There was an armchair. It had some years of service, including a lot of reclining. There was a built-in support in the mounting assembly for the seat, where a spring was located that facilitated a degree of reclining when the user leaned back.
The backrest and armrests were connected together to one structure. This structure was attached to the seat. The seat was a 1-cm thick plywood board, with integrated metal nuts for bolts holding the seat to the undercarriage and the armrests-backrest to the seat.
During reclining, all the bending force was focused to two triads of nuts, located relatively close together. The high stress concentration, coupled with the cyclical nature of the stress, led to material fatigue - delamination of the plywood. One day the plywood gave way.
After analysis of the nature of the fault a repair approach was chosen. The broken parts of the plywood were bridged over by a metal board, with built-in studs for the Z-plates for the armrests.
2mm thick perforated steel boards, used for connection of wooden trusses, were chosen as a relatively inexpensive, commercially readily available material.
Long-strip boards was used for each armrest mount, with a square board on each its end. The square boards were used as grips for the plywood, putting it in compressive stress to prevent further delamination and to act like overgrown washers.
Three holes were drilled into each long board in a triangle matching the holes in the armrest mountpoints. M6 threads were cut in them and M6 bolts were placed in.
Mounting holes were drilled to the plywood seat, 4 on each side of the strip, using the strip as a drill guide. Bolts were placed into the drilled holes in order to keep the board in place and avoid shifting and subsequent hole misalignment.
M5x25 hex-head bolts were chosen for joining the strip and the squares. The squares were slid between the polyurethane cushioning layer on the seat and the plywood base. Bolts were inserted through the strip-plywood-square sandwich, nuts were placed on them (a fairly difficult operation, using touch).
The sandwich structures were tightened using a wrench. The armrest holding adapters were placed on the protruding M6 bolts and attached with nuts.
The chair was then reassembled and the upholstery, partially removed to gain access to the upper side of the plywood board, was nailed back in place.
Load tests shown no deformation nor other degradation of the repaired structure.
Broken chair, detail
Position of armrest-mounting board
Holes for armrest-mounting board, detail of delamination
Armrest mounting boards in place
Armrest screws detail
Armrest mounting plates, attached