Fiber-reinforced thermoplastic composite structural members, aircraft including the same, and related methods

A method of forming a fiber-reinforced thermoplastic composite structural member from a first thermoplastic sub-structure and a second thermoplastic sub-structure, wherein the first thermoplastic sub-structure includes a first web, a first flange that extends away from the first web, and a first transition region between the first flange and the first web, wherein the second thermoplastic sub-structure includes a second web, a second flange that extends away from the second web, and a second transition region between the second flange and the second web, the method comprising:

• positioning the first thermoplastic sub-structure and the second thermoplastic sub-structure relative to one another, such that the first flange extends away from the second flange and also such that the first web faces the second web;
• compressing the first web and the second web together;
• and
• welding the first web and the second web to one another to at least partially define the fiber-reinforced thermoplastic composite structural member;
• wherein the welding the first web and the second web to one another includes melting a joining layer;
• wherein the first thermoplastic sub-structure includes a first thermoplastic resin that defines a first thermoplastic resin melt temperature;
• wherein the second thermoplastic sub-structure includes a second thermoplastic resin that defines a second thermoplastic melt temperature;
• wherein the joining layer includes a joining layer thermoplastic resin that defines a joining layer thermoplastic resin melt temperature that is less than the first thermoplastic resin melt temperature and less than the second thermoplastic resin melt temperature;
• and
• further wherein the melting the joining layer includes heating to a joining layer temperature that is greater than the joining layer thermoplastic resin melt temperature and less than both the first thermoplastic resin melt temperature and the second thermoplastic resin melt temperature.

A method of forming a fiber-reinforced thermoplastic composite structural member from a first thermoplastic sub-structure and a second thermoplastic sub-structure, wherein the first thermoplastic sub-structure includes a first web, a first flange that extends away from the first web, and a first transition region between the first flange and the first web, wherein the second thermoplastic sub-structure includes a second web, a second flange that extends away from the second web, and a second transition region between the second flange and the second web, the method comprising:

• positioning the first thermoplastic sub-structure and the second thermoplastic sub-structure relative to one another such that the first flange extends away from the second flange and also such that the first web faces the second web;
• compressing the first web and the second web together;
• and
• welding the first web and the second web to one another to at least partially define the fiber-reinforced thermoplastic composite structural member, wherein, during the welding the first web and the second web to one another, the method further includes at least one of:
• (i) thermally isolating the first flange from the first web;
• (ii) thermally isolating the first transition region from the first web;
• (iii) thermally isolating the second flange from the second web;
• and
• (iv) thermally isolating the second transition region from the second web.