Lap Joint

Definition - What does Lap Joint mean?

A lap joint is the joint between two pieces of metal in which the edges or ends are overlapped and fastened together to produce a continuous or flush surface.

Several types of lap joint techniques exist and are employed depending on the type of materials, shape, and thickness of the parts or the application of the jointed object. Lap joints are also applicable in wood and plastics.

Corrosionpedia explains Lap Joint

Full-lap and half-lap joints are the most commonly used joints. In the full-lap technique, no material is removed from any of the parts and the resulting joint is the combined thickness of the two. In the half-lap joint, some material is removed from each of the members and the joint is the thickness of the thickest part. If the two members have similar thicknesses, only half is removed from each.

A lap joint may be temporary or permanent. Temporary joints may be used in most engines and other forms of machinery where replacements are required or when two parts of the machine are joined together to achieve a particular task. This uses brackets and fastening components such as bolts, screws, nails and rivets. Permanent joints are formed using brazing, welding, soldering, flame joints and adhesives.

Advantages of lap joints include:

  • Easy to prepare (does not require cut faces to be parallel or perfectly flat)
  • Can be formed between two dissimilar metals, such as aluminum and copper
  • Accommodates different thicknesses (thinner piece must be welded on top)
  • Thin material such as diaphragms and foils can be joined

Disadvantages include:

  • Some instances of lower tensile strength
  • Less rigid than the base materials since the weld may act as a pivot
  • Overlaps may be undesirable for mechanical or aesthetic reasons.
  • Micro-cracks and cavity defects may occur if wrong welding speed is used.
  • Corrosion and fatigue cracking may occur on the shielded areas due to moisture retention

Lap joints are widely used in woodworking, plastics and metals. They are used to enable the fabrication of regularly and irregularly shaped parts, sheets and also when longer parts are required. Typical applications include aircraft fuselages and the advanced structural frames for cars and motorcycles.

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