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Material Testing Overview

Material Testing Overview

When selecting materials for an engineering application, critical mechanical properties of the material must be reviewed. There are many important mechanical, physical, and chemical properties when considering applications and selecting the optimal fastening system. Because of the highly technical nature of this subject, this blog content will take an overview approach and should not be considered an engineering resource. We simply wish to provide valuable content that hopefully helps you understand these concepts a little better and leads to further research and discovery. We offer the following thanks to information provided by our industry resources such as IFI (Industrial Fastener Institute), ASTM International (American Society for Testing and Materials), and previously published articles in NFDA magazines (National Fastener Distributors Association). Within these resources you will find supporting diagrams, charts, and analysis.

Ductility

Ductility is defined as the ability of a material to deform plastically in tension or shear before fracture. Its measurement is of interest to those conducting metal forming processes (ex. automobile body panels); to designers of machines and structures; and to those responsible for assessing the quality of a material as it is being produced. Two measures of ductility are Elongation and Reduction of Area. Elongation is defined as the increase in the gauge length of a test piece divided by the original gauge length. Reduction of Area is defined as the decrease in the cross-sectional area divided by the original cross-sectional area. The conventional means by which these ductility measures are obtained is by pulling a specimen in tension until fracture. ASTM E8 governs the determination of Ductility measures for Metals.

Elongation at Break

Elongation at Break or Elongation is one measure of ductility. The conventional means by which we obtain Elongation is by pulling a specimen in tension until fracture. Elongation is defined as the increase in the gauge length, of a test piece subjected to tensile forces divided by the original gauge length. Elongation is expressed as a percentage of the original gauge length and is given by: ΔL is the change in length of the original gauge length measured after the specimen fractures and the specimen is fitted together A punch is often used to apply the gauge marks to each specimen. The change in gauge length, is determined by carefully fitting the ends of the fractured specimen together and measuring the distance between the gauge marks. In reporting Elongation, both the original gauge length and percentage increase are to be reported. If any portion of the fracture occurs on or outside a gauge mark, Elongation may not be representative of the material. ASTM E8 governs the determination of Elongation for Metals.

Engineering Stress / True Stress

Engineering Stress (ES) is equivalent to the applied uniaxial tensile or compressive force at time divided by the original cross-sectional area of the specimen. True Stress (TS) is equivalent to the applied uniaxial tensile or compressive force at time. Ductile materials undergo plastic deformation prior to rupture or break. When a ductile material is loaded beyond its Ultimate Tensile Strength, necking occurs and the cross-sectional area and applied force both decreases. Thus, the Engineering Stress which is based on the original area decreases beyond the Ultimate Tensile Strength, whereas the True Stress increases, due to the necking or reduction of area that occurs to the specimen cross section.

Gauge Length

The Gauge Length is the original length of the portion of the specimen over which strain, elongation, or the change of length are determined. Testing standards often specify a gauge length selection based on the material being tested. For further information on the effect of different gauge lengths on test results, click contact your testing lab or refer to ASTM resources.

Offset Yield

The offset yield strength is reported as a stress (psi, MPa, etc.) and is defined as the point where a line drawn parallel to the modulus line intersects the stress-strain curve.

Proportional Limit / Elastic Limit

Once the applied stress exceeds the proportional limit of an elastoplastic material, the linear stress strain response transitions to a non-linear relationship. If the stress applied to a material never exceeds its proportional limit, then the specimen will return to its original length when all forces are removed. If the applied stress exceeds the proportional limit, then the material will have a permanent set and will not return to its original length.

The stress at which the onset of permanent deformation occurs is referred to as the proportional limit and is indicated as point A in the figure. The portion of the stress-strain diagram prior to the proportional limit is also defined as the elastic region and beyond the proportional limit the plastic region.

Shear Strength

The strength of a material, in general, is the value by which yielding, fracture or excessive deformation occurs in a load-carrying member. Shear strength is a material property that describes a material’s resistance against a shear load before the component fails in shear. The shear action or sliding failure described by shear strength occurs parallel to the direction of the force acting on a plane.  In construction, automotive, aerospace, and other engineering industries, knowing the shear strength of materials is vital for the design of mechanical and structural devices as well as the selection of materials to be used for an application. It is also a major consideration in sizing components or parts.

Stress Strain Curve

A stress strain curve relates the forces on a member to the deflections imposed by the forces. An XY graph of force versus deflection is one way to depict this relationship. When the size of the member is changed, however, a new graph will need to be drawn. Expressing the relationship in the form of a stress strain curve eliminates the need to redraw the graph each time member dimensions are changed. Stress is obtained by dividing the applied force by the original cross-sectional area of the member. Strain is obtained by dividing the change in length of the member by its original length.

Stress strain curves are frequently generated by universal testing machines equipped with an extensometer. The testing machine is used to load the member to failure and record the stress versus strain relationship.

Tensile Strength

The maximum stress applied in tension to a material before rupture is defined as the Ultimate Tensile Strength (UTS). Ultimate Tensile Strength is commonly reported in units of pounds per square inch (psi) or in Newtons per square mm (MPa, Mega Pascals).

Yield Point

The Yield Point is the stress at which there is an appreciable increase in strain with no increase in stress, with the limitation that if straining continues the stress will again increase. Few materials possess a Yield Point, the most common examples are low carbon steel. If Yield Point exists for a given material, it will be indicated as the point on a stress strain curve where a zero or negative slope occurs prior to the Ultimate Tensile Strength. Note that the heat treated, and higher carbon steels do not exhibit a Yield Point like low carbon fasteners do. ASTM E8 governs the determination of Yield Point for Metals.

Yield Strength

The Yield Strength is defined as the stress that will induce a specified permanent set, usually 0.2%, which is equivalent to a strain of 0.002. The Yield Strength is useful for materials with no Yield Point.

Extra Resources

Mechanical Properties of Metals – PDF

Mechanical Testing – Understanding the Basics

 

Ground – Fitted – Body Bound – Stripper Bolts

Applications
Many fastener applications require very tight tolerances where both thread-fit and the bolt grip dimensions are critical. Some common examples are open and boxed gearing, bearing installs, pulley systems, and cam alignment; where enhanced precision helps to hold together parts of machinery or similar applications to minimize vibration, slippage, or movement while conducting specific functions.
Common Names and Characteristics
These fasteners are referred to by several names such as Ground Body Bolts, Fitted Bolts, Body Bound Bolts, Stripper Bolts, Shoulder Bolts and Step-Down Bolts. They come in a variety of head styles including hex head, socket, slotted, flange, and torx. These fasteners can be found in both inch and metric sizes, in diameters up to 6”, and can be made from about any material. Although usually the application calls out for higher hardness’s, and increased tensiles and yields.
Methods of Manufacturing
Because of their ground body, tight tolerance bodies and ground finishes, these highly effective products are typically machined from bar on CNC or multi spindle machines, often with secondary grinding. It is also common to start with a forged or cold headed bolt and grind the body after. Because subsequent heat treating can cause expansion or contraction, it is prudent to manufacture the body dimension slightly oversize and allow for grinding to be the last step.
Other Benefits
These fasteners are the perfect blend of dowel pin and headed screws. In addition to tightening up fit and reducing vibration, less wear and tear of fasteners and adjoining parts is another benefit of Body and Fitted bolts. Other benefits include reduction in fatigue and torque failure, increase in clamp load, improving the ability to assemble diverse components, and ultimately ensuring peak performance.
If you have any questions regarding Body or Fitted bolts or would like to speak to a representative, then contact KJ Fasteners at 440-951-5095 or info@kjfasteners.com. K-J’s focus has always been to provide our valued customers with superior quality products and service.

Galling and Non Ferrous Fasteners

Galling and Non Ferrous Fasteners

Galling is one of the most common problems we see when tightening fasteners. Galling, also known as cold welding occurs when two metal surfaces slide against each other, resulting in metal from one surface becoming part of the other. This is problematic because it can lead to broken fasteners, weakened joints, and damaged threads. In some cases of galling, the fastener can be removed.

However, in more extreme cases the nut and bolt can become welded together and the installer will not be able to remove the fastener.

The science behind galling can be frustrating, and we have learned the hard way over the years. If you have experienced non-ferrous parts that have refused to play well together in your assembly or install sandbox, the following info may be of help.

Slow Down Installation Speed.

Friction and heat are the main contributors to galling. Slowing down during installation or removal will reduce the amount of heat and friction produced.

Use a Lubricant.

Reducing friction by using the proper lubricant is the most effective way to reduce galling. Even if the joint is permanent, lubrication is still required. We do not recommend spray film lubricant, instead opt for an anti-seize product.

Use a torque wrench and correct force.

Reducing contact stress will lessen the risk of galling. In order to prevent over-tightening, using a torque wrench to control the force is necessary. Ask KJ for torquing recommendations for your material application.

Ensure hardness difference of at least 50 Brinell between nut & bolt Surfaces treated so that they are harder, for example, hard chrome-plated, nitrided, carburised or cold worked surfaces are usually less prone to galling.

Avoid Damaged or Dirty Threads.

Dirt and other abrasive material between the mating surfaces will create friction and heat.

Use Extra Care With Lock Nuts.

Lock nuts are designed to add extra resistance and heat when installed, so it is recommended to avoid using them when you can. If necessary, make sure you are installing slowly to prevent extra heat production.

Dig deeper into mating materials and their compatibility… Understanding mating materials and their compatibility can help reduce galling. Mixing nut and bolt grades can be effective in preventing galling.

Proper fit

Having the proper fit is so important when it comes to preventing galling and minimizing thread burrs. The fit should be tight enough to prevent vibration and wear, but should still have enough clearance to prevent galling. You should keep the contact load on the components that slide to a minimum, and the contact area maximized.

Copper Nickel & Exotic Materials

Fasteners are used in a variety of different applications. But just because a fastener has standard dimensions doesn’t mean it is necessarily off the shelf. Many applications demand exotic materials where durability, tensile & torque strengths, and resistance to corrosion are critical. Exotic material fasteners are a crucial part in everything from food rendering equipment, injection molding machines, medical applications, marine installations, to oil and gas industries.

Our wide variety of special materials are preferred for assuring compatibility, preventing galling and seizing, guaranteeing peak performance in caustic and harsh conditions, and preserving the integrity and lifespan of your capital equipment. Just a few of the options we supply standard fasteners and per print specials in include: 17-4 PH, Alloy 2205, Monel Inconel, Hastelloy, Titanium and Silicone bronze. But copper nickel alloys have their own personality.

The combination of copper and nickel is ideal for applications that are exposed to water, particularly seawater. Copper nickel is resistant to corrosion due to its composition, making it the perfect choice for marine applications.

Copper nickel fasteners are also an excellent option for food rendering. The process of food rendering combines the abusive elements of pressure, friction, high repetition, and often hostile temperatures. The fasteners used for these machines must be able to withstand high volume cycles, abrasive contact, and various environments.

Copper nickel fasteners can also be used for energy generation applications. Because the oil and gas industries often handle temperatures that can get extremely hot or be installed in very cold parts of the world, it is important for fasteners to be able to withstand the changes without becoming worn over time. Copper nickel fasteners are an excellent option here, as well, due to their resilient composition.

Depending on what the job is, we can recommend the perfect fastener for your project regardless of what exotic material you require. If you are building something that needs a strong nonferrous fastener that can withstand your unique applications, we can help you determine the proper solution

Wheel Studs and Place Bolts

Wheel housings, drive trains, and rear end assemblies are crucial components on automobiles, but not all wheel studs and place bolts are created equally. When you initially purchase a vehicle, the standard bolts are manufactured specific to that vehicle. This means that if and when modifications are made to an automobile, the wheel studs may not be suited for vehicle updates. If you are a classic car enthusiast, do-it-yourselfer, or performance lover – this is even more important.

As the car changes, the amount of stress on the wheel studs can also change. So, upgrading wheel studs to a stronger and more durable pair can help in the long run. If modifications are made to a vehicle that drastically change the weight, stronger wheel studs are necessary. The right wheel stud and place bolt parts increase strength while driving and reduce seizing, making the vehicle drive smoothly and safely. Many consumers may not realize that wheel studs should be updated proactively and not wait until something goes wrong. Strengthening existing wheel studs and place bolts with a more durable pair can help prevent issues with the wheel. This is especially true when it comes to heavy tires or higher speeds that increase tension and torque, reliable performance plays even a bigger role.

Wheel studs also play a significant role in the quality of the drive during difficult road conditions. A bump in the road or uneven pavement can derail wheel studs, so proper installment is crucial to prevent damage to the vehicle. The wheel stud must be set to the proper torque for the type of automobile, otherwise it could cause issues that can cause the wheel to fall off completely.
 
Ultimately, wheel studs are an extremely important piece on a vehicle. They make sure the brake rotors are in the proper place and fasten the wheels to the automobile. When wheel studs break down, they can cause severe damage to the vehicle and even lead to dangerous car accidents.

And let’s not forget aesthetics. Yeah, you like to pop the hood and show it off, but don’t forget the under carriage. From top to bottom, front to back – your car is your passion! Whether cruisin, racing, or just looking for the smoothest everyday ride – it all starts with your Wheel housings, drive trains, and rear end assemblies.

Contact us today if you have any questions or would like to speak to a KJ Fasteners Representative.