5 min read

Exploring Substrate Repair Considerations in Collision Repair

Written by
Kirk Carlisle
Published on
June 28, 2023

In automotive collision repair, understanding the various substrates used in vehicle construction is essential. While many substrates are used in vehicle construction, we spend most of our time repairing just a few: mild steel, aluminum, high-strength steel, and ultra-high-strength steel. High and ultra-high-strength steels are members of the advanced high-strength steels family, including substrates like boron steel, dual-phase, and others. Our focus will stay on high and ultra as they are the most commonly encountered, and most others are always replace-only components. Let’s look at some of the differences between these substrates and how those differences can impact the repair methods required. 

So what is the most significant difference between these substrates? Strength! How do we measure strength? MPa! But what’s an MPa, and why do I care as a repairer?

What is a Megapascal (MPa)?

MPa stands for megapascal, a unit of measurement for stress or pressure. It is commonly used to express the strength or hardness of materials, including those used in automotive construction. In collision repair, understanding the MPa strength ratings of different substrates is essential for determining their repairability and the appropriate repair techniques for the material to be safely repaired.

The MPa strength rating indicates the material's ability to withstand forces and deformation. Higher MPa ratings indicate more robust and rigid materials, while lower MPa ratings suggest more flexible or malleable materials. The cliff notes version is that a higher MPa material is harder to dent and, therefore, more challenging to remove a dent from. 

Results of a metal tensile stress test.

Strength Ratings and Work Hardening

During the collision repair process, it is crucial to understand work hardening. Work hardening refers to a material's increased strength and hardness when deformed or manipulated. When a panel is damaged, it undergoes plastic deformation, altering its original shape and strength characteristics. The damaged area can then be further hardened when removing the damage. Improperly repaired panels can lead to inconsistent work hardening, resulting in new and potentially dangerous strength characteristics. In other words, we can easily change the crash characteristics of a vehicle when we repair a panel. 

A correctly repaired panel ensures that its strength and structural integrity are restored to match the original manufacturer's specifications as closely as possible. Failure to achieve this can compromise the vehicle's and its occupants' safety in the event of a subsequent collision. Therefore, obtaining the vehicle's construction materials sheet from the manufacturer or a reliable source like Alldata is crucial before commencing any repair. This sheet confirms whether the panel is repairable, lets you know the material you are working with, and provides the recommended repair methods specified by the manufacturer. Some estimating systems also try to provide this information, but the best practice is always to get this information from the manufacturer. 

Common Substrates

Now, let's delve into the common substrates used in automotive collision repair:

1. Mild Steel:

Mild steel has long been the conventional material in automotive manufacturing due to its moderate strength and cost-effectiveness. Mild steels generally have an MPa below 400 and are easy to form and work with. If you’ve ever given the back side of a dent a swift palm smack and had 80% of it pop out, you were likely smacking mild steel. 

2. Aluminum:

With its lightweight properties, aluminum is increasingly used in vehicle construction to enhance fuel efficiency and performance. Repairing aluminum panels requires specialized knowledge, tools, and techniques. In the case of aluminum, it work hardens at a much faster rate compared to mild steel. It's important to note that aluminum does not have a "memory" like mild steel, so there is no ‘popping out’ a dent. Therefore, heat is often used during the repair process to prevent cracking and ensure the metal can be shaped without compromising its structural integrity. Before working on an aluminum panel, it’s important to know the alloy's heat tolerance and to stay within the required heat range. If it is not adequately heated, aluminum can work to harden and crack. If it is overheated, the heat will anneal the alloy, causing it to lose the strength characteristics it was designed with.  

3. High-Strength Steel (HSS):

High-strength steel offers superior tensile strength compared to mild steel and is commonly used in critical areas of the vehicle's structure and panels that protect occupants, like doors. High-strength alloys are typically in the 400 - 900 MPa range and will behave differently than mild steel and aluminum under stress. Due to the higher strength, additional deformation can occur while the panel is under repair. Called “oil canning,” a dent will tend to spread outward during repair due to the high tension and rigidity of the panel. Higher-strength HSS components may sometimes necessitate specific repair techniques, such as controlled heat repairs, to avoid compromising the material's integrity. A baseball-sized dent on a high-strength steel panel is more difficult to repair than a baseball-sized dent on a mild steel panel. 

4. Ultra-High-Strength Steel (UHSS):

The general rule for UHSS was that repair was not allowed for quite a while. While still valid, there are some cases where a manufacturer allows limited repair to specific UHSS components. Ultra-high-strength steels will have an MPa above 800; they are very repair resistant. They are prone to cracking during repair, and repairers should not attempt to fix a cracked panel. You will typically find Ultra-high-strength steel in impact-absorbing and structural components. Before attempting any type of repair on UHSS, always reference the manufacturer guidelines for the component and replace the component if no additional guidance is available. 

In conclusion, as the automotive industry continues to evolve, so does the need for collision repair professionals to adapt and stay informed about the various substrates used in vehicle construction. Understanding the unique properties and repair considerations of materials like mild steel, aluminum, high-strength steel, and ultra-high-strength steel is crucial for safe and effective repairs. By following the recommended repair methods provided by the vehicle manufacturer and consulting the construction materials sheet, collision repair shops can confidently tackle repairs on these diverse substrates. App screenshots on iPhone. Can Help You!

At, we understand the challenges collision repair professionals face in navigating the complexities of modern vehicle repairs. That's why our Damage Appraisal App offers a powerful solution to streamline the repair process and enhance efficiency. With the app's ability to document a panel's substrate, capture accurate repair hours, and recommend additional operations specific to each material, you can ensure that your repairs are performed precisely and in accordance with industry standards.

By leveraging the capabilities of the Damage Appraisal App, you can deliver high-quality repairs while prioritizing the safety and satisfaction of your customers. Stay ahead of the curve and embrace the power of technology to elevate your collision repair business. You can try the app for free for 10 days at If you would like to see more, we’re happy to set up a live demo for you! Simply pick the time that's best for you at

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