What Is Tensile Testing? Methods, Results, and Hardware Applications

When purchasing chains, springs, or other metal hardware, many buyers ask suppliers for tensile test results. Tensile testing can measure material properties and show how a finished product behaves under a pulling force. However, material tests and finished-product tests use different methods, and their results mean different things. This article explains what tensile testing is, how it is performed, and how you should understand its use for chains, springs, hooks, connecting links, and other hardware products.

What Is Tensile Testing?

Tensile testing is a way to check how a material or product behaves when it is pulled. The test can tell you how strong a material is and how far it can stretch. It can also show whether a finished product changes shape under a set force and how much force causes it to fail.

When steel or wire is tested, the tensile test mainly measures material properties such as yield strength, tensile strength, and elongation. When a finished product such as welded chain, weldless chain, extension spring, S hook, quick link, or wire form (a part bent from wire) is tested, the result reflects the combined effects of the material, dimensions, welding, forming, and heat treatment.

These two types of test results cannot replace each other. Raw material that meets its tensile strength requirement does not guarantee that the finished chain or hardware will withstand the required load. In the same way, a finished product that passes a pull test does not tell you everything about the material itself. In production and quality inspection, we therefore check the material and the finished product separately. We do not use one material report as a substitute for all other inspections.

How Is Tensile Testing Performed?

The first step is to confirm what will be tested. Metal materials are normally tested according to standards such as ASTM E8/E8M or ISO 6892-1 (common standards for testing metal samples). For chains, springs, connecting parts, and other hardware products, the test method should match the product’s shape and intended use.

After you confirm what is being tested, the sample is placed in a universal testing machine (UTM), which is a machine that pulls the sample while measuring the force. The fixtures (the parts that hold the sample) must fit the product. Chains usually require pins or specially made fixtures. Hooks and connecting links should be pulled in the direction in which they will normally be used. For an extension spring, the fixtures need to hold both end hooks. If the sample is installed incorrectly, the result may not show how the product really performs.

Once the sample is ready, the machine increases the pulling force at the required speed. Some tests stop when the product reaches a set load. Others continue until the sample breaks or can no longer do its job. When you review the result, do not look only at the maximum force. You should also check for permanent deformation (a change in shape that remains after the force is removed) and see where the product changed shape or broke. For example, a welded chain that breaks at the weld may have a different problem from one that breaks elsewhere on the link.

What Results Can Tensile Testing Provide?

Before you review a test report, first confirm whether it describes material properties or the test results of a complete product.

If you are testing a material, you need to consider yield strength, tensile strength, elongation, and elastic modulus together. You should not judge the material only by whichever value is highest.

For a finished product, the focus changes. You need to look at the specified test load, maximum force, elongation or deformation, and how and where the product failed. Breaking force alone cannot tell you whether the product meets every requirement.

Tensile Strength Mainly Describes Material Performance

Tensile strength describes stress, not the total pulling force shown by the testing machine. It is calculated using the applied force and the sample’s original cross-sectional area (the area across the sample if you cut through it). The result is commonly expressed in MPa (megapascals) or N/mm² (newtons per square millimeter). Tensile strength is important when you are evaluating a material. For a finished product, however, its dimensions, shape, welding, heat treatment, and pulling direction also affect performance. You therefore cannot determine the working load limit (WLL), which is the maximum load allowed in normal use, from the material’s tensile strength alone.

Breaking Force Describes a Specific Sample or Finished Product

Breaking force is the actual force carried by a material sample or finished product when it fails. It is normally expressed in N (newtons) or kN (kilonewtons). Even when two products use the same material, their breaking forces may differ if their thickness, size, or shape is different.

For welded steel chain, breaking force depends on more than the material. Link diameter, link shape, weld quality, and heat treatment can all affect the result. Breaking force is also not the only value that matters for every product. An S hook or connecting link, for example, may deform or stop holding the connection before it actually breaks.

Test Load Is Not the Same as Working Load Limit

Proof load is a test force set by the applicable product standard. It may also be called a specified test load. During the test, the product must withstand this force without any change in shape, damage, or failure that the standard does not allow. Proof load is used for inspection. It is not the product’s working load limit.

The working load limit serves a different purpose. It is the maximum load that a product is rated to carry in normal service, so it is not the same as the specified test load or minimum breaking force. Without the applicable product standard and the manufacturer’s rated data, you should not calculate a working load limit from the result of a single breaking test. You should never put a product into service based only on such a calculation.

How Do Tensile Testing Priorities Differ Between Hardware Products?

Whether you are buying chains, springs, hooks, or connecting links, you can ask the manufacturer to use an appropriate load test to check the finished product. However, these products have different structures and uses, so the testing priorities and the way you interpret the results will also differ.

Chains

If you are purchasing chain products, the raw-material test report cannot replace the finished-chain test result. The finished chain also needs to be pulled and tested as a complete product. You should check the specified test load, minimum breaking force, and any stretching or change in the shape of the links. Testing a complete chain shows how the material, link dimensions, weld quality, and heat treatment work together. However, it cannot replace separate checks of the material, dimensions, and welds.

After you know which results to check, you also need to use the correct test standard. Standards vary with the chain grade, size, and intended use.

• Some welded carbon steel chains may be tested with reference to ASTM A413/A413M (a standard for certain welded carbon steel chains). Other chains should follow the relevant standards from ASTM International, the National Association of Chain Manufacturers (NACM), the International Organization for Standardization (ISO), or another suitable organization.
• If you are buying a light-duty weldless chain such as double loop chain, jack chain, or sash chain, you should not directly apply the requirements for transport chain or lifting chain. Other light-duty products, including single loop chain and hanging chain, also need test requirements that match their shape and intended use.

Welded and weldless chains have different structures and uses, so you should not judge them in the same way.

Springs

For an extension spring or tension spring, the main points are usually the force at a set length, initial tension (the force already present before the coils begin to separate), and whether the spring returns to its original shape after the force is removed. These results often tell you more about whether the spring can provide the required force in its working position than its maximum breaking force does.

However, you cannot use the wire’s tensile strength in place of these finished-spring results. Coil diameter, the number of active coils (the coils that move when the spring is loaded), heat treatment, and end hooks all affect the finished spring. You should also remember that a compression spring is pushed rather than pulled during normal use. It therefore needs a compression load test instead of a tensile test.

For these reasons, a spring should be tested according to its product drawing, agreed technical requirements, or an applicable industry standard.

Hooks, Connecting Links, and Formed Metal Parts

For finished products such as S hooks, quick links, connecting links, snap hooks, and wire forms, you need to check both the pulling force and any change in shape. A product may not break during the test, but it may already have bent, opened, or stopped working as intended.

To make these checks meaningful, the product also needs to be pulled in its normal direction of use. If the pulling direction or contact point is wrong, the result may show a problem with the test setup instead of the product. For custom hooks, connecting links, and wire forms, you first need to understand how the product will be installed and pulled in actual use. You can then decide whether the test should focus on maximum force, change in shape, or loss of connection.

For this reason, the final test method for hooks, connecting links, and formed metal parts should follow the relevant product standard or the technical requirements confirmed with the customer.

What Tensile Testing Information Should You Confirm When Purchasing?

When buying materials, chains, springs, or other hardware, you normally do not need every setting from the tensile testing machine. Test speed, fixtures, the maximum force the machine can apply, and the pulling direction can affect the result. For a routine purchasing check, however, you usually only need to confirm that the test method suits the product and review the main results generated by the machine, such as test load, maximum force, stretching, or change in shape.

With that in mind, you should first confirm that the item described in the report matches the product you are buying. You should then check whether the result meets the agreed requirement.

The specific results you check next depend on the product:

• If you are buying raw material, you should focus on tensile strength, yield strength, and elongation in the report.
• If you are buying chains, hooks, or connecting links, you should focus on the specified test load, breaking force, and deformation.
• For an extension spring test report, you should pay particular attention to the force at the specified length and the spring’s recovery after unloading.

If your order includes several sizes or grades, a passing result for one specification does not prove that the entire product range has passed. In this situation, you should ask the supplier to clearly identify which product specification corresponds to each test result.

Beyond matching each result to the correct specification, you may need to confirm more details when the product standard, contract, or target market has specific testing requirements. In this case, you should agree with the supplier on the sample quantity, third-party testing (testing by an independent laboratory), or any additional test conditions. For products intended for the United States, Europe, Australia, or another specific market, you should confirm in advance whether the project, customer, or industry requires a particular testing standard. This helps prevent the supplier from using a test method that does not meet the final acceptance requirement.

What Are the Limitations of Tensile Testing?

Tensile testing can help you understand a product’s strength, how far it stretches, and whether it changes shape. However, it also has several limitations:

• The test may destroy the sample. A breaking test pulls the sample until it fails, so manufacturers normally test only selected samples instead of every finished product.
• One test result cannot represent every product in a batch. It can help you judge whether the batch meets the required level, but normal differences in materials and production may cause individual products to produce slightly different results.
• The test result cannot fully represent performance in actual use. A conventional tensile test usually applies a steady force in one direction. In actual use, the product may also face impact, repeated loading, side forces, twisting, wear, or corrosion.

For these reasons, passing a tensile test does not mean that the product will perform in the same way under every working condition.

Frequently Asked Questions About Tensile Testing

What Is the Acceptance Standard for a Hardware Tensile Test?

There is no single passing value for every hardware tensile test. You first need to know whether the test is for a material or for a finished product such as a chain, spring, or hook. Metal materials are commonly tested according to methods such as ASTM E8/E8M or ISO 6892-1 (common standards for testing metal samples). Finished products must meet the relevant product standard, drawing, or order requirement. When a spring is pulled and tested, the force at the length shown on the drawing is often more important than a general material standard. A result is acceptable only when it meets the requirements for that particular product.

Can a Material Tensile Test Report Replace a Finished Hardware Test Report?

Usually, it cannot. A material report only tells you about the steel or wire itself. Once that material is made into a chain, spring, or hook, welding, forming, heat treatment, and product dimensions all affect how much force the finished product can carry. If you need to confirm how much pulling force the complete product can withstand, you need the result of a finished-product tensile test or, where appropriate, a product pull test (a pull test on the complete product).

Why Can Tensile Test Results Differ for the Same Product?

Even when products have the same name and dimensions, differences in material batches, welding, forming, and heat treatment can cause some variation. Test speed, fixtures, installation direction, and sample condition can also affect the result. When you compare several reports, you should first confirm that the product specification, test standard, and test method are the same. You can then compare the values.

How Can You Tell Whether a Supplier’s Tensile Test Report Is Reliable?

You should first check the product name, specification, and batch shown in the report to make sure it covers the product you are buying. You should then check whether the test date, test method, and results are complete. If the specification does not match your order or a value looks unusual, you should ask the supplier to explain it. If you need further verification, you can arrange third-party testing (testing by an independent laboratory).

How Are the Sample Quantity and Testing Frequency Determined?

There is no fixed sampling ratio for every product. The quantity and frequency depend on the product standard, order requirements, batch size, and intended use. If an order includes several sizes, grades, or production batches, one tested sample will not normally represent all of them. When no specific rule applies, you can agree with the supplier before placing the order whether samples will be selected by specification or by production batch.

Is a Result Still Valid If the Sample Breaks Near the Grip?

Not always. The grip is the part of the machine that holds the sample. If the sample breaks inside or near it, the sample may have been clamped too tightly, may have slipped, or may not have been straight. In this situation, you should not rely only on the maximum force recorded by the machine. You should check where the sample broke according to the test standard and test another sample if necessary.

What Determines the Cost and Lead Time of Tensile Testing?

Arrangements vary between suppliers. Tuohai Hardware’s chain factory has its own tensile testing equipment. For chains manufactured and supplied by us, we can perform the test at no additional charge when tensile testing is specified in the order. We can also provide the basic results generated by the equipment.

This arrangement applies to routine chain testing done in our own factory. If you need third-party testing, a special report, or additional test requirements, the cost and lead time will depend on the specific request. We do not manufacture heavy-duty anchor chains or oversized chains, so our testing experience cannot be used to estimate the testing cost or lead time for those products.

How Can You Tell Whether a Chain Tensile Testing Machine Is Suitable for the Product?

You should first confirm that the chain tensile testing machine can reach the required test load or expected breaking force. You should then check whether it has pins or fixtures (holding tools) that fit the chain size. During a chain proof load test, the result will not be reliable if the machine cannot apply enough force or the fixtures bend the links in an extra direction. For a routine purchase, you normally do not need to review every machine setting.

Can You Calculate a Chain’s Working Load Limit from Its Breaking Force?

You should not calculate a working load limit (WLL) directly from the result of a single chain breaking test. A chain’s WLL depends on the applicable product standard, chain grade, intended use, and the load rating provided by the manufacturer. You cannot determine it simply by dividing the breaking force by a safety factor (a number used to provide a safety margin). If the supplier does not provide a clear WLL, you should not make your own calculation and put the chain into service based on that figure.

Conclusion

Tensile testing shows how a material or finished product behaves under a pulling force. From our practical experience, clearly stating what you are testing and why is more important than simply looking for a higher breaking force. A result is useful only when the test method matches the product and its intended use.

If you still have questions about tensile testing, required loads, or test results, you can contact Tuohai Hardware. We can help you understand the testing requirements and results for chains, springs, and other metal hardware based on their specifications and intended uses. For products manufactured and supplied by us, we can also provide the relevant test records or test reports based on the order and your requirements.

For product specifications related to this topic, see Trailer Chains How to Attach Trailer Chains？.