Introduction:
A brazed joint is a type of joint formed by joining two metal surfaces using a filler material, typically a metal alloy with a lower melting point than the base metals being joined. The strength of a brazed joint is an important factor in determining its reliability and performance. The gap between the two joining surfaces can have a significant impact on the strength of the joint.

Explanation:
When considering the strength of a brazed joint, it is important to understand the role of the gap between the joining surfaces. The gap refers to the space between the two surfaces that are being joined together. Here, we will discuss the effect of the gap on the strength of the brazed joint.

1. Decreases with increase in gap:
- Initially, as the gap between the two surfaces increases, the overall strength of the brazed joint decreases.
- This is because a larger gap results in a smaller contact area between the surfaces, leading to a reduction in the amount of bonding that occurs during the brazing process.
- With a smaller contact area, there is less surface area available for the molten filler material to wet and bond to the base metals, resulting in a weaker joint.

2. Increases up to a certain gap:
- However, there is a point at which the strength of the brazed joint starts to increase with an increase in the gap between the two surfaces.
- This is because a larger gap allows for a greater amount of filler material to be introduced into the joint during the brazing process.
- The additional filler material helps to compensate for the reduced contact area, resulting in a stronger joint.
- The increased amount of filler material also provides more bonding between the base metals, enhancing the overall strength of the joint.

3. Decreases beyond a certain gap:
- Beyond a certain point, increasing the gap between the two surfaces further will lead to a decrease in the strength of the brazed joint.
- This is because excessive gaps can create voids or gaps within the joint, which can act as stress concentrators and weaken the joint.
- Additionally, a larger gap may result in an inadequate amount of filler material being present to fully bond the base metals together, resulting in a weaker joint.

Conclusion:
In conclusion, the strength of a brazed joint initially decreases with an increase in the gap between the two joining surfaces. However, up to a certain gap, the strength starts to increase as a larger gap allows for more filler material to be introduced. Beyond this point, the strength of the joint decreases due to the presence of excessive gaps and inadequate bonding. It is important to carefully consider the appropriate gap size to achieve optimal joint strength in brazing applications.