The most popular small, imported Japanese and German cars have failed special head-on crash tests, the Department of Transportation said yesterday in a warning to consumers.

The American-made Chevrolet Chevette and the Italian-made Fiat Strada passed the tests, DOT said.

The Japanese-built imports were singled out for criticism by Joan Claybrook, chief of the National Highway Transportation Safety Administration, the DOT agency that conducted the tests. She faulted their seat-belt units and their structural strength, saying she was "disappointed" in their crashworthiness.

Japanese automakers "have just not put their attention" into strengthening structure, she said.

Her pointed criticism of the small imports comes a month before the beginning of a new sales campaign, when the competition between American and Japanese cars may have farreaching political and economic importance.

Twelve 1980 subcompact and mini-compact cars selected by NHTSA were crashed into a wall at 35 miles an hour. Electronically wired dummies were seated in the cars with seat belts on. A failing grade means that human occupants would have suffered serious or fatal injuries.

The cars that failed, NHTSA said yesterday, were a Honda Civic 3-door hatchback, Toyota Corolla Tercel 2-door sedan, Datsun 310 2-door hatchback, Subaru GLF 4-door sedan, Honda Prelude 2-door coupe, Toyota Corolla 4-door sedan, VW Rabbit convertible, Audi 4000 4-door sedan, Mazda 626 2-door coupe and Datsun 200 SX 2-door HT.

The addition, a 1980 Ford Mustang and a Toyota Celica carrying seat-belted dummies were crashed head-on at 35 miles per hour -- a closing speed of 70 mph. The Mustang passed, the Toyota Celica did not.

"These test results clearly illustrate the emerging safety problem faced by consumers who are shifting to smaller cars to conserve fuel," Claybrook said.

Other test results show that in a serious collision between a subcompact and a full-size car, the occupants of the smaller car are eight times more likely to be killed than those in the larger car. However, an industry investment as little as $5 to $15 per car could produce substantial improvements in seatbelts and steering columns of the cars that failed the most recent tests, she said.

In an earlier series of tests, announced in February, failing marks were givin to a Ford Fiesta and Plymouth Champ, two foreign-made mini-compacts, and the VW Rabbit hardtop. A Mercury Bobcat and Toyota Celica 2-door liftback also failed.

A Plymouth Horizon, Dodge Omni, Ford Mustang, Mercury Capri, Ford Pinto, Chevrolet Citation, Pontiac Phoenix, Olds Omega, and Buick Skylark passed.

Claybrook said that the anticipated extensive redesign of automobiles for the American market between now and 1985 presents an excellent opportunity to build safer small cars.

The transportation Department's research vehicle, for instance, has "passed" a test crash at 49.8 miles per hour, she said, and its features could be adopted in production automobiles.

The 1980 model year cars are not required to pass the 35 mph test. The agency ran the cars into the wall to dramatize its concern about small car safety, and show consumers the differences between various models. Claybrook said.

"I would certainly want to buy one that passed," she said.

The crash test will not be in force until the 1982 model year.