system for mid-to-large-size vehicles and vehicles traveling long distances. Just one charge of 10 minutes allows for a distance up to 500 km (310.6 miles), meanwhile the H2 internal combustion engines are 25-30% more economical than battery packs or fuel cells when vehicle price and maintenance costs are factored in.

The new hydrogen engines will be installed in commercial vehicles.

To both accelerate commercialization and lower costs, HDI plans to leverage its current engine technology and facilities. The new hydrogen engines that will be produced will be installed on commercial vehicles, including large buses, trucks and construction equipment. HDI will unveil its prototype hydrogen-powered ICE power unit this year (2023), with plans for full-scale testing slated for 2024, and full-scale mass production planned for the following year in 2025.

Hydrogen internal combustion engines will be used in mid-to-large-sized commercial vehicles such as trucks, buses and construction equipment and mid-to-large-sized power generators,” said Kim Joong-soo, HDI’s Head of the Engine Department. “We will put in the utmost effort to realize carbon neutrality in response to the eco-friendly market by developing green hydrogen-related technologies in line with increasingly strict carbon emission regulations.

Source:Hydrogenfuelnews

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Today's KNOWLEDGE Share: ELASTOMERS (THERMOPLASTIC vs THERMOSET)

 Today's KNOWLEDGE Share:

ELASTOMERS (THERMOPLASTIC vs THERMOSET)

There are times when a thermoplastic elastomer (TPE) cannot match the performance of a thermoset rubber compound, and rubber must be used in a demanding application.

 

I completed a material review in which my client was trying to replace a nitrile rubber (NBR) seal with a thermoplastic elastomer (TPE). They were looking for a material that would offer cost and manufacturing advantages over the thermoset NBR material. The screening tests paralleled the application environment, and included chemical exposure at elevated temperature under conditions of dynamic actuation of the seal.

 

Both thermoset rubber and TPEs are elastomers. As elastomers, both classes of materials exhibit some characteristic properties:

•  Substantial amorphous content

•  Subambient glass transition, which allows substantial segmental molecular motion

•  Relatively low hardness values, meaning they are relatively soft

•  Comparatively low modulus values, meaning that they are flexible

•  High degree of stretch and elongation at break

•     Compared with non-elastomeric materials, are elastic, meaning they exhibit good recovery from stress

 

While both thermoset rubber and TPEs are elastomers, their structure, and as an extension, their performance properties can be dramatically different. Thermoset rubber compounds have a cross-linked molecular structure, with covalent bonds joining the individual polymer chains, essentially forming a molecular network. Conversely, TPEs are thermoplastic in nature, and do not have covalent bonds that join individual polymer chains. Instead, the polymer chains are held together by weak intermolecular forces, such as hydrogen bonding and van der Waals forces.

 

Based upon the difference in molecular structure, the two material classes can exhibit desparate physical properties. It is somewhat difficult to characterize all TPE materials and contrast them against all thermoset rubber materials. There are numerous subcategories within each type of material, and each has its unique characteristics, as well as strengths and weaknesses. However, given the fundamental molecular structure, some differences can be highlighted. The cross-linked structure of thermoset rubber affords them superior performance properties in three general areas compared with TPEs:

 • Superior chemical resistance

 • Higher thermal stability

 • Greater stress recovery, resulting in better compression set - creep and stress relaxation properties

 

In this case, it was concluded through testing and review that a TPE material was not suitable based upon the test and application conditions - specifically the chemical resistance and the stress recovery at elevated temperature. It was recommended that the incumbent NBR rubber compound be maintained in the sealing application.

 

Source:Jeffrey A.Jansen/The Madison Group

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Today's KNOWLEDGE Share:AUTOCLAVE CURING PROCESS:

 Today's KNOWLEDGE Share:

AUTOCLAVE CURING PROCESS:

Autoclave curing is a convective heat transfer process used for the curing of FRPs. In autoclave a closed vessel is maintained at a certain temperature and pressure for a definite time depending upon the type of resin under curing. The composite cured by autoclave technique can be prepared by the hand lay-up process or vacuum bagging process using prepregs. After preparation of the laminate stack, the component is placed in the autoclave chamber at a particular temperature and pressure for a particular time. Using certain optimized parameters, the components get cured/solidified. Mostly all types of composite laminates can be cured using cylindrical autoclaves. Large volumetric components of aircraft and wind energy generation wings can be easily cured using autoclave chambers. 

The autoclave curing chamber contains a pressure chamber in which components get cured under the required pressure and heat. The cylindrical shape of the vessel provides for both a flat and a cylindrical body for curing. For proper curing of components, the pressure should be maintained at a sustainable limit. The pressure vessel is made leakproof and the door is properly sealed after closing. The required pressure in the chamber is achieved using an air compressor mounted on the outer body of the chamber. For vacuum bagging, vacuum is maintained by two hose pipes connected to a vacuum compressor. Temperature sensors and thermocouples are placed inside the chamber for detecting temperature. Vessel pressure can be maintained by a safety valve that is mounted on the chamber to release excess pressure above the required level. For heating up the chamber, gas firing and electric heaters are used to complete polymerization/curing of polymer composites. Direct gas firing is mostly preferred for large volumetric components (aircraft and wind turbine blade), although direct heating systems are preferred for small components (automotive components). Large components require high thermal energy to spread over the surface of the components that can be only achieved by the gas firing method, whereas for small components the polymerization of components can be easily achieved by direct heating. The design of the gas firing tube should be done according to prescribed measurements to avoid leakage during operating hours. 

Bibliographical Reference:

Reinforced Polymer Composites: Processing, Characterization and Post Life Cycle Assessment - Page 82

Source:#managingcomposites/#thenativelab