What are Tensile Grips?
Performing tensile tests requires a firm grip on your samples. These grips come in a variety of styles, including manual, pneumatic, wedge, hydraulic, and specialty options for ropes, threads, webbing, and more. Selecting the most suitable grip is crucial for obtaining accurate measurements of tensile properties like tensile strength, yield, peak load, elongation, and tensile modulus.
ADMET tensile grips and fixtures are compatible with various testing machines, including those from Instron, MTS, Zwick, Shimadzu, and more.
This guide will explore some common issues experienced when using tensile grips, the different grip types, and their ideal tensile testing applications to help you make the best decision for your testing needs.
Common Issues with Tensile Grips
Specimen Slippage
Specimen slippage during tensile tests can occur for two main reasons: the design of the grips themselves and the material properties of your samples. More specifically, issues can occur due to:
Clamping Force: Insufficient clamping force can cause slippage. Consider switching to pneumatic grips, which offer a stronger grip than manual options. Most ADMET grips come in both manual and pneumatic versions.
Grip Jaw Coverage: Ensure the grip jaws cover at least three-quarters of your specimen’s surface area, as recommended by most tensile testing standards. This maximizes grip and minimizes slippage. ADMET offers a variety of grip jaw sizes to accommodate different specimen dimensions.
Jaw Surface Match: The type of jaw surface also plays a role. For optimal grip, consult with our Sales Engineers to discuss your specific sample material and testing application. They can help you select the most appropriate jaw surface type.
Specimen Break at Grip Face
Even if your specimen doesn’t slip, breaks at the grip faces can lead to inaccurate results. Here’s how to identify and address this issue:
Observe the Break: During testing, monitor how the specimen breaks. If it breaks right at the grip line, the recorded strength won’t be accurate.
Adjust Gripping Force: Reduce the manual clamping force if you’re using manual vise grips. For pneumatic grips, adjust the testing pressure to find the sweet spot that prevents slippage without causing breakage.
Check for Serrated Jaw Damage: Serrated jaws might damage the specimen, leading to breaks at the grip. If so, explore other jaw surfaces or custom-made serrated jaws for a better fit.
Consider an Alternative Grip Design: If the above solutions don’t work, consider grips designed to avoid jaw contact, like webbing or eccentric roller grips that are designed with a specimen tightening mechanism around a roller.
Our experienced engineers can help you find the right fixture quickly.
Don’t let the wrong grip keep you from testing accurately and reliably!
Specimens with Unusual Shapes and Sizes
What if your samples are oddly shaped, too big, or too small for standard grips? This can be a challenge, especially when testing finished products rather than just materials.
The good news: solutions exist! Here are two options to explore:
Non-Standard Grips: Many manufacturers offer grips beyond those listed in catalogs. Speak with a sales representative to discuss your specific needs.
Custom Grips: For truly unique samples, consider having custom grips engineered. This ensures a perfect fit for accurate testing.
For example, testing needles requires a special solution. Our engineers developed a Luer lock to adapter fixture to securely hold these slender samples.
Specialized fixture for tensile testing of Luer Locks
Tips for Finding the Right Grips for your Application
By considering these four key factors when choosing your grips, you can minimize common issues discussed above and ensure accurate tensile testing.
Understanding Different Grip Designs and Capacities
Depending on your testing needs, you may require a variety of grips to be on hand for testing a range of samples, or you may only need a single specific grip. In either case, it is important to understand the options available so you can choose the right style and capacity for your specimens.
Hydraulic Grips
Need to grip like a titan? Look no further than hydraulic grips. These champs can deliver up to 3,000 kN (675,000lbf) of gripping force, making them ideal for handling tough, rigid materials in tensile tests. Often paired with hydraulic machines and pumps, hydraulic grips ensure your samples stay firmly in place during even the most demanding tests.
Hydraulic tensile grips
Wedge Grips
When high clamping force is paramount, wedge grips are a frequently recommended solution. These workhorses utilize a self-intensifying mechanism that tightens as the test progresses, ideal for holding onto strong materials, and have capacities as high as 500 kN (112,500lbf).
ADMET offers a range of wedge grips to suit your needs:
Lower Capacity: Spring-operated with a handle for opening and closing the jaws.
Higher Capacity: May feature handle extrusions, retaining knobs, or doors for added security.
Moving Body: Minimize compressive loads on your samples.
High-Temperature: Designed for use in environmental chambers.
Wedge grips are popular fixtures and are commonly used for testing metals as they are the recommended choice for metal testing according to ASTM E8.
Wedge grips
Manual Vise Grips
Manual vise grips are a popular choice for tensile testing thanks to their:
Ease of Use: The simple tightening mechanism makes them perfect for quick setups.
Versatility: Their large opening width accommodates a wide range of sample sizes.
However, it’s important to remember that manual vise grips offer a lower clamping force (up to 50 kN) compared to pneumatic or self-tightening grips. If you’re testing materials requiring high force or encountering slippage issues, consider exploring alternative grip options.
Manual vise tensile testing grips
Pneumatic Grips
When testing stretchy materials like rubber, pneumatic vise grips and eccentric roller grips are your best bets. These grips have capacities up to 30 kN are specifically suited for ASTM D412 and ASTM D624 standards. Here’s why they excel:
Constant Clamping Force: They maintain a secure hold throughout the test, even as the material elongates significantly. This is crucial for accurate results.
Precise Alignment: Pneumatic grips use air pressure to move dual jaws that center the sample perfectly.
Adjustable Pressure: With pneumatic grips, you can fine-tune the air pressure to ensure a firm grip without damaging your sample.
By controlling clamping force and alignment, these grips ensure reliable testing of high-elongation materials.
Pneumatic tensile grips
Pinching Grips
When it comes to testing very small or oddly shaped samples, vise grips just won’t do. That’s where pinching grips come in! These mini-champs are perfect for:
Low-Capacity Testing: Ideal for small samples that traditional grips can’t handle.
Electronics and Biomaterials: Commonly used for testing delicate components and biomaterials.
Pinching grips come in capacities up to 2 kN and offer two deployment options:
Chain-Mounted: Provides flexibility in positioning your samples.
Pneumatically Controlled: Makes clamping easier and automatically aligns your sample for precise testing.
With their versatility and delicate touch, pinching grips are the go-to solution for small sample testing.
Pinching grip with lever arm
Webbing, Rope, and Thread Grips
For unique samples like rope, thread, webbing, and more, ADMET offers specialized grips:
Thread Grips: Ideal for delicate materials like cords, fine wires, and narrow ribbons with capacities from 200 N to 5 kN.
Rope Grips: Designed for thicker samples such as yarns, cords, and wire strands, with capacity up to 50 kN.
Self-Tightening Scissor Grips: Perfect for gripping thick elastomers or oddly shaped materials like plastic belts.
Webbing Grips: These grips feature a center slot and a rotating drum that tightens automatically as you pull the material. This prevents breakage at the grip point and is ideal for coated textiles that might slip from standard grips. They come in capacities from 5 kN to 200 kN.
Rope grips for tensile testing
Don't see what you need?
We can modify existing grips or engineer custom fixture solutions for you!
Picking the Best Grip Face
A convenient feature of many tensile grips lies in their interchangeable grip faces. This allows you to test a variety of samples using the same grip body, simply by swapping the face attachments.
While some ASTM standards recommend specific grip faces for certain materials, most leave the choice up to you. Here’s how to navigate this decision:
Know Your Material: Consider the characteristics of your sample material. Is it soft, hard, smooth, or textured?
Think About Dimensions: The size and shape of your specimen will also influence the best face type.
Need a Hand? Our sales engineers are happy to assist you in selecting the optimal grip face based on your specific sample and testing requirements.
The type of grip faces available include:
Blank: Often used with film and foils
Rubber: Recommended for biomaterials and paper specimens
Serrated/Pyramid: These are recommended for ASTM D412 rubber testing, ASTM D638 plastic testing, and ASTM E8 metal testing
Diamond: These jaws have a sandpaper-like surface due to their synthetic diamond coating
Wave: Used with flexible materials held in place between the 5mm waves
V-Jaws: Used for round samples
Line Contact: Made with a smooth, nickel-plated surface to provide a better grip for plastic films and sheet materials
ADMET also offers a quick change system for all the jaw surface types mentioned above. Users interested in a faster way to change from one jaw type to another can use the jaws with quick change carriers.
Specimen Material and Geometry and What They Mean
Before testing, each material requires specific preparation to achieve a standard geometry. This is typically outlined in testing standards and often involves creating “dog-bone” shaped flats or cylindrical tubes/bars. Sample prep can involve machining or die cutting. ADMET offers dies conforming to various standards to simplify this process. This standard ASTM D638 exemplifies the importance of precise sample prep. It specifies five different specimen dimensions that must be chosen based on the material’s characteristics.
ASTM D638 Cutting Die Specifications
The right grip ensures accurate results. Here’s a general guideline:
Hard Specimens (Metals, Composites, Hard Plastics): High clamping force grips like hydraulic or wedge grips are recommended to prevent slippage or premature breakage.
Soft Specimens (Rubbers, Elastomers): These offer more flexibility in grip choice, including manual vise grips, pneumatic grips, wedge grips, or eccentric roller designs. Manual or pneumatic grips with rubber-coated or serrated jaws are common and user-friendly options.
Some testing standards go beyond prescribing the test method and specify a dedicated fixture design. Examples include ASTM C297, ASTM D1414, and ASTM D3039.
Mounting Grips to your Testing System
A key advantage of tensile grips lies in their versatility. They’re not machine-specific, allowing you to use them with various UTM models. Often, ADMET grips are mounted on Instron, MTS, and Tinius Olsen machines using adapters.
These adapters can be:
Supplied with your grips: A convenient option to ensure compatibility.
Machined in-house: Our sales engineers can provide the necessary drawings for a perfect fit, if you prefer this route.
This flexibility ensures you can leverage your existing UTM setup while using the most suitable grips for your testing needs.
Male end adapter for mounting grips and fixtures
Conclusion
Selecting the optimal tensile grips requires considering several factors:
Maximum Force Capacity: Knowing the force required to test your samples is crucial.
Sample Characteristics: Material, dimensions, shape, and elongation properties all play a role.
Don’t Do It Alone!
Our experienced sales engineers are here to guide you. They can discuss grip options based on your testing goals. If needed, our team can also arrange sample testing at our facility to ensure a perfect fit.
Get in touch with us and we’ll be happy to help. Let us assist you in achieving accurate and reliable tensile testing results today!
Custom test fixture for ASTM C297
